ABSTRACT: When the heart is stimulated to perform work there is an increase in oxidative ATP production. The mechanism responsible for increasing respiration with increased work is not understood. The goal of this project is to determine what metabolite changes are responsible for stimulating oxygen consumption in the heart. The hypothesis that increased mitochondrial NADH is responsible for stimulating oxygen consumption will be tested. This hypothesis contrasts with older models which place emphasis on changes in phosphates. 31P NMR, surface fluorescence, and absorbance spectroscopy will primarily be used to monitor metabolites. Changes in levels of phosphates and mitochondrial redox state will be measured in the perfused heart as work is varied by increasing pacing, perfusion pressure, and administration of inotropic agents. The effects of a variety of substrates and inhibitors will be used to determine what enzymatic steps are important for respiratory control. The relation between phosphate levels mitochondrial redox state, and oxygen consumption will be investigated in isolated heart mitochondria. The effect of varying Ca++, and treating the heart with adrenergic agents prior to isolation of mitochondria will be studied. The gap between isolated mitochondria and the intact organ will be bridged with the quiescent and hyperpermeable heart preparations These systems allow a greater degree of control over the extramitochondrial environment than is possible with the beating heart. Data from isolated mitochondria, the quiescent and the hyperpermeable heart will be used to determine if changes in metabolites measured in the functioning heart are sufficient to explain increases in oxygen consumption. Changes in energy metabolism have been implicated in blood flow regulation, mechanical failure of the heart, and the ability of the heart to survive ischemia, anoxia, or cold cardioplegia. An understanding of the mechanism of respiratory control in normal myocardium is essential to interpreting changes which may be occurring in pathological conditions.