Preincubation of rat soleus muscle with 1 and 10 μm monensin for 2 h increased the subsequent basal 2-deoxyglucose uptake by muscle 76 and 121% respectively. Under the same conditions, monensin decreased the insulin-stimulated (1 mU/ml) 2-deoxyglucose uptake by 29 and 37% respectively. The monensin-induced augmentation of basal 2-deoxyglucose uptake was inhibited 92% by cytochalasin B suggesting that the uptake is mediated by glucose transporters. Monensin did not increase the cellular accumulation of l-glucose in muscle indicating that it does not affect the cell membrane integrity. Neither the stimulatory effect of monensin on basal 2-deoxyglucose uptake nor the opposite, inhibitory action of monensin on the insulin-stimulated 2-deoxyglucose uptake were influenced by the removal of Ca2+ from the medium or by dantrolene, an inhibitor of Ca2+ release from the sarcoplasmic reticulum, suggesting that the actions of monensin are not mediated by calcium. Monensin had no effect on muscle ATP concentration. The monensin-induced augmentation of basal 2-deoxyglucose uptake was neither associated with stimulation of muscle phosphatidylinositol 3-kinase activity nor inhibited by wortmannin, demonstrating that the increase in basal 2-deoxyglucose uptake is not mediated by activation of phosphatidylinositol 3-kinase. The inhibition of insulin-stimulated 2-deoxyglucose uptake by monensin was associated with a 31% decrease in the abundance of insulin receptors in muscles, a 64% decrease in the insulin-induced autophosphorylation of the insulin receptor β-subunit, and a 44% reduction of the insulin-stimulated phosphatidylinositol 3-kinase activity. Addition of monensin into the phosphatidylinositol 3-kinase reaction had no effect on the activity of the enzyme, demonstrating that the inhibition in monensin-treated muscles is indirect and occurs upstream of phosphatidylinositol 3-kinase. It is concluded that monensin has a dual effect on 2-deoxyglucose uptake by skeletal muscle: it stimulates basal uptake but inhibits the insulin-stimulated uptake. The primary cause of the latter, inhibitory effect of monensin is at the level of the insulin receptor.
Journal of Endocrinology (1997) 154, 85–93
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