EFFECT OF VANADATE ON GLUCOSE-TRANSPORTER (GLUT4) INTRINSIC ACTIVITY IN SKELETAL-MUSCLE PLASMA-MEMBRANE GIANT VESICLES

Maximally effective concentrations of vanadate (a phosphotyrosine phos phatase inhibitor) increase glucose transport in muscle less than maxi mal insulin stimulation. This might be due to vanadate-induced decreas ed intrinsic activity of GLUT4 accompanying GLUT4 translocation. Thus, the effect of vanadate (NaVO3) on glucose transporter (GLUT4) intrins ic activity (V-max = intrinsic activity x [GLUT4 protein]) was studied in muscle plasma membrane giant vesicles. Giant vesicles (average dia meter 7.6 mu m) were produced by collagenase treatment of rat skeletal muscle. The vesicles were incubated for 1.5 h with concentrations of vanadate ranging from 3 to 40 mmol l(-1) at 34 degrees C before being used for determination of glucose transport. The dose-response curve s howed that vanadate decreased the specific D-glucose uptake by a maxim um of 70% compared with a control preparation. The vanadate-induced de crease in glucose uptake was not due to a decrease in number of vesicl es. To further verify the apparent vanadate-induced decrease in GLUT4 intrinsic activity, the kinetics of glucose transport were also examin ed. In the presence of 10 mmol l(-1) vanadate the V-max and K-m were d ecreased (P < 0.05, n = 6) 55% and 60%, respectively, compared with co ntrol. The plasma membrane GLUT4 protein content was not changed in re sponse to vanadate. It is concluded that vanadate decreased glucose tr ansport per GLUT4 (intrinsic activity). This finding suggests that reg ulation of glucose transport in skeletal muscle can involve changes in GLUT4 intrinsic activity.