Objective: Studies have shown that the diabetic heart exhibits abnormalities in cellular ion transport, which can affect susceptibility to reperfusion-induced ventricular fibrillation (VF), tachycardia (VT) and functional derangements. It has been shown that "preconditioning" renders the heart very resistant to a subsequent prolonged ischemic episode. This phenomenon has been extensively studied in healthy myocardium, but such a study has not been previously done in diseased (hypertrophic or myopathic) hearts.
Methods: We studied the incidence of reperfusion-induced VF, VT, cardiac function, and ion shifts (Na+, K+, Ca2+, and Mg2+) induced by ischemia/reperfusion in isolated hearts from rats with streptozotocin-induced diabetes. Following 2, 4, 6, and 8 weeks of diabetes, hearts were isolated and subjected to 30 min global ischemia followed by reperfusion.
Results: In the 2-week diabetic group the total incidence of VF and VT was reduced from their non-diabetic age-matched control value of 100 and 100% to 42 (P < 0.05) and 42% (P < 0.05), respectively. Such a reduction in the incidence of VF and VT was not observed with progressive diabetes (4, 6, and 8 weeks). In the 2-week diabetics, the reduction in the VF and VT was reflected in the improvement of postischemic function, the reduction of ischemia and reperfusion-induced Na+ and Ca2+ gains, and the prevention in K+ and Mg2+ loss. This diabetes-induced initial protection was not seen in the 4- and 6-week diabetics, and a deterioration of postischemic function was observed in the 8-week diabetics. Four cycles of preconditioning, each consisting of 5 min ischemia followed by 10 min reperfusion, failed to reduce the incidence of VF and VT, improve cardiac function, and prevent ion shifts induced by 30 min ischemia followed by 30 min reperfusion in 4- and 8-week diabetics.
Conclusions: In the early phase of diabetes the heart is more resistant to ischemia/reperfusion than the non-diabetic heart. Preconditioning does not afford protection against a prolonged period of ischemia in diabetics, indicating that preconditioning may be a "healthy heart phenomenon".