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 at constant pH, [MgATP^2-], [K⁺], [CP^2-], [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.