The purpose of these studies is to establish a better understanding of energy metabolism in tissues in vivo. Toward this goal, this laboratory concentrates on the use of non-invasive and non-destructive optical and NMR techniques to evaluate the biochemical and physiological function of the heart with regard to energy metabolism. Of special interest to the laboratory is the control of oxidative phosphorylation and blood flow in the intact heart. The following major findings were made over the last year: 1) Our studies have revealed that the rate limiting step for maximal respiratory rate in isolated mitochondria is carbon substrate delivery. Intermediate rates of respiration are regulated by the interplay between the carbon substrate delivery and F1-ATPase activity. A new hypothesis concerning the regulation of oxidative phosphorylation has been developed concerning the interaction of metabolic substrate utilization and energy dissipation of the inner membrane by F1-ATPase. 2) An in vitro assay for F1-ATPase activity has been developed for rapid biopsy samples from the heart. These studies revealed for the first time that the F1-ATPase activity is up-regulated with increases in work in vivo. 3) Studies on the isolated perfused heart reveal that the cytosolic redox potential and the phosphorylation potential are not in equilibrium. This was performed using a new indicator of the cytosolic redox state 3-glycerol-phosphate. These data suggest that the effects of lactate and pyruvate on cardiac energetics must involve processes other than the cytosolic redox state alone. 4) Adenosine has been proposed a major feedback metabolite in the control of coronary blood flow. We have several new lines of evidence that adenosine action is not required in the control of coronary blood flow in response to workload. We have shown that the specific inhibitor, 8-theophylline, does not alter the relationship between oxygen consumption and blood flow in the intact heart. This occurred with no evidence of cardiac ischemia in the presence of adenosine action inhibition. We are currently evaluating other metabolites which might serve as appropriate feedback or feedforward regulators of coronary flow. 5) A model for the study of muscle energetics has been developed with permits the observation of chronic pacing training on the skeletal muscle of the rabbit in vivo. These studies have revealed that the transformation of muscles into cardiac muscle like fibers during chronic pacing can be the result of the metabolic deficit induced by the pacing.