Effects of external calcium reduction on biphasic potassium contractures and action of divalent cations on the calcium reduction in frog single twitch muscle fibers.

Abstract

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 80 mM K+ contractures was greatly potentiated by exposing the fiber to low Ca2+ solution for 30 sec, 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 3 mM Mg2+ or 0.5 mM Ni2+. The inhibition of the secondary component, especially the shortening of its time course, was reversed partially by 3 mM Mg2+ and almost completely by 0.5 mM Ni2+. The difference between the inhibitory effect of 10 mM Mg2+ or 1-3 mM 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 20 min, but it was observed in the presence of 3 mM 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.