Properties of caffeine- and potassium-contractures in fatigued frog single twitch muscle fibers.

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Abstract
The properties of caffeine contracture and potassium contracture in fatigued single fibers were examined in detail using frog semitendinosus muscle, Rana japonica. Fatigue was caused by repetitive stimulation at 2 Hz. The dose-response curve of caffeine contracture in the fatigued fibers was shifted toward the right; the threshold concentration of caffeine for the contracture in normal fibers was 1.5 mM, whereas that in fatigued fibers was 5 mM. However, in the presence of 25 mM K+ or 0.01% Triton X-100, caffeine contractures occurred sufficiently at the lower concentrations (3-5 mM) even in the fatigued fibers. Furthermore, in the fatigued fibers, the peak tension of the initial component of biphasic potassium contracture with 60 or 80 mM KC1 (C1 constant; 120 mM) was slightly inhibited, whereas the secondary component of the contracture was markedly inhibited. These results indicate that the permeability to caffeine of the transverse tubular membrane (T-membrane) of the fibers and the Ca influx in response to the direct depolarization of T-membrane with K+ are markedly inhibited in the fatigued fibers.

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