The comparative effects of [Ca²⁺] and [Mg²⁺] on tension generation in the fibers of skinned frog skeletal muscle and mechanically disrupted rat ventricular cardiac muscle

The effects of intracellular Mg²⁺ on Ca²⁺-activated isometric tension generation in rat cardiac muscle fibers and frog skeletal muscle were compared. The membranous sarcolemmal barrier was removed from rat cardiac muscle fibers by mechanical disruption and from frog skeletal muscle by skinning. Tension was recorded in the fibers in bathing solutions of different Ca²⁺ concentrations and either 5×10^-5 M or 1×10^-3 M Mg²⁺ concentration (the same concentrations used in a previous study on single skinned frog skeletal muscle fibers [3]). The amount of Ca²⁺ required to activate the muscle increased with Mg²⁺ concentration for both rat ventricular muscle and frog skeletal muscle. These data indicate that intracellular Mg²⁺ concentration could strongly modulate Ca²⁺-activated tension in cardiac muscle and that very similar molecular mechanisms are responsible for Ca²⁺-activated tension in rat ventricular muscle and frog skeletal muscle. The possible sites of action of Mg²⁺ on Ca²⁺-activated tension are discussed.