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 at constant pH, [MgATP2-], [K+], [CP2-], [CPK], and ionic strength. Ca2+ (or Sr2+) activated steady state tensions were recorded for three [Mg2+]'s: 5x10-5 M, 1 x 10-3 M, and 2x10-3 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+] = 5x10-5 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.
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