2 The Insulin-Like Effect of Muscle Contraction

Muscle contraction results in an increase in membrane permeability to glucose. The actual mechanism by which contractile activity increases membrane permeability is unknown. However, kinetic studies suggest that this increase is due to an increase in the number of glucose transporters associated with the plasma membrane. This is also suggested by the recent finding that cytochalsin B, which competitively inhibits the binding of glucose to the glucose transporter in the plasma membrane, prevents activation of the glucose transport process by muscle contraction. Unlike insulin-stimulated glucose transport, in which permeability is reversed rapidly upon removal of the insulin, the increase in membrane permeability following contractile activity can persist for many hours. It has also been reported that the stimulatory effects of insulin and contraction are additive, and that prostaglandin E2 augments the effect of insulin on glucose transport but has no effect on contraction-facilitated glucose transport. Collectively, these findings suggest that insulin and contractile activity increase membrane permeability to glucose by independent mechanisms. An increase in membrane permeability is only partially responsible for the increase in glucose uptake during exercise in vivo. With an increase in muscle activity, there is an increase in delivery of glucose and insulin to the muscle as a result of an increase in muscle blood flow. Glucose uptake may also be facilitated by an increase in the insulin sensitivity of the muscle. The increases in muscle blood flow and insulin sensitivity may be associated with activation of the kinin-prostaglandin system of the muscle. The increase in muscle insulin sensitivity may also involve an increase in insulin binding to its receptors on the sarcolemma. It should be noted that the increase in insulin binding associated with contractile activity requires the presence of epinephrine. Muscle glycogen may also affect the rate of glucose uptake during exercise. During prolonged, moderately intense exercise, glucose uptake increases as the muscle glycogen level declines. This increase in glucose uptake is inversely related to the glucose-6-phosphate concentration of the cell. During high-intensity exercise, the rate of glycogenolysis is rapid, resulting in the accumulation of glucose-6-phosphate and free glucose. Thus, it appears that the rate-limiting step in glucose uptake during exercise is shifted from transport to glucose phosphorylation, and that this shift is mediated by the intracellular glucose-6-phosphate concentration, which is influenced by the rate of muscle glycogen catabolism.