Changes in glycemia induced by exercise in rats: Contribution of hepatic glycogenolysis and gluconeogenesis

The participation of hepatic glycogenolysis and gluconeogenesis to the glyc emic changes promoted by exercise was investigated. For this purpose, we em ployed swimming rats (2.5% body weight extra load attached to the tail, at 24 degrees C) using a favorable condition to measure hepatic glycogenolysis (fed rats) and a favorable condition to measure hepatic gluconeogenesis (f asted rats). This experimental approach permits us to compare the contribut ion of hepatic glycogenolysis and gluconeogenesis to glucose changes for a specific schedule of exercise. The animals were investigated at rest, after 5 minutes of swimming and after swimming to exhaustion. Our results show t hat hepatic glycogen has a crucial role to determine hyperglycemia during e xercise. In contrast, hypoglycemia developed during exercise when glycogen was depleted. However, the ability of the liver to produce glucose from L-l actate, glycerol and L-glutamine was increased during exercise. Taken toghe ter, these findings suggest that the hepatic capacity to produce glucose fr om gluconeogenic substrates (except for L-alanine) was increased when hepat ic glycogen stores were depleted. Thus, the increased capacity to produce g lucose shown by livers from exercising rats must to be an important metabol ic adaptation to protect against severe hypoglycemia.