Changes in the calcium current of rat heart ventricular myocytes during development

Adapting to a new environment: postnatal maturation of the human cardiomyocyte

The postnatal mammalian heart undergoes remarkable developmental changes, which are stimulated by the transition from the intrauterine to extrauterine environment. With birth, increased oxygen levels promote metabolic, structural and biophysical maturation of cardiomyocytes, resulting in mature muscle with increased efficiency, contractility and electrical conduction. In this Topical Review article, we highlight key studies that inform our current understanding of human cardiomyocyte maturation. Collectively, these studies suggest that human atrial and ventricular myocytes evolve quickly within the first year but might not reach a fully mature adult phenotype until nearly the first decade of life. However, it is important to note that fetal, neonatal and paediatric cardiac physiology studies are hindered by a number of limitations, including the scarcity of human tissue, small sample size and a heavy reliance on diseased tissue samples, often without age-matched healthy controls. Future developmental studies are warranted to expand our understanding of normal cardiac physiology/pathophysiology and inform age-appropriate treatment strategies for cardiac disease.

Sellke F, Feng J. Potassium and Cardiac Surgery

Potassium homeostasis affects cardiac rhythm and contractility, along with vascular reactivity and vascular smooth muscle proliferation. This chapter will focus on potassium dynamics during and after cardiac surgery involving cardioplegic arrest and cardiopulmonary bypass (CPB). Hyperkalemic, hypothermic solutions are frequently used to induce cardioplegic arrest and protect the heart during cardiac surgery involving CPB. Common consequences of hyperkalemic cardioplegic arrest and reperfusion include microvascular dysfunction involving several organ systems and myocardial dysfunction. Immediately after CPB, blood potassium levels often drop precipitously due to a variety of factors, including CPB -induced electrolyte depletion and frequent, long-term administration of insulin during and after surgery. Meanwhile, some patients with pre-existing kidney dysfunction may experience postoperative hyperkalemia following cardioplegia. Any degree of postoperative hyper/hypokalemia significantly elevates the risk of cardiac arrythmias and subsequent myocardial failure. Therefore, proper management of blood potassium levels during and after cardioplegia/CPB is crucial for optimizing patient outcomes following cardiac surgery.

Cardiac Transverse Tubules in Physiology and Heart Failure

In mammalian cardiac myocytes, the plasma membrane includes the surface sarcolemma but also a network of membrane invaginations called transverse (t-) tubules. These structures carry the action potential deep into the cell interior, allowing efficient triggering of Ca2+ release and initiation of contraction. Once thought to serve as rather static enablers of excitation-contraction coupling, recent work has provided a newfound appreciation of the plasticity of the t-tubule network's structure and function. Indeed, t-tubules are now understood to support dynamic regulation of the heartbeat across a range of timescales, during all stages of life, in both health and disease. This review article aims to summarize these concepts, with consideration given to emerging t-tubule regulators and their targeting in future therapies.

Cardiac Toxicity of Cadmium Involves Complex Interactions Among Multiple Ion Currents in Rainbow Trout (Oncorhynchus mykiss) Ventricular Myocytes

Cadmium (Cd²⁺ ) is cardiotoxic to fish, but its effect on the electrical excitability of cardiac myocytes is largely unknown. To this end, we used the whole-cell patch-clamp method to investigate the effects of Cd²⁺ on ventricular action potentials (APs) and major ion currents in rainbow trout (Oncorhynchus mykiss) ventricular myocytes. Trout were acclimated to +4 °C, and APs were measured at the acclimated temperature and elevated temperature (+18 °C). Cd²⁺ (10, 20, and 100 µM) altered the shape of the ventricular AP in a complex manner. The early plateau fell to less positive membrane voltages, and the total duration of AP prolonged. These effects were obvious at both +4 °C and +18 °C. The depression of the early plateau is due to the strong Cd²⁺ -induced inhibition of the L-type calcium (Ca²⁺ ) current (I_CaL ), whereas the prolongation of the AP is an indirect consequence of the I_CaL inhibition: at low voltages of the early plateau, the delayed rectifier potassium (K⁺ ) current (I_Kr ) remains small, delaying repolarization of AP. Cd²⁺ reduced the density and slowed the kinetics of the Na⁺ current (I_Na ) but left the inward rectifier K⁺ current (I_K1 ) intact. These altered cellular and molecular functions can explain several Cd²⁺ -induced changes in impulse conduction of the fish heart, for example, slowed propagation of the AP in atrial and ventricular myocardia (inhibition of I_Na ), delayed relaxation of the ventricle (prolongation of ventricular AP duration), bradycardia, and atrioventricular block (inhibition of I_CaL ). These findings indicate that the cardiotoxicity of Cd²⁺ in fish involves multiple ion currents that are directly and indirectly altered by Cd²⁺ . Through these mechanisms, Cd²⁺ may trigger cardiac arrhythmias and impair myocardial contraction. Elevated temperature (+18 °C) slightly increases Cd²⁺ toxicity in trout ventricular myocytes. Environ Toxicol Chem 2021;40:2874-2885. © 2021 SETAC.

Calcium release-dependent inactivation precedes formation of the tubular system in developing rat cardiac myocytes

Developing cardiac myocytes undergo substantial structural and functional changes transforming the mechanism of excitation-contraction coupling from the embryonic form, based on calcium influx through sarcolemmal DHPR calcium channels, to the adult form, relying on local calcium release through RYR calcium channels of sarcoplasmic reticulum stimulated by calcium influx. We characterized day-by-day the postnatal development of the structure of sarcolemma, using techniques of confocal fluorescence microscopy, and the development of the calcium current, measured by the whole-cell patch-clamp in isolated rat ventricular myocytes. We characterized the appearance and expansion of the t-tubule system and compared it with the appearance and progress of the calcium current inactivation induced by the release of calcium ions from sarcoplasmic reticulum as structural and functional measures of direct DHPR-RYR interaction. The release-dependent inactivation of calcium current preceded the development of the t-tubular system by several days, indicating formation of the first DHPR-RYR couplons at the surface sarcolemma and their later spreading close to contractile myofibrils with the growing t-tubules. Large variability of both of the measured parameters among individual myocytes indicates uneven maturation of myocytes within the growing myocardium.

Cholesterol depletion impairs contractile machinery in neonatal rat cardiomyocytes

Cholesterol regulates numerous cellular processes. Depleting its synthesis in skeletal myofibers induces vacuolization and contraction impairment. However, little is known about how cholesterol reduction affects cardiomyocyte behavior. Here, we deplete cholesterol by incubating neonatal cardiomyocytes with methyl-beta-cyclodextrin. Traction force microscopy shows that lowering cholesterol increases the rate of cell contraction and generates defects in cell relaxation. Cholesterol depletion also increases membrane tension, Ca2+ spikes frequency and intracellular Ca2+ concentration. These changes can be correlated with modifications in caveolin-3 and L-Type Ca2+ channel distributions across the sarcolemma. Channel regulation is also compromised since cAMP-dependent PKA activity is enhanced, increasing the probability of L-Type Ca2+ channel opening events. Immunofluorescence reveals that cholesterol depletion abrogates sarcomeric organization, changing spacing and alignment of α-actinin bands due to increase in proteolytic activity of calpain. We propose a mechanism in which cholesterol depletion triggers a signaling cascade, culminating with contraction impairment and myofibril disruption in cardiomyocytes.

The cardiac L-type calcium channel distal carboxy terminus autoinhibition is regulated by calcium

The L-type calcium channel (LTCC) provides trigger Ca(2+) for sarcoplasmic reticulum Ca-release, and LTCC function is influenced by interacting proteins including the LTCC distal COOH terminus (DCT) and calmodulin. DCT is proteolytically cleaved and reassociates with the LTCC complex to regulate calcium channel function. DCT reduces LTCC barium current (I(Ba,L)) in reconstituted channel complexes, yet the contribution of DCT to LTCC Ca(2+) current (I(Ca,L)) in cardiomyocyte systems is unexplored. This study tests the hypothesis that DCT attenuates cardiomyocyte I(Ca,L). We measured LTCC current and Ca(2+) transients with DCT coexpressed in murine cardiomyocytes. We also heterologously coexpressed DCT and Ca(V)1.2 constructs with truncations corresponding to the predicted proteolytic cleavage site, Ca(V)1.2Δ1801, and a shorter deletion corresponding to well-studied construct, Ca(V)1.2Δ1733. DCT inhibited I(Ba,L) in cardiomyocytes, and in human embryonic kidney (HEK) 293 cells expressing Ca(V)1.2Δ1801 and Ca(V)1.2Δ1733. Ca(2+)-CaM relieved DCT block in cardiomyocytes and HEK cells. The selective block of I(Ba,L) combined with Ca(2+)-CaM effects suggested that DCT-mediated blockade may be relieved under conditions of elevated Ca(2+). We therefore tested the hypothesis that DCT block is dynamic, increasing under relatively low Ca(2+), and show that DCT reduced diastolic Ca(2+) at low stimulation frequencies but spared high frequency Ca(2+) entry. DCT reduction of diastolic Ca(2+) and relief of block at high pacing frequencies and under conditions of supraphysiological bath Ca(2+) suggests that a physiological function of DCT is to increase the dynamic range of Ca(2+) transients in response to elevated pacing frequencies. Our data motivate the new hypothesis that DCT is a native reverse use-dependent inhibitor of LTCC current.

Postnatal developmental decline in IK1 in mouse ventricular myocytes isolated by the Langendorff perfusion method: comparison with the chunk method

Expression and function of cardiac ion channels exhibit postnatal developmental changes, which, however, has not yet been proven in ventricular myocytes isolated using similar techniques. In this study, ventricular myocytes were enzymatically dissociated from mouse heart at different postnatal ages (including postnatal day 0) by similar techniques using Langendorff perfusion. Whole-cell patch-clamp experiments were performed to record action potentials, I (K1), I (Kr), I (Kur), I (ss), and I (Ca,L), in ventricular myocytes freshly isolated from postnatal days 0, 7, and 14 and adult mice. Viable ventricular myocytes of day-0 mouse heart exhibited spindle-shaped appearance having cell length of approximately 50 μm, which gradually developed to a rod-shaped one having clear cross striation with cell length of approximately 120 μm (adult). The action potential duration markedly shortened, while the resting membrane potential depolarized to a small but significant extent during postnatal development. I (K1) density was maximal in postnatal day-0 ventricular myocytes and gradually decreased during development, which was accompanied by postnatal depolarization of resting membrane potential. However, I (K1) density was markedly decreased by approximately 80% in postnatal day-0 ventricular myocytes, when isolated by the chunk method. Quantitative real-time polymerase chain reaction (PCR) and western blot analyses demonstrated higher Kir2.3 expression but lower expression levels of Kir2.1 and Kir2.2 in day-0 mouse ventricles, compared with those of day-14 and adult mouse ventricles. Whereas I (Kr) exhibited marked decrease during postnatal development, I (Kur), I (ss), and I (Ca,L) exhibited postnatal developmental increase. The present cell isolation method using the Langendorff perfusion thus found that, in mouse ventricles, I (K1) exhibited postnatal developmental decrease, associated with depolarization of resting potential.

Ionic mechanism of shock-induced arrhythmias: role of intracellular calcium

BACKGROUND

Strong electrical shocks can cause focal arrhythmias, the mechanism of which is not well known. Strong shocks have been shown to produce diastolic Ca(i)(2+) increase, which may initiate focal arrhythmias via spontaneous Ca(i)(2+) rise (SCR), activation of inward Na(+)/Ca(2+) exchange current (I(NCX)), and rise in membrane potential (V(m)). It can be hypothesized that this mechanism is responsible for generation of shock-induced arrhythmias.

OBJECTIVE

The purpose of this study was to examine the roles of SCRs and I(NCX) in shock-induced arrhythmias.

METHODS

The occurrence of SCRs during shock-induced arrhythmias was assessed in neonatal rat myocyte cultures.

RESULTS

Simultaneous V(m)-Ca(i)(2+) optical mapping at arrhythmia source demonstrated that V(m) upstrokes always preceded Ca(i)(2+) transients, and V(m)-Ca(i)(2+) delays were not different between arrhythmic and paced beats (5.5 ± 0.9 and 5.7 ± 0.4 ms, respectively, P = .5). Shocks caused gradual rise of diastolic Ca(i)(2+) consistent with membrane electroporation but no significant Ca(i)(2+) rises immediately before V(m) upstrokes. Application of the Ca(i)(2+) chelator BAPTA-AM (10 μmol/L) decreased the duration of shock-induced arrhythmias whereas application of the I(NCX) inhibitor KB-R7943 (2 μmol/L) increased it, indicating that, despite the absence of SCRs, changes in Ca(i)(2+) affected arrhythmias. It is hypothesized that this effect is mediated by Ca(i)(2+) inhibition of outward I(K1) current and destabilization of resting V(m). The possible role of I(K1) was supported by application of the I(K1) inhibitor BaCl(2) (0.2 mmol/L), which increased the arrhythmia duration.

CONCLUSION

Shock-induced arrhythmias in neonatal rat myocyte monolayers are not caused by SCRs and inward I(NCX). However, these arrhythmias depend on Ca(i)(2+) changes, possibly via Ca(i)(2+)-dependent modulation of outward I(K1) current.

RyRCa2+ leak limits cardiac Ca2+ window current overcoming the tonic effect of calmodulinin mice

Ca²⁺ mediates the functional coupling between L-type Ca²⁺ channel (LTCC) and sarcoplasmic reticulum (SR) Ca²⁺ release channel (ryanodine receptor, RyR), participating in key pathophysiological processes. This crosstalk manifests as the orthograde Ca²⁺-induced Ca²⁺-release (CICR) mechanism triggered by Ca²⁺ influx, but also as the retrograde Ca²⁺-dependent inactivation (CDI) of LTCC, which depends on both Ca²⁺ permeating through the LTCC itself and on SR Ca²⁺ release through the RyR. This latter effect has been suggested to rely on local rather than global Ca²⁺ signaling, which might parallel the nanodomain control of CDI carried out through calmodulin (CaM). Analyzing the CICR in catecholaminergic polymorphic ventricular tachycardia (CPVT) mice as a model of RyR-generated Ca²⁺ leak, we evidence here that increased occurrence of the discrete local SR Ca²⁺ releases through the RyRs (Ca²⁺ sparks) causea depolarizing shift in activation and a hyperpolarizing shift inisochronic inactivation of cardiac LTCC current resulting in the reduction of window current. Both increasing fast [Ca²⁺]_i buffer capacity or depleting SR Ca²⁺ store blunted these changes, which could be reproduced in WT cells by RyRCa²⁺ leak induced with Ryanodol and CaM inhibition.Our results unveiled a new paradigm for CaM-dependent effect on LTCC gating and further the nanodomain Ca²⁺ control of LTCC, emphasizing the importance of spatio-temporal relationships between Ca²⁺ signals and CaM function.

Contribution of sarcoplasmic reticulum Ca²+ release and Ca²+ transporters on sarcolemmal channels to Ca²+ transient in fetal mouse heart

Sarcoplasmic reticulum (SR) Ca release has been shown not to be the predominant mechanism responsible for excitation-contraction (E-C) coupling in fetal myocytes. However, most of the studies have been conducted either on primary cultures or acutely isolated cells, in which an apparent reduction of ryanodine receptor density have been reported. We aimed to elucidate the contribution of SR Ca release and Ca transporters on sarcolemmal channels to Ca transients in fetal mouse whole hearts. On embryonic day 13.5, ryanodine significantly reduced the amplitude of the Ca transient to 27.2 ± 4.4% of the control, and both nickel and SEA0400 significantly prolonged the time to peak from 84 ± 2 ms to 140 ± 5 ms and 129 ± 6 ms, respectively, whereas nifedipine did not alter it. Therefore, at early fetal stages, SR Ca release should be an important component of E-C coupling, and T-type Ca channel and reverse mode sodium-calcium exchanger (NCX)-mediated SR Ca release could be the predominant contributors. Using embryonic mouse cultured cardiomyocytes, we showed that both nifedipine and nickel inhibited the ability of NCX to extrude Ca from the cytosol. From these results, we propose a novel idea concerning E-C coupling in immature heart.

Functional and morphological maturation of implanted neonatal cardiomyocytes as a comparator for cell therapy

Knowledge of the rate of development of immature cardiomyocytes after implantation into a host heart is important for studies using cell therapy. To assess this functionally, we have implanted rat neonatal cardiomyocytes (NCMs) in normal and infarcted rat heart and re-isolated them for functional assessment. Maturation of implanted bone marrow stromal cells (BMSCs) was compared under similar conditions. NCMs from green fluorescent protein (GFP) transgenic rats were implanted into adult normal or infarcted rat hearts and re-isolated after 1, 2, or 4 weeks by standard enzymatic digestion. BMSCs labeled with DiI and iron oxide were implanted into rats with myocardial infarction and cells re-isolated 1, 2, 5, 6, and 16 weeks later. GFP-labeled myocytes approaching the adult morphology were detected 2 weeks after implantation of NCMs, but were significantly shorter than adult host myocytes and had reduced contractility. By 4 weeks after implantation, re-isolated GFP-labeled myocytes were close to the adult phenotype in contractile characteristics, although still significantly shorter. Infarction of the host did not alter the rate of maturation of implanted cells. After implantation of BMSCs, small numbers of functional DiI-labeled myocytes were re-isolated from 4/11 animals but were more mature than expected from the NCM studies. This adds evidence that BMSC-derived cardiomyocytes were not a result of transdifferentiation. The maturation rate of implanted NCMs represents a benchmark against which to evaluate the likely rate of formation of fully functional cardiomyocytes from implanted cells.

Inhibition of L-type calcium currents by salusin-beta in rat cardiac ventricular myocytes

Salusin-beta is a new regulatory peptide relevant to the cardiovascular system and exerts negative inotropic effect on ventricular muscle. The purpose of the present study was to determine whether salusin-beta can inhibit cardiac L-type calcium channel current (I(Ca,L)). Using whole-cell voltage-clamp techniques, I(Ca,L) was measured in ventricular myocytes isolated from 12 to 16 weeks rats. Salusin-beta dose-dependently and reversibly reduced the magnitude of I(Ca,L) in rat ventricular myocytes. Neither threshold potential nor the peak potential of current-voltage relationship was affected. Salusin-beta increased the rate of I(Ca,L) inactivation without altering its gating properties. These results suggest salusin-beta inhibited I(Ca,L) by increasing the rate of I(Ca,L) inactivation and the inhibition of L-type Ca(2+) channels induced by salusin-beta may contribute to its negative inotropic effect on ventricular muscle.

Ablation of triadin causes loss of cardiac Ca2+ release units, impaired excitation-contraction coupling, and cardiac arrhythmias

Heart muscle excitation-contraction (E-C) coupling is governed by Ca(2+) release units (CRUs) whereby Ca(2+) influx via L-type Ca(2+) channels (Cav1.2) triggers Ca(2+) release from juxtaposed Ca(2+) release channels (RyR2) located in junctional sarcoplasmic reticulum (jSR). Although studies suggest that the jSR protein triadin anchors cardiac calsequestrin (Casq2) to RyR2, its contribution to E-C coupling remains unclear. Here, we identify the role of triadin using mice with ablation of the Trdn gene (Trdn(-/-)). The structure and protein composition of the cardiac CRU is significantly altered in Trdn(-/-) hearts. jSR proteins (RyR2, Casq2, junctin, and junctophilin 1 and 2) are significantly reduced in Trdn(-/-) hearts, whereas Cav1.2 and SERCA2a remain unchanged. Electron microscopy shows fragmentation and an overall 50% reduction in the contacts between jSR and T-tubules. Immunolabeling experiments show reduced colocalization of Cav1.2 with RyR2 and substantial Casq2 labeling outside of the jSR in Trdn(-/-) myocytes. CRU function is impaired in Trdn(-/-) myocytes, with reduced SR Ca(2+) release and impaired negative feedback of SR Ca(2+) release on Cav1.2 Ca(2+) currents (I(Ca)). Uninhibited Ca(2+) influx via I(Ca) likely contributes to Ca(2+) overload and results in spontaneous SR Ca(2+) releases upon beta-adrenergic receptor stimulation with isoproterenol in Trdn(-/-) myocytes, and ventricular arrhythmias in Trdn(-/-) mice. We conclude that triadin is critically important for maintaining the structural and functional integrity of the cardiac CRU; triadin loss and the resulting alterations in CRU structure and protein composition impairs E-C coupling and renders hearts susceptible to ventricular arrhythmias.

Effects of unipolar stimulation on voltage and calcium distributions in the isolated rabbit heart

BACKGROUND

The effect of electric stimulation on the polarization of cardiac tissue (virtual electrode effect) is well known; the corresponding response of intracellular calcium concentration ([Ca(2+)](i)) and its dependence on coupling interval between conditioning stimulus (S1) and test stimulus (S2) has yet to be elucidated.

OBJECTIVE

Because uncovering the transmembrane potential (V(m))-[Ca(2+)](i) relationship during an electric shock is imperative for understanding arrhythmia induction and defibrillation, we aimed to study simultaneous V(m) and [Ca(2+)](i) responses to strong unipolar stimulation.

METHODS

We used a dual-camera optical system to image concurrently V (m) and [Ca(2+)](i) responses to unipolar stimulation (20 ms +/- 20 mA) in Langendorff-perfused rabbit hearts. RH-237 and Rhod-2 fluorescent dyes were used to measure V(m) and [Ca(2+)](i), respectively. The S1-S2 interval ranged from 10 to 170 ms to examine stimulation during the action potential.

RESULTS

The [Ca(2+)](i) deflections were less pronounced than changes in V(m) for all S1-S2 intervals. For cathodal stimulation, [Ca(2+)](i) at the central virtual cathode region increased with prolongation of S1-S2 interval. For anodal stimulation, [Ca(2+)](i) at the central virtual anode area decreased with shortening of the S1-S2 interval. At very short S1-S2 intervals (10-20 ms), when S2 polarization was superimposed on the S1 action potential upstroke, the [Ca(2+)](i) distribution did not follow V(m) and produced a more complex pattern. After S2 termination [Ca(2+)](i) exhibited three outcomes in a manner similar to V(m): non-propagating response, break stimulation, and make stimulation.

CONCLUSIONS

Changes in the [Ca(2+)](i) distribution correlate with the behavior of the V (m) distribution for S1-S2 coupling intervals longer than 20 ms; at shorter intervals S2 creates more heterogeneous [Ca(2+)](i) distribution in comparison with V(m). Stimulation in diastole and at very short coupling intervals caused V(m)-[Ca(2+)](i) uncoupling at the regions of positive polarization (virtual cathode).

Does contractile Ca2+ control calcineurin-NFAT signaling and pathological hypertrophy in cardiac myocytes?

In noncontractile cells, a sustained increase in total cytoplasmic Ca(2+) concentration is typically needed to activate the intracellular protein phosphatase calcineurin, leading to dephosphorylation of the transcription factor nuclear factor of activated T cells (NFAT), its nuclear translocation, and induction of gene expression. It remains a mystery exactly how Ca(2+)-dependent signaling pathways, such as that mediated by calcineurin-NFAT, are regulated in contracting cardiac myocytes given the highly specialized manner in which Ca(2+) concentration rhythmically cycles in excitation-contraction coupling. Here, we critically review evidence that supports the hypothesis that calcineurin-NFAT signaling is regulated by contractile Ca(2+) transients in cardiac myocytes.

Mathematical model of the neonatal mouse ventricular action potential

Therapies for heart disease are based largely on our understanding of the adult myocardium. The dramatic differences in action potential (AP) shape between neonatal and adult cardiac myocytes, however, indicate that a different set of molecular interactions in neonatal myocytes necessitates different treatment for newborns. Computational modeling is useful for synthesizing data to determine how interactions between components lead to systems-level behavior, but this technique has not been used extensively to study neonatal heart cell function. We created a mathematical model of the neonatal (day 1) mouse myocyte by modifying, on the basis of experimental data, the densities and/or formulations of ion transport mechanisms in an adult cell model. The new model reproduces the characteristic AP shape of neonatal cells, with a brief plateau phase and longer duration than the adult (action potential duration at 80% repolarization = 60.1 vs. 12.6 ms). The simulation results are consistent with experimental data, including 1) decreased density and altered inactivation of transient outward K+ currents, 2) increased delayed rectifier K+ currents, 3) Ca2+ entry through T-type as well as L-type Ca2+ channels, 4) increased Ca2+ influx through Na+/Ca2+ exchange, and 5) Ca2+ transients resulting from transmembrane Ca2+ entry rather than release from the sarcoplasmic reticulum (SR). Simulations performed with the model generated novel predictions, including increased SR Ca2+ leak and elevated intracellular Na+ concentration in neonatal compared with adult myocytes. This new model can therefore be used for testing hypotheses and obtaining a better quantitative understanding of differences between neonatal and adult physiology.

Cell culture modifies Ca2+ signaling during excitation-contraction coupling in neonate cardiac myocytes

In heart, the excitation-contraction coupling (ECC) mechanism changes during development. Primary cell culture has been used to study Ca(2+) signaling in newborn (NB) rat heart. In this work, the effects of cell culture on the action potential (AP) and ECC Ca(2+) signaling during development were investigated. Specifically, AP, Ca(2+) currents (I(Ca)), and ryanodine receptor (RyR) properties (i.e. density, distribution, and contribution to Ca(2+) transients and Ca(2+) sparks) were defined in cultured myocytes (CM) from 0-day-old NB rat at different times in culture (1-4 days). Compared with acutely dissociated myocytes (ADM) from NB of equivalent ages (1-4 days), CM showed lower RyR density (50% at 1 day, 25% at 4 days), but larger RyR contribution to the Ca(2+) transient (25% at 1 day, 57% at 4 days). Additionally, Ca(2+) sparks were larger, longer, wider, and more frequent in CM than in ADM. RyR cellular distribution also showed different arrangement. While in CM, RyRs were located peripherally, in ADM of equivalent ages a sarcomeric arrangement was predominant. Finally, CM showed a two-fold increase in sarcolemmal Ca(2+) entry during the AP. These results indicated that primary culture is a feasible model to study Ca(2+) signaling in heart; however, it does not precisely reproduce what occurs in ECC during development.

The recombinant dihydropyridine receptor II-III loop and partly structured 'C' region peptides modify cardiac ryanodine receptor activity

A physical association between the II-III loop of the DHPR (dihydropryidine receptor) and the RyR (ryanodine receptor) is essential for excitation-contraction coupling in skeletal, but not cardiac, muscle. However, peptides corresponding to a part of the II-III loop interact with the cardiac RyR2 suggesting the possibility of a physical coupling between the proteins. Whether the full II-III loop and its functionally important 'C' region (cardiac DHPR residues 855-891 or skeletal 724-760) interact with cardiac RyR2 is not known and is examined in the present study. Both the cardiac DHPR II-III loop (CDCL) and cardiac peptide (C(c)) activated RyR2 channels at concentrations >10 nM. The skeletal DHPR II-III loop (SDCL) activated channels at < or =100 nM and weakly inhibited at > or =1 microM. In contrast, skeletal peptide (C(s)) inhibited channels at all concentrations when added alone, or was ineffective if added in the presence of C(c). Ca2+-induced Ca2+ release from cardiac sarcoplasmic reticulum was enhanced by CDCL, SDCL and the C peptides. The results indicate that the interaction between the II-III loop and RyR2 depends critically on the 'A' region (skeletal DHPR residues 671-690 or cardiac 793-812) and also involves the C region. Structure analysis indicated that (i) both C(s) and C(c) are random coil at room temperature, but, at 5 degrees C, have partial helical regions in their N-terminal and central parts, and (ii) secondary-structure profiles for CDCL and SDCL are similar. The data provide novel evidence that the DHPR II-III loop and its C region interact with cardiac RyR2, and that the ability to interact is not isoform-specific.

Ca2+ currents in cardiac myocytes: Old story, new insights

Calcium is a ubiquitous second messenger which plays key roles in numerous physiological functions. In cardiac myocytes, Ca2+ crosses the plasma membrane via specialized voltage-gated Ca2+ channels which have two main functions: (i) carrying depolarizing current by allowing positively charged Ca2+ ions to move into the cell; (ii) triggering Ca2+ release from the sarcoplasmic reticulum. Recently, it has been suggested than Ca2+ channels also participate in excitation-transcription coupling. The purpose of this review is to discuss the physiological roles of Ca2+ currents in cardiac myocytes. Next, we describe local regulation of Ca2+ channels by cyclic nucleotides. We also provide an overview of recent studies investigating the structure-function relationship of Ca2+ channels in cardiac myocytes using heterologous system expression and transgenic mice, with descriptions of the recently discovered Ca2+ channels alpha(1D) and alpha(1E). We finally discuss the potential involvement of Ca2+ currents in cardiac pathologies, such as diseases with autoimmune components, and cardiac remodeling.

Electrophysiologic and mechanical evidence of superiority of hyperpolarizing versus depolarizing cardioplegia in protection of endothelium-derived hyperpolarizing factor–mediated endothelial function: a study in coronary resistance arteries

OBJECTIVE

The advantages of hyperpolarizing cardioplegia with potassium-channel openers versus depolarizing cardioplegia have been suggested but not demonstrated in coronary microarteries. This study examined the simultaneous electric and tonic alteration of coronary microarteries at the cellular level during and after exposure to depolarizing cardioplegia or hyperpolarizing cardioplegia, with emphasis on endothelium-derived hyperpolarizing factor-mediated relaxation and hyperpolarization.

METHODS

Porcine coronary microarteries (diameter, approximately 200-400 microm) were incubated with depolarizing cardioplegia (20 mmol/L KCl) or hyperpolarizing cardioplegia (10 micromol/L aprikalim) for 1 hour. Cellular membrane potential with a glass microelectrode in a coronary smooth muscle cell and isometric force of the muscle were simultaneously measured in a myograph.

RESULTS

Depolarizing cardioplegia incubation produced a stable contraction (from 4.9 +/- 0.3 mN to 7.3 +/- 0.4 mN) and depolarization (from -51 +/- 1 mV to -41 +/- 2 mV). In contrast, hyperpolarizing cardioplegia relaxed (from 4.8 +/- 0.3 mN to 3.5 +/- 0.3 mN) and hyperpolarized (from -51 +/- 2 mV to -56 +/- 1 mV) the smooth muscle. After exposure to depolarizing cardioplegia, the bradykinin-induced, endothelium-derived hyperpolarizing factor-mediated relaxation reduced from 66.2% +/- 5.0% to 18.4% +/- 3.7% (P <.001), and the membrane hyperpolarization reduced from 18 +/- 1 mV to 7 +/- 1 mV (P <.001) in the presence of indomethacin and N(G)-nitro-L-arginine. In contrast, hyperpolarizing cardioplegia did not affect the bradykinin-induced responses.

CONCLUSIONS

In the coronary microarteries, exposure to hyperpolarizing cardioplegia preserves whereas depolarizing cardioplegia reduces the endothelium-derived hyperpolarizing factor-mediated electric (hyperpolarization) and mechanical (relaxation) responses. Thus hyperpolarizing cardioplegia is superior to depolarizing cardioplegia in protecting the endothelial function in the coronary microcirculation.

Phosphoinositide 3-kinase regulates excitation-contraction coupling in neonatal cardiomyocytes

The phosphoinositide 3-kinase (PI3K) inhibitor LY-294002 decreased steady-state contraction in neonatal rat ventricular myocytes (NRVM). To determine whether the effect on steady-state contraction could be due to decreased intracellular Ca2+content, Ca2+content was assessed with fluorescent plate reader analysis by using the caffeine-releasable Ca2+stores as an index of sarcoplasmic reticulum (SR) Ca2+content. Caffeine-releasable Ca2+content was diminished in a dose-dependent manner with LY-294002, suggesting that the decrease in steady-state contraction was due to diminished intracellular Ca2+content. Activation of the L-type Ca2+channel by BAY K 8644 was attenuated by LY-294002, suggesting the effect of LY-294002 is to reduce Ca2+influx at this channel. To investigate whether additional proteins involved in excitation-contraction (EC) coupling are likewise regulated by PI3K activity, the effects of compounds acting at sarco(endo)plasmic reticulum Ca2+-ATPase (SERCA2a), the ryanodine receptor, and the Na/Ca exchanger (NCX) were compared with LY-294002. Inhibition of SERCA2a by thapsigargin increased basal Ca2+levels in contrast to LY-294002, indicating that SERCA2a activity is sustained in the presence of LY-294002. Ryanodine decreased SR Ca2+content. The additive effect with coadministration of LY-294002 could be attributed to a decrease in Ca2+influx at the L-type Ca2+channel. The NCX inhibitor Ni2+was used to investigate whether the decrease in intracellular Ca2+content with LY-294002 could be due to inhibition of the NCX reverse-mode activity. The minimal effect of LY-294002 with Ni2+suggests that the primary effect of LY-294002 on EC coupling occurs through inhibition of PI3K-mediated L-type Ca2+channel activity.

Developmental differences in L-type calcium current of human atrial myocytes

We investigated differences in L-type Ca2+ current (ICa) between infant (INF, 1-12 mo old), young adult (YAD, 14-18 yr old), and older adult (AD) myocytes from biopsies of right atrial appendages. Basal ICa was smaller in INF myocytes (1.2 +/- 0.1 pA/pF, n = 29, 6 +/- 1 mo old, 11 patients) than in YAD (2.5 +/- 0.2 pA/pF, n = 20, 16 +/- 1 yr old, 5 patients) or AD (2.6 +/- 0.3 pA/pF, n = 19, 66 +/- 3 yr old, 9 patients) myocytes (P < 0.05). Maximal ICa produced by isoproterenol (Iso) was similar in INF, YAD, and AD cells: 8.4 +/- 1.1, 9.6 +/- 1.0, and 9.2 +/- 1.3 pA/pF, respectively. Efficacy (Emax) was larger in INF (607 +/- 50%) than for YAD (371 +/- 29%) or AD (455 +/- 12%) myocytes. Potency (EC50) was 8- to 10-fold higher in AD (0.82 +/- 0.09 nM) or YAD (0.41 +/- 0.14 nM) than in INF (7.6 +/- 3.5 nM) myocytes. Protein levels were similar for Gialpha2 but much greater for Gialpha3 in INF than in AD or YAD atrial tissue. When Gialpha3 activity was inhibited by inclusion of a Gialpha3 COOH-terminal decapeptide in the pipette, basal ICa and the response to 10 nM Iso were increased in INF, but not in YAD, cells. We propose that basal ICa and the response to low-dose beta-adrenergic stimulation are inhibited in INF (but not YAD or AD) cells as a result of constitutive inhibitory effects of Gialpha3.

Developmental changes of intracellular Ca2+ transients in beating rat hearts

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

T-tubule function in mammalian cardiac myocytes

The transverse tubules (t-tubules) of mammalian cardiac ventricular myocytes are invaginations of the surface membrane. Recent studies have suggested that the structure and function of the t-tubules are more complex than previously believed; in particular, many of the proteins involved in cellular Ca2+ cycling appear to be concentrated at the t-tubule. Thus, the t-tubules are an important determinant of cardiac cell function, especially as the main site of excitation-contraction coupling, ensuring spatially and temporally synchronous Ca2+ release throughout the cell. Changes in t-tubule structure and protein expression occur during development and in heart failure, so that changes in the t-tubules may contribute to the functional changes observed in these conditions. The purpose of this review is to provide an overview of recent studies of t-tubule structure and function in cardiac myocytes.

Interactions between dihydropyridine receptors and ryanodine receptors in striated muscle

Excitation-contraction coupling in both skeletal and cardiac muscle depends on structural and functional interactions between the voltage-sensing dihydropyridine receptor L-type Ca(2+) channels in the surface/transverse tubular membrane and ryanodine receptor Ca(2+) release channels in the sarcoplasmic reticulum membrane. The channels are targeted to either side of a narrow junctional gap that separates the external and internal membrane systems and are arranged so that bi-directional structural and functional coupling can occur between the proteins. There is strong evidence for a physical interaction between the two types of channel protein in skeletal muscle. This evidence is derived from studies of excitation-contraction coupling in intact myocytes and from experiments in isolated systems where fragments of the dihydropyridine receptor can bind to the ryanodine receptors in sarcoplasmic reticulum vesicles or in lipid bilayers and alter channel activity. Although micro-regions that participate in the functional interactions have been identified in each protein, the role of these regions and the molecular nature of the protein-protein interaction remain unknown. The trigger for Ca(2+) release through ryanodine receptors in cardiac muscle is a Ca(2+) influx through the L-type Ca(2+) channel. The Ca(2+) entering through the surface membrane Ca(2+) channels flows directly onto underlying ryanodine receptors and activates the channels. This was thought to be a relatively simple system compared with that in skeletal muscle. However, complexities are emerging and evidence has now been obtained for a bi-directional physical coupling between the proteins in cardiac as well as skeletal muscle. The molecular nature of this coupling remains to be elucidated.

Alpha(1H) mRNA in single skeletal muscle fibres accounts for T-type calcium current transient expression during fetal development in mice

Calcium channels are essential for excitation-contraction coupling and muscle development. At the end of fetal life, two types of Ca(2+) currents can be recorded in muscle cells. Whereas L-type Ca(2+) channels have been extensively studied, T-type channels have been poorly characterized in skeletal muscle. We describe here the functional and molecular properties of T-type calcium channels in developing mouse skeletal muscle. The T-type current density increased transiently during prenatal myogenesis with a maximum at embryonic day E16 followed by a drastic decrease until birth. This current showed similar electrophysiological and pharmacological properties at all examined stages. It displayed a wide window current centred at about -35 and -55 mV in 10 and 2 mM external Ca(2+), respectively. Activation and inactivation kinetics were fast (3 and 16 ms, respectively). The current was inhibited by nickel and amiloride with an IC(50) of 5.4 and 156 microM, respectively, values similar to those described for cloned T-type alpha(1H) channels. Whole muscle tissue RT-PCR analysis revealed mRNAs corresponding to alpha(1H) and alpha(1G) subunits in the fetus but not in the adult. However, single-fibre RT-PCR demonstrated that only alpha(1H) mRNA was present in prenatal fibres, suggesting that the alpha(1G) transcript present in muscle tissue must be expressed by non-skeletal muscle cells. Altogether, these results demonstrate that the alpha(1H) subunit generates functional T-type calcium channels in developing skeletal muscle fibres and suggest that these channels are involved in the early stages of muscle differentiation.

Gene expression of SERCA2a and L- and T-type Ca channels during human heart development

In this study we report, for the first time, on the gene expression of human cardiac SERCA2a, L-type (alpha(1C)) and T-type (alpha(1H)) Ca channels during development, using RNase protection assay, relative quantitative RT-PCR and Western blot. Human hearts during early gestation (8- to 20-wk gestation), neonatal (1- to 4-d-old) and adult (18- to 48-year-old) stages were used. The results show that T-type Ca channel alpha(1H) subunit mRNA decreased and that L-type Ca channel alpha(1C) subunit mRNA increased with development. While the levels of sarcoplasmic reticulum ATPase (SERCA2a) mRNA did not significantly change with development, its protein levels increased with development. In conclusion, SERCA2a, L-type and T-type Ca channel transcripts were detected as early as 8-wk gestation. Defining the profile of Ca handling proteins during development is important to the understanding of excitation-contraction (EC)-coupling of the developing human heart.

L-type but not T-type calcium current changes during postnatal development in rabbit sinoatrial node

Although the neonatal sinus node beats at a faster rate than the adult, when a sodium current (I(Na)) present in the newborn is blocked, the spontaneous rate is slower in neonatal myocytes than in adult myocytes. This suggests a possible functional substitution of I(Na) by another current during development. We used ruptured [T-type calcium current (I(Ca,T))] and perforated [L-type calcium current (I(Ca,L))] patch clamps to study developmental changes in calcium currents in sinus node cells from adult and newborn rabbits. I(Ca,T) density did not differ with age, and no significant differences were found in the voltage dependence of activation or inactivation. I(Ca,L) density was lower in the adult than newborn (12.1 +/- 1.4 vs. 17.6 +/- 2.5 pA/pF, P = 0.049). However, activation and inactivation midpoints were shifted in opposite directions, reducing the potential contribution during late diastolic depolarization in the newborn (activation midpoints -17.3 +/- 0.8 and -22.3 +/- 1.4 mV in the newborn and adult, respectively, P = 0.001; inactivation midpoints -33.4 +/- 1.4 and -28.3 +/- 1.7 mV for the newborn and adult, respectively, P = 0.038). Recovery of I(Ca,L) from inactivation was also slower in the newborn. The results suggest that a smaller but more negatively activating and rapidly recovering I(Ca,L) in the adult sinus node may contribute to the enhanced impulse initiation at this age in the absence of I(Na).

Loss of annexin A7 leads to alterations in frequency-induced shortening of isolated murine cardiomyocytes

Annexin A7 has been proposed to function in the fusion of vesicles, acting as a Ca(2+) channel and as Ca(2+)-activated GTPase, thus inducing Ca(2+)/GTP-dependent secretory events. To understand the function of annexin A7, we have performed targeted disruption of the Anxa7 gene in mice. Matings between heterozygous mice produced offspring showing a normal Mendelian pattern of inheritance, indicating that the loss of annexin A7 did not interfere with viability in utero. Mice lacking annexin A7 showed no obvious phenotype and were fertile. To assay for exocytosis, insulin secretion from isolated islets of Langerhans was examined. Ca(2+)-induced and cyclic AMP-mediated potentiation of insulin secretion was unchanged in the absence of annexin A7, suggesting that it is not directly implicated in vesicle fusion. Ca(2+) regulation studied in isolated cardiomyocytes, showed that while cells from early embryos displayed intact Ca(2+) homeostasis and expressed all of the components required for excitation-contraction coupling, cardiomyocytes from adult Anxa7(-/-) mice exhibited an altered cell shortening-frequency relationship when stimulated with high frequencies. This suggests a function for annexin A7 in electromechanical coupling, probably through Ca(2+) homoeostasis.

Characterization of nifedipine-resistant calcium current in neonatal rat ventricular cardiomyocytes

Calcium current was recorded from ventricular cardiomyocytes of rats at various stages of postnatal development using the whole cell patch-clamp technique. In cultured 3-day-old neonatal cells, the current carried by Ca(2+) or Ba(2+) (5 mM) was not completely inhibited by 2 microM nifedipine. A residual current was activated in the same voltage range as the L-type, nifedipine-sensitive Ca(2+) current, but its steady-state inactivation was negatively shifted by 16 mV. This nifedipine-resistant calcium current was not further inhibited by other organic calcium current antagonists such as PN200-110, verapamil, and diltiazem nor by nickel, omega-conotoxin, or tetrodotoxin. It was completely blocked by cadmium and increased by isoproterenol and forskolin. This current was >20% of total calcium current in ventricular myocytes freshly isolated from neonatal rats, and it decreased during postnatal maturation, disappearing at the adult stage. This suggests that this current could be caused by an isoform of the L-type calcium channel expressed in a way that reflects the developmental stage of the rat heart.

Nonlinear changes of transmembrane potential during defibrillation shocks: role of Ca(2+) current

Defibrillation shocks induce complex nonlinear changes of transmembrane potential (DeltaV(m)). To elucidate the ionic mechanisms of nonlinear DeltaV(m), we studied the effects of ionic channel blockers on DeltaV(m) in geometrically defined myocyte cultures. Experiments were carried out in cell strands with widths of 0.2 mm (narrow strands) and 0.8 mm (wide strands) produced using a technique of directed cell growth. Uniform-field shocks were applied across strands during the action potential (AP) plateau, and the distribution of shock-induced DeltaV(m) was measured using an optical mapping technique. Nifedipine and 4-aminopyridine were applied to inhibit the L-type calcium current (I:(Ca)) and the transient outward current (I:(to)), respectively. In control conditions, the distribution of DeltaV(m) across cell strands was highly asymmetrical with a large ratio of negative to positive DeltaV(m) (DeltaV(-)(m)/DeltaV(+)(m)) measured at the opposite strand borders. Application of nifedipine caused a large increase of DeltaV(+)(m) and a decrease of DeltaV(-)(m)/DeltaV(+)(m), indicating involvement of I:(Ca) in the asymmetrical DeltaV(m), likely as a result of the outward flow of I:(Ca) when V(m) exceeded the I:(Ca) reversal potential. DeltaV(-)(m) decreased in the narrow strands but remained unchanged in the wide strands, indicating that the changes of DeltaV(-)(m) were caused by electrotonic interaction with an area of depolarization. 4-Aminopyridine did not change DeltaV(-)(m)/DeltaV(+)(m). These results provide evidence that (1) the asymmetry of shock-induced DeltaV(m) during the AP plateau is due to outward flow of I:(Ca) in the depolarized portions of the strands, (2) I:(to) is not involved in the mechanism of DeltaV(m) asymmetry, and (3) the effects of drugs on DeltaV(m) are modulated by the tissue geometry.

Inhibition by nickel of the L-type Ca channel in guinea pig ventricular myocytes and effect of internal cAMP

The characteristics of nickel (Ni) block of L-type Ca current (I(Ca, L)) were studied in whole cell patch-clamped guinea pig cardiac myocytes at 37 degrees C in the absence and presence of 100 microM cAMP in the pipette solution. Ni block of peak I(Ca,L) had a dissociation constant (K(d)) of 0.33 +/- 0.03 mM in the absence of cAMP, whereas in the presence of cAMP, the K(d) was 0.53 +/- 0.05 mM (P = 0.006). Ni blocked Ca entry via Ca channels (measured as I(Ca, L) integral over 50 ms) with similar kinetics (K(d) of 0.35 +/- 0.03 mM in cAMP-free solution and 0.30 +/- 0.02 mM in solution with cAMP, P = not significant). Under both conditions, 5 mM Ni produced a maximal block that was complete for the first pulse after application. Ni block of I(Ca,L) was largely use independent. Ni (0. 5 mM) induced a positive shift (4 to 6 mV) in the activation curve of I(Ca,L). The block of I(Ca,L) by 0.5 mM Ni was independent of prepulse membrane potential (over the range of -120 to -40 mV). Ni (0.5 mM) also induced a significant shift in I(Ca,L) inactivation: by 6 mV negative in cAMP-free solution and by 4 mV positive in cells dialyzed with 100 microM cAMP. These data suggest that, in addition to blocking channel conductance by binding to a site in the channel pore, Ni may bind to a second site that influences the voltage-dependent gating of the L-type Ca channel. They also suggest that Ca channel phosphorylation causes a conformational change that alters some effects of Ni. The results may be relevant to excitation-contraction coupling studies, which have employed internal cAMP dialysis, and where Ni has been used to block I(Ca,L) and Ca entry into cardiac cells.

Antiarrhythmic drug therapy in the neonate

The neonate presents a challenge for the practitioner considering antiarrhythmic therapy: pharmacokinetics are different than in older children or adults; and the arrhythmia substrate may also differ, with respect to issues of ion channel and autonomic nervous system development. This paper reviews the need for antiarrhythmic drug therapy in the neonate, developmental aspects of pharmacokinetics, autonomic regulation and cellular electrophysiology and the antiarrhythmics available today with an emphasis on newer drug therapy.

Developmental regulation of the L-type calcium channel alpha1C subunit expression in heart

We used Northern analyses, RNase protection assays and immunoblot analyses to examine the relationship among developmental age of the heart, abundance of mRNA and L-type calcium channel alpha1C subunit protein, and to establish the size of the native protein in heart. Northern analysis, RNase protection assays, and immunoblots were used to study RNA and protein from rat heart of various ages. In fetal and adult ventricles there was a predominant 8.3-kb transcript for the alpha1C subunit with no change in transcript size during development. RNase protection assays demonstrated a 2-fold increase in abundance of the DHP receptor message during postnatal development. Immunoblots identified a 240 kD protein, corresponding to the predicted molecular mass of the full length alpha1C subunit. No change in size of protein for the alpha1C subunit was observed at any developmental stage and there was no evidence for a truncated isoform. There was an approximate 2-fold increase in alpha1C subunit protein in ventricular homogenates during postnatal development. Thus, in the developing rat heart, alterations in calcium channel properties during development appear to result neither from alternative splicing that produces a smaller transcript for the alpha1C subunit nor from expression of a truncated protein, but at least in part from transcriptionally-regulated expression of the 240 kDa polypepde.

Long-term myocardial preservation: beneficial and additive effects of polarized arrest (Na+-channel blockade), Na+/H+-exchange inhibition, and Na+/K+/2Cl- -cotransport inhibition combined with calcium desensitization

BACKGROUND

Polarized arrest, induced by tetrodotoxin (TTX) at an optimal concentration of 22 micromol/L, has been shown to reduce ionic imbalance and improve myocardial preservation compared with hyperkalemic (depolarized) arrest. Additional pharmacologic manipulation of ionic changes (involving inhibition of Na+ influx by the Na+/H+ exchanger [HOE694] and Na+/K+/2Cl- cotransporter [furosemide], and calcium desensitization [BDM]) may further improve long-term preservation. In this study, we (i) established optimal concentrations of each drug, (ii) determined additive effects of optimal concentrations of each drug and (iii) compared our optimal preservation solution to an established depolarizing cardioplegia (St Thomas' Hospital solution No 2: STH2) used during long-term hypothermic storage for clinical transplantation.

METHODS

The isolated working rat heart, perfused with Krebs Henseleit (KH) buffer was used; cardiac function was measured after 20 min aerobic working mode perfusion. The hearts (n=6/group) were arrested with a 2 ml infusion (for 30 sec) of the polarizing (control) solution (22 micromol/L TTX in KH) or control+drug and subjected to 5 hr or 8 hr of storage at 7.5 degrees C in the arresting solution. Postischemic function during reperfusion was measured (expressed as percentage of preischemic function).

RESULTS

Dose-response studies established optimal concentrations of HOE694 (10 micromol/L), furosemide (1.0 micromol/L) and BDM (30 mmol/L) in the polarizing (control) solution. Sequential addition to the control solution (Group I) of optimal concentrations of HOE694 (Group II), furosemide (Group III), and BDM (Group IV) were compared with STH2 (Group V); postischemic recovery of aortic flow was 29+/-7%, 49+/-6%*, 56+/-2%*, 76+/-3%*, and 25+/-6%, respectively (*P<0.05 vs. I and V). Creatine kinase leakage was lowest, and myocardial ATP content was highest in Group IV.

CONCLUSIONS

A polarizing preservation solution (KH+TTX) containing HOE694, furosemide, and BDM significantly enhanced long-term preservation compared with an optimized depolarizing solution (STH2) used clinically for long-term donor heart preservation.

Polarization and myocardial protection

Heart surgery or transplantation generally involve global ischemia, and techniques have been developed to protect the myocardium from ischemic and reperfusion injury. Hyperkalemic cardioplegia has been the gold standard for myocardial protection for years, but patients undergoing surgery almost invariably have some postoperative dysfunction. One factor may be the depolarizing nature of hyperkalemia, which results in continuing transmembrane fluxes and metabolism, even during hypothermic ischemia. A potentially beneficial alternative to hyperkalemic (depolarizing) cardioplegia is arrest in a "hyperpolarized" or "polarized" state, which maintains the myocardial membrane potential at or near the resting potential. This should minimize transmembrane fluxes and metabolic demand and improve myocardial protection. These alternative concepts have recently been investigated by using adenosine and potassium-channel openers (which are thought to induce hyperpolarized arrest) or the sodium-channel blocker tetrodotoxin (which induces polarized arrest), and results have been beneficial compared with the results of hyperkalemia. Additional studies are needed before experimental promise can become clinical reality.

Developments in cardioprotection: "polarized" arrest as an alternative to "depolarized" arrest

During cardiac surgery or cardiac transplantation, the heart is subjected to varying periods of global ischemia. The heart must be protected during this ischemic period to avoid additional injury, and techniques have been developed that delay ischemic injury and minimize reperfusion injury. Almost universally, this involves using a hyperkalemic cardioplegic solution and these solutions have become the gold standard for myocardial protection for more than 20 years. Despite the extensive and continued research aimed at improving these basic hyperkalemic cardioplegic solutions, patients undergoing surgery almost invariably experience some degree of postoperative dysfunction. It is likely that this relates to the depolarizing nature of hyperkalemic solutions, which results in ionic imbalance caused by continuing transmembrane fluxes and the consequent maintenance of high energy phosphate metabolism, even during hypothermic ischemia. A potentially beneficial alternative to hyperkalemic cardioplegia is to arrest the heart in a "hyperpolarized" or "polarized" state, which maintains the membrane potential of the arrested myocardium at or near to the resting membrane potential. At these potentials, transmembrane fluxes will be minimized and there should be little metabolic demand, resulting in improved myocardial protection. Recent studies have explored these alternative concepts for myocardial protection. The use of compounds such as adenosine or potassium channel openers, which are thought to induce hyperpolarized arrest, have demonstrated improved protection after normothermic, or short periods of hypothermic, ischemia when compared to hyperkalemic (depolarized) arrest. Similarly, studies from our own laboratory, in which the sodium channel blocker, tetrodotoxin, was used to induce polarized arrest (demonstrated by direct measurement of membrane potential during ischemia) was also shown to provide better recovery of function after 5 hours of long-term hypothermic (7.5 degrees C) storage. These promising initial studies need to be consolidated before experimental promise becomes clinical reality.

Neuropeptide Y contributes to innervation-dependent increase in I(Ca, L) via ventricular Y2 receptors

The developmental increase in L-type Ca current (I(Ca,L)) density in the rat ventricle is reproduced in vitro by culturing neonatal myocytes with sympathetic neurons. We tested whether this effect of sympathetic innervation results from a chronic or sustained action of neurally released neuropeptide Y (NPY). Ventricular myocytes from newborn rats were cultured in serum-free medium with or without sympathetic neurons, NPY, or NPY analogs. Ca currents were measured in single myocytes at room temperature using the perforated patch clamp. In all cell groups (control, innervated, or NPY treated), the current-voltage relation for I(Ca,L) was represented by a bell-shaped curve with maximal value near 0 mV. The current density at 0 mV normalized to that of corresponding mean control values was 1.63 +/- 0.12 and 1.52 +/- 0.16 for innervated and NPY-treated myocytes, respectively. Both groups differed significantly from control (P < 0.05). NPY analogs exhibited the following rank order of effectiveness: NPY >/= NPY-(13-36) >/= PYY >> [Leu31Pro34]NPY, suggesting that the NPY effect occurs via a Y2-receptor subtype. In confirmation, chronic treatment of innervated cultures with a Y2-selective NPY antagonist prevented the innervation-dependent increase in I(Ca,L). These results indicate that sympathetic innervation contributes to the developmental increase in I(Ca,L) via neurally released NPY acting at Y2 receptors on the ventricular myocytes.

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.

Long-term myocardial preservation: effects of hyperkalemia, sodium channel, and Na/K/2Cl cotransport inhibition on extracellular potassium accumulation during hypothermic storage

OBJECTIVES

We previously demonstrated improved myocardial preservation with polarized (tetrodotoxin-induced), compared with depolarized (hyperkalemia-induced), arrest and hypothermic storage. This study was undertaken to determine whether polarized arrest reduced ionic imbalance during ischemic storage and whether this was influenced by Na+/K +/2Cl- cotransport inhibition.

METHODS

We used the isolated crystalloid perfused working rat heart preparation (1) to measure extracellular K+ accumulation (using a K+-sensitive intramyocardial electrode) during ischemic (control), depolarized (K+ 16 mmol/L), and polarized (tetrodotoxin, 22 micromol/L) arrest and hypothermic (7.5 degrees C) storage (5 hours), (2) to determine dose-dependent (0.1, 1.0, 10 and 100 micromol/L) effects of the Na +/K+/2Cl- cotransport inhibitor, furosemide, on extracellular K+ accumulation during polarized arrest and 7.5 degrees C storage, and (3) to correlate extracellular K+ accumulation to postischemic recovery of cardiac function.

RESULTS

Characteristic triphasic profiles of extracellular K+ accumulation were observed in control and depolarized arrested hearts; a significantly attenuated profile with polarized arrested hearts demonstrated reduced extracellular K+ accumulation, correlating with higher postischemic function (recovery of aortic flow was 54% +/-4% [P =.01] compared with 39% +/-3% and 32% +/-3% in depolarized and control hearts, respectively). Furosemide (0.1, 1.0, 10, and 100 micromol/L) modified extracellular K+ accumulation by -18%, -38%, -0.2%, and +9%, respectively, after 30 minutes and by -4%, -27%, +31%, and +42%, respectively, after 5 hours of polarized storage. Recovery of aortic flow was 53% +/-4% (polarized arrest alone), 56% +/-8%, 70% +/-2% (P =.04 vs control), 69% +/-4% (P =.04 vs control), and 65% +/-3% ( P =. 04 vs control), respectively.

CONCLUSIONS

Polarized arrest was associated with a reduced ionic imbalance (demonstrated by reduced extracellular K+ accumulation) and improved recovery of cardiac function. Further attenuation of extracellular K + accumulation (by furosemide) resulted in additional recovery.

Relationship between L-type Ca2+ current and unitary sarcoplasmic reticulum Ca2+ release events in rat ventricular myocytes

1. The time courses of Ca2+ current and Ca2+ spark occurrence were determined in single rat ventricular myocytes voltage clamped with patch pipettes containing 0.1 microM fluo-3. Acquisition of line-scan images on a laser scanning confocal microscope was synchronized with measurement of Cd2+-sensitive Ca2+ currents. In most cells, individual Ca2+ sparks were observed by reducing Ca2+ current density with nifedipine (0.1-8 microM). 2. Ca2+ sparks elicited by depolarizing voltage-clamp pulses had a peak [Ca2+] amplitude of 289 +/- 3 nM with a decay half-time of 20.8 +/- 0.2 ms and a full width at half-maximum of 1.40 +/- 0.03 microm (mean +/- s. e.m., n = 345), independent of the membrane potential. 3. The time between the beginning of a depolarization and the initiation of each Ca2+ spark was calculated and data were pooled to construct waiting time histograms. Exponential functions were fitted to these histograms and to the decaying phase of the Ca2+ current. This analysis showed that the time constants describing Ca2+ current and Ca2+ spark occurrence at membrane potentials between -30 mV and +30 mV were not significantly different. At +50 mV, in the absence of nifedipine, the time constant describing Ca2+ spark occurrence was significantly larger than the time constant of the Ca2+ current. 4. A simple model is developed using Poisson statistics to relate macroscopic Ca2+ current to the opening of single L-type Ca2+ channels at the dyad junction and to the time course of Ca2+ spark occurrence. The model suggests that the time courses of macroscopic Ca2+ current and Ca2+ spark occurrence should be closely related when opening of a single L-type Ca2+ channel initiates a Ca2+ spark. By comparison with the data, the model suggests that Ca2+ sparks are initiated by the opening of a single L-type Ca2+ channel at all membrane potentials encountered during an action potential.

Effect of phosphorylation on L-type calcium current in ventricular myocytes dialysed with proteolytic enzymes

1. L-Type Ca2+ channels play important roles in cardiac excitation and conduction. The present study used the whole-cell patch-clamp technique to investigate properties of Ca2+ channels in guinea-pig isolated ventricular myocytes. The effects of internal application of the proteolytic enzymes trypsin and carboxypeptidase (CBP) on the whole-cell L-type Ca2+ current (ICa) were determined. When the effects of the enzymes on ICa had reached steady state, the effects of isoprenaline (ISP) or 2,3-butane-dione monoxime (BDM), which increase and decrease channel phosphorylation, respectively, were examined. The effects of these agents were compared with those observed in the absence of enzyme pretreatment. 2. The amplitude and inactivation characteristics of ICa during depolarizing voltage-clamp commands to +10 mV (0.1 Hz) were determined at 37 degrees C. 3. Trypsin and CBP (both at concentrations of 1 mg/mL in the pipette solution) increased the amplitude of ICa 4.2- and 2.8-fold, respectively, and each enzyme increased the time constant of the slowly inactivating current by 50%. 4. Trypsin decreased the potential at which ICa was half maximally activated from (mean +/- SD) -1.4 +/- 2.2 mV (n = 9) to -11.3 +/- 2.5 mV (n = 7). Although CBP increased ICa amplitude, it did not shift the half-maximal activation voltage. Maximum conductance was increased 5.3-fold by trypsin and 2.2-fold by CBP. 5. Isoprenaline (1 mumol/L) had no effects in myocytes dialysed with trypsin, but significantly increased the current in myocytes dialysed with CBP by 8%. 6. At 12 mmol/L, BDM had no effect on current amplitude in the presence of trypsin, but decreased the time constant of slow inactivation to control values. After dialysis with CBP, BDM significantly decreased the maximum current by 11% and also decreased the rate of slow inactivation towards control values. 7. These data suggest that trypsin and CBP may have digested a part of the calcium channel that normally restricts current flow, but to different extents. The enzymes interacted with BDM and ISP in a fashion suggesting that two sites may influence the amplitude of the current and at least two other sites may influence the time course of the slowly inactivating current.