On the antiarrhythmic mechanisms of magnesium in cardiac Purkinje fibers.

The antiarrhythmic actions of high [Mg]o (8 mM) were studied in sheep cardiac Purkinje fibers. At 0.1 microM, strophanthidin inhibits the Na-K pump (intracellular sodium activity aiNa and force increase) and increasing Mg shortens the action potential, decreases force and increases aiNa, as in control. At 1 microM, strophanthidin increases aiNa and force, induces oscillatory potentials (Vos) and arrhythmias: increasing Mg reduces Vos and abolishes arrhythmias but increases aiNa further. High Mg also slows or stops spontaneous activity induced by norepinephrine by a positive shift of the threshold, and decreases Vos in high [Ca]o. Thus, antiarrhythmic mechanisms of high [Mg]o include a shift in threshold potential and a decrease in Vos but not a removal of Na pump inhibition.