Lactate:Glycolytic End Product and Oxidative Substrate During Sustained Exercise in Mammals — The “Lactate Shuttle”

Abstract

Results of kinetic tracer studies on several mammalian species during rest and prolonged sustained exercise indicate that lactate is a metabolic intermediate which is very active in supplying carbon for a number of important physiological processes. Lactate is a product of glycolysis and glycogenolysis, and it is a precursor for glucose and glycogen resynthesis. Additionally, lactate produced at some sites during exercise is also a substrate for oxidative energy transduction at other sites. This “shuttling of lactate” through the interstitium and vasculature can be quantitatively as important as the release of glucose from the liver for supplying oxidizable substrate. Because much of the glycolytic and gluconeogenic flux passes through the lactate pool, the metabolism of lactate emerges as a critically important component in the overall integration and regulation of intermediary metabolism. This dynamic role of lactate in mammals, as described here, is different from the stagnant role of lactate portrayed from measurements of lactate concentration in the blood and muscle of mammals and other species. Rather than a dead-end metabolite which accumulates as the resuit of muscle anoxia during exercise and waits until the recovery (“O₂ debt”) period to be returned to glucose and glycogen, lactate is a dynamic metabolite which turns over rapidly and which can participate in a number of processes during rest, exercise, and recovery from exercise.