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Data from Ahlborg et al. J Clin Invest 1974;53:1080-1090.

glycogen breakdown in muscle, and FFA release from adipocytes. At first glance decreasing insulin secretion might seem deleterious for glucose consumption in muscle. However, the decrease in insulin concentration only decreases glucose uptake by nonworking muscles. Mobilization of GLUT4 and transport of glucose across the sarcolemma is stimulated by muscular contractions independently of insulin. Glucose metabolism in working muscles is therefore not limited by membrane transport but, rather, by phosphofructokinase, which in turn is responsive to a variety of intracellular metabolites that coordinate its activity with energy demand.

Increased hepatic glucose production results primarily from the combined effects of the fall in insulin secretion and the rise in glucagon secretion with some contribution from catecholamines. The contributions of the increased

FIGURE 10 Changes in concentration of insulin and counter-regulatory hormones during prolonged moderate exercise. Values shown are the means obtained for eight young men exercising on a bicycle ergometer at ~50% of maximum oxygen consumption. (Drawn from data of Davis SN, Galassetti P, Wasserman DH, Tate D. Effects of gender on neuroendocrine and metabolic counterregulatory responses to exercise in normal man. J Clin Endocrinol Metab 2000;85:224-230).

Data from Ahlborg et al. J Clin Invest 1974;53:1080-1090.

FIGURE 10 Changes in concentration of insulin and counter-regulatory hormones during prolonged moderate exercise. Values shown are the means obtained for eight young men exercising on a bicycle ergometer at ~50% of maximum oxygen consumption. (Drawn from data of Davis SN, Galassetti P, Wasserman DH, Tate D. Effects of gender on neuroendocrine and metabolic counterregulatory responses to exercise in normal man. J Clin Endocrinol Metab 2000;85:224-230).

secretion of GH and Cortisol to this effect are unlikely to be important initially, but with sustained exercise the contributions of both are likely to increase. Actions of both hormones on adipocytes increase the output of FFA and glycerol and decrease glucose utilization by adipocytes andnonworkingmuscles. Additionally, the increased cortisol would be expected to increase the expression of gluconeogenic enzymes in the liver.

Glycogen reserves of nonworking muscles may provide an important source of carbohydrate for working muscles during sustained exercise and for restoring muscle glycogen after exercise. Epinephrine and norepi-nephrine stimulate glycogenolysis in nonworking as well as working muscles. Glucose phosphate produced from glycogen can be completely broken down to carbon dioxide and water in working muscles, but nonworking muscles convert it to pyruvate and lactate, which escape into the blood. Liver then reconverts these 3-carbon acids to glucose, which is returned to the circulation and selectively taken up by the working muscles (Fig. 11).

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