Effects of External Calcium Reduction on Biphasic Potassium Contractures and Action of Divalent Cations on the Calcium Reduction in Frog Single Twitch Muscle Fibers

The effects of external Ca2+ reduction on the biphasic potassium (K) contractures and the action of divalent cations under conditions of the Ca2+ reduction were examined in detail, using frog single twitch muscle fibers. The peak tension of the initial component of 80mM K+ contractures was greatly potentiated by exposing the fiber to low Ca2+ solution for 30sec, and the degree of this potentiation was decreased with increasing the exposing time. In contrast, the peak tension of the secondary component was rapidly inhibited by Ca2+ reduction. The potentiation of the initial component was removed by 3mM Mg2+ or 0.5mM Ni2+. The inhibition of the secondary component, especially the shortening of its time course, was reversed partially by 3mM Mg2+ and almost completely by 0.5mM Ni2+. The difference between the inhibitory effect of 10mM Mg2+ or 1-3mM Ni2+ on the initial component and that on the secondary component was also demonstrated. The tension development of the secondary component was completely inhibited by external Ca2+ reduction for 20min, but it was observed in the presence of 3mM Mg2+, although its time course was shorter. These results indicate that the actions of Mg2+ and Ni2+ on the initial component differ from those on the secondary component and suggest the possible mechanisms of the actions of these divalent cations on the biphasic K contractures.