Differential electrophysiologic effects of global and regional ischemia and reperfusion in perfused rat hearts. Effects of Mg2+ concentration.

The effects of regional and global ischemia on cellular electrical activity and on arrhythmias induced by reperfusion were studied at different Mg2+ concentrations (Mg2+o, 0, 1.2, and 4.8 mM) in perfused rat hearts. Surface electrograms and transmembrane potentials were recorded during control, 10 min of ischemia (perfusion arrest or coronary ligation), and reperfusion. Increasing Mg2+o from 0-4.8 mM decreased heart rate, did not alter action potential morphology, and had a strong antiarrhythmic action on reperfusion following coronary ligation. At low and normal Mg2+o, the incidence of tachyarrhythmias was between 70 and 80%. Global ischemia led to progressive atrioventricular block and the final ventricular beating rate was similar at all Mg2+o despite unequal initial values. The severity of arrhythmias was similar to that found after regional ischemia in Mg2+o = 0, but much lower at normal and high Mg2+o. The resting depolarization induced by coronary ligation decreased as Mg2+o was raised, but such a relation was not seen during global ischemia where the depolarization was less marked. The action potential duration did not vary with the ventricular rate between 160 and 380 beats per min but increased considerably when sinus rate was markedly slowed (40 to 80 bpm) by raising Mg2+o to 9.6 mM. Our data show that a high Mg2+o exerts a strong protection against reperfusion arrhythmias regardless of the type of ischemia. Modulation of the sinus rhythm by Mg2+ may contribute to its protective effect by decreasing K+o accumulation and Na+i loading during ischemia.