Sodium-calcium exchange in heart: membrane currents and changes in [Ca2+]i

Involvement of the Na+/Ca2+ Exchanger in the Automaticity of the Cardiomyocytes from the Guinea Pig Pulmonary Vein but Not the Sinus Node

Fluorescence imaging analysis was performed in cardiomyocytes from the sinus node, the orthotopic pacemaker, and the pulmonary vein, a potential ectopic pacemaker that may cause atrial fibrillation, focusing on the role of the Na+/Ca2+ exchanger (NCX). Isolated cardiomyocytes from the guinea pig pulmonary vein and sinus node showing automaticity were loaded with fluorescence probes for analysis. Inhibition of NCX by SEA0400 decreased the Ca2+ transient frequency in the pulmonary vein cardiomyocytes but not in the sinus node. The basal intracellular Ca2+ concentration, as well as the number of Ca2+ sparks in the subsarcolemmal region, was higher in the pulmonary vein cardiomyocytes than in the sinus node. By contrast, the intracellular Na+ concentration was not different between the pulmonary vein and sinus node cardiomyocytes. The equilibrium potential for NCX (ENCX) was estimated to be less negative in the pulmonary vein cardiomyocytes than in the sinus node. In conclusion, the forward mode NCX is involved in spontaneous activity in the pulmonary vein cardiomyocytes but not in the sinus node; this is probably because the Ca2+ supply and the driving force for the forward mode NCX are both larger in the pulmonary vein cardiomyocytes than in the sinus node.

Functional coupling between Piezo1 and TRPM4 influences the electrical activity of HL-1 atrial myocytes

The transient receptor potential melastatin 4 (TRPM4) channel contributes extensively to cardiac electrical activity, especially cardiomyocyte action potential formation. Mechanical stretch can induce changes in heart rate and rhythm, and the mechanosensitive channel Piezo1 is expressed in many cell types within the myocardium. Our previous study showed that TRPM4 and Piezo1 are closely co-localized in the t-tubules of ventricular cardiomyocytes and contribute to the Ca2+-dependent signalling cascade that underlies hypertrophy in response to mechanical pressure overload. However, there was no direct evidence showing that Piezo1 activation was related to TRPM4 activation in situ. In the present study, we employed the HL-1 mouse atrial myocyte-like cell line as an in vitro model to investigate whether Piezo1-TRPM4 coupling can affect action potential properties. We used the small molecule Piezo1 agonist, Yoda1, as a surrogate for mechanical stretch to activate Piezo1 and detected the action potential changes in HL-1 cells using FluoVolt, a fluorescent voltage sensitive dye. Our results demonstrate that Yoda1-induced activation of Piezo1 changes the action potential frequency in HL-1 cells. This change in action potential frequency is reduced by Piezo1 knockdown using small intefering RNA. Importantly knockdown or pharmacological inhibition of TRPM4 significantly affected the degree to which Yoda1-evoked Piezo1 activation influenced action potential frequency. Thus, the present study provides in vitro evidence of a functional coupling between Piezo1 and TRPM4 in a cardiomyocyte-like cell line. The coupling of a mechanosensitive Ca2+ permeable channel and a Ca2+-activated TRP channel probably represents a ubiquitous model for the role of TRP channels in mechanosensory transduction. KEY POINTS: The transient receptor potential melastatin 4 (TRPM4) and Piezo1 channels have been confirmed to contribute to the Ca2+-dependent signalling cascade that underlies cardiac hypertrophy in response to mechanical pressure overload. However, there was no direct evidence showing that Piezo1 activation was related to TRPM4 activation in situ. We employed the HL-1 mouse atrial myocyte-like cell line as an in vitro model to investigate the effect of Piezo1-TRPM4 coupling on cardiac electrical properties. The results show that both pharmacological and genetic inhibition of TRPM4 significantly affected the degree to which Piezo1 activation influenced action potential frequency in HL-1 cells. Our findings provide in vitro evidence of a functional coupling between Piezo1 and TRPM4 in a cardiomyocyte-like cell line. The coupling of a mechanosensitive Ca2+ permeable channel and a Ca2+-activated TRP channel probably represents a ubiquitous model for the role of TRP channels in mechanosensory transduction in various (patho)physiological processes.

Cardiac monoamine oxidase-A inhibition protects against catecholamine-induced ventricular arrhythmias via enhanced diastolic calcium control

AIMS

A mechanistic link between depression and risk of arrhythmias could be attributed to altered catecholamine metabolism in the heart. Monoamine oxidase-A (MAO-A), a key enzyme involved in catecholamine metabolism and longstanding antidepressant target, is highly expressed in the myocardium. The present study aimed to elucidate the functional significance and underlying mechanisms of cardiac MAO-A in arrhythmogenesis.

METHODS AND RESULTS

Analysis of the TriNetX database revealed that depressed patients treated with MAO inhibitors had a lower risk of arrhythmias compared with those treated with selective serotonin reuptake inhibitors. This effect was phenocopied in mice with cardiomyocyte-specific MAO-A deficiency (cMAO-Adef), which showed a significant reduction in both incidence and duration of catecholamine stress-induced ventricular tachycardia compared with wild-type mice. Additionally, cMAO-Adef cardiomyocytes exhibited altered Ca2+ handling under catecholamine stimulation, with increased diastolic Ca2+ reuptake, reduced diastolic Ca2+ leak, and diminished systolic Ca2+ release. Mechanistically, cMAO-Adef hearts had reduced catecholamine levels under sympathetic stress, along with reduced levels of reactive oxygen species and protein carbonylation, leading to decreased oxidation of Type II PKA and CaMKII. These changes potentiated phospholamban (PLB) phosphorylation, thereby enhancing diastolic Ca2+ reuptake, while reducing ryanodine receptor 2 (RyR2) phosphorylation to decrease diastolic Ca2+ leak. Consequently, cMAO-Adef hearts exhibited lower diastolic Ca2+ levels and fewer arrhythmogenic Ca2+ waves during sympathetic overstimulation.

CONCLUSION

Cardiac MAO-A inhibition exerts an anti-arrhythmic effect by enhancing diastolic Ca2+ handling under catecholamine stress.

Calcium Buffering in the Heart in Health and Disease

Changes of intracellular Ca²⁺ concentration regulate many aspects of cardiac myocyte function. About 99% of the cytoplasmic calcium in cardiac myocytes is bound to buffers, and their properties will therefore have a major influence on Ca²⁺ signaling. This article considers the fundamental properties and identities of the buffers and how to measure them. It reviews the effects of buffering on the systolic Ca²⁺ transient and how this may change physiologically, and in heart failure and both atrial and ventricular arrhythmias, as well. It is concluded that the consequences of this strong buffering may be more significant than currently appreciated, and a fuller understanding is needed for proper understanding of cardiac calcium cycling and contractility.

Use of Isolated Adult Myocytes to Evaluate Cardiotoxicity. II. Preparation and Properties*

The preparation and properties of isolated adult cardiac myocytes are reviewed, with the goal being to evaluate their usefulness as a model system for measuring cardiotoxicity. Some important factors in cell isolation methodology which impact on the quality of the preparation are identified, along with criteria for assessing the quality of cells after isolation. By all criteria, myocytes isolated by good procedures appear to largely retain their original properties. Moreover, the distinctive behavior of adult myocytes under metabolic stress endows them with a particular usefulness as monitors of toxicity. Overall, we conclude that the art of adult heart cell isolation and culture is now sufficiently advanced for either freshly isolated cells in suspension or cells in culture to be a useful model system for toxicity studies.

The Control of Diastolic Calcium in the Heart: Basic Mechanisms and Functional Implications

Normal cardiac function requires that intracellular Ca²⁺ concentration be reduced to low levels in diastole so that the ventricle can relax and refill with blood. Heart failure is often associated with impaired cardiac relaxation. Little, however, is known about how diastolic intracellular Ca²⁺ concentration is regulated. This article first discusses the reasons for this ignorance before reviewing the basic mechanisms that control diastolic intracellular Ca²⁺ concentration. It then considers how the control of systolic and diastolic intracellular Ca²⁺ concentration is intimately connected. Finally, it discusses the changes that occur in heart failure and how these may result in heart failure with preserved versus reduced ejection fraction.

Ups and downs of calcium in the heart

Contraction and relaxation of the heart result from cyclical changes of intracellular Ca²⁺ concentration ([Ca²⁺ ]_i ). The entry of Ca²⁺ into the cell via the L-type Ca²⁺ current leads to the release of more from the sarcoplasmic reticulum (SR). Compared to other regulatory mechanisms such as phosphorylation, Ca²⁺ signalling is very rapid. However, since Ca²⁺ cannot be destroyed, Ca²⁺ signalling can only be controlled by pumping across membranes. In the steady state, on each beat, the amount of Ca²⁺ released from the SR must equal that taken back and influx and efflux across the sarcolemma must be equal. Any imbalance in these fluxes will result in a change of SR Ca²⁺ content and this provides a mechanism for regulation of SR Ca²⁺ content. These flux balance considerations also explain why simply potentiating Ca²⁺ release from the SR has no maintained effect on the amplitude of the Ca²⁺ transient. A low diastolic [Ca²⁺ ]_i is essential for cardiac relaxation, but the factors that control diastolic [Ca²⁺ ]_i are poorly understood. Recent work suggests that flux balance is also important here. In particular, decreasing SR function decreases the amplitude of the systolic Ca²⁺ transient and the resulting decrease of Ca²⁺ efflux results in an increase of diastolic [Ca²⁺ ]_i to maintain total efflux.

Calcium and Excitation-Contraction Coupling in the Heart

Cardiac contractility is regulated by changes in intracellular Ca concentration ([Ca²⁺]_i). Normal function requires that [Ca²⁺]_i be sufficiently high in systole and low in diastole. Much of the Ca needed for contraction comes from the sarcoplasmic reticulum and is released by the process of calcium-induced calcium release. The factors that regulate and fine-tune the initiation and termination of release are reviewed. The precise control of intracellular Ca cycling depends on the relationships between the various channels and pumps that are involved. We consider 2 aspects: (1) structural coupling: the transporters are organized within the dyad, linking the transverse tubule and sarcoplasmic reticulum and ensuring close proximity of Ca entry to sites of release. (2) Functional coupling: where the fluxes across all membranes must be balanced such that, in the steady state, Ca influx equals Ca efflux on every beat. The remainder of the review considers specific aspects of Ca signaling, including the role of Ca buffers, mitochondria, Ca leak, and regulation of diastolic [Ca²⁺]_i.

In silico investigation of the short QT syndrome, using human ventricle models incorporating electromechanical coupling

INTRODUCTION

Genetic forms of the Short QT Syndrome (SQTS) arise due to cardiac ion channel mutations leading to accelerated ventricular repolarization, arrhythmias and sudden cardiac death. Results from experimental and simulation studies suggest that changes to refractoriness and tissue vulnerability produce a substrate favorable to re-entry. Potential electromechanical consequences of the SQTS are less well-understood. The aim of this study was to utilize electromechanically coupled human ventricle models to explore electromechanical consequences of the SQTS.

METHODS AND RESULTS

The Rice et al. mechanical model was coupled to the ten Tusscher et al. ventricular cell model. Previously validated K(+) channel formulations for SQT variants 1 and 3 were incorporated. Functional effects of the SQTS mutations on [Ca(2+)] i transients, sarcomere length shortening and contractile force at the single cell level were evaluated with and without the consideration of stretch-activated channel current (I sac). Without I sac, at a stimulation frequency of 1Hz, the SQTS mutations produced dramatic reductions in the amplitude of [Ca(2+)] i transients, sarcomere length shortening and contractile force. When I sac was incorporated, there was a considerable attenuation of the effects of SQTS-associated action potential shortening on Ca(2+) transients, sarcomere shortening and contractile force. Single cell models were then incorporated into 3D human ventricular tissue models. The timing of maximum deformation was delayed in the SQTS setting compared to control.

CONCLUSION

The incorporation of I sac appears to be an important consideration in modeling functional effects of SQT 1 and 3 mutations on cardiac electro-mechanical coupling. Whilst there is little evidence of profoundly impaired cardiac contractile function in SQTS patients, our 3D simulations correlate qualitatively with reported evidence for dissociation between ventricular repolarization and the end of mechanical systole.

Triple threat: the Na+/Ca2+ exchanger in the pathophysiology of cardiac arrhythmia, ischemia and heart failure

The Na(+)/Ca(2+) exchanger (NCX) is the main Ca(2+) extrusion mechanism of the cardiac myocyte and thus is crucial for maintaining Ca(2+) homeostasis. It is involved in the regulation of several parameters of cardiac excitation contraction coupling, such as cytosolic Ca(2+) concentration, repolarization and contractility. Increased NCX activity has been identified as a mechanism promoting heart failure, cardiac ischemia and arrhythmia. Transgenic mice as well as pharmacological interventions have been used to support the idea of using NCX inhibition as a future pharmacological strategy to treat cardiovascular disease.

Na+/Ca2+ exchanger is a determinant of excitation-contraction coupling in human embryonic stem cell-derived ventricular cardiomyocytes

In adult cardiomyocytes (CMs), the Na(+)/Ca(2+) exchanger (NCX) is a well-defined determinant of Ca(2+) homeostasis. Developmentally, global NCX knockout in mice leads to abnormal myofibrillar organization, electrical defects, and early embryonic death. Little is known about the expression and function of NCX in human heart development. Self-renewable, pluripotent human embryonic stem cells (hESCs) can serve as an excellent experimental model. However, hESC-derived CMs are highly heterogeneous. A stably lentivirus-transduced hESC line (MLC2v-dsRed) was generated to express dsRed under the transcriptional control of the ventricular-restricted myosin light chain-2v (MLC2v) promoter. Electrophysiologically, dsRed+ cells differentiated from MLC2vdsRed hESCs displayed ventricular action potentials (AP), exclusively. Neither atrial nor pacemaker APs were observed. While I(Ca-L), I(f), and I(Kr) were robustly expressed, I(Ks) and I(K1) were absent in dsRed+ ventricular hESCCMs. Upon differentiation (7+40 to +90 days), the basal [Ca(2+)](i), Ca(2+) transient amplitude, maximum upstroke, and decay velocities significantly increased (P < 0.05). The I(Ca-L) antagonizer nifedipine (1 microM) decreased the Ca(2+) transient amplitude (to approximately 30%) and slowed the kinetics (by approximately 2-fold), but Ca(2+) transients could still be elicited even after complete ICa-L blockade, suggesting the presence of additional Ca(2+) influx(es). Indeed, Ni(2+)-sensitive INCX could be recorded in 7+40- and +90-day dsRed+ hESC-CMs, and its densities increased from -1.2 +/- 0.6 pA/pF at -120 mV and 3.6 +/- 1.0 pA/pF at 60 mV by 6- and 2-folds, respectively. With higher [Ca(2+)](i), 7+90-day ventricular hESC-CMs spontaneously but irregularly fired transients upon a single stimulus under an external Na(+)-free condition; however, without extracellular Na(+), nifedipine could completely inhibit Ca(2+) transients. We conclude that I(NCX) is functionally expressed in developing ventricular hESC-CMs and contributes to their excitation-contraction coupling.

Action potential energy efficiency varies among neuron types in vertebrates and invertebrates

The initiation and propagation of action potentials (APs) places high demands on the energetic resources of neural tissue. Each AP forces ATP-driven ion pumps to work harder to restore the ionic concentration gradients, thus consuming more energy. Here, we ask whether the ionic currents underlying the AP can be predicted theoretically from the principle of minimum energy consumption. A long-held supposition that APs are energetically wasteful, based on theoretical analysis of the squid giant axon AP, has recently been overturned by studies that measured the currents contributing to the AP in several mammalian neurons. In the single compartment models studied here, AP energy consumption varies greatly among vertebrate and invertebrate neurons, with several mammalian neuron models using close to the capacitive minimum of energy needed. Strikingly, energy consumption can increase by more than ten-fold simply by changing the overlap of the Na(+) and K(+) currents during the AP without changing the APs shape. As a consequence, the height and width of the AP are poor predictors of energy consumption. In the Hodgkin-Huxley model of the squid axon, optimizing the kinetics or number of Na(+) and K(+) channels can whittle down the number of ATP molecules needed for each AP by a factor of four. In contrast to the squid AP, the temporal profile of the currents underlying APs of some mammalian neurons are nearly perfectly matched to the optimized properties of ionic conductances so as to minimize the ATP cost.

The mechanism and significance of the slow changes of ventricular action potential duration following a change of heart rate

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.

Modulation of sarcoplasmic reticulum function by PST2744 [istaroxime; (E,Z)-3-((2-aminoethoxy)imino) androstane-6,17-dione hydrochloride)] in a pressure-overload heart failure model

PST2744 [Istaroxime; (E,Z)-3-((2-aminoethoxy)imino) androstane-6,17-dione hydrochloride)] is a novel inotropic agent that enhances sarco(endo)plasmic reticulum Ca(2+) ATPase (SERCA) 2 activity. We investigated the istaroxime effect on Ca(2+) handling abnormalities in myocardial hypertrophy/failure (HF). Guinea pig myocytes were studied 12 weeks after aortic banding (AoB) and compared with those of sham-operated animals (sham). The gain of calcium-induced Ca(2+) release (CICR), sarcoplasmic reticulum (SR) Ca(2+) content, Na(+)/Ca(2+) exchanger (NCX) function, and the rate of SR reloading after caffeine-induced depletion (SR Ca(2+) uptake, measured during NCX blockade) were evaluated by measurement of cytosolic Ca(2+) and membrane currents. HF characterization: AoB caused hypertrophy and failure in 100 and 25% of animals, respectively. Although CICR gain during constant pacing was preserved, SR Ca(2+) content and SR Ca(2+) uptake were strongly depressed. Resting Ca(2+) and the slope of the Na(+)/Ca(2+) exchanger current (I(NCX))/Ca(2+) relationship were unchanged by AoB. Istaroxime effects: CICR gain, SR Ca(2+) content, and SR Ca(2+) uptake rate were increased by istaroxime in sham myocytes and, to a significantly larger extent, in AoB myocytes; this led to almost complete recovery of SR Ca(2+) uptake in AoB myocytes. Istaroxime increased resting Ca(2+) and the slope of the I(NCX)/Ca(2+) relationship similarly in sham and AoB myocytes. Istaroxime failed to increase SERCA activity in skeletal muscle microsomes devoid of phospholamban. Thus, clear-cut abnormalities in Ca(2+) handling occurred in this model of hypertrophy, with mild decompensation. Istaroxime enhanced SR function more in HF myocytes than in normal ones; almost complete drug-induced recovery suggests a purely functional nature of SR dysfunction in this HF model.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

NCX current in the murine embryonic heart: development-dependent regulation by Na+

Due to its high functional expression, I(NCX) may serve as an important mechanism to ensure intracellular Ca2+ homeostasis, especially during the early embryonic stage. At the fetal stage I(NCX) density is significantly decreased but underlies regulatory processes, that is, regulation by Na+.

Na/Ca Exchange: Regulator of Intracellular Calcium and Source of Arrhythmias in the Heart

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.

Functional expression of the Na+/Ca2+ exchanger in the embryonic mouse heart

The Na(+)/Ca(2+) exchanger (NCX) is one of the earliest functional genes and is currently assumed to compensate at least in part for the rudimentary sarcoplasmic reticulum in the developing mouse heart. However, to date little is known about the functional expression of NCX during development. This prompted us to investigate the NCX current (I(NCX)) in very early (embryonic day E8.5-E9.5 post coitum), early (E10.5-E11.5), middle (E13.5) and late (E16.5) stage mouse embryonic cardiomyocytes. For standard I(NCX) measurements, [Ca(2+)](i) was buffered to 150 nmol/l and voltage ramps were applied from +60 mV to -120 mV. At very early stages of development, we observed a prominent role of the I(NCX) Ca(2+) inward mode in elevating the cytosolic Ca(2+) concentration ([Ca(2+)](i)). Accordingly, a high I(NCX) density was observed (+60 mV: 4.6+/-0.7 pA/pF, n=14). Likewise, we found a strong Ca(2+) outward mode of I(NCX) (-120 mV: -3.9+/-0.7 pA/pF, n=14). At later stages, however, I(NCX) Ca(2+) inward mode was reduced by 54+/-6% (n=15, p<0.0001) in ventricular and 68+/-10% (n=9, p<0.0006) in atrial cells. For the outward mode, a reduction by 43+/-10% (n=15, p<0.01) in ventricular and 62+/-11% (n=9, p<0.004) in atrial cardiomyocytes was observed. By contrast, NCX isoform expression and the reversal potential did not significantly change during development. Thus, NCX displays a prominent Ca(2+) inward and outward mode during early embryonic heart development pointing to its important contribution to maintain [Ca(2+)](i) homeostasis. The functional and protein expression of NCX declines during further development.

Simulation of five intracellular Ca2+-regulation mechanisms in response to voltage-clamp pulses

We report the simulation of five C2+ regulation mechanisms in response to voltage-clamp pulses applied at the membrane. We obtain the currents generated and also record the C2+ concentration changes in the whole cell during and after the pulse. Our simulator includes models for the L-type voltage-dependent C2+ channels, the C2+ pump, the Na+/C2+ exchanger and a fast buffering system, along with diffusion assuming a spheric cell shape.

Influence of age and run training on cardiac Na+/Ca2+ exchange

Effects of age and training on myocardial Na+/Ca2+ exchange were examined in young sedentary (YS; 14-15 mo), aged sedentary (AS; 27-31 mo), and aged trained (AT; 8- to 11-wk treadmill run training) male Fischer Brown Norway rats. Whole heart performance and isolated cardiocyte Na+/Ca2+ exchange characteristics were measured. At the whole heart level, a small but significant slowing of late isovolumic left ventricular (LV) relaxation, which may be indicative of altered Na+/Ca2+ exchange activity, was seen in hearts from AS rats. This subtle impairment in relaxation was not observed in hearts from AT rats. At the single-cardiocyte level, late action potential duration was prolonged, resting membrane potential was more positive, and overshoot potential was greater in cardiocytes from AS rats than from YS rats (P < 0.05). Training did not influence any of these age-related action potential characteristics. In electrically paced cardiocytes, neither shortening nor intracellular Ca2+ concentration ([Ca2+]i) dynamics was influenced by age or training. Similarly, neither age nor training influenced the rate of [Ca2+]i clearance via forward (Nain+ /Caout2+) Na+/Ca2+ exchange after caffeine-induced Ca2+ release from the sarcoplasmic reticulum or cardiac Na+/Ca2+ exchanger protein (NCX1) expression. However, when whole cell patch-clamp techniques combined with fluorescence microscopy were used to evaluate the ability of Na+/Ca2+ exchange to alter cytosolic [Ca2+] ([Ca2+]c) under conditions where membrane potential (Vm) and internal and external [Na+] and [Ca2+] could be controlled, we observed age-associated increases in forward Na+/Ca2+ exchange-mediated [Ca2+]c clearance (P < 0.05) that were not influenced by training. The age-related increase in forward Na+/Ca2+ exchange activity provides a hypothetical explanation for the late action potential prolongation observed in this study.

The voltage-sensitive release mechanism of excitation contraction coupling in rabbit cardiac muscle is explained by calcium-induced calcium release

The putative voltage-sensitive release mechanism (VSRM) was investigated in rabbit cardiac myocytes at 37 degrees C with high resistance microelectrodes to minimize intracellular dialysis. When the holding potential was adjusted from -40 to -60 mV, the putative VSRM was expected to operate alongside CICR. Under these conditions however, we did not observe a plateau at positive potentials of the cell shortening versus voltage relationship. The threshold for cell shortening changed by -10 mV, but this resulted from a similar change of the threshold for activation of inward current. Cell shortening under conditions where the putative VSRM was expected to operate was blocked in a dose dependent way by nifedipine and CdCl2 and blocked completely by NiCl2. "Tail contractions" persisted in the presence of nifedipine and CdCl2 but were blocked completely by NiCl2. Block of early outward current by 4-aminopyridine and 4-acetoamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid (SITS) demonstrated persisting inward current during test depolarizations despite the presence of nifedipine and CdCl2. Inward current did not persist in the presence of NiCl2. A tonic component of cell shortening that was prominent during depolarizations to positive potentials under conditions selective for the putative VSRM was sensitive to rapidly applied changes in superfusate [Na+] and to the outward Na+/Ca2+ exchange current blocking drug KB-R7943. This component of cell shortening was thought to be the result of Na+/Ca2+ exchange-mediated excitation contraction coupling. Cell shortening recorded under conditions selective for the putative VSRM was increased by the enhanced state of phosphorylation induced by isoprenaline (1 microM) and by enhancing sarcoplasmic reticulum Ca2+ content by manipulation of the conditioning steps. Under these conditions, cell shortening at positive test depolarizations was converted from tonic to phasic. We conclude that the putative VSRM is explained by CICR with the Ca2+ "trigger" supplied by unblocked L-type Ca2+ channels and Na+/Ca2+ exchange.

Amiloride and KB-R7943 in outward Na+ /Ca2+ exchange current in guinea pig ventricular myocytes

The reverse mode of Na+/Ca2+ exchange represents an important pathway in inducing Ca2+ overload during ischemia and reperfusion. The inhibitory effects of amiloride and KB-R7943 on Na+/Ca2+ exchange current (INa/Ca) were investigated in guinea pig ventricular myocytes. Whole-cell patch clamp techniques were used under bidirectional ionic conditions and 25 mM of Na+ in pipette solution. At +50 mV, amiloride 10, 30, and 100 microM inhibited the outward INa/Ca by 15, 23, and 41%, respectively; at -80 mV, it inhibited inward INa/Ca by 6, 15, and 23%, respectively. Its inhibitory effect on outward INa/Ca was greater than that on inward INa/Ca. At +50 mV, KB-R7943 1 and 10 microM inhibited the outward INa/Ca by 29 and 61%, respectively; at -80 mV, it inhibited inward INa/Ca by 22 and 57%, respectively. KB-R7943 inhibited both directions of the exchange current with an equal potency. The data suggest that KB-R7943 is not a selective inhibitor on reverse mode of Na+/Ca2+ exchange.

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.

Sarcoplasmic reticulum Ca(2+) release causes myocyte depolarization. Underlying mechanism and threshold for triggered action potentials

Spontaneous sarcoplasmic reticulum (SR) Ca(2+) release causes delayed afterdepolarizations (DADs) via Ca(2+)-induced transient inward currents (I:(ti)). However, no quantitative data exists regarding (1) Ca(2+) dependence of DADs, (2) Ca(2+) required to depolarize the cell to threshold and trigger an action potential (AP), or (3) relative contributions of Ca(2+)-activated currents to DADs. To address these points, we evoked SR Ca(2+) release by rapid application of caffeine in indo 1-AM-loaded rabbit ventricular myocytes and measured caffeine-induced DADs (cDADs) with whole-cell current clamp. The SR Ca(2+) load of the myocyte was varied by different AP frequencies. The cDAD amplitude doubled for every 88+/-8 nmol/L of Delta[Ca(2+)](i) (simple exponential), and the Delta[Ca(2+)](i) threshold of 424+/-58 nmol/L was sufficient to trigger an AP. Blocking Na(+)-Ca(2+) exchange current (I(Na/Ca)) by removal of [Na](o) and [Ca(2+)](o) (or with 5 mmol/L Ni(2+)) reduced cDADs by >90%, for the same Delta[Ca(2+)](i). In contrast, blockade of Ca(2+)-activated Cl(-) current (I(Cl(Ca))) with 50 micromol/L niflumate did not significantly alter cDADs. We conclude that DADs are almost entirely due to I(Na/Ca), not I(Cl(Ca)) or Ca(2+)-activated nonselective cation current. To trigger an AP requires 30 to 40 micromol/L cytosolic Ca(2+) or a [Ca(2+)](i) transient of 424 nmol/L. Current injection, simulating I(ti)s with different time courses, revealed that faster I:(ti)s require less charge for AP triggering. Given that spontaneous SR Ca(2+) release occurs in waves, which are slower than cDADs or fast I(ti)s, the true Delta[Ca(2+)](i) threshold for AP activation may be approximately 3-fold higher in normal myocytes. This provides a safety margin against arrhythmia in normal ventricular myocytes.

Prevention of monensin-induced hyperpolarization in NG108-15 cells

The NG 108-15 (neuroblastoma X glioma hybrid) cell line was used as an in vitro neuronal model to evaluate potential antagonists of the Na+-selective carboxylic ionophore monensin. Changes in membrane electrical characteristics induced by monensin with and without the simultaneous administration of antagonists were measured using intracellular microelectrode techniques. Bath application of monensin (3 microM) produced a hyperpolarization of approximately = 35 mV. Monensin also altered the generation of action potentials in response to electrical stimulation in 14 of 24 (58%) exposed cells, as evident in a partial or complete loss of action potentials or in an alteration of action potential waveform. The antagonists used were Na+-K+ pump inhibitor ouabain (1-3 microM), the Ca2+dependent K+ channel blocker quinine (3-30 microM) or drugs known to influence Ca2+ signaling in cells, i.e., trifluoperazine (3-10 microM), verapamil (1-10 microM) or chlorpromazine (3-30 microM). On a molar basis, ouabain was the most and trifluoperazine the least effective of the antagonists. Quinine, verapamil and chlorpromazine all prevented the development of the hyperpolarization in an approximate concentration-dependent manner. However, none of these drugs was able to block the effects of monensin on action potentials. Indeed, high concentrations of the antagonists that were most effective in preventing the hyperpolarization accentuated impairments in action potential generation and also reduced input resistance in many cells. Thus, none of these antagonists appears suitable for transition to in vivo antidotal protection studies.

Comparative effects of carboxylic ionophores on membrane potential and resistance of NG108-15 cells

Comparative analyses were conducted to determine the effects of Na(+) (monensin, MON), K(+) (nigericin, NIG) and Ca(2+) (A23187) selective carboxylic ionophores on differentiated NG108-15 (neuroblastoma X glioma hybrid) cells. Alterations in membrane potential (V(m)), input resistance (Rin) and electrically induced action potential generation were measured using intracellular microelectrode techniques in cells treated with 0.1-30 microM MON and NIG and 0.1-10 microM A23187. Responses to the ionophores were similar in that membrane hyperpolarization and unchanged R(in) predominated with all three compounds. However, significant differences between the ionophores were also detected. MON- and A23187-induced hyperpolarization was generally maintained throughout the 24-min superfusion whereas that produced by NIG diminished with time or was replaced by depolarization. In addition, action potential generation was blocked by NIG, whereas MON had no effect and action potential alterations were evident only with the highest A23187 concentration (10 microM). This study represents the initial comprehensive analysis of the effects of carboxylic ionophores on membrane electrical characteristics of an intact cell system and forms the basis for subsequent work using NG108-15 cells as a model system to evaluate potential therapeutic treatments against the carboxylic ionophores.

Changes in Ca(2+) cycling proteins underlie cardiac action potential prolongation in a pressure-overloaded guinea pig model with cardiac hypertrophy and failure

Ventricular arrhythmias are common in both cardiac hypertrophy and failure; cardiac failure in particular is associated with a significant increase in the risk of sudden cardiac death. We studied the electrophysiologic changes in a guinea pig model with aortic banding resulting in cardiac hypertrophy at 4 weeks and progressing to cardiac failure at 8 weeks using whole-cell patch-clamp and biochemical techniques. Action potential durations (APDs) were significantly prolonged in banded animals at 4 and 8 weeks compared with age-matched sham-operated animals. APDs at 50% and 90% repolarization (APD(50) and APD(90) in ms) were the following: 4 week, banded, 208+/-51 and 248+/-49 (n = 15); 4 week, sham, 189+/-68 and 213+/-69 (n = 16); 8 week, banded, 197+/-40 and 226+/-40 (n = 21); and 8 week, sham, 156+/-42 and 189+/-45 (n = 22), respectively; P<0.05 comparing banded versus sham-operated animals. We observed no significant differences in the K(+) currents between the 2 groups of animals at 4 and 8 weeks. However, banded animals exhibited a significant increase in Na(+) and Na(+)-Ca(2+) exchange current densities compared with controls. Furthermore, we have found a significant attenuation in the Ca(2+)-dependent inactivation of the L-type Ca(2+) current in the banded compared with sham-operated animals, likely as a result of the significant downregulation of the sarcoplasmic reticulum Ca(2+) ATPase, which has been documented previously in the heart failure animals. Our data provide an alternate mechanism for APD prolongation in cardiac hypertrophy and failure and support the notion that there is close interaction between Ca(2+) handling and action potential profile.

Intracellular Ca2+ oscillations drive spontaneous contractions in cardiomyocytes during early development

Activity of cardiac pacemaker cells is caused by a balanced interplay of ion channels. However, it is not known how the rhythmic beating is initiated during early stages of cardiomyogenesis, when the expression of ion channels is still incomplete. Based on the observation that early-stage embryonic stem cell-derived cardiomyocytes continuously contracted in high extracellular K+ solution, here we provide experimental evidence that the spontaneous activity of these cells is not generated by transmembrane ion currents, but by intracellular [Ca2+]i oscillations. This early activity was clearly independent of voltage dependent L-type Ca2+ channels and the interplay between these and ryanodine sensitive Ca2+ stores. We also show that intracellular Ca2+ oscillations evoke small membrane depolarizations and that these can trigger L-type Ca2+ channel driven action potentials.

Na+-Ca2+ exchange and Ca2+ efflux in transfected Chinese hamster ovary cells

The objective of this study was to assess the contribution of Na+-Ca2+ exchange activity to Ca2+ efflux at various cytosolic Ca2+ concentrations ([Ca2+]i) in transfected Chinese hamster cells expressing the bovine cardiac Na+-Ca2+ exchanger. Ionomycin was added to fura-2 loaded cells and the resulting [Ca2+]i transient was monitored in Ca2+-free media with or without extracellular Na+. The presence of Na+ reduced both the amplitude and duration of the [Ca2+]i transient. Na+ had similar effects when the peak of the [Ca2+]i transient was buffered to 100 nM by cytosolic EGTA, or when Ca2+ was slowly released from internal stores with thapsigargin. Ca2+ efflux following ionomycin addition was directly measured with extracellular fura-2 and followed a biphasic time course (t(1/2) approximately = 10 s and 90s). The proportion of total efflux owing to the rapid phase was increased by Na+ and reduced by EGTA-loading. Na+ accelerated the initial rate of Ca2+ efflux by 65% in unloaded cells but only by 16% in EGTA-loaded cells. In both cases, the stimulation by Na+ was less than expected, given the pronounced effects of Na+ on the [Ca2+]i transient. We conclude that the exchanger contributes importantly to Ca2+ efflux activity at all [Ca2+]i values above 40 nM. We also suggest that Ca2+ efflux pathways may involve non-cytosolic or local routes of Ca2+ traffic.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Significance of Na/Ca exchange for Ca2+ buffering and electrical activity in mouse pancreatic beta-cells

We have combined the patch-clamp technique with microfluorimetry of the cytoplasmic Ca2+ concentration ([Ca2+]i) to characterize Na/Ca exchange in mouse beta-cells and to determine its importance for [Ca2+]i buffering and shaping of glucose-induced electrical activity. The exchanger contributes to Ca2+ removal at [Ca2+]i above 1 microM, where it accounts for >35% of the total removal rate. At lower [Ca2+]i, thapsigargin-sensitive Ca2+-ATPases constitute a major (70% at 0.8 microM [Ca2+]i) mechanism for Ca2+ removal. The beta-cell Na/Ca exchanger is electrogenic and has a stoichiometry of three Na+ for one Ca2+. The current arising from its operation reverses at approximately -20 mV (current inward at more negative voltages), has a conductance of 53 pS/pF (14 microM [Ca2+]i), and is abolished by removal of external Na+ or by intracellularly applied XIP (exchange inhibitory peptide). Inhibition of the exchanger results in shortening (50%) of the bursts of action potentials of glucose-stimulated beta-cells in intact islets and a slight (5 mV) hyperpolarization. Mathematical simulations suggest that the stimulatory action of glucose on beta-cell electrical activity may be accounted for in part by glucose-induced reduction of the cytoplasmic Na+ concentration with resultant activation of the exchanger.

Overexpression of the cardiac Na+/Ca2+ exchanger increases susceptibility to ischemia/reperfusion injury in male, but not female, transgenic mice

Influx of Ca2+ into myocytes via Na+/Ca2+ exchange may be stimulated by the high levels of intracellular Na+ and the changes in membrane potential known to occur during ischemia/reperfusion. This increased influx could, in turn, lead to Ca2+ overload and injury. Overexpression of the cardiac Na+/Ca2+ exchanger therefore may increase susceptibility to ischemia/reperfusion injury. To test this hypothesis, the hearts of male and female transgenic mice, overexpressing the Na+/Ca2+ exchange protein, and hearts of their wild-type littermates, were perfused with Krebs-Henseleit buffer and subjected to 20 minutes of ischemia and 40 minutes of reperfusion. Preischemic left ventricular developed pressures and +dP/dtmax, as well as -dP/dtmin, were higher in the male transgenic hearts compared with wild-type, implying a role for Na+/Ca2+ exchange in the contraction, as well as the relaxation, phases of the cardiac beat. Postischemic function was lower in male transgenic than in male wild-type hearts (7+/-2% versus 32+/-6% of preischemic function), but there was no difference between female transgenic and female wild-type hearts, both at approximately 30% of preischemic function. To assess whether this male/female difference was due to female-specific hormones such as estrogen, the hearts of bilaterally ovariectomized and sham-operated transgenic females were subjected to the same protocol. The functional recoveries of ovariectomized female transgenic hearts were lower (17+/-3% of preischemic function) than those of wild-type and sham-operated transgenic females. The lower postischemic functional recovery in the male transgenic and female ovariectomized transgenic hearts correlated with lower recoveries of the energy metabolites, ATP and phosphocreatine, as measured by 31P nuclear magnetic resonance spectroscopy. Alternans were observed during reperfusion in male transgenic and female ovariectomized transgenic hearts only, consistent with intracellular Ca2+ overload. Western analyses showed that alterations in the expression of the Na+/Ca2+ exchange or L-type Ca2+ channel proteins were not responsible for the protection observed in the female transgenic hearts. In conclusion, in males, overexpression of the Na+/Ca2+ exchanger reduced postischemic recovery of both contractile function and energy metabolites, indicating that the Na+/Ca2+ exchanger may play a role in ischemia/reperfusion injury. From the studies of females, however, it appears that this exacerbation of ischemia/reperfusion injury by overexpression of the Na+/Ca2+ exchanger can be overcome partially by female-specific hormones such as estrogen.

Shortening and [Ca2+] dynamics of left ventricular myocytes isolated from exercise-trained rats

The effects of run endurance training and fura 2 loading on the contractile function and Ca2+ regulation of rat left ventricular myocytes were examined. In myocytes not loaded with fura 2, the maximal extent of myocyte shortening was reduced with training under our pacing conditions [0.5 Hz at 2.0 and 0.75 mM external Ca2+ concentration ([Ca2+]o)], although training had no effect on the temporal characteristics. The "light" loading of myocytes with fura 2 markedly suppressed (approximately 50%) maximal shortening in the sedentary and trained groups, although the temporal characteristics of myocyte shortening were significantly prolonged in the trained group. No discernible differences in the dynamic characteristics of the intracellular Ca2+ concentration ([Ca2+]) transient were detected at 2.0 mM [Ca2+]o, although peak [Ca2+] and rate of [Ca2+] rise during caffeine contracture were greater in the trained state at 0.75 mM [Ca2+]o. We conclude that training induced a diminished myocyte contractile function under the conditions studied here and a more effective coupling of inward Ca2+ current to sarcoplasmic reticulum Ca2+ release at low [Ca2+]o, and that fura 2 and its loading vehicle DMSO significantly alter the intrinsic characteristics of myocyte contractile function and Ca2+ regulation.

Na+/Ca2+ exchange in neonatal rat heart cells: antisense inhibition and protein half-life

Cardiac Na+/Ca2+ exchanger (NCX) protein half-life (t1/2) and antisense knockdown were studied in primary cultured neonatal rat cardiomyocytes. Protein t1/2 was determined using [35S]methionine with a pulse-chase protocol. The 35S signal in NCX was identified by immunoprecipitation and Western blotting. The t1/2 of NCX protein was 33 h. Low concentrations (0.5 microM) of chimeric, phosphorothioated antisense oligodeoxynucleotides (AS-oligos) targeted to the region around the start codon of NCX1 transcript were used to knock down NCX protein and activity. Control myocytes (no oligos or scrambled oligos for at least 4 days) exhibited spontaneous Ca2+ transients (measured with fura 2). The sustained ("diastolic") Ca2+ concentration in the cytosol ([Ca2+]cyt) of control cells was unaffected by cyclopiazonic acid (CPA) plus caffeine (Caf), which promote depletion of sarcoplasmic reticular Ca2+ stores, but [Ca2+]cyt rose in control cells when external Na+ was removed. In contrast, approximately 60% of cells treated with AS-oligos for at least 4 days did not exhibit spontaneous Ca2+ transients or respond to Na+-free medium; however, CPA + Caf did induce a prolonged elevation in [Ca2+]cyt in these cells. In all cells, 50 mM K+ increased [Ca2+]cyt. NCX protein was reduced by approximately 50% in cells treated with AS-oligos for 7 days but was not reduced after only 2 days. These biochemical data are consistent with the physiological evidence of NCX knockdown in approximately 60% of cells.

The sodium-calcium ion membrane exchanger: physiologic significance and pharmacologic implications

The Na(+)-Ca2+ exchanger is a non-ATP-dependent protein that, under steady-state conditions, extrudes Ca2+ from the interior of the cell into the extracellular space via facilitated transport. The activity of the exchanger seems to be reduced in myocardial ischemia, leading to increased intracellular Ca2+ in the ischemic heart, which can result in arrhythmia, myocardial stunning, and necrosis. In contrast, congestive heart failure and myocardial hypertrophy are associated with increased exchanger activity and a decreased inotropic state. Pharmacologic agents are being developed to modulate sodium ion levels in the cell, which could enhance or reduce sodium-calcium exchange as needed in various pathophysiologic states. At this time there are no available drugs that act specifically on the Na(+)-Ca2+ exchanger itself. The exchanger has been cloned, and inhibitory peptides of the exchanger may soon be available for possible use in treatment of congestive heart failure.

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.

Contribution of reverse-mode sodium-calcium exchange to contractions in failing human left ventricular myocytes

OBJECTIVE

To examine the contribution of reverse mode sodium-calcium (Na-Ca) exchange to contractions in isolated left-ventricular myocytes from failing human heart.

METHODS

Low resistance patch pipettes were used to dialyze cells with Na-free or high-Na pipette solution ([Na]pipette = 0 and 20 mmol/L, respectively) to reduce or enhance Na-Ca exchange. Whole-cell membrane-potential, membrane-current and cell-shortening data were simultaneously acquired during whole-cell voltage clamp protocols. Thapsigargin (100 nmol/L) and nifedipine (1 mumol/L) were also used to inhibit sarcoplasmic reticulum (SR) Ca-ATPase and L-type Ca channels, respectively.

RESULTS

Two types of contractions were observed. Rapid phasic contractions were seen in both Na-free and high-Na cells. Slow tonic contractions were seen only in high-Na cells. Phasic contractions demonstrated bell-shaped voltage dependence over the voltage range that corresponds to the activity of the L-type Ca channel. Although the voltage dependence of phasic contractions were similar Na-free and high-Na cells, phasic contractions in high-Na cells were larger than phasic contractions in Na-free cells. Phasic contractions were sensitive to inhibition of SR Ca-ATPase and L-type Ca channels. Tonic contractions were not inhibited by either thapsigargin or nifedipine. In thapsigargin-treated high-Na cells, tonic contraction magnitude increased exponentially with test-potential.

CONCLUSIONS

The increases in phasic contraction magnitude observed in high-Na cells compared to Na-free cells were most likely due to increased SR Ca loading resulting from increased reverse-mode Na-Ca exchange. Our results also suggest that tonic contractions in high-Na cells were mediated by Ca entry via reverse-mode Na-Ca exchange and were not the result of either SR Ca release or L-type Ca channel activity.

Reduced outward K+ conductances generate depolarizing after-potentials in rat supraoptic nucleus neurones

1. Whole-cell patch clamp recordings were obtained from sixty-five rat supraoptic nucleus (SON) neurones in brain slices to investigate ionic mechanisms underlying depolarizing after-potentials (DAPs). When cells were voltage clamped around -58 mV, slow inward currents mediating DAPs (IDAP), evoked by three brief depolarizing pulses, had a peak of 17 +/- 1 pA (mean +/- S.E.M.) and lasted for 2.8 +/- 0.1 s. 2. No significant differences in the amplitude and duration were observed when one to three preceding depolarizing pulses were applied, although there was a tendency for twin pulses to evoke larger IDAP than a single pulse. The IDAP was absent when membrane potentials were more negative than -70 mV. In the range -70 to -50 mV, IDAP amplitudes and durations increased as the membrane became more depolarized, with an activation threshold of -65.7 +/- 0.7 mV. 3. IDAP with normal amplitude and duration could be evoked during the decay of a preceding IDAP. As frequencies of depolarizing pulses rose from 2 to 20 Hz, the times to peak IDAP amplitude were reduced but the amplitudes and durations did not change. 4. A consistent reduction in membrane conductance during the IDAP was observed in all SON neurones tested, and averaged 34.6 +/- 3.3%. Small hyperpolarizing pulses used to measure membrane conductances appeared not to disturb major ionic mechanisms underlying IDAP, since the slope and duration of IDAP with and without test pulses were similar. 5. The IDAP had an averaged reversal potential of -87.4 +/- 1.6 mV, which was close to the K+ equilibrium potential. An elevation in [K+]o reduced or abolished the IDAP, and shifted its reversal potential toward more positive levels. Perifusion of slices with 7.5-10 mM TEA, a K+ channel blocker, reversibly suppressed the IDAP. 6. Both Na+ and Ca2+ currents failed to induce an IDAP-like current during perifusion of slices with media containing high [K+]o or TEA. However, the IDAP was abolished by replacing external Ca2+ with Co2+, or replacing 82% of external Na+ with choline or Li+. Perifusion of slices with media containing 1-2 microM TTX also reduced IDAP by 55.5 +/- 9.0%. 7. These results suggest that the generation of DAPs in SON neurones mainly involves a reduction in outward K+ current(s), which probably has little or no inactivation and can be inhibited by [Ca2+]i transients, due to Ca2+ influx during action potentials and Ca2+ release from internal stores. Na+ influx might provide a permissive influence for Ca(2+)-induced reduction of K+ conductances and/or help to raise [Ca2+]i via reverse-mode Ca(2+)-Na+ exchange. Other conductances, making minor contributions to the IDAP, may also be involved.

The role of L-type Ca2+ current and Na+ current-stimulated Na/Ca exchange in triggering SR calcium release in guinea-pig cardiac ventricular myocytes

OBJECTIVE

This study examines the relative ability of sodium current (INa)-stimulated reverse mode Na/Ca exchange and the L-type calcium current (ICa) to trigger calcium-induced calcium release (CICR) in guinea-pig ventricular myocytes.

METHODS

Cytosolic Ca2+ transients were recorded from enzymatically dissociated guinea-pig ventricular myocytes using Indo-1. Macroscopic membrane currents were simultaneously recorded using the whole-cell patch-clamp technique.

RESULTS

At room temperature (22-25 degrees C) Ca2+ transients were associated with the activation of INa, ICa or INa plus ICa in combination. However, after ICa was blocked by verapamil (10 microM), no Ca2+ transient could be evoked by the activation of INa alone at either -40 or +5 mV. Similar results were obtained with 5 and 8 mM intracellular sodium, and when the temperature of the bathing solution was raised to 35 degrees C and cAMP (10 microM) added to the pipette solution.

CONCLUSIONS

From consideration of the relative magnitudes of the Ca2+ influx via ICa and Na/Ca exchange and thermodynamic considerations, we suggest that ICa is the major source of 'trigger' calcium for CICR (and cardiac contraction) under normal conditions. Although the Na/Ca exchanger was incapable of triggering CICR under the conditions of these experiments, we suggest that it may become more important when cytosolic Ca2+ is elevated, a condition which will also lead to decrease the amplitude of ICa.

Na(+)-Ca2+ exchange currents in cortical neurons: concomitant forward and reverse operation and effect of glutamate

Na(+)-Ca2+ exchanger-associated membrane currents were studied in cultured murine neocortical neurons, using whole-cell recording combined with intracellular perfusion. A net inward current specifically associated with forward (Na+(o)-Ca2+(i)) exchange was evoked at -40 mV by switching external 140 mM Li+ to 140 mM Na+. The voltage dependence of this current was consistent with that predicted for 3Na+:1Ca2+ exchange. As expected, the current depended on internal Ca2+, and could be blocked by intracellular application of the exchanger inhibitory peptide, XIP. Raising internal Na+ from 3 to 20 mM or switching the external solution from 140 mM Li+ to 30 mM Na+ activated outward currents, consistent with reverse (Na+(i)-Ca2+(o)) exchange. An external Ca2(+)-sensitive current was also identified as associated with reverse Na(+)-Ca2+ exchange based on its internal Na+ dependence and sensitivity to XIP. Combined application of external Na+ and Ca2+ in the absence of internal Na+ triggered a 3.3-fold larger inward current than the current activated in the presence of 3 mM internal Na+, raising the intriguing possibility that Na(+)-Ca2+ exchangers might concurrently operate in both the forward and the reverse direction, perhaps in different subcellular locations. With this idea in mind, we examined the effect of excitotoxic glutamate receptor activation on exchanger operation. After 3-5 min of exposure to 100-200 microM glutamate, the forward exchanger current was significantly increased even when external Na+ was reduced to 100 mM, and the external Ca2(+)-activated reverse exchanger current was eliminated.

Regulation of spontaneous transient outward potassium currents in human coronary arteries

BACKGROUND

Spontaneous transient outward potassium currents (STOCs) induce myogenic relaxation in small cerebral vessels. We found STOCs in human coronary artery vascular smooth muscle cells (VSMCs) and studied their regulation.

METHODS AND RESULTS

K+ currents were recorded in human coronary VSMCs by current- and voltage-clamp techniques. STOCs were recorded in the presence of 200 mumol/L Cd2+ and 10 mumol/L verapamil, which block voltage-dependent Ca2+ channels. STOCs were inhibited by iberiotoxin (100 nmol/L), a selective blocker of Ca(2+)-activated potassium channels (BKCa), and disappeared in a Ca(2+)-free bath. Iberiotoxin depolarized the VSMCs within 20 minutes from -44 +/- 7 to -18 +/- 5 mV (n = 17). The Ca2+ ionophore A23187 increased intracellular Ca2+ and stimulated whole-cell BKCa current. Depletion of Ca2+ from the sarcoplasmic reticulum with caffeine (4 mmol/L) abolished STOCs for several minutes. Ryanodine (50 mumol/L) transiently stimulated STOCs but then completely inhibited STOCs within 10 minutes. The firing frequency of STOCs was directly correlated with intracellular Na+ concentrations from 0 to 24 mmol/L. Lowering intracellular Na+ to zero abolished STOCs. We next gave monensin (30 mumol/L) to increase intracellular Na+. This maneuver resulted in an increase in whole-cell current fluctuations and STOCs. Monensin-induced STOCs were abolished by either lowering extracellular Ca2+ to zero or chelating Ca2+ intracellularly with BAPTA-AM (30 mumol/L).

CONCLUSIONS

STOCs resulted from BKCa activity and were dependent on extracellular Ca2+ but not significantly on voltage-dependent Ca2+ channels. STOCs were dependent on intracellular Na+ and intracellular calcium store refilling state. We suggest that Ca2+ entry into the cell through reverse-mode Na+/Ca2+ exchange determines calcium store refilling, which in turn regulates STOC generation in human coronary VSMCs.

The heart and development

The perspective from which the developing heart is viewed can lead to differing conclusions about the effects of development on cardiac function. The hearts of the embryo, fetus and adult, viewed from a global perspective, sustain the circulation through the same basic mechanisms of developing pressure and ejecting blood. The failure of the embryonic heart to perform these tasks results in growth failure, edema, and embryonic death, just as in the infant and adult such failure results in premature death. Furthermore, from the viewpoint of gross anatomy, following embryonic morphogenesis, the developing and adult hearts appear in general to be structurally similar, differing only in size and mass. However, a closer view shows, in the molecular and structural makeup of the myocardium, richly complex changes that can modulate the basic physiological properties of the cardiac myocyte. This article focuses on how these changes and the effects of birth and development alter ventricular function.

Na+/Ca2+ exchange currents and SR Ca2+ contents in postinfarction myocytes

Myocytes isolated from rat hearts 3 wk after myocardial infarction (MI) had lower peak cytosolic free Ca2+ concentration ([Ca2+]i) and reduced maximal extent of cell shortening during contraction, but Ca2+ entry via L-type Ca2+ channels was normal. In the current study using whole cell patch-clamp technique, reverse Na+/Ca2+ exchange current (INa/Ca; 3 Na+ out:1 Ca2+ in) was measured in myocytes in which Na+, K+, and Ca2+ currents were blocked or minimized. Steady-state outward currents measured under these conditions increased with depolarization or with elevation of extracellular Ca2+ concentration ([Ca2+]o) from 1.8 to 5.0 mM, but were inhibited by 5 mM Ni2+ or by reduction of [Ca2+]i to near zero. In addition, reduction of cytosolic free Na+ concentration or of [Ca2+]i also decreased the amplitude of the outward current. These characteristics indicate the outward current was INa/Ca operating in reverse mode. Reverse INa/Ca was significantly lower in MI myocytes, especially at more positive voltages. In addition, sarcoplasmic reticulum (SR)-releasable Ca2+ content as estimated by integrating forward INa/Ca during caffeine-induced SR Ca2+ release was also significantly lower in MI myocytes. Depressed Na+/Ca2+ exchange activity may contribute to abnormal [Ca2+]i dynamics in MI myocytes.

Evidence for functional relevance of an enhanced expression of the Na(+)-Ca2+ exchanger in failing human myocardium

BACKGROUND

The present study aimed at investigating the expression of the Na(+)-Ca2+ exchanger and its functional role in human failing myocardium.

METHODS AND RESULTS

Na(+)-Ca2+ exchanger mRNA and protein levels were examined in nonfailing (NF, n = 8) and failing human myocardium (New York Heart Association functional class IV) with idiopathic dilated cardiomyopathy (DCM, n = 8) or ischemic heart disease (ICM, n = 6). The inotropic effect of the Na+ channel activator BDF 9148 was determined in electrically driven left ventricular papillary muscle strip preparations (1 Hz, 37 degrees C) from nonfailing (n = 8) and failing (n = 8) human hearts. Na(+)-Ca2+ exchanger mRNA levels were significantly increased, by 79% (P < .001) in DCM and by 58% (P < .01) in ICM compared with NF; protein levels increased by 36% (P < .001) and by 20% (P < .05), respectively. BDF 9148 increased the force of contraction concentration dependently, with a similar maximal effect in NYHA class IV and NF, but was more potent in NYHA class IV as demonstrated by a significantly smaller (P < .01) EC50 value (NYHA class IV, 0.18 [0.16 to 0.22] mumol/L; NF, 1.65 [1.3 to 3.0] mumol/L). In NYHA class IV, BDF 9148 (0.1 mumol/L) restored the positive force-frequency relationship and reduced the frequency-dependent increase in diastolic tension in relation to force of contraction.

CONCLUSIONS

The increased expression of the Na(+)-Ca2+ exchanger is a possible explanation for the increased inotropic potency of the Na+ channel activator BDF 9148 in failing human myocardium. The increase in exchanger molecules could be of functional relevance for the modulation of cardiac contractility by agents that increase the intracellular Na+ concentration. Enhancement of Na(+)-Ca2+ exchanger activity might be a powerful mechanism for increasing cardiac contractility in chronic heart failure.