Carbohydrate handling in the hind leg muscle of exercising rats

The arterio-venous concentrations of oxygen, glucose and lactate, as well as blood flow and muscle levels of lactate, glucose, hexose-phosphates and glycogen of rat hind leg muscle were determined under basal conditions and under mild and intense exercise, as well as during post-exercise recovery. During intense exercise and fatigue glycogen is practically exhausted, providing glycosyl residues to the hexose-phosphates pool in addition to increased glucose uptake from the blood. The result is the production of huge amounts of lactate, which accumulates in muscle and the skin, and buildup the arterial concentrations. During recovery lactate is slowly disposed of, and the muscle takes up large amounts of glucose which is stored into glycogen, with fully reinstated glucose oxidation. The data shown suggest that the shift from oxidative to mainly anaerobic utilization of glucose is not as streamlined as is usually assumed, since the results found here hint at the wasteful utilization of glycogen-derived hexose skeletons for other synthetic pathways. Glucose, nevertheless, is of paramount importance as energy staple to sustain untrained intense exercise in the rat.