This project will use NMR spectroscopy and other methods to examine the role of changes in energetically important metabolites (e.g., PCr, ATP, inorganic phosphate, lactic acid, pH) in the regulation of mammalian skeletal muscle metabolism. The major specific aim is to test competing models of the control of respiration in intact muscle. This will be accomplished by examining the relationships between cytoplasmic phosphate metabolites, mitochondrial NADH/NAD ratio (measured by surface fluorescence) and respiration in cat fast (biceps) and slow-twitch (soleus) muscles at rest and during and after stimulation. Additional studies will examine the effects of experimentally-imposed changes in pH, PCr, ATP, and substrate supply on these relationships. These experiments will also examine the effects of imposed changes in pH and phosphate metabolites on regulation of glycolysis and force development. Additional experiments will examine whether there is functionally-- significant compartmentation of adenine nucleotides in muscle, test the validity of 1H-NMR methods for measurement of lactic acid, and define the relationships between pH and lactic acid during recovery after muscle stimulation. A key feature of these studies compared to other NMR studies of muscle is the use of well-characterized perfused muscle preparations, which enables experimental manipulations not possible in muscles in situ. The results will advance our basic understanding of muscle metabolism, as well as provide essential background for the rational design and interpretation of clinical NMR studies of muscle.