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Jin Q, Greenstein JL, Winslow RL. Estimating the probability of early afterdepolarizations and predicting arrhythmic risk associated with long QT syndrome type 1 mutations. Biophys J 2023; 122:4042-4056. [PMID: 37705243 PMCID: PMC10598291 DOI: 10.1016/j.bpj.2023.09.001] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/10/2023] [Revised: 08/29/2023] [Accepted: 09/08/2023] [Indexed: 09/15/2023] Open
Abstract
Early afterdepolarizations (EADs) are action potential (AP) repolarization abnormalities that can trigger lethal arrhythmias. Simulations using biophysically detailed cardiac myocyte models can reveal how model parameters influence the probability of these cellular arrhythmias; however, such analyses can pose a huge computational burden. We have previously developed a highly simplified approach in which logistic regression models (LRMs) map parameters of complex cell models to the probability of ectopic beats. Here, we extend this approach to predict the probability of EADs (P(EAD)) as a mechanistic metric of arrhythmic risk. We use the LRM to investigate how changes in parameters of the slow-activating delayed rectifier current (IKs) affect P(EAD) for 17 different long QT syndrome type 1 (LQTS1) mutations. In this LQTS1 clinical arrhythmic risk prediction task, we compared P(EAD) for these 17 mutations with two other recently published model-based arrhythmia risk metrics (AP morphology metric across populations of myocyte models and transmural repolarization prolongation based on a one-dimensional [1D] tissue-level model). These model-based risk metrics yield similar prediction performance; however, each fails to stratify clinical risk for a significant number of the 17 studied LQTS1 mutations. Nevertheless, an interpretable ensemble model using multivariate linear regression built by combining all of these model-based risk metrics successfully predicts the clinical risk of 17 mutations. These results illustrate the potential of computational approaches in arrhythmia risk prediction.
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Affiliation(s)
- Qingchu Jin
- Department of Biomedical Engineering and Institute for Computational Medicine, Johns Hopkins University, Baltimore, Maryland
| | - Joseph L Greenstein
- Department of Biomedical Engineering and Institute for Computational Medicine, Johns Hopkins University, Baltimore, Maryland
| | - Raimond L Winslow
- Department of Biomedical Engineering and Institute for Computational Medicine, Johns Hopkins University, Baltimore, Maryland.
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2
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Neumann J, Hofmann B, Dhein S, Gergs U. Role of Dopamine in the Heart in Health and Disease. Int J Mol Sci 2023; 24:ijms24055042. [PMID: 36902474 PMCID: PMC10003060 DOI: 10.3390/ijms24055042] [Citation(s) in RCA: 9] [Impact Index Per Article: 9.0] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/04/2023] [Revised: 02/25/2023] [Accepted: 03/02/2023] [Indexed: 03/08/2023] Open
Abstract
Dopamine has effects on the mammalian heart. These effects can include an increase in the force of contraction, and an elevation of the beating rate and the constriction of coronary arteries. Depending on the species studied, positive inotropic effects were strong, very modest, or absent, or even negative inotropic effects occurred. We can discern five dopamine receptors. In addition, the signal transduction by dopamine receptors and the regulation of the expression of cardiac dopamine receptors will be of interest to us, because this might be a tempting area of drug development. Dopamine acts in a species-dependent fashion on these cardiac dopamine receptors, but also on cardiac adrenergic receptors. We will discuss the utility of drugs that are currently available as tools to understand cardiac dopamine receptors. The molecule dopamine itself is present in the mammalian heart. Therefore, cardiac dopamine might act as an autocrine or paracrine compound in the mammalian heart. Dopamine itself might cause cardiac diseases. Moreover, the cardiac function of dopamine and the expression of dopamine receptors in the heart can be altered in diseases such as sepsis. Various drugs for cardiac and non-cardiac diseases are currently in the clinic that are, at least in part, agonists or antagonists at dopamine receptors. We define the research needs in order to understand dopamine receptors in the heart better. All in all, an update on the role of dopamine receptors in the human heart appears to be clinically relevant, and is thus presented here.
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Affiliation(s)
- Joachim Neumann
- Institut für Pharmakologie und Toxikologie, Medizinische Fakultät, Martin-Luther-Universität Halle-Wittenberg, 06097 Halle, Germany
- Correspondence: ; Tel.: +49-345-557-1686; Fax: +49-345-557-1835
| | - Britt Hofmann
- Herzchirurgie, Medizinische Fakultät, Martin-Luther-Universität Halle-Wittenberg, 06097 Halle, Germany
| | - Stefan Dhein
- Medizinische Fakultät, Rudolf-Boehm-Institut für Pharmakologie und Toxikologie, Universität Leipzig, 04107 Leipzig, Germany
| | - Ulrich Gergs
- Institut für Pharmakologie und Toxikologie, Medizinische Fakultät, Martin-Luther-Universität Halle-Wittenberg, 06097 Halle, Germany
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3
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Del Rivero Morfin PJ, Marx SO, Ben-Johny M. Sympathetic Nervous System Regulation of Cardiac Calcium Channels. Handb Exp Pharmacol 2023. [PMID: 36592229 DOI: 10.1007/164_2022_632] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/03/2023]
Abstract
Calcium influx through voltage-gated calcium channels, Cav1.2, in cardiomyocytes initiates excitation-contraction coupling in the heart. The force and rate of cardiac contraction are modulated by the sympathetic nervous system, mediated substantially by changes in intracellular calcium. Norepinephrine released from sympathetic neurons innervating the heart and epinephrine secreted by the adrenal chromaffin cells bind to β-adrenergic receptors on the sarcolemma of cardiomyocytes initiating a signaling cascade that generates cAMP and activates protein kinase A, the targets of which control calcium influx. For decades, the mechanisms by which PKA regulated calcium channels in the heart were not known. Recently, these mechanisms have been elucidated. In this chapter, we will review the history of the field and the studies that led to the identification of the evolutionarily conserved process.
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Affiliation(s)
- Pedro J Del Rivero Morfin
- Department of Physiology and Cellular Biophysics, Columbia University Vagelos College of Physicians and Surgeons, New York, NY, USA
| | - Steven O Marx
- Division of Cardiology, Department of Medicine, Columbia University Vagelos College of Physicians and Surgeons, New York, NY, USA. .,Department of Pharmacology and Molecular Signaling, Columbia University Vagelos College of Physicians and Surgeons, New York, NY, USA.
| | - Manu Ben-Johny
- Department of Physiology and Cellular Biophysics, Columbia University Vagelos College of Physicians and Surgeons, New York, NY, USA.
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4
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Abstract
Each heartbeat is initiated by the action potential, an electrical signal that depolarizes the plasma membrane and activates a cycle of calcium influx via voltage-gated calcium channels, calcium release via ryanodine receptors, and calcium reuptake and efflux via calcium-ATPase pumps and sodium-calcium exchangers. Agonists of the sympathetic nervous system bind to adrenergic receptors in cardiomyocytes, which, via cascading signal transduction pathways and protein kinase A (PKA), increase the heart rate (chronotropy), the strength of myocardial contraction (inotropy), and the rate of myocardial relaxation (lusitropy). These effects correlate with increased intracellular concentration of calcium, which is required for the augmentation of cardiomyocyte contraction. Despite extensive investigations, the molecular mechanisms underlying sympathetic nervous system regulation of calcium influx in cardiomyocytes have remained elusive over the last 40 years. Recent studies have uncovered the mechanisms underlying this fundamental biologic process, namely that PKA phosphorylates a calcium channel inhibitor, Rad, thereby releasing inhibition and increasing calcium influx. Here, we describe an updated model for how signals from adrenergic agonists are transduced to stimulate calcium influx and contractility in the heart.
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Affiliation(s)
- Arianne Papa
- Department of Physiology and Cellular Biophysics, Vagelos College of Physicians and Surgeons, Columbia University, New York, NY, USA
| | - Jared Kushner
- Division of Cardiology, Department of Medicine, Vagelos College of Physicians and Surgeons, Columbia University, New York, NY, USA;
| | - Steven O Marx
- Division of Cardiology, Department of Medicine, Vagelos College of Physicians and Surgeons, Columbia University, New York, NY, USA;
- Department of Molecular Pharmacology and Therapeutics, Vagelos College of Physicians and Surgeons, Columbia University, New York, NY, USA
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5
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Abstract
Calmodulin (CaM) regulation of voltage-gated calcium (CaV) channels is a powerful Ca2+ feedback mechanism that adjusts Ca2+ influx, affording rich mechanistic insights into Ca2+ decoding. CaM possesses a dual-lobed architecture, a salient feature of the myriad Ca2+-sensing proteins, where two homologous lobes that recognize similar targets hint at redundant signaling mechanisms. Here, by tethering CaM lobes, we demonstrate that bilobal architecture is obligatory for signaling to CaV channels. With one lobe bound, CaV carboxy tail rearranges itself, resulting in a preinhibited configuration precluded from Ca2+ feedback. Reconstitution of two lobes, even as separate molecules, relieves preinhibition and restores Ca2+ feedback. CaV channels thus detect the coincident binding of two Ca2+-free lobes to promote channel opening, a molecular implementation of a logical NOR operation that processes spatiotemporal Ca2+ signals bifurcated by CaM lobes. Overall, a unified scheme of CaV channel regulation by CaM now emerges, and our findings highlight the versatility of CaM to perform exquisite Ca2+ computations.
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Tomek J, Bub G. Hypertension-induced remodelling: on the interactions of cardiac risk factors. J Physiol 2017; 595:4027-4036. [PMID: 28217927 PMCID: PMC5471416 DOI: 10.1113/jp273043] [Citation(s) in RCA: 40] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/16/2016] [Accepted: 01/25/2017] [Indexed: 12/19/2022] Open
Abstract
Hypertension induces considerable cardiac remodelling, such as hypertrophy, interstitial fibrosis, and abnormal activity of the cardiac sympathetic nervous system, which are established risk factors in several highly dangerous heart diseases, such as ventricular fibrillation and congestive heart failure. All these risk factors and heart diseases are studied extensively in isolation, but to our knowledge, there is no comprehensive review of their interactions. At the same time, there is growing evidence suggesting that such interactions are numerous and that a successful therapy against a particular condition may have unexpectedly weak effects on mortality, as treated patients may die of a different cause exacerbated by the therapy. In this article, we present a multiscale review of the literature focusing on the relationships between the above‐mentioned risk factors and heart diseases, and introduce a framework that gives insight into their possible interactions. We use this framework to demonstrate that conditions such as fibrosis and elevated activity of the sympathetic nervous system may be compensatory, rather than purely pathological, mechanisms in certain contexts. Finally, we show why the described mechanisms are relevant not only in hypertension, but also in the case of healed myocardial infarction.
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Affiliation(s)
- Jakub Tomek
- Department of Physiology Anatomy and Genetics, University of Oxford, Oxford, UK
| | - Gil Bub
- Department of Physiology, McGill University, Canada
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7
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Weiss S, Oz S, Benmocha A, Dascal N. Regulation of cardiac L-type Ca²⁺ channel CaV1.2 via the β-adrenergic-cAMP-protein kinase A pathway: old dogmas, advances, and new uncertainties. Circ Res 2013; 113:617-31. [PMID: 23948586 DOI: 10.1161/circresaha.113.301781] [Citation(s) in RCA: 78] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
In the heart, adrenergic stimulation activates the β-adrenergic receptors coupled to the heterotrimeric stimulatory Gs protein, followed by subsequent activation of adenylyl cyclase, elevation of cyclic AMP levels, and protein kinase A (PKA) activation. One of the main targets for PKA modulation is the cardiac L-type Ca²⁺ channel (CaV1.2) located in the plasma membrane and along the T-tubules, which mediates Ca²⁺ entry into cardiomyocytes. β-Adrenergic receptor activation increases the Ca²⁺ current via CaV1.2 channels and is responsible for the positive ionotropic effect of adrenergic stimulation. Despite decades of research, the molecular mechanism underlying this modulation has not been fully resolved. On the contrary, initial reports of identification of key components in this modulation were later refuted using advanced model systems, especially transgenic animals. Some of the cardinal debated issues include details of specific subunits and residues in CaV1.2 phosphorylated by PKA, the nature, extent, and role of post-translational processing of CaV1.2, and the role of auxiliary proteins (such as A kinase anchoring proteins) involved in PKA regulation. In addition, the previously proposed crucial role of PKA in modulation of unstimulated Ca²⁺ current in the absence of β-adrenergic receptor stimulation and in voltage-dependent facilitation of CaV1.2 remains uncertain. Full reconstitution of the β-adrenergic receptor signaling pathway in heterologous expression systems remains an unmet challenge. This review summarizes the past and new findings, the mechanisms proposed and later proven, rejected or disputed, and emphasizes the essential issues that remain unresolved.
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Affiliation(s)
- Sharon Weiss
- Department of Physiology and Pharmacology, Sackler School of Medicine, Tel Aviv University, Tel-Aviv, Israel.
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Rosa AO, Movafagh S, Cleemann L, Morad M. Hypoxic regulation of cardiac Ca2+ channel: possible role of haem oxygenase. J Physiol 2012; 590:4223-37. [PMID: 22753548 DOI: 10.1113/jphysiol.2012.236570] [Citation(s) in RCA: 15] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/12/2022] Open
Abstract
Acute and chronic hypoxias are common cardiac diseases that lead often to arrhythmia and impaired contractility. At the cellular level it is unclear whether the suppression of cardiac Ca(2+) channels (Ca(V)1.2) results directly from oxygen deprivation on the channel protein or is mediated by intermediary proteins affecting the channel. To address this question we measured the early effects of hypoxia (5-60 s, P(O(2)) < 5 mmHg) on Ca(2+) current (I(Ca)) and tested the involvement of protein kinase A (PKA) phosphorylation, Ca(2+)/calmodulin-mediated signalling and the haem oxygenase (HO) pathway in the hypoxic regulation of Ca(V)1.2 in rat and cat ventricular myocytes and HEK-293 cells. Hypoxic suppression of ICa) and Ca(2+) transients was significant within 5 s and intensified in the following 50 s, and was reversible. Phosphorylation by cAMP or the phosphatase inhibitor okadaic acid desensitized I(Ca) to hypoxia, while PKA inhibition by H-89 restored the sensitivity of I(Ca) to hypoxia. This phosphorylation effect was specific to Ca(2+), but not Ba(2+) or Na(+), permeating through the channel. CaMKII inhibitory peptide and Bay K8644 reversed the phosphorylation-induced desensitization to hypoxia. Mutation of CAM/CaMKII-binding motifs of the α(1c) subunit of Ca(V)1.2 fully desensitized the Ca(2+) channel to hypoxia. Rapid application of HO inhibitors (zinc protoporphyrin (ZnPP) and tin protoporphyrin (SnPP)) suppressed the channel in a manner similar to acute hypoxia such that: (1) I(Ca) and I(Ba) were suppressed within 5 s of ZnPP application; (2) PKA activation and CaMKII inhibitors desensitized I(Ca), but not I(Ba), to ZnPP; and (3) hypoxia failed to further suppress I(Ca) and I(Ba) in ZnPP-treated myocytes. We propose that the binding of HO to the CaM/CaMKII-specific motifs on Ca(2+) channel may mediate the rapid response of the channel to hypoxia.
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Affiliation(s)
- Angelo O Rosa
- Cardiac Signaling Center of University of South Carolina, Medical University of South Carolina and Clemson University, Charleston, SC 29245, USA
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9
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Single-channel monitoring of reversible L-type Ca(2+) channel Ca(V)α(1)-Ca(V)β subunit interaction. Biophys J 2012; 101:2661-70. [PMID: 22261054 DOI: 10.1016/j.bpj.2011.09.063] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 07/19/2011] [Accepted: 09/06/2011] [Indexed: 11/24/2022] Open
Abstract
Voltage-dependent Ca(2+) channels are heteromultimers of Ca(V)α(1) (pore), Ca(V)β- and Ca(V)α(2)δ-subunits. The stoichiometry of this complex, and whether it is dynamically regulated in intact cells, remains controversial. Fortunately, Ca(V)β-isoforms affect gating differentially, and we chose two extremes (Ca(V)β(1a) and Ca(V)β(2b)) regarding single-channel open probability to address this question. HEK293α(1C) cells expressing the Ca(V)1.2 subunit were transiently transfected with Ca(V)α(2)δ1 alone or with Ca(V)β(1a), Ca(V)β(2b), or (2:1 or 1:1 plasmid ratio) combinations. Both Ca(V)β-subunits increased whole-cell current and shifted the voltage dependence of activation and inactivation to hyperpolarization. Time-dependent inactivation was accelerated by Ca(V)β(1a)-subunits but not by Ca(V)β(2b)-subunits. Mixtures induced intermediate phenotypes. Single channels sometimes switched between periods of low and high open probability. To validate such slow gating behavior, data were segmented in clusters of statistically similar open probability. With Ca(V)β(1a)-subunits alone, channels mostly stayed in clusters (or regimes of alike clusters) of low open probability. Increasing Ca(V)β(2b)-subunits (co-)expressed (1:2, 1:1 ratio or alone) progressively enhanced the frequency and total duration of high open probability clusters and regimes. Our analysis was validated by the inactivation behavior of segmented ensemble averages. Hence, a phenotype consistent with mutually exclusive and dynamically competing binding of different Ca(V)β-subunits is demonstrated in intact cells.
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10
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Blayney LM, Lai FA. Ryanodine receptor-mediated arrhythmias and sudden cardiac death. Pharmacol Ther 2009; 123:151-77. [PMID: 19345240 PMCID: PMC2704947 DOI: 10.1016/j.pharmthera.2009.03.006] [Citation(s) in RCA: 75] [Impact Index Per Article: 5.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/27/2009] [Accepted: 03/03/2009] [Indexed: 12/25/2022]
Abstract
The cardiac ryanodine receptor-Ca2+ release channel (RyR2) is an essential sarcoplasmic reticulum (SR) transmembrane protein that plays a central role in excitation–contraction coupling (ECC) in cardiomyocytes. Aberrant spontaneous, diastolic Ca2+ leak from the SR due to dysfunctional RyR2 contributes to the formation of delayed after-depolarisations, which are thought to underlie the fatal arrhythmia that occurs in both heart failure (HF) and in catecholaminergic polymorphic ventricular tachycardia (CPVT). CPVT is an inherited disorder associated with mutations in either the RyR2 or a SR luminal protein, calsequestrin. RyR2 shows normal function at rest in CPVT but the RyR2 dysfunction is unmasked by physical exercise or emotional stress, suggesting abnormal RyR2 activation as an underlying mechanism. Several potential mechanisms have been advanced to explain the dysfunctional RyR2 observed in HF and CPVT, including enhanced RyR2 phosphorylation status, altered RyR2 regulation at luminal/cytoplasmic sites and perturbed RyR2 intra/inter-molecular interactions. This review considers RyR2 dysfunction in the context of the structural and functional modulation of the channel, and potential therapeutic strategies to stabilise RyR2 function in cardiac pathology.
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Affiliation(s)
- Lynda M Blayney
- Wales Heart Research Institute, Cardiff University School of Medicine, Cardiff CF144XN, UK.
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11
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Faber GM, Rudy Y. Calsequestrin mutation and catecholaminergic polymorphic ventricular tachycardia: a simulation study of cellular mechanism. Cardiovasc Res 2007; 75:79-88. [PMID: 17531962 PMCID: PMC2030636 DOI: 10.1016/j.cardiores.2007.04.010] [Citation(s) in RCA: 24] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 02/02/2007] [Revised: 03/15/2007] [Accepted: 04/13/2007] [Indexed: 11/19/2022] Open
Abstract
OBJECTIVES Patients with a missense mutation of the calsequestrin 2 gene (CASQ2) are at risk for catecholaminergic polymorphic ventricular tachycardia. This mutation (CASQ2(D307H)) results in decreased ability of CASQ2 to bind Ca2+ in the sarcoplasmic reticulum (SR). In this theoretical study, we investigate a potential mechanism by which CASQ2(D307H) manifests its pro-arrhythmic consequences in patients. METHODS Using simulations in a model of the guinea pig ventricular myocyte, we investigate the mutation's effect on SR Ca2+ storage, the Ca2+ transient (CaT), and its indirect effect on ionic currents and membrane potential. We model the effects of isoproterenol (ISO) on Ca(V)1.2 (the L-type Ca2+ current, I(Ca(L))) and other targets of beta-adrenergic stimulation. RESULTS ISO increases I(Ca(L)), prolonging action potential (AP) duration (Control: 172 ms, +ISO: 207 ms, at cycle length of 1500 ms) and increasing CaT (Control: 0.79 microM, +ISO: 1.61 microM). ISO increases I(Ca(L)) by reducing the fraction of channels which undergo voltage-dependent inactivation and increasing transitions from a non-conducting to conducting mode of channel gating. CASQ2(D307H) reduces SR storage capacity, thereby reducing the magnitude of CaT (Control: 0.79 microM, CASQ2(D307H): 0.52 microM, at cycle length of 1500 ms). The combined effect of CASQ2(D307H) and ISO elevates SR free Ca2+ at a rapid rate, leading to store-overload-induced Ca2+ release and delayed afterdepolarization (DAD). If resting membrane potential is sufficiently elevated, the Na+-Ca2+ exchange-driven DAD can trigger I(Na) and I(Ca(L)) activation, generating a triggered arrhythmogenic AP. CONCLUSIONS The CASQ2(D307H) mutation manifests its pro-arrhythmic consequences due to store-overload-induced Ca2+ release and DAD formation due to excess free SR Ca2+ following rapid pacing and beta-adrenergic stimulation.
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Affiliation(s)
- Gregory M. Faber
- Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH 44106-7207
| | - Yoram Rudy
- Cardiac Bioelectricity and Arrhythmia Center and Department of Biomedical Engineering, Washington University, St. Louis, MO 63130-4899
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12
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Abstract
As more detailed molecular information accumulates on the biology of the heart and other complex systems in health and disease, the need for new integrative analyses and tools is growing. Systems biology and bioengineering seek to use high-throughput technologies and integrative computational analysis to construct networks of the interactions between molecular components in the system, to develop systems models of their functionally integrated biological properties, and to incorporate these systems models into structurally integrated multi-scale models for predicting clinical phenotypes. This review gives examples of recent applications using these approaches to elucidate the electromechanical function of the heart in aging and disease.
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Affiliation(s)
- Andrew D McCulloch
- Department of Bioengineering, University of California San Diego, 9500 Gioman Drive, La Jolla, CA 92093-0412, USA.
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13
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Greenstein JL, Hinch R, Winslow RL. Mechanisms of excitation-contraction coupling in an integrative model of the cardiac ventricular myocyte. Biophys J 2005; 90:77-91. [PMID: 16214852 PMCID: PMC1367039 DOI: 10.1529/biophysj.105.065169] [Citation(s) in RCA: 112] [Impact Index Per Article: 5.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
It is now well established that characteristic properties of excitation-contraction (EC) coupling in cardiac myocytes, such as high gain and graded Ca(2+) release, arise from the interactions that occur between L-type Ca(2+) channels (LCCs) and nearby ryanodine-sensitive Ca(2+) release channels (RyRs) in localized microdomains. Descriptions of Ca(2+)-induced Ca(2+) release (CICR) that account for these local mechanisms are lacking from many previous models of the cardiac action potential, and those that do include local control of CICR are able to reconstruct properties of EC coupling, but require computationally demanding stochastic simulations of approximately 10(5) individual ion channels. In this study, we generalize a recently developed analytical approach for deriving simplified mechanistic models of CICR to formulate an integrative model of the canine cardiac myocyte which is computationally efficient. The resulting model faithfully reproduces experimentally measured properties of EC coupling and whole cell phenomena. The model is used to study the role of local redundancy in L-type Ca(2+) channel gating and the role of dyad configuration on EC coupling. Simulations suggest that the characteristic steep rise in EC coupling gain observed at hyperpolarized potentials is a result of increased functional coupling between LCCs and RyRs. We also demonstrate mechanisms by which alterations in the early repolarization phase of the action potential, resulting from reduction of the transient outward potassium current, alters properties of EC coupling.
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Affiliation(s)
- Joseph L Greenstein
- The Center for Cardiovascular Bioinformatics and Modeling and The Whitaker Biomedical Engineering Institute, The Johns Hopkins University Whiting School of Engineering and School of Medicine, Baltimore, Maryland, USA.
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14
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Luvisetto S, Fellin T, Spagnolo M, Hivert B, Brust PF, Harpold MM, Stauderman KA, Williams ME, Pietrobon D. Modal gating of human CaV2.1 (P/Q-type) calcium channels: I. The slow and the fast gating modes and their modulation by beta subunits. ACTA ACUST UNITED AC 2005; 124:445-61. [PMID: 15504896 PMCID: PMC2234000 DOI: 10.1085/jgp.200409034] [Citation(s) in RCA: 35] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/30/2022]
Abstract
The single channel gating properties of human CaV2.1 (P/Q-type) calcium channels and their modulation by the auxiliary β1b, β2e, β3a, and β4a subunits were investigated with cell-attached patch-clamp recordings on HEK293 cells stably expressing human CaV2.1 channels. These calcium channels showed a complex modal gating, which is described in this and the following paper (Fellin, T., S. Luvisetto, M. Spagnolo, and D. Pietrobon. 2004. J. Gen. Physiol. 124:463–474). Here, we report the characterization of two modes of gating of human CaV2.1 channels, the slow mode and the fast mode. A channel in the two gating modes differs in mean closed times and latency to first opening (both longer in the slow mode), in voltage dependence of the open probability (larger depolarizations are necessary to open the channel in the slow mode), in kinetics of inactivation (slower in the slow mode), and voltage dependence of steady-state inactivation (occurring at less negative voltages in the slow mode). CaV2.1 channels containing any of the four β subtypes can gate in either the slow or the fast mode, with only minor differences in the rate constants of the transitions between closed and open states within each mode. In both modes, CaV2.1 channels display different rates of inactivation and different steady-state inactivation depending on the β subtype. The type of β subunit also modulates the relative occurrence of the slow and the fast gating mode of CaV2.1 channels; β3a promotes the fast mode, whereas β4a promotes the slow mode. The prevailing mode of gating of CaV2.1 channels lacking a β subunit is a gating mode in which the channel shows shorter mean open times, longer mean closed times, longer first latency, a much larger fraction of nulls, and activates at more positive voltages than in either the fast or slow mode.
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Affiliation(s)
- Siro Luvisetto
- Dept. of Biomedical Sciences, University of Padova, Viale G. Colombo 3, 35121 Padova, Italy
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15
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Fellin T, Luvisetto S, Spagnolo M, Pietrobon D. Modal gating of human CaV2.1 (P/Q-type) calcium channels: II. the b mode and reversible uncoupling of inactivation. ACTA ACUST UNITED AC 2005; 124:463-74. [PMID: 15504897 PMCID: PMC2234006 DOI: 10.1085/jgp.200409035] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
The single channel gating properties of human CaV2.1 (P/Q-type) calcium channels were investigated with cell-attached patch-clamp recordings on HEK293 cells stably expressing these calcium channels. Human CaV2.1 channels showed a complex modal gating, which is described in this and the preceding paper (Luvisetto, S., T. Fellin, M. Spagnolo, B. Hivert, P.F. Brust, M.M. Harpold, K.A. Stauderman, M.E. Williams, and D. Pietrobon. 2004. J. Gen. Physiol. 124:445-461). Here, we report the characterization of the so-called b gating mode. A CaV2.1 channel in the b gating mode shows a bell-shaped voltage dependence of the open probability, and a characteristic low open probability at high positive voltages, that decreases with increasing voltage, as a consequence of both shorter mean open time and longer mean closed time. Reversible transitions of single human CaV2.1 channels between the b gating mode and the mode of gating in which the channel shows the usual voltage dependence of the open probability (nb gating mode) were much more frequent (time scale of seconds) than those between the slow and fast gating modes (time scale of minutes; Luvisetto et al., 2004), and occurred independently of whether the channel was in the fast or slow mode. We show that the b gating mode produces reversible uncoupling of inactivation in human CaV2.1 channels. In fact, a CaV2.1 channel in the b gating mode does not inactivate during long pulses at high positive voltages, where the same channel in both fast-nb and slow-nb gating modes inactivates relatively rapidly. Moreover, a CaV2.1 channel in the b gating mode shows a larger availability to open than in the nb gating modes. Regulation of the complex modal gating of human CaV2.1 channels could be a potent and versatile mechanism for the modulation of synaptic strength and plasticity as well as of neuronal excitability and other postsynaptic Ca2+-dependent processes.
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Affiliation(s)
- Tommaso Fellin
- Dept. of Biomedical Sciences, University of Padova, Viale G. Colombo, 3 35121 Padova, Italy
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16
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Winslow RL, Cortassa S, Greenstein JL. Using models of the myocyte for functional interpretation of cardiac proteomic data. J Physiol 2005; 563:73-81. [PMID: 15611013 PMCID: PMC1665560 DOI: 10.1113/jphysiol.2004.080457] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2004] [Accepted: 12/15/2004] [Indexed: 01/03/2023] Open
Abstract
There has been significant progress towards the development of highly integrative computational models of the cardiac myocyte over the past decade. Models now incorporate descriptions of voltage-gated ionic currents and membrane transporters, mechanisms of calcium-induced calcium release and intracellular calcium cycling, mitochondrial ATP production and its coupling to energy-requiring membrane transport processes and mechanisms of force generation. There is an extensive literature documenting both the reconstructive and predictive abilities of these models and there is no question that an interplay between quantitative modelling and experimental investigation has become a central component of modern cardiovascular research. As data regarding the cardiovascular proteome in both health and disease emerge, integrative models of the myocyte are becoming useful tools for interpreting the functional significance of changes in protein expression and post-translational modifications (PTMs). Data of particular importance include information on: (a) changes of expressed protein level, (b) changes of protein PTMs, (c) protein localization, and (d) protein-protein interactions, as it is often possible to incorporate and interpret the functional significance of such findings using computational models. We provide two examples of how models may be used in this fashion. In the first example, we show how information on altered expression of the sarcoplasmic reticulum Ca2+-ATPase, when interpreted through the use of a computational model, has provided key insights into fundamental mechanisms regulating cardiac action potential duration. In the second example, we show how information on the effects of phosphorylation of L-type Ca2+ channels, when interpreted through the use of a model, provides insights on how this post-translational modification alters the properties of excitation-contraction coupling and risk for arrhythmia.
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Affiliation(s)
- Raimond L Winslow
- Center for Cardiovascular Bioinformatics and Modelling, The Johns Hopkins University School of Medicine and Whiting School of Engineering, Baltimore, MD, USA.
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17
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Tanskanen AJ, Greenstein JL, O'Rourke B, Winslow RL. The role of stochastic and modal gating of cardiac L-type Ca2+ channels on early after-depolarizations. Biophys J 2004; 88:85-95. [PMID: 15501946 PMCID: PMC1201453 DOI: 10.1529/biophysj.104.051508] [Citation(s) in RCA: 124] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Certain signaling events that promote L-type Ca2+ channel (LCC) phosphorylation, such as beta-adrenergic stimulation or an increased expression of Ca(2+)/calmodulin-dependent protein kinase II, promote mode 2 gating of LCCs. Experimental data suggest the hypothesis that these events increase the likelihood of early after-depolarizations (EADs). We test this hypothesis using an ionic model of the canine ventricular myocyte incorporating stochastic gating of LCCs and ryanodine-sensitive calcium release channels. The model is extended to describe myocyte responses to the beta-adrenergic agonist isoproterenol. Results demonstrate that in the presence of isoproterenol the random opening of a small number of LCCs gating in mode 2 during the plateau phase of the action potential (AP) can trigger EADs. EADs occur randomly, where the likelihood of these events increases as a function of the fraction of LCCs gating in mode 2. Fluctuations of the L-type Ca2+ current during the AP plateau lead to variability in AP duration. Consequently, prolonged APs are occasionally observed and exhibit an increased likelihood of EAD formation. These results suggest a novel stochastic mechanism, whereby phosphorylation-induced changes in LCC gating properties contribute to EAD generation.
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Affiliation(s)
- Antti J Tanskanen
- Center for Cardiovascular Bioinformatics and Modeling, The Johns Hopkins University School of Medicine and Whiting School of Engineering, Baltimore, Maryland 21218, USA.
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18
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Hinch R, Greenstein JL, Tanskanen AJ, Xu L, Winslow RL. A simplified local control model of calcium-induced calcium release in cardiac ventricular myocytes. Biophys J 2004; 87:3723-36. [PMID: 15465866 PMCID: PMC1304886 DOI: 10.1529/biophysj.104.049973] [Citation(s) in RCA: 101] [Impact Index Per Article: 5.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022] Open
Abstract
Calcium (Ca2+)-induced Ca2+ release (CICR) in cardiac myocytes exhibits high gain and is graded. These properties result from local control of Ca2+ release. Existing local control models of Ca2+ release in which interactions between L-Type Ca2+ channels (LCCs) and ryanodine-sensitive Ca2+ release channels (RyRs) are simulated stochastically are able to reconstruct these properties, but only at high computational cost. Here we present a general analytical approach for deriving simplified models of local control of CICR, consisting of low-dimensional systems of coupled ordinary differential equations, from these more complex local control models in which LCC-RyR interactions are simulated stochastically. The resulting model, referred to as the coupled LCC-RyR gating model, successfully reproduces a range of experimental data, including L-Type Ca2+ current in response to voltage-clamp stimuli, inactivation of LCC current with and without Ca2+ release from the sarcoplasmic reticulum, voltage-dependence of excitation-contraction coupling gain, graded release, and the force-frequency relationship. The model does so with low computational cost.
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Affiliation(s)
- R Hinch
- Mathematical Institute, University of Oxford, Oxford, United Kingdom.
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GREENSTEIN JOSEPHL, TANSKANEN ANTTIJ, WINSLOW RAIMONDL. Modeling the actions of beta-adrenergic signaling on excitation--contraction coupling processes. Ann N Y Acad Sci 2004; 1015:16-27. [PMID: 15201146 PMCID: PMC1201510 DOI: 10.1196/annals.1302.002] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
Activation of the beta-adrenergic (beta-AR) signaling pathway enhances cardiac function through protein kinase A (PKA)-mediated phosphorylation of target proteins involved in the process of excitation-contraction (EC) coupling. Experimental studies of the effects of beta-AR stimulation on EC coupling have yielded complex results, including increased, decreased, or unchanged EC coupling gain. In this study, we extend a previously developed model of the canine ventricular myocyte describing local control of sarcoplasmic reticulum (SR) calcium (Ca(2+)) release to include the effects of beta-AR stimulation. Incorporation of phosphorylation-dependent effects on model membrane currents and Ca(2+)-cycling proteins yields changes of action potential (AP) and Ca(2+) transients in agreement with those measured experimentally in response to the nonspecific beta-AR agonist isoproterenol (ISO). The model reproduces experimentally observed alterations in EC coupling gain in response to beta-AR agonists and predicts the specific roles of L-type Ca(2+) channel (LCC) and SR Ca(2+) release channel phosphorylation in altering the amplitude and shape of the EC coupling gain function. The model also indicates that factors that promote mode 2 gating of LCCs, such as beta-AR stimulation or activation of the Ca(2+)/calmodulin-dependent protein kinase II (CaMKII), may increase the probability of occurrence of early after-depolarizations (EADs), due to the random, long-duration opening of LCC gating in mode 2.
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Affiliation(s)
| | | | - RAIMOND L. WINSLOW
- Address for correspondence: Prof. Raimond L. Winslow, Ph.D., Center for Cardiovascular Bioinformatics and Modeling, Dept. of Biomedical Engineering, The Johns Hopkins University School of Medicine, and Whiting School of Engineering, Clark Hall, 3400 N. Charles St., Baltimore, MD 21218. Fax: 410-516-5294.
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20
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Saucerman JJ, McCulloch AD. Mechanistic systems models of cell signaling networks: a case study of myocyte adrenergic regulation. PROGRESS IN BIOPHYSICS AND MOLECULAR BIOLOGY 2004; 85:261-78. [PMID: 15142747 DOI: 10.1016/j.pbiomolbio.2004.01.005] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/28/2022]
Abstract
Signal transduction networks coordinate a wide variety of cellular functions, including gene expression, metabolism, and cell fate processes. Understanding biological networks quantitatively is a major challenge to post-genomic biology, and mechanistic systems models will be crucial for this task. Here, we review approaches towards developing mechanistic systems models of established cell signaling networks. The ability of mechanistic system models to generate testable biological hypotheses and experimental strategies is discussed. As a case study of model development and analysis, we examined the functional roles of phospholamban, the L-type calcium channel, the ryanodine receptor, and troponin I phosphorylation upon beta-adrenergic stimulation in the rat ventricular myocyte. Model analysis revealed that while protein kinase A-mediated phosphorylation of the ryanodine receptor greatly increases its calcium sensitivity, calcium autoregulation may adapt quickly by negating potential increases in contractility. Systematic combinations of in silico perturbations supported the conclusion that phospholamban phosphoregulation is the primary mechanism for increased sarcoplasmic reticulum load and calcium relaxation rate during beta-adrenergic stimulation, while both phospholamban and the L-type calcium channel contribute to increased systolic calcium. Combined with detailed experimental studies, mechanistic systems models will be valuable for developing a quantitative understanding of cell signaling networks.
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Affiliation(s)
- Jeffrey J Saucerman
- Department of Bioengineering, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA 92093-0412, USA
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21
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Greenstein JL, Winslow RL. An integrative model of the cardiac ventricular myocyte incorporating local control of Ca2+ release. Biophys J 2002; 83:2918-45. [PMID: 12496068 PMCID: PMC1201479 DOI: 10.1016/s0006-3495(02)75301-0] [Citation(s) in RCA: 157] [Impact Index Per Article: 7.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
The local control theory of excitation-contraction (EC) coupling in cardiac muscle asserts that L-type Ca(2+) current tightly controls Ca(2+) release from the sarcoplasmic reticulum (SR) via local interaction of closely apposed L-type Ca(2+) channels (LCCs) and ryanodine receptors (RyRs). These local interactions give rise to smoothly graded Ca(2+)-induced Ca(2+) release (CICR), which exhibits high gain. In this study we present a biophysically detailed model of the normal canine ventricular myocyte that conforms to local control theory. The model formulation incorporates details of microscopic EC coupling properties in the form of Ca(2+) release units (CaRUs) in which individual sarcolemmal LCCs interact in a stochastic manner with nearby RyRs in localized regions where junctional SR membrane and transverse-tubular membrane are in close proximity. The CaRUs are embedded within and interact with the global systems of the myocyte describing ionic and membrane pump/exchanger currents, SR Ca(2+) uptake, and time-varying cytosolic ion concentrations to form a model of the cardiac action potential (AP). The model can reproduce both the detailed properties of EC coupling, such as variable gain and graded SR Ca(2+) release, and whole-cell phenomena, such as modulation of AP duration by SR Ca(2+) release. Simulations indicate that the local control paradigm predicts stable APs when the L-type Ca(2+) current is adjusted in accord with the balance between voltage- and Ca(2+)-dependent inactivation processes as measured experimentally, a scenario where common pool models become unstable. The local control myocyte model provides a means for studying the interrelationship between microscopic and macroscopic behaviors in a manner that would not be possible in experiments.
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Affiliation(s)
- Joseph L Greenstein
- Department of Biomedical Engineering, Center for Computational Medicine and Biology, The Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA
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22
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Michels G, Matthes J, Handrock R, Kuchinke U, Groner F, Cribbs LL, Pereverzev A, Schneider T, Perez-Reyes E, Herzig S. Single-channel pharmacology of mibefradil in human native T-type and recombinant Ca(v)3.2 calcium channels. Mol Pharmacol 2002; 61:682-94. [PMID: 11854450 DOI: 10.1124/mol.61.3.682] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
To study the molecular pharmacology of low-voltage-activated calcium channels in biophysical detail, human medullary thyroid carcinoma (hMTC) cells were investigated using the single-channel technique. These cells had been reported to express T-type whole-cell currents and a Ca(v)3.2 (or alpha 1H) channel subunit. We observed two types of single-channel activity that were easily distinguished based on single-channel conductance, voltage dependence of activation, time course of inactivation, rapid gating kinetics, and the response to the calcium agonist (S)-Bay K 8644. Type II channels had biophysical properties (activation, inactivation, conductance) typical for high-voltage-activated calcium channels. They were markedly stimulated by 1 microM (S)-Bay K 8644, allowing to identify them as L-type channels. The channel termed type I is a low-voltage-activated, small-conductance (7.2 pS) channel that inactivates rapidly and is not modulated by (S)-Bay K 8644. Type I channels are therefore classified as T-type channels. They were strongly inhibited by 10 microM mibefradil. Mibefradil block was caused by changes in two gating parameters: a pronounced reduction in fraction of active sweeps and a slight shortening of the open-state duration. Single recombinant low-voltage-activated T-type calcium channels were studied in comparison, using human embryonic kidney 293 cells overexpressing the pore-forming Ca(v)3.2 subunit. Along all criteria examined (mechanisms of block, extent of block), recombinant Ca(v)3.2 interact with mibefradil in the same way as their native counterparts expressed in hMTC cells. In conclusion, the pharmacologic phenotype of these native human T-type channels--as probed by mibefradil--is similar to recombinant human Ca(v)3.2.
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Affiliation(s)
- Guido Michels
- Department of Pharmacology, University of Cologne, Cologne, Germany
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23
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Abstract
Voltage-dependent L-type Ca(2+) channels are multisubunit transmembrane proteins, which allow the influx of Ca(2+) (I:(Ca)) essential for normal excitability and excitation-contraction coupling in cardiac myocytes. A variety of different receptors and signaling pathways provide dynamic regulation of I:(Ca) in the intact heart. The present review focuses on recent evidence describing the molecular details of regulation of L-type Ca(2+) channels by protein kinase A (PKA) and protein kinase C (PKC) pathways. Multiple G protein-coupled receptors act through cAMP/PKA pathways to regulate L-type channels. ss-Adrenergic receptor stimulation results in a marked increase in I:(Ca), which is mediated by a cAMP/PKA pathway. Growing evidence points to an important role of localized signaling complexes involved in the PKA-mediated regulation of I:(Ca), including A-kinase anchor proteins and binding of phosphatase PP2a to the carboxyl terminus of the alpha(1C) (Ca(v)1.2) subunit. Both alpha(1C) and ss(2a) subunits of the channel are substrates for PKA in vivo. The regulation of L-type Ca(2+) channels by Gq-linked receptors and associated PKC activation is complex, with both stimulation and inhibition of I:(Ca) being observed. The amino terminus of the alpha(1C) subunit is critically involved in PKC regulation. Crosstalk between PKA and PKC pathways occurs in the modulation of I:(Ca). Ultimately, precise regulation of I:(Ca) is needed for normal cardiac function, and alterations in these regulatory pathways may prove important in heart disease.
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Affiliation(s)
- T J Kamp
- Department of Medicine, University of Wisconsin, Madison, WI 53792-3248, USA.
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24
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Mitarai S, Kaibara M, Yano K, Taniyama K. Two distinct inactivation processes related to phosphorylation in cardiac L-type Ca(2+) channel currents. Am J Physiol Cell Physiol 2000; 279:C603-10. [PMID: 10942710 DOI: 10.1152/ajpcell.2000.279.3.c603] [Citation(s) in RCA: 31] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
We investigated the inactivation process of macroscopic cardiac L-type Ca(2+) channel currents using the whole cell patch-clamp technique with Na(+) as the current carrier. The inactivation process of the inward currents carried by Na(+) through the channel consisted of two components >0 mV. The time constant of the faster inactivating component (30.6 +/- 2.2 ms at 0 mV) decreased with depolarization, but the time constant of the slower inactivating component (489 +/- 21 ms at 0 mV) was not significantly influenced by the membrane potential. The inactivation process in the presence of isoproterenol (100 nM) consisted of a single component (538 +/- 60 ms at 0 mV). A protein kinase inhibitor, H-89, decreased the currents and attenuated the effects of isoproterenol. In the presence of cAMP (500 microM), the inactivation process consisted of a single slow component. We propose that the faster inactivating component represents a kinetic of the dephosphorylated or partially phosphorylated channel, and phosphorylation converts the kinetics into one with a different voltage dependency.
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Affiliation(s)
- S Mitarai
- Department of Pharmacology, Nagasaki University, School of Medicine, Nagasaki 8528523, Japan
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25
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Kreuzberg U, Theissen P, Schicha H, Schröder F, Mehlhorn U, de Vivie ER, Bokník P, Neumann J, Grohé C, Herzig S. Single-channel activity and expression of atrial L-type Ca(2+) channels in patients with latent hyperthyroidism. Am J Physiol Heart Circ Physiol 2000; 278:H723-30. [PMID: 10710339 DOI: 10.1152/ajpheart.2000.278.3.h723] [Citation(s) in RCA: 16] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Patients with "latent hyperthyroidism" (suppressed thyroid-stimulating hormone and normal circulating thyroid hormones) are at risk to develop atrial fibrillation. In animal models, hyperthyroidism is associated with increased cardiac L-type Ca(2+) current. Therefore, we assessed L-type channel function and expression in right atria from patients undergoing cardiac surgery. Single L-type channels were studied in the cell-attached condition. Voltage dependence of gating was similar in patients with and without latent hyperthyroidism. With use of a pulse protocol leading to maximum channel availability, single-channel activity was further analyzed. Average peak current was significantly enhanced in latent hyperthyroidism, mainly because of an increased channel availability (P < 0.05). Protein expression was analyzed by Western blot. In latent hyperthyroidism, expression of Ca(2+) channel alpha(1)-subunits was increased more than threefold (P < 0.01). In contrast, sarco(endo)plasmic reticulum Ca(2+)-ATPase and phospholamban levels were not significantly changed. We only observed a trend toward increased sarco(endo)plasmic reticulum Ca(2+)-ATPase expression (P = 0.085). Function and expression of human atrial L-type Ca(2+) channels are increased in latent hyperthyroidism. These endocrine effects on the heart may be clinically relevant.
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Affiliation(s)
- U Kreuzberg
- Department of Pharmacology, University of Cologne, 50931 Cologne, Germany
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26
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Abstract
Anion transport proteins in mammalian cells participate in a wide variety of cell and intracellular organelle functions, including regulation of electrical activity, pH, volume, and the transport of osmolites and metabolites, and may even play a role in the control of immunological responses, cell migration, cell proliferation, and differentiation. Although significant progress over the past decade has been achieved in understanding electrogenic and electroneutral anion transport proteins in sarcolemmal and intracellular membranes, information on the molecular nature and physiological significance of many of these proteins, especially in the heart, is incomplete. Functional and molecular studies presently suggest that four primary types of sarcolemmal anion channels are expressed in cardiac cells: channels regulated by protein kinase A (PKA), protein kinase C, and purinergic receptors (I(Cl.PKA)); channels regulated by changes in cell volume (I(Cl.vol)); channels activated by intracellular Ca(2+) (I(Cl.Ca)); and inwardly rectifying anion channels (I(Cl.ir)). In most animal species, I(Cl.PKA) is due to expression of a cardiac isoform of the epithelial cystic fibrosis transmembrane conductance regulator Cl(-) channel. New molecular candidates responsible for I(Cl.vol), I(Cl.Ca), and I(Cl.ir) (ClC-3, CLCA1, and ClC-2, respectively) have recently been identified and are presently being evaluated. Two isoforms of the band 3 anion exchange protein, originally characterized in erythrocytes, are responsible for Cl(-)/HCO(3)(-) exchange, and at least two members of a large vertebrate family of electroneutral cotransporters (ENCC1 and ENCC3) are responsible for Na(+)-dependent Cl(-) cotransport in heart. A 223-amino acid protein in the outer mitochondrial membrane of most eukaryotic cells comprises a voltage-dependent anion channel. The molecular entities responsible for other types of electroneutral anion exchange or Cl(-) conductances in intracellular membranes of the sarcoplasmic reticulum or nucleus are unknown. Evidence of cardiac expression of up to five additional members of the ClC gene family suggest a rich new variety of molecular candidates that may underlie existing or novel Cl(-) channel subtypes in sarcolemmal and intracellular membranes. The application of modern molecular biological and genetic approaches to the study of anion transport proteins during the next decade holds exciting promise for eventually revealing the actual physiological, pathophysiological, and clinical significance of these unique transport processes in cardiac and other mammalian cells.
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Affiliation(s)
- J R Hume
- Department of Physiology, University of Nevada School of Medicine, Reno, Nevada, USA.
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27
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Herzig S, Neumann J. Effects of serine/threonine protein phosphatases on ion channels in excitable membranes. Physiol Rev 2000; 80:173-210. [PMID: 10617768 DOI: 10.1152/physrev.2000.80.1.173] [Citation(s) in RCA: 202] [Impact Index Per Article: 8.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
This review deals with the influence of serine/threonine-specific protein phosphatases on the function of ion channels in the plasma membrane of excitable tissues. Particular focus is given to developments of the past decade. Most of the electrophysiological experiments have been performed with protein phosphatase inhibitors. Therefore, a synopsis is required incorporating issues from biochemistry, pharmacology, and electrophysiology. First, we summarize the structural and biochemical properties of protein phosphatase (types 1, 2A, 2B, 2C, and 3-7) catalytic subunits and their regulatory subunits. Then the available pharmacological tools (protein inhibitors, nonprotein inhibitors, and activators) are introduced. The use of these inhibitors is discussed based on their biochemical selectivity and a number of methodological caveats. The next section reviews the effects of these tools on various classes of ion channels (i.e., voltage-gated Ca(2+) and Na(+) channels, various K(+) channels, ligand-gated channels, and anion channels). We delineate in which cases a direct interaction between a protein phosphatase and a given channel has been proven and where a more complex regulation is likely involved. Finally, we present ideas for future research and possible pathophysiological implications.
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Affiliation(s)
- S Herzig
- Institut für Pharmakologie, Universität Köln, Köln, Germany.
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28
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Pluzhnikov K, Nosyreva E, Shevchenko L, Kokoz Y, Schmalz D, Hucho F, Grishin E. Analysis of ectatomin action on cell membranes. EUROPEAN JOURNAL OF BIOCHEMISTRY 1999; 262:501-6. [PMID: 10336635 DOI: 10.1046/j.1432-1327.1999.00426.x] [Citation(s) in RCA: 28] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/20/2022]
Abstract
Ectatomin (m = 7928 Da) is a toxic component from the Ectatomma tuberculatum ant venom containing two homologous polypeptide chains (37 and 34 residues) linked to each other by a disulfide bond. In aqueous solution it forms a four alpha-helix bundle. At concentrations of 0.05-0.1 microm, ectatomin forms channels in cellular and artificial bilayer membranes. Immunochemical analysis of the intracellular distribution of ectatomin showed that the toxin gets efficiently inserted into the plasma membrane at a concentration of 5 x 10-7 m and does not penetrate inside the cell. The effect of ectatomin on cardiac L-type calcium current was studied. Calcium currents (ICa) in isolated rat cardiac ventricular myocytes were measured using the whole-cell perforated patch-clamp technique. It was shown that ectatomin at concentrations of 0.01-10 nm inhibited ICa after a latency of few seconds. ICa was decreased twofold by 10 nm ectatomin. However, the most prominent effect of ectatomin was observed after stimulation of ICa by isoproterenol, an agonist of beta-adrenoreceptors, or forskolin, a stimulator of adenylate cyclase. At a concentration of 1 nm, ectatomin abolished the isoproterenol- and forskolin-sensitive components of ICa. The inhibitory effect of ectatomin was partially reversed by subsequent application of 2 microm of forskolin, whereas subsequent isoproterenol application did not produce the same effect.
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Affiliation(s)
- K Pluzhnikov
- Laboratory of neuroreceptors and neuroregulators, Shemyakin and Ovchinnikov Institute of Bioorganic Chemistry, RAS, Moscow, Russia
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29
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Hirano Y, Yoshinaga T, Murata M, Hiraoka M. Prepulse-induced mode 2 gating behavior with and without beta-adrenergic stimulation in cardiac L-type Ca channels. THE AMERICAN JOURNAL OF PHYSIOLOGY 1999; 276:C1338-45. [PMID: 10362596 DOI: 10.1152/ajpcell.1999.276.6.c1338] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Mode 2 gating of L-type Ca channels is characterized by high channel open probability (NPo) and long openings. In cardiac myocytes, this mode is evoked physiologically in two apparently different circumstances: membrane depolarization (prepulse facilitation) and activation of protein kinase A. To examine whether the phosphorylation mechanism is involved during prepulse-induced facilitation of cardiac L-type Ca channels, we used isolated guinea pig ventricular myocytes to analyze depolarization-induced modal gating behavior under different basal levels of phosphorylation. In control, NPo measured at 0 mV was augmented as the duration of prepulse to +100 mV was prolonged from 50 to 400 ms. This was due to the induction of mode 2 gating behavior clustered at the beginning of test pulses. Analysis of open time distribution revealed that the prepulse evoked an extra component, the time constant of which is not dependent on prepulse duration. When isoproterenol (1 microM) was applied to keep Ca channels at an enhanced level of phosphorylation, basal NPo without prepulse was increased by a factor of 3.6 +/- 2.2 (n = 6). Under these conditions, prepulse further increased NPo by promoting long openings with the same kinetics of transition to mode 2 gating (tau congruent with 200 ms at +100 mV). Likewise, recovery from mode 2 gating, as estimated by the decay of averaged unitary current, was not affected after beta-stimulation (tau congruent with 25 ms at 0 mV). The kinetic behavior independent from the basal level of phosphorylation or activity of cAMP-dependent protein kinase suggests that prepulse facilitation of the cardiac Ca channel involves a mechanism directly related to voltage-dependent conformational change rather than voltage-dependent phosphorylation.
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Affiliation(s)
- Y Hirano
- Department of Cardiovascular Diseases, Medical Research Institute, Tokyo Medical and Dental University, Tokyo 113, Japan.
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30
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Schröder F, Herzig S. Effects of beta2-adrenergic stimulation on single-channel gating of rat cardiac L-type Ca2+ channels. THE AMERICAN JOURNAL OF PHYSIOLOGY 1999; 276:H834-43. [PMID: 10070066 DOI: 10.1152/ajpheart.1999.276.3.h834] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Cardiac L-type Ca2+ channels can be stimulated by activation of beta2-adrenoceptors. We intended to determine how the gating behavior at the single-channel level (cell-attached configuration) is affected after selective stimulation of beta2-adrenoceptors. Rat cardiomyocytes were exposed to zinterol, a beta2-agonist (n = 7), isoproterenol (n = 6), a nonselective agonist, 8-bromo-cAMP (n = 6), and a combination of isoproterenol and ICI-118551 (n = 8), a selective beta2-receptor antagonist, or isoproterenol and CGP-20712A, a beta1-selective antagonist (n = 7). In all groups the ensemble-average current and the availability of the channels to open on depolarization were increased in a similar fashion. In addition, the open probability (Po) within active sweeps was elevated. However, zinterol exerted this effect in a unique manner. It elevated Po not by shortening closed times but solely by reducing active sweeps with very low Po and a short burst duration. All zinterol effects were abolished by ICI-118551 (n = 5) and mimicked by isoproterenol plus CGP-20712A (n = 7). We conclude that beta2-adrenoceptor activation of L-type channels differs qualitatively from the classical cAMP-dependent mechanism.
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Affiliation(s)
- F Schröder
- Department of Pharmacology, University of Cologne, 50931 Cologne, Germany
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31
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Schröder F, Handrock R, Beuckelmann DJ, Hirt S, Hullin R, Priebe L, Schwinger RH, Weil J, Herzig S. Increased availability and open probability of single L-type calcium channels from failing compared with nonfailing human ventricle. Circulation 1998; 98:969-76. [PMID: 9737516 DOI: 10.1161/01.cir.98.10.969] [Citation(s) in RCA: 173] [Impact Index Per Article: 6.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
BACKGROUND The role of the L-type calcium channel in human heart failure is unclear, on the basis of previous whole-cell recordings. METHODS AND RESULTS We investigated the properties of L-type calcium channels in left ventricular myocytes isolated from nonfailing donor hearts (n= 16 cells) or failing hearts of transplant recipients with dilated (n=9) or ischemic (n=7) cardiomyopathy. The single-channel recording technique was used (70 mmol/L Ba2+). Peak average currents were significantly enhanced in heart failure (38.2+/-9.3 fA) versus nonfailing control hearts (13.2+/-4.5 fA, P=0.02) because of an elevation of channel availability (55.9+/-6.7% versus 26.4+/-5.3%, P=0.001) and open probability within active sweeps (7.36+/-1.51% versus 3.18+/-1.33%, P=0.04). These differences closely resembled the effects of a cAMP-dependent stimulation with 8-Br-cAMP (n= 11). Kinetic analysis of the slow gating shows that channels from failing hearts remain available for a longer time, suggesting a defect in the dephosphorylation. Indeed, the phosphatase inhibitor okadaic acid was unable to stimulate channel activity in myocytes from failing hearts (n=5). Expression of calcium channel subunits was measured by Northern blot analysis. Expression of alpha1c- and beta-subunits was unaltered. Whole-cell current measurements did not reveal an increase of current density in heart failure. CONCLUSIONS Individual L-type calcium channels are fundamentally affected in severe human heart failure. This is probably important for the impairment of cardiac excitation-contraction coupling.
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Affiliation(s)
- F Schröder
- Department of Pharmacology, University of Cologne, Germany
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32
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Obejero-Paz CA, Auslender M, Scarpa A. PKC activity modulates availability and long openings of L-type Ca2+ channels in A7r5 cells. THE AMERICAN JOURNAL OF PHYSIOLOGY 1998; 275:C535-43. [PMID: 9688608 DOI: 10.1152/ajpcell.1998.275.2.c535] [Citation(s) in RCA: 56] [Impact Index Per Article: 2.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/08/2023]
Abstract
The possibility that protein kinase C (PKC) could control the activity of L-type Ca2+ channels in A7r5 vascular smooth muscle-derived cells in the absence of agonist stimulation was investigated using the patch-clamp technique. Consistent with the possibility that L-type Ca2+ channels are maximally phosphorylated by PKC under these conditions, we show that 1) activation of PKC with the phorbol ester phorbol 12,13-dibutyrate was ineffective in modulating whole cell and single-channel currents, 2) inhibition of PKC activity with staurosporine or chelerythrine inhibited channel activity, 3) inhibition of protein phosphatases by intracellular dialysis of okadaic acid did not affect whole cell currents, and 4) the inhibitory effect of staurosporine was absent in the presence of okadaic acid. The inhibition of Ca2+ currents by PKC inhibitors was due to a decrease in channel availability and long open events, whereas the voltage dependence of the open probability and the single-channel conductance were not affected. The evidence suggests that in resting, nonstimulated A7r5 cells there is a high level of PKC activity that modulates the gating of L-type Ca2+ channels.
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Affiliation(s)
- C A Obejero-Paz
- Department of Physiology and Biophysics, Case Western Reserve University, Cleveland, Ohio 44106, USA
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33
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Mikala G, Klöckner U, Varadi M, Eisfeld J, Schwartz A, Varadi G. cAMP-dependent phosphorylation sites and macroscopic activity of recombinant cardiac L-type calcium channels. Mol Cell Biochem 1998; 185:95-109. [PMID: 9746216 DOI: 10.1023/a:1006878106672] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The involvement of cAMP-dependent phosphorylation sites in establishing the basal activity of cardiac L-type Ca2+ channels was studied in HEK 293 cells transiently cotransfected with mutants of the human cardiac alpha1 and accessory subunits. Systematic individual or combined elimination of high consensus protein kinase A (PKA) sites, by serine to alanine substitutions at the amino and carboxyl termini of the alpha1 subunit, resulted in Ca2+ channel currents indistinguishable from those of wild type channels. Dihydropyridine (DHP)-binding characteristics were also unaltered. To explore the possible involvement of nonconsensus sites, deletion mutants were used. Carboxyl-terminal truncations of the alpha1 subunit distal to residue 1597 resulted in increased channel expression and current amplitudes. Modulation of PKA activity in cells transfected with the wild type channel or any of the mutants did not alter Ca2+ channel functions suggesting that cardiac Ca2+ channels expressed in these cells behave, in terms of lack of PKA control, like Ca2+ channels of smooth muscle cells.
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Affiliation(s)
- G Mikala
- Institute of Molecular Pharmacology and Biophysics, University of Cincinnati, College of Medicine, OH 45267-0828, USA
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34
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Handrock R, Schröder F, Hirt S, Haverich A, Mittmann C, Herzig S. Single-channel properties of L-type calcium channels from failing human ventricle. Cardiovasc Res 1998; 37:445-55. [PMID: 9614499 DOI: 10.1016/s0008-6363(97)00257-5] [Citation(s) in RCA: 25] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/07/2023] Open
Abstract
OBJECTIVE The aim of our study was to analyse the single-channel properties of L-type calcium channels from failing human heart and to compare them to the respective animal data. Furthermore, we intended to evaluate the feasibility of future single-channel studies on the role of calcium channels in the pathophysiology of heart failure. METHODS Single L-type calcium channels were recorded in ventricular myocytes from explanted failing human heart, using the cell-attached configuration of the patch-clamp technique. RESULTS One or more successful registrations of calcium channels could be obtained in 11 of 19 cell isolations. Determination of single-channel conductance yielded a mean value of 16.6 +/- 1.2 pS (70 mM Ba2+ as the charge carrier) under control conditions and 23.7 +/- 2.8 pS in presence of the calcium-channel agonist FPL 64176. The rapid gating process could be described by a C<-->C<-->O gating scheme. Slow gating analysis revealed a highly significant clustering of active and non-active sweeps. CONCLUSION Single-channel measurements of L-type calcium channels in human failing ventricle are feasible and reproducible despite the varying patient characteristics. Their channel properties are qualitatively comparable to those found in other mammals. Whether there are quantitative differences due to the underlying heart failure can be elucidated in further studies.
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Affiliation(s)
- R Handrock
- Department of Pharmacology, University of Cologne, Germany
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35
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Zhong J, Hwang TC, Adams HR, Rubin LJ. Reduced L-type calcium current in ventricular myocytes from endotoxemic guinea pigs. THE AMERICAN JOURNAL OF PHYSIOLOGY 1997; 273:H2312-24. [PMID: 9374768 DOI: 10.1152/ajpheart.1997.273.5.h2312] [Citation(s) in RCA: 43] [Impact Index Per Article: 1.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/05/2023]
Abstract
The circulatory response to gram-negative sepsis and its experimental counterpart, endotoxemia, includes a profound dysfunction in myocardial contractility that is resident to the myocyte and associated with reduced systolic free intracellular Ca2+ concentration ([Ca2+]i). We explored the possibility that decreased systolic [Ca2+]i in endotoxemic myocytes is correlated with reduced L-type Ca2+ current (ICa,L). Ventricular myocytes were isolated from guinea pigs 4 h after an intraperitoneal injection of Escherichia coli lipopolysaccharide (LPS; 4 mg/kg). Membrane potentials and Ca2+ currents were measured using whole cell patch-clamp methods. The action potential duration of endotoxemic myocytes was significantly shorter than control values (time to 50% repolarization: LPS, 314 +/- 23 ms; control, 519 +/- 36 ms, P < 0.05). Correspondingly, endotoxemic myocytes demonstrated significantly reduced peak ICa,L density (3.5 +/- 0.2 pA/pF) and Ba2+ current (IBa) density (7.3 +/- 0.5 pA/pF) compared with respective values of control myocytes (ICa,L) density 6.1 +/- 0.3 pA/pF, IBa density 11.3 +/- 0.8 pA/pF; P < 0.05). Endotoxemia-induced reduction in peak ICa,L could not be attributed to alterations in current-voltage relationships, steady-state activation and inactivation, or recovery from inactivation. The beta-adrenoceptor agonist isoproterenol, but not the Ca2+ channel activator BAY K 8644, reversed the LPS-induced reduction in peak ICa,L, cell contraction, and systolic [Ca2+]i. These data demonstrate that part of the host response to endotoxemia involves diminished sarcolemmal ICa,L of ventricular myocytes.
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Affiliation(s)
- J Zhong
- Department of Veterinary Biomedical Sciences, College of Veterinary Medicine, Columbia, Missouri, USA
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36
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Campbell DL, Stamler JS, Strauss HC. Redox modulation of L-type calcium channels in ferret ventricular myocytes. Dual mechanism regulation by nitric oxide and S-nitrosothiols. J Gen Physiol 1996; 108:277-93. [PMID: 8894977 PMCID: PMC2229328 DOI: 10.1085/jgp.108.4.277] [Citation(s) in RCA: 331] [Impact Index Per Article: 11.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Download PDF] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The effects of NO-related activity and cellular thiol redox state on basal L-type calcium current, ICa,L, in ferret right ventricular myocytes were studied using the patch clamp technique. SIN-1, which generates both NO. and O2-, either inhibited or stimulated ICa,L. In the presence of superoxide dismutase only inhibition was seen. 8-Br-cGMP also inhibited ICa,L, suggesting that the NO inhibition is cGMP-dependent. On the other hand, S-nitrosothiols (RSNOs), which donate NO+, stimulated ICa,L. RSNO effects were not dependent upon cell permeability, modulation of SR Ca2+ release, activation of kinases, inhibition of phosphatases, or alterations in cGMP levels. Similar activation of ICa,L by thiol oxidants, and reversal by thiol reductants, identifies an allosteric thiol-containing "redox switch" on the L-type calcium channel subunit complex by which NO/O2- and NO+ transfer can exert effects opposite to those produced by NO. In sum, our results suggest that: (a) both indirect (cGMP-dependent) and direct (S-nitrosylation/oxidation) regulation of ventricular ICa,L, and (b) sarcolemma thiol redox state may be an important determinant of ICa,L activity.
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Affiliation(s)
- D L Campbell
- Department of Pharmacology, Duke University Medical Center, Durham, North Carolina 27710, USA
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37
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Brink PR, Ramanan SV, Christ GJ. Human connexin 43 gap junction channel gating: evidence for mode shifts and/or heterogeneity. THE AMERICAN JOURNAL OF PHYSIOLOGY 1996; 271:C321-31. [PMID: 8760061 DOI: 10.1152/ajpcell.1996.271.1.c321] [Citation(s) in RCA: 68] [Impact Index Per Article: 2.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
The gating parameters of human connexin 43 (Cx43) gap junction channels were determined using dual whole cell patch clamp and methods designed for analysis of multichannel recordings. Under steady-state conditions, the mean open time (MOT) of Cx43 gap junction channels was computed and it ranged from 0.43 to 5.25 s. The computed mean closed times (MCT) varied from 0.21 to 1.49 s. Analysis showed that, while the MOT declined with increasing transjunctional voltage (Vj), the apparent decline in the MCT with Vj was not statistically significant. The calculated open probabilities ranged from 0.50 to 0.95. Inspection of the data showed that there was a prolonged decay in junctional current, which had a time course of 60-150 s. The analysis excluded the possibility of a homogeneous voltage inactivated/deactivated population of independent and identical Cx43 gap junction channels. The analysis provided evidence for a homogeneous population of Cx43 channels, which can mode shift under the influence of voltage. The latter case cannot be distinguished from a heterogeneous population of Cx43 channels in which one population is voltage inactivated/deactivated and another is unaffected or weakly inactivated/deactivated by voltage.
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Affiliation(s)
- P R Brink
- Department of Physiology and Biophysics, State University of New York at Stony Brook 11794, USA
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38
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Allen TJ. Temperature dependence of macroscopic L-type calcium channel currents in single guinea pig ventricular myocytes. J Cardiovasc Electrophysiol 1996; 7:307-21. [PMID: 8777479 DOI: 10.1111/j.1540-8167.1996.tb00532.x] [Citation(s) in RCA: 29] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 02/02/2023]
Abstract
INTRODUCTION Lowering temperature greatly reduces calcium influx through calcium channels. Studies on a number of tissues demonstrate that the peak inward current, ICa, exhibits Q10 values ranging from 1.8 to 3.5; however, it remains unclear which component(s) of calcium channel gating may give rise to this large temperature sensitivity. Components of gating that may affect channel availability include phosphorylation and changes in [Ca2+]i, processes that vary in pertinence depending on the channel examined. This study addresses this problem by examining the temperature sensitivity (from 34 degrees to 14 degrees C) of cardiac ICa under control conditions, during attenuation or activation of protein kinase A (PKA) activity, and when intracellular [Ca2+] has been elevated. METHODS AND RESULTS ICa was studied using the whole cell configuration of the patch champ technique. In control, lowering temperature from 34 degrees to 24 degrees C resulted in a shift in the potential for maximum slope (Va) and the peak current (Ymax) toward more positive membrane potentials. The Q10 values for the decrease in Ymax and the macroscopic slope conductance (Gmax), which reflects the number of available channels, were 3.15 +/- 0.19 and 2.57 +/- 0.13, respectively. At 0 mV the Ca2+ current decayed biexponentially, and the two time constants (tau 1 and tau 2) showed Q10 values of 1.79 +/- 0.21 and 2.06 +/- 0.38, while their contribution to the total current (I1 and I2) showed a Q10 of 5.99 +/- 0.83 and 1.61 +/- 0.22. In myocytes loaded with inhibitors of the PKA cycle sufficient to inhibit the increase of ICa to 1 microM isoprenaline, the Q10 values for some of the kinetic parameters were increased with the Q10 for I1 increasing to 17.06 +/- 3.48. Stimulation of ICa by exposing myocytes to 1 microM isoprenaline reduced the temperature sensitivity of Ymax, Gmax and I1, yielding respective values of 2.00 +/- 0.18, 1.85 +/- 0.07, and 2.04 +/- 0.15. Raising [Ca2+]i to enhance Ca2+i-dependent inactivation, while affecting inactivation and activation kinetics, affected temperature sensitivity little compared to control. The Q10 for time to peak changed little under experimental conditions (2.3 to 2.4) CONCLUSIONS Increasing the phosphorylated states of calcium channels, but not Ca2+i-dependent inactivation, reduces temperature sensitivity of certain gating parameters. The data suggest that the rate of the transitions between the unavailable and also between the various closed states are changed in the opposite direction to that induced by PKA-dependent phosphorylation. Processes, e.g., inhibitory mechanisms, may be involved to maintain channels in unavailable or "unphosphorylated" states, and it may be these that contribute to the high Q10 of macroscopic channel currents.
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Affiliation(s)
- T J Allen
- British Heart Foundation Research Group, Department of Physiology, School of Veterinary Science, Bristol, United Kingdom
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An RH, Davies MP, Doevendans PA, Kubalak SW, Bangalore R, Chien KR, Kass RS. Developmental changes in beta-adrenergic modulation of L-type Ca2+ channels in embryonic mouse heart. Circ Res 1996; 78:371-8. [PMID: 8593695 DOI: 10.1161/01.res.78.3.371] [Citation(s) in RCA: 41] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/31/2023]
Abstract
In the adult mammalian myocardium, cellular Ca2+ entry is regulated by the sympathetic nervous system. L-type Ca2+ channel currents are markedly increased by beta-adrenergic (beta-A) agonists, which contribute to changes in pacing and contractile activity of the heart. In the developing mammalian heart, the regulation of Ca2+ entry by this enzyme cascade has not been clearly established, because changes in receptor density and coupling to downstream elements of the signaling cascade are known to occur during embryogenesis. In this study, we systematically investigated the regulation of L-type Ca2+ channel currents during development of the murine embryonic heart. We used conventional whole-cell and perforated-patch-clamp procedures to study modulation of L- type Ca2+ channel currents and to assay functional activity of distinct steps in the beta-A signaling cascade in murine embryonic myocytes at different stages of gestation. Our data indicate that the L-type Ca2+ channels in early-stage (day-11 to -13) myocytes are unresponsive to either isoproterenol or cAMP. L-type Ca2+ channels in late-stage (day-17 to -19) murine myocytes, however, exhibit responses to isoproterenol and cAMP similar to responses in adult cells, providing evidence that the beta-A cascade becomes functionally active during this period of embryonic development. We found that L-type Ca2+ channel activity in early-stage cells is increased by cell dialysis with the catalytic subunit of cAMP-dependent protein kinase (cA-PK) and that dialysis of early-stage cells with the holoenzyme of cA-PK restores functional responses to forskolin and cAMP, but not to isoproterenol. Our results provide strong evidence that a key factor in the early-stage insensitivity of L-type Ca2+ channels to cAMP is the absence, or low expression level, of the holoenzyme of cA-PK but that in addition, another element in the signaling cascade upstream from adenylate cyclase is expressed at a nonfunctional level or is uncoupled from the cascade and thus contributes to L-type Ca2+ channel insensitivity to beta-A agonists in early stages of the developing murine heart.
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Affiliation(s)
- R H An
- Department of Physiology, University of Rochester Medical Center, New York 14642-8642, USA
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40
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Herzig S. Ca2+ channel activation by CGP 48506, a new positive inotropic benzodiazocine derivative. Eur J Pharmacol 1996; 295:113-7. [PMID: 8925868 DOI: 10.1016/0014-2999(95)00673-7] [Citation(s) in RCA: 6] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/03/2023]
Abstract
Effects on L-type Ca2+ channels of a new positive inotropic compound, the active (+)-enantiomer of the Ca2+ sensitizer 5-methyl-6-phenyl-1,3,5,6,-tetrahydro-3,6,-methano-1,5-benzodiazocine -2,4-dione (CGP 48506), were studied in guinea-pig cardiomyocytes. Whole-cell currents (physiological solutions, 2 mM Ca2+) were enhanced approximately 1.8-fold (10(-4) M, n = 7). Slowing of (de)activation kinetics became apparent under conditions where K+ currents were fully eliminated and Ca(2+)-dependent inactivation was minimized (n = 7). Single-channel current (70 mM Ba2+) and mean open time were increased approximately 2.5-fold (10(-4) M, n = 5), because the drug specifically enhanced sweeps containing long openings (mode 2). Therefore, CGP 48506 stimulates Ca2+ channels in a manner reminiscent of, but not identical to chemically distinct activators like Bay K 8644.
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Affiliation(s)
- S Herzig
- Department of Pharmacology, University of Kiel, Germany
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41
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Groschner K, Schuhmann K, Baumgartner W, Pastushenko V, Schindler H, Romanin C. Basal dephosphorylation controls slow gating of L-type Ca2+ channels in human vascular smooth muscle. FEBS Lett 1995; 373:30-4. [PMID: 7589427 DOI: 10.1016/0014-5793(95)01012-4] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
The role of cellular phosphatase activity in regulation of smooth muscle L-type Ca2+ channels was investigated using tautomycin, a potent and specific inhibitor of serin/threonin phosphatases type 1 and 2A. Tautomycin (1-100 nM) inhibited Ca2+ channel activity in smooth muscle cells isolated from human umbilical vein. Tautomycin-induced inhibition of Ca2+ channel activity was due to a reduction of channel availability which originated mainly from prolongation of the lifetime of unavailable states of the channel. Pretreatment of smooth muscle cells with the protein kinase inhibitor H-7 (10 microM) prevented the inhibitory effect of tautomycin. Our results suggest modulation of slow gating between available and unavailable states as a mechanism of phosphorylation-dependent down-regulation of Ca2+ channels in vascular smooth muscle.
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Affiliation(s)
- K Groschner
- Institut für Pharmakologie und Toxikologie, Karl-Franzens-Universität Graz, Austria
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42
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Wiechen K, Yue DT, Herzig S. Two distinct functional effects of protein phosphatase inhibitors on guinea-pig cardiac L-type Ca2+ channels. J Physiol 1995; 484 ( Pt 3):583-92. [PMID: 7623278 PMCID: PMC1157945 DOI: 10.1113/jphysiol.1995.sp020688] [Citation(s) in RCA: 53] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023] Open
Abstract
1. The effects of the phosphatase inhibitors okadaic acid and calyculin A on single guinea-pig ventricular L-type Ca2+ channels were studied. The inactive derivative norokadaone was used as a negative control. 2. The two known effects of cAMP-dependent stimulation are mimicked by the phosphatase inhibitors to a varying extent. Only okadaic acid promotes the high-activity gating mode ('mode 2'), while calyculin A increases channel availability to a larger extent. As revealed by kinetic analysis of slow gating, the two phosphatase inhibitors retard a slow rate constant, which is assumed to represent exit from the available state by dephosphorylation. Norokadaone was inactive in both regards. 3. Mode 2 gating elicited by very positive prepulses is augmented by okadaic acid, and mode 2 lifetime is prolonged. Calyculin A fails to affect these parameters. Thus, voltage-facilitated mode 2 gating reveals the same pharmacological properties as the mode 2 sweeps observed using conventional pulse protocols. 4. The results are interpreted in terms of the different sensitivity of protein phosphatase subtypes towards the inhibitors: channel availability appears to be controlled by a phosphorylation site dephosphorylated by a type 1-like phosphatase, while mode 2 gating is coupled to a distinct site, dephosphorylated by a type 2A-like phosphatase.
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Affiliation(s)
- K Wiechen
- Department of Pharmacology, University of Kiel, Germany
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43
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Allen TJ, Chapman RA. The effect of a chemical phosphatase on single calcium channels and the inactivation of whole-cell calcium current from isolated guinea-pig ventricular myocytes. Pflugers Arch 1995; 430:68-80. [PMID: 7545282 DOI: 10.1007/bf00373841] [Citation(s) in RCA: 25] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023]
Abstract
A chemical phosphatase, butanedione monoxime (BDM, at 12-20 mM), reduced open probability (P0) of single cardiac L-type Ca2+ channels in cell-attached patches from guinea-pig ventricular myocytes, without effect on the amplitude of single-channel current, the mean open time or the mean shorter closed time, but it increased mean longer closed time and caused a fall in channel availability. A decrease in the mean time between first channel opening and last closing within a trace was principally due to an inhibition of the longer periods of activity. As a result, the time course of the mean currents, which resolved into an exponentially declining and a sustained component, was changed by an increase in the rate of the exponential phase and a profound reduction of the sustained current. Essentially similar results were obtained when studying whole-cell Ba2+ currents. The inactivation of the whole-cell Ca2+ currents was composed of two exponentially declining components with the slower showing a significantly greater sensitivity to BDM, an effect that was much more pronounced in myocytes exposed to isoprenaline with adenosine 5'-O-(3-thiotriphosphate) (ATP[gamma S]) in the pipette solution. The actions of BDM, which are the opposite of those produced by isoprenaline, suggest that the level of phosphorylation affects processes involved in the slow regulation of channel activity under basal conditions and that several sites (and probably several kinases) are involved. Channels with an inherently slow inactivation would seem to be converted into channels with a rapid inactivation by a dephosphorylation process.
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Affiliation(s)
- T J Allen
- British Heart Foundation Research Group, School of Veterinary Science, Bristol, UK
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44
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Herzig S, Meier A, Pfeiffer M, Neumann J. Stimulation of protein phosphatases as a mechanism of the muscarinic-receptor-mediated inhibition of cardiac L-type Ca2+ channels. Pflugers Arch 1995; 429:531-8. [PMID: 7617443 DOI: 10.1007/bf00704158] [Citation(s) in RCA: 42] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Abstract
Acetylcholine decreases currents through cardiac L-type Ca2+ channels after stimulation with agents which elevate levels of cyclic adenosine monophosphate, such as isoproterenol, but there is still a controversy over the mechanisms of this muscarinic effect. We tested the hypothesis of whether, after isoproterenol stimulation, protein phosphatases are activated by acetylcholine. Whole-cell currents were recorded from guinea-pig ventricular myocytes. The effect of 10(-5) M acetylcholine on currents induced by 10(-8) M isoproterenol was studied in the absence or presence of protein phosphatase inhibitors. Three agents reduced the acetylcholine response: okadaic acid (3 or 9 x 10(-6) M) and cantharidin (3 x 10(-6) M) added to the pipette solution, and bath-applied fluoride (3 mM). In contrast, pipette application of other phosphatase inhibitors, namely the inhibitor PPI2 (1000 U/ml), ciclosporin (10(-5) M), or calyculin A (10(-6) M) did not significantly diminish the acetylcholine effect. Interestingly, there was no correlation between the effects of the compounds on basal Ca2+ current and their interference with the muscarinic response. An activation of type 2A phosphatases by acetylcholine would explain these findings. Indeed, okadaic acid is 3 orders of magnitude more potent in vitro in its inhibition of this isoform (purified from cardiac myocytes) than is calyculin A, while type-1 phosphatases are inhibited equally. The data support the attractive possibility that stimulation of protein phosphatases is part of the signal transduction cascade of Ca2+ channel inhibition by acetylcholine.
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Affiliation(s)
- S Herzig
- Institut für Pharmakologie, Universität Kiel, Germany
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45
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Campbell DL, Strauss HC. Regulation of calcium channels in the heart. ADVANCES IN SECOND MESSENGER AND PHOSPHOPROTEIN RESEARCH 1995; 30:25-88. [PMID: 7695992 DOI: 10.1016/s1040-7952(05)80004-7] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [MESH Headings] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 01/26/2023]
Affiliation(s)
- D L Campbell
- Department of Pharmacology, Duke University Medical Center, Durham, North Carolina 27710
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Lu C, Kumar R, Akita T, Joyner RW. Developmental changes in the actions of phosphatase inhibitors on calcium current of rabbit heart cells. Pflugers Arch 1994; 427:389-98. [PMID: 7971137 DOI: 10.1007/bf00374252] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/28/2023]
Abstract
We used whole-cell voltage clamp to compare the modulation of calcium current density (ICa, picoampere per picofarad) of freshly isolated, adult and newborn rabbit heart in response to intracellular application of microcystin and okadaic acid, both of which block phosphatase activity of phosphatase type 1 and 2A. Newborn cells showed a much larger response to the intracellular application of either microcystin or okadaic acid than did adult cells. In newborn cells, the application of microcystin produced an increase in ICa which appeared to maximize ICa, as shown by the rise in ICa to levels which could be reached by application of 10 microM forskolin or by the intracellular application of 200 microM 3',5'-cyclic adenosine monophosphate (cAMP). In adult cells, the maximal response to microcystin was considerably less than that obtainable with forskolin or cAMP. After achieving a maximal response with microcystin, the addition of forskolin increased ICa further in adult cells but elicited no additional response in newborn cells. The treatment of cells with 0.1 microM isoproterenol, a concentration approximately equal to that required for a half-maximal response, strongly potentiated the effect of microcystin in newborn cells, but not in adult cells. We propose that newborn rabbit heart cells compared with adult rabbit heart cells have a greater level of protein phosphatase activity (perhaps combined with a somewhat greater kinase activity), a greater proportion of the protein phosphatase activity in the form of protein phosphatase type 1 (which is inhibited by isoproterenol) and a greater dependence on the inhibition of protein phosphatase as a mechanism of action of isoproterenol, compared with the increase in kinase activity on calcium channels.
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Affiliation(s)
- C Lu
- Todd Franklin Cardiac Research Laboratory, Department of Pediatrics, Emory University, Atlanta, GA 30323
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Affiliation(s)
- H C Hartzell
- Department of Anatomy and Cell Biology, Emory University School of Medicine, Atlanta, Georgia 30322
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