pH-dependent modulation of intracellular free magnesium ions with ion-selective electrodes in papillary muscle of guinea pig

A change in pH can alter the intracellular concentration of electrolytes such as intracellular Ca²⁺ and Na⁺ ([Na⁺] _i) that are important for the cardiac function. For the determination of the role of pH in the cardiac magnesium homeostasis, the intracellular Mg²⁺ concentration ([Mg²⁺]_i), membrane potential and contraction in the papillary muscle of guinea pigs using ion-selective electrodes changing extracellular pH ([pH]_o) or intracellular pH ([pH]_i) were measured in this study. A high CO ₂-induced low [pH]_o causes a significant increase in the [Mg²⁺]_i and [Na⁺]_i, which was accompanied by a decrease in the membrane potential and twitch force. The high [pH]_o had the opposite effect. These effects were reversible in both the beating and quiescent muscles. The low [pH]_o-induced increase in [Mg²⁺]_i occurred in the absence of [Mg²⁺] _o. The [Mg²⁺]; was increased by the low [pH]_i induced by propionate. The [Mg²⁺]_i was increased by the low [pH]_i induced by NH₄Cl-prepulse and decreased by the recovery of [pH]_i induced by the removal of NH₄Cl. These results suggest that the pH can modulate [Mg²⁺]_i with a reverse relationship in heart, probably by affecting the intracellular Mg ²⁺ homeostasis, but not by Mg²⁺ transport across the sarcolemma.