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Dowrick JM, Tran K, Garrett AS, Anderson AJ, Nielsen PMF, Taberner AJ, Han JC. Work-loop contractions reveal that the afterload-dependent time course of cardiac Ca 2+ transients is modulated by preload. J Appl Physiol (1985) 2022; 133:663-675. [PMID: 35771221 PMCID: PMC9762964 DOI: 10.1152/japplphysiol.00137.2022] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/11/2023] Open
Abstract
Preload and afterload dictate the dynamics of the cyclical work-loop contraction that the heart undergoes in vivo. Cellular Ca2+ dynamics drive contraction, but the effects of afterload alone on the Ca2+ transient are inconclusive. To our knowledge, no study has investigated whether the putative afterload dependence of the Ca2+ transient is preload dependent. This study is designed to provide the first insight into the Ca2+ handling of cardiac trabeculae undergoing work-loop contractions, with the aim to examine whether the conflicting afterload dependency of the Ca2+ transient can be accounted for by considering preload under isometric and physiological work-loop contractions. Thus, we subjected ex vivo rat right-ventricular trabeculae, loaded with the fluorescent dye Fura-2, to work-loop contractions over a wide range of afterloads at two preloads while measuring stress, length changes, and Ca2+ transients. Work-loop control was implemented with a real-time Windkessel model to mimic the contraction patterns of the heart in vivo. We extracted a range of metrics from the measured steady-state twitch stress and Ca2+ transients, including the amplitudes, time courses, rates of rise, and integrals. Results show that parameters of stress were afterload and preload dependent. In contrast, the parameters associated with Ca2+ transients displayed a mixed dependence on afterload and preload. Most notably, its time course was afterload dependent, an effect augmented at the greater preload. This study reveals that the afterload dependence of cardiac Ca2+ transients is modulated by preload, which brings the study of Ca2+ transients during isometric contractions into question when aiming to understand physiological Ca2+ handling.NEW & NOTEWORTHY This study is the first examination of Ca2+ handling in trabeculae undergoing work-loop contractions. These data reveal that reducing preload diminishes the influence of afterload on the decay phase of the cardiac Ca2+ transient. This is significant as it reconciles inconsistencies in the literature regarding the influence of external loads on cardiac Ca2+ handling. Furthermore, these findings highlight discrepancies between Ca2+ handling during isometric and work-loop contractions in cardiac trabeculae operating at their optimal length.
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Affiliation(s)
- Jarrah M. Dowrick
- 1Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
| | - Kenneth Tran
- 1Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
| | - Amy S. Garrett
- 1Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
| | - Alex J. Anderson
- 1Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
| | - Poul M. F. Nielsen
- 1Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand,2Department of Engineering Science, University of Auckland, Auckland, New Zealand
| | - Andrew J. Taberner
- 1Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand,2Department of Engineering Science, University of Auckland, Auckland, New Zealand
| | - June-Chiew Han
- 1Auckland Bioengineering Institute, University of Auckland, Auckland, New Zealand
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Averin AS, Andreeva LA, Popova SS, Kosarsky LS, Anufriev AI, Nenov MN, Nakipova OV. α1-Adrenergic receptor regulates papillary muscle and aortic segment contractile function via modulation of store-operated Ca 2+ entry in long-tailed ground squirrels Urocitellus undulatus. J Comp Physiol B 2021; 191:10.1007/s00360-021-01394-6. [PMID: 34297192 DOI: 10.1007/s00360-021-01394-6] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/05/2020] [Revised: 06/28/2021] [Accepted: 07/13/2021] [Indexed: 10/20/2022]
Abstract
The effect of phenylephrine (PE) on right ventricle papillary muscle (PM) and aortic segment (AS) contractile activity was studied in long-tailed ground squirrels Urocitellus undulatus during summer activity, torpor and interbout active (IBA) periods in comparison to rat. We found that PE (10 μM) exerts positive inotropic effect on ground squirrel PM that was blocked by α1-AR inhibitor-prazosin. PE differently affected frequency dependence of PM contraction in ground squirrels and rats. PE significantly increased the force of PM contraction in summer and hibernating ground squirrels including both torpor and IBA predominantly at the range of low stimulation frequencies (0.003-0.1 Hz), while in rat PM it was evident only at high stimulation frequency range (0.2-1.0 Hz). Further, it was found that PE vasoconstrictor effect on AS contractility is significantly higher in ground squirrels of torpid state compared to IBA and summer periods. Overall vasoconstrictor effect of PE was significantly higher in AS of ground squirrels of all periods compared to rats. Positive inotropic effect of PE on PM along with its vasoconstrictor effect on AS of ground squirrels was not affected by pretreatment with inhibitors of L-type Ca2+ channels, or Na+/Ca2+ exchanger or Ca2+-ATPase but was completely blocked by an inhibitor of store-operated Ca2+ entry (SOCE)-2-APB, suggesting the involvement of SOCE in the mechanisms underlying PE action on ground squirrel cardiovascular system. Obtained results support an idea about the significant role of alpha1-AR in adaptive mechanisms critical for the maintaining of cardiovascular contractile function in long-tailed ground squirrel Urocitellus undulatus.
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Affiliation(s)
- Alexey S Averin
- Institute of Cell Biophysics Federal Research Center "Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences", Institutskaya 3, Pushchino, Moscow region, Russia, 142290
| | - Ludmila A Andreeva
- Institute of Cell Biophysics Federal Research Center "Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences", Institutskaya 3, Pushchino, Moscow region, Russia, 142290
| | - Svetlana S Popova
- Institute of Theoretical and Experimental Biophysics of Russian Academy of Science, Institutskaya 3, Pushchino, Moscow region, Russia, 142290
| | - Leonid S Kosarsky
- Institute of Cell Biophysics Federal Research Center "Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences", Institutskaya 3, Pushchino, Moscow region, Russia, 142290
| | - Andrey I Anufriev
- Yakutsk Branch, Siberian Division, Institute of Biology, Russian Academy of Sciences, Yakutsk, Russia, 677891
| | - Miroslav N Nenov
- Institute of Theoretical and Experimental Biophysics of Russian Academy of Science, Institutskaya 3, Pushchino, Moscow region, Russia, 142290.
- Alzheimer's Center at Temple, Lewis Katz School of Medicine, Temple University, Philadelphia, PA, 19140, USA.
| | - Olga V Nakipova
- Institute of Cell Biophysics Federal Research Center "Pushchino Scientific Center for Biological Research of the Russian Academy of Sciences", Institutskaya 3, Pushchino, Moscow region, Russia, 142290
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Fletcher S, Maddock H, James RS, Wallis R, Gharanei M. The cardiac work-loop technique: An in vitro model for identifying and profiling drug-induced changes in inotropy using rat papillary muscles. Sci Rep 2020; 10:5258. [PMID: 32210283 PMCID: PMC7093439 DOI: 10.1038/s41598-020-58935-2] [Citation(s) in RCA: 3] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/12/2019] [Accepted: 11/26/2019] [Indexed: 11/09/2022] Open
Abstract
The cardiac work-loop technique closely mimics the intrinsic in vivo movement and characteristics of cardiac muscle function. In this study, six known inotropes were profiled using the work-loop technique to evaluate the potential of this method to predict inotropy. Papillary muscles from male Sprague-Dawley rats were mounted onto an organ bath perfused with Krebs-Henseleit buffer. Following optimisation, work-loop contractions were performed that included an initial stabilisation period followed by vehicle control or drug administration. Six known inotropes were tested: digoxin, dobutamine, isoprenaline, flecainide, verapamil and atenolol. Muscle performance was evaluated by calculating power output during work-loop contraction. Digoxin, dobutamine and isoprenaline caused a significant increase in power output of muscles when compared to vehicle control. Flecainide, verapamil and atenolol significantly reduced power output of muscles. These changes in power output were reflected in alterations in work loop shapes. This is the first study in which changes in work-loop shape detailing for example the activation, shortening or passive re-lengthening have been linked to the mechanism of action of a compound. This study has demonstrated that the work-loop technique can provide an important novel method with which to assess detailed mechanisms of drug-induced effects on cardiac muscle contractility.
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Affiliation(s)
- Sophie Fletcher
- Centre for Sport, Exercise and Life Sciences, Coventry University, Coventry, United Kingdom.,InoCardia Ltd, Technocentre, Puma Way, Coventry, CV1 2TT, UK
| | - Helen Maddock
- Centre for Sport, Exercise and Life Sciences, Coventry University, Coventry, United Kingdom. .,InoCardia Ltd, Technocentre, Puma Way, Coventry, CV1 2TT, UK.
| | - Rob S James
- Centre for Sport, Exercise and Life Sciences, Coventry University, Coventry, United Kingdom
| | - Rob Wallis
- InoCardia Ltd, Technocentre, Puma Way, Coventry, CV1 2TT, UK
| | - Mayel Gharanei
- Centre for Sport, Exercise and Life Sciences, Coventry University, Coventry, United Kingdom.,InoCardia Ltd, Technocentre, Puma Way, Coventry, CV1 2TT, UK
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4
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Mullins PD, Bondarenko VE. Mathematical model for β1-adrenergic regulation of the mouse ventricular myocyte contraction. Am J Physiol Heart Circ Physiol 2020; 318:H264-H282. [DOI: 10.1152/ajpheart.00492.2019] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/02/2023]
Abstract
The β1-adrenergic regulation of cardiac myocyte contraction plays an important role in regulating heart function. Activation of this system leads to an increased heart rate and stronger myocyte contraction. However, chronic stimulation of the β1-adrenergic signaling system can lead to cardiac hypertrophy and heart failure. To understand the mechanisms of action of β1-adrenoceptors, a mathematical model of cardiac myocyte contraction that includes the β1-adrenergic system was developed and studied. The model was able to simulate major experimental protocols for measurements of steady-state force-calcium relationships, cross-bridge release rate and force development rate, force-velocity relationship, and force redevelopment rate. It also reproduced quite well frequency and isoproterenol dependencies for intracellular Ca2+ concentration ([Ca2+]i) transients, total contraction force, and sarcomere shortening. The mathematical model suggested the mechanisms of increased contraction force and myocyte shortening on stimulation of β1-adrenergic receptors is due to phosphorylation of troponin I and myosin-binding protein C and increased [Ca2+]i transient resulting from activation of the β1-adrenergic signaling system. The model was used to simulate work-loop contractions and estimate the power during the cardiac cycle as well as the effects of 4-aminopyridine and tedisamil on the myocyte contraction. The developed mathematical model can be used further for simulations of contraction of ventricular myocytes from genetically modified mice and myocytes from mice with chronic cardiac diseases. NEW & NOTEWORTHY A new mathematical model of mouse ventricular myocyte contraction that includes the β1-adrenergic system was developed. The model simulated major experimental protocols for myocyte contraction and predicted the effects of 4-aminopyridine and tedisamil on the myocyte contraction. The model also allowed for simulations of work-loop contractions and estimation of the power during the cardiac cycle.
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Affiliation(s)
- Paula D. Mullins
- Department of Mathematics, University of North Georgia, Blue Ridge, Georgia
- Department of Mathematics and Statistics and Neuroscience Institute, Georgia State University, Atlanta, Georgia
| | - Vladimir E. Bondarenko
- Department of Mathematics and Statistics and Neuroscience Institute, Georgia State University, Atlanta, Georgia
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5
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Tran K, Taberner AJ, Loiselle DS, Han JC. Energetics Equivalent of the Cardiac Force-Length End-Systolic Zone: Implications for Contractility and Economy of Contraction. Front Physiol 2020; 10:1633. [PMID: 32038302 PMCID: PMC6985585 DOI: 10.3389/fphys.2019.01633] [Citation(s) in RCA: 7] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2019] [Accepted: 12/26/2019] [Indexed: 11/23/2022] Open
Abstract
We have recently demonstrated the existence of a region on the cardiac mechanics stress-length plane, which we have designated "The cardiac end-systolic zone." The zone is defined as the area on the pressure-volume (or stress-length) plane within which all stress-length contraction profiles reach their end-systolic points. It is enclosed by three boundaries: the isometric end-systolic relation, the work-loop (shortening) end-systolic relation, and the zero-active stress isotonic end-systolic relation. The existence of this zone reflects the contraction-mode dependence of the cardiac end-systolic force-length relations, and has been confirmed in a range of cardiac preparations at the whole-heart, tissue and myocyte levels. This finding has led us to speculate that a comparable zone prevails for cardiac metabolism. Specifically, we hypothesize the existence of an equivalent zone on the energetics plane (heat vs. stress), and that it can be attributed to the recently-revealed heat of shortening in cardiac muscle. To test these hypotheses, we subjected trabeculae to both isometric contractions and work-loop contractions over wide ranges of preloads and afterloads. We found that the heat-stress relations for work-loop contractions were distinct from those of isometric contractions, mirroring the contraction mode-dependence of the stress-length relation. The zone bounded by these contraction-mode dependent heat-stress relations reflects the heat of shortening. Isoproterenol-induced enhancement of contractility led to proportional increases in the zones on both the mechanics and energetics planes, thereby supporting our hypothesis.
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Affiliation(s)
- Kenneth Tran
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
| | - Andrew J. Taberner
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
- Department of Engineering Science, The University of Auckland, Auckland, New Zealand
| | - Denis S. Loiselle
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
- Department of Physiology, The University of Auckland, Auckland, New Zealand
| | - June-Chiew Han
- Auckland Bioengineering Institute, The University of Auckland, Auckland, New Zealand
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Predictivity of in vitro non-clinical cardiac contractility assays for inotropic effects in humans — A literature search. J Pharmacol Toxicol Methods 2015; 75:62-9. [DOI: 10.1016/j.vascn.2015.05.009] [Citation(s) in RCA: 18] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/15/2015] [Revised: 05/09/2015] [Accepted: 05/22/2015] [Indexed: 11/18/2022]
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7
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Investigation into the cardiotoxic effects of doxorubicin on contractile function and the protection afforded by cyclosporin A using the work-loop assay. Toxicol In Vitro 2014; 28:722-31. [DOI: 10.1016/j.tiv.2014.01.011] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 05/19/2013] [Revised: 10/04/2013] [Accepted: 01/27/2014] [Indexed: 11/23/2022]
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8
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van Eif VWW, Bogaards SJP, van der Laarse WJ. Intrinsic cardiac adrenergic (ICA) cell density and MAO-A activity in failing rat hearts. J Muscle Res Cell Motil 2013; 35:47-53. [DOI: 10.1007/s10974-013-9373-6] [Citation(s) in RCA: 12] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/15/2013] [Accepted: 12/06/2013] [Indexed: 12/26/2022]
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9
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Bardswell SC, Cuello F, Kentish JC, Avkiran M. cMyBP-C as a promiscuous substrate: phosphorylation by non-PKA kinases and its potential significance. J Muscle Res Cell Motil 2011; 33:53-60. [PMID: 22089698 DOI: 10.1007/s10974-011-9276-3] [Citation(s) in RCA: 33] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/14/2011] [Accepted: 11/04/2011] [Indexed: 11/26/2022]
Abstract
It is now generally accepted that phosphorylation of cMyBP-C is critically important in maintaining normal cardiac function. Although much of the work to date on phospho-regulation of cMyBP-C has focused on the role of protein kinase A (PKA, also known as cAMP-dependent protein kinase), recent evidence suggests that a number of non-PKA serine/threonine kinases, such as Ca(2+)/calmodulin-dependent protein kinase II, protein kinase C, protein kinase D and the 90-kDa ribosomal S6 kinase are also capable of targeting this key regulatory sarcomeric protein. This article reviews such evidence and proposes a hypothetical role for some of the pertinent signalling pathways in phospho-regulation of cMyBP-C in the setting of heart failure.
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Affiliation(s)
- Sonya C Bardswell
- Cardiovascular Division, King's College London British Heart Foundation Centre of Research Excellence, The Rayne Institute, St Thomas' Hospital, London, UK
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10
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Taberner AJ, Han JC, Loiselle DS, Nielsen PMF, Nielsen PMF. An innovative work-loop calorimeter for in vitro measurement of the mechanics and energetics of working cardiac trabeculae. J Appl Physiol (1985) 2011; 111:1798-803. [PMID: 21903883 DOI: 10.1152/japplphysiol.00752.2011] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/22/2022] Open
Abstract
We describe a unique work-loop calorimeter with which we can measure, simultaneously, the rate of heat production and force-length work output of isolated cardiac trabeculae. The mechanics of the force-length work-loop contraction mimic those of the pressure-volume work-loops experienced by the heart. Within the measurement chamber of a flow-through microcalorimeter, a trabecula is electrically stimulated to respond, under software control, in one of three modes: fixed-end, isometric, or isotonic. In each mode, software controls the position of a linear motor, with feedback from muscle force, to adjust muscle length in the desired temporal sequence. In the case of a work-loop contraction, the software achieves seamless transitions between phases of length control (isometric contraction, isometric relaxation, and restoration of resting muscle length) and force control (isotonic shortening). The area enclosed by the resulting force-length loop represents the work done by the trabecula. The change of enthalpy expended by the muscle is given by the sum of the work term and the associated amount of evolved heat. With these simultaneous measurements, we provide the first estimation of suprabasal, net mechanical efficiency (ratio of work to change of enthalpy) of mammalian cardiac trabeculae. The maximum efficiency is at the vicinity of 12%.
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Affiliation(s)
- Andrew J Taberner
- Auckland Bioengineering Institute, The Univ. of Auckland, Auckland, New Zealand.
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11
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Gender comparison of contractile performance and beta-adrenergic response in isolated rat cardiac trabeculae. J Comp Physiol B 2007; 178:307-13. [PMID: 18030479 DOI: 10.1007/s00360-007-0223-y] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/07/2007] [Revised: 10/29/2007] [Accepted: 11/06/2007] [Indexed: 10/22/2022]
Abstract
It is known that gender can affect susceptibility to development of various cardiomyopathies. However, it is unclear whether basic mechanical contractile function of the myocardium differs between genders, whether they respond differently to stressors, or both. To test for a possible gender factor, contractile parameters of healthy, isolated myocardium were investigated under near physiological conditions. Right ventricular ultra-thin trabeculae from young adult LBN-f1 rats were electrically stimulated to isometrically contract at 37 degrees C. No differences were found in developed force or kinetic parameters. In each muscle, the force-frequency relationship was measured at 4, 6, and 8 Hz, encompassing most of the in vivo range. Again, no differences were observed in force-frequency behavior; developed force rose from 21.6 +/- 4.0 at 4 Hz to 30.3 +/- 5.8 mN/mm(2) at 8 Hz in females and from 23.4 +/- 3.4 to 29.8 +/- 3.4 mN/mm(2 )in males. The response to beta-adrenergic stimulation was similar; at 1 microM isoproterenol, developed force increased to 34.5 +/- 6.2 mN/mm(2) in females and 32.3 +/- 3.2 mN/mm(2) in males (female vs. male, not significant). We conclude that basic mechanical performance of healthy isolated myocardium under physiological conditions is not different between males and females, and a different response to stress must underlie gender-based differences in cardiac performance.
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12
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Alvarez BV, Kieller DM, Quon AL, Markovich D, Casey JR. Slc26a6: a cardiac chloride-hydroxyl exchanger and predominant chloride-bicarbonate exchanger of the mouse heart. J Physiol 2004; 561:721-34. [PMID: 15498800 PMCID: PMC1665392 DOI: 10.1113/jphysiol.2004.077339] [Citation(s) in RCA: 62] [Impact Index Per Article: 3.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022] Open
Abstract
Bicarbonate facilitate more than 50% of pH recovery in the acidotic myocardium, and have roles in cardiac hypertrophy and steady-state pH regulation. To determine which bicarbonate transporters are responsible for this activity, we measured the expression levels of all known HCO3(-)-anion exchange proteins in mouse heart, by quantitative real time RT-PCR. Bicarbonate-anion exchangers are members of either the SLC4A or the SLC26A gene families. In neonatal and adult myocardium, AE1 (Slc4a1), AE2 (Slc4a2), AE3 (Slc4a3) (AE3fl and AE3c variants), Slc26a3 and Slc26a6 were expressed. Adult hearts expressed Slc26a3 and Slc4a1-3 mRNAs at similar levels, while Slc26a6 mRNA was about seven-fold higher than AE3, which was more abundant than any other. Immunohistochemistry revealed that Slc26a6 and AE3 are present in the plasma membrane of ventricular myocytes. Slc26a6 expression levels were higher in ventricle than atrium, whereas AE3 was detected only in ventricle. Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchange activity of SLC26A6 and AE3 were investigated in transfected HEK293 cells, using intracellular fluorescence measurements of 2',7'-bis (2-carboxyethyl)-5(6)-carboxyfluorescein (BCECF), to monitor intracellular pH (pH(i)). Rates of pH(i) change were measured under HCO3(-)-containing (Cl(-)-HCO(3)(-)) or nominally HCO3(-)-free (Cl(-)-OH(-)) conditions. HCO3(-) fluxes were similar for cells expressing AE3fl, SLC26A6 or Slc26a3, suggesting that they have similar transport activity. However, only SLC26A6 and Slc26a3 functioned as Cl(-)-OH(-) exchangers. Activation of alpha-adrenergic receptors, which stimulates protein kinase C, inhibited SLC26A6 Cl(-)-HCO(3)(-) exchange activity. We conclude that Slc26a6 is the predominant Cl(-)-HCO(3)(-) and Cl(-)-OH(-) exchanger of the myocardium and that Slc26a6 is negatively regulated upon alpha-adrenergic stimulation.
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Affiliation(s)
- Bernardo V Alvarez
- CIHR Membrane Protein Research Group, Department of Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 2H7
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13
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Layland J, Kentish JC. Myofilament-based relaxant effect of isoprenaline revealed during work-loop contractions in rat cardiac trabeculae. J Physiol 2002; 544:171-82. [PMID: 12356890 PMCID: PMC2290578 DOI: 10.1113/jphysiol.2002.022855] [Citation(s) in RCA: 14] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
In cardiac muscle, beta-adrenergic stimulation increases contractile force and accelerates relaxation. The relaxant effect is thought to be due primarily to stimulation of Ca(2+) uptake into the sarcoplasmic reticulum (SR), although changes in myofilament properties may also contribute. The present study investigated the contribution of the myofilaments to the beta-adrenergic response in isolated rat cardiac trabeculae undergoing either isometric or work-loop contractions (involving simultaneous force generation and shortening) at different stimulation frequencies (range 0.25-4.5 Hz). SR-dependent effects were eliminated by treatment with ryanodine (1 microM) and cyclopiazonic acid (30 microM). In isometric contractions during SR inhibition, isoprenaline increased the force but did not alter the time course of the twitch. In contrast, in work-loop contractions, the positive inotropic effect was accompanied by a reduced diastolic force between beats, most apparent at higher frequencies (e.g. diastolic stress fell from 58.6 +/- 5.5 to 28.8 +/- 5.8 mN mm(-2) at 1.5 Hz). This relaxant effect contributed to a beta-adrenoceptor-mediated increase in net work and power output at higher frequencies, by reducing the amount of work required to re-lengthen the muscle. Consequently, the frequency for maximum power output increased from 1.1 +/- 0.1 to 1.6 +/- 0.1 Hz. We conclude that the contribution of myofilament properties to the relaxant effect of beta-stimulation may be of greater significance when force and length are changing simultaneously (as occurs in the heart) than during force development under isometric conditions.
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Affiliation(s)
- Joanne Layland
- Centre for Cardiovascular Biology and Medicine, Kings College London, St Thomas's Campus, Lambeth Palace Road, UK.
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