Low sodium inotropy is accompanied by diastolic Ca2+ gain and systolic loss in isolated guinea-pig ventricular myocytes

Mechanisms of flecainide induced negative inotropy: An in silico study

It is imperative to develop better approaches to predict how antiarrhythmic drugs with multiple interactions and targets may alter the overall electrical and/or mechanical function of the heart. Safety Pharmacology studies have provided new insights into the multi-target effects of many different classes of drugs and have been aided by the addition of robust new in vitro and in silico technology. The primary focus of Safety Pharmacology studies has been to determine the risk profile of drugs and drug candidates by assessing their effects on repolarization of the cardiac action potential. However, for decades experimental and clinical studies have described substantial and potentially detrimental effects of Na⁺ channel blockers in addition to their well-known conduction slowing effects. One such side effect, associated with administration of some Na⁺ channel blocking drugs is negative inotropy. This reduces the pumping function of the heart, thereby resulting in hypotension. Flecainide is a well-known example of a Na⁺ channel blocking drug, that exhibits strong rate-dependent block of I_Na and may cause negative cardiac inotropy. While the phenomenon of Na⁺ channel suppression and resulting negative inotropy is well described, the mechanism(s) underlying this effect are not. Here, we set out to use a modeling and simulation approach to reveal plausible mechanisms that could explain the negative inotropic effect of flecainide. We utilized the Grandi-Bers model [1] of the cardiac ventricular myocyte because of its robust descriptions of ion homeostasis in order to characterize and resolve the relative effects of QRS widening, flecainide off-target effects and changes in intracellular Ca²⁺ and Na⁺ homeostasis. The results of our investigations and predictions reconcile multiple data sets and illustrate how multiple mechanisms may play a contributing role in the flecainide induced negative cardiac inotropic effect.

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.

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.

Regulation of cardiac L-type Ca2+ current in Na+-Ca2+ exchanger knockout mice: functional coupling of the Ca2+ channel and the Na+-Ca2+ exchanger

L-type Ca2+ current (I(Ca)) is reduced in myocytes from cardiac-specific Na+-Ca2+ exchanger (NCX) knockout (KO) mice. This is an important adaptation to prevent Ca2+ overload in the absence of NCX. However, Ca2+ channel expression is unchanged, suggesting that regulatory processes reduce I(Ca). We tested the hypothesis that an elevation in local Ca2+ reduces I(Ca) in KO myocytes. In patch-clamped myocytes from NCX KO mice, peak I(Ca) was reduced by 50%, and inactivation kinetics were accelerated as compared to wild-type (WT) myocytes. To assess the effects of cytosolic Ca2+ concentration on I(Ca), we used Ba2+ instead of Ca2+ as the charge carrier and simultaneously depleted sarcoplasmic reticular Ca2+ with thapsigargin and ryanodine. Under these conditions, we observed no significant difference in Ba2+ current between WT and KO myocytes. Also, dialysis with the fast Ca2+ chelator BAPTA eliminated differences in both I(Ca) amplitude and decay kinetics between KO and WT myocytes. We conclude that, in NCX KO myocytes, Ca2+-dependent inactivation of I(Ca) reduces I(Ca) amplitude and accelerates current decay kinetics. We hypothesize that the elevated subsarcolemmal Ca2+ that results from the absence of NCX activity inactivates some L-type Ca2+ channels. Modulation of subsarcolemmal Ca2+ by the Na+-Ca2+ exchanger may be an important regulator of excitation-contraction coupling.

Cardiac Na+-Ca2+ exchanger current induced by tyrphostin tyrosine kinase inhibitors

Tyrosine kinase (TK) inhibitors genistein and tyrphostin A23 (A23) inhibited Ca(2+) currents in guinea-pig ventricular myocytes investigated under standard whole-cell conditions (K(+)-free Tyrode's superfusate; EGTA-buffered (pCa-10.5) Cs(+) dialysate). However, the inhibitors (100 microM) also induced membrane currents that reversed between -40 and 0 mV, and the objective of the present study was to characterize these currents. Genistein-induced current behaved like Cl(-) current, and was unaffected by either the addition of divalent cations (0.5 mM Cd(2+); 3 mM Ni(2+)) that block the Na(+)-Ca(2+) exchanger (NCX), or the removal of external Na(+) and Ca(2+). A23-induced current was independent of Cl(-) driving force, and strongly suppressed by addition of Cd(2+) and Ni(2+), and by removal of either external Na(+) or Ca(2+). These and other results suggested that A23 activated an NCX current driven by submembrane Na(+) and Ca(2+) concentrations higher than those in the bulk cytoplasm. Improved control of intracellular Na(+) and Ca(2+) concentrations was obtained by suppressing cation influx (10 microM verapamil) and raising dialysate Na(+) to 7 mM and dialysate pCa to 7. Under these conditions, stimulation by A23 was described by the Hill equation with EC(50) 68 +/- 4 microM and coefficient 1.1, tyrphostin A25 was as effective as A23, and TK-inactive tyrphostin A1 was ineffective. Phosphotyrosyl phosphatase inhibitor orthovanadate (1 mM) antagonized the action of 100 microM A23. The results suggest that activation of cardiac NCX by A23 is due to inhibition of genistein-insensitive TK.

Intracellular Ca2+ concentration and rate adaptation of the cardiac action potential

Influx of Ca(2+) ions through the cardiac plasma membrane contributes to the shaping of the action potential plateau and acts as trigger for the release of Ca(2+) ions from the sarcoplasmic reticulum and the initiation of the contractile process. The increased intracellular Ca(2+) concentration feeds back on the channels and transporters in the plasma membrane and modulates the electrical activity. This interaction and its change with rate of pacing is the topic of this review, which is subdivided in three parts. In part I a description is given of different channels and transporters that carry Ca(2+) ions, or are activated-modulated by intracellular Ca(2+) ions. In part II an analysis is given of the changes in action potential duration and shape when stimuli are applied in the relative refractory period (electrical restitution) and when rate is suddenly increased and kept at the higher level until steady-state is obtained. A description of experimental findings in each case is followed by a discussion of possible mechanisms. Part III deals with physiopathological aspects of Ca(2+) handling and discusses recent information on hypertrophy, heart failure and atrial fibrillation.

Sarcoplasmic Reticulum Calcium Content Fluctuation Is the Key to Cardiac Alternans

The aim of this work was to investigate whether beat-to-beat alternation in the amplitude of the systolic Ca 2+ transient (Ca 2+ alternans) is due to changes of sarcoplasmic reticulum (SR) Ca 2+ content, and if so, whether the alternans arises due to a change in the gain of the feedback controlling SR Ca 2+ content. We found that, in rat ventricular myocytes, stimulating with small (20 mV) depolarizing pulses produced alternans of the amplitude of the Ca 2+ transient. Confocal measurements showed that the larger transients resulted from propagation of Ca 2+ waves. SR Ca 2+ content (measured from caffeine-evoked membrane currents) alternated in phase with the alternans of Ca 2+ transient amplitude. After a large transient, if SR Ca 2+ content was elevated by brief exposure of the cell to a Na + -free solution, then the alternans was interrupted and the next transient was also large. This shows that changes of SR Ca 2+ content are sufficient to produce alternans. The dependence of Ca 2+ transient amplitude on SR content was steeper under alternating than under control conditions. During alternation, the Ca 2+ efflux from the cell was also a steeper function of SR Ca 2+ content than under control. We attribute these steeper relationships to the fact that the larger responses in alternans depend on wave propagation and that wave propagation is a steep function of SR Ca 2+ content. In conclusion, alternans of systolic Ca 2+ appears to depend on alternation of SR Ca 2+ content. This, in turn results from the steep dependence on SR Ca 2+ content of Ca 2+ release and therefore Ca 2+ efflux from the cell as a consequence of wave propagation.

Positive inotropic effect of endothelin-1 in the neonatal mouse right ventricle

In neonatal mouse right ventricles, endothelin-1 (ET-1, 1-300 nM) induced a dose-dependent increase in twitch contractions and the dose-response curve was shifted to the right by BQ-123 (10 microM), an endothelin ET(A) receptor antagonist. The ET-1 (100 nM)-induced positive inotropy was accompanied by an increase in [Ca(2+)](i) transients without any change in the [Ca(2+)](i)-force relationship. Ryanodine (1 microM) partially decreased the [Ca(2+)](i) transients and contractile force, but did not affect the ET-1 (100 nM)-induced positive inotropy. Reduction of [Na(+)](o) elicited an increase in contractile force, and this effect was significantly inhibited by KB-R7943 (30 microM), an inhibitor of the Na(+)-Ca(2+) exchanger. KB-R7943 (30 microM) almost completely suppressed the positive inotropic effect of ET-1. Activation of protein kinase C (PKC) by phorbol 12,13-dibutylate (100 nM) decreased the contractile force, an effect which was suppressed by bisindolylmaleimide I (3 microM). On the other hand, the ET-1-induced positive inotropic effect was unaffected by bisindolylmaleimide I (3 microM). These results suggest that the positive inotropic effect of ET-1 in neonatal mouse right ventricles is caused by the increase in [Ca(2+)](i) transients through activation of the endothelin ET(A) receptor and the increase in Ca(2+) influx via the Na(+)-Ca(2+) exchanger during an action potential. Furthermore, the ET-1-induced positive inotropy is independent of the effects of PKC, which makes it distinct from the ET-1-mediated pathways reported for cardiac tissues in other species.

Heart failure -- a challenge to our current concepts of excitation-contraction coupling

Development of novel therapeutic strategies for congestive heart failure (CHF) seems to be hampered by insufficient knowledge of the molecular machinery of excitation-contraction (EC) coupling in both normal and failing hearts. Cardiac hypertrophy and failure represent a multitude of cardiac phenotypes, and available invasive and non-invasive techniques, briefly reviewed here, allow proper quantification of myocardial function in experimental models even in rats and mice. Both reduced fractional shortening and reduced velocity of contraction characterize myocardial failure. Only when myocardial function is depressed in vivo can meaningful studies be done in vitro of contractility and EC coupling. Also, we point out potential limitations with the whole cell patch clamp technique. Two main factors stand out as explanations for myocardial failure. First, a basic feature of CHF seems to be a reduced Ca(2+) load of the sarcoplasmic reticulum (SR) mainly due to a low phosphorylation level of phospholamban. Second, there seems to be a defect of the trigger mechanism of Ca(2+) release from the SR. We argue that this defect only becomes manifest in the presence of reduced Ca(2+) reuptake capacity of the SR and that it may not be solely attributable to reduced gain of the Ca(2+)-induced Ca(2+) release (CICR). We list several possible explanations for this defect that represent important avenues for future research.

Role of the Na/Ca exchanger in arrhythmias in compensated hypertrophy

Sudden, presumably arrhythmic, death is common in heart failure patients. Although total mortality is highest in end-stage failure, the fraction of sudden death in total mortality is higher in the early stages. In each of these stages various, not necessarily identical, ionic mechanisms may contribute to arrhythmogenesis. Dogs with chronic complete atrioventricular block (6-8 weeks) have an increased risk for arrhythmias and sudden death and have compensated biventricular hypertrophy. In this animal model, Ca(2+) release from the sarcoplasmic reticulum (SR) is not reduced. For low frequencies of stimulation, the SR Ca(2+) content is increased, related to a higher activity of the Na/Ca exchanger. Spontaneous Ca(2+) release induces inward Na/Ca exchange current, which can lead to delayed afterdepolarizations (DADs) triggering a new action potential. Such arrhythmogenic DADs and ectopic beats also can be observed in vivo during monophasic action potential recording. They appear after pacing protocols, and/or administration of ouabain, which result in contractile potentiation, suggestive of a enhanced sarcoplasmic reticulum Ca(2+) content. Other arrhythmogenic mechanisms related to increased dispersion of repolarization also can be identified in vivo. Downregulation of delayed K(+) currents is an important factor in prolongation of action potentials. In conclusion, in this animal model of compensated hypertrophy, Ca(2+) handling is different from end-stage heart failure. It is possible that arrhythmogenic mechanisms related to a higher Ca(2+) load contribute to the high incidence of sudden death in stages of compensated hypertrophy before overt heart failure. However, more than one ionic remodeling process is likely to be present, and different cellular mechanisms of arrhythmias can coexist.