Mechanical responses generated by alternating current or repetitive square pulses in the frog skeletal muscle.

It is well known fact that alternating current(AC)of 50c/sec is more effective than repetitive square pulses(SP)to generate the contraction of skeletal muscle,suggesting that AC has an action for muscle to contract,besides the generation of action potential.Some experiments have been done on the effectiveness of the internal current for the initiation of contraction (CSAPO and SUZUKI,1958;CSAPO,1959).On the contrary,TAYLOR(1953),STENKNUDSEN(1954,1960)and WATANABE(1958)has pointed out that the applied current did not act on the contractile substance directly but through depolarization of membrane.And then,HODGKIN and HOROWICZ(1960)successfully determined the quantitative relation between depolarization and tension of potassium contracture in the single muscle fibre.On the other hand,KATZ and Lou(1947),STEN-KNUDSEN(1954)and CSAPO and WILKIE(1956)showed the contraction of non-propagating muscle in excess potassium solution by the application of AC.They suggested that,in excess potassium solution, not the spike potential but the local response would be the cause of contraction.But no information has been submitted whether or not the local responses are also repeated in normal Ringer solution during repetitive stimulation.The answer to this question was clearly given in the previous paper (MASHIMA and WASHIO,1968).Namely,the electrical responses evoked by AC or SP were separated into successive three phases;the phase of spike potentials,the phase of alternating spike and local responses and the phase of summated local responses. Recently,however,COSTANTIN and PODOLSKY(1967)have shown the evidence that the skinned fibre could be contracted by electrical current and LEE et al.(1966)also have found that the isolated sarcoplasmic reticulum could release Ca++by electrical stimulation.These facts imply the possibility that