The lactate shuttle during exercise and recovery.

Most (75%+) of the lactate formed during sustained, steady-rate exercise is removed by oxidation during exercise, and only a minor fraction (approximately 20%) is converted to glucose. Significant lactate extraction occurs during net lactate release from active skeletal muscle; the total lactate extraction approximates half the net chemical release. Of the lactate which appears in blood, most of this will be removed and combusted by oxidative (muscle) fibers in the active bed and the heart. The "shuttling" of oxidizable substrate in the form of lactate from areas of high glycogenolytic rate to areas of high cellular respiration through the interstitium and vasculature appears to represent an important means by which substrate is distributed, metabolic "waste" is removed, and the functions of various tissues are coordinated during exercise. During recovery from sustained exhausting exercise, most of the lactate accumulated during exercise will continue to be removed by direct oxidation. However, as the muscle respiratory rate declines in recovery, lactate becomes the preferred substrate for hepatic gluconeogenesis. Practically all of the newly formed liver glucose will be released into the circulation to serve as a precursor for cardiac and skeletal muscle glycogen repletion. Liver glycogen depots will not be restored, and muscle glycogen will not be completely restored until refeeding. This is because the diversion of lactate carbon to oxidation during exercise and recovery represents an irreversible loss of gluconeogenic precursor and because the processes of protein proteolysis and gluconeogenesis from amino acids are insufficient to achieve complete glycogen restitution after exhausting exercise.(ABSTRACT TRUNCATED AT 250 WORDS)