Role of voltage-sensitive release mechanism in depression of cardiac contraction in myopathic hamsters

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

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

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.

Normal contractions triggered by I(Ca,L) in ventricular myocytes from rats with postinfarction CHF

Attenuated L-type Ca(2+) current (I(Ca,L)), or current-contraction gain have been proposed to explain impaired cardiac contractility in congestive heart failure (CHF). Six weeks after coronary artery ligation, which induced CHF, left ventricular myocytes from isoflurane-anesthetized rats were current or voltage clamped from -70 mV. In both cases, contraction and contractility were attenuated in CHF cells compared with cells from sham-operated rats when cells were only minimally dialyzed using high-resistance microelectrodes. With patch pipettes, cell dialysis caused attenuation of contractions in sham cells, but not CHF cells. Stepping from -50 mV, the following variables were not different between sham and CHF, respectively: peak I(Ca,L) (4.5 +/- 0.3 vs. 3.8 +/- 0.3 pApF(-1) at 23 degrees C and 9.4 +/- 0.5 vs. 8.4 +/- 0.5 pApF(-1) at 37 degrees C), the bell-shaped voltage-contraction relationship in Cs(+) solutions (fractional shortening, 15.2 +/- 1.0% vs. 14.3 +/- 0.7%, respectively, at 23 degrees C and 7.5 +/- 0.4% vs. 6.7 +/- 0.5% at 37 degrees C) and the sigmoidal voltage-contraction relationship in K(+) solutions. Caffeine-induced Ca(2+) release and sarcoplasmic reticulum Ca(2+)-ATPase-to-phospholamban ratio were not different. Thus CHF contractions triggered by I(Ca,L) were normal, and the contractile deficit was only seen in undialyzed cardiomyocytes stimulated from -70 mV.

Cardiac excitation-contraction coupling: role of membrane potential in regulation of contraction

The steps that couple depolarization of the cardiac cell membrane to initiation of contraction remain controversial. Depolarization triggers a rise in intracellular free Ca(2+) which activates contractile myofilaments. Most of this Ca(2+) is released from the sarcoplasmic reticulum (SR). Two fundamentally different mechanisms have been proposed for SR Ca(2+) release: Ca(2+)-induced Ca(2+) release (CICR) and a voltage-sensitive release mechanism (VSRM). Both mechanisms operate in the same cell and may contribute to contraction. CICR couples the release of SR Ca(2+) closely to the magnitude of the L-type Ca(2+) current. In contrast, the VSRM is graded by membrane potential rather than Ca(2+) current. The electrophysiological and pharmacological characteristics of the VSRM are strikingly different from CICR. Furthermore, the VSRM is strongly modulated by phosphorylation and provides a new regulatory mechanism for cardiac contraction. The VSRM is depressed in heart failure and may play an important role in contractile dysfunction. This review explores the operation and characteristics of the VSRM and CICR and discusses the impact of the VSRM on our understanding of cardiac excitation-contraction coupling.