Glucosidic pathways of glycogen breakdown and glucose production by muscle from postexercised frogs.

Muscle and body glucose in frogs increases markedly during the initial hour of recovery after strenuous exercise. The liver is not the major source responsible for this accumulation. This is indicated by the stability of liver glycogen levels after exercise and by the observation that hepatectomized and normal frogs accumulate similar amounts of glucose in their muscles and body during recovery. The renal contribution cannot account for this increase in body glucose. Most of the glucose that accumulates in the body after exercise has a muscular origin, as indicated by the facts that two-thirds of the body glucose is found in muscle and that the intracellular levels of muscle glucose are much higher than those of the plasma. The glucose that accumulates outside muscle may also have a muscular origin. The glucosidic pathways of glycogen breakdown are the only metabolic avenue with sufficient capacity to account for the amount of glucose accumulated in muscle during the first hour of recovery. These results indicate that the ability of an isolated preparation of frog muscle to liberate glucose during recovery from exercise (Fournier et al. J. Biol. Chem. 267: 8234-8238, 1992) is not an artifactual metabolic curiosity but rather a metabolic reality that takes place in vivo. Glucose accumulation during recovery is thought to facilitate the metabolic transition of frog carbohydrate metabolism from a catabolic state, characteristic of exercise, to an anabolic one.