Exposure of the isolated, glucose-perfused rat heart to buffer containing 0.4 mM tolbutamide resulted in significant changes in both energy metabolism and myocardial contractility. In the nondiabetic, tolbutamide mediated only small increases in mechanical function at low atrial filling pressure, but this effect increased with increasing preload. By contrast, the stimulation of mechanical function resulting from exposure of the diabetic heart to tolbutamide was independent of preload. As a result, the tolbutamide-mediated, positive inotropic effect in the diabetic heart was greater at lower, but not higher, preload values than the effect in the nondiabetic. Moreover, the changes in energy metabolism initiated by tolbutamide were considerably larger in the diabetic. The most prominent effect was the mobilization of glycogen by tolbutamide in the diabetic, which was considerably greater than that observed in the nondiabetic. The drug also enhanced glucose utilization. The net effect of sulfonylurea exposure was to shift from preferential use of fatty acids as an energy source for contraction to use of glucose. Since the most prominent effect of the drug in the diabetic was the stimulation of glycogenolysis, it is concluded that tolbutamide can dramatically alter the metabolism of a tissue without acting through insulin.

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