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

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.