Biphasic Potassium Contractures in Frog Single Twitch Muscle Fibers and Effects of Various Agents on the Contractures

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Abstract
The characteristics and the mechanism of the biphasic K 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, R. japonica. Larger diameters (75-100 .mu.m) of single fibers are required as one of the conditions for obtaining a biphasic K contracture. The effects of Ca removal from the external medium and the various agents, i.e., 2.5 .mu.M dantrolene, 5 mM Mn 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 Ca-free Ringer solution or with dantrolene or Mn ions mainly inhibited the secondary component, while it slightly or moderately inhibited the initial component in keeping with its steepness. Procaine, verapamil and lincomycin mainly inhibited the initial component, changing slightly or moderately the secondary component. The excitation-contraction coupling, i.e., the activation process of the initial component differs from that of the secondary component.

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