EFFECT OF MONENSIN ON 2-DEOXYGLUCOSE UPTAKE, THE INSULIN-RECEPTOR ANDPHOSPHATIDYLINOSITOL 3-KINASE ACTIVITY IN RAT MUSCLE

Preincubation of rat soleus muscle with land 10 mu M monensin for 2 h increased the subsequent basal 2-deoxyglucose uptake by muscle 76 and 121% respectively. Under the same conditions, monensin decreased the i nsulin-stimulated (1 mU/ml) 2-deoxyglucose uptake by 29 and 37% respec tively. The monensin-induced augmentation of basal 2-deoxyglucose upta ke was inhibited 92% by cytochalasin B suggesting that the uptake is m ediated by glucose transporters. Monensin did not increase the cellula r accumulation of L-glucose in muscle indicating that it does not affe ct the cell membrane integrity. Neither the stimulatory effect of mone nsin on basal 2-deoxyglucose uptake nor the opposite, inhibitory actio n of monensin on the insulin-stimulated 2-deoxyglucose uptake were inf luenced by the removal of Ca2+ from the medium or by dantrolene, an in hibitor of Ca2+ release from the sarcoplasmic reticulum, suggesting th at the actions of monensin are not mediated by calcium. Monensin had n o effect on muscle ATP concentration. The monensin-induced augmentatio n of basal 2-deoxyglucose uptake was neither associated with stimulati on of muscle phosphatidylinositol 3-kinase activity nor inhibited by w ortmannin, demonstrating that the increase in basal 2-deoxyglucose upt ake is not mediated by activation of phosphatidylinositol 3-kinase. Th e inhibition of insulin-stimulated 2-deoxyglucose uptake by monensin w as associated with a 31% decrease in the abundance of insulin receptor s in muscles, a 64% decrease in the insulin-induced autophosphorylatio n of the insulin receptor beta-subunit, and a 44% reduction of the ins ulin-stimulated phosphatidylinositol 3-kinase activity. Addition of mo nensin into the phosphatidylinositol 3-kinase reaction had no effect o n the activity of the enzyme, demonstrating that the inhibition in mon ensin-treated muscles is indirect and occurs upstream of phosphatidyli nositol 3-kinase. It is concluded that monensin has a dual effect on 2 -deoxyglucose uptake by skeletal muscle: it stimulates basal uptake bu t inhibits the insulin-stimulated uptake, The primary cause of the lat ter, inhibitory effect of monensin is at the level of the insulin rece ptor.