On the source of Ca(2+) activating the tonic component of contraction of myocytes of guinea pig heart

Deciphering Cardiac Biology and Disease by Single-Cell Transcriptomic Profiling

By detecting minute molecular changes in hundreds to millions of single cells, single-cell RNA sequencing allows for the comprehensive characterization of the diversity and dynamics of cells in the heart. Our understanding of the heart has been transformed through the recognition of cellular heterogeneity, the construction of regulatory networks, the building of lineage trajectories, and the mapping of intercellular crosstalk. In this review, we introduce cardiac progenitors and their transcriptional regulation during embryonic development, highlight cellular heterogeneity and cell subtype functions in cardiac health and disease, and discuss insights gained from the study of pluripotent stem-cell-derived cardiomyocytes.

Defective excitation-contraction coupling in hearts of rats with congestive heart failure

AIM

We examined the cellular basis for depressed cardiac contractility in rats with congestive heart failure (CHF) secondary to myocardial infarction.

METHODS

Six weeks after ligation of the left coronary artery, CHF was confirmed by haemodynamic measures and echocardiographic demonstration of reduced myocardial contractility in vivo. Papillary muscles from CHF animals developed less force than those from sham operated (SHAM) animals. Cell shortening was measured in isolated ventricular myocytes voltage-clamped with high resistance electrodes. Ca2+ transients were measured in fluo-4 loaded myocytes.

RESULTS

Contractions triggered by depolarizing test steps from a post conditioning potential of -70 mV were significantly smaller and had significantly reduced velocity of shortening in CHF compared with SHAM myocytes. However, contractions initiated from -40 mV, were similar in amplitude and velocity of shortening in CHF and SHAM cells. L-type Ca2+ current was not significantly different between CHF and SHAM cells, whether activated from -70 or -40 mV. Therefore, in SHAM cells, excitation-contraction coupling exhibited higher gain when contractions were initiated from negative (-70 mV), as compared with depolarized potentials (-40 mV). However, in CHF myocytes, excitation-contraction coupling gain was selectively depressed with steps from -70 mV. This depression of gain in CHF was not accompanied by a significant reduction in sarcoplasmic reticulum Ca2+ content. Isoproterenol increased Ca2+ transients less in CHF than SHAM myocytes.

CONCLUSION

In this post-infarction model of CHF, the contractile deficit was voltage dependent and the gain of excitation-contraction coupling was selectively depressed for contractions initiated negative to -40 mV.

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.

Tonic component of myocardial contraction

Calcium transients and contractions of cardiac myocytes consist of phasic component, relaxing spontaneously independently of membrane voltage and of the tonic component (TC) relaxing only upon repolarization. Experimental data reviewed in this article suggest that most Ca(2+) activating TC is released from sarcoplasmic reticulum (SR) via the ryanodine receptors (RyRs). Most likely these RyRs are activated by sustained Ca(2+) influx. However, its route may differ depending on species and state of the cells. It seems that in rat RyRs responsible for TC are activated by the sustained Ca(2+) current. In guinea-pig the blockers of Ca(2+) current or reverse mode Na(+)/Ca(2+) exchange do not inhibit TC, so these routes seem unlikely. In myocytes of the failing human hearts TC is activated mostly via the reverse mode Na(+)/Ca(2+) exchange and contribution of SR is negligible. The mechanism of TC in the normal human cardiomyocytes has not been investigated. Thus, despite investigation of TC for half a century many problems concerning the mechanism of its activation and maintenance as well as its physiological meaning remain unsolved.

Differential effects of phosphodiesterase-sensitive and -resistant analogs of cAMP on initiation of contraction in cardiac ventricular myocytes

Amplitudes of cardiac contractions initiated by Ca2+-induced Ca2+ release (CICR) are proportional to the magnitude of Ca2+ current (ICa-L). However, large contractions accompanied by little inward current have been reported in some but not all studies in which cells were dialyzed with different analogs of cAMP. This study compares the effects of different phosphodiesterase (PDE)-resistant and PDE-sensitive analogs of cAMP on CICR, and investigates whether cAMP sensitizes CICR so that small currents induce large contractions. Experiments were conducted in voltage-clamped guinea pig ventricular myocytes at 37 degrees C, with different analogs of cAMP added to patch pipette solutions. With PDE sensitive Tris-cAMP, contraction-voltage relations were bell-shaped and proportional to ICa-L. In contrast, dialysis with PDE-resistant dibutyryl-cAMP resulted in sigmoidal contraction-voltage relations and large responses with little inward current. Similarly, in cells loaded with fura-2, large Ca2+ transients were elicited with little inward current in cells dialyzed with PDE-resistant but not PDE-sensitive cAMP. However, large transients were observed with PDE-sensitive cAMP when PDE was inhibited with 3-isobutyl-1-methylxanthine. When the amplitude of ICa-L was varied by partial block with Cd2+, or by partial inactivation, CICR remained proportional to the amplitude of ICa-L. Thus, cAMP altered the relationship between Ca2+ transients and membrane potential but did not sensitize conventional CICR coupled to ICa-L. Our results show that effects of different analogs of cAMP on contraction depend on the PDE resistance of the analog tested. Furthermore, PDE can play a major role in modulating cardiac contraction by altering the relationship between membrane potential and Ca2+ release.

Differential effects of docosahexaenoic acid on contractions and L-type Ca2+ current in adult cardiac myocytes

Beneficial effects of n-3 polyunsaturated fatty acids in Ca2+ overload have been attributed to blockade of L-type Ca2+ current (I(Ca-L)). However, cardiac contractions may be maintained despite block of I(Ca-L).

OBJECTIVE

This study investigates the cellular basis by which docosahexaenoic acid (DHA), a representative n-3 polyunsaturated fatty acid, inhibits I(Ca-L) while preserving contraction.

METHODS

Experiments were conducted in adult guinea pig ventricular myocytes with Na+ currents blocked. Contractions initiated by the voltage-sensitive release mechanism (VSRM) and calcium-induced calcium release (CICR) triggered by I(Ca-L), were activated separately with voltage clamp techniques.

RESULTS

DHA (10 microM) inhibited I(Ca-L) and CICR contractions but not VSRM contractions. CICR contractions exhibited a bell-shaped voltage-dependence. However, in the presence of DHA, only contractions with a sigmoidal voltage-dependence characteristic of the VSRM remained. These contractions exhibited inactivation properties characteristic of the VSRM. DHA abolished I(Ca-L) elicited by test steps from -40 mV. Block was voltage-dependent, as residual I(Ca-L) was elicited by steps from -70 mV. Cd2+ inhibited residual current, but not contractions initiated by the same activation steps.

CONCLUSION

Preservation of VSRM contractions during block of I(Ca-L), may explain the ability of n-3 polyunsaturated fatty acids to inhibit Ca2+ influx while preserving cardiac contractile function.