Measurements of Ca2+ entry and sarcoplasmic reticulum Ca2+ content during the cardiac cycle in guinea pig and rat ventricular myocytes

Mitochondrial Membrane Intracellular Communication in Healthy and Diseased Myocardium

Research efforts in the twenty-first century have been paramount to the discovery and development of novel pharmacological treatments in a variety of diseases resulting in improved life expectancy. Yet, cardiac disease remains a leading cause of morbidity and mortality worldwide. Over time, there has been an expansion in conditions such as atrial fibrillation (AF) and heart failure (HF). Although past research has elucidated specific pathways that participate in the development of distinct cardiac pathologies, the exact mechanisms of action leading to disease remain to be fully characterized. Protein turnover and cellular bioenergetics are integral components of cardiac diseases, highlighting the importance of mitochondria and endoplasmic reticulum (ER) in driving cellular homeostasis. More specifically, the interactions between mitochondria and ER are crucial to calcium signaling, apoptosis induction, autophagy, and lipid biosynthesis. Here, we summarize mitochondrial and ER functions and physical interactions in healthy physiological states. We then transition to perturbations that occur in response to pathophysiological challenges and how this alters mitochondrial–ER and other intracellular organelle interactions. Finally, we discuss lifestyle interventions and innovative therapeutic targets that may be used to restore beneficial mitochondrial and ER interactions, thereby improving cardiac function.

Effects of amiodarone on rodent ventricular cardiomyocytes: Novel perspectives from a cellular model of Long QT Syndrome Type 3

AIMS

Amiodarone (AMIO) is currently used in medical practice to reverse ventricular tachycardia. Here we determine the effects of AMIO in the electromechanical properties of isolated left ventricle myocyte (LVM) from mice and guinea pig and in a cellular model of Long QT Syndrome Type 3 (LQTS-3) using anemone neurotoxin 2 (ATX II), which induces increase of late sodium current in LVM.

MAIN METHODS AND KEY FINDINGS

Using patch-clamp technique, fluorescence imaging to detect cellular Ca²⁺ transient and sarcomere detection systems we evaluate the effect of AMIO in healthy LVM. AMIO produced a significant reduction in the percentage of sarcomere shortening (0.1, 1 and 10 μM) in a range of pacing frequencies, however, without significant attenuation of Ca²⁺ transient. Also, 10 μM of AMIO caused the opposite effect on action potential repolarization of mouse and guinea pig LVM. When LVM from mouse and guinea pig were paced in a range of pacing frequencies and exposed to ATX (10 nM), AMIO (10 μM) was only able to abrogate electromechanical arrhythmias in LVM from guinea pig at lower pacing frequency.

SIGNIFICANCE

AMIO has negative inotropic effect with opposite effect on action potential waveform in mouse and guinea pig LVM. Furthermore, the antiarrhythmic action of AMIO in LQTS-3 is species and frequency-dependent, which indicates that AMIO may be beneficial for some types of arrhythmias related to late sodium current.

Species-Dependent Mechanisms of Cardiac Arrhythmia: A Cellular Focus

Although ventricular arrhythmia remains a leading cause of morbidity and mortality, available antiarrhythmic drugs have limited efficacy. Disappointing progress in the development of novel, clinically relevant antiarrhythmic agents may partly be attributed to discrepancies between humans and animal models used in preclinical testing. However, such differences are at present difficult to predict, requiring improved understanding of arrhythmia mechanisms across species. To this end, we presently review interspecies similarities and differences in fundamental cardiomyocyte electrophysiology and current understanding of the mechanisms underlying the generation of afterdepolarizations and reentry. We specifically highlight patent shortcomings in small rodents to reproduce cellular and tissue-level arrhythmia substrate believed to be critical in human ventricle. Despite greater ease of translation from larger animal models, discrepancies remain and interpretation can be complicated by incomplete knowledge of human ventricular physiology due to low availability of explanted tissue. We therefore point to the benefits of mathematical modeling as a translational bridge to understanding and treating human arrhythmia.

Therapeutic cardiac-targeted delivery of miR-1 reverses pressure overload-induced cardiac hypertrophy and attenuates pathological remodeling

Background

MicroRNAs (miRNAs) play a key role in the development of heart failure, and recent studies have shown that the muscle‐specific miR‐1 is a key regulator of cardiac hypertrophy. We tested the hypothesis that chronic restoration of miR‐1 gene expression in vivo will regress hypertrophy and protect against adverse cardiac remodeling induced by pressure overload.

Methods and Results

Cardiac hypertrophy was induced by left ventricular pressure overload in male Sprague‐Dawley rats subjected to ascending aortic stenosis. When the hypertrophy was established at 2 weeks after surgery, the animals were randomized to receive either an adeno‐associated virus expressing miR‐1 (AAV9.miR‐1) or green fluorescent protein (GFP) as control (AAV9.GFP) via a single‐bolus tail‐vein injection. Administration of miR‐1 regressed cardiac hypertrophy (left ventricular posterior wall thickness,; 2.32±0.08 versus 2.75±0.07 mm, P<0.001) and (left ventricular septum wall thickness, 2.23±0.06 versus 2.54±0.10 mm, P<0.05) and halted the disease progression compared with control‐treated animals, as assessed by echocardiography (fractional shortening, 37.60±5.01% versus 70.68±2.93%, P<0.05) and hemodynamic analyses (end‐systolic pressure volume relationship/effective arterial elastance, 1.87±0.46 versus 0.96±0.38, P<0.05) after 7 weeks of treatment. Additionally, miR‐1 replacement therapy lead to a marked reduction of myocardial fibrosis, an improvement in calcium handling, inhibition of apoptosis, and inactivation of the mitogen‐activated protein kinase signaling pathways, suggesting a favorable effect on preventing the maladaptive ventricular remodeling. We also identified and validated a novel bona fide target of miR‐1, Fibullin‐2 (Fbln2), a secreted protein implicated in extracellular matrix remodeling.

Conclusions

Taken together, our findings suggest that restoration of miR‐1 gene expression is a potential novel therapeutic strategy to reverse pressure‐induced cardiac hypertrophy and prevent maladaptive cardiac remodeling.

Regulation of ion gradients across myocardial ischemic border zones: a biophysical modelling analysis

The myocardial ischemic border zone is associated with the initiation and sustenance of arrhythmias. The profile of ionic concentrations across the border zone play a significant role in determining cellular electrophysiology and conductivity, yet their spatial-temporal evolution and regulation are not well understood. To investigate the changes in ion concentrations that regulate cellular electrophysiology, a mathematical model of ion movement in the intra and extracellular space in the presence of ionic, potential and material property heterogeneities was developed. The model simulates the spatial and temporal evolution of concentrations of potassium, sodium, chloride, calcium, hydrogen and bicarbonate ions and carbon dioxide across an ischemic border zone. Ischemia was simulated by sodium-potassium pump inhibition, potassium channel activation and respiratory and metabolic acidosis. The model predicted significant disparities in the width of the border zone for each ionic species, with intracellular sodium and extracellular potassium having discordant gradients, facilitating multiple gradients in cellular properties across the border zone. Extracellular potassium was found to have the largest border zone and this was attributed to the voltage dependence of the potassium channels. The model also predicted the efflux of [Formula: see text] from the ischemic region due to electrogenic drift and diffusion within the intra and extracellular space, respectively, which contributed to [Formula: see text] depletion in the ischemic region.

Energetic study of cardioplegic hearts under ischaemia/reperfusion and [Ca(2+)] changes in cardiomyocytes of guinea-pig: mitochondrial role

AIM

To study the role of mitochondria in the recovery of guinea-pig hearts exposed to high-K(+)-cardioplegia (CPG) and ischaemia/reperfusion (I/R) METHODS: We measured contractility and heat release in perfused guinea-pig hearts and cytosolic and mitochondrial Ca(2+) by epifluorescence and confocal microscopy in isolated cardiomyocytes loaded with Fluo-4 or Rhod-2.

RESULTS

In hearts, CPG increased the postischaemic contractile recovery, and this was potentiated by the mNCX blocker clonazepam and the mKATP opener diazoxide, which also prevented the fall in muscle economy. Moreover, CPG prevented the stunning induced by ouabain, which was reduced by clonazepam. In cardiomyocytes, CPG increased fluorescent signals of cytosolic and mitochondrial Ca(2+), while the addition of a mNCX blocker (CGP37157) increased cytosolic but reduced mitochondrial [Ca(2+)]. Ouabain in CPG increased cytosolic Ca(2+) and resting heat, but the addition of CGP37157 reduced them, as well as mitochondrial Ca(2+).

CONCLUSIONS

CPG, diazoxide and clonazepam improve postischaemic recovery, respectively, by increasing the Ca(2+) cycling and by reducing the mitochondrial Ca(2+) uptake either by uniporter or by mNCX. The mitochondria compete with the leaky sarcoplasmic reticulum (SR) as sink of Ca(2+) in guinea-pig hearts, affecting the postischaemic contractility. CPG also prevented the ouabain-induced dysfunction by avoiding the Ca(2+) overload. Ouabain reduced the synergism between CPG and clonazepam suggesting that [Na(+)]i and SR load influence the mNCX role.

Genetic background affects function and intracellular calcium regulation of mouse hearts

AIMS

The genetic background is currently under close scrutiny when determining cardiovascular disease progression and response to therapy. However, this factor is rarely considered in physiological studies, where it could influence the normal behaviour and adaptive responses of the heart. We aim to test the hypothesis that genetic strain variability is associated with differences in excitation-contraction coupling mechanisms, in particular those involved in cytoplasmic Ca(2+) regulation, and that they are concomitant to differences in whole-heart function and cell morphology.

METHODS AND RESULTS

We studied 8- to 10-week-old male C57BL/6, BALB/C, FVB, and SV129 mice. Echocardiography and radiotelemetry were used to assess cardiac function in vivo. FVB mice had increased left ventricular ejection fraction and fractional shortening with significantly faster heart rate (HR) and lack of diurnal variation of HR. Confocal microscopy, sarcomere length tracking, and epifluorescence were used to investigate cell volume, t-tubule density, contractility, and Ca(2+) handling in isolated ventricular myocytes. Sarcomere relaxation and time-to-peak of the Ca(2+) transient were prolonged in BALB/C myocytes, with more frequent Ca(2+) sparks and significantly higher sarcoplasmic reticulum (SR) Ca(2+) leak. There were no strain differences in the contribution of different Ca(2+) extrusion mechanisms. SV129 had reduced SR Ca(2+) leak with elevated SR Ca(2+) content and smaller cell volume and t-tubule density compared with myocytes from other strains.

CONCLUSION

These results demonstrate that a different genetic background is associated with physiological differences in cardiac function in vivo and differences in morphology, contractility, and Ca(2+) handling at the cellular level.

Quantification of calcium entry at the T-tubules and surface membrane in rat ventricular myocytes

The action potential of cardiac ventricular myocytes is characterized by its long duration, mainly due to Ca flux through L-type Ca channels. Ca entry also serves to trigger the release of Ca from the sarcoplasmic reticulum. The aim of this study was to investigate the role of cell membrane invaginations called transverse (T)-tubules in determining Ca influx and action potential duration in cardiac ventricular myocytes. We used the whole cell patch clamp technique to record electrophysiological activity in intact rat ventricular myocytes (i.e., from the T-tubules and surface sarcolemma) and in detubulated myocytes (i.e., from the surface sarcolemma only). Action potentials were significantly shorter in detubulated cells than in control cells. In contrast, resting membrane potential and action potential amplitude were similar in control and detubulated myocytes. Experiments under voltage clamp using action potential waveforms were used to quantify Ca entry via the Ca current. Ca entry after detubulation was reduced by approximately 60%, a value similar to the decrease in action potential duration. We calculated that Ca influx at the T-tubules is 1.3 times that at the cell surface (4.9 vs. 3.8 micromol/L cytosol, respectively) during a square voltage clamp pulse. In contrast, during a cardiac action potential, Ca entry at the T-tubules is 2.2 times that at the cell surface (3.0 vs. 1.4 micromol/L cytosol, respectively). However, more Ca entry occurs per microm(2) of junctional membrane at the cell surface than in the T-tubules (in nM/microm(2): 1.43 vs. 1.06 during a cardiac action potential). This difference is unlikely to be due to a difference in the number of Ca channels/junction at each site because we estimate that the same number of Ca channels is present at cell surface and T-tubule junctions ( approximately 35). This study provides the first evidence that the T-tubules are a key site for the regulation of action potential duration in ventricular cardiac myocytes. Our data also provide the first direct measurements of T-tubular Ca influx, which are consistent with the idea that cardiac excitation-contraction coupling largely occurs at the T-tubule dyadic clefts.

The effects of overexpression of the Na+/Ca2+ exchanger on calcium regulation in hypertrophied mouse cardiac myocytes

In cardiac hypertrophy and failure it has been shown that the amount of Na/Ca exchanger protein can increase. Several studies have investigated this modification in overt heart failure. However, the role of Na/Ca exchanger overexpression during the development of hypertrophy is unknown. To address this question we investigated Ca2+ regulation in an early stage of cardiac hypertrophy before signs of heart failure occurred and evaluated the role of Na/Ca exchanger overexpression. Cardiac hypertrophy was induced by a constant infusion of angiotensin II (Ang, 1 microg/min/kg) via an osmotic pump for 14 days. Thereafter, ventricular myocytes from either wild type (NON) or transgenic mice overexpressing the Na/Ca exchanger (TR) were isolated. Myocytes were loaded with indo-1 AM or fluo-4 AM to monitor cytoplasmic [Ca2+] with all experiments performed at 37 degrees C. In myocytes exposed to Ang there was an increase in cell capacitance of more than 20% indicating cellular hypertrophy. Ca2+ transients were prolonged in hypertrophied NON myocytes but not in TR myocytes. Action potentials had a less negative plateau in TR myocytes. Sarcoplasmic reticulum (SR) Ca2+ content, measured using rapid caffeine application, was greater in TR myocytes but unaffected by hypertrophy. Ca2+ spark frequency was significantly greater in TR. Na/Ca exchanger overexpression prevented the prolongation of the Ca2+ transient observed in hypertrophy and maintained a similar SR Ca2+ leak suggesting a compensatory role in Ca2+ regulation in hypertrophied cardiac myocytes from transgenic mice. We suggest this compensatory effect is mediated by increased SR Ca2+ content and faster Ca2+ removal via the Na/Ca exchanger.

Ascending aortic stenosis selectively increases action potential-induced Ca2+ influx in epicardial myocytes of the rat left ventricle

A decrease of the transient outward potassium current (Ito) has been observed in cardiac hypertrophy and contributes to the altered shape of the action potential (AP) of hypertrophied ventricular myocytes. Since the shape and duration of the ventricular AP are important determinants of the Ca2+ influx during the AP (QCa), we investigated the effect of ascending aortic stenosis (AS) on QCa in endo- and epicardial myocytes of the left ventricular free wall using the AP voltage-clamp technique. In sham-operated animals, QCa was significantly larger in endocardial compared to epicardial myocytes (803 +/- 65 fC pF(-1), n = 27 vs. 167 +/- 32 fC pF(-1), n = 38, P < 0.001). Ascending aortic stenosis significantly increased QCa in epicardial myocytes (368 +/- 54 fC pF(-1), n = 42, P < 0.05), but did not alter QCa in endocardial myocytes (696 +/- 65 fC pF(-1), n = 26). Peak and current-voltage relation of the AP-induced Ca2+ current were unaffected by AS. However, the time course of the current-voltage relation was significantly prolonged in epicardial myocytes of AS animals. Model calculations revealed that the increase in QCa can be ascribed to a prolonged opening of the activation gate, whereas an increase in inactivation prevents an excessive increase in QCa. In conclusion, AS significantly increased AP-induced Ca2+ influx in epicardial but not in endocardial myocytes of the rat left ventricle.

Behavior of Ca(2+) waves in multicellular preparations from guinea pig ventricle

Ca(+) waves have been implicated in Ca(2+) overload-induced cardiac arrhythmias. To deepen understanding of the behavior of Ca(2+) waves in a multicellular system, consecutive two-dimensional Ca(2+) images were obtained with a confocal microscope from surface cells of guinea pig ventricular papillary muscles loaded with fluo 3 or rhod 2. In intact muscles, no Ca(2+) waves were detected under the resting condition, whereas they were frequently observed during the rest immediately after high-frequency stimulations where cytoplasmic Ca(2+) concentration and Ca(2+) stored in the sarcoplasmic reticulum (SR) were gradually decreasing. The intervals of Ca(2+) waves increased as they occurred later, their amplitudes and velocities remaining unchanged. A SERCA inhibitor reversibly prolonged the wave intervals. In Na(+)-free/Ca(2+)-free medium where neither Ca(2+) influx nor Na(+)/Ca(2+) exchange took place, recurrent Ca(2+) waves emerged at constant intervals in each cell. These results are consistent with the conclusion that the loading level of the SR is critical for induction of Ca(2+) waves. Each cell independently exhibited its own regular rhythm of Ca(2+) wave with a distinct interval. These waves propagated in either direction along the longitudinal axis within a muscle cell, but seldom beyond the cell boundary. In contrast, in partially damaged muscles that showed spontaneous Ca(2+) waves at rest in normal Krebs solution, their propagation often was unidirectional, decreasing in frequency. In these cases, however, Ca(2+) waves rarely moved beyond the cellular boundary. The gradient of the cytoplasmic Ca(2+) concentration was suggested to be the cause of the one-way propagation.

Role of the Na+–H+exchanger (NHE1) in heart muscle function during transient acidosis. A study in papillary muscles from rat and guinea pig hearts

The sodium–hydrogen exchanger (NHE) helps the cell to recover from intracellular acidosis. In this study, we have investigated the effect of HOE 642 (a specific NHE1 blocker) on papillary muscles from rats and guinea pigs during transient acidosis and PKC activation by recording developed force (DF), action potential characteristics, and electrical conductance (stimulus–response interval). Two protocols were used, with or without HOE 642 (10–5mol/L): papillary muscle was exposed (i) for 15 min to a glucose-free, nonoxygenated HEPES buffer containing lactate (20 mmol/L) (pH 6.8) followed by 15 min recovery or (ii) to a PKC activator (phorbolmyristate acetate (PMA) (10–9mol/L)) for 30 min. The DF after acidification remained significantly decreased in the NHE-blocked papillary muscles. During recovery from acidosis, papillary muscles exposed to HOE 642 remained at a higher electrical resistance. The present study shows that post-acidotic continued depression of DF and change in tissue electrophysiological properties might occur as a result of blocking the NHE. During infarct development, the tissue-protecting effect of NHE blockade has been well documented. When acidosis or reduced contractile function is present, however, blocking NHE by HOE 642 might not improve the situation.Key words: sodium–hydrogen exchange (NHE), HOE 642 (cariporide), gap junction, PKC, acidosis.

Species-independent metabolic response to an increase of [Ca(2+)](i) in quiescent cardiac muscle

1. The aim of the present investigation was to contrast the Ca2+ dependence of cardiac energy metabolism in two species with differential reliance on extracellular Ca2+ for excitation-contraction coupling. 2. We measured energy expenditure as the rate of oxygen consumption (Vo2) of isolated, Langendorff-perfused hearts of rats and guinea-pigs during KCl arrest. In parallel experiments, we indexed intracellular Ca2+ concentration ([Ca2+]i) of isolated right-ventricular trabeculae, using the Ca2+ fluorophore fura-2 and ratiometric spectrofluorometry. By varying extracellular Na+ concentration ([Na+]o), Vo2-[Na+]o and [Ca2+]i-[Na+]o relationships were constructed for each species. 3. Reduction of [Na+]o during K+ arrest caused pronounced species-dependent elevations of both Vo2 and [Ca2+]i. Despite the species dependence of both Vo2 and [Ca2+]i on [Na+]o, a single species-independent Vo2-[Ca2+]i relationship obtained. 4. We infer that elevation of the metabolic rate of the arrested heart above its basal value is determined primarily by [Ca2+]i and is not species dependent.

Electrophysiologic characterization of dronedarone in guinea pig ventricular cells

The electrophysiological properties of dronedarone (SR33589), a noniodinated amiodarone-like agent, were studied on action potential (AP) and contraction of papillary muscle and on membrane ionic currents, Ca2+ transient, and shortening of ventricular cells of the guinea pig heart. In multicellular preparations, dronedarone (3, 10, and 30 microM) decreased maximum rate of rise of AP (dV/dt max) with a concentration- and frequency-dependent relationship; resting potential was not modified and AP amplitude was decreased only at 30 microM. The effects of dronedarone on AP durations (APDs) at different percentages of repolarization were not significantly changed, except for a slight decrease in APD30 and APD50 at the highest concentration. In isolated ventricular myocytes, dronedarone inhibited rapidly activating delayed-rectifier K+ current (I(Kr)) (median inhibitory concentration [IC50] </= 3 microM voltage-independent); slowly activating delayed-rectifier K+ current (I(Ks)) (IC50 approximately/= 10 microM voltage-dependent and time-, frequency-, or use-independent); and inward rectifier potassium current (I(K1)) (IC50 >/= 30 microM). Dronedarone blocked L-type Ca2+ current (I(Ca(L))) (IC50 = 0.18 +/- 0.018 microM at a stimulation frequency of 0.033 Hz) in a use- and frequency-dependent manner. Simultaneously to these electrophysiological effects, dronedarone reduced contraction amplitudes of papillary muscle and decreased Ca2+ transient and shortening of ventricular myocytes. The results show that dronedarone is a multichannel blocker because it decreases dV/dt max (I(Na)), I(Ca(L)), I(Kr), I(Ks), and I(K1). These effects are accompanied by a reduction in free intracellular calcium and contraction amplitudes. Dronedarone does not significantly change APD whatever the stimulation frequency. Our data demonstrate that the acute electrophysiological characteristics of dronedarone, despite absence of iodine in its molecular structure, are very similar to those of amiodarone in cardiac ventricle.

Regulation of cardiac excitation-contraction coupling by action potential repolarization: role of the transient outward potassium current (I(to))

The cardiac action potential (AP) is critical for initiating and coordinating myocyte contraction. In particular, the early repolarization period of the AP (phase 1) strongly influences the time course and magnitude of the whole-cell intracellular Ca(2+) transient by modulating trans-sarcolemmal Ca(2+) influx through L-type Ca(2+) channels (I(Ca,L)) and Na-Ca exchangers (I(Ca,NCX)). The transient outward potassium current (I(to)) has kinetic properties that make it especially effective in modulating the trajectory of phase 1 repolarization and thereby cardiac excitation-contraction coupling (ECC). The magnitude of I(to) varies greatly during cardiac development, between different regions of the heart, and is invariably reduced as a result of heart disease, leading to corresponding variations in ECC. In this article, we review evidence supporting a modulatory role of I(to) in ECC through its influence on I(Ca,L), and possibly I(Ca,NCX). We also discuss differential effects of I(to) on ECC between different species, between different regions of the heart and in heart disease.

Overexpression of SERCA2a accelerates repolarisation in rabbit ventricular myocytes

Overexpression of the sarcoplasmic reticulum Ca ATPase (SERCA2a) produces positive inotropism and it has been proposed as a promising strategy to counteract defective excitation-contraction coupling in the failing heart. However, the effects of overexpressing SERCA2a on action potential duration (APD), which can affect diastolic parameters in the heart, is unknown. We, therefore, investigated the relationship between SERCA2a overexpression and APD in adult rabbit ventricular myocytes which were cultured for 48 h. Overexpression of SERCA2a was achieved by infection with an adenovirus carrying both SERCA2a and GFP independently driven by CMV promoters, Ad.SERCA2a. Myocytes infected with Ad.GFP only and/or non-infected myocytes were used as controls. Electrophysiological measurements were taken using switch clamping with 15-25 M Omega resistance microelectrodes. In Ad.SERCA2a infected myocytes, APD was significantly reduced compared with both groups of control cells at 0.5 Hz (APD50 (ms) non-infected: 481+/-98, n=12; Ad.GFP: 464+/-85, n=11; Ad.SERCA2a: 285+/-69, n=13 (mean+/-S.E.M.) and at 1 Hz (APD50 (ms) non-infected: 375+/-64, n=22; Ad.GFP: 363+/-47, n=18; Ad.SERCA2a: 231+/-54, n=24). Using AP voltage-clamping, we recorded a 0.2 mM Cd-sensitive current which can be ascribed to Ca current flowing during the AP. The integral of this current was reduced in Ad.SERCA2a myocytes compared with control (non-infected charge (pC): 27.5+/-4.2, n=8; Ad.SERCA2a: 15.5+/-4.1, n=11; P<0.01). Using AP clamping during the loading protocol, to take into account changes in APD, SR Ca content (assessed by integrating a 20 mM caffeine-induced inward current) was significantly larger in Ad.SERCA2a compared with both controls (SR Ca content (microM/l non-mitochondrial volume): non-infected: 25.5+/-7, n=8; Ad.GFP: 25.7+/-11, n=6; Ad.SERCA2a: 80.5+/-19, n=8). In conclusion, this study shows that SR Ca content is increased despite decreased Ca entry after overexpression of SERCA2a, and this can lead to positive inotropism. This effect coupled with shorter APD may be a useful therapeutic modality in heart failure.

Effects of Na(+)/Ca(2+)-exchanger overexpression on excitation-contraction coupling in adult rabbit ventricular myocytes

The Na(+)/Ca(2+)-exchanger (NCX) is the main mechanism by which Ca(2+) is transported out of the ventricular myocyte. NCX levels are raised in failing human heart, and the consequences of this for excitation-contraction coupling are still debated. We have increased NCX levels in adult rabbit myocytes by adenovirally-mediated gene transfer and examined the effects on excitation-contraction coupling after 24 and 48 h. Infected myocytes were identified through expression of green fluorescent protein (GFP), transfected under a separate promoter on the same viral construct. Control experiments were done with both non-infected myocytes and those infected with adenovirus expressing GFP only. Contraction amplitude was markedly reduced in NCX-overexpressing myocytes at either time point, and neither increasing frequency nor raising extracellular Ca(2+) could reverse this depression. Resting membrane potential and action potential duration were largely unaffected by NCX overexpression, as was peak Ca(2+) entry via the L-type Ca(2+) channel. Systolic and diastolic Ca(2+) levels were significantly reduced, with peak systolic Ca(2+) in NCX-overexpressing myocytes lower than diastolic levels in control cells at 2 m m extracellular Ca(2+). Both cell relengthening and the decay of the Ca(2+) transient were significantly slowed. Sarcoplasmic reticulum (SR) Ca(2+) stores were completely depleted in a majority of myocytes, and remained so despite increasingly vigorous loading protocols. Depressed contractility following NCX overexpression is therefore related to decreased SR Ca(2+) stores and low diastolic Ca(2+) levels rather than reduced Ca(2+) entry.

Alterations in action potential profile enhance excitation-contraction coupling in rat cardiac myocytes

Action potential (AP) prolongation typically occurs in heart disease due to reductions in transient outward potassium currents (Ito), and is associated with increased Ca2+ transients. We investigated the underlying mechanisms responsible for enhanced Ca2+ transients in normal isolated rat ventricular myocytes in response to the AP changes that occur following myocardial infarction. Normal myocytes stimulated with a train of long post-myocardial infarction (MI) APs showed a 2.2-fold elevation of the peak Ca2+ transient and a 2.7-fold augmentation of fractional cell shortening, relative to myocytes stimulated with a short control AP. The steady-state Ca2+ load of the sarcoplasmic reticulum (SR) was increased 2.0-fold when myocytes were stimulated with trains of long post-MI APs (111 +/- 21.6 micromol l(-1)) compared with short control APs (56 +/- 7.2 micromol l(-1)). Under conditions of equal SR Ca2+ load, long post-MI APs still resulted in a 1.7-fold increase in peak [Ca2+]i and a 3.8-fold increase in fractional cell shortening relative to short control APs, establishing that changes in the triggering of SR Ca2+ release are largely responsible for elevated Ca2+ transients following AP prolongation. Fractional SR Ca2+ release calculated from the measured SR Ca2+ load and the integrated SR Ca2+ fluxes was 24 +/- 3 and 11 +/- 2 % following post-MI and control APs, respectively. The fractional release (FR) of Ca2+ from the SR divided by the integrated L-type Ca2+ flux (FR/[integral]FCa,L) was increased 1.2-fold by post-MI APs compared with control APs. Similar increases in excitation-contraction (E-C) coupling gains were observed establishing enhanced E-C coupling efficiency. Our findings demonstrate that AP prolongation alone can markedly enhance E-C coupling in normal myocytes through increases in the L-type Ca2+ current (ICa,L) trigger combined with modest enhancements in Ca2+ release efficiency. We propose that such changes in AP profile in diseased myocardium may contribute significantly to alterations in E-C coupling independent of other biochemical or genetic changes.

SERCA2A overexpression decreases the incidence of aftercontractions in adult rabbit ventricular myocytes

K. Davia, E. Bernobich, H. K. Ranu, F. del Monte, C. M. N. Terracciano, K. T. MacLeod, D. L. Adamson, B. Chaudhri, R. J. Hajjar and S. E. Harding. SERCA2a Overexpression Decreases the Incidence of Aftercontractions in Adult Rabbit Ventricular Myocytes. Journal of Molecular and Cellular Cardiology (2001) 33, 1005-1015. Slow relaxation and poor contractile response to increasing stimulation frequency in failing human heart have been strongly linked to a decrease in the activity of the sarcoplasmic reticulum (SR) Ca(2+)-ATPase (SERCA2a). Restoration of SERCA2a levels using gene transfer has beneficial effects on contractile function but, like beta -adrenoceptor stimulation, could potentially produce excess SR Ca(2+), arrhythmias and cell death. We have examined the effects of SERCA2a overexpression in adult rabbit cardiac myocytes, and compared changes in relaxation with those following beta -adrenoceptor stimulation. Myocytes were infected with an adenovirus carrying both SERCA2a and green fluorescent protein (GFP) for positive identification of infected cells. Myocyte survival was significantly enhanced in the infected cultures. There was a reduction in both time-to-peak contraction and time-to-50% relaxation (R50) 48 h after infection. Time-to-90% relaxation (R90) was particularly improved (non-infected 516+/-41 ms, AD.SERCA2a-GFP 230+/-23 ms, n=7 preparations, P<0.001). There was also a decreased incidence of aftercontractions in Ad.SERCA2a-GFP infected myocytes (21+/-5%v 41+/-4% in controls, P<0.01). This contrasts with beta -adrenoceptor stimulation, which reduced R50 but prolonged R90 by 158+/-76 ms (P<0.02, n=16). At higher stimulation frequencies (2-3 Hz) contraction amplitude and SR calcium content were increased and diastolic contracture was reduced following SERCA2a overexpression. Overall, increasing levels of SERCA2a resulted in an improvement in systolic and diastolic function and a reduction in cell death and arrhythmic aftercontractions. SERCA2a overexpression therefore lacks the detrimental effects associated with some other inotropic interventions.

Evidence for the action potential mediating the changes to contraction observed in cardiac hypertrophy in the rabbit

BACKGROUND

We investigated the effects of cardiac hypertrophy on intracellular calcium (Ca(2+)) homeostasis, the amounts of proteins involved in calcium regulation and the influence of the action potential on such changes.

METHODS

Cardiac hypertrophy was induced in rabbits by constriction of the ascending aorta. They were kept for 6 weeks then the heart was removed and left ventricular myocytes isolated. A portion of these myocytes was immediately frozen and stored for subsequent protein analyses using Western blotting.

RESULTS

After aortic banding, cardiac myocyte two-dimensional area and membrane capacitance were increased by 53% and 23% respectively. Hypertrophy prolonged cell contraction and relaxation and the corresponding Indo-1 Ca(2+) transients. Hypertrophied cells displayed longer action potentials but Ca(2+) current densities were unchanged compared with myocytes from sham hearts. If Ca(2+) was released from the sarcoplasmic reticulum using rapid cooling, so bypassing the normal mechanisms involved in excitation-contraction coupling, then no functional differences between hypertrophied and control cells could be observed. Western blot analysis showed that the amounts of sarcoplasmic reticulum Ca(2+) ATPase, its regulatory protein phospholamban and the sodium/calcium exchanger were unchanged whereas the amount of calsequestrin was increased by 65% and the alpha(1) subunit of the sodium/potassium ATPase was reduced by 72%. These changes do not appear to evoke functional consequences under these conditions.

CONCLUSION

In this model of cardiac hypertrophy, the increase in action potential duration is responsible for changes in contraction and relaxation.

Na(+)/Ca(2+)-exchange activity regulates contraction and SR Ca(2+) content in rainbow trout atrial myocytes

We have used the whole cell configuration of the patch-clamp technique to measure sarcolemmal Ca(2+) transport by the Na(+)/Ca(2+) exchanger (NCX) and its contribution to the activation and relaxation of contraction in trout atrial myocytes. In contrast to mammals, cell shortening continued, increasing at membrane potentials above 0 mV in trout atrial myocytes. Furthermore, 5 microM nifedipine abolished L-type Ca(2+) current (I(Ca)) but only reduced cell shortening and the Ca(2+) carried by the tail current to 66 +/- 5 and 67 +/- 6% of the control value. Lowering of the pipette Na(+) concentration from 16 to 10 or 0 mM reduced Ca(2+) extrusion from the cell from 2.5 +/- 0.2 to 1.0 +/- 0.2 and 0.5 +/- 0.06 amol/pF. With 20 microM exchanger inhibitory peptide (XIP) in the patch pipette Ca(2+) extrusion 20 min after patch break was 39 +/- 8% of its initial value. With 16, 10, and 0 mM Na(+) in the pipette, the sarcoplasmic reticulum (SR) Ca(2+) content was 47 +/- 4, 29 +/- 6, and 10 +/- 3 amol/pF, respectively. Removal of Na(+) from or inclusion of 20 microM XIP in the pipette gradually eliminated the SR Ca(2+) content. Whereas I(Ca) was the same at -10 or +10 mV, Ca(2+) extrusion from the cell and the SR Ca(2+) content at -10 mV were 65 +/- 7 and 80 +/- 4% of that at +10 mV. The relative amount of Ca(2+) extruded by the NCX (about 55%) and taken up by the SR (about 45%) was, however, similar with depolarizations to -10 and +10 mV. We conclude that modulation of the NCX activity critically determines Ca(2+) entry and cell shortening in trout atrial myocytes. This is due to both an alteration of the transsarcolemmal Ca(2+) transport and a modulation of the SR Ca(2+) content.

Na(+)-K(+)-ATPase alpha(2)-isoform expression in guinea pig hearts during transition from compensation to decompensation

Disturbance in ionic gradient across sarcolemma may lead to arrhythmias. Because Na(+)-K(+)-ATPase regulates intracellular Na(+) and K(+) concentrations, and therefore intracellular Ca(2+) concentration homeostasis, our aim was to determine whether changes in the Na(+)-K(+)-ATPase alpha-isoforms in guinea pigs during transition from compensated (CLVH) to decompensated left ventricular hypertrophy (DLVH) were concomitant with arrhythmias. After 12- and 20-mo aortic stenosis, CLVH and DLVH were characterized by increased mean arterial pressure (30% and 52.7%, respectively). DLVH differed from CLVH by significantly increased end-diastolic pressure (34%), decreased sarco(endo)plasmic reticulum Ca(2+)-ATPase (-75%), and increased Na(+)/Ca(2+) exchanger (25%) mRNA levels and by the occurrence of ventricular arrhythmias. The alpha-isoform (mRNA and protein levels) was significantly lower in DLVH (2.2 +/- 0.2- and 1. 4 +/- 0.15-fold, respectively, vs. control) than in CLVH (3.5 +/- 0. 4- and 2.2 +/- 0.13-fold, respectively) and was present in sarcolemma and T tubules. Changes in the levels of alpha(1)- and alpha(3)-isoform in CLVH and DLVH appear physiologically irrelevant. We suggest that the increased level of alpha(2)-isoform in CLVH may participate in compensation, whereas its relative decrease in DLVH may enhance decompensation and arrhythmias.

Sub-second quenched-flow/X-ray microanalysis shows rapid Ca2+ mobilization from cortical stores paralleled by Ca2+ influx during synchronous exocytosis in Paramecium cells

Though only actual local free Ca2+ concentrations, [Ca2+], rather than total Ca concentrations, [Ca], govern cellular responses, analysis of total calcium fluxes would be important to fully understand the very complex Ca2+ dynamics during cell stimulation. Using Paramecium cells we analyzed Ca2+ mobilization from cortical stores during synchronous (< or = 80 ms) exocytosis stimulation, by quenched-flow/cryofixation, freeze-substitution (modified for Ca retention) and X-ray microanalysis which registers total calcium concentrations, [Ca]. When the extracellular free calcium concentration, [Ca2+]e, is adjusted to approximately 30 nM, i.e. slightly below the normal free intracellular calcium concentration, [Ca2+]i = 65 nM, exocytosis stimulation causes release of 52% of calcium from stores within 80 ms. At higher extracellular calcium concentration, [Ca2+]e = 500 microM, Ca2+ release is counterbalanced by influx into stores within the first 80 ms, followed by decline of total calcium, [Ca], in stores to 21% of basal values within 1 s. This includes the time required for endocytosis coupling (350 ms), another Ca2+-dependent process. To confirm that Ca2+ mobilization from stores is superimposed by rapid Ca2+ influx and/or uptake into stores, we substituted Sr2+ for Ca2+ in the medium for 500 ms, followed by 80 ms stimulation. This reveals reduced Ca signals, but strong Sr signals in stores. During stimulation, Ca2+ is spilled over preformed exocytosis sites, particularly with increasing extracellular free calcium, [Ca2+]e. Cortically enriched mitochondria rapidly gain Ca signals during stimulation. Balance calculations indicate that total Ca2+ flux largely exceeds values of intracellular free calcium concentrations locally required for exocytosis (as determined previously). Our approach and some of our findings appear relevant also for some other secretory systems.

Inhibition of Ca(2+) sparks by ruthenium red in permeabilized rat ventricular myocytes

We have compared the effects of the sarcoplasmic reticulum (SR) Ca(2+) release inhibitor, ruthenium red (RR), on single ryanodine receptor (RyR) channels in lipid bilayers, and on Ca(2+) sparks in permeabilized rat ventricular myocytes. Ruthenium red at 5 microM inhibited the open probability (P(o)) of RyRs approximately 20-50-fold, without significantly affecting the conductance or mean open time of the channel. At the same concentration, RR inhibited the frequency of Ca(2+) sparks in permeabilized myocytes by approximately 10-fold, and reduced the amplitude of large amplitude events (with most probable localization on the line scan) by approximately 3-fold. According to our theoretical simulations, performed with a numerical model of Ca(2+) spark formation, this reduction in Ca(2+) spark amplitude corresponds to an approximately 4-fold decrease in Ca(2+) release flux underlying Ca(2+) sparks. Ruthenium red (5 microM) increased the SR Ca(2+) content by approximately 2-fold (from 151 to 312 micromol/l cytosol). Considering the degree of inhibition of local Ca(2+) release events, the increase in SR Ca(2+) load by RR, and the lack of effects of RR on single RyR open time and conductance, we have estimated that Ca(2+) sparks under normal conditions are generated by openings of at least 10 single RyRs.

Beat-to-beat repolarization variability in ventricular myocytes and its suppression by electrical coupling

Single ventricular myocytes paced at a constant rate and held at a constant temperature exhibit beat-to-beat variations in action potential duration (APD). In this study we sought to quantify this variability, assess its mechanism, and determine its responsiveness to electrotonic interactions with another myocyte. Interbeat APD(90) (90% repolarization) of single cells was normally distributed. We thus quantified APD(90) variability as the coefficient of variability, CV = (SD/mean APD(90)) x 100. The mean +/- SD of the CV in normal solution was 2.3 +/- 0.9 (132 cells). Extracellular TTX (13 microM) and intracellular EGTA (14 mM) both significantly reduced the CV by 44 and 26%, respectively. When applied in combination the CV fell by 54%. In contrast, inhibition of the rapid delayed rectifier current with L-691,121 (100 nM) increased the CV by 300%. The CV was also significantly reduced by 35% when two normal myocytes were electrically connected with a junctional resistance (R(j)) of 100 MOmega. Electrical coupling (R(j) = 100 MOmega) of a normal myocyte to one producing early afterdepolarization (EAD) completely blocked EAD formation. These results indicate that beat-to-beat APD variability is likely mediated by stochastic behavior of ion channels and that electrotonic interactions act to limit temporal dispersion of refractoriness, a major contributor to arrhythmogenesis.

Na+-Ca2+ exchange current from rabbit isolated atrioventricular nodal and ventricular myocytes compared using action potential and ramp waveforms

We measured and compared Na-Ca exchanger current (INa-Ca) from rabbit isolated ventricular and atrioventricular (AV) nodal myocytes, using action potential (AP) and ramp voltage commands. Whole cell patch-clamp recordings were made at 35-37 degrees C; INa-Ca was measured as 5 mM nickel (Ni)- sensitive current with major interfering voltage and calcium-activated currents blocked. In ventricular cells a 2-s descending ramp elicited INa-Ca showing outward rectification and a reversal potential (Erev) of -13.1 +/- 1. 2 mV (n = 12; mean +/- SEM). With a ventricular AP as the voltage command, the profile of INa-Ca followed the applied waveform closely. The current-voltage relation during AP repolarization was almost linear and showed an Erev of -38.3 +/- 5.3 mV (n = 6). As INa-Ca depended on the applied voltage waveform, comparisons between the two cell types utilized the same command waveform (a series of AV nodal APs). In ventricular myocytes this elicited INa-Ca that reversed near -38 mV and was inwardly directed during the pacemaker potential. This command was also applied to AV node cells; mean INa-Ca density at all voltages encompassed by the AP (-70 to +30 mV) did not differ significantly from that in ventricular myocytes (P > 0.05, ANOVA). This finding was confirmed using brief (250 ms) voltage ramp protocols (P > 0.1 ANOVA). These data represent the first direct measurements of AV nodal INa-Ca and suggest that the exchanger may be functionally expressed to similar levels in the two cell types. They may also suggest a possible role for INa-Ca during the pacemaker potential in AV node as inward INa-Ca was observed over the pacemaker potential range even with bulk internal Ca buffered to a low level.

Stretch-activated whole cell currents in adult rat cardiac myocytes

Mechanoelectric transduction can initiate cardiac arrhythmias. To examine the origins of this effect at the cellular level, we made whole cell voltage-clamp recordings from acutely isolated rat ventricular myocytes under controlled strain. Longitudinal stretch elicited noninactivating inward cationic currents that increased the action potential duration. These stretch-activated currents could be blocked by 100 microM Gd(3+) but not by octanol. The current-voltage relationship was nearly linear, with a reversal potential of approximately -6 mV in normal Tyrode solution. Current density varied with sarcomere length (SL) according to I (pA/pF) = 8.3 - 5.0 SL (microm). Repeated attempts to record single channel currents from stretch-activated ion channels failed, in accord with the absence of such data from the literature. The inability to record single channel currents may be a result of channels being located on internal membranes such as the T tubules or, possibly, inactivation of the channels by the mechanics of patch formation.

Ca2+-activated non-selective cation current in rabbit ventricular myocytes

Oscillatory currents (OCs) were studied in isolated rabbit ventricular myocytes with whole cell mode voltage clamp using Na+-free intracellular and extracellular solutions under conditions where K+ currents were anticipated to be eliminated or minimized. All OCs were dependent on release of Ca2+ from the sarcoplasmic reticulum (SR) because they were associated with intracellular Ca2+ ([Ca2+]i) transients, and were suppressed by high concentrations of BAPTA (20 mmol l-1) or pretreatment with the SR antagonist agents ryanodine (10 micromol l-1) or thapsigargin (1 micromol l-1). The reversal potential (Vrev) for OCs shifted with changes in the calculated Vrev for Cl- (ECl) but was between ECl and the calculated Vrev for elemental monovalent cations (ECat), indicating that more than one Ca2+-activated current contributed to OCs. Addition of the Ca2+-activated Cl- current (ICl(Ca)) antagonist, niflumic acid, shifted the OC Vrev to ECat, suggesting that ICl(Ca) and a Ca2+-activated non-selective cation current (ICAN) contributed to the observed OCs. A reduced niflumic acid-insensitive Ca2+-activated OC persisted following marked symmetrical reduction of Cl- in the intracellular and extracellular solutions. Subsequent removal of all extracellular monovalent cations, by N-methyl-D-glucamine (NMDG) substitution, eliminated OCs and the inward holding current suggesting that ICAN and ICl(Ca) accounted for all or most of the Ca2+-activated OC in the absence of Na+. The OC Vrev was equal to ECl in the absence of monovalent elemental cations. Under these conditions niflumic acid eliminated all OCs. Macroscopic OC is partially due to ICAN in rabbit ventricular myocytes.

Ca(2+) influx through Ca(2+) channels in rabbit ventricular myocytes during action potential clamp: influence of temperature

Ca(2+) influx via Ca(2+) current (I(Ca)) during the action potential (AP) was determined at 25 degrees C and 35 degrees C in isolated rabbit ventricular myocytes using AP clamp. Contaminating currents through Na(+) and K(+) channels were eliminated by using Na(+)- and K(+)-free solutions, respectively. DIDS (0.2 mmol/L) was used to block Ca(2+)-activated chloride current (I(Cl(Ca))). When the sarcoplasmic reticulum (SR) was depleted of Ca(2+) by preexposure to 10 mmol/L caffeine, total Ca(2+) entry via I(Ca) during the AP was approximately 12 micromol/L cytosol (at both 25 degrees C and 35 degrees C). Similar Ca(2+) influx at 35 degrees C and 25 degrees C resulted from a combination of higher and faster peak I(Ca), offset by more rapid I(Ca) inactivation at 35 degrees C. During repeated AP clamps, the SR gradually fills with Ca(2+), and consequent SR Ca(2+) release accelerates I(Ca) inactivation during the AP. During APs and contractions in steady state, total Ca(2+) influx via I(Ca) was reduced by approximately 50% but was again unaltered by temperature (5.6+/-0.2 micromol/L cytosol at 25 degrees C, 6.0+/-0.2 micromol/L cytosol at 35 degrees C). Thus, SR Ca(2+) release is responsible for sufficient I(Ca) inactivation to cut total Ca(2+) influx in half. However, because of the kinetic differences in I(Ca), the amount of Ca(2+) influx during the first 10 ms, which presumably triggers SR Ca(2+) release, is much greater at 35 degrees C. I(Ca) during a first pulse, given just after the SR was emptied with caffeine, was subtracted from I(Ca) during each of 9 subsequent pulses, which loaded the SR. These difference currents reflect I(Ca) inactivation due to SR Ca(2+) release and thus indicate the time course of local [Ca(2+)] in the subsarcolemmal space near Ca(2+) channels produced by SR Ca(2+) release (eg, maximal at 20 ms after the AP activation at 35 degrees C). Furthermore, the rate of change of this difference current may reflect the rate of SR Ca(2+) release as sensed by L-type Ca(2+) channels. These results suggest that peak SR Ca(2+) release occurs within 2.5 or 5 ms of AP upstroke at 35 degrees C and 25 degrees C, respectively. I(Cl(Ca)) might also indicate local [Ca(2+)], and at 35 degrees C in the absence of DIDS (when I(Cl(Ca)) is prominent), peak I(Cl(Ca)) also occurred at a time comparable to the peak I(Ca) difference current. We conclude that SR Ca(2+) release decreases the Ca(2+) influx during the AP by approximately 50% (at both 25 degrees C and 35 degrees C) and that changes in I(Ca) (and I(Cl(Ca))), which depend on SR Ca(2+) release, provide information about local subsarcolemmal [Ca(2+)].

Relationship between transient outward K+ current and Ca2+ influx in rat cardiac myocytes of endo- and epicardial origin

1. The transient outward K+ current (Ito) is a major repolarizing ionic current in ventricular myocytes of several mammals. Recently it has been found that its magnitude depends on the origin of the myocyte and is regulated by a number of physiological and pathophysiological signals. 2. The relationship between the magnitude of Ito, action potential duration (APD) and Ca2+ influx (QCa) was studied in rat left ventricular myocytes of endo- and epicardial origin using whole-cell recordings and the action potential voltage-clamp method. 3. Under control conditions, in response to a depolarizing voltage step to +40 mV, Ito averaged 12.1 +/- 2.6 pA pF-1 in endocardial (n = 11) and 24.0 +/- 2.6 pA pF-1 in epicardial myocytes (n = 12; P < 0.01). APD90 (90 % repolarization) was twice as long in endocardial myocytes, whereas QCa inversely depended on the magnitude of Ito. L-type Ca2+ current density was similar in myocytes from both regions. 4. To determine the effects of controlled reductions of Ito on QCa, recordings were repeated in the presence of increasing concentrations of the Ito inhibitor 4-aminopyridine. 5. Inhibition of Ito by as little as 20 % more than doubled QCa in epicardial myocytes, whereas it had only a minor effect on QCa in myocytes of endocardial origin. Further inhibition of Ito led to a progressive increase in QCa in epicardial myocytes; at 90 % inhibition of Ito, QCa was four times larger than the control value. 6. We conclude that moderate changes in the magnitude of Ito strongly affect QCa primarily in epicardial regions. An alteration of Ito might therefore allow for a regional regulation of contractility during physiological and pathophysiological adaptations.

Ca(2+) uptake in the sarcoplasmic reticulum from the systemic heart of octopod cephalopods

We have measured Ca(2+) uptake in crude homogenates of heart tissue, as well as cell shortening and ionic currents in isolated myocytes exposed to caffeine, to characterize Ca(2+) uptake in the sarcoplasmic reticulum (SR) of the systemic heart of octopus. The maximal rate of SR Ca(2+) uptake in crude homogenates of octopus heart was 43+/−4 (mean +/− s.e.m., N=7), compared with 28+/−2 nmol min(−)(1)mg(−)(1) protein (N=4) in homogenates of rat heart. The Ca(2+)-dependency of SR Ca(2+) uptake was similar for the two species, with a Ca(2+) activity at half-maximal uptake rate (pCa(50)) of 6.04+/−0.02 for octopus and 6.02+/−0.05 for rat. Exposure of isolated myocytes to 10 mmol l(−)(1) caffeine resulted in cell shortening to 53+/−2 % of the resting cell length and an inward trans-sarcolemmal ionic current. The charge carried by this current was 3.28+/−0.70 pC pF(−)(1) (mean +/− s.e.m., N=5) corresponding to extrusion of 34.0+/−0.7 amol Ca(2+)pF(−)(1) from the cell by Na(+)/Ca(2+) exchange. This is approximately 50 times more than the Ca(2+) carried by the Ca(2+) current elicited by a 200 ms depolarization from −80 to 0 mV and corresponds to an increase in the total intracellular [Ca(2+)] of 404+/−86 (μ)mol l(−)(1) non-mitochondrial volume due to Ca(2+) release from the SR. Thus, we find that at 20 degrees C in the SR both Ca(2+) content and Ca(2+) uptake rate in the systemic heart of octopus are comparable with or larger than the corresponding values obtained in the rat heart. These results support the argument that the SR may play an important role in the regulation of contraction in the systemic heart of cephalopods.

Tetracaine can inhibit contractions initiated by a voltage-sensitive release mechanism in guinea-pig ventricular myocytes

1. Effects of tetracaine on membrane currents and cell shortening were measured with high resistance electrodes, single-electrode voltage clamp (switch clamp) and a video edge detector at 37 C in cardiac ventricular myocytes. 2. Sequential voltage steps from -65 mV to -40 and 0 mV were used to activate two mechanisms of excitation-contraction (EC) coupling separately. The step to -40 mV activated the voltage-sensitive release mechanism (VSRM); the step to 0 mV1 activated Ca2+-induced Ca2+ release (CICR) coupled to inward Ca2+ current (IL). 3. Exposure to 100-300 microM tetracaine inhibited VSRM contractions but not CICR contractions. Inhibition of VSRM contractions was independent of INa blockade. In contrast, 100 microM Cd2+ blocked IL and CICR contractions, but not VSRM contractions. Simultaneous application of both agents blocked both mechanisms of EC coupling. 4. Contraction-voltage relationships were sigmoidal when the VSRM was available. However, when the VSRM was inhibited with 100-300 microM tetracaine, contraction-voltage relationships became bell-shaped. The tetracaine-insensitive contractions were abolished by 0.1 microM ryanodine, indicating that they were dependent on release of SR Ca2+. 5. At a higher concentration (1 mM) tetracaine also inhibited IL and contractions triggered by IL; however, the time course of effects on IL and associated contractions were different than for VSRM contractions. 6. With continuous application of tetracaine, the VSRM remained inhibited although SR Ca2+ stores increased 4-fold as assessed with caffeine. CICR contractions were not inhibited and maximum amplitude of contraction was not reduced. 7. Rapid application of tetracaine just before and during test steps also inhibited VSRM contractions, but without significantly affecting sarcoplasmic reticulum (SR) Ca2+ stores or CICR contractions. Maximum amplitude of contraction was reduced. 8. Rapid application of tetracaine (100-300 microM) allows preferential inhibition of the VSRM and provides a pharmacological method to assess the contribution of the VSRM to EC coupling.

Tight coupling between the rate of rise of Ca2+ transient and peak twitch contraction in guinea-pig papillary muscle

We evaluated the relationship between cytoplasmic Ca2+ concentration ([Ca2+]i) and force in guinea-pig papillary muscles loaded with a fluorescent Ca2+ indicator, fura-PE3. In the absence of ryanodine, [Ca2+]i transient and force were altered by changing extracellular Ca2+ concentration and stimulation frequency, and also by adding methyl-1,4-dihydro-2,6-dimethyl-3-nitro-4-(2-trifluoromethylphenyl)-pyri dine-5-carboxylate (Bay K 8644) or ouabain. Under these conditions, the peak force correlated linearly with the maximal rate of rise of [Ca2+]i (gamma = 0.948) more than the peak [Ca2+]i transient (gamma = 0.737). Ryanodine inhibited the increase in the maximal rate of rise of [Ca2+]i resulting in abolishment of the correlation between force and the maximal rate of rise of [Ca2+]i. These results suggest that the maximal rate of rise of [Ca2+]i reflects Ca2+ release from the sarcoplasmic reticulum, and this fraction of [Ca2+]i is crucial for determining the amplitude of twitch contractions when the sarcoplasmic reticulum is intact in guinea-pig papillary muscle.

Na+/Ca2+ exchange current in ventricular myocytes of fish heart: contribution to sarcolemmal Ca2+ influx

Influx of extracellular Ca2+ plays a major role in the activation of contraction in fish cardiac cells. The relative contributions of Na+/Ca2+ exchange and L-type Ca2+ channels to Ca2+ influx are, however, unknown. Using a physiological action potential as the command pulse in voltage-clamped heart cells, we examined sarcolemmal Ca2+ influx through Na+/Ca2+ exchange and L-type Ca2+ channels in crucian carp (Carassius carassius L.) ventricular myocytes. When other cation conductances were blocked, a Ni2+-sensitive current with the characteristic voltage- and time-dependent properties of the Na+/Ca2+ exchange current could be distinguished. At the maximum overshoot voltage of the ventricular action potential (+40 mV; [Na+]i=10 mmol l-1), the density of the Na+/Ca2+ exchange current was 2.99+/−0.27 pA pF-1 for warm-acclimated fish (23 degrees C) and 2.38+/−0.42 pA pF-1 for cold-acclimated fish (4 degrees C) (means +/− s.e.m., N=5-6; not significantly different, P=0.26). The relative contributions of the Na+/Ca2+ exchanger and L-type Ca2+ channels to Ca2+ influx were estimated using two partly different methods. Integration of the Ni2+-sensitive Na+/Ca2+ exchange current and the verapamil- and Cd2+-sensitive L-type Ca2+ current suggests that, during the action potential, approximately one-third of the activating Ca2+ comes through Na+/Ca2+ exchange and approximately two-thirds through L-type Ca2+ channels. An alternative method of analysis, using the inward tail current as a measure of the total sarcolemmal Ca2+ flux from which the Ni2+-sensitive Na+/Ca2+ exchange current was subtracted to obtain the Ca2+ influx through the channels, suggests that L-type Ca2+ channels and Na+/Ca2+ exchange are almost equally important in the activation of contraction. Furthermore, the time course of cell shortening is not adequately explained by sarcolemmal Ca2+ influx through the channels alone, but is well approximated by the sum of Ca2+ influx through the channels and the exchanger. The present results indicate that reverse Na+/Ca2+ exchange in crucian carp ventricular myocytes has sufficient capacity to trigger contraction and suggest that the exchange current makes a significant contribution to contractile Ca2+ during the physiological action potential. The relative significance of channels and exchanger molecules in sarcolemmal Ca2+ entry into crucian carp ventricular myocytes was unaffected by thermal acclimation when determined at 22 degrees C.

CaM kinase augments cardiac L-type Ca2+ current: a cellular mechanism for long Q-T arrhythmias

Early afterdepolarizations (EAD) caused by L-type Ca2+ current (ICa, L) are thought to initiate long Q-T arrhythmias, but the role of intracellular Ca2+ in these arrhythmias is controversial. Rabbit ventricular myocytes were stimulated with a prolonged EAD-containing action potential-clamp waveform to investigate the role of Ca2+/calmodulin-dependent protein kinase II (CaM kinase) in ICa,L during repolarization. ICa,L was initially augmented, and augmentation was dependent on Ca2+ from the sarcoplasmic reticulum because the augmentation was prevented by ryanodine or thapsigargin. ICa,L augmentation was also dependent on CaM kinase, because it was prevented by dialysis with the inhibitor peptide AC3-I and reconstituted by exogenous constitutively active CaM kinase when Ba2+ was substituted for bath Ca2+. Ultrastructural studies confirmed that endogenous CaM kinase, L-type Ca2+ channels, and ryanodine receptors colocalized near T tubules. EAD induction was significantly reduced in current-clamped cells dialyzed with AC3-I (4/15) compared with cells dialyzed with an inactive control peptide (11/15, P = 0.013). These findings support the hypothesis that EADs are facilitated by CaM kinase.

Different tolerance to hypothermia and rewarming of isolated rat and guinea pig hearts

We examined the effect of hypothermia and rewarming on myocardial function and calcium control in Langendorff-perfused hearts from rat and guinea pig. Both rat and guinea pig hearts demonstrated a rise in myocardial calcium ([Ca]total) in response to hypothermic perfusion (40 min, 10 degrees C), which was accompanied by an increase in left ventricular end diastolic pressure (LVEDP). The elevation in [Ca]total was severalfold higher in guinea pig than in rat hearts, reaching 12.9 +/- 0.8 and 3.1 +/- 0.6 micromol.g dry wt-1, respectively. The rise in LVEDP, however, was comparable in the two species: 62.5 +/- 2.5 (guinea pig) and 52.5 +/- 5.1 mm Hg (rat). Following rewarming, [Ca]total remained elevated in guinea pig, whereas a moderate decline in [Ca]total was observed in the rat (13.6 +/- 1.9 and 2.2 +/- 0.3 micromol.g dry wt-1, respectively). Posthypothermic values of LVEDP were also significantly higher in guinea pig compared to rat hearts (42.5 +/- 6.8 vs 20.5 +/- 5.1 mm Hg, P < 0.027). Furthermore, whereas rat hearts demonstrated a 78 +/- 7% recovery of left ventricular developed pressure, there was only a 15 +/- 7% recovery in guinea pig hearts. Measurements of tissue levels of high energy phosphates and glycogen utilization indicated a higher metabolic requirement in guinea pig than in rat hearts in order to oppose the hypothermia-induced calcium load. Thus, we conclude that isolated guinea pig hearts are more sensitive to a hypothermic insult than rat hearts.

Calmodulin kinase inhibition prevents development of the arrhythmogenic transient inward current

Although it is widely accepted that afterdepolarizations initiate arrhythmias when action potentials are prolonged, the underlying mechanisms are unclear. In this study, we tested the hypothesis that action potential prolongation would raise intracellular calcium and thereby activate the arrhythmogenic transient inward current (Iti). Furthermore, given that Iti can be activated by sarcoplasmic reticulum Ca2+ release, we tested the hypothesis that inhibition of calmodulin (CaM) kinase would prevent Iti. Isolated rabbit ventricular myocytes were studied with whole-cell-mode voltage clamp. Stimulation with a prolonged action potential clamp, under near-physiological conditions, increased [Ca2+]i. Iti was reproducibly induced in 60 of 60 cells, but Iti was not seen with the use of a shorter action potential waveform (n=12). Iti was associated with a secondary elevation in [Ca2+]i. When [Ca2+]i buffering was enhanced by dialysis with BAPTA (20 mmol/L, n=9), no Iti was present. The Na+/Ca2+ exchanger was likely responsible for Iti, because Iti was inhibited by the Na+/Ca2+ exchanger inhibitory peptide XIP (10 micromol/L, n=6), but not by an inactive scrambled peptide (10 micromol/L, n=5) or by the Cl- current antagonist niflumic acid (10 to 40 micromol/L, n=9). Activator Ca2+ from the sarcoplasmic reticulum was essential for development of Iti, because it was prevented by pretreatment with ryanodine (10 micromol/L, n=6) or thapsigargin (1 micromol/L, n=6). Two different CaM kinase inhibitory peptides (n=16) and a CaM inhibitory peptide (n=4) completely suppressed Iti. These results are consistent with the hypothesis that CaM kinase plays a role in arrhythmias related to increased [Ca2+]i.

Localization and quantitation of cardiac annexins II, V, and VI in hypertensive guinea pigs

Annexins are characterized by Ca2+-dependent binding to phospholipids. Annexin II mainly participates in cell-cell adhesion and signal transduction, whereas annexins V and VI also seem to regulate intracellular calcium cycling. Their abundance and localization were determined in left ventricle (LV) and right ventricle (RV) from hypertensive guinea pigs, during the transition from compensatory hypertrophy to heart failure. Immunoblot analysis of annexins II, V, and VI revealed an increased accumulation (2.6-, 1.45-, and 2.3-fold, respectively) in LV from hypertensive guinea pigs and no modification in RV. Immunofluorescent labeling of annexins II, V, and VI; of Na+-K+-ATPase; and of sarcomeric alpha-actinin showed that in control LV and RV, 1) annexin II is present in nonmuscle cells; 2) annexins V and VI are mainly observed in the sarcolemma and intercalated disks of myocytes; 3) annexins II, V, and VI strongly label endothelial cells and adventitia of coronary arteries; and 4) annexin VI is present in the media. At the onset of heart failure, the most striking changes are the increased protein accumulation in LV and the very strong labeling of annexins II, V, and VI in interstitial tissue, suggesting a role in fibrosis development and cardiac remodeling.

The sarcoplasmic reticulum and the Na+/Ca2+ exchanger both contribute to the Ca2+ transient of failing human ventricular myocytes

Our objective was to determine the respective roles of the sarcoplasmic reticulum (SR) and the Na+/Ca2+ exchanger in the small, slowly decaying Ca2+ transients of failing human ventricular myocytes. Left ventricular myocytes were isolated from explanted hearts of patients with severe heart failure (n=18). Cytosolic Ca2+, contraction, and action potentials were measured by using indo-1, edge detection, and patch pipettes, respectively. Selective inhibitors of SR Ca2+ transport (thapsigargin) and reverse-mode Na+/Ca2+ exchange activity (No. 7943, Kanebo Ltd) were used to define the respective contribution of these processes to the Ca2+ transient. Ca2+ transients and contractions induced by action potentials (AP transients) at 0.5 Hz exhibited phasic and tonic components. The duration of the tonic component was determined by the action potential duration. Ca2+ transients induced by caffeine (Caf transients) exhibited only a phasic component with a rapid rate of decay that was dependent on extracellular Na+. The SR Ca2+-ATPase inhibitor thapsigargin abolished the phasic component of the AP Ca2+ transient and of the Caf transient but had no significant effect on the tonic component of the AP transient. The Na+/Ca2+ exchange inhibitor No. 7943 eliminated the tonic component of the AP transient and reduced the magnitude of the phasic component. In failing human myocytes, Ca2+ transients and contractions exhibit an SR-related, phasic component and a slow, reverse-mode Na+/Ca2+ exchange-related tonic component. These findings suggest that Ca2+ influx via reverse-mode Na+/Ca2+ exchange during the action potential may contribute to the slow decay of the Ca2+ transient in failing human myocytes.

Quantification of Ca2+ uptake in the sarcoplasmic reticulum of trout ventricular myocytes

We measured Ca2+ uptake by the sarcoplasmic reticulum (SR) in trout ventricular myocytes, measuring indo 1 fluorescence in permeabilized cells or ionic currents in single myocytes subjected to voltage clamp. Titration of the SR Ca2+ pumps with thapsigargin gave a pump site density of 454 pmol/mg cell protein. Lowering the temperature from 20 degreesC to 10 or 5 degreesC reduced the SR Ca2+ uptake rate in permeabilized myocytes by 50 and 63%, respectively. Surprisingly, Ca2+ leak from the SR also decreased with decreasing temperatures. Exposure of single myocytes to 10 mM caffeine (Caf) induced a cell contracture and an inward ionic current. Neither contracture nor current decreased significantly after rest periods of 120 and 320 s. The inward current was due to Ca2+ extrusion by the Na+/Ca2+ exchanger (NCX), and the time integral of the exchange current (INCX) was used to calculate the SR Ca2+ content. This gave a steady-state SR Ca2+ content of 22.5 +/- 2.8 amol Ca2+/pF or 750 microM. When the SR was loaded by depolarizing the cell to +50 mV, the Ca2+ content increased with increasing length of the depolarization, reaching a maximum of 52.0 +/- 5.9 amol Ca2+/pF. When the cell was depolarized to different voltages for 3 s, a subsequent Caf-induced INCX increased with increasing voltage. At +100 mV, the Ca2+ content was 36.6 +/- 3.8 amol/pF, giving a maximal SR Ca2+ uptake rate of 12.2 +/- 1.2 amol Ca2+. pF-1. s-1 or 417 microM/s. We conclude that maximal SR Ca2+ content and Ca2+ uptake rates can be estimated using specific SR Ca2+ loading protocols. Contrary to the general assumption that contraction in lower vertebrates depends largely on transsarcolemmal Ca2+ fluxes, we found that although the L-type Ca2+ current is insufficient to fully activate contraction, the SR is capable of participating in the regulation of the cytosolic Ca2+ during the excitation-contraction coupling in trout ventricular myocytes.

In vivo left ventricular function and collagen expression in aldosterone/salt-induced hypertension

Cardiac fibrosis is linked to aldosterone-induced hypertension, but the effects on in vivo left ventricular (LV) function are not established. We studied the relations between in vivo LV function and aldosterone/salt cardiac fibrosis. Adult guinea pigs (GPs) were treated for 3 months with an aldosterone infusion and high-salt diet. This treatment induced arterial hypertension (+35%) and moderate LV hypertrophy (LVH; +60%) without right ventricular (RV) hypertrophy. Echo-Doppler LV assessment demonstrated unaltered cardiac output, stroke volume, or LV relaxation. Type I collagen messenger RNA (mRNA) was significantly increased in both ventricles (LV, +48%; RV, +77%) and accompanied by a significant increase in total collagen deposition (LV, from 0.52% in controls to 4.4% in treated GPs; RV, from 0.82 to 5.5% in treated GPs). Plasma norepinephrine levels increased 2.6-fold (p < 0.01) and correlated with the increase in collagen deposition in both ventricles. Collagen content was not correlated with hypertension or LVH. We conclude that aldosterone administration induces cardiac collagen accumulation and a sympathetic stimulation, which might preserve systolic and diastolic function.

Na+-Ca2+ exchange and sarcoplasmic reticular Ca2+ regulation in ventricular myocytes from transgenic mice overexpressing the Na+-Ca2+ exchanger

1. The contribution of the sarcoplasmic reticulum (SR) and Na+-Ca2+ exchanger to intracellular Ca2+ regulation in mouse cardiac myocytes was investigated by measuring contraction after variable rest intervals, rapid cooling contractures (RCCs) and fast application of caffeine. The results obtained showed differences from other species in the roles played by the SR and the Na+-Ca2+ exchanger. They suggest that in mouse ventricular myocytes there is significant Ca2+ entry via the exchanger during rest and during the latter part of the Ca2+ transient. 2. In cardiac myocytes isolated from transgenic mice overexpressing the cardiac Na+-Ca2+ exchanger the time to peak and relaxation of twitches and RCCs were faster than in control littermates. The decline of Ca2+, assessed by indo-1 fluorescence, was faster in transgenic myocytes even in the absence of Na+ and Ca2+ in the superfusing solution. This suggests that SR Ca2+ uptake is faster in these myocytes. However, no difference in the expression of SERCA2a, phospholamban or calsequestrin measured with Western blotting could be found in the two groups. 3. We measured SR Ca2+ content by integrating the caffeine-induced transient inward current. The amount of Ca2+ stored in the SR of transgenic mouse myocytes was 69 % greater than in non-transgenic littermates. The increased SR Ca2+ content may be responsible for the faster rate of SR Ca2+ release and uptake in cells from transgenic mice. 4. We performed experiments to assess whether the reversal potential of the Na+-Ca2+ exchanger (ENa-Ca) was different in transgenic cardiac cells. We measured a Ni2+-sensitive current elicited by voltage ramps in non-dialysed myocytes. The current-voltage relationship showed no difference in the reversal potential of the Na+-Ca2+ exchanger in transgenic and control myocytes. This suggests that the effects on the SR Ca2+ content in transgenic cardiac myocytes can be ascribed to the overexpression of the exchanger and are not secondary to changes in intracellular diastolic Ca2+ and Na+.

Control of maximum sarcoplasmic reticulum Ca load in intact ferret ventricular myocytes. Effects Of thapsigargin and isoproterenol

In steady state, the Ca content of the sarcoplasmic reticulum (SR) of cardiac myocytes is determined by a balance among influx and efflux pathways. The SR Ca content may be limited mainly by the ATP-supplied chemical potential that is inherent in the gradient between SR and cytosol. That is, forward Ca pumping from cytosol to SR may be opposed by energetically conservative reverse pumping dependent on intra-SR free [Ca]. On the other hand, SR Ca loading may be limited by dissipative pathways (pump slippage and/or pump-independent leak). To assess how SR Ca content is limited, we loaded voltage-clamped ferret ventricular myocytes cumulatively with known amounts of Ca via L-type Ca channels (ICa), using Na-free solutions to prevent Na/Ca exchange. We then measured the maximal resulting caffeine-released SR Ca content under control conditions, as well as when SR Ca pumping was accelerated by isoproterenol (1 micro M) or slowed by thapsigargin (0.2-0.4 micro M). Under control conditions, SR Ca content reached a limit of 137 micro mol.liter cytosol-1 (nonmitochondrial volume) when measured by integrating caffeine-induced Na/Ca exchange currents lintegraINaCaXdt) and of 119 micro mol.liter cytosol-1 when measured using fluorescence signals dependent on changes in cytosolic free Ca ([Ca]i). When Ca-ATPase pumping rate was slowed 39% by thapsigargin, the maximal SR Ca content decreased by 5 (integralINaCaXdt method) or 23% (fluorescence method); when pumping rate was increased 74% by isoproterenol, SR Ca content increased by 10% (fluorescence method) or 20% (integralINaCaXdt method). The relative stability of the SR Ca load suggests that dissipative losses have only a minor influence in setting the SR Ca content. Indeed, it appears that the SR Ca pump in intact cells can generate a [Ca] gradient approaching the thermodynamic limit.

Mitochondrial calcium in relaxed and tetanized myocardium

The elemental composition of rat cardiac muscle was determined with electron probe x-ray microanalysis (EPMA) of rapidly frozen papillary muscles and trabeculae incubated with ryanodine (1 microM) in either 1.2 or 10 mM [Ca2+]o-containing solutions, paced at 0.6 Hz or tetanized at 10 Hz. Total mitochondrial calcium increased significantly, by 4.2 mmol/kg dry weight during a 7 s tetanus, only in muscles tetanized in the presence of 10 mM [Ca2+]o when cytoplasmic Ca2+ is 1-4 microM (Backx, P. H., W.-D. Gao, M. D. Azan-Backx, and E. Marban. 1995. The relationship between contractile force and intracellular [Ca2+] in intact rat trabeculae. J. Gen. Physiol. 105:1-19). Comparison of total mitochondrial with free mitochondrial Ca2+ reported in the literature indicates that the total/free ratio is approximately 6000 at physiological or near-physiological levels of total mitochondrial calcium. Increases in free mitochondrial [Ca2+] consistent with regulation of mitochondrial enzymes should be associated with increases in total mitochondrial calcium detectable with EPMA. However, such increases in mitochondrial calcium occur only as the result of prolonged, unphysiological elevations of cytosolic [Ca2+].