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

Submaximum and maximum forces of the cardiac muscle contractile apparatus, activated by Ca²⁺ or Sr²⁺, were determined as a function of Mg²⁺ concentration. Apical left ventricular tissue from Sprague-Dawley rats was broken by homogenization into small bundles of fibers with disrupted sarcolemmas (skinned). Tension generation was activated by and graded according to the concentration of Ca²⁺ or Sr²⁺ in solutions bathing the skinned fibers and measured with a photodiode force transducer. Steady-state tensions for various levels of activation at each of four concentrations of Mg²⁺ (5 x 10^-5, 1 x 10^-3, 5 x 10^-3, and 10 x 10^-3 M) in the bathing solutions were analyzed. Other bathing solution constituents and parameters mimicked significant normal intracellular conditions while providing adequate buffering of [H⁺], [Ca²⁺], and [MgATP^2-] (magnesium adenosine triphosphate). To assess changes in sensitivity of the mechanical system to activation by Ca²⁺ (or Sr²⁺), each submaximum tension was expressed as a percentage of the given fiber bundle's maximum force at saturating [Ca²⁺] (or [Sr²⁺]) at the same [Mg²⁺]. When plotted as saturation curves these data demonstrate that increasing [Mg²⁺] depresses Ca²⁺ sensitivity of the force-generating mechanism. The Ca²⁺ and Sr²⁺ sensitivity of the cardiac force-generating apparatus is very similar at every [Mg²⁺], indicating that the magnitude of Mg²⁺ effect is similar for both types of activation. However, absolute maximum tensions at saturating activating cation concentration increased as [Mg²⁺] increased; the effect of Mg²⁺ on maximum force was proportionately the same for Ca²⁺ and Sr²⁺ activation. But because saturating [Ca²⁺] always resulted in a lower maximum force than saturating [Sr²⁺], this site of Ca²⁺-Mg²⁺ interaction appears distinct from the one influencing Ca²⁺ sensitivity.