Skeletal muscle energetics can be studied noninvasively at rest, during exercise, and in recovery using phosphorus nuclear magnetic resonance (31P-NMR). In resting muscle, inorganic phosphate (Pi) and total cellular phosphate concentration are regulated by Na+-dependent Pi transport. Insulin was shown to stimulate Pi uptake in G-8 muscle cells, in isolated rat soleus muscle, and in human muscle in vivo under conditions of hyperinsulinemic-euglycemic clamp. The relationship between plasma Pi and intracellular muscle Pi was examined in a group of patients with elevated plasma Pi resulting from renal failure. The total creatine content of muscle cells is controlled by an active creatine uptake in which β2-receptor stimulation and the activity of the Na+-K+-ATPase play a significant role. Recovery after exercise is entirely oxidative; the rate of ATP synthesis is largely controlled by ADP, the concentration of which is determined by the creatine kinase equilibrium that includes the concentration of H+. At the onset of aerobic dynamic exercise, ATP is maintained largely by glycolysis, producing lactic acid, and by phosphocreatine breakdown. After vasodilation, ATP synthesis becomes predominantly oxidative. The above processes can be quantitatively evaluated by 31P-NMR.

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