Effect of SCN on potassium contracture in twitch muscle fibers of the frog.

The effect of SCN on potassium contracture, especially the time course and the mechanical inactivation of the contracture, was investigated using frog twitch muscle fibers. SCN increased the magnitude and the rate of rise of the potassium contracture tension and prolonged its time course. These effects of SCN depended on the concentration of K+ in the external medium and on the duration of pretreatment of the fibers with SCN-Ringer solution. The potentiating effect of SCN on the potassium contracture tension was pronounced at lower and moderate concentrations of K+ and this effect attained a maximum within 1 min after the pretreatment. In the contracture induced by exposure of the fibers to K-SCN-solution without the pretreatment, the time course of the contracture, especially the retardation of the spontaneous relaxation, was marked at higher concentrations of K+. This retarding effect of SCN attained a maximum at more than 10 min after the pretreatment with SCN-Ringer solution. SCN shifted the mechanical inactivation curve of potassium contracture toward lower concentrations of K+, as in the case of the activation curve, and markedly increased the rate of the inactivation induced by conditioning with 15 mM K+. In addition, SCN delayed the recovery of potassium contracture from the mechanical inactivation induced by preceding K-SCN-contracture. On the basis of these results, the sites and the mechanism of action of SCN on potassium contracture are discussed.