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 Ca(2+) 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 Mg(2+) concentration ([Mg(2+)](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 [Mg(2+)](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 [Mg(2+)](i) occurred in the absence of [Mg(2+)](o). The [Mg(2+)](i) was increased by the low [pH](i) induced by propionate. The [Mg(2+)](i) was increased by the low [pH](i) induced by NH(4)Cl-prepulse and decreased by the recovery of [pH](i) induced by the removal of NH(4)Cl. These results suggest that the pH can modulate [Mg(2+)](i) with a reverse relationship in heart, probably by affecting the intracellular Mg(2+) homeostasis, but not by Mg(2+) transport across the sarcolemma.