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

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