The effects of reducing the extracellular calcium concentration on the twitch in isolated frog's skeletal muscle fibres.

When muscle fibres isolated from the frog's semitendinosus are placed in a calcium-free, bicarbonate buffered Ringer's solution the twitch declines in an irregular stepwise fashion and disappears usually within 1 to 9 min. There is often an initial period of twitch potentiation when the fibres are exposed to 0-Ca2+. Although considerably shorter than the time in 0-Ca2+ required to deplete intracellular calcium stores, the time required to eliminate the twitch is longer than estimates of the minimum time required to remove calcium from the fluid in the t-tubular network by free diffusion. When the calcium concentration was only partially reduced the twitch was potentiated at concentrations between 10 and 50% of the usual concentration in Ringer's. At lower calcium concentrations the potentiation is followed by a reduction, and in some fibres the twitch was eliminated without completely removing the calcium ions from the bathing solution. The results support the hypothesis that there is a store of calcium ions bound to the t-tubular membranes ("trigger calcium") which is required for excitation-contraction coupling during the twitch.