Kinetics and compartmentation of energy metabolism in intact skeletal muscle determined from super(18)O labeling of metabolite phosphoryls

Analyses of isolated intact diaphragm muscle show that at rest only about 30% of the total cellular P sub(i) is metabolically reactive as indicated by super(18)O incorporation from ( super(18)O)water, whereas up to 90% becomes metabolically active incrementally with contractile frequency. Kinetics of ( gamma - super(18)O)ATP appearance show that about 90% of the cellular ATP is metabolically active and suggest slowly and rapidly metabolizing compartments of ATP in resting muscle and only rapidly metabolizing compartments in contracting muscle. Rates of ( super(18)O)creatine phosphate (( super(18)O)CrP) appearance are consistent with creatine kinase-catalyzed phosphoryl exchange functioning in an obligatory phosphoryl shuttle system.