Glycogen depletion in exercising rats infused with glucose, lactate, or pyruvate

The influence of an intravenous infusion of glucose, lactate, or pyruvate on the work capacity and rates of glycogen depletion in the liver and hindlimb skeletal muscles of rats has been studied. Running time to exhaustion at a speed of 21 m/min on a treadmill at a +10 degrees incline was prolonged by glucose infusion but shortened by lactate or pyruvate infusions. Blood glucose concentrations were only lowered in the terminal stages of the exercise, whereas at this point lactate was elevated. Declines in liver and muscle glycogen concentrations were retarded by glucose infusion but accelerated by either lactate or pyruvate infusions. In all cases a marked depletion of both muscle and liver glycogen existed at the point of exhaustion. It is concluded from the lack of any major elevation of glucose, lactate, or pyruvate in the blood of the rats during running that these materials were taken up and oxidized, presumably by the working muscles. These observations suggest that skeletal muscle can take up large amounts of glucose and when it is supplied from an external source this can exert a glycogen sparing effect on the liver and working skeletal muscles.