Time course of changes in gluconeogenic enzyme activities during exercise and recovery.

Gluconeogenic enzymes were assayed after varying periods of exercise and recovery to determine how rapidly changes occur and whether they persist after the cessation of exercise. Untrained male rats (250 g) ran on a treadmill at 28 m/min and were killed after varying periods of exercise and recovery. Livers were quickly removed and analyzed for maximal enzyme activities (saturating levels of substrate) and submaximal activities (low-substrate concentrations). The most significant enzyme changes during exercise were increased maximal activity of phosphoenolpyruvate carboxykinase (PEPCK) and decreased submaximal activity of phosphofructokinase (PFK). Submaximal PFK activity was decreased by 30 min of exercise and remained at that low level up to exhaustion (172 +/- 16 min). Changes in submaximal PFK activity are in response to decreased concentrations of fructose-2,6-bisphosphate that were decreased to approximately one-tenth the control value after 30 min of exercise and remained low throughout exercise and 1 h of recovery. The PEPCK activity progressively increased during exercise and was highest at exhaustion. The cAMP level was significantly elevated in liver of rats exercised for 30 min and continued to rise with duration. Six hours after exercise PEPCK and submaximal PFK activities were the same in control and exercised-rested rats. The change in PEPCK activity is consistent with an increase in the rate of enzyme synthesis and/or a decrease in enzyme degradation during exercise, whereas the lowered activity of PFK likely reflects covalent modification of 6-phospho-fructo-2-kinase/fructose-2,6-bisphosphatase.