Post-exercise lactate metabolism: a comparative review of sites, pathways, and regulation.
Most vertebrates utilize supplemental lactate production to support the energetic demands of vigorous, brief exercise. Despite similar patterns of accumulation, there appears to be a trichotomy with regards to lactate processing post-exercise. Most fish retain most of their lactate intramuscularly, using it for in situ glycogen replenishment. Recent evaluation of fish muscle concludes that pyruvate kinase reversal is a probable gluconeogenic pathway. Amphibians and reptiles also utilize lactate as a muscle glyconeogenic substrate, but lactate is not sequestered post-exercise. None of these groups utilize hepatic gluconeogenesis to any significant extent post-exercise, and muscle glucose uptake is limited. Lactate oxidation plays a major role post-exercise in mammals, with hepatic and muscular gluco- and glyconeogenesis contributing to a lesser extent. Glucocorticoids may regulate lactate release from fish muscle, although catecholamines may influence glyconeogenesis in reptile muscle. Insulin affects lactate metabolism indirectly through its effects on muscle glucose metabolism.