Influence of magnesium on chloride-induced calcium release in skinned muscle fibers.

Abstract

Chloride-induced Ca release in skinned frog muscle fibers was studied by measuring isometric force transients and 45Ca loss from fiber to washout solutions. Skinned fibers prepared from muscles soaked in normal Ringer solution made large force transients in 120 mM Cl solution with 5 mM ATP and 1 mM Mg, but 3 mM Mg was inhibitory. Mg inhibition was antagonized by low temperature and by Cd, agents which slow active Ca uptake by the sarcoplasmic reticulum (SR). In low Mg2+, Cl stimulated rapid 45Ca release from the SR in sufficient amounts to account for the force response. The increased 45Ca release was inhibited by EGTA [ethylene glycol bis(.beta.-aminoethyl ether) tetraacetic acid], suggesting that release requires free Ca under these conditions. The 45Ca initially released was partially reaccumulated later. Reaccumulation was increased in higher Mg2+. These results provide additional evidence that the Ca uptake rate is an important determinant of net release, and suggest that Mg2+ acts primarily on this mechanism. Skinned fibers prepared from muscles soaked in low Cl solutions could give force responses to Cl solutions with 3 mM and 6 mM Mg. The Cl stimulus apparently varies with the [Cl] gradient across the internal membranes, and supports the hypothesis that applied Cl causes membrane depolarization.