The role of sarcolemmal Ca2+-ATPase in the regulation of resting calcium concentration in rat ventricular myocytes

Sex- and age-dependent susceptibility to ventricular arrhythmias in the rat heart ex vivo

The incidence of life-threatening ventricular arrhythmias, the most common cause of sudden cardiac death (SCD), depends largely on the arrhythmic substrate that develops in the myocardium during the aging process. There is a large deficit of comparative studies on the development of this substrate in both sexes, with a particular paucity of studies in females. To identify the substrates of arrhythmia, fibrosis, cardiomyocyte hypertrophy, mitochondrial density, oxidative stress, antioxidant defense and intracellular Ca2+ signaling in isolated cardiomyocytes were measured in the hearts of 3- and 24-month-old female and male rats. Arrhythmia susceptibility was assessed in ex vivo perfused hearts after exposure to isoproterenol (ISO) and hydrogen peroxide (H2O2). The number of ventricular premature beats (PVBs), ventricular tachycardia (VT) and ventricular fibrillation (VF) episodes, as well as intrinsic heart rate, QRS and QT duration, were measured in ECG signals recorded from the surfaces of the beating hearts. After ISO administration, VT/VFs were formed only in the hearts of males, mainly older ones. In contrast, H2O2 led to VT/VF formation in the hearts of rats of both sexes but much more frequently in older males. We identified several components of the arrhythmia substrate that develop in the myocardium during the aging process, including high spontaneous ryanodine receptor activity in cardiomyocytes, fibrosis of varying severity in different layers of the myocardium (nonheterogenic fibrosis), and high levels of oxidative stress as measured by nitrated tyrosine levels. All of these elements appeared at a much greater intensity in male individuals during the aging process. On the other hand, in aging females, antioxidant defense at the level of H2O2 detoxification, measured as glutathione peroxidase expression, was weaker than that in males of the same age. We showed that sex has a significant effect on the development of an arrhythmic substrate during aging. This substrate determines the incidence of life-threatening ventricular arrhythmias in the presence of additional stimuli with proarrhythmic potential, such as catecholamine stimulation or oxidative stress, which are constant elements in the pathomechanism of most cardiovascular diseases.

Reduced Plasma-Membrane Calcium ATPase Activity and Extracellular Acidification Trigger Presynaptic Homeostatic Potentiation at the Mouse Neuromuscular Junction

At the vertebrate neuromuscular junction (NMJ), presynaptic homeostatic potentiation (PHP) refers to an increase in neurotransmitter release that restores the strength of synaptic transmission following a blockade of nicotinic acetylcholine receptors (nAChRs). Mechanisms informing the presynaptic terminal of the loss of postsynaptic receptivity remain poorly understood. Previous research at the mouse NMJ suggests that extracellular protons may function as a retrograde signal that triggers an upregulation of neurotransmitter output (measured by quantal content, QC) through the activation of acid-sensing ion channels (ASICs). We further investigated the pH-dependency of PHP in an ex-vivo mouse muscle preparation. We observed that increasing the buffering capacity of the perfusion saline with HEPES abolishes PHP and that acidifying the saline from pH 7.4 to pH 7.2-7.1 increases QC, demonstrating the necessity and sufficiency of extracellular acidification for PHP. We then sought to uncover how the blockade of nAChRs leads to the pH decrease. Plasma-membrane calcium ATPase (PMCA), a calcium-proton antiporter, is known to alkalize the synaptic cleft following neurotransmission in a calcium-dependent manner. We hypothesize that since nAChR blockade reduces postsynaptic calcium entry, it also reduces the alkalizing activity of the PMCA, thereby causing acidosis, ASIC activation, and QC upregulation. In line with this hypothesis, we found that pharmacological inhibition of the PMCA with carboxyeosin induces QC upregulation and that this effect requires functional ASICs. We also demonstrated that muscles pre-treated with carboxyeosin fail to generate PHP. These findings suggest that reduced PMCA activity causes presynaptic homeostatic potentiation by activating ASICs at the mouse NMJ.

InsP3R-RyR channel crosstalk augments sarcoplasmic reticulum Ca2+ release and arrhythmogenic activity in post-MI pig cardiomyocytes

Ca²⁺ transients (CaT) underlying cardiomyocyte (CM) contraction require efficient Ca²⁺ coupling between sarcolemmal Ca²⁺ channels and sarcoplasmic reticulum (SR) ryanodine receptor Ca²⁺ channels (RyR) for their generation; reduced coupling in disease contributes to diminished CaT and arrhythmogenic Ca²⁺ events. SR Ca²⁺ release also occurs via inositol 1,4,5-trisphosphate receptors (InsP₃R) in CM. While this pathway contributes negligeably to Ca²⁺ handling in healthy CM, rodent studies support a role in altered Ca²⁺ dynamics and arrhythmogenic Ca²⁺ release involving InsP₃R crosstalk with RyRs in disease. Whether this mechanism persists in larger mammals with lower T-tubular density and coupling of RyRs is not fully resolved. We have recently shown an arrhythmogenic action of InsP₃-induced Ca²⁺ release (IICR) in end stage human heart failure, often associated with underlying ischemic heart disease (IHD). How IICR contributes to early stages of disease is however not determined but highly relevant. To access this stage, we chose a porcine model of IHD, which shows substantial remodelling of the area adjacent to the infarct. In cells from this region, IICR preferentially augmented Ca²⁺ release from non-coupled RyR clusters that otherwise showed delayed activation during the CaT. IICR in turn synchronised Ca²⁺ release during the CaT but also induced arrhythmogenic delayed afterdepolarizations and action potentials. Nanoscale imaging identified co-clustering of InsP₃Rs and RyRs, thereby allowing Ca²⁺-mediated channel crosstalk. Mathematical modelling supported and further delineated this mechanism of enhanced InsP₃R-RyRs coupling in MI. Our findings highlight the role of InsP₃R-RyR channel crosstalk in Ca²⁺ release and arrhythmia during post-MI remodelling.

Regulation of APD and Force by the Na+/Ca2+ Exchanger in Human-Induced Pluripotent Stem Cell-Derived Engineered Heart Tissue

The physiological importance of NCX in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is not well characterized but may depend on the relative strength of the current, compared to adult cardiomyocytes, and on the exact spatial arrangement of proteins involved in Ca2+ extrusion. Here, we determined NCX currents and its contribution to action potential and force in hiPSC-CMs cultured in engineered heart tissue (EHT). The results were compared with data from rat and human left ventricular tissue. The NCX currents in hiPSC-CMs were larger than in ventricular cardiomyocytes isolated from human left ventricles (1.3 ± 0.2 pA/pF and 3.2 ± 0.2 pA/pF for human ventricle and EHT, respectively, p < 0.05). SEA0400 (10 µM) markedly shortened the APD90 in EHT (by 26.6 ± 5%, p < 0.05) and, to a lesser extent, in rat ventricular tissue (by 10.7 ± 1.6%, p < 0.05). Shortening in human left ventricular preparations was small and not different from time-matched controls (TMCs; p > 0.05). Force was increased by the NCX block in rat ventricle (by 31 ± 5.4%, p < 0.05) and EHT (by 20.8 ± 3.9%, p < 0.05), but not in human left ventricular preparations. In conclusion, hiPSC-CMs possess NCX currents not smaller than human left ventricular tissue. Robust NCX block-induced APD shortening and inotropy makes EHT an attractive pharmacological model.

The α2-isoform of the Na+/K+-ATPase protects against pathological remodeling and β-adrenergic desensitization after myocardial infarction

The role of the Na⁺/K⁺-ATPase (NKA) in heart failure associated with myocardial infarction (MI) is poorly understood. The elucidation of its precise function is hampered by the existence of two catalytic NKA isoforms (NKA-α1 and NKA-α2). Our aim was to analyze the effects of an increased NKA-α2 expression on functional deterioration and remodeling during long-term MI treatment in mice and its impact on Ca²⁺ handling and inotropy of the failing heart. Wild-type (WT) and NKA-α2 transgenic (TG) mice (TG-α2) with a cardiac-specific overexpression of NKA-α2 were subjected to MI injury for 8 wk. As examined by echocardiography, gravimetry, and histology, TG-α2 mice were protected from functional deterioration and adverse cardiac remodeling. Contractility and Ca²⁺ transients (Fura 2-AM) in cardiomyocytes from MI-treated TG-α2 animals showed reduced Ca²⁺ amplitudes during pacing or after caffeine application. Ca²⁺ efflux in cardiomyocytes from TG-α2 mice was accelerated and diastolic Ca²⁺ levels were decreased. Based on these alterations, sarcomeres exhibited an enhanced sensitization and thus increased contractility. After the acute stimulation with the β-adrenergic agonist isoproterenol (ISO), cardiomyocytes from MI-treated TG-α2 mice responded with increased sarcomere shortenings and Ca²⁺ peak amplitudes. This positive inotropic response was absent in cardiomyocytes from WT-MI animals. Cardiomyocytes with NKA-α2 as predominant isoform minimize Ca²⁺ cycling but respond to β-adrenergic stimulation more efficiently during chronic cardiac stress. These mechanisms might improve the β-adrenergic reserve and contribute to functional preservation in heart failure.NEW & NOTEWORTHY Reduced systolic and diastolic calcium levels in cardiomyocytes from NKA-α2 transgenic mice minimize the desensitization of the β-adrenergic signaling system. These effects result in an improved β-adrenergic reserve and prevent functional deterioration and cardiac remodeling.

Novel in vivo and in vitro mechanisms of positive inotropic effect of atractylodin

This study was to investigate the inotropic effect of atractylodin and its underlying mechanism. The cardiac pressure-volume loop (P-V loop), Langendroff-perfused isolated rat heart, patch-clamp, Ca2+ transient and western blot techniques were used. The results demonstrated that atractylodin (3 mg/kg, ip) remarkably increased the left ventricular stroke work, cardiac output, stroke volume, heart rate, ejection fraction, end-systolic pressure, peak rates of rise and fall of left ventricular pressures (+dP/dtmax , -dP/dtmax ), the slopes of end-systolic pressure-volume relationship (also named as end-systolic elastance, Ees) and reducing end-systolic volume and end-diastolic volume in the in vivo rat study. Also, atractylodin (3 mg/kg, ip) significantly decreased diastolic blood pressure and the arterial elastance (Ea) without significant systolic blood pressure change. In addition, atractylodin (0.1, 1, 10 µmol/L) also increased the isolated rat heart left ventricular developed pressure which is the difference between the systolic and diastolic pressure in non-pacing and pacing modes. Furthermore, JMV-2959 (1 μmol/L), a ghrelin receptor unbiased antagonist, blocked the increased left ventricular developed pressure of atractylodin in isolated rat hearts. Finally, atractylodin (5 µmol/L) increased the amplitude of Ca2+ transient by enhancing SERCA2a activity, the sarcoplasmic reticulum Ca2+ content and the phosphorylation of phospholamban at 16-serine. These results demonstrated that atractylodin had a positive inotropic effect by enhancing SERCA2a activity which might be mediated by acting ghrelin receptor in myocardium. In conclusion, atractylodin which had the positive inotropic effect and decreased diastolic blood pressure might serve as an agent for the treatment of heart failure in clinical settings.

Increased sarcoplasmic/endoplasmic reticulum calcium ATPase 2a activity underlies the mechanism of the positive inotropic effect of ivabradine

NEW FINDINGS

What is the central question of this study? The therapeutic effect of ivabradine on patients with chronic heart failure and chronic stable angina pectoris is mediated through a reduction in heart rate: what are the haemodynamic characteristics and the mechanism of the inotropic effect? What is the main finding and its importance? Ivabradine has a positive inotropic effect and lowers the heart rate both in vivo and in vitro. These effects are likely mediated by ivabradine's significant increase of the fast component rate constant mediated by sarcoplasmic/endoplasmic reticulum calcium ATPase 2a and decrease of the slow component rate constant that is mediated by the Na⁺ /Ca²⁺ exchanger and sarcolemmal Ca²⁺ -ATPase during the Ca²⁺ transient decay phase.

ABSTRACT

Ivabradine's therapeutic effect is mediated by a reduction of the heart rate; however, its haemodynamic characteristics and the mechanism of its inotropic effect are poorly understood. We aimed to investigate the positive inotropic effect of ivabradine and its underlying mechanism. The results demonstrated that ivabradine increased the positive inotropy of the rat heart in vivo by increasing the stroke work, cardiac output, stroke volume, end-diastolic volume, end-systolic pressure, ejection fraction, ±dP/dt_max , left ventricular end-systolic elastance and systolic blood pressure without altering the diastolic blood pressure and arterial elastance. This inotropic effect was observed in both non-paced and paced rat isolated heart. Ivabradine increased the Ca²⁺ transient amplitude and the reuptake rates of sarcoplasmic/endoplasmic reticulum calcium ATPase 2a (SERCA2a), lowered the diastolic Ca²⁺ level and suppressed the combined extrusion rate of the Na⁺ /Ca²⁺ exchanger and the sarcolemmal Ca²⁺ -ATPase. In addition, ivabradine widened the action potential duration, hyperpolarized the resting membrane potential, increased sarcoplasmic reticulum Ca²⁺ content and reduced Ca²⁺ leak. Overall, ivabradine had a positive inotropic effect brought about by enhanced SERCA2a activity, which might be mediated by increased phospholamban phosphorylation. The positive inotropic effect along with the lowered heart rate underlies ivabradine's therapeutic effect in heart failure.

Rolipram, a PDE4 Inhibitor, Enhances the Inotropic Effect of Rat Heart by Activating SERCA2a

This study was designed to investigate the hemodynamic effect of rolipram, a phosphodiesterase type 4 (PDE4) inhibitor, in normal rat hearts both in vivo and in vitro and its underlying mechanism. The pressure-volume loop, isolated heart, and Ca²⁺ transients triggered by field stimulation or caffeine were used to analyze the hemodynamic mechanism of rolipram. The results demonstrated that rolipram (3 mg/kg, ip) significantly increased the in vivo rat heart contractility by enhancing stroke work, cardiac output, stroke volume, end-systolic volume, end-diastolic volume, end-systolic pressure, heart rate, ejection fraction, peak rate of rise of left pressure (+dp/dt_max), the slopes of end-systolic pressure-volume relationship (slope of ESPVR) named as left ventricular end-systolic elastance, and reduced the slopes of end-diastolic pressure-volume relationship (slope of EDPVR). Meanwhile, the systolic blood pressure, diastolic blood pressure, and pulse pressure were significantly enhanced by rolipram. Also, rolipram deviated normal ventricular-arterial coupling without changing the arterial elastance. Furthermore, rolipram (0.1, 1, 10 μM) also exerted positive inotropic effect in isolated rat hearts by increasing the left ventricular development pressure, and +dp/dt_max in non-paced and paced modes. Rolipram (10 μM) increased the SERCA2a activity, Ca²⁺ content, and Ca²⁺ leak rate without changing diastolic Ca²⁺ level. Rolipram had significant positive inotropic effect with less effect on peripheral vascular elastance and its underlying mechanism was mediated by increasing SERCA2a activity. PDE4 inhibition by rolipram resulted in a positive inotropic effect and might serve as a target for developing agents for the treatment of heart failure in clinical settings.

Dipeptidyl peptidase-4 independent cardiac dysfunction links saxagliptin to heart failure

Saxagliptin treatment has been associated with increased rate of hospitalization for heart failure in type 2 diabetic patients, though the underlying mechanism(s) remain elusive. To address this, we assessed the effects of saxagliptin on human atrial trabeculae, guinea pig hearts and cardiomyocytes. We found that the primary target of saxagliptin, dipeptidyl peptidase-4, is absent in cardiomyocytes, yet saxagliptin internalized into cardiomyocytes and impaired cardiac contractility via inhibition of the Ca²⁺/calmodulin-dependent protein kinase II-phospholamban-sarcoplasmic reticulum Ca²⁺-ATPase 2a axis and Na⁺-Ca²⁺ exchanger function in Ca²⁺ extrusion. This resulted in reduced sarcoplasmic reticulum Ca²⁺ content, diastolic Ca²⁺ overload, systolic dysfunction and impaired contractile force. Furthermore, saxagliptin reduced protein kinase C-mediated delayed rectifier K⁺ current that prolonged action potential duration and consequently QTc interval. Importantly, saxagliptin aggravated pre-existing cardiac dysfunction induced by ischemia/reperfusion injury. In conclusion, our novel results provide mechanisms for the off-target deleterious effects of saxagliptin on cardiac function and support the outcome of SAVOR-TIMI 53 trial that linked saxagliptin with the risk of heart failure.

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.

The Plasma Membrane Calcium ATPases and Their Role as Major New Players in Human Disease

The Ca²⁺ extrusion function of the four mammalian isoforms of the plasma membrane calcium ATPases (PMCAs) is well established. There is also ever-increasing detail known of their roles in global and local Ca²⁺ homeostasis and intracellular Ca²⁺ signaling in a wide variety of cell types and tissues. It is becoming clear that the spatiotemporal patterns of expression of the PMCAs and the fact that their abundances and relative expression levels vary from cell type to cell type both reflect and impact on their specific functions in these cells. Over recent years it has become increasingly apparent that these genes have potentially significant roles in human health and disease, with PMCAs1-4 being associated with cardiovascular diseases, deafness, autism, ataxia, adenoma, and malarial resistance. This review will bring together evidence of the variety of tissue-specific functions of PMCAs and will highlight the roles these genes play in regulating normal physiological functions and the considerable impact the genes have on human disease.

Sarcolemmal distribution of ICa and INCX and Ca2+ autoregulation in mouse ventricular myocytes

This study shows that in contrast to the rat, mouse ventricular Na⁺/Ca²⁺ exchange current density is lower in the t-tubules than in the surface sarcolemma and Ca²⁺ current is predominantly located in the t-tubules. As a consequence, the t-tubules play a role in recovery (autoregulation) from reduced, but not increased, sarcoplasmic reticulum Ca²⁺ release.

The balance of Ca²⁺ influx and efflux regulates the Ca²⁺ load of cardiac myocytes, a process known as autoregulation. Previous work has shown that Ca²⁺ influx, via L-type Ca²⁺ current (I_Ca), and efflux, via the Na⁺/Ca²⁺ exchanger (NCX), occur predominantly at t-tubules; however, the role of t-tubules in autoregulation is unknown. Therefore, we investigated the sarcolemmal distribution of I_Ca and NCX current (I_NCX), and autoregulation, in mouse ventricular myocytes using whole cell voltage-clamp and simultaneous Ca²⁺ measurements in intact and detubulated (DT) cells. In contrast to the rat, I_NCX was located predominantly at the surface membrane, and the hysteresis between I_NCX and Ca²⁺ observed in intact myocytes was preserved after detubulation. Immunostaining showed both NCX and ryanodine receptors (RyRs) at the t-tubules and surface membrane, consistent with colocalization of NCX and RyRs at both sites. Unlike I_NCX, I_Ca was found predominantly in the t-tubules. Recovery of the Ca²⁺ transient amplitude to steady state (autoregulation) after application of 200 µM or 10 mM caffeine was slower in DT cells than in intact cells. However, during application of 200 µM caffeine to increase sarcoplasmic reticulum (SR) Ca²⁺ release, DT and intact cells recovered at the same rate. It appears likely that this asymmetric response to changes in SR Ca²⁺ release is a consequence of the distribution of I_Ca, which is reduced in DT cells and is required to refill the SR after depletion, and NCX, which is little affected by detubulation, remaining available to remove Ca²⁺ when SR Ca²⁺ release is increased.

NEW & NOTEWORTHY This study shows that in contrast to the rat, mouse ventricular Na⁺/Ca²⁺ exchange current density is lower in the t-tubules than in the surface sarcolemma and Ca²⁺ current is predominantly located in the t-tubules. As a consequence, the t-tubules play a role in recovery (autoregulation) from reduced, but not increased, sarcoplasmic reticulum Ca²⁺ release.

Omega-3 Fatty Acids Do Not Protect Against Arrhythmias in Acute Nonreperfused Myocardial Infarction Despite Some Antiarrhythmic Effects

Ventricular arrhythmias are an important cause of mortality in the acute myocardial infarction (MI). To elucidate the effect of the omega-3 polyunsaturated fatty acids (PUFAs) on ventricular arrhythmias in acute nonreperfused MI, rats were fed with normal or eicosapentaenoic acid (EPA) or docosahexaenoic acid (DHA)-enriched diet for 3 weeks. Subsequently the rats were subjected to either MI induction or sham operation. ECG was recorded for 6 h after the operation and episodes of ventricular tachycardia/fibrillation (VT/VF) were identified. Six hours after MI epicardial monophasic action potentials (MAPs) were recorded, cardiomyocyte Ca(2+) handling was assessed and expression of proteins involved in Ca(2+) turnover was studied separately in non-infarcted left ventricle wall and infarct borderzone. EPA and DHA had no effect on occurrence of post-MI ventricular arrhythmias or mortality. Nevertheless, DHA but not EPA prevented Ca(2+) overload in LV cardiomiocytes and improved rate of Ca(2+) transient decay, protecting PMCA and SERCA function. Moreover, both EPA and DHA prevented MI-induced hyperphosphorylation of ryanodine receptors (RyRs) as well as dispersion of action potential duration (APD) in the left ventricular wall. In conclusion, EPA and DHA have no antiarrhythmic effect in the non-reperfused myocardial infarction in the rat, although these omega-3 PUFAs and DHA in particular exhibit several potential antiarrhythmic effects at the subcellular and tissue level, that is, prevent MI-induced abnormalities in Ca(2+) handling and APD dispersion. In this context further studies are needed to see if these potential antiarrhythmic effects could be utilized in the clinical setting. J. Cell. Biochem. 117: 2570-2582, 2016. © 2016 Wiley Periodicals, Inc.

Development and characterization of a novel fluorescent indicator protein PMCA4-GCaMP2 in cardiomyocytes

Isoform 4 of the plasma membrane calcium/calmodulin dependent ATPase (PMCA4) has recently emerged as an important regulator of several key pathophysiological processes in the heart, such as contractility and hypertrophy. However, direct monitoring of PMCA4 activity and assessment of calcium dynamics in its vicinity in cardiomyocytes are difficult due to the lack of molecular tools. In this study, we developed novel calcium fluorescent indicators by fusing the GCaMP2 calcium sensor to the N-terminus of PMCA4 to generate the PMCA4-GCaMP2 fusion molecule. We also identified a novel specific inhibitor of PMCA4, which might be useful for studying the role of this molecule in cardiomyocytes and other cell types. Using an adenoviral system we successfully expressed PMCA4-GCaMP2 in both neonatal and adult rat cardiomyocytes. This fusion molecule was correctly targeted to the plasma membrane and co-localised with caveolin-3. It could monitor signal oscillations in electrically stimulated cardiomyocytes. The PMCA4-GCaMP2 generated a higher signal amplitude and faster signal decay rate compared to a mutant inactive PMCA4(mut)GCaMP2 fusion protein, in electrically stimulated neonatal and adult rat cardiomyocytes. A small molecule library screen enabled us to identify a novel selective inhibitor for PMCA4, which we found to reduce signal amplitude of PMCA4-GCaMP2 and prolong the time of signal decay (Tau) to a level comparable with the signal generated by PMCA4(mut)GCaMP2. In addition, PMCA4-GCaMP2 but not the mutant form produced an enhanced signal in response to β-adrenergic stimulation. Together, the PMCA4-GCaMP2 and PMCA4(mut)GCaMP2 demonstrate calcium dynamics in the vicinity of the pump under active or inactive conditions, respectively. In summary, the PMCA4-GCaMP2 together with the novel specific inhibitor provides new means with which to monitor calcium dynamics in the vicinity of a calcium transporter in cardiomyocytes and may become a useful tool to further study the biological functions of PMCA4. In addition, similar approaches could be useful for studying the activity of other calcium transporters during excitation-contraction coupling in the heart.

Cardiac Na+-Ca2+ exchanger: dynamics of Ca2+-dependent activation and deactivation in intact myocytes

Cardiac Na(+)-Ca(2+) exchange (NCX) activity is regulated by [Ca(2+)]i. The physiological role and dynamics of this process in intact cardiomyocytes are largely unknown. We examined NCX Ca(2+) activation in intact rabbit and mouse cardiomyocytes at 37°C. Sarcoplasmic reticulum (SR) function was blocked, and cells were bathed in 2 mm Ca(2+). We probed Ca(2+) activation without voltage clamp by applying Na(+)-free (0 Na(+)) solution for 5 s bouts, repeated each 10 s, which should evoke [Ca(2+)]i transients due to Ca(2+) influx via NCX. In rested rabbit myocytes, Ca(2+) influx was undetectable even after 0 Na(+) applications were repeated for 2-5 min or more, suggesting that NCX was inactive. After external electric field stimulation pulses were applied, to admit Ca(2+) via L-type Ca(2+) channels, 0 Na(+) bouts activated Ca(2+) influx efficaciously, indicating that NCX had become active. Calcium activation increased with more field pulses, reaching a maximum typically after 15-20 pulses (1 Hz). At rest, NCX deactivated with a time constant typically of 20-40 s. An increase in [Na(+)]i, either in rabbit cardiomyocytes as a result of inhibition of Na(+)-K(+) pumping, or in mouse cardiomyocytes where normal [Na(+)]i is higher vs. rabbit, sensitized NCX to self-activation by 0 Na(+) bouts. In experiments with the SR functional but initially empty, the activation time course was slowed. It is possible that the SR initially accumulated Ca(2+) that would otherwise cause activation. We modelled Ca(2+) activation as a fourth-order highly co-operative process ([Ca]i required for half-activation K0.5act = 375 nm), with dynamics severalfold slower than the cardiac cycle. We incorporated this NCX model into an established ventricular myocyte model, which allowed us to predict responses to twitch stimulation in physiological conditions with the SR intact. Model NCX fractional activation increased from 0.1 to 1.0 as the frequency was increased from 0.2 to 2 Hz. By adjusting Ca(2+) activation on a multibeat time scale, NCX might better maintain a stable long-term Ca(2+) balance while contributing to the ability of myocytes to produce Ca(2+) transients over a wide range of intensity.

Enhanced sarcoplasmic reticulum Ca2+ leak and increased Na+-Ca2+ exchanger function underlie delayed afterdepolarizations in patients with chronic atrial fibrillation

BACKGROUND

Delayed afterdepolarizations (DADs) carried by Na(+)-Ca(2+)-exchange current (I(NCX)) in response to sarcoplasmic reticulum (SR) Ca(2+) leak can promote atrial fibrillation (AF). The mechanisms leading to delayed afterdepolarizations in AF patients have not been defined.

METHODS AND RESULTS

Protein levels (Western blot), membrane currents and action potentials (patch clamp), and [Ca(2+)](i) (Fluo-3) were measured in right atrial samples from 76 sinus rhythm (control) and 72 chronic AF (cAF) patients. Diastolic [Ca(2+)](i) and SR Ca(2+) content (integrated I(NCX) during caffeine-induced Ca(2+) transient) were unchanged, whereas diastolic SR Ca(2+) leak, estimated by blocking ryanodine receptors (RyR2) with tetracaine, was ≈50% higher in cAF versus control. Single-channel recordings from atrial RyR2 reconstituted into lipid bilayers revealed enhanced open probability in cAF samples, providing a molecular basis for increased SR Ca(2+) leak. Calmodulin expression (60%), Ca(2+)/calmodulin-dependent protein kinase-II (CaMKII) autophosphorylation at Thr287 (87%), and RyR2 phosphorylation at Ser2808 (protein kinase A/CaMKII site, 236%) and Ser2814 (CaMKII site, 77%) were increased in cAF. The selective CaMKII blocker KN-93 decreased SR Ca(2+) leak, the frequency of spontaneous Ca(2+) release events, and RyR2 open probability in cAF, whereas protein kinase A inhibition with H-89 was ineffective. Knock-in mice with constitutively phosphorylated RyR2 at Ser2814 showed a higher incidence of Ca(2+) sparks and increased susceptibility to pacing-induced AF compared with controls. The relationship between [Ca(2+)](i) and I(NCX) density revealed I(NCX) upregulation in cAF. Spontaneous Ca(2+) release events accompanied by inward I(NCX) currents and delayed afterdepolarizations/triggered activity occurred more often and the sensitivity of resting membrane voltage to elevated [Ca(2+)](i) (diastolic [Ca(2+)](i)-voltage coupling gain) was higher in cAF compared with control.

CONCLUSIONS

Enhanced SR Ca(2+) leak through CaMKII-hyperphosphorylated RyR2, in combination with larger I(NCX) for a given SR Ca(2+) release and increased diastolic [Ca(2+)](i)-voltage coupling gain, causes AF-promoting atrial delayed afterdepolarizations/triggered activity in cAF patients.

Calcium signaling in closely related protozoan groups (Alveolata): non-parasitic ciliates (Paramecium, Tetrahymena) vs. parasitic Apicomplexa (Plasmodium, Toxoplasma)

The importance of Ca2+-signaling for many subcellular processes is well established in higher eukaryotes, whereas information about protozoa is restricted. Recent genome analyses have stimulated such work also with Alveolates, such as ciliates (Paramecium, Tetrahymena) and their pathogenic close relatives, the Apicomplexa (Plasmodium, Toxoplasma). Here we compare Ca2+ signaling in the two closely related groups. Acidic Ca2+ stores have been characterized in detail in Apicomplexa, but hardly in ciliates. Two-pore channels engaged in Ca2+-release from acidic stores in higher eukaryotes have not been stingently characterized in either group. Both groups are endowed with plasma membrane- and endoplasmic reticulum-type Ca2+-ATPases (PMCA, SERCA), respectively. Only recently was it possible to identify in Paramecium a number of homologs of ryanodine and inositol 1,3,4-trisphosphate receptors (RyR, IP3R) and to localize them to widely different organelles participating in vesicle trafficking. For Apicomplexa, physiological experiments suggest the presence of related channels although their identity remains elusive. In Paramecium, IP3Rs are constitutively active in the contractile vacuole complex; RyR-related channels in alveolar sacs are activated during exocytosis stimulation, whereas in the parasites the homologous structure (inner membrane complex) may no longer function as a Ca2+ store. Scrutinized comparison of the two closely related protozoan phyla may stimulate further work and elucidate adaptation to parasitic life. See also "Conclusions" section.

Preserved cardiomyocyte function and altered desmin pattern in transgenic mouse model of dilated cardiomyopathy

Taking advantage of the unique model of slowly developing dilated cardiomyopathy in mice with cardiomyocyte-specific transgenic overexpression of activated Gαq protein (Tgαq*44 mice) we analyzed the contribution of the cardiomyocyte malfunction, fibrosis and cytoskeleton remodeling to the development of heart failure in this model. Left ventricular (LV) in vivo function, myocardial fibrosis, cytoskeletal proteins expression and distribution, Ca(2+) handling and contractile function of isolated cardiomyocytes were evaluated at the stages of the early, compensated, and late, decompensated heart failure in 4-, 12- and 14-month-old Tgαq*44 mice, respectively, and compared to age-matched wild-type FVB mice. In the 4-month-old Tgαq*44 mice significant myocardial fibrosis, moderate myocyte hypertrophy and increased expression of regularly arranged and homogenously distributed desmin accompanied by increased phosphorylation of desmin chaperone protein, αB-crystallin, were found. Cardiomyocyte shortening, Ca(2+) handling and LV function were not altered. At 12 and 14 months of age, Tgαq*44 mice displayed progressive deterioration of the LV function. The contractile performance of isolated myocytes was still preserved, and the amplitude of Ca(2+) transients was even increased probably due to impairment of Na(+)/Ca(2+) exchanger function, while fibrosis was more extensive than in younger mice. Moreover, substantial disarrangement of desmin distribution accompanied by decreasing phosphorylation of αB-crystallin appeared. In Tgαq*44 mice disarrangement of desmin, at least partly related to inadequate phosphorylation of αB-crystallin seems to be importantly involved in the progressive deterioration of contractile heart function.

Extracellular pH dynamics of retinal horizontal cells examined using electrochemical and fluorometric methods

Extracellular H(+) has been hypothesized to mediate feedback inhibition from horizontal cells onto vertebrate photoreceptors. According to this hypothesis, depolarization of horizontal cells should induce extracellular acidification adjacent to the cell membrane. Experiments testing this hypothesis have produced conflicting results. Studies examining carp and goldfish horizontal cells loaded with the pH-sensitive dye 5-hexadecanoylaminofluorescein (HAF) reported an extracellular acidification on depolarization by glutamate or potassium. However, investigations using H(+)-selective microelectrodes report an extracellular alkalinization on depolarization of skate and catfish horizontal cells. These studies differed in the species and extracellular pH buffer used and the presence or absence of cobalt. We used both techniques to examine H(+) changes from isolated catfish horizontal cells under identical experimental conditions (1 mM HEPES, no cobalt). HAF fluorescence indicated an acidification response to high extracellular potassium or glutamate. However, a clear extracellular alkalinization was found using H(+)-selective microelectrodes under the same conditions. Confocal microscopy revealed that HAF was not localized exclusively to the extracellular surface, but rather was detected throughout the intracellular compartment. A high degree of colocalization between HAF and the mitochondrion-specific dye MitoTracker was observed. When HAF fluorescence was monitored from optical sections from the center of a cell, glutamate produced an intracellular acidification. These results are consistent with a model in which depolarization allows calcium influx, followed by activation of a Ca(2+)/H(+) plasma membrane ATPase. Our results suggest that HAF is reporting intracellular pH changes and that depolarization of horizontal cells induces an extracellular alkalinization, which may relieve H(+)-mediated inhibition of photoreceptor synaptic transmission.

Intracellular [Na(+)] modulates synergy between Na(+)/Ca (2+) exchanger and L-type Ca (2+) current in cardiac excitation-contraction coupling during action potentials

Excitation-contraction coupling (ECC) in cardiac myocytes involves triggering of Ca(2+) release from the sarcoplasmic reticulum (SR) by L-type Ca channels, whose activity is strongly influenced by action potential (AP) profile. The contribution of Ca(2+) entry via the Na(+)/Ca(2+) exchanger (NCX) to trigger SR Ca(2+) release during ECC in response to an AP remains uncertain. To isolate the contribution of NCX to SR Ca(2+) release, independent of effects on SR Ca(2+) load, Ca(2+) release was determined by recording Ca(2+) spikes using confocal microscopy on patch-clamped rat ventricular myocytes with [Ca(2+)](i) fixed at 150 nmol/L. In response to AP clamps, normalized Ca(2+) spike amplitudes (ΔF/F (0)) increased sigmoidally and doubled as [Na(+)](i) was elevated from 0 to 20 mmol/L with an EC(50) of ~10 mmol/L. This [Na(+)](i)-dependence was independent of I (Na) as well as SR Ca(2+) load, which was unchanged under our experimental conditions. However, NCX inhibition using either KB-R7943 or XIP reduced ΔF/F (0) amplitude in myocytes with 20 mmol/L [Na(+)](i), but not with 5 mmol/L [Na(+)](i). SR Ca(2+) release was complete before the membrane repolarized to -15 mV, indicating Ca(2+) entry into the dyad (not reduced extrusion) underlies [Na(+)](i)-dependent enhancement of ECC. Because I (Ca,L) inhibition with 50 mmol/L Cd(2+) abolished Ca(2+) spikes, our results demonstrate that during cardiac APs, NCX enhances SR Ca(2+) release by synergistically increasing the efficiency of I (Ca,L)-mediated ECC.

Calcium signaling dysfunction in heart disease

In the heart, Ca(2+) is crucial for the regulation of contraction and intracellular signaling, processes, which are vital to the functioning of the healthy heart. Ca(2+) -activated signaling pathways must function against a background of large, rapid, and tightly regulated changes in intracellular free Ca(2+) concentrations during each contraction and relaxation cycle. This review highlights a number of proteins that regulate signaling Ca(2+) in both normal and pathological conditions including cardiac hypertrophy and heart failure, and discusses how these pathways are not regulated by the marked elevation in free intracellular calcium ([Ca(2+) ](i)) during contraction but require smaller sustained increases in Ca(2+) concentration. In addition, we present published evidence that the pool of Ca(2+) that regulates signaling is compartmentalized into distinct cellular microdomains and is thus distinct from that regulating contraction.

The plasma membrane calcium ATPase modulates calcium homeostasis, intracellular signaling events and function in platelets

BACKGROUND

The plasma membrane calcium ATPase (PMCA) regulates localized signaling events in a variety of cell types, although its functional role in platelets remains undefined.

OBJECTIVES

To investigate the role of PMCA in determining platelet intracellular calcium concentration ([Ca²(+) ](i) ) at rest and following agonist stimulation, and to define the corresponding effects upon different stages of platelet activation.

METHODS

[Ca²(+) ](i) was continuously measured in Fura-2-loaded platelets and in vitro and in vivo functional analyses performed in the presence of the PMCA inhibitor carboxyeosin (CE).

RESULTS

Concentrations of CE that selectively inhibited Ca²(+) extrusion through PMCA were established in human platelets. [Ca²(+) ](i) was elevated by CE in resting platelets, although collagen-stimulated Ca²(+) release was reduced. Impaired Ca²(+) mobilization upon agonist stimulation was accompanied by reduced dense granule secretion and impaired platelet aggregation. Platelet aggregation responses were also reduced in PMCA4(-/-) mice and in an in vivo mouse model of platelet thromboembolism. Conversely, inhibition of PMCA promoted the early and later stages of platelet activation, observed as enhanced adhesion to fibrinogen, and accelerated clot retraction. Investigations into the signaling mechanisms underlying CE-mediated inhibition of platelet aggregation implicated cGMP-independent vasodilator-stimulated phosphoprotein phosphorylation.

CONCLUSIONS

Disruption of PMCA activity perturbs platelet Ca²(+) homeostasis and function in a time-dependent manner, demonstrating that PMCA differentially regulates Ca²(+) -dependent signaling events, and hence function, throughout the platelet activation process.

Cardiotoxicity of the anticancer therapeutic agent bortezomib

Recent case reports provided alarming signals that treatment with bortezomib might be associated with cardiac events. In all reported cases, patients experiencing cardiac problems were previously or concomitantly treated with other chemotherapeutics including cardiotoxic anthracyclines. Therefore, it is difficult to distinguish which components of the therapeutic regimens contribute to cardiotoxicity. Here, we addressed the influence of bortezomib on cardiac function in rats that were not treated with other drugs. Rats were treated with bortezomib at a dose of 0.2 mg/kg thrice weekly. Echocardiography, histopathology, and electron microscopy were used to evaluate cardiac function and structural changes. Respiration of the rat heart mitochondria was measured polarographically. Cell culture experiments were used to determine the influence of bortezomib on cardiomyocyte survival, contractility, Ca(2+) fluxes, induction of endoplasmic reticulum stress, and autophagy. Our findings indicate that bortezomib treatment leads to left ventricular contractile dysfunction manifested by a significant drop in left ventricle ejection fraction. Dramatic ultrastructural abnormalities of cardiomyocytes, especially within mitochondria, were accompanied by decreased ATP synthesis and decreased cardiomyocyte contractility. Monitoring of cardiac function in bortezomib-treated patients should be implemented to evaluate how frequently cardiotoxicity develops especially in patients with pre-existing cardiac conditions, as well as when using additional cardiotoxic drugs.

Calcium balance and mechanotransduction in rat cochlear hair cells

Auditory transduction occurs by opening of Ca(2+)-permeable mechanotransducer (MT) channels in hair cell stereociliary bundles. Ca(2+) clearance from bundles was followed in rat outer hair cells (OHCs) using fast imaging of fluorescent indicators. Bundle deflection caused a rapid rise in Ca(2+) that decayed after the stimulus, with a time constant of about 50 ms. The time constant was increased by blocking Ca(2+) uptake into the subcuticular plate mitochondria or by inhibiting the hair bundle plasma membrane Ca(2+) ATPase (PMCA) pump. Such manipulations raised intracellular Ca(2+) and desensitized the MT channels. Measurement of the electrogenic PMCA pump current, which saturated at 18 pA with increasing Ca(2+) loads, indicated a maximum Ca(2+) extrusion rate of 3.7 fmol x s(-1). The amplitude of the Ca(2+) transient decreased in proportion to the Ca(2+) concentration bathing the bundle and in artificial endolymph (160 mM K(+), 20 microM Ca(2+)), Ca(2+) carried 0.2% of the MT current. Nevertheless, MT currents in endolymph displayed fast adaptation with a submillisecond time constant. In endolymph, roughly 40% of the MT current was activated at rest when using 1 mM intracellular BAPTA compared with 12% with 1 mM EGTA, which enabled estimation of the in vivo Ca(2+) load as 3 pA at rest. The results were reproduced by a model of hair bundle Ca(2+) diffusion, showing that the measured PMCA pump density could handle Ca(2+) loads incurred from resting and maximal MT currents in endolymph. The model also indicated the endogenous mobile buffer was equivalent to 1 mM BAPTA.

Reduced expression of the Ca(2+) transporter protein PMCA2 slows Ca(2+) dynamics in mouse cerebellar Purkinje neurones and alters the precision of motor coordination

Cerebellar Purkinje neurones (PNs) express high levels of the plasma membrane calcium ATPase, PMCA2, a transporter protein critical for the clearance of calcium from excitable cells. Genetic deletion of one PMCA2 encoding gene in heterozygous PMCA2 knock-out (PMCA2(+/-) mice enabled us to determine how PMCA2 influences PN calcium regulation without the complication of the severe morphological changes associated with complete PMCA2 knock-out (PMCA2(-/-) in these cells. The PMCA2(+/-) cerebellum expressed half the normal levels of PMCA2 and this nearly doubled the time taken for PN dendritic calcium transients to recover (mean fast and slow recovery times increased from 70 ms to 110 ms and from 600 ms to 1100 ms). The slower calcium recovery had distinct consequences for PMCA2(+/-) PN physiology. The PNs exhibited weaker climbing fibre responses, prolonged outward Ca(2+)-dependent K(+) current (mean fast and slow recovery times increased from 136 ms to 192 ms and from 595 ms to 1423 ms) and a slower mean frequency of action potential firing (7.4 Hz compared with 15.8 Hz). Our findings were consistent with prolonged calcium accumulation in the cytosol of PMCA2(+/-) Purkinje neurones. Although PMCA2(+/-) mice exhibited outwardly normal behaviour and little change in their gait pattern, when challenged to run on a narrow beam they exhibited clear deficits in hindlimb coordination. Training improved the motor performance of both PMCA2(+/-) and wild-type mice, although PMCA2(+/-) mice were always impaired. We conclude that reduced calcium clearance perturbs calcium dynamics in PN dendrites and that this is sufficient to disrupt the accuracy of cerebellar processing and motor coordination.

Assessment of the contribution of the plasma membrane calcium ATPase, PMCA, calcium transporter to synapse function using patch clamp electrophysiology and fast calcium imaging

The plasma membrane calcium ATPase, or PMCA, functions to extrude calcium out of cells as a key component necessary for adequate calcium homeostasis in all cells. However, calcium is particularly important at synapses between neurons, where communication relies on the controlled rise and fall in presynaptic calcium that precedes the release of neurotransmitter. Here we show how to infer the real-time contribution of PMCA-mediated calcium extrusion to this presynaptic calcium dynamic and how this influences the properties of the synapse. To do this we have taken advantage of a well-studied synapse in the cerebellum. We use electrophysiology to assess the timing of short-term facilitation at this synapse in the presence and absence of PMCA2 using PMCA2 knockout mice and pharmacology and fast calcium imaging to measure the presynaptic calcium dynamics. These approaches are all highly applicable to other synapses and can help determine the contribution of PMCA, and other transporters or exchangers, to the calcium dynamics that underpin reliable synaptic transmission.

Individual effect of components of defibrillation waveform on the contractile function and intracellular calcium dynamics of cardiomyocytes

OBJECTIVES

Although electrical shock is a unique and effective treatment for fatal arrhythmia, it produces myocardial dysfunction closely related to the intensity of shock delivered. The isolated contribution of defibrillator components to postshock contractile impairment is not yet securely established. We sought to evaluate contractile function in cardiomyocytes following electrical shocks with different peak currents, energies, and durations. We hypothesized that peak current may play a more important role than energy in determining postshock dysfunction. Prolongation of the duration may reduce contractile impairment.

DESIGN

Prospective, randomized, controlled study.

SETTING

University-affiliated research institute.

SUBJECTS

Male albino Sprague-Dawley rats.

INTERVENTIONS

We assigned 324 cardiomyocytes isolated from adult male rats to 11 groups having different waveforms (triangular and square), peak currents (derived from peak voltage gradients of 25 V/cm, 35.4 V/cm, 50 V/cm, 70.7 V/cm, and 100 V/cm), and durations (10 and 20 msecs) of shocks delivered. One single shock was given to each cardiomyocyte, and length shortening and Ca transients were recorded optically with fura-2 loading.

MEASUREMENTS AND MAIN RESULTS

Increase of peak current and corresponding energy caused more cells to have irregular beating (p < .001) and reduced length shortening (p < .001). This was associated with increased Ca abnormality (p < .05). Increasing peak current independent of energy significantly impaired postshock contractile function (p < .05), whereas the change of energy alone did not. Prolongation of duration independent of energy and peak current reduced postshock contractile impairment (p < .05).

CONCLUSIONS

Peak current may play a more determinative role in producing postshock contractile dysfunction than does energy.

Increased O2 consumption in excitation-contraction coupling in hypertrophied rat heart slices related to increased Na+ -Ca2+ exchange activity

The goal of our study was to evaluate the origin of the increased O(2) consumption in electrically stimulated left ventricular slices of isoproterenol-induced hypertrophied rat hearts with normal left ventricular pressure. O(2) consumption per minute (mVO(2)) of mechanically unloaded left ventricular slices was measured in the absence and presence of 1-Hz field stimulation. Basal metabolic mVO(2), i.e., mVO(2) without electrical stimulation, was significantly smaller, but mVO(2) for the total Ca(2+) handling in excitation-contraction coupling (E-C coupling mVO(2)), i.e., delta mVO(2) (=mVO(2) with stimulation - mVO(2) without stimulation), was significantly larger in the hypertrophied heart. Furthermore, the fraction of E-C coupling mVO(2) was markedly altered in the hypertrophied heart. Namely, mVO(2) consumed by sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2) was depressed by 40%; mVO(2) consumed by the Na(+)/K(+)-ATPase (NKA)-Na(+)/Ca(2+) exchange (NCX) coupling was increased by 100%. The depressed mVO(2) consumption by SERCA2 was supported by lower protein expressions of phosphorylated-Ser(16) phospholamban and SERCA2. The increase in NKA-NCX coupling mVO(2) was supported by marked augmentation of NCX current. However, the increase in NCX current was not due to the increase in NCX1 protein expression, but was attributable to attenuation of the intrinsic inactivation mechanisms. The present results demonstrated that the altered origin of the increased E-C coupling mVO(2) in hypertrophy was derived from decreased SERCA2 activity (1ATP: 2Ca(2+)) and increased NCX activity coupled to NKA activity (1ATP: Ca(2+)). Taken together, we conclude that the energetically less efficient Ca(2+) extrusion pathway evenly contributes to Ca(2+) handling in E-C coupling in the present hypertrophy model.

Free radicals mediate postshock contractile impairment in cardiomyocytes

OBJECTIVE

Previous studies demonstrated myocardial dysfunction after electrical shock and indicated it may be related to free radicals. Whether the free radicals are generated after electrical shock has not been documented at the cellular level. This study was to investigate whether electrical shock generates intracellular free radicals inside cardiomyocytes and to evaluate whether reducing intracellular free radicals by pretreatment of ascorbic acid would reduce the contractile dysfunction after electrical shock.

DESIGN

Randomized prospective animal study.

SETTING

University affiliated research laboratory.

SUBJECTS

Sprague-Dawley rats.

INTERVENTIONS

Cardiomyocytes isolated from adult male rats were divided into four groups: (1) electrical shock alone; (2) electrical shock pretreated with ascorbic acid; (3) pretreated with ascorbic acid alone; and (4) control. Ascorbic acid (0.2 mM) was administrated in the perfusate of the ascorbic acid + electrical shock and ascorbic acid groups. A 2-J electrical shock was delivered to the electrical shock and ascorbic acid + electrical shock groups.

MEASUREMENTS AND MAIN RESULTS

DCFH-DA-loaded cardiomyocytes showed increased intracellular free radicals after electrical shock. The contractions and Ca2+ transients were recorded optically with fura-2 loading. Within 4 mins after electrical shock in the electrical shock group, the length shortening decreased from 8.4% +/- 2.5% to 5.6% +/- 3.4% (p = 0.000) and the Ca2+ transient decreased from 1.15 +/- 0.13 au to 1.08 +/- 0.1 au (p = 0.038). Compared with control, a significant difference in length shortening (p = 0.001) but not Ca2+ transient (p = 0.052) was noted. In the presence of ascorbic acid, electrical shock did not affect length shortening and Ca2+ transient.

CONCLUSION

Electrical shock generates free radicals inside the cardiomyocyte, and causes contractile impairment and associated decrease of Ca transient. Administering ascorbic acid may improve such damage by eliminating free radicals.

Na(+)/Ca(2+) exchanger inhibition exerts a positive inotropic effect in the rat heart, but fails to influence the contractility of the rabbit heart

BACKGROUND AND PURPOSE

The Na(+)/Ca(2+) exchanger (NCX) may play a key role in myocardial contractility. The operation of the NCX is affected by the action potential (AP) configuration and the intracellular Na(+) concentration. This study examined the effect of selective NCX inhibition by 0.1, 0.3 and 1.0 microM SEA0400 on the myocardial contractility in the setting of different AP configurations and different intracellular Na(+) concentrations in rabbit and rat hearts.

EXPERIMENTAL APPROACH

The concentration-dependent effects of SEA0400 on I(Na/Ca) were studied in rat and rabbit ventricular cardiomyocytes using a patch clamp technique. Starling curves were constructed for isolated, Langendorff-perfused rat and rabbit hearts. The cardiac sarcolemmal NCX protein densities of both species were compared by immunohistochemistry.

KEY RESULTS

SEA0400 inhibited I(Na/Ca) with similar efficacy in the two species; there was no difference between the inhibitions of the forward or reverse mode of the NCX in either species. SEA0400 increased the systolic and the developed pressure in the rat heart in a concentration-dependent manner, for example, 1.0 microM SEA0400 increased the maximum systolic pressures by 12% relative to the control, whereas it failed to alter the contractility in the rabbit heart. No interspecies difference was found in the cardiac sarcolemmal NCX protein densities.

CONCLUSIONS AND IMPLICATIONS

NCX inhibition exerted a positive inotropic effect in the rat heart, but it did not influence the contractility of the rabbit heart. This implies that the AP configuration and the intracellular Na(+) concentration may play an important role in the contractility response to NCX inhibition.

Modulation of extracellular proton fluxes from retinal horizontal cells of the catfish by depolarization and glutamate

Self-referencing H(+)-selective microelectrodes were used to measure extracellular proton fluxes from cone-driven horizontal cells isolated from the retina of the catfish (Ictalurus punctatus). The neurotransmitter glutamate induced an alkalinization of the area adjacent to the external face of the cell membrane. The effect of glutamate occurred regardless of whether the external solution was buffered with 1 mM HEPES, 3 mM phosphate, or 24 mM bicarbonate. The AMPA/kainate receptor agonist kainate and the NMDA receptor agonist N-methyl-D-aspartate both mimicked the effect of glutamate. The effect of kainate on proton flux was inhibited by the AMPA/kainate receptor blocker CNQX, and the effect of NMDA was abolished by the NMDA receptor antagonist DAP-5. Metabotropic glutamate receptor agonists produced no alteration in proton fluxes from horizontal cells. Depolarization of cells either by increasing extracellular potassium or directly by voltage clamp also produced an alkalinization adjacent to the cell membrane. The effects of depolarization on proton flux were blocked by 10 microM nifedipine, an inhibitor of L-type calcium channels. The plasmalemma Ca(2+/)H(+) ATPase (PMCA) blocker 5(6)-carboxyeosin also significantly reduced proton flux modulation by glutamate. Our results are consistent with the hypothesis that glutamate-induced extracellular alkalinizations arise from activation of the PMCA pump following increased intracellular calcium entry into cells. This process might help to relieve suppression of photoreceptor neurotransmitter release that results from exocytosed protons from photoreceptor synaptic terminals. Our findings argue strongly against the hypothesis that protons released by horizontal cells act as the inhibitory feedback neurotransmitter that creates the surround portion of the receptive fields of retinal neurons.

Plasma membrane calcium ATPase and its relationship to nitric oxide signaling in the heart

The plasma membrane calcium/calmodulin-dependent ATPase (PMCA) is a ubiquitously expressed calcium-extruding enzymatic pump. In the majority of cells the main function of PMCA is as the only system to extrude calcium from the cytosol, however, in the excitable cells of the heart it has only a minor role in the bulk removal of calcium compared to the sodium-calcium exchanger. There is increasing evidence to suggest that PMCA has an additional role as a potential modulator of a number of signal transduction pathways. Of key interest in the heart is the functional interaction between the calcium/calmodulin-dependent enzyme neuronal nitric oxide synthase (nNOS) and isoform 4 of PMCA. Nitric oxide production from nNOS is known to be important in the regulation of excitation-contraction (EC) coupling and subsequently contractility. This article will focus on recent evidence suggesting that PMCA4 has a regulatory role in the nitric oxide signaling pathway in the heart.

Role of Ca2+ Transporters and Channels in the Cardiac Cell Volume Regulation

Na+/K+ pump is one of key mechanisms to maintain cell volume. When it is inhibited, cells are at risk of swelling. However, in guinea pig ventricular myocytes, the cell area as an index of cell volume was almost constant during 90 min Na+/K+ pump blockade with 40 microM ouabain despite the marked membrane depolarization. In this study, involvements of Ca2+ transporters and channels in the cardiac cell volume regulation were proposed by conducting the computer simulation in parallel with the experimental validation.

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.

Neuronal Nitric Oxide Synthase Signaling in the Heart Is Regulated by the Sarcolemmal Calcium Pump 4b

Background— Neuronal nitric oxide synthase (nNOS) has recently been shown to be a major regulator of cardiac contractility. In a cellular system, we have previously shown that nNOS is regulated by the isoform 4b of plasma membrane calcium/calmodulin-dependent ATPase (PMCA4b) through direct interaction mediated by a PDZ domain (PSD 95, Drosophilia Discs large protein and Zona occludens-1) on nNOS and a cognate ligand on PMCA4b. It remains unknown, however, whether this interaction has physiological relevance in the heart in vivo.

Methods and Results— We generated 2 strains of transgenic mice overexpressing either human PMCA4b or PMCA ct120 in the heart. PMCA ct120 is a highly active mutant form of the pump that does not interact with or modulate nNOS function. Calcium was extruded normally from PMCA4b-overexpressing cardiomyocytes, but in vivo, overexpression of PMCA4b reduced the β-adrenergic contractile response. This attenuated response was not observed in ct120 transgenic mice. Treatment with a specific nNOS inhibitor ( N ω-propyl- l -arginine) reduced the β-adrenergic response in wild-type and ct120 transgenic mice to levels comparable to those of PMCA4b transgenic animals. No differences in lusitropic response were observed in either transgenic strain compared with wild-type littermates.

Conclusions— These data demonstrate the physiological relevance of the interaction between PMCA4b and nNOS and suggests its signaling role in the heart.

Presynaptic plasma membrane Ca2+ ATPase isoform 2a regulates excitatory synaptic transmission in rat hippocampal CA3

Plasma membrane calcium ATPase isoforms (PMCAs) are expressed in a wide variety of tissues where cell-specific expression provides ample opportunity for functional diversity amongst these transporters. The PMCAs use energy derived from ATP to extrude submicromolar concentrations of intracellular Ca2+ ([Ca2+]i) out of the cell. Their high affinity for Ca2+ and the speed with which they remove [Ca2+]i depends upon splicing at their carboxy (C)-terminal site. Here we provide biochemical and functional evidence that a brain-specific, C-terminal truncated and therefore fast variant of PMCA2, PMCA2a, has a role at hippocampal CA3 synapses. PMCA2a was enriched in forebrain synaptosomes, and in hippocampal CA3 it colocalized with the presynaptic marker proteins synaptophysin and the vesicular glutamate transporter 1, but not with the postsynaptic density protein PSD-95. PMCA2a also did not colocalize with glutamic acid decarboxylase-65, a marker of GABA-ergic terminals, although it did localize to a small extent with parvalbumin-positive presumed inhibitory terminals. Pharmacological inhibition of PMCA increased the frequency but not the amplitude of mEPSCs with little effect on mIPSCs or paired-pulse depression of evoked IPSCs. However, inhibition of PMCA activity did enhance the amplitude and slowed the recovery of paired-pulse facilitation (PPF) of evoked EPSCs. These results indicated that fast PMCA2a-mediated clearance of [Ca2+]i from presynaptic excitatory terminals regulated excitatory synaptic transmission within hippocampal CA3.

Ionic mechanisms of cardiac cell swelling induced by blocking Na+/K+ pump as revealed by experiments and simulation

Although the Na(+)/K(+) pump is one of the key mechanisms responsible for maintaining cell volume, we have observed experimentally that cell volume remained almost constant during 90 min exposure of guinea pig ventricular myocytes to ouabain. Simulation of this finding using a comprehensive cardiac cell model (Kyoto model incorporating Cl(-) and water fluxes) predicted roles for the plasma membrane Ca(2+)-ATPase (PMCA) and Na(+)/Ca(2+) exchanger, in addition to low membrane permeabilities for Na(+) and Cl(-), in maintaining cell volume. PMCA might help maintain the [Ca(2+)] gradient across the membrane though compromised, and thereby promote reverse Na(+)/Ca(2+) exchange stimulated by the increased [Na(+)](i) as well as the membrane depolarization. Na(+) extrusion via Na(+)/Ca(2+) exchange delayed cell swelling during Na(+)/K(+) pump block. Supporting these model predictions, we observed ventricular cell swelling after blocking Na(+)/Ca(2+) exchange with KB-R7943 or SEA0400 in the presence of ouabain. When Cl(-) conductance via the cystic fibrosis transmembrane conductance regulator (CFTR) was activated with isoproterenol during the ouabain treatment, cells showed an initial shrinkage to 94.2 +/- 0.5%, followed by a marked swelling 52.0 +/- 4.9 min after drug application. Concomitantly with the onset of swelling, a rapid jump of membrane potential was observed. These experimental observations could be reproduced well by the model simulations. Namely, the Cl(-) efflux via CFTR accompanied by a concomitant cation efflux caused the initial volume decrease. Then, the gradual membrane depolarization induced by the Na(+)/K(+) pump block activated the window current of the L-type Ca(2+) current, which increased [Ca(2+)](i). Finally, the activation of Ca(2+)-dependent cation conductance induced the jump of membrane potential, and the rapid accumulation of intracellular Na(+) accompanied by the Cl(-) influx via CFTR, resulting in the cell swelling. The pivotal role of L-type Ca(2+) channels predicted in the simulation was demonstrated in experiments, where blocking Ca(2+) channels resulted in a much delayed cell swelling.

Cardiac function of two ecologically distinct Neotropical freshwater fish: Curimbata, Prochilodus lineatus (Teleostei, Prochilodontidae), and trahira, Hoplias malabaricus (Teleostei, Erythrinidae)

An isometric muscle preparation was used to investigate the importance of the ventricular sarcoplasmic reticulum (SR) and extracellular Ca(2+) (2.5 up to 10.5 mM) to force generation at 25 degrees C (acclimation temperature) in two ecologically distinct Neotropical teleost fish: Curimbata (active species), and trahira (sedentary species). The post-rest force was studied with and without 10 muM ryanodine in the medium. The positive inotropism observed for both species in response to increases on extracellular Ca(2+) reflected a greater Ca(2+) influx through sarcolemma, as well as an increase in Ca(2+) liberation from the SR by the Ca(2+)-induced Ca(2+) release mechanism. The significant post-rest potentiation recorded for the curimbata and trahira control preparations (3.22+/-0.24 to 6.55+/-0.77 mN mm(-2) and 0.74+/-0.07 to 2.26+/-0.26 mN mm(-2), respectively), was completely inhibited by the addition of ryanodine to the bathing medium, suggesting a potential functionality of SR for both species. Considering the differences in these species habitats, modes of life and levels of activity and the fact of a probable SR Ca(2+) cycling in a physiological temperature, we suggest that the functionality of the SR in these species is probably related to their phylogeny.

Plasma membrane calcium ATPase is concentrated in the head of sea urchin spermatozoa

Plasma membrane Ca2+ATPases (PMCAs) export Ca2+ from cells in a highly regulated manner, providing fine-tuning to the maintenance of intracellular Ca2+ concentrations. There are few studies of PMCAs in spermatozoa, which is surprising considering the importance of this enzyme in all cell types. Here we describe the primary structure and localization of the PMCA of sea urchin spermatozoa (suPMCA). The suPMCA is 1,154 amino acids and has 56% identity and 76% similarity to all 4 human PMCA isoforms. The suPMCA shares the features of a typical PMCA, including domains for calmodulin binding, ATP binding, ATPase phosphorylation, and 10 putative transmembrane segments with two large cytoplasmic loops. Southern blots show that suPMCA is a single copy gene. Treatment of live sea urchin sperm with the PMCA inhibitor, 5-(-6)-carboxyeosin, results in elevations of intracellular Ca2+ and loss of flagellar motility. Immunoblotting and immunoflorescence show that suPMCA is concentrated in the sperm head plasma membrane. In previous work, we showed that a plasma membrane K+ dependent Na+/Ca2+ exchanger (suNCKX), which also keeps Ca2+ low in these cells, is concentrated in the sperm flagellum. Thus, the sperm head and flagellum localize different gene products, both functioning to keep intracellular Ca2+ low, while the sperm swims in seawater containing 10 mM Ca2+.

Sodium calcium exchange as a target for antiarrhythmic therapy

In search of better antiarrhythmic therapy, targeting the Na/Ca exchanger is an option to be explored. The rationale is that increased activity of the Na/Ca exchanger has been implicated in arrhythmogenesis in a number of conditions. The evidence is strong for triggered arrhythmias related to Ca2+ overload, due to increased Na+ load or during adrenergic stimulation; the Na/Ca exchanger may be important in triggered arrhythmias in heart failure and in atrial fibrillation. There is also evidence for a less direct role of the Na/Ca exchanger in contributing to remodelling processes. In this chapter, we review this evidence and discuss the consequences of inhibition of Na/Ca exchange in the perspective of its physiological role in Ca2+ homeostasis. We summarize the current data on the use of available blockers of Na/Ca exchange and propose a framework for further study and development of such drugs. Very selective agents have great potential as tools for further study of the role the Na/Ca exchanger plays in arrhythmogenesis. For therapy, they may have their specific indications, but they carry the risk of increasing Ca2+ load of the cell. Agents with a broader action that includes Ca2+ channel block may have advantages in other conditions, e.g. with Ca2+ overload. Additional actions such as block of K+ channels, which may be unwanted in e.g. heart failure, may be used to advantage as well.

Collaborative effect of SERCA and PMCA in cytosolic calcium homeostasis in human platelets

Intracellular free Ca2+ concentration ([Ca2+]c) is finely regulated by several mechanisms that either increase or reduce [Ca2+]c. Two different Ca2+ pumps have been described so far as the main mechanisms for Ca2+ removal from the cytosol, either by its sequestration into the stores, mediated by the sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA) or by Ca2+ extrusion to the extracellular medium, by the plasma membrane Ca2+-ATPase (PMCA). We have used inhibitors of these pumps to analyze their Ca2+ clearance efficacy in human platelets stimulated by the physiological agonist thrombin. Results demonstrate that, after platelet stimulation with thrombin, activation of SERCA precedes that of PMCA, although the ability of PMCA to remove Ca2+ from the cytosol last longer than that of SERCA. The efficacy of SERCA and PMCA removing Ca2+ from the cytosol is reduced when the concentration of thrombin increases. This phenomenon correlates with the greater increase in [Ca2+]c induced by higher concentrations of thrombin, which further confirms that SERCA and PMCA activities are regulated by [Ca2+]c.

Genetic manipulation of cardiac Na+/Ca2+ exchange expression

The Na+/Ca2+ exchanger (NCX) is the primary Ca2+ extrusion mechanism in cardiomyocytes. To further investigate the role of NCX in excitation-contraction coupling and Ca2+ homeostasis, we created murine models with altered expression levels of NCX. Homozygous overexpression of NCX resulted in mild cardiac hypertrophy. Decline of the Ca2+ transient and relaxation of contraction were increased and the reverse mode of NCX was augmented. Overexpression also led to a higher susceptibility to ischemia-reperfusion injury and to a greater ability of NCX to trigger Ca2+-induced Ca2+ release. Furthermore, an increase in peak L-type Ca2+ current was observed suggesting a direct influence of NCX on L-type Ca2+ current. Whereas global knockout of NCX led to prenatal death, a recently generated cardiac-specific NCX knockout mouse was viable with surprisingly normal contractile properties. Expression levels of other Ca2+-handling proteins were not altered. Ca2+ influx in these animals is limited by a decrease of peak L-type Ca2+ current. An alternative Ca2+ efflux mechanism, presumably the plasma membrane Ca2+-ATPase, is sufficient to maintain Ca2+-homeostasis in the NCX knockout mice.

Mechanism of activation of the tonic component of contraction in myocytes of guinea pig heart

AIM

Contractions of myocytes of guinea pig heart consist of a phasic component relaxing independently on the voltage and a tonic component relaxing upon repolarization. We found previously that Ca(2+) activating the tonic component is released from the sarcoplasmic reticulum. In this study, we analysed the mechanism of activation and maintenance of this release.

METHODS

Experiments were performed at 37 degrees C in ventricular myocytes of guinea pig heart. Voltage-clamped myocytes were stimulated by the pulses of the duration of 300 ms to 15-45 s from the holding potential of -40 to +5 mV. [Ca(2+)](i) was monitored as fluorescence of Indo-1 and contractions were recorded with the TV edge-tracking system.

RESULTS

Myocytes responded to the short and long pulses with phasic contraction or Ca(2+) transient followed by the sustained contraction or increase in [Ca(2+)](i). Repolarization brought about relaxation. 10 mmol L(-1) Ni(2+) blocking Na(+)/Ca(2+) exchange superfused during the tonic component increased its amplitude. Superfusion of Ca(2+)-free solution during sustained contraction brought about relaxation both in normal cells and in cells superfused with Ni(2+) despite preserved sarcoplasmic reticulum Ca(2+) content assessed with caffeine spritz. Relaxing effect of Ca(2+)-free solution was not affected by carboxyeosin, a blocker of sarcolemmal Ca(2+)-ATPase. Tonic component of contraction and of Ca(2+) transient was inhibited by 200 micromol L(-1) ryanodine, a blocker of Ca(2+) release channels of the sarcoplasmic reticulum and by 20 micromol L(-1) nifedipine, a blocker of L-type I(Ca).

CONCLUSION

Tonic component of contraction results from Ca(2+) release via the sarcoplasmic reticulum Ca(2+) channels activated by sustained, nifedipine-sensitive and Ni(2+)-insensitive Ca(2+) influx. Alternatively, the SR Ca(2+) release is activated by voltage, the dihydropyridine receptors acting like voltage sensors.