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Uptake of radiolabelled deoxyglucose out of proportion to reduced coronary flow demonstrated by positron emission tomography has been used to identify reversibly ischemic, viable myocardium. For this concept to be applied reliably in the clinical setting, factors that may depress glucose availability independent of tissue viability, such as adrenergic stimulation and substrate competition, must be examined. Accordingly, we studied the effect of catecholamine stimulation by dopamine on myocardial glucose uptake in vivo using chronically instrumented, intact dogs and positron emission tomography. We measured myocardial activity of [2-18F]-2-deoxyglucose (FDG) and 82Rb in glucose-loaded animals randomly studied during dopamine infusion, during insulin infusion, and then during their combined infusion. Myocardial FDG uptake was significantly decreased when animals were treated with dopamine, compared with treatment in the same animals with insulin. When insulin was added to the dopamine infusion, myocardial FDG uptake was restored. In contrast, myocardial activity of 82Rb, which is taken up in proportion to coronary flow, was similar under all three experimental conditions. Plasma glucose, free fatty acid, and lactate concentrations were determined before and during each infusion. The depression of myocardial FDG activity seen during dopamine infusion and its reversal with addition of insulin can be explained on the basis of effects of these hormones on substrate availability and competition.