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Effect of intravenous caffeine on liver glycogenolysis during prolonged exercise.

Ingestion of caffeine has been reported to enhance performance during prolonged endurance exercise. This effect on performance has been attributed to a caffeine-induced enhancement of lipid oxidation, thereby sparing muscle glycogen. The purpose of this experiment was to determine the effect of caffeine on the rate of liver glycogenolysis during exercise. Adult male rats were given an intravenous injection of caffeine (5 mg X kg-1) or of 0.9% NaCl. One h later they were run on a motor driven treadmill at 28 m X min-1 up a 15% grade for 45 or 90 min. Liver and muscles were frozen, and blood was collected for analysis. No significant differences were found between caffeine- and saline-injected rats in liver or muscle glycogen, plasma free fatty acid, insulin, glucagon, blood glucose, or blood glycerol. Liver adenosine 3',5'-cyclic monophosphate was essentially the same in caffeine- and saline-injected rats after 90 min of exercise. In a separate study, injection of 5 or 25 mg caffeine per kg body weight immediately prior to a 60-min bout of exercise had no effect on the rate of liver muscle glycogenolysis or on any of the hormones and metabolites studied. We conclude that in the rat, blood levels of caffeine in the range of 3.7 to 29 micrograms X ml-1 have no effect on liver adenosine 3',5'-cyclic monophosphate or the rate of liver glycogenolysis during prolonged treadmill running.