Energy sources in fully aerobic rest-work transitions: a new role for glycolysis

Rate of O2 consumption (VO2), intracellular PO2, lactate extraction, and tissue contents of phosphocreatine, creatine, ATP, lactate, and pyruvate were measured during rest-work transitions in dog gracilis muscles. Samples were taken at rest and after 5, 10, 15, 30, 60, and 180 s of twitch contraction at 4/s. There was no anoxia at any time or location [companion paper, Am. J. Physiol. 248 (Heart Circ. Physiol. 17): H914-H921, 1985]. Energy was supplied by a continuous utilization of phosphocreatine stores, a slow rise in VO2, and two distinct bursts of glycolysis. Glycolytic rate was independent of tissue PO2 and VO2 and was poorly correlated with phosphocreatine under the aerobic conditions of our experiments. The glycolytic bursts served as a source of ATP, and the bulk of the lactate formed remained in the tissue. Lactate efflux was a small fraction of tissue lactate; it depended on blood flow but not on tissue lactate content. At steady state a small glycolytic flux maintained the high tissue lactate mainly by matching rates of pyruvate production and oxidation. We propose that this flux and the high tissue lactate concentration buffer cytosolic redox and/or substrate supply in support of the aerobic functions of mitochondria during exercise.