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.