To study the effects of leucine, glucose, and insulin on myocardial protein degradation in fed and fasted nutritional states, we developed and validated a sensitive method for measuring rates of total protein degradation in rat isolated left atrial preparations. Fasting resulted in a progressive decrease in myocardial protein breakdown to 71% of control over a 24-hour period, with no further reduction in degradation rate between 24 and 72 hours of fasting. Insulin (100 mU/ml) suppressed atrial protein degradation by 38% in fed animals (P< 0.001) and by 51% in fasted animals (P< 0.001). Glucose alone had no effect on protein degradation in either nutritional state. At 5 times normal plasma levels, leucine suppressed protein breakdown by 21% in fed and by 15% in fasted animals. The decrease in degradation induced by fasting and the absence of an effect of glucose are in contrast to the behavior reported for skeletal muscle.Circ Res 46: 000-000, 1980