Maximally effective concentrations of vanadate (a phosphotyrosine phosphatase inhibitor) increase glucose transport in muscle less than maximal insulin stimulation. This might be due to vanadate-induced decreased intrinsic activity of GLUT4 accompanying GLUT4 translocation. Thus, the effect of vanadate (NaVO3) on glucose transporter (GLUT4) intrinsic activity (V(max) = intrinsic activity x [GLUT4 protein]) was studied in muscle plasma membrane giant vesicles. Giant vesicles (average diameter 7.6 microns) 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 showed that vanadate decreased the specific D-glucose uptake by a maximum of 70% compared with a control preparation. The vanadate-induced decrease in glucose uptake was not due to a decrease in number of vesicles. To further verify the apparent vanadate-induced decrease in GLUT4 intrinsic activity, the kinetics of glucose transport were also examined. In the presence of 10 mmol l-1 vanadate the V(max) and K(m) were decreased (P < 0.05, n = 6) 55% and 60%, respectively, compared with control. The plasma membrane GLUT4 protein content was not changed in response to vanadate. It is concluded that vanadate decreased glucose transport per GLUT4 (intrinsic activity). This finding suggests that regulation of glucose transport in skeletal muscle can involve changes in GLUT4 intrinsic activity.