Role of Calcium Currents in E-C Coupling and Evolution of the T-System and SR in Developing Cultured Embryonic Amphibian Skeletal Muscle Cells

Adult frog phasic skeletal muscle cells have slow inward calcium current (ICa) (Stanfield, 1977) carried through L-type voltage dependent calcium channels. It has been suggested that ICa may play a role in E-C coupling (Cota & Stefani, 1981, 1989). However, phasic skeletal muscle cells contract for several minutes after the extracellular Ca2+ concentration ([Ca2+]o) is lowered to <10-8 M (Armstrong et al, 1972). Therefore, extracellular Ca2+ (Ca2+o) is not essential for contraction in these fibers. It has been also shown, by blocking Ica that Ica is not essential for triggering contraction (Gonzalez-Serratos et al., 1982). These results have led to the conclusion that Ica has no obvious role in E-C coupling in adult amphibian phasic skeletal muscle. The question arises then as to what is the biological role Ica in phasic skeletal muscle? We have observed that embryonic skeletal muscle cells are capable of contracting during the first day of development in culture (Cordoba-Rodriguez, et al., 1996), before the T-system and the sarcoplasmic reticulum (SR)may have fully developed (Flucher, et al., 1993).

Calcium currents in normal and hypertrophied isolated feline ventricular myocytes

The magnitude and kinetics of the slow inward calcium current (Isi) were compared in single right ventricular myocytes that were isolated from normal cats and cats with right ventricular hypertrophy. Peak inward current density was greater in hypertrophy than normal myocytes (-20.4 +/- 15.3 vs. -10.4 +/- 8.8 microA/cm2, P less than 0.05). When we blocked early outward currents with intracellular CsCl, however, the peak magnitude of Isi was shown to be similar in hypertrophy and normal myocytes (-16.4 +/- 11.2 vs. -12.7 +/- 3.0 microA/cm2, P = NS). The increased net inward current in hypertrophy was thus due to a decrease in Cs-sensitive early outward current rather than an increase in the magnitude of Isi. The fast component of inactivation of Isi was similar in hypertrophy and normal myocytes, but the slow component was delayed in hypertrophy (slow time constant; tau slow = 75.9 +/- 14.7 ms vs. tau slow = 60.6 +/- 4.9 ms, P less than 0.05). These abnormalities of Isi may contribute to the prolonged duration of the action potential and of contraction in hypertrophied myocardium, but a defect in excitation-contraction coupling distal to Isi appears to produce the diminished magnitude of contraction.

Stimulation frequency and external ionic composition control the repriming of caffeine-induced contractures in frog skeletal muscle

The effect of stimulation rate and of external ionic composition on the repriming period of contractures induced by 6 mM caffeine was tested on isolated skeletal muscle fibres of the frog (Rana ridibunda). The repriming period, which was 11.2 ± 0.1 min (mean ± SEM, n = 9) on quiescent fibres, was shortened in fibres stimulated at a frequency ranging from 3 to 12 min−1 (optimal rate, 8 min−1; full repriming 5.7 ± 0.2 min; n = 10). A 10-fold increase in the extracellular calcium concentration shortened the repriming period on both stimulated and quiescent fibres, whereas decreasing external calcium (1/10) delayed it. In a Na+-free solution (Li+ substituted) the repriming period of stimulated fibres was markedly delayed (14 min), whereas quiescent fibres never recover more than 10% of their ability to develop subsequent caffeine contractures. In contrast, with a 35% Na solution, the repriming period was greatly shortened (stimulated, 5.4 ± 0.2 min, n = 7; quiescent, 6.2 ± 0.5 min, n = 8). It is concluded that repriming depends on three mechanisms that seem to refill a calcium store and trigger recovery: the slow inward calcium current, a Na+–Ca2+ exchange, and perhaps a passive Ca2 influx.

A new calcium current underlying the plateau of the cardiac action potential

A small and very slow inward calcium current has been identified in isolated single ventricular cells using TTX and Cd 2+ to block the sodium and fast calcium currents. Activation requires about 300 ms at the threshold potential of —60 mV, decreasing to 80 ms at the peak current voltage of —30 mV. Inactivation is five to ten times longer. Half steady-state activation and inactivation are at — 50 and — 45 mV respectively. The current is distinctively different in both its kinetics and pharmacology from the conventional calcium current described in single heart cells. It is proposed that it contributes significant current to help maintain a major portion of the long ventricular action potential.