EFFECTS OF EXTRACELLULAR MAGNESIUM MANIPULATION ON REPERFUSION-INDUCED ARRHYTHMIAS AND MYOCARDIAL ION SHIFTS IN ISOLATED ISCHEMIC-REPERFUSED RAT HEARTS

Isolated rat hearts were subjected to global ischemia followed by repe rfusion, and a reduction in the incidence of reperfusion-induced ventr icular fibrillation and ventricular tachycardia was brought about by i ncreasing the extracellular Mg concentration in the perfusion buffer. Thus the incidence of ventricular fibrillation was reduced from its co ntrol value of 100% in 1.2 mM Mg to 83% by 2.4 mM Mg (P = N.S.), to 42 % by 3.6 mM Mg (P < .05), to 17% by 4.8 mM Mg (P <.001) and to 17% by 9.6 mM Mg (P < .001). The corresponding values for ventricular tachyca rdia were 100% (control, 1.2 mM Mg) vs. 92% (P = N.S.), 50% (P < .05), 25% (P < .01) and 25% (P < .01), respectively. In further studies, ex tracellular Ca was reduced by 50% (1.2 mM) in the perfusion buffer jus t before ischemia and during reperfusion. The incidence of ventricular fibrillation was reduced from its control value of 83% in 1.2 mM Mg t o 75% by 1.8 mM Mg (P = N.S.), to 33% by 2.4 mM Mg (P < .05), to 17% b y 3.6 mM Mg (P < .01) and to 8% by 4.8 mM Mg (P < .01). The incidence of ventricular tachycardia followed the same pattern. Myocardial Na, K , Ca and Mg were measured by atomic absorption spectrophotometer after the removal of ions from the extracellular space. In controls, 30 min of ischemia resulted in 3- and 4-fold accumulation of myocardial Na a nd Ca, respectively, and during reperfusion these values were similar to the values for 30-min ischemia. Elevated extracellular Mg attenuate d ischemia and reperfusion-induced myocardial Na and Ca gain in both t he 2.4 and the 1.2 mM extracellular Ca-perfused groups. Thirty minutes of ischemia followed by reperfusion resulted in a 50% decrease in bot h myocardial K and myocardial Mg. When extracellular Mg was increased in the perfusate, myocardial K and Mg losses were reduced. Our results show that the manipulation of Mg in the perfusate can modify both the ischemia and the reperfusion-induced ion shifts and protect the heart against arrhythmias.