Glucose Autoregulates Its Uptake in Skeletal Muscle

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Abstract
Preexposure to a low concentration of glucose upregulates glucose transport into skeletal muscle, whereas exposure to a high concentration of glucose has the opposite effect. This autoregulatory process occurs independently of insulin, and the mechanism by which it operates is incompletely understood. Activation of the energy-sensing enzyme AMP-activated protein kinase (AMPK) has been shown to increase insulin-independent glucose transport into skeletal muscle in response to such stimuli as exercise and hypoxia. In the present study, we examined whether AMPK could also mediate glucose autoregulation. The activity of the α2 isoform of AMPK and 2-deoxyglucose uptake were assessed in incubated rat extensor digitorum longus muscle after preincubation for 4 h in media containing 0, 3, 6, or 25 mmol/l glucose. The principal findings were as follows. First, AMPK activity was highest in muscles incubated with no added glucose, and it decreased as the concentration of glucose was increased. In keeping with these findings, the concentration of malonyl CoA was increased, and acetyl CoA carboxylase phosphorylation at serine 79 was decreased as the medium glucose concentration was raised. Second, decreases in AMPK activity at the higher glucose concentrations correlated closely with decreases in glucose transport (2-deoxyglucose uptake), measured during a subsequent 20-min incubation at 6 mmol/l glucose (r2 = 0.93, P < 0.001). Third, the decrease in AMPK activity at the higher glucose concentrations was not associated with changes in whole-tissue concentrations of creatine phosphate or adenine nucleotides; however, it did correlate with increases in the rate of glycolysis, as estimated by lactate release. The results suggest that glucose autoregulates its own transport into skeletal muscle by a mechanism involving AMPK. They also suggest that this autoregulatory mechanism is not paralleled by changes in whole-tissue concentrations of creatine phosphate ATP, or AMP, but they leave open the possibility that alterations in a cytosolic pool of these compounds play a regulatory role.

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