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

The dependence of the occurrence of biphasic contractures on potassium chloride concentration, and the activation curve, the inactivation curve and the time-dependence curve of inactivation of the initial component of potassium 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 about 75-100 micrometers 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 (about 50-60 mM K) was higher than that of the secondary component (about 32 mM K) and also the time course of the inactivation, induced by conditioning depolarization, on the initial component did not coincide with that of the secondary component. From these results, it was concluded that the activation and inactivation processes of the excitation-contraction coupling of the initial component is different from those of the secondary component and that the efficiency of the coupling of the initial component is less than that of the secondary component. In addition, it was also indicated that each time-dependence curve of the inactivation of the initial and secondary components is biphasic, consisting of the first phase and the second phase, and that these phases have the characteristics of inactivation 1 and inactivation 2, respectively, which was proposed by us.