Characterization of the effects of Mg2+ on Ca2+- and Sr2+-activated tension generation of skinned skeletal muscle fibers

Changes in Mg2+] in a millimolar range have a significant inverse effect on the Ca2+- (or Sr2+)activated tension generation of skeletal muscle fibers. Single frog (Rana pipiens) semitendinosus muscle fibers were "skinned" (sarcolemma removed) and contracted isometrically in bathing solutions of varying Ca2+] or Sr2+] and Mg2+] but a constant pH, MgATP2-], K+], CP2-], CPK], and ionic strength. Ca2+- (or Sr2+- )activated steady-state tensions were recorded for three Mg2+]'s: 5 X 10(-5)M, 1 X 10(-3) M, and 2 X 10(-3) M; and these tensions were expressed as the percentages of maximum tension generation of the fibers for the same Mg2+]. Maximum tension was not affected by Mg2+] within Ca2+-activating or Sr2+-activating sets of solutions; however, the submaximum Ca2+-(or Sr2+)activated tension is strongly affected in an inverse fashion by increasing Mg2+]. Mg2+ behaves as a competitive inhibitor of Ca2+ and also affects the degree of cooperativity in the system. At Mg2+] = 5 X 10(-5)M the shape of tension versus Ca2+] (or Sr2+]) curve showed evidence of cooperativity of Ca2+ (or Sr2+) binding or activation of the contractile system. As Mg2+] increased, the apparent affinity for Ca2+ or Sr2+ and cooperativity of the contractile system declined. The effect on cooperativity suggests that as Mg2+] decreases a threshold for Ca2+ activation appears.