Regulation of hepatic lactate balance during exercise.

The rate of exchange of lactate across the liver gives important insights into intracellular processes during muscular work. At the onset of exercise hepatic glycogenolysis increases rapidly, resulting in high rates of glycolytic flux and a transient rise in lactate output. With increasing exercise duration, gluconeogenesis is accelerated and the liver gradually shifts from a lactate-producing to a lactate-consuming state. Exercise-induced changes in hormone levels are critical in the regulation of hepatic glycogenolysis and gluconeogenesis and, therefore, net hepatic lactate balance. The fall in insulin stimulates hepatic glycogenolysis, glycolytic flux, and, as a result, hepatic lactate output. On the other hand, the stimulatory effects of glucagon on gluconeogenesis elicit an increase in hepatic lactate uptake. The rise in epinephrine may regulate gluconeogenesis during prolonged exercise by stimulating peripheral lactate mobilization, thereby providing gluconeogenic substrate to the liver. Chronic hepatic-denervation leads to an increase in gluconeogenesis and net hepatic lactate uptake at rest without altering total glucose production. However, the response to exercise is unaffected by the absence of hepatic nerves. Hence, the direction and magnitude of the hepatic lactate balance during exercise yields important information regarding flux through the gluconeogenic and glycolytic pathways, such that high rates of gluconeogenesis correspond to accelerated rates of hepatic lactate uptake and high rates of hepatic glycolytic flux lead to increased rates of hepatic lactate output.