Mobilization of glucose from the liver during exercise and replenishment afterward.

The liver is anatomically well situated to regulate blood glucose. It is positioned downstream from the pancreas, which releases the key regulatory hormones glucagon and insulin. It is also just downstream from the gut, permitting efficient extraction of ingested glucose and preventing large excursions in systemic glucose after a glucose-rich meal. The position of the liver is not as well situated from the standpoint of experimentation and clinical assessment, as its primary blood supply is impossible to access in conscious human subjects. Over the last 20 years, to study hepatic glucose metabolism during and after exercise, we have utilized a conscious dog model which permits sampling of the blood that perfuses (portal vein, artery) and drains (hepatic vein) the liver. Our work has demonstrated the key role of exercise-induced changes in glucagon and insulin in stimulating hepatic glycogenolysis and gluconeogenesis during exercise. Recently we showed that portal venous infusion of the pharmacological agent 5'-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside leads to a marked increase in hepatic glucose production. Based on this, we propose that the concentration of AMP may be a component of a physiological pathway for stimulating hepatic glucose production during exercise. Insulin-stimulated hepatic glucose uptake is increased following exercise by an undefined mechanism that is independent of liver glycogen content. The fate of glucose taken up by the liver is critically dependent on hepatic glycogen stores, however, as glycogen deposition is greatly facilitated by prior glycogen depletion.