Actions of peroxovanadate or tungstate on glucose transport by isolated rat adipocytes

Insulin-like action of chromate on glucose transport in isolated rat adipocytes

The effects of chromium compounds on 3-O-methylglucose (3-O-MG) transport were studied in isolated rat adipocytes. Sodium chromate significantly stimulated 3-O-MG uptake into adipocytes in a dose-dependent manner without altering the equilibrium space of 3-O-MG in adipocytes. The stimulatory effect reached the maximum at 300 microM, and the effect was 60-70% of the maximal insulin effect that was obtained with 20 nM insulin. The chromate concentration achieving a half-maximal effect was estimated at 50 microM. The effect of the combination of 1 mM chromate and 20 nM insulin was equipotent to that of 20 nM insulin alone, which showed that these two effects were not additive. The stimulatory effects of 1 mM chromate and 20 nM insulin were entirely abolished in adipocytes deprived of ATP, which indicated that these effects were completely ATP-dependent. Judging from experiments using various chromium compounds, CrO4(2-) was responsible for the insulinomimetic action. These results indicate that the action of CrO4(2-) is exerted through a mechanism analogous to that of insulin action, and that CrO4(2-) is a novel and useful tool for studying issues involved in insulin actions.

Inhibitory effects of diazoxide or polymyxin B on glucose transport by isolated rat erythrocytes or adipocytes

The inhibitory effects of diazoxide or polymyxin B on 3-O-methylglucose uptake were studied in isolated rat erythrocytes or adipocytes to elucidate the mechanisms of the actions of these agents. One to three mmol/L diazoxide significantly inhibited 3-O-methylglucose uptake into erythrocytes by 11-33% without altering the equilibrium space, while 0.3 mmol/L diazoxide did not. The inhibitory effect was exerted in a dose-dependent manner in this concentration range. To test whether polymyxin B affects the process of insulin action or the glucose transport activity recruited by insulin, adipocytes prestimulated with insulin and exposed to 2 mmol/L potassium cyanide (KCN) were employed since the cells, on which glucose transporters recruited by insulin were located quiescently, were useful to estimate the effect of an agent on glucose transport activity per se. Polymyxin B (100 micrograms/mL) inhibited the insulin-stimulated uptake activity in this transport system by 22.5% while it inhibited the insulin-stimulated uptake activity in intact adipocytes which were not exposed to KCN by 32.2%. These results suggest that diazoxide inhibits the function of the erythrocyte glucose transporter, GLUT1 (classified by Bell et al.), and indicate that the inhibition of the glucose transport activity recruited by insulin is the major effect of polymyxin B (100 micrograms/mL) and the inhibition of the process of insulin action is rather small.

An extrapancreatic action of diazoxide to inhibit glucose transport activity on adipocytes

The effect of diazoxide on 3-O-methylglucose (3-O-MG) transport was studied in isolated rat adipocytes to elucidate its extrapancreatic action. Diazoxide (0.3-3 mmol/L) significantly inhibited 3-O-MG uptake into adipocytes in a basal state or an insulin-stimulated state. The inhibitory effect was mainly due to the inhibition of insulin responsiveness for 3-O-MG uptake. The insulin responsiveness is determined by the capacity in the process of insulin action and in the final glucose transport activity, and diazoxide mainly inhibited the 3-O-MG transport activity itself. Based on these findings, this extrapancreatic action of diazoxide is considered to contribute partially to raising the blood glucose level in children receiving the drug. Diazoxide, as a glucose transport inhibitor, may be a useful tool for studying the issues related to glucose transport or insulin action.