Caffeine and the myoplasmic calcium removal mechanisms in cut frog skeletal muscle fibres.

Antipyrylazo III myoplasmic calcium transients were recorded in cut skeletal muscle fibres of the frog (Rana esculenta), using the double vaseline-gap voltage-clamp system. Intracellular calcium removal mechanisms were analysed, using a slightly modified model taken from the literature. Parameter values reported here are generally consistent with those obtained by the original model. Caffeine (0.5 mmol.l-1) moderately enhanced the overall myoplasmic calcium removal. In particular, the rate constant of the non-saturable uptake increased by 51% on the average, but there was a considerable fiber-to-fiber variation. The kinetic features of the binding sites representing the saturable uptake did not change significantly while the concentration of the available sites decreased by 36%. It is concluded that the caffeine-induced changes of the calcium removal components can be explained by supposing an increased resting myoplasmatic Ca2+ concentration in the presence of the drug.