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Development an ovine myocardial infarction- induced heart failure model and characterisation of altered calcium homeostasis mechanisms. Eur Heart J 2021. [DOI: 10.1093/eurheartj/ehab724.3306] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/13/2022] Open
Abstract
Abstract
Introduction
Ventricular arrhythmias (VA) and heart failure (HF) are the major complications following myocardial infarction (MI). In both conditions, there is a key role for perturbed calcium homeostasis of which the underlying mechanisms remain unclear. A preclinical model that faithfully presents most of the features of MI-induced HF has been lacking. The complexity of this syndrome means that animal modelling is difficult. As the hearts of large animals share many electrophysiological similarities to humans, ovine modelling of cardiac diseases could better reflect human pathologies than in small mammals.
Question
Is it possible to develop a clinically relevant ovine model with moderate cardiac dysfunction following myocardial infarction?
Methods
MI was induced in sheep by inflating an angioplasty balloon distal to the second diagonal branch of the left anterior descending artery for 90 min. Cardiac function was monitored for 20 weeks using electrocardiography (ECG), echocardiography, blood biochemical analysis, and subjective signs of cardiac deterioration (lethargy, dyspnoea, and cough). 20 weeks post-MI, the animals were humanely killed and single left ventricular myocytes were isolated from the infarct border zone. Changes in cellular electrophysiology and intracellular calcium concentration were monitored using whole-cell patch technique in voltage-clamp mode and the calcium sensitive fluorescent indicator Fura-2 (K5 salt).
Results
By using minimally invasive procedures, we obtained a survival rate of 80% (n=15). During surgery, our data show clinical features of ischaemia, including changes in the ECG features (elevation of the ST and T segment, left bundle branch block and/or pathological Q waves) and elevation of the cardiac biomarker such as troponin I. Following MI, we observed a decline in ejection fraction (−25±3%, p<0.0001) and an increase in whole animal arrhythmias (incidence of VA 72 hours post-MI, ∼70%). On cardiac removal apico-septal transmural necrosis / scarring was evident. Importantly, the L-type calcium current (ICaL) was decreased in MI cells compared to healthy cells (−1.87±0.73 pA/pF, p<0.05), but isoprenaline had no effet on ICaL (0.48±1.2 pA/pF, p=0.70) in MI cells. Moreover, the amplitude of the systolic calcium transient (−0.33±0.1 F/F0, p<0.05) and the sarcoplasmic reticulum calcium content (−23±7 μmol/L, p<0.01) were also decreased. The shortening velocity of the sarcomere was also decreased in MI cells (−0.55±0.18 μm/s, p<0.01).
Conclusion
We successfully established an ovine MI model using minimally invasive procedure which displays a moderately impaired cardiac function, reduced contractility, and pro-arrhythmic electrophysiological remodelling. Future analysis will examine the role of the L-type calcium channel with respect to the excitation-contraction coupling process and myocyte contractility and how we can improve therapeutic strategies towards VA and HF.
Funding Acknowledgement
Type of funding sources: Foundation. Main funding source(s): British Heart Foundation
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Bringing European physiologists together. Acta Physiol (Oxf) 2018; 222:e13043. [PMID: 29377580 DOI: 10.1111/apha.13043] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022]
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Reproducibility of science: Fraud, impact factors and carelessness. J Mol Cell Cardiol 2017; 114:364-368. [PMID: 29079076 PMCID: PMC6565841 DOI: 10.1016/j.yjmcc.2017.10.009] [Citation(s) in RCA: 23] [Impact Index Per Article: 3.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/03/2017] [Revised: 10/19/2017] [Accepted: 10/21/2017] [Indexed: 12/22/2022]
Abstract
There is great concern that results published in a large fraction of biomedical papers may not be reproducible. This article reviews the evidence for this and considers some of the factors that are responsible and how the problem may be solved. One issue is scientific fraud. This, in turn, may result from pressures put on scientists to succeed including the need to publish in "high impact" journals. I emphasise the importance of judging the quality of the science itself as opposed to using surrogate metrics. The other factors discussed include problems of experimental design and statistical analysis of the work. It is important that these issues are addressed by the scientific community before others impose draconian regulations.
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Poster session 1Cell growth, differentiation and stem cells - Heart72Understanding the metabolism of cardiac progenitor cells: a first step towards controlling their proliferation and differentiation?73Expression of pw1/peg3 identifies a new cardiac adult stem cell population involved in post-myocardial infarction remodeling74Long-term stimulation of iPS-derived cardiomyocytes using optogenetic techniques to promote phenotypic changes in E-C coupling75Benefits of electrical stimulation on differentiation and maturation of cardiomyocytes from human induced pluripotent stem cells76Constitutive beta-adrenoceptor-mediated cAMP production controls spontaneous automaticity of human induced pluripotent stem cell-derived cardiomyocytes77Formation and stability of T-tubules in cardiomyocytes78Identification of miRNAs promoting human cardiomyocyte proliferation by regulating Hippo pathway79A direct comparison of foetal to adult epicardial cell activation reveals distinct differences relevant for the post-injury response80Role of neuropilins in zebrafish heart regeneration81Highly efficient immunomagnetic purification of cardiomyocytes derived from human pluripotent stem cells82Cardiac progenitor cells posses a molecular circadian clock and display large 24-hour oscillations in proliferation and stress tolerance83Influence of sirolimus and everolimus on bone marrow-derived mesenchymal stem cell biology84Endoglin is important for epicardial behaviour following cardiac injuryCell death and apoptosis - Heart87Ultrastructural alterations reflecting Ca2+ handling and cell-to-cell coupling disorders precede occurrence of severe arrhythmias in intact animal heart88Urocortin-1 promotes cardioprotection through ERK1/2 and EPAC pathways: role in apoptosis and necrosis89Expression p38 MAPK and Cas-3 in myocardium LV of rats with experimental heart failure at melatonin and enalapril introductionTranscriptional control and RNA species - Heart92Accumulation of beta-amyloid 1-40 in HF patients: the role of lncRNA BACE1-AS93Role of miR-182 in zebrafish and mouse models of Holt-Oram syndrome94Mir-27 distinctly regulates muscle-enriched transcription factors and growth factors in cardiac and skeletal muscle cells95AF risk factors impair PITX2 expression leading to Wnt-microRNA-ion channel remodelingCytokines and cellular inflammation - Heart98Post-infarct survival depends on the interplay of monocytes, neutrophils and interferon gamma in a mouse model of myocardial Infarction99Inflammatory cd11b/c cells play a protective role in compensated cardiac hypertrophy by promoting an orai3-related pro-survival signal100Anti-inflammatory effects of endothelin receptor blockade in the atrial tissue of spontaneously hypertensive rats101Mesenchymal stromal cells reduce NLRP3 inflammasome activity in Coxsackievirus B3-induced myocarditis102Mesenchymal stromal cells modulate monocytes trafficking in Coxsackievirus B3-induced myocarditis103The impact of regulatory T lymphocytes on long-term mortality in patients with chronic heart failure104Temporal dynamics of dendritic cells after ST-elevation myocardial infarction relate with improvement of myocardial functionGrowth factors and neurohormones - Heart107Preconditioning of hypertrophied heart: miR-1 and IGF-1 crosstalk108Modulation of catecholamine secretion from human adrenal chromaffin cells by manipulation of G protein-coupled receptor kinase-2 activity109Evaluation of cyclic adenosin-3,5- monophosphate and neurohormones in patients with chronic heart failureNitric oxide and reactive oxygen species - Heart112Hydrogen sulfide donor inhibits oxidative and nitrosative stress, cardiohemodynamics disturbances and restores cNOS coupling in old rats113Role and mechanisms of action of aldehydes produced by monoamine oxidase A in cardiomyocyte death and heart failure114Exercise training has contrasting effects in myocardial infarction and pressure-overload due to different endothelial nitric oxide synthase regulation115S-Nitroso Human Serum Albumin dose-dependently leads to vasodilation and alters reactive hyperaemia in coronary arteries of an isolated mouse heart model116Modulating endothelial nitric oxide synthase with folic acid attenuates doxorubicin-induced cardiomyopathy119Effects of long-term very high intensity exercise on aortic structure and function in an animal model120Electron paramagnetic resonance spectroscopy quantification of nitrosylated hemoglobin (HbNO) as an index of vascular nitric oxide bioavailability in vivo121Deletion of repressor activator protein 1 impairs acetylcholine-induced relaxation due to production of reactive oxygen speciesExtracellular matrix and fibrosis - Heart124MicroRNA-19b is associated with myocardial collagen cross-linking in patients with severe aortic stenosis. Potential usefulness as a circulating biomarker125A new ex vivo model to study cardiac fibrosis126Heterogeneity of fibrosis and fibroblast differentiation in the left ventricle after myocardial infarction127Effect of carbohydrate metabolism degree compensation to the level of galectin-3 changes in hypertensive patients with chronic heart failure and type 2 diabetes mellitus128Statin paradox in association with calcification of bicuspid aortic valve interstitial cells129Cardiac function remains impaired despite reversible cardiac fibrosis after healed experimental viral myocarditisIon channels, ion exchangers and cellular electrophysiology - Heart132Identifying a novel role for PMCA1 (Atp2b1) in heart rhythm instability133Mutations of the caveolin-3 gene as a predisposing factor for cardiac arrhythmias134The human sinoatrial node action potential: time for a computational model135iPSC-derived cardiomyocytes as a model to dissect ion current alterations of genetic atrial fibrillation136Postextrasystolic potentiation in healthy and diseased hearts: effects of the site of origin and coupling interval of the preceding extrasystole137Absence of Nav1.8-based (late) sodium current in rabbit cardiomyocytes and human iPSC-CMs138hiPSC-derived cardiomyocytes from Brugada Syndrome patients without identified mutations do not exhibit cellular electrophysiological abnormalitiesMicrocirculation141Atherogenic indices, collagen type IV turnover and the development of microvascular complications- study in diabetics with arterial hypertension142Changes in the microvasculature and blood viscosity in women with rheumatoid arthritis, hypercholesterolemia and hypertensionAtherosclerosis145Shear stress regulates endothelial autophagy: consequences on endothelial senescence and atherogenesis146Obstructive sleep apnea causes aortic remodeling in a chronic murine model147Aortic perivascular adipose tissue displays an aged phenotype in early and late atherosclerosis in ApoE-/- mice148A systematic evaluation of the cellular innate immune response during the process of human atherosclerosis149Inhibition of Coagulation factor Xa increases plaque stability and attenuates the onset and progression of atherosclerotic plaque in apolipoprotein e-deficient mice150Regulatory CD4+ T cells from patients with atherosclerosis display pro-inflammatory skewing and enhanced suppression function151Hypoxia-inducible factor (HIF)-1alpha regulates macrophage energy metabolism by mediating miRNAs152Extracellular S100A4 is a key player of smooth muscle cell phenotypic transition: implications in atherosclerosis153Microparticles of healthy origins improve atherosclerosis-associated endothelial progenitor cell dysfunction via microRNA transfer154Arterial remodeling and metabolism impairment in early atherosclerosis155Role of pannexin1 in atherosclerotic plaque formationCalcium fluxes and excitation-contraction coupling158Amphiphysin II induces tubule formation in cardiac cells159Interleukin 1 beta regulation of connexin 43 in cardiac fibroblasts and the effects of adult cardiac myocyte:fibroblast co-culture on myocyte contraction160T-tubular electrical defects contribute to blunted beta-adrenergic response in heart failure161Beat-to-beat variability of intracellular Ca2+ dynamics of Purkinje cells in the infarct border zone of the mouse heart revealed by rapid-scanning confocal microscopy162The efficacy of late sodium current blockers in hypertrophic cardiomyopathy is dependent on genotype: a study on transgenic mouse models with different mutations163Synthesis of cADPR and NAADP by intracellular CD38 in heart: role in inotropic and arrhythmogenic effects of beta-adrenoceptor signalingContractile apparatus166Towards an engineered heart tissue model of HCM using hiPSC expressing the ACTC E99K mutation167Diastolic mechanical load delays structural and functional deterioration of ultrathin adult heart slices in culture168Structural investigation of the cardiac troponin complex by molecular dynamics169Exercise training restores myocardial and oxidative skeletal muscle function from myocardial infarction heart failure ratsOxygen sensing, ischaemia and reperfusion172A novel antibody specific to full-length stromal derived factor-1 alpha reveals that remote conditioning induces its cleavage by endothelial dipeptidyl peptidase 4173Attenuation of myocardial and vascular arginase activity by vagal nerve stimulation via a mechanism involving alpha-7 nicotinic receptor during cardiac ischemia and reperfusion174Novel nanoparticle-mediated medicine for myocardial ischemia-reperfusion injury simultaneously targeting mitochondrial injury and myocardial inflammation175Acetylcholine plays a key role in myocardial ischaemic preconditioning via recruitment of intrinsic cardiac ganglia176The role of nitric oxide and VEGFR-2 signaling in post ischemic revascularization and muscle recovery in aged hypercholesterolemic mice177Efficacy of ischemic preconditioning to protect the human myocardium: the role of clinical conditions and treatmentsCardiomyopathies and fibrosis180Plakophilin-2 haploinsufficiency leads to impaired canonical Wnt signaling in ARVC patient181Improved technique for customized, easier, safer and more reliable transverse aortic arch banding and debanding in mice as a model of pressure overload hypertrophy182Late sodium current inhibitors for the treatment of inducible obstruction and diastolic dysfunction in hypertrophic cardiomyopathy: a study on human myocardium183Angiotensin II receptor antagonist fimasartan has protective role of left ventricular fibrosis and remodeling in the rat ischemic heart184Role of High-Mobility Group Box 1 (HMGB1) redox state on cardiac fibroblasts activities and heart function after myocardial infarction185Atrial remodeling in hypertrophic cardiomyopathy: insights from mouse models carrying different mutations in cTnT186Electrophysiological abnormalities in ventricular cardiomyocytes from a Maine Coon cat with hypertrophic cardiomyopathy: effects of ranolazine187ZBTB17 is a novel cardiomyopathy candidate gene and regulates autophagy in the heart188Inhibition of SRSF4 in cardiomyocytes induces left ventricular hypertrophy189Molecular characterization of a novel cardiomyopathy related desmin frame shift mutation190Autonomic characterisation of electro-mechanical remodeling in an in-vitro leporine model of heart failure191Modulation of Ca2+-regulatory function by three novel mutations in TNNI3 associated with severe infant restrictive cardiomyopathyAging194The aging impact on cardiac mesenchymal like stromal cells (S+P+)195Reversal of premature aging markers after bariatric surgery196Sex-associated differences in vascular remodeling during aging: role of renin-angiotensin system197Role of the receptor for advanced glycation end-products (RAGE) in age dependent left ventricle dysfunctionsGenetics and epigenetics200hsa-miR-21-5p as a key factor in aortic remodeling during aneurysm formation201Co-inheritance of mutations associated with arrhythmogenic and hypertrophic cardiomyopathy in two Italian families202Lamin a/c hot spot codon 190: form various amino acid substitutions to clinical effects203Treatment with aspirin and atorvastatin attenuate cardiac injury induced by rat chest irradiation: Implication of myocardial miR-1, miR-21, connexin-43 and PKCGenomics, proteomics, metabolomics, lipidomics and glycomics206Differential phosphorylation of desmin at serines 27 and 31 drives the accumulation of preamyloid oligomers in heart failure207Potential role of kinase Akt2 in the reduced recovery of type 2 diabetic hearts subjected to ischemia / reperfusion injury208A proteomics comparison of extracellular matrix remodelling in porcine coronary arteries upon stent implantationMetabolism, diabetes mellitus and obesity211Targeting grk2 as therapeutic strategy for cancer associated to diabetes212Effects of salbutamol on large arterial stiffness in patients with metabolic syndrome213Circulating microRNA-1 and microRNA-133a: potential biomarkers of myocardial steatosis in type 2 diabetes mellitus214Anti-inflammatory nutrigenomic effects of hydroxytyrosol in human adipocytes - protective mechanisms of mediterranean diets in obesity-related inflammation215Alterations in the metal content of different cardiac regions within a rat model of diabetic cardiomyopathyTissue engineering218A novel conductive patch for application in cardiac tissue engineering219Establishment of a simplified and improved workflow from neonatal heart dissociation to cardiomyocyte purification and characterization220Effects of flexible substrate on cardiomyocytes cell culture221Mechanical stretching on cardiac adipose progenitors upregulates sarcomere-related genes. Cardiovasc Res 2016. [DOI: 10.1093/cvr/cvw135] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/12/2022] Open
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The effect of 2,5-di-(tert-butyl)-1,4-benzohydroquinone (TBQ) on intracellular Ca2+ handling in rat ventricular myocytes. Cell Calcium 2015; 58:208-14. [PMID: 26120055 PMCID: PMC4509554 DOI: 10.1016/j.ceca.2015.05.002] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2015] [Revised: 05/20/2015] [Accepted: 05/25/2015] [Indexed: 11/16/2022]
Abstract
The suitability of TBQ as a specific inhibitor of SERCA was investigated. TBQ decreased SERCA activity in a concentration dependent manner. TBQ inhibited the calcium current. TBQ activated an outward current consistent with an ATP-dependent potassium channel. TBQ cannot be used as a specific inhibitor of SERCA in rat cardiac myocytes.
2,5-Di-(tert-butyl)-1,4-benzohydroquinone (TBQ) is a reversible inhibitor of SERCA, potentially making it a useful tool to study the effects of SERCA inhibition in cardiac cells. However, it is unknown if TBQ also has effects on other components of ventricular Ca handling. The aim of these experiments was to characterise the effects of TBQ on Ca handling in rat ventricular myocytes and assess its suitability as a specific inhibitor of SERCA. This was achieved by voltage clamp via perforated patch and [Ca2+]i measurement using Fluo-3 AM. TBQ produced a fully reversible, concentration dependent decrease in the rate of systolic Ca decay. 10 μM TBQ decreased the amplitude of the systolic Ca transient by 48 ± 5% and the rate of decay by 54 ± 6%. SR Ca content was also reduced by 62 ± 4%. However, 10 μM TBQ also decreased the peak L-type Ca current by 23 ± 7%. At higher concentrations (100 μM), TBQ also activated an outward current with a current–voltage relationship consistent with a potassium current. This outward current was abolished by Glibenclamide (100 μM). These data show that TBQ can be used to reversibly inhibit SERCA. However, at concentrations that decrease SERCA activity, TBQ also decreases the L-type Ca current and (at higher concentrations) activates an outward current which appears to be an ATP dependent potassium current. We conclude that TBQ cannot be used as a specific inhibitor of SERCA in rat ventricular myocytes.
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Do calcium waves propagate between cells and synchronize alternating calcium release in rat ventricular myocytes? J Physiol 2012; 590:6353-61. [PMID: 23090944 DOI: 10.1113/jphysiol.2012.245241] [Citation(s) in RCA: 9] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
The aim was to investigate the propagation of Ca(2+) waves between cells and determine whether this synchronizes alternating Ca(2+) release between cells. Experiments were carried out on electrically coupled cell pairs; spontaneous Ca(2+) waves were produced by elevating external Ca(2+). There was a significant difference in the ability of these waves to propagate between cells depending on the orientation of the pairs. Although almost all pairs connected by side-to-side contacts showed propagating Ca(2+) release, this was very uncommon in end-to-end cell pairs. Confocal studies showed that there was a gap at the intercalated disc consisting of cell membranes and a region of cytoplasm devoid of sarcoplasmic reticulum. This gap was 2.3 μm in length and is suggested to interfere with Ca(2+) wave propagation. The gap measured was much smaller between side-to-side contacts: 1.5 μm and so much less likely to interfere with propagation. Subsequent experiments investigated the synchronization between cells of Ca(2+) alternans produced by small depolarizing pulses. Although this alternation results from beat-to-beat alternation of intracellular Ca(2+) wave propagation, there was no evidence that propagation of Ca(2+) waves between cells contributed to synchronization of this alternans.
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Transverse tubules are a common feature in large mammalian atrial myocytes including human. Am J Physiol Heart Circ Physiol 2011; 301:H1996-2005. [PMID: 21841013 DOI: 10.1152/ajpheart.00284.2011] [Citation(s) in RCA: 118] [Impact Index Per Article: 9.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 01/06/2023]
Abstract
Transverse (t) tubules are surface membrane invaginations that are present in all mammalian cardiac ventricular cells. The apposition of L-type Ca(2+) channels on t tubules with the sarcoplasmic reticulum (SR) constitutes a "calcium release unit" and allows close coupling of excitation to the rise in systolic Ca(2+). T tubules are virtually absent in the atria of small mammals, and therefore Ca(2+) release from the SR occurs initially at the periphery of the cell and then propagates into the interior. Recent work has, however, shown the occurrence of t tubules in atrial myocytes from sheep. As in the ventricle, Ca(2+) release in these cells occurs simultaneously in central and peripheral regions. T tubules in both the atria and the ventricle are lost in disease, contributing to cellular dysfunction. The aim of this study was to determine if the occurrence of t tubules in the atrium is restricted to sheep or is a more general property of larger mammals including humans. In atrial tissue sections from human, horse, cow, and sheep, membranes were labeled using wheat germ agglutinin. As previously shown in sheep, extensive t-tubule networks were present in horse, cow, and human atrial myocytes. Analysis shows half the volume of the cell lies within 0.64 ± 0.03, 0.77 ± 0.03, 0.84 ± 0.03, and 1.56 ± 0.19 μm of t-tubule membrane in horse, cow, sheep, and human atrial myocytes, respectively. The presence of t tubules in the human atria may play an important role in determining the spatio-temporal properties of the systolic Ca(2+) transient and how this is perturbed in disease.
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Changes of SERCA activity have only modest effects on sarcoplasmic reticulum Ca2+ content in rat ventricular myocytes. J Physiol 2011; 589:4723-9. [PMID: 21825024 DOI: 10.1113/jphysiol.2011.211052] [Citation(s) in RCA: 44] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/17/2022] Open
Abstract
Changes of the activity of the sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) affect the amplitude of the systolic Ca(2+) transient and thence cardiac contractility. This is thought to be due to alterations of SR Ca(2+) content. Recent work on mice in which the expression of SERCA is decreased found that a large reduction of SERCA expression resulted in a proportionately much smaller decrease of SR Ca(2+) content. The aim of the current work was to investigate the quantitative nature of the dependence of both the amplitude of the systolic Ca(2+) transient and SR Ca(2+) content on SERCA activity during acute partial inhibition of SERCA. Experiments were performed on rat ventricular myocytes. Brief application of thapsigargin (1 μm) resulted in a decrease of SERCA activity as measured from the rate of decay of the systolic Ca(2+) transient. This was accompanied by a decrease in the amplitude of the systolic Ca(2+) transient which was linearly related to that of SERCA activity. However, the fractional decrease in the SR Ca(2+) content was much less than that of SERCA activity. On average SR Ca(2+) content was proportional to SERCA activity raised to the 0.38 ± 0.07 power. This shallow dependence of SR content on SERCA activity arises because Ca(2+) release is a steep function of SR Ca(2+) content. In contrast SR Ca(2+) content was increased 4.59 ± 0.40 (n = 8)-fold by decreasing ryanodine receptor opening with tetracaine (1 mm). Therefore a modest decrease of SR Ca(2+) content results in a proportionately larger fall of Ca(2+) release from the SR which can balance a larger initiating decrease of SERCA. In conclusion, the shallow dependence of SR Ca(2+) content on SERCA activity is expected for a system in which small changes of SR Ca(2+) content produce larger effects on the amplitude of the systolic Ca(2+) transient.
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Abstract
Cardiac contraction is activated by an increase of intracellular calcium concentration ([Ca(2+)](i)), most of which comes from the sarcoplasmic reticulum (SR) where it is released, via the ryanodine receptor (RyR), in response to Ca(2+) entering the cell on the L-type Ca(2+) current. This phenomenon is termed Ca(2+)-induced Ca(2+) release (CICR). However, under certain circumstances, the SR can become overloaded with Ca(2+) and once a threshold SR Ca(2+) content is reached Ca(2+) is released spontaneously. Such spontaneous Ca(2+) release from the SR propagates as a Ca(2+) wave by CICR. Some of the Ca(2+) released during a wave is removed from the cell on the electrogenic Na - Ca exchanger resulting in depolarization. This is the cellular mechanism producing delayed afterdepolarizations and is common to those arrhythmias produced by digitalis toxicity and right ventricular outflow tract tachycardia. More recently it has been suggested that arrhythmogenic Ca(2+) waves can also occur if the properties of the RyR are altered, resulting in increase of RyR open probability, for example by phosphorylation. However, in this review experimental evidence will be presented to support the view that such arrhythmias still require a threshold SR Ca(2+) content to be exceeded and that this threshold is decreased by increasing RyR open probability.
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Abstract
In cardiac muscle, although most of the calcium that activates contraction comes from the sarcoplasmic reticulum (SR), a significant fraction (up to 30%, depending on the species) enters from outside the cell and is then pumped out at the end of systole. Although some of this calcium influx is required to trigger calcium release from the SR, the bulk serves to reload the cell (and thence the SR) with calcium to replace the calcium that is pumped out of the cell. An alternative strategy would be for the heart to have a much smaller calcium influx balancing a smaller efflux. We demonstrate that this would result in a slowing of inotropic responses due to changes of SR calcium content. We conclude that the large sarcolemmal calcium fluxes facilitate rapid changes of contractility.
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The mechanism and significance of the slow changes of ventricular action potential duration following a change of heart rate. Exp Physiol 2009; 94:520-8. [PMID: 19270038 DOI: 10.1113/expphysiol.2008.044008] [Citation(s) in RCA: 44] [Impact Index Per Article: 2.9] [Reference Citation Analysis] [Abstract] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022]
Abstract
This article reviews the effects of changes of heart rate on the ventricular action potential duration. These can be divided into short term (fractions of a second), resulting from the kinetics of recovery of membrane currents, through to long term (up to days), resulting from changes of protein expression. We concentrate on the medium-term changes (time course of the order of 100 s). These medium-term changes occur in isolated tissues and in the intact human heart. They may protect against cardiac arrhythmias. Finally, we discuss the cellular mechanisms responsible for these changes.
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Alternans of cardiac calcium cycling in a cluster of ryanodine receptors: a simulation study. Am J Physiol Heart Circ Physiol 2008; 295:H598-609. [PMID: 18515647 DOI: 10.1152/ajpheart.01086.2007] [Citation(s) in RCA: 37] [Impact Index Per Article: 2.3] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Mechanical alternans in cardiac muscle is associated with intracellular Ca(2+) alternans. Mechanisms underlying intracellular Ca(2+) alternans are unclear. In previous experimental studies, we produced alternans of systolic Ca(2+) under voltage clamp, either by partially inhibiting the Ca(2+) release mechanism, or by applying small depolarizing pulses. In each case, alternans relied on propagating waves of Ca(2+) release. The aim of this study is to investigate by computer modeling how alternans of systolic Ca(2+) is produced. A mathematical model of a cardiac cell with 75 coupled elements is developed, with each element contains L-type Ca(2+) current, a subspace into which Ca release takes place, a cytoplasmic space, sarcoplasmic reticulum (SR) release channels [ryanodine receptor (RyR)], and uptake sites (SERCA). Interelement coupling is via Ca(2+) diffusion between neighboring subspaces via cytoplasmic spaces and network SR spaces. Small depolarizing pulses were simulated by step changes of cell membrane potential (20 mV) with random block of L-type channels. Partial inhibition of the release mechanism is mimicked by applying a reduction of RyR open probability in response to full stimulation by L-type channels. In both cases, systolic alternans follow, consistent with our experimental observations, being generated by propagating waves of Ca(2+) release and sustained through alternation of SR Ca(2+) content. This study provides novel and fundamental insights to understand mechanisms that may underlie intracellular Ca(2+) alternans without the need for refractoriness of L-type Ca or RyR channels under rapid pacing.
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The control of sarcoplasmic reticulum Ca content in cardiac muscle. Cell Calcium 2008; 38:391-6. [PMID: 16139353 DOI: 10.1016/j.ceca.2005.06.017] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/20/2005] [Accepted: 06/28/2005] [Indexed: 11/15/2022]
Abstract
Most of the calcium that activates contraction in the heart comes from the sarcoplasmic reticulum (SR) and it is therefore essential to control the SR Ca content. SR Ca content reflects the balance between uptake (via the SR Ca-ATPase, SERCA) and release, largely via the ryanodine receptor (RyR). Unwanted changes of SR Ca are prevented because, for example, an increase of SR Ca content increases the amplitude of the systolic Ca transient and this, in turn, results in increased loss of Ca from and decreased Ca entry into the cell thereby restoring cell and SR Ca towards control levels. We discuss the parameters that affect the steady level of SR Ca and how these may change in heart failure. Finally, we discuss disordered Ca regulation with particular emphasis on the condition of alternans where successive heartbeats alternate in amplitude. This behaviour can be explained by excessive feedback gain in the processes controlling SR Ca.
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Regulation of systolic [Ca2+]i and cellular Ca2+ flux balance in rat ventricular myocytes by SR Ca2+, L-type Ca2+ current and diastolic [Ca2+]i. J Physiol 2007; 585:579-92. [PMID: 17932152 DOI: 10.1113/jphysiol.2007.141473] [Citation(s) in RCA: 59] [Impact Index Per Article: 3.5] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 02/02/2023] Open
Abstract
The force-frequency response is an important physiological mechanism regulating cardiac output changes and is accompanied in vivo by beta-adrenergic stimulation. We sought to determine the role of sarcoplasmic reticulum (SR) Ca2+ content and L-type current (ICa-L) in the frequency response of the systolic Ca2+ transient alone and during beta-adrenergic stimulation. Experiments (on single rat ventricular myocytes) were designed to be as physiological as possible. Under current clamp stimulation SR Ca2+ content increased in line with stimulation frequency (1-8 Hz) but the systolic Ca2+ transient was maximal at 6 Hz. Under voltage clamp, increasing frequency decreased both systolic Ca2+ transient and ICa-L. Normalizing peak ICa-L by altering the test potential decreased the Ca2+ transient amplitude less than an equivalent reduction achieved through changes in frequency. This suggests that, in addition to SR Ca2+ content and ICa-L, another factor, possibly refractoriness of Ca2+ release from the SR contributes. Under current clamp, beta-adrenergic stimulation (isoprenaline, 30 nm) increased both the Ca2+ transient and the SR Ca2+ content and removed the dependence of both on frequency. In voltage clamp experiments, beta-adrenergic stimulation still increased SR Ca2+ content yet there was an inverse relation between frequency and Ca2+ transient amplitude and ICa-L. Diastolic [Ca2+]i increased with stimulation frequency and this contributed substantially (69.3 +/- 6% at 8 Hz) to the total Ca2+ efflux from the cell. We conclude that Ca2+ flux balance is maintained by the combination of increased efflux due to elevated diastolic [Ca2+]i and a decrease of influx on IC-L) on each pulse.
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Analysis of cellular calcium fluxes in cardiac muscle to understand calcium homeostasis in the heart. Cell Calcium 2007; 42:503-12. [PMID: 17509680 DOI: 10.1016/j.ceca.2007.04.002] [Citation(s) in RCA: 71] [Impact Index Per Article: 4.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 02/05/2007] [Revised: 03/23/2007] [Indexed: 11/24/2022]
Abstract
Central to controlling intracellular calcium concentration ([Ca(2+)](i)) are a number of Ca(2+) transporters and channels with the L-type Ca(2+) channel, Na(+)-Ca(2+) exchanger and sarcoplasmic reticulum Ca(2+)-ATPase (SERCA) being of particular note in the heart. This review concentrates on the regulation of [Ca(2+)](i) in cardiac muscle and the homeostatic mechanisms employed to ensure that the heart can operate under steady-state conditions on a beat by beat basis. To this end we discuss the relative importance of various sources and sinks of Ca(2+) responsible for initiating contraction and relaxation in cardiac myocytes and how these can be manipulated to regulate the Ca(2+) content of the major Ca(2+) store, the sarcoplasmic reticulum (SR). We will present a simple feedback system detailing how such control can be achieved and highlight how small perturbations to the steady-state operation of the feedback loop can be both beneficial physiologically and underlie changes in systolic Ca(2+) in ageing and heart disease. In addition to manipulating the amplitude of the normal systolic Ca(2+) transient, the tight regulation of SR Ca(2+) content is also required to prevent the abnormal, spontaneous or diastolic release of Ca(2+) from the SR. Such diastolic events are a major factor contributing to the genesis of cardiac arrhythmias in disease situations and in recently identified familial mutations in the SR Ca(2+) release channel (ryanodine receptor, RyR). How such diastolic release arises and potential mechanisms for controlling this will be discussed.
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Na/Ca Exchange: Regulator of Intracellular Calcium and Source of Arrhythmias in the Heart. Ann N Y Acad Sci 2007; 1099:315-25. [PMID: 17446473 DOI: 10.1196/annals.1387.033] [Citation(s) in RCA: 47] [Impact Index Per Article: 2.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/12/2022]
Abstract
The major effect of Na/Ca exchange (NCX) on the systolic Ca transient is secondary to its effect on the Ca content of the sarcoplasmic reticulum (SR). SR Ca content is controlled by a mechanism in which an increase of SR Ca produces an increase in the amplitude of the systolic Ca transient. This, in turn, increases Ca efflux on NCX as well as decreasing entry on the L-type current resulting in a decrease of both cell and SR Ca content. This control mechanism also changes the response to other maneuvers that affect excitation-contraction coupling. For example, potentiating the opening of the SR Ca release channel (ryanodine receptor, RyR) with caffeine produces an immediate increase in the amplitude of the systolic Ca transient. However, this increases efflux of Ca from the cell on NCX and then decreases SR Ca content until a new steady state is reached. Changing the activity of NCX (by decreasing external Na) changes the level of SR Ca reached by this mechanism. If the cell and SR are overloaded with Ca then Ca waves appear during diastole. These waves activate the electrogenic NCX and thereby produce arrhythmogenic-delayed afterdepolarizations. A major challenge is how to remove this arrhythmogenic Ca release without compromising the normal systolic release. We have found that application of tetracaine to decrease RyR opening can abolish diastolic release while simultaneously potentiating the systolic release.
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Does the adenosine A2A receptor stimulate the ryanodine receptor? Cardiovasc Res 2007; 73:247-8; author reply 249-50. [PMID: 17137567 DOI: 10.1016/j.cardiores.2006.10.023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 10/13/2006] [Accepted: 10/27/2006] [Indexed: 11/28/2022] Open
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Reducing ryanodine receptor open probability as a means to abolish spontaneous Ca2+ release and increase Ca2+ transient amplitude in adult ventricular myocytes. Circ Res 2006; 98:1299-305. [PMID: 16614307 DOI: 10.1161/01.res.0000222000.35500.65] [Citation(s) in RCA: 74] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The aim of this work was to investigate whether it is possible to remove arrhythmogenic Ca2+ release from the sarcoplasmic reticulum that occurs in calcium overload without compromising normal systolic release. Exposure of rat ventricular myocytes to isoproterenol (1 micromol/L) resulted in an increased amplitude of the systolic Ca2+ transient and the appearance of waves of diastolic Ca2+ release. Application of tetracaine (25 to 50 micromol/L) decreased the frequency or abolished the diastolic Ca2+ release. This was accompanied by an increase in the amplitude of the systolic Ca2+ transient. Cellular Ca2+ flux balance was investigated by integrating Ca2+ entry (on the L-type Ca2+ current) and efflux (on Na-Ca2+ exchange). Isoproterenol increased Ca2+ influx but failed to increase Ca2+ efflux during systole (because of the abbreviation of the duration of the Ca2+ transient). To match this increased influx the bulk of Ca2+ efflux occurred via Na-Ca2+ exchange during a diastolic Ca2+ wave. Subsequent application of tetracaine increased systolic Ca2+ efflux and abolished the diastolic efflux. The increase of systolic efflux in tetracaine resulted from both increased amplitude and duration of the systolic Ca2+ transient. In the presence of isoproterenol, those Ca2+ transients preceded by diastolic release were smaller than those where no diastolic release had occurred. When tetracaine was added, the amplitude of the Ca2+ transient was similar to those in isoproterenol with no diastolic release and larger than those preceded by diastolic release. We conclude that tetracaine increases the amplitude of the systolic Ca2+ transient by removing the inhibitory effect of diastolic Ca2+ release.
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A mechanism distinct from the L-type Ca current or Na-Ca exchange contributes to Ca entry in rat ventricular myocytes. Cell Calcium 2006; 39:417-23. [PMID: 16563501 DOI: 10.1016/j.ceca.2006.01.011] [Citation(s) in RCA: 15] [Impact Index Per Article: 0.8] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/17/2005] [Revised: 01/21/2006] [Accepted: 01/25/2006] [Indexed: 10/24/2022]
Abstract
The aim of this paper was to characterize the pathways that allow Ca(2+) ions to enter the cell at rest. Under control conditions depolarization produced an increase of intracellular Ca concentration ([Ca(2+)](i)) that increased with depolarization up to about 0 mV and then declined. During prolonged depolarization the increase of [Ca(2+)](i) decayed. This increase of [Ca(2+)](i) was inhibited by nifedipine and the calculated rate of entry of Ca increased on depolarization and then declined with a similar time course to the inactivation of the L-type Ca current. We conclude that this component of change of [Ca(2+)](i) is due to the L-type Ca current. If intracellular Na was elevated then only part of the change of [Ca(2+)](i) was inhibited by nifedipine. The nifedipine-insensitive component increased monotonically with depolarization and showed no relaxation on prolonged depolarization. This component appears to result from Na-Ca exchange (NCX). When the L-type current and NCX were both inhibited (nifedipine and Na-free solution) then depolarization decreased and hyperpolarization increased [Ca(2+)](i). These changes of [Ca(2+)](i) were unaffected by modifiers of B-type Ca channels such as chlorpromazine and AlF(3) but were abolished by gadolinium ions. We conclude that, in addition to L-type Ca channels and NCX, there is another pathway for entry of Ca(2+) into the ventricular myocyte but this is distinct from the previously reported B-type channel.
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Mechanisms underlying enhanced cardiac excitation contraction coupling observed in the senescent sheep myocardium. J Mol Cell Cardiol 2005; 37:1171-81. [PMID: 15572047 DOI: 10.1016/j.yjmcc.2004.09.005] [Citation(s) in RCA: 39] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Received: 05/25/2004] [Revised: 07/30/2004] [Accepted: 09/14/2004] [Indexed: 11/27/2022]
Abstract
Ageing related stiffening of the vascular system is believed to be in part responsible for a number of clinical outcomes including hypertension and heart failure. In the present study, we sought to determine whether there are alterations in cardiac excitation contraction coupling that may help compensate for the increased vessel stiffness. Experiments were performed on single cardiac myocytes isolated from young (18 months) and aged (>8 years) sheep. Intracellular Ca(2+) concentration, action potentials, L-type Ca(2+) currents and SR Ca(2+) content were measured at 23 degrees C. With ageing, cell capacitance increased by 26% indicating cellular hypertrophy. Action potential duration (APD90) (590 +/- 21 vs. 726 +/- 36 ms), Ca(2+) transient amplitude (112 +/- 15 vs. 202 +/- 25 nmol l(-1)) and fractional cell shortening (by 37%) also increased in the aged hearts (all values P < 0.05). The larger Ca(2+) transient amplitude observed under current clamp conditions was maintained under voltage clamp control; however, SR Ca(2+) content was identical. Both the peak L-type Ca(2+) current (2.8 +/- 0.3 vs. 4.9 +/- 0.5 pA pF(-1)) and integrated Ca(2+) entry (5.1 +/- 0.7 vs. 7.9 +/- 0.8 micromol l(-1), all P < 0.01) were greater in aged cells. In this study we show that in the ageing ovine myocardium, the amplitude of the systolic Ca(2+) transient is increased. The larger Ca(2+) transients cannot simply be explained by changes in APD and we suggest that the greater inward L-type Ca(2+) current provides a more effective trigger for calcium-induced-calcium release from the SR whilst maintaining a stable SR Ca(2+) content. These changes in cardiac excitation contraction coupling may help maintain cardiac output in the face of increased great vessel stiffness.
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Abstract
[Ca2+]i is used as a signal in many tissues. In this review we discuss the mechanisms that regulate [Ca2+]i and, importantly, what determines their stability. Brief mention is made of the effects of feedback gain and delays on stability. The control of cytoplasmic Ca concentration is shown to be generally stable as Ca pumping is essentially an instantaneous function of [Ca2+]i. In contrast, regulation of the Ca content of intracellular stores may be less stable. One example of this is instability in the control of sarcoplasmic reticulum (SR) Ca content in cardiac muscle. An increase of SR Ca content increases the systolic Ca transient amplitude. This in turn decreases Ca influx into the cell and increases efflux, thereby restoring SR Ca to control levels. This feedback system has an inherent delay and is potentially unstable if the gain is increased beyond a certain level. This instability produces Ca transients of alternating amplitude and may contribute to the clinical syndrome of pulsus alternans.
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Physiological and pathological modulation of ryanodine receptor function in cardiac muscle. Cell Calcium 2004; 35:583-9. [PMID: 15110148 DOI: 10.1016/j.ceca.2004.01.012] [Citation(s) in RCA: 17] [Impact Index Per Article: 0.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/01/2003] [Accepted: 01/12/2004] [Indexed: 11/22/2022]
Abstract
Calcium release from the sarcoplasmic reticulum (SR) in cardiac muscle occurs through a specialised release channel, the ryanodine receptor, RyR, via the process of Ca-induced Ca release (CICR). The open probability of the RyR is increased by elevation of cytoplasmic Ca concentration ([Ca(2+)](i)). However, in addition to Ca, other modulators affect the RyR open probability. Agents which increase the RyR opening during systole produce a transient increase of systolic [Ca(2+)](i) followed by a return to the initial level due to a compensating decrease of SR Ca content. Increasing RyR opening during diastole decreases SR Ca content and thereby decreases systolic [Ca(2+)](i). We therefore conclude that potentiation of RyR opening will, if anything, decrease systolic [Ca(2+)](i). The effects of specific examples of modulators of the RyR, such as phosphorylation, metabolic changes, heart failure and polyunsaturated fatty acids, are discussed.
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Interplay between SERCA and sarcolemmal Ca2+ efflux pathways controls spontaneous release of Ca2+ from the sarcoplasmic reticulum in rat ventricular myocytes. J Physiol 2004; 559:121-8. [PMID: 15194743 PMCID: PMC1665077 DOI: 10.1113/jphysiol.2003.058917] [Citation(s) in RCA: 41] [Impact Index Per Article: 2.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/25/2023] Open
Abstract
Waves of calcium-induced calcium release occur in a variety of cell types and have been implicated in the origin of cardiac arrhythmias. We have investigated the effects of inhibiting the SR Ca(2+)-ATPase (SERCA) with the reversible inhibitor 2',5'-di(tert-butyl)-1,4-benzohydroquinone (TBQ) on the properties of these waves. Cardiac myocytes were voltage clamped at a constant potential between -65 and -40 mV and spontaneous waves evoked by increasing external Ca(2+) concentration to 4 mm. Application of 100 microm TBQ decreased the frequency of waves. This was associated with increases of resting [Ca(2+)](i), the time constant of decay of [Ca(2+)](i) and the integral of the accompanying Na(+)-Ca(2+) exchange current. There was also a decrease in propagation velocity of the waves. There was an increase of the calculated Ca(2+) efflux per wave. The SR Ca(2+) content when a wave was about to propagate decreased to 91.7 +/- 3.2%. The period between waves increased in direct proportion to the Ca(2+) efflux per wave meaning that TBQ had no effect on the Ca(2+) efflux per unit time. We conclude that (i) decreased wave frequency is not a direct consequence of decreased Ca(2+) pumping by SERCA between waves but, rather, to more Ca(2+) loss on each wave; (ii) inhibiting SERCA increases the chance of spontaneous Ca(2+) release propagating at a given SR content.
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pH-dependent and -independent effects inhibit Ca(2+)-induced Ca2+ release during metabolic blockade in rat ventricular myocytes. J Physiol 2003; 550:413-8. [PMID: 12766236 PMCID: PMC2343052 DOI: 10.1113/jphysiol.2003.042846] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/08/2022] Open
Abstract
We have investigated the role of changes of intracellular pH (pHi) in the effects of metabolic blockade (cyanide plus 2-deoxyglucose) on Ca2+ release from the sarcoplasmic reticulum (SR) in rat ventricular myocytes. pHi and cell length were measured simultaneously. Metabolic blockade decreased the frequency of Ca2+ waves, an effect previously shown to be due to inhibition of Ca2+ release from the SR. This was accompanied by an intracellular acidification. Intracellular acidification was produced in the absence of metabolic inhibition by application of sodium butyrate. A maintained intracellular acidosis produced a decrease of wave frequency. A hysteresis between pHi and wave frequency was observed such that during the onset of the acidification the wave frequency decreased more than in the steady state. Comparison of the steady state relationship between pHi and wave frequency showed that the decrease of wave frequency produced by metabolic blockade was greater than could be accounted for simply by the accompanying decrease of pHi. In other experiments the buffering power of the solution was increased. Under these conditions, metabolic blockade produced no change of pHi but the decrease of wave frequency persisted. We conclude that, although intracellular acidification occurs during metabolic blockade, it is not responsible for most of the inhibition of Ca2+ release from the SR.
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A new technique for simultaneous and in situ measurements of Ca2+ signals in arteriolar smooth muscle and endothelial cells. Cell Calcium 2003; 34:27-33. [PMID: 12767890 DOI: 10.1016/s0143-4160(03)00019-8] [Citation(s) in RCA: 42] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/18/2022]
Abstract
We report here the first local and global Ca(2+) measurements made from in situ terminal arterioles. The advantages of the method are that there is minimal disturbance to the vessels, which retain their relationship to the tissue they are supplying (rat ureter) and the small size of vessel that can be studied. Good loading with the Ca(2+) indicator, Fluo-4 was obtained, and confocal sectioning through the tissue enabled vascular smooth muscle and endothelial cells to be clearly seen, along with red blood cells, nerve endings and the ureteric smooth muscle cells. We find the terminal arterioles to be extremely active, both spontaneously and in response to nor-adrenaline stimulation, with Ca(2+) sparks occurring in the vascular myocytes and Ca(2+) puffs in the endothelial cells. Even under resting conditions, endothelial cells produced oscillations and waves, which could pass from cell to cell, whereas the vascular myocytes only produced waves in response to agonist stimulation, and with no increase in the frequency of Ca(2+) sparks, and no spread from cell to cell. We compare our data to those obtained in dissected intact vessels and single cells. We conclude that this approach is a convenient and useful method for studying inter- and intracellular Ca(2+) signalling events and communication between cell types, particularly in very small vessels.
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MESH Headings
- Action Potentials/drug effects
- Action Potentials/physiology
- Aniline Compounds
- Animals
- Arterioles/cytology
- Arterioles/metabolism
- Calcium/analysis
- Calcium/metabolism
- Calcium Signaling/drug effects
- Calcium Signaling/physiology
- Endothelium, Vascular/cytology
- Endothelium, Vascular/drug effects
- Endothelium, Vascular/metabolism
- Histocytochemistry/instrumentation
- Histocytochemistry/methods
- Microscopy, Confocal/instrumentation
- Microscopy, Confocal/methods
- Muscle Contraction/drug effects
- Muscle Contraction/physiology
- Muscle, Smooth, Vascular/cytology
- Muscle, Smooth, Vascular/drug effects
- Muscle, Smooth, Vascular/metabolism
- Norepinephrine/pharmacology
- Rats
- Ureter/blood supply
- Ureter/cytology
- Vasoconstriction/drug effects
- Vasoconstriction/physiology
- Xanthenes
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Abstract
We have investigated the possibility that some component of calcium release from the cardiac sarcoplasmic reticulum (SR) may occur directly in response to the surface membrane action potential rather than by calcium induced calcium release (CICR). Experiments were performed on rat ventricular myocytes and intracellular calcium concentration ([Ca(2+)](i)) measured with fluo-3. In order to mimic physiological conditions, experiments were performed at 37 degrees C, using the perforated patch technique (to avoid intracellular dialysis) with pulses from -80 to 0 mV. The addition of 500 microM Cd(2+) to inhibit the L-type Ca current reduced the rate of increase of the Ca transient to 2.8 +/- 1% of control. When experiments were performed with Na-free solutions in the pipette, Cd(2+) abolished the transient completely suggesting that the residual Ca entry was on Na-Ca exchange. The addition of Ni(2+) produced a concentration dependent inhibition of the Ca transient with 5 mM being sufficient to completely inhibit the transient. The inhibitory effects of Ni(2+) were unaffected by prior exposure to isoprenaline. These results provide no evidence for a voltage activated calcium release mechanism in cardiac muscle and are consistent with SR Ca(2+) release being triggered by a process of Ca(2+) induced Ca(2+) release.
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Abstract
This issue of The Journal of Physiology includes a series of review articles arising from a symposium held at the joint meeting of the UK, German and Scandinavian Physiological Societies. The articles focus on different aspects of the cellular control of contraction. The basic mechanism of cardiac excitation-contraction coupling ('calcium-induced calcium release') is now reasonably well-established. Calcium enters the cell from the extracellular fluid via the voltage-dependent L-type Ca(2+) channel. This results in a 'trigger' increase of [Ca(2+)](i) in the space between the sarcolemma and sarcoplasmic reticulum (SR) and this leads to the opening of the SR Ca(2+) release channel or 'ryanodine receptor' (RyR). As exemplified by the papers from the symposium, much current work is focused on how this mechanism is modified in different circumstances. These include autonomic modulation, but also pathological conditions such as cardiac hypertrophy and failure, a recurrent theme in several of these papers.
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PH-induced changes in calcium: functional consequences and mechanisms of action in guinea pig portal vein. Am J Physiol Heart Circ Physiol 2002; 283:H2518-26. [PMID: 12427597 DOI: 10.1152/ajpheart.01102.2001] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
The effects of changing extracellular (pH(o)) and intracellular pH (pH(i)) on force and the mechanisms involved in the guinea pig portal vein were investigated to better understand the control of tone in this vessel. When pH(o) was altered, the effects on force and calcium were the same irrespective of whether force had been produced spontaneously by high-K depolarization or by norepinephrine; alkalinization increased tone, and acidification reduced it. Because pH(o) changes also lead to changes in pH(i), we determined whether the effects on force could be explained by these induced pH(i) changes. It was found, however, that only with spontaneous activity did intracellular alkalinization increase force. In depolarized preparations, force was decreased, and, with norepinephrine, force was initially decreased and then increased. Thus the effects of pH(o) cannot be explained solely by changes in pH(i). The role of the sarcoplasmic reticulum (SR) and surface membrane Ca(2+)-ATPase on the mechanism were investigated and shown not to be involved. Therefore, it is concluded that both pH(o) and pH(i) can have powerful modulatory effects on portal vein tone, that these effects are not identical, and that they are likely to be due to effects of pH on ion channels rather than the SR or plasma membrane Ca(2+)-ATPase.
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Depressed ryanodine receptor activity increases variability and duration of the systolic Ca2+ transient in rat ventricular myocytes. Circ Res 2002; 91:585-93. [PMID: 12364386 DOI: 10.1161/01.res.0000035527.53514.c2] [Citation(s) in RCA: 136] [Impact Index Per Article: 6.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
Sarcoplasmic reticulum (SR) Ca2+ release, through the ryanodine receptor (RyR), is essential for the systolic Ca2+ transient and thus the cardiac contractile function. The aim of this study was to examine the effects on the spatial organization of the systolic Ca2+ transient of depressing RyR open probability (P(o)) with tetracaine or intracellular acidification. Voltage-clamped, fluo-3-loaded myocytes were studied using confocal microscopy. Depressing RyR P(o) increased the variability of the Ca2+ transient amplitude between different regions of the cell. This variability often produced alternans with a region producing large and small transients alternately. In addition, the raising phase of the Ca2+ transient became biphasic. The initial phase was constant but the second was variable and propagated as a wave through part of the cell. That both phases involved SR Ca2+ release was shown by their reduction by caffeine. Regional [Ca2+]i alternans was accompanied by a much smaller degree of alternans at the whole cell level. We suggest that, in tetracaine or acidosis, the initial phase of the Ca2+ transient results from Ca2+ release via RyRs directly activated by adjacent L-type Ca2+ channels. At some sites, this will activate neighboring RyRs and a Ca2+ wave will propagate via activation of other RyRs. This work is the first demonstration that decreased RyR P(o) alone can produce disarray of the Ca2+ release process and initiate alternans.
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Integrative analysis of calcium signalling in cardiac muscle. FRONTIERS IN BIOSCIENCE : A JOURNAL AND VIRTUAL LIBRARY 2002; 7:d843-52. [PMID: 11897570 DOI: 10.2741/trafford] [Citation(s) in RCA: 27] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
This review discusses the control of the amplitude of the cardiac systolic Ca transient. The Ca transient arises largely from release from the sarcoplasmic reticulum (SR). Release is triggered by calcium-induced calcium release (CICR) whereby the entry of a small amount of Ca on the L-type Ca current, "the trigger", results in the release of much more Ca from the SR. There are three potential control points: (1) the Ca content of the SR; (2) the properties of the SR Ca release channel or ryanodine receptor (RyR); (3) the amplitude of the L-type Ca current. The data reviewed show that the Ca content of the SR has pronounced effects on systolic [Ca2+]i and, reciprocally, the amount of Ca released from the SR affects sarcolemmal Ca fluxes thereby "autoregulating" SR content. Modulation of the ryanodine receptor has no steady-state effect due to compensating changes of SR Ca content. An increase of the L-type Ca current results in an abrupt increase of systolic [Ca2+]i with little change of SR content. This is because of a coordinated increase of both the trigger and loading function of the Ca current. These results emphasise the importance of considering all aspects of Ca handling in the context of SR Ca release and thus the regulation of the systolic Ca transient and contraction in cardiac muscle.
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The role of intracellular Ca buffers in determining the shape of the systolic Ca transient in cardiac ventricular myocytes. Pflugers Arch 2001; 442:96-100. [PMID: 11374074 DOI: 10.1007/s004240000509] [Citation(s) in RCA: 26] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/27/2022]
Abstract
We have studied the kinetics of decay of cardiac Ca transients elicited by either caffeine or electrical stimulation. The decay of the caffeine-evoked increase of intracellular Ca concentration ([Ca2+]i) could not be fit by a single exponential. A two exponential fit showed an initial rapid component. The rate of decay of total Ca was calculated from measured free Ca and the buffering properties of the cell. There was no initial rapid phase of decay of total Ca. We conclude that the rapid phase of decay of free Ca is due to a decrease of Ca buffering power at elevated [Ca2+]i. In contrast, the decay of the Ca transient, produced by voltage-clamp depolarization or field stimulation was fit by a single exponential. We suggest that these apparently simple kinetics arise because a tendency to saturation at elevated [Ca2+]i of the sarcoplasmic reticulum Ca-ATPase offsets the decrease of Ca buffering power. These data show the importance of Ca buffers as well as transporters in determining the kinetics of changes of [Ca2+]i.
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The effects of exogenous calcium buffers on the systolic calcium transient in rat ventricular myocytes. Biophys J 2001; 80:1915-25. [PMID: 11259304 PMCID: PMC1301380 DOI: 10.1016/s0006-3495(01)76161-9] [Citation(s) in RCA: 32] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 10/21/2022] Open
Abstract
The aim of this work was to characterize the effects that two commonly used "caged" calcium buffers (NP-EGTA and nitr-5) have on the amplitude and time course of decay of the calcium transient. We made quantitative measurements of both free and total calcium using the measured buffering properties of the cell. Intracellular calcium concentration ([Ca(2+)](i)) was measured with fluo-3 in rat ventricular myocytes. Incorporation of the buffer NP-EGTA decreased both the amplitude and rate of decay of the caffeine response. The slowing could be quantitatively accounted for by the measured increased buffering. These effects were removed by photolysis of NP-EGTA. Similar results were obtained with nitr-5 except that the effects were not completely removed by photolysis. This was shown to be due to the persistence of a component of the increased buffering after photolysis. Both buffers decreased the amplitude of the systolic calcium transient. However, although nitr-5 produced a simple slowing of the decay, NP-EGTA resulted in an initial rapid phase of decay. This rapid phase of decay is attributed to calcium binding to NP-EGTA. This work represents the first quantitative analysis of the effects that extra buffering by a fast and a slow calcium chelator may have on the calcium transient.
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38
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Abstract
1. The role of the sarcoplasmic reticulum (SR) was investigated in spontaneous and agonist-induced uterine Ca2+ transients, by combining low- (mag-fluo-4) and high-affinity (fura-2) indicators to measure intraluminal SR ([Ca2+]L) and cytosolic ([Ca2+]i) calcium concentration, simultaneously, in single smooth muscle cells from pregnant rat uterus. 2. Carbachol or ATP, in the absence of extracellular Ca2+, decreased [Ca2+]L and increased [Ca2+]i. Although some replenishment (around 50 %) occurred in its absence, extracellular Ca2+ was required for full replenishment of the SR Ca2+. 3. In 4/15 cells, ATP evoked oscillations of [Ca2+]i. These were accompanied by successive release and re-uptake of SR Ca2+. Inhibition of the SR Ca2+-ATPase with thapsigargin abolished the oscillations and luminal changes. 4. Spontaneous [Ca2+]i transients produced no detectable changes in [Ca2+]L. The larger [Ca2+]i transients evoked by high-K+ depolarisation increased [Ca2+]L. Spontaneous activity was inhibited when [Ca2+]L was increased. 5. These data show that it is possible to simultaneously measure SR and cytosolic [Ca2+], and to investigate their response to agonist application and spontaneous activity.
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39
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Coordinated control of cell Ca(2+) loading and triggered release from the sarcoplasmic reticulum underlies the rapid inotropic response to increased L-type Ca(2+) current. Circ Res 2001; 88:195-201. [PMID: 11157672 DOI: 10.1161/01.res.88.2.195] [Citation(s) in RCA: 94] [Impact Index Per Article: 4.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Submit a Manuscript] [Subscribe] [Scholar Register] [Indexed: 11/16/2022]
Abstract
The aim of this study was to investigate how sarcoplasmic reticulum (SR) Ca(2+) content and systolic Ca(2+) are controlled when Ca(2+) entry into the cell is varied. Experiments were performed on voltage-clamped rat and ferret ventricular myocytes loaded with fluo-3 to measure intracellular Ca(2+) concentration ([Ca(2+)](i)). Increasing external Ca(2+) concentration ([Ca(2+)](o)) from 1 to 2 mmol/L increased the amplitude of the systolic Ca(2+) transient with no effect on SR Ca(2+) content. This constancy of SR content is shown to result because the larger Ca(2+) transient activates a larger Ca(2+) efflux from the cell that balances the increased influx. Decreasing [Ca(2+)](o) to 0.2 mmol/L decreased systolic Ca(2+) but produced a small increase of SR Ca(2+) content. This increase of SR Ca(2+) content is due to a decreased release of Ca(2+) from the SR resulting in decreased loss of Ca(2+) from the cell. An increase of [Ca(2+)](o) has two effects: (1) increasing the fraction of SR Ca(2+) content, which is released on depolarization and (2) increasing Ca(2+) entry into the cell. The results of this study show that the combination of these effects results in rapid changes in the amplitude of the systolic Ca(2+) transient. In support of this, the changes of amplitude of the transient occur more quickly following changes of [Ca(2+)](o) than following refilling of the SR after depletion with caffeine. We conclude that the coordinated control of increased Ca(2+) entry and greater fractional release of Ca(2+) is an important factor in regulating excitation-contraction coupling.
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Abstract
Changes in the behavior of the sarcoplasmic reticulum (SR) in rat ventricular myocytes were investigated under conditions of metabolic inhibition using laser-scanning confocal microscopy to measure intracellular Ca(2+) and the perforated patch-clamp technique to measure SR Ca(2+) content. Metabolic inhibition had several effects on SR function, including reduced frequency of spontaneous releases of Ca(2+) (sparks and waves of Ca(2+)-induced Ca(2+) release), increased SR Ca(2+) content (79.4+/-5.7 to 115.2+/-6.6 micromol/L cell volume [mean+/-SEM; P:<0.001]), and, after a wave of Ca(2+) release, slower reuptake of Ca(2+) into the SR (rate constant of fall of Ca(2+) reduced from 8.5+/-1.1 s(-)(1) in control to 5.2+/-0.4 s(-)(1) in metabolic inhibition [P:<0.01]). Inhibition of L-type Ca(2+) channels with Cd(2+) (100 micromol/L) did not reproduce the effects of metabolic inhibition on spontaneous Ca(2+) sparks. These results are evidence of inhibition of both Ca(2+) release and reuptake mechanisms. Reduced frequency of release could be attributable to either of these effects, but the increased SR Ca(2+) content at the time of reduced frequency of spontaneous release of Ca(2+) shows that the dominant effect of metabolic inhibition is to inhibit release of Ca(2+) from the SR, allowing the accumulation of greater than normal amounts of Ca(2+). In the context of ischemia, this extra accumulation of Ca(2+) would present a risk of potentially arrhythmogenic, spontaneous release of Ca(2+) on reperfusion of the tissue.
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The effect of acidosis on systolic Ca2+ and sarcoplasmic reticulum calcium content in isolated rat ventricular myocytes. J Physiol 2000; 529 Pt 3:661-8. [PMID: 11118496 PMCID: PMC2270229 DOI: 10.1111/j.1469-7793.2000.00661.x] [Citation(s) in RCA: 61] [Impact Index Per Article: 2.5] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/27/2022] Open
Abstract
We have investigated the mechanisms responsible for the changes of systolic Ca2+ that occur in voltage-clamped rat ventricular myocytes during acidosis produced by application of the weak acid butyrate (30 mM). Intracellular pH regulation was inhibited with dimethylamiloride (bicarbonate-free solution). The application of butyrate produced an intracellular acidification of 0.33 pH units. This was accompanied by a decrease in systolic Ca2+ to about 50% of control. However, within 2 min, systolic Ca2+ returned to control levels. The decrease in systolic Ca2+ was accompanied by a decrease in the Na+-Ca2+ exchange current observed on repolarisation so that the calculated Ca2+ efflux on Na+-Ca2+ exchange was less than the entry on the L-type Ca2+ current. The magnitude of the Na+-Ca2+ exchange current recovered along with systolic Ca2+ until it equalled the Ca2+ entry on the L-type Ca2+ current. From the measurement of Ca2+ fluxes, it was calculated that, during acidosis, the cell gains 121.6+/-16.2 micromol l(-1) of Ca2+. This is equal to the measured increase of sarcoplasmic reticulum (SR) calcium content obtained by applying caffeine (20 mM) and integrating the resulting Na+-Ca2+ exchange current. We conclude that the recovery of the amplitude of the systolic Ca2+ transient is due to decreased SR calcium release, resulting in reduced Ca2+ efflux from the cell leading to increased SR calcium content.
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Abstract
The control of intracellular calcium is central to regulation of contractile force in cardiac muscle. This review illustrates how analysis of the control of calcium requires an integrated approach in which several systems are considered. Thus, the calcium content of the sarcoplasmic reticulum (SR) is a major determinant of the amount of Ca(2+) released from the SR and the amplitude of the Ca(2+) transient. The amplitude of the transient, in turn, controls Ca(2+) fluxes across the sarcolemma and thence SR content. This control of SR content influences the response to maneuvers that modify, for example, the properties of the SR Ca(2+) release channel or ryanodine receptor. Specifically, modulation of the open probability of the ryanodine receptor produces only transient effects on the Ca(2+) transient as a result of changes of SR content. These interactions between various Ca(2+) fluxes are modified by the Ca(2+) buffering properties of the cell. Finally, we predict that, under some conditions, the above interactions can result in instability (such as alternans) rather than ordered control of contractility.
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The effects of low concentrations of caffeine on spontaneous Ca release in isolated rat ventricular myocytes. Cell Calcium 2000; 28:269-76. [PMID: 11032782 DOI: 10.1054/ceca.2000.0156] [Citation(s) in RCA: 82] [Impact Index Per Article: 3.4] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/18/2022]
Abstract
We have investigated the effects on spontaneous SR Ca release of modulating the sarcoplasmic reticulum ryanodine receptor (RyR) with low (<0.5 mM) concentrations of caffeine. Experiments were performed on isolated rat ventricular myocytes. Intracellular Ca concentration was measured with Indo-1 or Fluo-3 in voltage-clamped cells. Spontaneous Ca release was produced by elevating external Ca to 5 mM. Caffeine application increased the frequency of spontaneous release. Both the magnitude of the spontaneous Ca transients and the integral of the resulting Na-Ca exchange current were decreased by caffeine. The combination of increased frequency of spontaneous release and decreased Ca efflux per event meant that the Ca efflux per unit time was unaffected by low concentrations of caffeine. The SR Ca content was reduced by caffeine. The extra Ca efflux calculated from the Na-Ca exchange current integrals occurring during the initial burst of spontaneous activity on application of caffeine accounted for this reduction of SR Ca content. In contrast to these maintained effects on spontaneous release, caffeine had only transient effects on stimulated Ca release produced by depolarizing pulses. We conclude that stimulation of the RyR results in spontaneous release at SR Ca contents lower than those at which release would normally occur. Therefore, the balance between normal and spontaneous Ca release can be shifted by modulation of the RyR. This will have important consequences for arrhythmogenesis due to spontaneous Ca release.
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No Role for the Ryanodine Receptor in Regulating Cardiac Contraction? NEWS IN PHYSIOLOGICAL SCIENCES : AN INTERNATIONAL JOURNAL OF PHYSIOLOGY PRODUCED JOINTLY BY THE INTERNATIONAL UNION OF PHYSIOLOGICAL SCIENCES AND THE AMERICAN PHYSIOLOGICAL SOCIETY 2000; 15:275-279. [PMID: 11390926 DOI: 10.1152/physiologyonline.2000.15.5.275] [Citation(s) in RCA: 10] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 11/22/2022]
Abstract
Cardiac contraction is initiated by Ca(2+) leaving the sarcoplasmic reticulum through the ryanodine receptor (RyR). Although opening of the RyR can be modified by various ligands, these have no maintained effect on contraction. We conclude that modulation of the RyR controls sarcoplasmic reticulum Ca(2+) content rather than cytoplasmic Ca(2+) concentration.
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Abstract
The aim of this work was to obtain the first quantitative measurements of Ca2+ influx and efflux in quiescent cardiac cells. The relationship between free and total Ca2+ was obtained during a caffeine application. This buffering curve was then used to calculate changes of total Ca2+ from measurements of free cytosolic [Ca2+] ([Ca2+]i) made with Indo-1. The rate of Ca2+ removal from the cytoplasm was calculated by differentiating total Ca2+ with respect to time. The dependence of d(total Ca2+)/dt on [Ca2+]i was hyperbolic. Inhibition of either Na+-Ca2+ exchange (by addition of 10 mmol l(-1) NiCl2 or removal of external Na+) or the sarcolemmal Ca2+-activated adenosine triphosphatase (Ca2+-ATPase) (with carboxyeosin) decreased the calculated efflux. In both cases, the main effect was on the apparent maximum rate (Vmax) with little effect on the Michaelis-Menten constant (Km). These results suggest that the Na+-Ca2+ exchange and Ca2+-ATPase have very similar affinities for [Ca2+]i and that their fractional contributions do not change over the systolic range of [Ca2+]i. Ca2+ influx was quantified in two ways. The first method was to extrapolate the curve relating Ca2+ efflux to [Ca2+]i to zero [Ca2+]i. This gave a value of 4.49+/-0.54 micromol l(-1) s(-1) which was reduced to zero by either removal of external Ca2+ or addition of Ni2+. In other experiments external Ca2+ was removed and the maximum rate of fall of total Ca2+ calculated as 2.53+/-0.93 micromol l(-1) s(-1). This approach can be used to provide a quantitative analysis of the control of resting [Ca2+]i.
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Modulation of CICR has no maintained effect on systolic Ca2+: simultaneous measurements of sarcoplasmic reticulum and sarcolemmal Ca2+ fluxes in rat ventricular myocytes. J Physiol 2000; 522 Pt 2:259-70. [PMID: 10639102 PMCID: PMC2269754 DOI: 10.1111/j.1469-7793.2000.t01-2-00259.x] [Citation(s) in RCA: 143] [Impact Index Per Article: 6.0] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/26/2022] Open
Abstract
1. The effects of modulating Ca2+-induced Ca2+ release (CICR) in single cardiac myocytes were investigated using low concentrations of caffeine (< 500 microM) in reduced external Ca2+ (0.5 mM). Caffeine produced a transient potentiation of systolic [Ca2+]i (to 800 % of control) which decayed back to control levels. 2. Caffeine decreased the steady-state sarcoplasmic reticulum (SR) Ca2+ content. As the concentration of caffeine was increased, both the potentiation of the systolic Ca2+ transient and the decrease in SR Ca2+ content were increased. At higher concentrations, the potentiating effect decayed more rapidly but the rate of recovery on removal of caffeine was unaffected. 3. A simple model in which caffeine produces a fixed increase in the fraction of SR Ca2+ which is released could account qualitatively but not quantitatively for the above results. 4. The changes in total [Ca2+] during systole were obtained using measurements of the intracellular Ca2+ buffering power. Caffeine initially increased the fractional release of SR Ca2+. This was followed by a decrease to a level greater than that under control conditions. The fraction of systolic Ca2+ which was pumped out of the cell increased abruptly upon caffeine application but then recovered back to control levels. The increase in fractional loss is due to the fact that, as the cytoplasmic buffers become saturated, a given increase in systolic total [Ca2+] produces a larger increase in free [Ca2+] and thence of Ca2+ efflux. 5. These results confirm that modulation of the ryanodine receptor has no maintained effect on systolic Ca2+ and show the interdependence of SR Ca2+ content, cytoplasmic Ca2+ buffering and sarcolemmal Ca2+ fluxes. Such analysis is important for understanding the cellular basis of inotropic interventions in cardiac muscle.
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Effects of mefloquine on cardiac contractility and electrical activity in vivo, in isolated cardiac preparations, and in single ventricular myocytes. Br J Pharmacol 2000; 129:323-30. [PMID: 10694239 PMCID: PMC1571844 DOI: 10.1038/sj.bjp.0703060] [Citation(s) in RCA: 34] [Impact Index Per Article: 1.4] [Reference Citation Analysis] [Abstract] [Key Words] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/06/1999] [Revised: 09/07/1999] [Accepted: 10/29/1999] [Indexed: 11/09/2022] Open
Abstract
1. To examine the possible cardiotoxicity of the antimalarial drug mefloquine, increasing doses (0.3 - 30 mg kg(-1)) were given i.v. to anaesthetized guinea-pigs. Mefloquine did not alter ECG intervals significantly but gradually increased systolic blood pressure (at 3 mg kg(-1)) then had a depressor effect (at 10 mg kg(-1)). Death due to profound hypotension, probably resulting from cardiac contractile failure or AV block, occurred after either 10 mg kg(-1) (2/6) or 30 mg kg(-1) (4/6) mefloquine. 2. In isolated cardiac preparations mefloquine (3 - 100 microM) did not alter the effective refractory period but at the higher concentrations resting tension increased. Developed tension was reduced by 100 microM mefloquine in left atria (from 5.8+/-1.7 to 2.2+/-0.4 mN) whereas in papillary muscles although 30 microM mefloquine reduced developed tension (from 2. 6+/-0.5 to 1.1+/-0.1 mN) subsequent addition of 100 microM caused a marked, but not sustained, positive inotropic effect (from 1.2+/-0.1 to 3.8+/-0.8 mN). 3. In single ventricular myocytes, mefloquine (10 microM) shortened action potential duration (e.g. APD(90) from 285+/-29 to 141+/-12 ms) and reduced the amplitude of the systolic Ca(2+) transient. 4. These effects were accompanied by a decrease in the L-type Ca(2+) current. These results indicate that the main adverse effect of mefloquine on the heart is a negative inotropic action. This action can be explained by blockade of L-type Ca(2+) channels.
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Abstract
1. The mechanisms responsible for removing calcium ions from the cytoplasm were investigated in single rat uterine myocytes using indo-1. 2. Trains of depolarizing voltage-clamp pulses increased [Ca2+]i. The rate of decay of [Ca2+]i was slowed by inhibition of the sarcoplasmic reticulum (SR) Ca2+-ATPase with cyclopiazonic acid (CPA). However, if the sarcolemmal Na+-Ca2+ exchanger and Ca2+-ATPase were inhibited then recovery of [Ca2+]i was abolished showing that the SR Ca2+-ATPase alone cannot produce decay of [Ca2+]i. 3. In another series of experiments, Ca2+ release from the SR was induced with carbachol in a Ca2+-free solution. Under these conditions responses to repeated applications of carbachol could be obtained. In the presence of CPA, however, only the first application was effective. This suggests that the SR Ca2+-ATPase sequesters a significant amount of Ca2+ into the SR. 4. CPA slowed the rate of decay of [Ca2+]i following carbachol addition by > 50 %. Again, however, after a brief transient fall, decay was abolished when the Na+-Ca2+ exchanger and sarcolemmal Ca2+-ATPase were inhibited. 5. These data show that, although the SR Ca2+-ATPase contributes to the decay of [Ca2+]i, it cannot function effectively in the absence of Ca2+ removal from the cell. These data are discussed in the context of the superficial buffer barrier model in which Ca2+ is taken up into the SR and then released very close to sarcolemmal Ca2+ extrusion sites, i.e. the SR acting in series with the surface membrane extrusion mechanisms. We also suggest that the amount of filling of the SR influences the rate of Ca2+ removal.
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