Role of cardiac work in regulating myocardial biochemical characteristics.

The purpose of this study was to determine the extent to which functional demand regulates the biochemical character and enzyme capacities of the rat myocardium. Hearts from donor rats were heterotopically transplanted onto the abdominal aorta and inferior vena cava of isogenic recipients. The procedure results in a perfused but nonpumping heart that has a reduced heart rate (HR) and performs essentially no stroke work (SW). After 30 days, metabolic enzyme activities (phosphorylase, 6-phosphofructokinase, citrate synthase, and 3-hydroxyacyl-CoA dehydrogenase) were significantly lower (40-60%) in the nonworking heart. Specific sarcoplasmic reticulum Ca2(+)-adenosinetriphosphatase (ATPase) activity was unchanged, but activity per gram of heart was 41% lower. Myosin isozymes were 58% V1, 21% V2, and 21% V3 in the nonworking heart compared with 100% V1 in the working heart. Myosin and myofibrillar ATPase activities each decreased by 28%. These findings suggest that both HR and SW play major and specific roles in regulating myocardial biochemical capacities and determining the myosin phenotype.