The activation and inactivation in biphasic potassium contractures in frog single twitch muscle fibers.

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Abstract
The dependence of the occurrence of biphasic contractures on KCl concentration and the activation curve, the inactivation curve and the time-dependence curve of inactivation of the initial component of K contracture and those curves of the secondary component were examined using single twitch fibers of the frog semitendinosus muscles. When single fibers having diameters of .apprx. 75-100 .mu.m were used, typical biphasic contractures were clearly observed at 70-80 mM K. At 100 mM K the initial component was difficult to distinguish from the secondary component because of the fusion of both components and at 60 and 40 mM K the initial component hardly occurred and the secondary component alone was observed. The threshold of the activation curve of the initial component (.apprx. 50-60 mM K) was higher than that of the secondary component (.apprx. 32 mM K) and also the time course of the inactivation, induced by conditioning depolarization, of the initial component did not coincide with that of the secondary component. The activation and inactivation processes of the excitation-contraction coupling of the initial component are different from those of the secondary component and the efficiency of the coupling of the initial component is less than that of the secondary component. Each time-dependence curve of the inactivation of the initial and secondary components is apparently biphasic, consisting of the 1st phase and the 2nd phase and that these phases have the characteristics of inactivation 1 and inactivation 2, respectively.

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