Magnesium and intracellular sodium activity in cardiac Purkinje fibers.

The actions of magnesium on intracellular sodium activity (aiNa), transmembrane potentials and contractile force were studied in sheep cardiac Purkinje fibers perfused in vitro. Increasing [Mg]o from 1.05 to 2-8 mM shortens the action potential, decreases force and increases aiNa in active (and quiescent) fibers. High Ca (8.1 mM) increases force and decreases aiNa: in its presence, 8 mM Mg ('high Mg') decreases force as well as the action potential duration and still increases aiNa. Manganese decreases force and aiNa: the effects of high Mg are not changed by Mn-induced blockade of the slow channel. Even in the presence of 0.25 mM lidocaine (which decreases action potential duration, force and aiNa), high Mg increases aiNa, has little effect on the action potential duration and increases the threshold requirements. In the absence of [Ca]o, 1.35 mM Sr increases the action potential duration as well as aiNa and reduces force: high Mg markedly decreases aiNa, force and action potential duration. Thus, Mg decreases the duration of the action potential (as Ca does) but (in contrast to Ca) decreases contractile force (possibly by acting on intracellular structures) and increases intracellular Na activity. These effects are not dependent on Ca or Na influx and show that the increase in aiNa by Mg is the net result of opposing actions.