Opposite effects of acute hypoglycemia and acute hyperglycemia on glucose transport and glucose transporters in perfused rat skeletal muscle
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This study was undertaken to characterize the effects of glycemia per se (glucose effectiveness) on muscle glucose transport. Isolated rat hindlimbs were perfused in situ for 2 h with perfusate containing either low (2 mmol/l, n = 7), normal (6.5 mmol/l, n = 6), or high (20 mmol/l, n = 6) concentrations of glucose, without insulin, to simulate hypo-, eu-, and hyperglycemic conditions. The effect of varying glucose concentrations on muscle glucose transport was assessed by an ensuing 30-min perfusion with 5.5 mmol/l glucose perfusate without insulin. The 2-h of low glucose perfusion induced significant increases in both muscle glucose clearance (approximately 2.3-fold, P < 0.01) and plasma membrane GLUT4 content (approximately 20%, P < 0.05) relative to normal. In contrast, high glucose perfusion decreased glucose clearance (approximately 1.7-fold, P < 0.01) and plasma membrane GLUT4 content (approximately 20%, P < 0.05). Glucose extraction during the following 30-min perfusion was 2.5-fold greater (P < 0.0001) in the low group and threefold less (P < 0.0001) in the high group, relative to normal. 2-[3H]deoxyglucose-6-phosphate content in both red (soleus) and white (extensor digitorum longus) muscles increased approximately twofold after 2 h of low glucose perfusion (P < 0.0001) and decreased > or =2-fold after high glucose perfusion (P < 0.0001), relative to normal. It is concluded that glycemia regulates glucose transport in skeletal muscle independently of insulin, achieved at least partially via changes in plasma membrane GLUT4. We propose that high glucose levels can acutely downregulate GLUT4 and glucose clearance, thus limiting excessive glucose uptake in muscle. Conversely, low glucose-induced upregulation of muscle glucose clearance and GLUT4 can compensate for reduced glucose availability in the circulation.
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