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 act ivity and on arrhythmias induced by reperfusion were stud led at diffe rent Mg2+ concentrations (Mg-o(2+), 0, 1.2, and 4.8 mM) in perfused ra t hearts. Surface electrograms and transmembrane potentials were recor ded during control, 10 min of ischemia (perfusion arrest or coronary l igation), and reperfusion. Increasing Mg-o(2+) from 0-4.8 mM decreased heart rate, did not alter action potential morphology, and had a stro ng antiarrhythmic action on reperfusion following coronary ligation. A t low and normal Mg-o(2+), the incidence of tachyarrhythmias was betwe en 70 and 80%. Global ischemia led to progressive atrioventricular blo ck and the final ventricular beating rate was similar at all Mg-o(2+) despite unequal initial values. The severity of arrhythmias was simila r to that found after regional ischemia in Mg-o(2+) = 0, but much lowe r at normal and high Mg-o(2+). The resting depolarization induced by c oronary ligation decreased as Mg-o(2+) 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 ventricul ar rate between 160 and 380 beats per min but increased considerably w hen sinus rate was markedly slowed (40 to 80 bpm) by raising Mg-o(2+) to 9.6 mM. Our data show that a high Mg-o(2+) exerts a strong protecti on against reperfusion arrhythmias regardless of the type of ischemia. Modulation of the sinus rhythm by Mg2+ may contribute to its protecti ve effect by decreasing K-o(+) accumulation and Na-i(+) loading during ischemia.