Abstract
The characteristics and the mechanism of the biphasic potassium contractures, i.e., an initial rapid contraction (initial component) and a slow secondary contraction (secondary component) were investigated in detail using single twitch fibers from the semitendinosus muscles of the frog, Rana japonica. The present experiments confirmed the results of COSTANTIN (1971) and STEFANI and CHIARANDINI (1973) and further demonstrated that larger diameters (75-100 μm) of single fibers are required as one of the conditions for obtaining a biphasic potassium contracture. In addition, the effects of calcium removal from the external medium and the various agents, i.e., 2.5 μm dantrolene, 5 mM manganese ions, 7.5 mm procaine, 0.5 mM verapamil and 6. 5 mM lincomycin on the initial and secondary components were examined. The pretreatment of a single fiber with calcium-free Ringer solution or with dantrolene or manganese ions mainly inhibited the secondary component, while it slightly or moderately inhibited the initial component in keeping with its steepness. In contrast, procaine, verapamil and lincomycin mainly inhibited the initial component, changing slightly or moderately the secondary component. It could be concluded, therefore, that the excitation-contraction coupling, i.e., the activation process of the initial component differs from that of the secondary component.
