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Petkova MA, Dobrzynski H. Do human sinoatrial node cells have t-tubules? TRANSLATIONAL RESEARCH IN ANATOMY 2021. [DOI: 10.1016/j.tria.2021.100131] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022] Open
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Kohajda Z, Loewe A, Tóth N, Varró A, Nagy N. The Cardiac Pacemaker Story-Fundamental Role of the Na +/Ca 2+ Exchanger in Spontaneous Automaticity. Front Pharmacol 2020; 11:516. [PMID: 32410993 PMCID: PMC7199655 DOI: 10.3389/fphar.2020.00516] [Citation(s) in RCA: 11] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/01/2019] [Accepted: 04/01/2020] [Indexed: 01/01/2023] Open
Abstract
The electrophysiological mechanism of the sinus node automaticity was previously considered exclusively regulated by the so-called "funny current". However, parallel investigations increasingly emphasized the importance of the Ca2+-homeostasis and Na+/Ca2+ exchanger (NCX). Recently, increasing experimental evidence, as well as insight through mechanistic in silico modeling demonstrates the crucial role of the exchanger in sinus node pacemaking. NCX had a key role in the exciting story of discovery of sinus node pacemaking mechanisms, which recently settled with a consensus on the coupled-clock mechanism after decades of debate. This review focuses on the role of the Na+/Ca2+ exchanger from the early results and concepts to recent advances and attempts to give a balanced summary of the characteristics of the local, spontaneous, and rhythmic Ca2+ releases, the molecular control of the NCX and its role in the fight-or-flight response. Transgenic animal models and pharmacological manipulation of intracellular Ca2+ concentration and/or NCX demonstrate the pivotal function of the exchanger in sinus node automaticity. We also highlight where specific hypotheses regarding NCX function have been derived from computational modeling and require experimental validation. Nonselectivity of NCX inhibitors and the complex interplay of processes involved in Ca2+ handling render the design and interpretation of these experiments challenging.
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Affiliation(s)
- Zsófia Kohajda
- MTA-SZTE Research Group of Cardiovascular Pharmacology, Hungarian Academy of Sciences, Szeged, Hungary.,Department of Pharmacology and Pharmacotherapy, Faculty of Medicine, University of Szeged, Szeged, Hungary
| | - Axel Loewe
- Institute of Biomedical Engineering, Karlsruhe Institute of Technology (KIT), Karlsruhe, Germany
| | - Noémi Tóth
- Department of Pharmacology and Pharmacotherapy, Faculty of Medicine, University of Szeged, Szeged, Hungary
| | - András Varró
- MTA-SZTE Research Group of Cardiovascular Pharmacology, Hungarian Academy of Sciences, Szeged, Hungary.,Department of Pharmacology and Pharmacotherapy, Faculty of Medicine, University of Szeged, Szeged, Hungary
| | - Norbert Nagy
- MTA-SZTE Research Group of Cardiovascular Pharmacology, Hungarian Academy of Sciences, Szeged, Hungary.,Department of Pharmacology and Pharmacotherapy, Faculty of Medicine, University of Szeged, Szeged, Hungary
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Abstract
A progressive decline in maximum heart rate (mHR) is a fundamental aspect of aging in humans and other mammals. This decrease in mHR is independent of gender, fitness, and lifestyle, affecting in equal measure women and men, athletes and couch potatoes, spinach eaters and fast food enthusiasts. Importantly, the decline in mHR is the major determinant of the age-dependent decline in aerobic capacity that ultimately limits functional independence for many older individuals. The gradual reduction in mHR with age reflects a slowing of the intrinsic pacemaker activity of the sinoatrial node of the heart, which results from electrical remodeling of individual pacemaker cells along with structural remodeling and a blunted β-adrenergic response. In this review, we summarize current evidence about the tissue, cellular, and molecular mechanisms that underlie the reduction in pacemaker activity with age and highlight key areas for future work.
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Affiliation(s)
- Colin H Peters
- Department of Physiology and Biophysics, University of Colorado Anschutz Medical Campus, Aurora, Colorado 80045, USA; , ,
| | - Emily J Sharpe
- Department of Physiology and Biophysics, University of Colorado Anschutz Medical Campus, Aurora, Colorado 80045, USA; , ,
| | - Catherine Proenza
- Department of Physiology and Biophysics, University of Colorado Anschutz Medical Campus, Aurora, Colorado 80045, USA; , ,
- Department of Cardiology, University of Colorado Anschutz Medical Campus, Aurora, Colorado 80045, USA
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Vinogradova TM, Tagirova Sirenko S, Lakatta EG. Unique Ca 2+-Cycling Protein Abundance and Regulation Sustains Local Ca 2+ Releases and Spontaneous Firing of Rabbit Sinoatrial Node Cells. Int J Mol Sci 2018; 19:ijms19082173. [PMID: 30044420 PMCID: PMC6121616 DOI: 10.3390/ijms19082173] [Citation(s) in RCA: 12] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/24/2018] [Revised: 07/20/2018] [Accepted: 07/23/2018] [Indexed: 11/16/2022] Open
Abstract
Spontaneous beating of the heart pacemaker, the sinoatrial node, is generated by sinoatrial node cells (SANC) and caused by gradual change of the membrane potential called diastolic depolarization (DD). Submembrane local Ca2+ releases (LCR) from sarcoplasmic reticulum (SR) occur during late DD and activate an inward Na+/Ca2+ exchange current, which accelerates the DD rate leading to earlier occurrence of an action potential. A comparison of intrinsic SR Ca2+ cycling revealed that, at similar physiological Ca2+ concentrations, LCRs are large and rhythmic in permeabilized SANC, but small and random in permeabilized ventricular myocytes (VM). Permeabilized SANC spontaneously released more Ca2+ from SR than VM, despite comparable SR Ca2+ content in both cell types. In this review we discuss specific patterns of expression and distribution of SR Ca2+ cycling proteins (SR Ca2+ ATPase (SERCA2), phospholamban (PLB) and ryanodine receptors (RyR)) in SANC and ventricular myocytes. We link ability of SANC to generate larger and rhythmic LCRs with increased abundance of SERCA2, reduced abundance of the SERCA inhibitor PLB. In addition, an increase in intracellular [Ca2+] increases phosphorylation of both PLB and RyR exclusively in SANC. The differences in SR Ca2+ cycling protein expression between SANC and VM provide insights into diverse regulation of intrinsic SR Ca2+ cycling that drives automaticity of SANC.
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Affiliation(s)
- Tatiana M Vinogradova
- Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, NIH, 251 Bayview Blvd, Room 8B-123, Baltimore, MD 21224, USA.
| | - Syevda Tagirova Sirenko
- Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, NIH, 251 Bayview Blvd, Room 8B-123, Baltimore, MD 21224, USA.
| | - Edward G Lakatta
- Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, NIH, 251 Bayview Blvd, Room 8B-123, Baltimore, MD 21224, USA.
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Sirenko SG, Yang D, Maltseva LA, Kim MS, Lakatta EG, Maltsev VA. Spontaneous, local diastolic subsarcolemmal calcium releases in single, isolated guinea-pig sinoatrial nodal cells. PLoS One 2017; 12:e0185222. [PMID: 28945810 PMCID: PMC5612473 DOI: 10.1371/journal.pone.0185222] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/30/2017] [Accepted: 09/10/2017] [Indexed: 11/30/2022] Open
Abstract
Uptake and release calcium from the sarcoplasmic reticulum (SR) (dubbed “calcium clock”), in the form of spontaneous, rhythmic, local diastolic calcium releases (LCRs), together with voltage-sensitive ion channels (membrane clock) form a coupled system that regulates the action potential (AP) firing rate. LCRs activate Sodium/Calcium exchanger (NCX) that accelerates diastolic depolarization and thus participating in regulation of the time at which the next AP will occur. Previous studies in rabbit SA node cells (SANC) demonstrated that the basal AP cycle length (APCL) is tightly coupled to the basal LCR period (time from the prior AP-induced Ca2+ transient to the diastolic LCR occurrence), and that this coupling is further modulated by autonomic receptor stimulation. Although spontaneous LCRs during diastolic depolarization have been reported in SANC of various species (rabbit, cat, mouse, toad), prior studies have failed to detect LCRs in spontaneously beating SANC of guinea-pig, a species that has been traditionally used in studies of cardiac pacemaker cell function. We performed a detailed investigation of whether guinea-pig SANC generate LCRs and whether they play a similar key role in regulation of the AP firing rate. We used two different approaches, 2D high-speed camera and classical line-scan confocal imaging. Positioning the scan-line beneath sarcolemma, parallel to the long axis of the cell, we found that rhythmically beating guinea-pig SANC do, indeed, generate spontaneous, diastolic LCRs beneath the surface membrane. The average key LCR characteristics measured in confocal images in guinea-pig SANC were comparable to rabbit SANC, both in the basal state and in the presence of β-adrenergic receptor stimulation. Moreover, the relationship between the LCR period and APCL was subtended by the same linear function. Thus, LCRs in guinea-pig SANC contribute to the diastolic depolarization and APCL regulation. Our findings indicate that coupled-clock system regulation of APCL is a general, species-independent, mechanism of pacemaker cell normal automaticity. Lack of LCRs in prior studies is likely explained by technical issues, as individual LCRs are small stochastic events occurring mainly near the cell border.
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Affiliation(s)
- Syevda G. Sirenko
- Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, Maryland, United States of America
| | - Dongmei Yang
- Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, Maryland, United States of America
| | - Larissa A. Maltseva
- Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, Maryland, United States of America
| | - Mary S. Kim
- Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, Maryland, United States of America
| | - Edward G. Lakatta
- Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, Maryland, United States of America
| | - Victor A. Maltsev
- Laboratory of Cardiovascular Science, National Institute on Aging, National Institutes of Health, Baltimore, Maryland, United States of America
- * E-mail:
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Abstract
Cardiac arrhythmias can follow disruption of the normal cellular electrophysiological processes underlying excitable activity and their tissue propagation as coherent wavefronts from the primary sinoatrial node pacemaker, through the atria, conducting structures and ventricular myocardium. These physiological events are driven by interacting, voltage-dependent, processes of activation, inactivation, and recovery in the ion channels present in cardiomyocyte membranes. Generation and conduction of these events are further modulated by intracellular Ca2+ homeostasis, and metabolic and structural change. This review describes experimental studies on murine models for known clinical arrhythmic conditions in which these mechanisms were modified by genetic, physiological, or pharmacological manipulation. These exemplars yielded molecular, physiological, and structural phenotypes often directly translatable to their corresponding clinical conditions, which could be investigated at the molecular, cellular, tissue, organ, and whole animal levels. Arrhythmogenesis could be explored during normal pacing activity, regular stimulation, following imposed extra-stimuli, or during progressively incremented steady pacing frequencies. Arrhythmic substrate was identified with temporal and spatial functional heterogeneities predisposing to reentrant excitation phenomena. These could arise from abnormalities in cardiac pacing function, tissue electrical connectivity, and cellular excitation and recovery. Triggering events during or following recovery from action potential excitation could thereby lead to sustained arrhythmia. These surface membrane processes were modified by alterations in cellular Ca2+ homeostasis and energetics, as well as cellular and tissue structural change. Study of murine systems thus offers major insights into both our understanding of normal cardiac activity and its propagation, and their relationship to mechanisms generating clinical arrhythmias.
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Affiliation(s)
- Christopher L-H Huang
- Physiological Laboratory and the Department of Biochemistry, University of Cambridge, Cambridge, United Kingdom
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Allogeneic transplantation of iPS cell-derived cardiomyocytes regenerates primate hearts. Nature 2016; 538:388-391. [PMID: 27723741 DOI: 10.1038/nature19815] [Citation(s) in RCA: 541] [Impact Index Per Article: 67.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/07/2015] [Accepted: 08/31/2016] [Indexed: 12/25/2022]
Abstract
Induced pluripotent stem cells (iPSCs) constitute a potential source of autologous patient-specific cardiomyocytes for cardiac repair, providing a major benefit over other sources of cells in terms of immune rejection. However, autologous transplantation has substantial challenges related to manufacturing and regulation. Although major histocompatibility complex (MHC)-matched allogeneic transplantation is a promising alternative strategy, few immunological studies have been carried out with iPSCs. Here we describe an allogeneic transplantation model established using the cynomolgus monkey (Macaca fascicularis), the MHC structure of which is identical to that of humans. Fibroblast-derived iPSCs were generated from a MHC haplotype (HT4) homozygous animal and subsequently differentiated into cardiomyocytes (iPSC-CMs). Five HT4 heterozygous monkeys were subjected to myocardial infarction followed by direct intra-myocardial injection of iPSC-CMs. The grafted cardiomyocytes survived for 12 weeks with no evidence of immune rejection in monkeys treated with clinically relevant doses of methylprednisolone and tacrolimus, and showed electrical coupling with host cardiomyocytes as assessed by use of the fluorescent calcium indicator G-CaMP7.09. Additionally, transplantation of the iPSC-CMs improved cardiac contractile function at 4 and 12 weeks after transplantation; however, the incidence of ventricular tachycardia was transiently, but significantly, increased when compared to vehicle-treated controls. Collectively, our data demonstrate that allogeneic iPSC-CM transplantation is sufficient to regenerate the infarcted non-human primate heart; however, further research to control post-transplant arrhythmias is necessary.
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Joung B, Chen PS. Function and dysfunction of human sinoatrial node. Korean Circ J 2015; 45:184-91. [PMID: 26023305 PMCID: PMC4446811 DOI: 10.4070/kcj.2015.45.3.184] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/17/2015] [Revised: 03/23/2015] [Accepted: 03/26/2015] [Indexed: 11/11/2022] Open
Abstract
Sinoatrial node (SAN) automaticity is jointly regulated by a voltage (cyclic activation and deactivation of membrane ion channels) and Ca2+ clocks (rhythmic spontaneous sarcoplasmic reticulum Ca2+ release). Using optical mapping in Langendorff-perfused canine right atrium, we previously demonstrated that the β-adrenergic stimulation pushes the leading pacemaker to the superior SAN, which has the fastest activation rate and the most robust late diastolic intracellular calcium (Cai) elevation. Dysfunction of the superior SAN is commonly observed in animal models of heart failure and atrial fibrillation (AF), which are known to be associated with abnormal SAN automaticity. Using the 3D electroanatomic mapping techniques, we demonstrated that superior SAN served as the earliest atrial activation site (EAS) during sympathetic stimulation in healthy humans. In contrast, unresponsiveness of superior SAN to sympathetic stimulation was a characteristic finding in patients with AF and SAN dysfunction, and the 3D electroanatomic mapping technique had better diagnostic sensitivity than corrected SAN recovery time testing. However, both tests have significant limitations in detecting patients with symptomatic sick sinus syndrome. Recently, we reported that the location of the EAS can be predicted by the amplitudes of P-wave in the inferior leads. The inferior P-wave amplitudes can also be used to assess the superior SAN responsiveness to sympathetic stimulation. Inverted or isoelectric P-waves at baseline that fail to normalize during isoproterenol infusion suggest SAN dysfunction. P-wave morphology analyses may be helpful in determining the SAN function in patients at risk of symptomatic sick sinus syndrome.
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Affiliation(s)
- Boyoung Joung
- Division of Cardiology, Department of Medicine, Yonsei University College of Medicine, Seoul, Korea
| | - Peng-Sheng Chen
- The Krannert Institute of Cardiology and the Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, IN, USA
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Verkerk AO, van Borren MMGJ, van Ginneken ACG, Wilders R. Ca(2+) cycling properties are conserved despite bradycardic effects of heart failure in sinoatrial node cells. Front Physiol 2015; 6:18. [PMID: 25698973 PMCID: PMC4313601 DOI: 10.3389/fphys.2015.00018] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/09/2014] [Accepted: 01/12/2015] [Indexed: 01/01/2023] Open
Abstract
BACKGROUND In animal models of heart failure (HF), heart rate decreases due to an increase in intrinsic cycle length (CL) of the sinoatrial node (SAN). Pacemaker activity of SAN cells is complex and modulated by the membrane clock, i.e., the ensemble of voltage gated ion channels and electrogenic pumps and exchangers, and the Ca(2+) clock, i.e., the ensemble of intracellular Ca(2+) ([Ca(2+)]i) dependent processes. HF in SAN cells results in remodeling of the membrane clock, but few studies have examined its effects on [Ca(2+)]i homeostasis. METHODS SAN cells were isolated from control rabbits and rabbits with volume and pressure overload-induced HF. [Ca(2+)]i concentrations, and action potentials (APs) and Na(+)-Ca(2+) exchange current (INCX) were measured using indo-1 and patch-clamp methodology, respectively. RESULTS The frequency of spontaneous [Ca(2+)]i transients was significantly lower in HF SAN cells (3.0 ± 0.1 (n = 40) vs. 3.4 ± 0.1 Hz (n = 45); mean ± SEM), indicating that intrinsic CL was prolonged. HF slowed the [Ca(2+)]i transient decay, which could be explained by the slower frequency and reduced sarcoplasmic reticulum (SR) dependent rate of Ca(2+) uptake. Other [Ca(2+)]i transient parameters, SR Ca(2+) content, INCX density, and INCX-[Ca(2+)]i relationship were all unaffected by HF. Combined AP and [Ca(2+)]i recordings demonstrated that the slower [Ca(2+)]i transient decay in HF SAN cells may result in increased INCX during the diastolic depolarization, but that this effect is likely counteracted by the HF-induced increase in intracellular Na(+). β-adrenergic and muscarinic stimulation were not changed in HF SAN cells, except that late diastolic [Ca(2+)]i rise, a prominent feature of the Ca(2+) clock, is lower during β-adrenergic stimulation. CONCLUSIONS HF SAN cells have a slower [Ca(2+)]i transient decay with limited effects on pacemaker activity. Reduced late diastolic [Ca(2+)]i rise during β-adrenergic stimulation may contribute to an impaired increase in intrinsic frequency in HF SAN cells.
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Affiliation(s)
- Arie O Verkerk
- Department of Anatomy, Embryology and Physiology, Academic Medical Center, University of Amsterdam Amsterdam, Netherlands
| | - Marcel M G J van Borren
- Department of Anatomy, Embryology and Physiology, Academic Medical Center, University of Amsterdam Amsterdam, Netherlands ; Laboratory of Clinical Chemistry and Haematology, Rijnstate Hospital Arnhem, Netherlands
| | - Antoni C G van Ginneken
- Department of Anatomy, Embryology and Physiology, Academic Medical Center, University of Amsterdam Amsterdam, Netherlands
| | - Ronald Wilders
- Department of Anatomy, Embryology and Physiology, Academic Medical Center, University of Amsterdam Amsterdam, Netherlands
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Mesirca P, Torrente AG, Mangoni ME. Functional role of voltage gated Ca(2+) channels in heart automaticity. Front Physiol 2015; 6:19. [PMID: 25698974 PMCID: PMC4313592 DOI: 10.3389/fphys.2015.00019] [Citation(s) in RCA: 89] [Impact Index Per Article: 9.9] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/21/2014] [Accepted: 01/12/2015] [Indexed: 01/08/2023] Open
Abstract
Pacemaker activity of automatic cardiac myocytes controls the heartbeat in everyday life. Cardiac automaticity is under the control of several neurotransmitters and hormones and is constantly regulated by the autonomic nervous system to match the physiological needs of the organism. Several classes of ion channels and proteins involved in intracellular Ca(2+) dynamics contribute to pacemaker activity. The functional role of voltage-gated calcium channels (VGCCs) in heart automaticity and impulse conduction has been matter of debate for 30 years. However, growing evidence shows that VGCCs are important regulators of the pacemaker mechanisms and play also a major role in atrio-ventricular impulse conduction. Incidentally, studies performed in genetically modified mice lacking L-type Cav1.3 (Cav1.3(-/-)) or T-type Cav3.1 (Cav3.1(-/-)) channels show that genetic inactivation of these channels strongly impacts pacemaking. In cardiac pacemaker cells, VGCCs activate at negative voltages at the beginning of the diastolic depolarization and importantly contribute to this phase by supplying inward current. Loss-of-function of these channels also impairs atrio-ventricular conduction. Furthermore, inactivation of Cav1.3 channels promotes also atrial fibrillation and flutter in knockout mice suggesting that these channels can play a role in stabilizing atrial rhythm. Genomic analysis demonstrated that Cav1.3 and Cav3.1 channels are widely expressed in pacemaker tissue of mice, rabbits and humans. Importantly, human diseases of pacemaker activity such as congenital bradycardia and heart block have been attributed to loss-of-function of Cav1.3 and Cav3.1 channels. In this article, we will review the current knowledge on the role of VGCCs in the generation and regulation of heart rate and rhythm. We will discuss also how loss of Ca(2+) entry through VGCCs could influence intracellular Ca(2+) handling and promote atrial arrhythmias.
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Affiliation(s)
- Pietro Mesirca
- Laboratory of Excellence in Ion Channel Science and Therapeutics, Département de Physiologie, Institut de Génomique Fonctionnelle Montpellier, France ; UMR-5203, Centre National de la Recherche Scientifique, Universités de Montpellier 1 and 2 Montpellier, France ; INSERM U 1191, Département de Physiologie, Universités de Montpellier 1 and 2 Montpellier, France
| | - Angelo G Torrente
- Laboratory of Excellence in Ion Channel Science and Therapeutics, Département de Physiologie, Institut de Génomique Fonctionnelle Montpellier, France ; UMR-5203, Centre National de la Recherche Scientifique, Universités de Montpellier 1 and 2 Montpellier, France ; INSERM U 1191, Département de Physiologie, Universités de Montpellier 1 and 2 Montpellier, France
| | - Matteo E Mangoni
- Laboratory of Excellence in Ion Channel Science and Therapeutics, Département de Physiologie, Institut de Génomique Fonctionnelle Montpellier, France ; UMR-5203, Centre National de la Recherche Scientifique, Universités de Montpellier 1 and 2 Montpellier, France ; INSERM U 1191, Département de Physiologie, Universités de Montpellier 1 and 2 Montpellier, France
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Ben-Ari M, Schick R, Barad L, Novak A, Ben-Ari E, Lorber A, Itskovitz-Eldor J, Rosen MR, Weissman A, Binah O. From beat rate variability in induced pluripotent stem cell-derived pacemaker cells to heart rate variability in human subjects. Heart Rhythm 2014; 11:1808-1818. [PMID: 25052725 PMCID: PMC4283811 DOI: 10.1016/j.hrthm.2014.05.037] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/27/2014] [Indexed: 01/27/2023]
Abstract
BACKGROUND We previously reported that induced pluripotent stem cell-derived cardiomyocytes manifest beat rate variability (BRV) resembling heart rate variability (HRV) in the human sinoatrial node. We now hypothesized the BRV-HRV continuum originates in pacemaker cells. OBJECTIVE To investigate whether cellular BRV is a source of HRV dynamics, we hypothesized 3 levels of interaction among different cardiomyocyte entities: (1) single pacemaker cells, (2) networks of electrically coupled pacemaker cells, and (3) the in situ sinoatrial node. METHODS We measured BRV/HRV properties in single pacemaker cells, induced pluripotent stem cell-derived contracting embryoid bodies (EBs), and electrocardiograms from the same individual. RESULTS Pronounced BRV/HRV was present at all 3 levels. The coefficient of variance of interbeat intervals and Poincaré plot indices SD1 and SD2 for single cells were 20 times greater than those for EBs (P < .05) and the in situ heart (the latter two were similar; P > .05). We also compared BRV magnitude among single cells, small EBs (~5-10 cells), and larger EBs (>10 cells): BRV indices progressively increased with the decrease in the cell number (P < .05). Disrupting intracellular Ca(2+) handling markedly augmented BRV magnitude, revealing a unique bimodal firing pattern, suggesting that intracellular mechanisms contribute to BRV/HRV and the fractal behavior of heart rhythm. CONCLUSION The decreased BRV magnitude in transitioning from the single cell to the EB suggests that the HRV of in situ hearts originates from the summation and integration of multiple cell-based oscillators. Hence, complex interactions among multiple pacemaker cells and intracellular Ca(2+) handling determine HRV in humans and cardiomyocyte networks.
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Affiliation(s)
- Meital Ben-Ari
- The Sohnis Family Stem Cells Center, Technion, Haifa, Israel
- The Rappaport Institute, Technion, Haifa, Israel
- Ruth & Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel
| | - Revital Schick
- The Sohnis Family Stem Cells Center, Technion, Haifa, Israel
- The Rappaport Institute, Technion, Haifa, Israel
- Ruth & Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel
| | - Lili Barad
- The Sohnis Family Stem Cells Center, Technion, Haifa, Israel
- The Rappaport Institute, Technion, Haifa, Israel
- Ruth & Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel
| | - Atara Novak
- The Sohnis Family Stem Cells Center, Technion, Haifa, Israel
- The Rappaport Institute, Technion, Haifa, Israel
- Ruth & Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel
| | - Erez Ben-Ari
- Department of Electrical Engineering, Technion, Haifa, Israel
| | - Avraham Lorber
- Ruth & Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel
- Department of Pediatric Cardiology, Rambam Medical Center
| | - Joseph Itskovitz-Eldor
- The Sohnis Family Stem Cells Center, Technion, Haifa, Israel
- Ruth & Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel
- Department of Obstetrics and Gynecology, Rambam Medical Center, Haifa, Israel
| | - Michael R Rosen
- Department of Pharmacology, College of Physicians and Surgeons of Columbia University, New York, USA
| | - Amir Weissman
- Ruth & Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel
- Department of Obstetrics and Gynecology, Rambam Medical Center, Haifa, Israel
| | - Ofer Binah
- The Sohnis Family Stem Cells Center, Technion, Haifa, Israel
- The Rappaport Institute, Technion, Haifa, Israel
- Ruth & Bruce Rappaport Faculty of Medicine, Technion, Haifa, Israel
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12
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Mechanisms of beat-to-beat regulation of cardiac pacemaker cell function by Ca²⁺ cycling dynamics. Biophys J 2014; 105:1551-61. [PMID: 24094396 DOI: 10.1016/j.bpj.2013.08.024] [Citation(s) in RCA: 28] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/06/2013] [Revised: 07/08/2013] [Accepted: 08/15/2013] [Indexed: 01/31/2023] Open
Abstract
Whether intracellular Ca(2+) cycling dynamics regulate cardiac pacemaker cell function on a beat-to-beat basis remains unknown. Here we show that under physiological conditions, application of low concentrations of caffeine (2-4 mM) to isolated single rabbit sinoatrial node cells acutely reduces their spontaneous action potential cycle length (CL) and increases Ca(2+) transient amplitude for several cycles. Numerical simulations, using a modified Maltsev-Lakatta coupled-clock model, faithfully reproduced these effects, and also the effects of CL prolongation and dysrhythmic spontaneous beating (produced by cytosolic Ca(2+) buffering) and an acute CL reduction (produced by flash-induced Ca(2+) release from a caged Ca(2+) buffer), which we had reported previously. Three contemporary numerical models (including the original Maltsev-Lakatta model) failed to reproduce the experimental results. In our proposed new model, Ca(2+) releases acutely change the CL via activation of the Na(+)/Ca(2+) exchanger current. Time-dependent CL reductions after flash-induced Ca(2+) releases (the memory effect) are linked to changes in Ca(2+) available for pumping into sarcoplasmic reticulum which, in turn, changes the sarcoplasmic reticulum Ca(2+) load, diastolic Ca(2+) releases, and Na(+)/Ca(2+) exchanger current. These results support the idea that Ca(2+) regulates CL in cardiac pacemaker cells on a beat-to-beat basis, and suggest a more realistic numerical mechanism of this regulation.
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Maltsev VA, Yaniv Y, Maltsev AV, Stern MD, Lakatta EG. Modern perspectives on numerical modeling of cardiac pacemaker cell. J Pharmacol Sci 2014; 125:6-38. [PMID: 24748434 DOI: 10.1254/jphs.13r04cr] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/25/2022] Open
Abstract
Cardiac pacemaking is a complex phenomenon that is still not completely understood. Together with experimental studies, numerical modeling has been traditionally used to acquire mechanistic insights in this research area. This review summarizes the present state of numerical modeling of the cardiac pacemaker, including approaches to resolve present paradoxes and controversies. Specifically we discuss the requirement for realistic modeling to consider symmetrical importance of both intracellular and cell membrane processes (within a recent "coupled-clock" theory). Promising future developments of the complex pacemaker system models include the introduction of local calcium control, mitochondria function, and biochemical regulation of protein phosphorylation and cAMP production. Modern numerical and theoretical methods such as multi-parameter sensitivity analyses within extended populations of models and bifurcation analyses are also important for the definition of the most realistic parameters that describe a robust, yet simultaneously flexible operation of the coupled-clock pacemaker cell system. The systems approach to exploring cardiac pacemaker function will guide development of new therapies such as biological pacemakers for treating insufficient cardiac pacemaker function that becomes especially prevalent with advancing age.
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Affiliation(s)
- Victor A Maltsev
- Laboratory of Cardiovascular Science, Intramural Research Program, National Institute on Aging, NIH, USA
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Monfredi O, Maltsev VA, Lakatta EG. Modern concepts concerning the origin of the heartbeat. Physiology (Bethesda) 2014; 28:74-92. [PMID: 23455768 DOI: 10.1152/physiol.00054.2012] [Citation(s) in RCA: 68] [Impact Index Per Article: 6.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Physiological processes governing the heart beat have been under investigation for several hundred years. Major advances have been made in the recent past. A review of the present paradigm is presented here, including a look back at important steps that led us to where we are today, alongside a glimpse into the exciting future of pacemaker research.
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Affiliation(s)
- Oliver Monfredi
- Institute of Cardiovascular Sciences, University of Manchester, Manchester, UK
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15
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Wilson CM, Stecyk JAW, Couturier CS, Nilsson GE, Farrell AP. Phylogeny and effects of anoxia on hyperpolarization-activated cyclic nucleotide-gated channel gene expression in the heart of a primitive chordate, the Pacific hagfish (Eptatretus stoutii). ACTA ACUST UNITED AC 2013; 216:4462-72. [PMID: 23997200 DOI: 10.1242/jeb.094912] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The aneural heart of the Pacific hagfish, Eptatretus stoutii, varies heart rate fourfold during recovery from anoxia. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, which play an important role in establishing the pacemaker rate of vertebrate hearts, were postulated to be present in this ancestral vertebrate heart, and it was also theorized that changes in hagfish heart rate with oxygen availability involved altered HCN expression. Partial gene cloning revealed six HCN isoforms in the hagfish heart. Hagfish representatives of HCN2, HCN3 and HCN4 were discovered, with HCN2 and HCN3 existing as isoforms designated as HCN2a, HCN2b, HCN3a, two paralogs of HCN3b, and HCN3c. Phylogenetic analysis revealed HCN3b and HCN3c to be ancestral, followed by HCN3a, HCN4 and HCN2. Moreover, HCN3a expression was dominant in both the atrial and ventricular chambers, suggesting that the HCN4 dominance in adult mammalian hearts appeared after hagfish divergence. HCN expression was higher in the atrium than in the ventricle, as might be expected given that atrial beating rate is known to be faster than the ventricular rate. In addition, mRNA expression under normoxic conditions was compared with that following 24 h of anoxia, and either a 2-h or 36-h recovery in normoxic water. In the ventricle, anoxia decreased HCN3a but not HCN4 expression. In contrast, atrial HCN3a expression significantly increased following 2 h of recovery, before returning to control levels following 36 h of recovery, possibly contributing to heart rate changes previously observed under these conditions.
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Affiliation(s)
- Christopher M Wilson
- Department of Zoology, University of British Columbia, Vancouver, BC, Canada, V6T 1Z4
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16
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SK4 Ca2+ activated K+ channel is a critical player in cardiac pacemaker derived from human embryonic stem cells. Proc Natl Acad Sci U S A 2013; 110:E1685-94. [PMID: 23589888 DOI: 10.1073/pnas.1221022110] [Citation(s) in RCA: 49] [Impact Index Per Article: 4.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/01/2023] Open
Abstract
Proper expression and function of the cardiac pacemaker is a critical feature of heart physiology. Two main mechanisms have been proposed: (i) the "voltage-clock," where the hyperpolarization-activated funny current If causes diastolic depolarization that triggers action potential cycling; and (ii) the "Ca(2+) clock," where cyclical release of Ca(2+) from Ca(2+) stores depolarizes the membrane during diastole via activation of the Na(+)-Ca(2+) exchanger. Nonetheless, these mechanisms remain controversial. Here, we used human embryonic stem cell-derived cardiomyocytes (hESC-CMs) to study their autonomous beating mechanisms. Combined current- and voltage-clamp recordings from the same cell showed the so-called "voltage and Ca(2+) clock" pacemaker mechanisms to operate in a mutually exclusive fashion in different cell populations, but also to coexist in other cells. Blocking the "voltage or Ca(2+) clock" produced a similar depolarization of the maximal diastolic potential (MDP) that culminated by cessation of action potentials, suggesting that they converge to a common pacemaker component. Using patch-clamp recording, real-time PCR, Western blotting, and immunocytochemistry, we identified a previously unrecognized Ca(2+)-activated intermediate K(+) conductance (IK(Ca), KCa3.1, or SK4) in young and old stage-derived hESC-CMs. IK(Ca) inhibition produced MDP depolarization and pacemaker suppression. By shaping the MDP driving force and exquisitely balancing inward currents during diastolic depolarization, IK(Ca) appears to play a crucial role in human embryonic cardiac automaticity.
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17
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Yaniv Y, Maltsev VA, Ziman BD, Spurgeon HA, Lakatta EG. The "funny" current (I(f)) inhibition by ivabradine at membrane potentials encompassing spontaneous depolarization in pacemaker cells. Molecules 2012; 17:8241-54. [PMID: 22777191 PMCID: PMC4523892 DOI: 10.3390/molecules17078241] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/21/2012] [Revised: 07/03/2012] [Accepted: 07/04/2012] [Indexed: 11/16/2022] Open
Abstract
Recent clinical trials have shown that ivabradine (IVA), a drug that inhibits the funny current (I(f)) in isolated sinoatrial nodal cells (SANC), decreases heart rate and reduces morbidity and mortality in patients with cardiovascular diseases. While IVA inhibits I(f), this effect has been reported at essentially unphysiological voltages, i.e., those more negative than the spontaneous diastolic depolarization (DD) between action potentials (APs). We tested the relative potency of IVA to block I(f) over a wide range of membrane potentials, including those that encompass DD governing to the SANC spontaneous firing rate. A clinically relevant IVA concentration of 3 μM to single, isolated rabbit SANC slowed the spontaneous AP firing rate by 15%. During voltage clamp the maximal I(f) was 18 ± 3 pA/pF (at -120 mV) and the maximal I(f) reduction by IVA was 60 ± 8% observed at -92 ± 4 mV. At the maximal diastolic depolarization (~-60 mV) I(f) amplitude was only -2.9 ± 0.4 pA/pF, and was reduced by only 41 ± 6% by IVA. Thus, I(f) amplitude and its inhibition by IVA at physiologically relevant membrane potentials are substantially less than that at unphysiological (hyperpolarized) membrane potentials. This novel finding more accurately describes how IVA affects SANC function and is of direct relevance to numerical modeling of SANC automaticity.
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Affiliation(s)
| | | | | | | | - Edward G. Lakatta
- Author to whom correspondence should be addressed; ; Tel.: +1-410-558-8202; Fax: +1-410-558-8150
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18
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Tao T, Paterson DJ, Smith NP. A model of cellular cardiac-neural coupling that captures the sympathetic control of sinoatrial node excitability in normotensive and hypertensive rats. Biophys J 2011; 101:594-602. [PMID: 21806927 DOI: 10.1016/j.bpj.2011.05.069] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/14/2011] [Revised: 05/12/2011] [Accepted: 05/31/2011] [Indexed: 11/16/2022] Open
Abstract
Hypertension is associated with sympathetic hyperactivity. To represent this neural-myocyte coupling, and to elucidate the mechanisms underlying sympathetic control of the cardiac pacemaker, we developed a new (to our knowledge) cellular mathematical model that incorporates signaling information from cell-to-cell communications between the sympathetic varicosity and sinoatrial node (SAN) in both normotensive (WKY) and hypertensive (SHR) rats. Features of the model include 1), a description of pacemaker activity with specific ion-channel functions and Ca(2+) handling elements; 2), dynamic β-adrenergic modulation of the excitation of the SAN; 3), representation of ionic activity of sympathetic varicosity with NE release dynamics; and 4), coupling of the varicosity model to the SAN model to simulate presynaptic transmitter release driving postsynaptic excitability. This framework captures neural-myocyte coupling and the modulation of pacemaking by nitric oxide and cyclic GMP. It also reproduces the chronotropic response to brief sympathetic stimulations. Finally, the SHR model quantitatively suggests that the impairment of cyclic GMP regulation at both sides of the sympathetic cleft is crucial for development of the autonomic phenotype observed in hypertension.
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Affiliation(s)
- T Tao
- Computing Laboratory, University of Oxford, Oxford, United Kingdom
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19
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Liu J, Sirenko S, Juhaszova M, Ziman B, Shetty V, Rain S, Shukla S, Spurgeon HA, Vinogradova TM, Maltsev VA, Lakatta EG. A full range of mouse sinoatrial node AP firing rates requires protein kinase A-dependent calcium signaling. J Mol Cell Cardiol 2011; 51:730-9. [PMID: 21840316 DOI: 10.1016/j.yjmcc.2011.07.028] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/28/2011] [Revised: 06/24/2011] [Accepted: 07/26/2011] [Indexed: 11/24/2022]
Abstract
Recent perspectives on sinoatrial nodal cell (SANC)(*) function indicate that spontaneous sarcoplasmic reticulum (SR) Ca(2+) cycling, i.e. an intracellular "Ca(2+) clock," driven by cAMP-mediated, PKA-dependent phosphorylation, interacts with an ensemble of surface membrane electrogenic molecules ("surface membrane clock") to drive SANC normal automaticity. The role of AC-cAMP-PKA-Ca(2+) signaling cascade in mouse, the species most often utilized for genetic manipulations, however, has not been systematically tested. Here we show that Ca(2+) cycling proteins (e.g. RyR2, NCX1, and SERCA2) are abundantly expressed in mouse SAN and that spontaneous, rhythmic SR generated local Ca(2+) releases (LCRs) occur in skinned mouse SANC, clamped at constant physiologic [Ca(2+)]. Mouse SANC also exhibits a high basal level of phospholamban (PLB) phosphorylation at the PKA-dependent site, Serine16. Inhibition of intrinsic PKA activity or inhibition of PDE in SANC, respectively: reduces or increases PLB phosphorylation, and markedly prolongs or reduces the LCR period; and markedly reduces or accelerates SAN spontaneous firing rate. Additionally, the increase in AP firing rate by PKA-dependent phosphorylation by β-adrenergic receptor (β-AR) stimulation requires normal intracellular Ca(2+) cycling, because the β-AR chronotropic effect is markedly blunted when SR Ca(2+) cycling is disrupted. Thus, AC-cAMP-PKA-Ca(2+) signaling cascade is a major mechanism of normal automaticity in mouse SANC.
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Affiliation(s)
- Jie Liu
- Laboratory of Cardiovascular Science, Intramural Research Program, Gerontology Research Center, National Institute on Aging, National Institutes of Health, Baltimore MD 21224, USA
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20
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Itzhaki I, Rapoport S, Huber I, Mizrahi I, Zwi-Dantsis L, Arbel G, Schiller J, Gepstein L. Calcium handling in human induced pluripotent stem cell derived cardiomyocytes. PLoS One 2011; 6:e18037. [PMID: 21483779 PMCID: PMC3069979 DOI: 10.1371/journal.pone.0018037] [Citation(s) in RCA: 145] [Impact Index Per Article: 11.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/20/2010] [Accepted: 02/23/2011] [Indexed: 11/19/2022] Open
Abstract
BACKGROUND The ability to establish human induced pluripotent stem cells (hiPSCs) by reprogramming of adult fibroblasts and to coax their differentiation into cardiomyocytes opens unique opportunities for cardiovascular regenerative and personalized medicine. In the current study, we investigated the Ca(2+)-handling properties of hiPSCs derived-cardiomyocytes (hiPSC-CMs). METHODOLOGY/PRINCIPAL FINDINGS RT-PCR and immunocytochemistry experiments identified the expression of key Ca(2+)-handling proteins. Detailed laser confocal Ca(2+) imaging demonstrated spontaneous whole-cell [Ca(2+)](i) transients. These transients required Ca(2+) influx via L-type Ca(2+) channels, as demonstrated by their elimination in the absence of extracellular Ca(2+) or by administration of the L-type Ca(2+) channel blocker nifedipine. The presence of a functional ryanodine receptor (RyR)-mediated sarcoplasmic reticulum (SR) Ca(2+) store, contributing to [Ca(2+)](i) transients, was established by application of caffeine (triggering a rapid increase in cytosolic Ca(2+)) and ryanodine (decreasing [Ca(2+)](i)). Similarly, the importance of Ca(2+) reuptake into the SR via the SR Ca(2+) ATPase (SERCA) pump was demonstrated by the inhibiting effect of its blocker (thapsigargin), which led to [Ca(2+)](i) transients elimination. Finally, the presence of an IP3-releasable Ca(2+) pool in hiPSC-CMs and its contribution to whole-cell [Ca(2+)](i) transients was demonstrated by the inhibitory effects induced by the IP3-receptor blocker 2-Aminoethoxydiphenyl borate (2-APB) and the phospholipase C inhibitor U73122. CONCLUSIONS/SIGNIFICANCE Our study establishes the presence of a functional, SERCA-sequestering, RyR-mediated SR Ca(2+) store in hiPSC-CMs. Furthermore, it demonstrates the dependency of whole-cell [Ca(2+)](i) transients in hiPSC-CMs on both sarcolemmal Ca(2+) entry via L-type Ca(2+) channels and intracellular store Ca(2+) release.
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MESH Headings
- Animals
- Biological Transport
- Calcium/metabolism
- Calcium Channels, L-Type/genetics
- Calcium Channels, L-Type/metabolism
- Cell Differentiation
- Cell Line
- Gene Expression Regulation
- Humans
- Induced Pluripotent Stem Cells/cytology
- Inositol 1,4,5-Trisphosphate/metabolism
- Inositol 1,4,5-Trisphosphate Receptors/genetics
- Inositol 1,4,5-Trisphosphate Receptors/metabolism
- Intracellular Space/metabolism
- Mice
- Myocytes, Cardiac/cytology
- Myocytes, Cardiac/enzymology
- Myocytes, Cardiac/metabolism
- Ryanodine Receptor Calcium Release Channel/genetics
- Ryanodine Receptor Calcium Release Channel/metabolism
- Sarcolemma/metabolism
- Sarcoplasmic Reticulum Calcium-Transporting ATPases/genetics
- Sarcoplasmic Reticulum Calcium-Transporting ATPases/metabolism
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Affiliation(s)
- Ilanit Itzhaki
- Sohnis Family Research Laboratory for Cardiac Electrophysiology and Regenerative Medicine, Bruce Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, Haifa, Israel
| | - Sophia Rapoport
- Department of Biophysics Physiology, Technion - Israel Institute of Technology, Haifa, Israel
| | - Irit Huber
- Sohnis Family Research Laboratory for Cardiac Electrophysiology and Regenerative Medicine, Bruce Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, Haifa, Israel
| | - Itzhak Mizrahi
- Sohnis Family Research Laboratory for Cardiac Electrophysiology and Regenerative Medicine, Bruce Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, Haifa, Israel
| | - Limor Zwi-Dantsis
- Sohnis Family Research Laboratory for Cardiac Electrophysiology and Regenerative Medicine, Bruce Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, Haifa, Israel
| | - Gil Arbel
- Sohnis Family Research Laboratory for Cardiac Electrophysiology and Regenerative Medicine, Bruce Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, Haifa, Israel
| | - Jackie Schiller
- Department of Biophysics Physiology, Technion - Israel Institute of Technology, Haifa, Israel
| | - Lior Gepstein
- Sohnis Family Research Laboratory for Cardiac Electrophysiology and Regenerative Medicine, Bruce Rappaport Faculty of Medicine, Technion - Israel Institute of Technology, Haifa, Israel
- * E-mail:
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Abstract
Recent evidence indicates that the voltage clock (cyclic activation and deactivation of membrane ion channels) and Ca(2+) clocks (rhythmic spontaneous sarcoplasmic reticulum Ca(2+) release) jointly regulate sinoatrial node (SAN) automaticity. However, the relative importance of the voltage clock and Ca(2+) clock for pacemaking was not revealed in sick sinus syndrome. Previously, we mapped the intracellular calcium (Ca(i)) and membrane potentials of the normal intact SAN simultaneously using optical mapping in Langendorff-perfused canine right atrium. We demonstrated that the sinus rate increased and the leading pacemaker shifted to the superior SAN with robust late diastolic Ca(i) elevation (LDCAE) during β-adrenergic stimulation. We also showed that the LDCAE was caused by spontaneous diastolic sarcoplasmic reticulum (SR) Ca(2+) release and was closely related to heart rate changes. In contrast, in pacing induced canine atrial fibrillation and SAN dysfunction models, Ca(2+) clock of SAN was unresponsiveness to β-adrenergic stimulation and caffeine. Ryanodine receptor 2 (RyR2) in SAN was down-regulated. Using the prolonged low dose isoproterenol together with funny current block, we produced a tachybradycardia model. In this model, chronically elevated sympathetic tone results in abnormal pacemaking hierarchy in the right atrium, including suppression of the superior SAN and enhanced pacemaking from ectopic sites. Finally, if the LDCAE was too small to trigger an action potential, then it induced only delayed afterdepolarization (DAD)-like diastolic depolarization (DD). The failure of DAD-like DD to consistently trigger a sinus beat is a novel mechanism of atrial arrhythmogenesis. We conclude that dysfunction of both the Ca(2+) clock and the voltage clock are important in sick sinus syndrome.
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Affiliation(s)
- Boyoung Joung
- Division of Cardiology, Department of Medicine, Yonsei University College of Medicine, Seoul, Korea
| | - Peng-Sheng Chen
- The Krannert Institute of Cardiology and Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, Indiana, USA
| | - Shien-Fong Lin
- The Krannert Institute of Cardiology and Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, Indiana, USA
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22
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Robinson RB. The long and short of calcium-dependent automaticity in the sinoatrial node. Am J Physiol Heart Circ Physiol 2011; 300:H31-2. [DOI: 10.1152/ajpheart.01083.2010] [Citation(s) in RCA: 4] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Affiliation(s)
- Richard B. Robinson
- Department of Pharmacology and Center for Molecular Therapeutics, Columbia University, New York, New York
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23
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Cheng H, Smith GL, Hancox JC, Orchard CH. Inhibition of spontaneous activity of rabbit atrioventricular node cells by KB-R7943 and inhibitors of sarcoplasmic reticulum Ca(2+) ATPase. Cell Calcium 2010; 49:56-65. [PMID: 21163524 PMCID: PMC3048929 DOI: 10.1016/j.ceca.2010.11.008] [Citation(s) in RCA: 21] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/20/2010] [Revised: 11/11/2010] [Accepted: 11/17/2010] [Indexed: 11/26/2022]
Abstract
The atrioventricular node (AVN) can act as a subsidiary cardiac pacemaker if the sinoatrial node fails. In this study, we investigated the effects of the Na–Ca exchange (NCX) inhibitor KB-R7943, and inhibition of the sarcoplasmic reticulum calcium ATPase (SERCA), using thapsigargin or cyclopiazonic acid (CPA), on spontaneous action potentials (APs) and [Ca2+]i transients from cells isolated from the rabbit AVN. Spontaneous [Ca2+]i transients were monitored from undialysed AVN cells at 37 °C using Fluo-4. In separate experiments, spontaneous APs and ionic currents were recorded using the whole-cell patch clamp technique. Rapid application of 5 μM KB-R7943 slowed or stopped spontaneous APs and [Ca2+]i transients. However, in voltage clamp experiments in addition to blocking NCX current (INCX) KB-R7943 partially inhibited L-type calcium current (ICa,L). Rapid reduction of external [Na+] also abolished spontaneous activity. Inhibition of SERCA (using 2.5 μM thapsigargin or 30 μM CPA) also slowed or stopped spontaneous APs and [Ca2+]i transients. Our findings are consistent with the hypothesis that sarcoplasmic reticulum (SR) Ca2+ release influences spontaneous activity in AVN cells, and that this occurs via [Ca2+]i-activated INCX; however, the inhibitory action of KB-R7943 on ICa,L means that care is required in the interpretation of data obtained using this compound.
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Affiliation(s)
- Hongwei Cheng
- School of Physiology and Pharmacology, Medical Sciences Building, University of Bristol, UK
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24
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Himeno Y, Toyoda F, Satoh H, Amano A, Cha CY, Matsuura H, Noma A. Minor contribution of cytosolic Ca2+ transients to the pacemaker rhythm in guinea pig sinoatrial node cells. Am J Physiol Heart Circ Physiol 2010; 300:H251-61. [PMID: 20952667 DOI: 10.1152/ajpheart.00764.2010] [Citation(s) in RCA: 49] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/07/2023]
Abstract
The question of the extent to which cytosolic Ca(2+) affects sinoatrial node pacemaker activity has been discussed for decades. We examined this issue by analyzing two mathematical pacemaker models, based on the "Ca(2+) clock" (C) and "membrane clock" (M) hypotheses, together with patch-clamp experiments in isolated guinea pig sinoatrial node cells. By applying lead potential analysis to the models, the C mechanism, which is dependent on potentiation of Na(+)/Ca(2+) exchange current via spontaneous Ca(2+) release from the sarcoplasmic reticulum (SR) during diastole, was found to overlap M mechanisms in the C model. Rapid suppression of pacemaker rhythm was observed in the C model by chelating intracellular Ca(2+), whereas the M model was unaffected. Experimental rupturing of the perforated-patch membrane to allow rapid equilibration of the cytosol with 10 mM BAPTA pipette solution, however, failed to decrease the rate of spontaneous action potential within ∼30 s, whereas contraction ceased within ∼3 s. The spontaneous rhythm also remained intact within a few minutes when SR Ca(2+) dynamics were acutely disrupted using high doses of SR blockers. These experimental results suggested that rapid disruption of normal Ca(2+) dynamics would not markedly affect spontaneous activity. Experimental prolongation of the action potentials, as well as slowing of the Ca(2+)-mediated inactivation of the L-type Ca(2+) currents induced by BAPTA, were well explained by assuming Ca(2+) chelation, even in the proximity of the channel pore in addition to the bulk cytosol in the M model. Taken together, the experimental and model findings strongly suggest that the C mechanism explicitly described by the C model can hardly be applied to guinea pig sinoatrial node cells. The possible involvement of L-type Ca(2+) current rundown induced secondarily through inhibition of Ca(2+)/calmodulin kinase II and/or Ca(2+)-stimulated adenylyl cyclase was discussed as underlying the disruption of spontaneous activity after prolonged intracellular Ca(2+) concentration reduction for >5 min.
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Affiliation(s)
- Yukiko Himeno
- Department of Diabetes and Clinical Nutrition, Faculty of Medicine, Kyoto University, Kyoto, Japan.
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25
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Imtiaz MS, von der Weid PY, Laver DR, van Helden DF. SR Ca2+ store refill—a key factor in cardiac pacemaking. J Mol Cell Cardiol 2010; 49:412-26. [DOI: 10.1016/j.yjmcc.2010.03.015] [Citation(s) in RCA: 22] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 01/06/2010] [Revised: 03/18/2010] [Accepted: 03/19/2010] [Indexed: 01/01/2023]
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26
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Kurata Y, Matsuda H, Hisatome I, Shibamoto T. Roles of hyperpolarization-activated current If in sinoatrial node pacemaking: insights from bifurcation analysis of mathematical models. Am J Physiol Heart Circ Physiol 2010; 298:H1748-60. [PMID: 20363885 DOI: 10.1152/ajpheart.00729.2009] [Citation(s) in RCA: 20] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 12/20/2022]
Abstract
To elucidate the roles of hyperpolarization-activated current (I(f)) in sinoatrial node (SAN) pacemaking, we theoretically investigated 1) the effects of I(f) on stability and bifurcation during hyperpolarization of SAN cells; 2) combined effects of I(f) and the sustained inward current (I(st)) or Na(+) channel current (I(Na)) on robustness of pacemaking against hyperpolarization; and 3) whether blocking I(f) abolishes pacemaker activity under certain conditions. Bifurcation analyses were performed for mathematical models of rabbit SAN cells; equilibrium points (EPs), periodic orbits, and their stability were determined as functions of parameters. Unstable steady-state potential region determined with applications of constant bias currents shrunk as I(f) density increased. In the central SAN cell, the critical acetylcholine concentration at which bifurcations, to yield a stable EP and quiescence, occur was increased by smaller I(f), but decreased by larger I(f). In contrast, the critical acetylcholine concentration and conductance of gap junctions between SAN and atrial cells at bifurcations progressively increased with enhancing I(f) in the peripheral SAN cell. These effects of I(f) were significantly attenuated by eliminating I(st) or I(Na), or by accelerating their inactivation. Under hyperpolarized conditions, blocking I(f) abolished SAN pacemaking via bifurcations. These results suggest that 1) I(f) itself cannot destabilize EPs; 2) I(f) improves SAN cell robustness against parasympathetic stimulation via preventing bifurcations in the presence of I(st) or I(Na); 3) I(f) dramatically enhances peripheral cell robustness against electrotonic loads of the atrium in combination with I(Na); and 4) pacemaker activity of hyperpolarized SAN cells could be abolished by blocking I(f).
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Affiliation(s)
- Yasutaka Kurata
- Dept. of Physiology, Kanazawa Medical Univ., 1-1 Daigaku, Uchinada-machi, Kahoku-gun, Ishikawa 920-0293, Japan.
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Efimov IR, Fedorov VV, Joung B, Lin SF. Mapping cardiac pacemaker circuits: methodological puzzles of the sinoatrial node optical mapping. Circ Res 2010; 106:255-71. [PMID: 20133911 DOI: 10.1161/circresaha.109.209841] [Citation(s) in RCA: 45] [Impact Index Per Article: 3.2] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Historically, milestones in science are usually associated with methodological breakthroughs. Likewise, the advent of electrocardiography, microelectrode recordings and more recently optical mapping have ushered in new periods of significance of advancement in elucidating basic mechanisms in cardiac electrophysiology. As with any novel technique, however, data interpretation is challenging and should be approached with caution, as it cannot be simply extrapolated from previously used methodologies and with experience and time eventually becomes validated. A good example of this is the use of optical mapping in the sinoatrial node (SAN): when microelectrode and optical recordings are obtained from the same site in myocardium, significantly different results may be noted with respect to signal morphology and as a result have to be interpreted by a different set of principles. Given the rapid spread of the use of optical mapping, careful evaluation must be made in terms of methodology with respect to interpretation of data gathered by optical sensors from fluorescent potential-sensitive dyes. Different interpretations of experimental data may lead to different mechanistic conclusions. This review attempts to address the origin and interpretation of the "double component" morphology in the optical action potentials obtained from the SAN region. One view is that these 2 components represent distinctive signals from the SAN and atrial cells and can be fully separated with signal processing. A second view is that the first component preceding the phase 0 activation represents the membrane currents and intracellular calcium transients induced diastolic depolarization from the SAN. Although the consensus from both groups is that ionic mechanisms, namely the joint action of the membrane and calcium automaticity, are important in the SAN function, it is unresolved whether the double-component originates from the recording methodology or represents the underlying physiology. This overview aims to advance a common understanding of the basic principles of optical mapping in complex 3D anatomic structures.
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Affiliation(s)
- Igor R Efimov
- Department of Biomedical Engineering, Washington University, One Brookings Dr, St Louis, MO 63130, USA.
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28
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Lakatta EG, Maltsev VA, Vinogradova TM. A coupled SYSTEM of intracellular Ca2+ clocks and surface membrane voltage clocks controls the timekeeping mechanism of the heart's pacemaker. Circ Res 2010; 106:659-73. [PMID: 20203315 PMCID: PMC2837285 DOI: 10.1161/circresaha.109.206078] [Citation(s) in RCA: 453] [Impact Index Per Article: 32.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/01/2023]
Abstract
Ion channels on the surface membrane of sinoatrial nodal pacemaker cells (SANCs) are the proximal cause of an action potential. Each individual channel type has been thoroughly characterized under voltage clamp, and the ensemble of the ion channel currents reconstructed in silico generates rhythmic action potentials. Thus, this ensemble can be envisioned as a surface "membrane clock" (M clock). Localized subsarcolemmal Ca(2+) releases are generated by the sarcoplasmic reticulum via ryanodine receptors during late diastolic depolarization and are referred to as an intracellular "Ca(2+) clock," because their spontaneous occurrence is periodic during voltage clamp or in detergent-permeabilized SANCs, and in silico as well. In spontaneously firing SANCs, the M and Ca(2+) clocks do not operate in isolation but work together via numerous interactions modulated by membrane voltage, subsarcolemmal Ca(2+), and protein kinase A and CaMKII-dependent protein phosphorylation. Through these interactions, the 2 subsystem clocks become mutually entrained to form a robust, stable, coupled-clock system that drives normal cardiac pacemaker cell automaticity. G protein-coupled receptors signaling creates pacemaker flexibility, ie, effects changes in the rhythmic action potential firing rate, by impacting on these very same factors that regulate robust basal coupled-clock system function. This review examines evidence that forms the basis of this coupled-clock system concept in cardiac SANCs.
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Affiliation(s)
- Edward G Lakatta
- Laboratory of Cardiovascular Science, Gerontology Research Center, National Institute on Aging/NIH, 5600 Nathan Shock Dr., Baltimore, MD 21224-6825, USA.
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Abstract
During a normal lifetime, the heart may beat over 2 billion times, but the mechanisms by which the heart beats are initiated remain a subject of intense investigation. Since the discovery of a pacemaker current (I(f)) in 1978, multiple studies have shown that rhythmic changes in membrane voltage (the "membrane voltage clock") underlie the mechanisms of automaticity. The I(f) is a depolarization current activated during hyperpolarization. Therefore, when the cardiac cells recover, the I(f) is activated and slowly depolarizes the cell membrane, leading to the onset of action potential. Recent studies, however, suggest that increased intracellular Ca (Ca(i)) induced by spontaneous rhythmic sarcoplasmic reticulum Ca release (the "calcium clock") is also jointly responsible for the initiation of the heart beat. Elevated Ca(i) activates another ionic current (the sodium-calcium exchanger current or I(NCX)), leading to spontaneous phase 4 depolarization. Under normal conditions, both clocks are needed to initiate the heart beat. Malfunction of the clocks is associated with sinus node dysfunction in heart failure and atrial fibrillation. More studies are needed to determine how both clocks work together to initiate heart beat under normal and disease conditions.
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Affiliation(s)
- Peng-Sheng Chen
- Krannert Institute of Cardiology and Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, IN 46202, USA.
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Maltsev VA, Lakatta EG. Yin and yang of the cardiac pacemaker clock system in health and disease. Heart Rhythm 2009; 7:96-8. [PMID: 20129289 DOI: 10.1016/j.hrthm.2009.11.001] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/30/2009] [Indexed: 11/24/2022]
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Vinogradova TM, Lakatta EG. Regulation of basal and reserve cardiac pacemaker function by interactions of cAMP-mediated PKA-dependent Ca2+ cycling with surface membrane channels. J Mol Cell Cardiol 2009; 47:456-74. [PMID: 19573534 PMCID: PMC2757791 DOI: 10.1016/j.yjmcc.2009.06.014] [Citation(s) in RCA: 63] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/03/2009] [Accepted: 06/23/2009] [Indexed: 01/01/2023]
Abstract
Decades of intensive research of primary cardiac pacemaker, the sinoatrial node, have established potential roles of specific membrane channels in the generation of the diastolic depolarization, the major mechanism allowing sinoatrial node cells to generate spontaneous beating. During the last three decades, multiple studies made either in the isolated sinoatrial node or sinoatrial node cells have demonstrated a pivotal role of Ca(2+) and, specifically Ca(2+) release from sarcoplasmic reticulum, for spontaneous beating of cardiac pacemaker. Recently, spontaneous, rhythmic local subsarcolemmal Ca(2+) releases from ryanodine receptors during late half of the diastolic depolarization have been implicated as a vital factor in the generation of sinoatrial node cell spontaneous firing. Local Ca(2+) releases are driven by a unique combination of high basal cAMP production by adenylyl cyclases, high basal cAMP degradation by phosphodiesterases and a high level of cAMP-mediated PKA-dependent phosphorylation. These local Ca(2+) releases activate an inward Na(+)-Ca(2+) exchange current which accelerates the terminal diastolic depolarization rate and, thus, controls the spontaneous pacemaker firing. Both the basal primary pacemaker beating rate and its modulation via beta-adrenergic receptor stimulation appear to be critically dependent upon intact RyR function and local subsarcolemmal sarcoplasmic reticulum generated Ca(2+) releases. This review aspires to integrate the traditional viewpoint that has emphasized the supremacy of the ensemble of surface membrane ion channels in spontaneous firing of the primary cardiac pacemaker, and these novel perspectives of cAMP-mediated PKA-dependent Ca(2+) cycling in regulation of the heart pacemaker clock, both in the basal state and during beta-adrenergic receptor stimulation.
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Affiliation(s)
- Tatiana M Vinogradova
- Laboratory of Cardiovascular Science, Gerontology Research Center, NIA, NIH, 5600 Nathan Shock Drive, Baltimore, MD 21224-6825, USA
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van Borren MMGJ, Verkerk AO, Wilders R, Hajji N, Zegers JG, Bourier J, Tan HL, Verheijck EE, Peters SLM, Alewijnse AE, Ravesloot JH. Effects of muscarinic receptor stimulation on Ca2+ transient, cAMP production and pacemaker frequency of rabbit sinoatrial node cells. Basic Res Cardiol 2009; 105:73-87. [PMID: 19639379 PMCID: PMC2789936 DOI: 10.1007/s00395-009-0048-9] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 03/04/2009] [Revised: 07/08/2009] [Accepted: 07/10/2009] [Indexed: 12/29/2022]
Abstract
We investigated the contribution of the intracellular calcium (Cai2+) transient to acetylcholine (ACh)-mediated reduction of pacemaker frequency and cAMP content in rabbit sinoatrial nodal (SAN) cells. Action potentials (whole cell perforated patch clamp) and Cai2+ transients (Indo-1 fluorescence) were recorded from single isolated rabbit SAN cells, whereas intracellular cAMP content was measured in SAN cell suspensions using a cAMP assay (LANCE®). Our data show that the Cai2+ transient, like the hyperpolarization-activated “funny current” (If) and the ACh-sensitive potassium current (IK,ACh), is an important determinant of ACh-mediated pacemaker slowing. When If and IK,ACh were both inhibited, by cesium (2 mM) and tertiapin (100 nM), respectively, 1 μM ACh was still able to reduce pacemaker frequency by 72%. In these If and IK,ACh-inhibited SAN cells, good correlations were found between the ACh-mediated change in interbeat interval and the ACh-mediated change in Cai2+ transient decay (r2 = 0.98) and slow diastolic Cai2+ rise (r2 = 0.73). Inhibition of the Cai2+ transient by ryanodine (3 μM) or BAPTA-AM (5 μM) facilitated ACh-mediated pacemaker slowing. Furthermore, ACh depressed the Cai2+ transient and reduced the sarcoplasmic reticulum (SR) Ca2+ content, all in a concentration-dependent fashion. At 1 μM ACh, the spontaneous activity and Cai2+ transient were abolished, but completely recovered when cAMP production was stimulated by forskolin (10 μM) and IK,ACh was inhibited by tertiapin (100 nM). Also, inhibition of the Cai2+ transient by ryanodine (3 μM) or BAPTA-AM (25 μM) exaggerated the ACh-mediated inhibition of cAMP content, indicating that Cai2+ affects cAMP production in SAN cells. In conclusion, muscarinic receptor stimulation inhibits the Cai2+ transient via a cAMP-dependent signaling pathway. Inhibition of the Cai2+ transient contributes to pacemaker slowing and inhibits Cai2+-stimulated cAMP production. Thus, we provide functional evidence for the contribution of the Cai2+ transient to ACh-induced inhibition of pacemaker activity and cAMP content in rabbit SAN cells.
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Affiliation(s)
- Marcel M G J van Borren
- Heart Failure Research Center, Academic Medical Center, University of Amsterdam, Amsterdam, The Netherlands.
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Chen B, Wu Y, Mohler PJ, Anderson ME, Song LS. Local control of Ca2+-induced Ca2+ release in mouse sinoatrial node cells. J Mol Cell Cardiol 2009; 47:706-15. [PMID: 19615376 DOI: 10.1016/j.yjmcc.2009.07.007] [Citation(s) in RCA: 31] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 06/06/2009] [Revised: 07/06/2009] [Accepted: 07/06/2009] [Indexed: 01/01/2023]
Abstract
Emerging evidence from large animal models implicates Ca2+ regulation, particularly intracellular sarcoplasmic reticulum (SR) Ca2+ release, as essential for sinoatrial node (SAN) automaticity. However, despite the apparent importance of SR Ca2+ release to SAN cell function it is uncertain how SR Ca2+ release is controlled in SAN cells from mouse. Understanding mouse SAN SR Ca2+ release mechanism will allow improved understanding of results in studies on SAN from genetic mouse models of Ca2+ homeostatic proteins. Here we investigated the functional relationship between sarcolemmal Ca2+ influx and SR Ca2+ release at the level of single SAN cell, using simultaneous patch-clamp current recording and high resolution confocal Ca2+ imaging techniques. In mouse SAN cells, both Ca2+ channel currents and triggered SR Ca2+ transients displayed bell-shaped, graded function with the membrane potential. Moreover, the gain function for Ca2+-induced Ca2+ release (CICR) displayed a monotonically decreasing function with strong voltage dependence, consistent with a "local control" mechanism for CICR. In addition, we observed numerous discrete Ca2+ sparks at the voltage range of diastolic depolarization, in sharp contrast to the much lower frequency of sparks observed at resting potentials. We concluded that the "local control" mechanism of CICR is responsible for both local Ca2+ release during diastolic depolarization and the synchronized Ca2+ transients observed during action potential in SAN cells.
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Affiliation(s)
- Biyi Chen
- Division of Cardiovascular Medicine, Department of Internal Medicine, University of Iowa Carver College of Medicine, 285 Newton Road, Iowa City, IA 52242, USA
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Abstract
Recent evidence indicates that the voltage (cyclic activation and deactivation of membrane ion channels) and Ca(2+) clocks (rhythmic spontaneous sarcoplasmic reticulum Ca(2+) release) jointly regulate sinoatrial node (SAN) automaticity. Since the intact SAN is a heterogeneous structure that includes multiple different cell types interacting with each other, the relative importance of the voltage and Ca(2+) clocks for pacemaking may be variable in different regions of the SAN. Recently, we performed optical mapping in isolated and Langendorff-perfused canine right atria. We mapped the intracellular calcium (Ca(i)) and membrane potentials of the intact SAN simultaneously. Using previously described criteria of the timing of the late diastolic Ca(i) elevation (LDCAE) relative to the action potential upstroke to detect Ca(2+) clock activity, we demonstrated that the sinus rate increased and the leading pacemaker shifted to the superior SAN with the robust LDCAE during beta-adrenergic stimulation. We also showed that the LDCAE was caused by spontaneous diastolic SR Ca(2+) release and was closely related with heart rate changes. We conclude that the Ca(2+) and voltage clocks work synergistically to generate SAN automaticity.
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Affiliation(s)
- Boyoung Joung
- Krannert Institute of Cardiology and the Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, USA
| | - Masahiro Ogawa
- Department of Cardiology, Fukuoka University School of Medicine, Fukuoka, Japan
| | - Shien-Fong Lin
- Krannert Institute of Cardiology and the Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, USA
| | - Peng-Sheng Chen
- Krannert Institute of Cardiology and the Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, USA
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Lakatta EG, DiFrancesco D. What keeps us ticking: a funny current, a calcium clock, or both? J Mol Cell Cardiol 2009; 47:157-70. [PMID: 19361514 DOI: 10.1016/j.yjmcc.2009.03.022] [Citation(s) in RCA: 205] [Impact Index Per Article: 13.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/05/2009] [Revised: 03/10/2009] [Accepted: 03/19/2009] [Indexed: 12/14/2022]
Affiliation(s)
- Edward G Lakatta
- Laboratory of Cardiovascular Science, National Institute on Aging, Intramural Research Program, NIH, 5600 Nathan Shock Drive, Baltimore, MD 21224-6825, USA.
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Joung B, Tang L, Maruyama M, Han S, Chen Z, Stucky M, Jones LR, Fishbein MC, Weiss JN, Chen PS, Lin SF. Intracellular calcium dynamics and acceleration of sinus rhythm by beta-adrenergic stimulation. Circulation 2009; 119:788-96. [PMID: 19188501 DOI: 10.1161/circulationaha.108.817379] [Citation(s) in RCA: 83] [Impact Index Per Article: 5.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
BACKGROUND Recent evidence indicates that membrane voltage and Ca2+ clocks jointly regulate sinoatrial node (SAN) automaticity. Here we test the hypothesis that sinus rate acceleration by beta-adrenergic stimulation involves synergistic interactions between these clock mechanisms. METHODS AND RESULTS We simultaneously mapped intracellular calcium (Ca(i)) and membrane potential in 25 isolated canine right atrium, using previously described criteria of the timing of late diastolic Ca(i) elevation (LDCAE) relative to the action potential upstroke to detect the Ca2+ clock. Before isoproterenol, the earliest pacemaking site occurred in the inferior SAN, and LDCAE was observed in only 4 of 25 preparations. Isoproterenol infusion (1 micromol/L) increased sinus rate and shifted pacemaking site to superior SAN, concomitant with the appearance of LDCAE preceding the action potential upstroke by 98+/-31 ms. Caffeine had similar effects, whereas sarcoplasmic reticulum Ca2+ depletion with ryanodine and thapsigargin prevented isoproterenol-induced LDCAE and blunted sinus rate acceleration. Ca(i) transient relaxation time during isoproterenol was shorter in superior SAN (124+/-34 ms) than inferior SAN (138+/-24 ms; P=0.01) or right atrium (164+/-33 ms; P=0.001) and was associated with a lower sarcoplasmic reticulum Ca2+ ATPase pump to phospholamban protein ratio in SAN than in right atrium. Hyperpolarization-activated pacemaker current (I(f)) blockade with ZD 7288 modestly blunted but did not prevent LDCAE or sinus rate acceleration by isoproterenol. CONCLUSIONS Acceleration of the Ca2+ clock in the superior SAN plays an important role in sinus acceleration during beta-adrenergic stimulation, interacting synergistically with the voltage clock to increase sinus rate.
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Affiliation(s)
- Boyoung Joung
- Krannert Institute of Cardiology and the Division of Cardiology, Department of Medicine, Indiana University School of Medicine, Indianapolis, USA
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Abstract
The heart automaticity is a fundamental physiological function in higher organisms. The spontaneous activity is initiated by specialized populations of cardiac cells generating periodical electrical oscillations. The exact cascade of steps initiating the pacemaker cycle in automatic cells has not yet been entirely elucidated. Nevertheless, ion channels and intracellular Ca(2+) signaling are necessary for the proper setting of the pacemaker mechanism. Here, we review the current knowledge on the cellular mechanisms underlying the generation and regulation of cardiac automaticity. We discuss evidence on the functional role of different families of ion channels in cardiac pacemaking and review recent results obtained on genetically engineered mouse strains displaying dysfunction in heart automaticity. Beside ion channels, intracellular Ca(2+) release has been indicated as an important mechanism for promoting automaticity at rest as well as for acceleration of the heart rate under sympathetic nerve input. The potential links between the activity of ion channels and Ca(2+) release will be discussed with the aim to propose an integrated framework of the mechanism of automaticity.
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Affiliation(s)
- Matteo E Mangoni
- Institute of Functional Genomics, Department of Physiology, Centre National de la Recherche Scientifique UMR5203, INSERM U661, University of Montpellier I and II, Montpellier, France.
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Dun W, Boyden PA. The Purkinje cell; 2008 style. J Mol Cell Cardiol 2008; 45:617-24. [PMID: 18778712 DOI: 10.1016/j.yjmcc.2008.08.001] [Citation(s) in RCA: 55] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/18/2008] [Revised: 07/22/2008] [Accepted: 08/01/2008] [Indexed: 11/26/2022]
Abstract
Cardiac Purkinje fibers, due to their unique anatomical location, cell structure and electrophysiologic characteristics, play an important role in cardiac conduction and arrhythmogenesis. Purkinje cell action potentials are longer than their ventricular counterpart, and display two levels of resting potential. Purkinje cells provide for rapid propagation of the cardiac impulse to ventricular cells and have pacemaker and triggered activity, which differs from ventricular cells. Additionally, a unique intracellular Ca2+ release coordination has been revealed recently for the normal Purkinje cell. However, since the isolation of single Purkinje cells is difficult, particularly in small animals, research using Purkinje cells has been restricted. This review concentrates on comparison of Purkinje and ventricular cells in the morphology of the action potential, ionic channel function and molecular determinants by summarizing our present day knowledge of Purkinje cells.
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Affiliation(s)
- Wen Dun
- Department of Pharmacology, Center for Molecular Therapeutics, Columbia University, New York, NY, USA
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Kurata Y, Matsuda H, Hisatome I, Shibamoto T. Regional difference in dynamical property of sinoatrial node pacemaking: role of na+ channel current. Biophys J 2008; 95:951-77. [PMID: 18390617 PMCID: PMC2440451 DOI: 10.1529/biophysj.107.112854] [Citation(s) in RCA: 43] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/15/2007] [Accepted: 03/25/2008] [Indexed: 02/03/2023] Open
Abstract
To elucidate the regional differences in sinoatrial node pacemaking mechanisms, we investigated 1), bifurcation structures during current blocks or hyperpolarization of the central and peripheral cells, 2), ionic bases of regional differences in bifurcation structures, and 3), the role of Na(+) channel current (I(Na)) in peripheral cell pacemaking. Bifurcation analyses were performed for mathematical models of the rabbit sinoatrial node central and peripheral cells; equilibrium points, periodic orbits, and their stability were determined as functions of parameters. Structural stability against applications of acetylcholine or electrotonic modulations of the atrium was also evaluated. Blocking L-type Ca(2+) channel current (I(Ca,L)) stabilized equilibrium points and abolished pacemaking in both the center and periphery. Critical acetylcholine concentration and gap junction conductance for pacemaker cessation were higher in the periphery than in the center, being dramatically reduced by blocking I(Na). Under hyperpolarized conditions, blocking I(Na), but not eliminating I(Ca,L), abolished peripheral cell pacemaking. These results suggest that 1), I(Ca,L) is responsible for basal pacemaking in both the central and peripheral cells, 2), the peripheral cell is more robust in withstanding hyperpolarizing loads than the central cell, 3), I(Na) improves the structural stability to hyperpolarizing loads, and 4), I(Na)-dependent pacemaking is possible in hyperpolarized peripheral cells.
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Affiliation(s)
- Yasutaka Kurata
- Department of Physiology, Kanazawa Medical University, Ishikawa 920-0293, Japan.
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40
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Lakatta EG, Vinogradova TM, Maltsev VA. The missing link in the mystery of normal automaticity of cardiac pacemaker cells. Ann N Y Acad Sci 2008; 1123:41-57. [PMID: 18375576 PMCID: PMC8407086 DOI: 10.1196/annals.1420.006] [Citation(s) in RCA: 40] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Earlier studies of the initiating event of normal automaticity of the heart's pacemaker cells, inspired by classical quantitative membrane theory, focused upon ion currents (IK, I f) that determine the maximum diastolic potential and the early phase of the spontaneous diastolic depolarization (DD). These early DD events are caused by the prior action potential (AP) and essentially reflect a membrane recovery process. Events following the recovery process that ignite APs have not been recognized and remained a mystery until recently. These critical events are linked to rhythmic intracellular signals initiated by Ca2+ clock (i.e., sarcoplasmic reticulum [SR] cycling Ca2+). Sinoatrial cells, regardless of size, exhibit intense ryanodine receptor (RyR), Na+/Ca2+ exchange (NCX)-1, and SR Ca2+ ATPase-2 immunolabeling and dense submembrane NCX/RyR colocalization; Ca2+ clocks generate spontaneous stochastic but roughly periodic local subsarcolemmal Ca2+ releases (LCR). LCRs generate inward currents via NCX that exponentially accelerate the late DD. The timing and amplitude of LCR/I NCX-coupled events control the timing and amplitude of the nonlinear terminal DD and therefore ultimately control the chronotropic state by determining the timing of the I CaL activation that initiates the next AP. LCR period is precisely controlled by the kinetics of SR Ca2+ cycling, which, in turn, are regulated by 1) the status of protein kinase A-dependent phosphorylation of SR Ca2+ cycling proteins; and 2) membrane ion channels ensuring the Ca2+ homeostasis and therefore the Ca2+ available to Ca2+ clock. Thus, the link between early DD and next AP, missed in earlier studies, is ensured by a precisely physiologically regulated Ca2+ clock within pacemaker cells that integrates multiple Ca2+-dependent functions and rhythmically ignites APs during late DD via LCRs-I NCX coupling.
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Affiliation(s)
- Edward G Lakatta
- National Institutes of Health, National Institute on Aging, Laboratory of Cardiovascular Science, Baltimore, MD 21224, USA
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Du YM, Nathan RD. Simulated ischemia enhances L-type calcium current in pacemaker cells isolated from the rabbit sinoatrial node. Am J Physiol Heart Circ Physiol 2007; 293:H2986-94. [PMID: 17766479 DOI: 10.1152/ajpheart.00491.2007] [Citation(s) in RCA: 8] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Ischemic-like conditions (a glucose-free, pH 6.6 Tyrode solution bubbled with 100% N2) enhance L-type Ca current ( ICa,L) in single pacemaker cells (PCs) isolated from the rabbit sinoatrial node (SAN). In contrast, studies of ventricular myocytes have shown that acidic extracellular pH, as employed in our “ischemic” Tyrode, reduces ICa,L. Therefore, our goal was to explain why ICa,Lis increased by “ischemia” in SAN PCs. The major findings were the following: 1) blockade of Ca-induced Ca release with ryanodine, exposure of PCs to BAPTA-AM, or replacement of extracellular Ca2+with Ba2+failed to prevent the ischemia-induced enhancement of ICa,L; 2) inhibition of protein kinase A with H-89, or calcium/calmodulin-dependent protein kinase II with KN-93, reduced ICa,Lbut did not prevent its augmentation by ischemia; 3) ischemic Tyrode or pH 6.6 Tyrode shifted the steady-state inactivation curve in the positive direction, thereby reducing inactivation; 4) ischemic Tyrode increased the maximum conductance but did not affect the activation curve; 5) in rabbit atrial myocytes isolated and studied with exactly the same techniques used for SAN PCs, ischemic Tyrode reduced the maximum conductance and shifted the activation curve in the positive direction; pH 6.6 Tyrode also shifted the steady-state inactivation curve in the positive direction. We conclude that the acidic pH of ischemic Tyrode enhances ICa,Lin SAN PCs, because it increases the maximum conductance and reduces inactivation. Furthermore, the opposite results obtained with rabbit atrial myocytes cannot be explained by differences in cell isolation or patch-clamp techniques.
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Affiliation(s)
- Yi-Mei Du
- Department of Cell Physiology and Molecular Biophysics, Texas Tech University Health Sciences Center, Lubbock, TX 79430, USA.
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42
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Abstract
Triggered activity in cardiac muscle and intracellular Ca2+ have been linked in the past. However, today not only are there a number of cellular proteins that show clear Ca2+ dependence but also there are a number of arrhythmias whose mechanism appears to be linked to Ca2+-dependent processes. Thus we present a systematic review of the mechanisms of Ca2+ transport (forward excitation-contraction coupling) in the ventricular cell as well as what is known for other cardiac cell types. Second, we review the molecular nature of the proteins that are involved in this process as well as the functional consequences of both normal and abnormal Ca2+ cycling (e.g., Ca2+ waves). Finally, we review what we understand to be the role of Ca2+ cycling in various forms of arrhythmias, that is, those associated with inherited mutations and those that are acquired and resulting from reentrant excitation and/or abnormal impulse generation (e.g., triggered activity). Further solving the nature of these intricate and dynamic interactions promises to be an important area of research for a better recognition and understanding of the nature of Ca2+ and arrhythmias. Our solutions will provide a more complete understanding of the molecular basis for the targeted control of cellular calcium in the treatment and prevention of such.
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Affiliation(s)
- Henk E D J Ter Keurs
- Department of Medicine, Physiology and Biophysics, University of Calgary, Alberta, Canada
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Haverinen J, Vornanen M. Temperature acclimation modifies sinoatrial pacemaker mechanism of the rainbow trout heart. Am J Physiol Regul Integr Comp Physiol 2007; 292:R1023-32. [PMID: 17008459 DOI: 10.1152/ajpregu.00432.2006] [Citation(s) in RCA: 61] [Impact Index Per Article: 3.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The hypothesis of pacemaker level origin of thermal compensation in heart rate was tested by recording action potentials (AP) in intact sinoatrial tissue and enzymatically isolated pacemaker cells of rainbow trout acclimated at 4°C (cold) and 18°C (warm). With electrophysiological recordings, the primary pacemaker was located at the base of the sinoatrial valve, where a morphologically distinct ring of tissue comprising myocytes and neural elements was found by histological examination. Intrinsic beating rate of this pacemaker was higher in cold-acclimated (46 ± 6 APs/min) than warm-acclimated trout (38 ± 3 APs/min; P < 0.05), and a similar difference was seen in beating rate of isolated pacemaker cells (44 ± 6 vs. 38 ± 6 APs/min; P < 0.05), supporting the hypothesis that thermal acclimation modifies the intrinsic pacemaker mechanism of fish heart. Inhibition of sarcoplasmic reticulum (SR) with 10 μM ryanodine and 1 μM thapsigargin did not affect heart rate in either warm- or cold-acclimated trout at 11°C but reduced heart rate in warm-acclimated trout from 74 ± 2 to 42 ± 6 APs/min ( P < 0.05) at 18°C. At 11°C, a half-maximal blockade of the delayed rectifier K+current ( IKr) with 0.1 μM E-4031 reduced heart rate more in warm-acclimated (from 45 ± 1 to 24 ± 5 APs/min) than cold-acclimated trout (56 ± 3 vs. 48 ± 2 APs/min), whereas IKrdensity was higher and AP duration less in cold-acclimated trout ( P > 0.05). Collectively, these findings suggest that a cold-induced increase in AP discharge frequency is at least partly due to higher density of the IKrin the cold-acclimated trout, whereas contribution of SR Ca2+release to thermal compensation of heart rate is negligible.
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Affiliation(s)
- Jaakko Haverinen
- Department of Biology, University of Joensuu, PO Box 111, 80101 Joensuu, Finland
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44
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Maltsev VA, Lakatta EG. Cardiac pacemaker cell failure with preserved I(f), I(CaL), and I(Kr): a lesson about pacemaker function learned from ischemia-induced bradycardia. J Mol Cell Cardiol 2006; 42:289-94. [PMID: 17188292 PMCID: PMC1868668 DOI: 10.1016/j.yjmcc.2006.11.009] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 11/17/2006] [Accepted: 11/17/2006] [Indexed: 10/23/2022]
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45
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Maltsev VA, Vinogradova TM, Lakatta EG. The emergence of a general theory of the initiation and strength of the heartbeat. J Pharmacol Sci 2006; 100:338-69. [PMID: 16799255 DOI: 10.1254/jphs.cr0060018] [Citation(s) in RCA: 86] [Impact Index Per Article: 4.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/24/2022] Open
Abstract
Sarcoplasmic reticulum (SR) Ca(2+) cycling, that is, the Ca(2+) clock, entrained by externally delivered action potentials has been a major focus in ventricular myocyte research for the past 5 decades. In contrast, the focus of pacemaker cell research has largely been limited to membrane-delimited pacemaker mechanisms (membrane clock) driven by ion channels, as the immediate cause for excitation. Recent robust experimental evidence, based on confocal cell imaging, and supported by numerical modeling suggests a novel concept: the normal rhythmic heart beat is governed by the tight integration of both intracellular Ca(2+) and membrane clocks. In pacemaker cells the intracellular Ca(2+) clock is manifested by spontaneous, rhythmic submembrane local Ca(2+) releases from SR, which are tightly controlled by a high degree of basal and reserve PKA-dependent protein phosphorylation. The Ca(2+) releases rhythmically activate Na(+)/Ca(2+) exchange inward currents that ignite action potentials, whose shape and ion fluxes are tuned by the membrane clock which, in turn, sustains operation of the intracellular Ca(2+) clock. The idea that spontaneous SR Ca(2+) releases initiate and regulate normal automaticity provides the key that reunites pacemaker and ventricular cell research, thus evolving a general theory of the initiation and strength of the heartbeat.
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Affiliation(s)
- Victor A Maltsev
- Laboratory of Cardiovascular Science, Gerontology Research Center, National Institute on Aging Intramural Research Program, National Institutes of Health, Baltimore, MD 21224, USA
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Maltsev VA, Vinogradova TM, Bogdanov KY, Lakatta EG, Stern MD. Diastolic calcium release controls the beating rate of rabbit sinoatrial node cells: numerical modeling of the coupling process. Biophys J 2004; 86:2596-605. [PMID: 15041695 PMCID: PMC1304106 DOI: 10.1016/s0006-3495(04)74314-3] [Citation(s) in RCA: 55] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
Recent studies employing Ca2+ indicators and confocal microscopy demonstrate substantial local Ca2+ release beneath the cell plasma membrane (subspace) of sinoatrial node cells (SANCs) occurring during diastolic depolarization. Pharmacological and biophysical experiments have suggested that the released Ca2+ interacts with the plasma membrane via the ion current (INaCa) produced by the Na+/Ca2+ exchanger and constitutes an important determinant of the pacemaker rate. This study provides a numerical validation of the functional importance of diastolic Ca2+ release for rate control. The subspace Ca2+ signals in rabbit SANCs were measured by laser confocal microscopy, averaged, and calibrated. The time course of the subspace [Ca2+] displayed both diastolic and systolic components. The diastolic component was mainly due to the local Ca2+ releases; it was numerically approximated and incorporated into a SANC cellular electrophysiology model. The model predicts that the diastolic Ca2+ release strongly interacts with plasma membrane via INaCa and thus controls the phase of the action potential upstroke and ultimately the final action potential rate.
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Affiliation(s)
- Victor A Maltsev
- Gerontology Research Center, Intramural Research Program, National Institute on Aging, National Institutes of Health, Baltimore, Maryland 21224-6825, USA
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Hilgemann DW. New insights into the molecular and cellular workings of the cardiac Na+/Ca2+exchanger. Am J Physiol Cell Physiol 2004; 287:C1167-72. [PMID: 15475515 DOI: 10.1152/ajpcell.00288.2004] [Citation(s) in RCA: 54] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/08/2023]
Abstract
The cardiac Na+/Ca2+exchanger (NCX1) is almost certainly the major Ca2+extrusion mechanism in cardiac myocytes, although the driving force for Ca2+extrusion is quite small. To explain multiple recent results, it is useful to think of the exchanger as a slow Ca2+buffer that can reverse its function multiple times during the excitation-contraction cycle (ECC). An article by the group of John Reeves brings new insights to this function by analyzing the role of regulatory domains of NCX1 that mediate its activation by a rise of cytoplasmic Ca2+. It was demonstrated that the gating reactions are operative just in the physiological range of Ca2+changes, a few fold above resting Ca2+level, and that they prevent the exchanger from damping out the influence of mechanisms that transiently increase Ca2+levels. Furthermore, exchangers with deleted regulatory domains are shown to reduce resting Ca2+to lower levels than achieved by wild-type exchangers. A study by the group of Kenneth Philipson demonstrated that the NCX1 regulatory domain can bind and respond to Ca2+changes on the time scale of the ECC in rat myocytes. At the same time, studies of transgenic mice and NCX1 knockout mice generated by the Philipson group revealed that large changes of NCX1 activity have rather modest effects on ECC. Simple simulations predict these results very well: murine cardiac ECC is very sensitive to small changes of the Na+gradient, very sensitive to changes of the sarcoplasmic reticulum Ca2+pump activity, and very insensitive to changes of NCX1 activity. It is speculated that the NCX1 gating reactions not only regulate coupled 3Na+:1Ca2+exchange but also control the exchanger’s Na+leak function that generates background Na+influx and depolarizing current in cardiac myocytes.
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Affiliation(s)
- Donald W Hilgemann
- Department of Physiology, University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390-9040, USA.
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Lancaster MK, Jones SA, Harrison SM, Boyett MR. Intracellular Ca2+ and pacemaking within the rabbit sinoatrial node: heterogeneity of role and control. J Physiol 2004; 556:481-94. [PMID: 14724216 PMCID: PMC1664947 DOI: 10.1113/jphysiol.2003.057372] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
Recent studies have proposed that release of calcium from the sarcoplasmic reticulum (SR) modulates the spontaneous activity of the sinoatrial node (SAN). Previously we have shown that several calcium regulatory proteins are expressed at a lower level in the centre of the SAN compared with the periphery. Such differences may produce heterogeneity of intracellular calcium handling and pacemaker activity across the SAN. Selective isolations showed that the centre of the SAN is composed of smaller cells than the periphery. Measurements of cytosolic calcium in spontaneously beating cells showed that diastolic calcium, systolic calcium, the calcium transient amplitude and spontaneous rate were greater in larger (likely to be peripheral) cells compared with smaller (likely to be central) SAN cells. The SR calcium content was greater in larger cells, although SR recruitment was more efficient in smaller cells. The sodium-calcium exchanger and sarcolemmal calcium ATPase had a lower activity and the exchanger was responsible for a larger proportion of sarcolemmal calcium extrusion in smaller cells compared with larger cells. Ryanodine had a greater effect on the spontaneous calcium transient in larger cells compared with smaller cells, and slowed pacemaker activity in larger cells but not smaller cells, thus abolishing the difference in cycle length. This study shows heterogeneity of intracellular calcium regulation within the SAN and this contributes to differences in pacemaker activity between cells from across the SAN. The smallest central cells of the leading pacemaker region of the SAN do not require SR calcium for spontaneous activity nor does disruption of the SR alter pacemaking in these primary pacemaker cells.
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Affiliation(s)
- Matthew K Lancaster
- School of Sport and Exercise Sciences, University of Leeds, Leeds LS2 9JT, UK.
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Sarai N, Matsuoka S, Kuratomi S, Ono K, Noma A. Role of individual ionic current systems in the SA node hypothesized by a model study. THE JAPANESE JOURNAL OF PHYSIOLOGY 2003; 53:125-34. [PMID: 12877768 DOI: 10.2170/jjphysiol.53.125] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/05/2022]
Abstract
This paper discusses the development of a cardiac sinoatrial (SA) node pacemaker model. The model successfully reconstructs the experimental action potentials at various concentrations of external Ca2+ and K+. Increasing the amplitude of L-type Ca2+ current (I(CaL)) prolongs the duration of the action potential and thereby slightly decreases the spontaneous rate. On the other hand, a negative voltage shift of I(CaL) gating by a few mV markedly increases the spontaneous rate. When the amplitude of sustained inward current (I(st)) is increased, the spontaneous rate is increased irrespective of the I(CaL) amplitude. Increasing [Ca2+](o) shortens the action potential and increases the spontaneous rate. When the spontaneous activity is stopped by decreasing I(CaL) amplitude, the resting potential is nearly constant (-35 mV) over 1-15 mM [K+](o) as observed in the experiment. This is because the conductance of the inward background non-selective cation current balances with the outward [K+](o)-dependent K+ conductance. The unique role of individual voltage- and time-dependent ion channels is clearly demonstrated and distinguished from that of the background current by calculating an instantaneous zero current potential ("lead potential") during the course of the spontaneous activity.
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Affiliation(s)
- Nobuaki Sarai
- Department of Physiology and Biophysics, Kyoto University Graduate School of Medicine, Yoshidakonoe-cho, Sakyo-ku, Kyoto 606-8501, Japan
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Hanck DA, Martin RL, Tytgat J, Ulens C. Newly Cloned Threshold Channels. Compr Physiol 2002. [DOI: 10.1002/cphy.cp020118] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
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