Glucose transport in human platelets and its inhibition by forskolin

Hexose transport in washed human platelets and its inhibition by forskolin was examined by using 3-O-methylglucose (MG). MG influx in platelets exhibited saturable kinetics with the Km of 1.74 mM and the Vmax of 0.242 mumol/ml of cells X sec at 20 degrees C. Forskolin was found to reduce the Vmax without appreciably affecting the Km, suggesting a noncompetitive mode of inhibition. Forskolin inhibited MG influx with an IC50 value of approximately 2 microM. Forskolin also inhibited the influx of other carbohydrates, including galactose, fructose and ribose. However, forskolin had no effect on adenosine transport. MG transport was abolished instantaneously by forskolin before a significant increase in cellular cyclic AMP content. While both forskolin and prostaglandin l2 stimulated cyclic AMP formation severalfold, only forskolin inhibited MG influx. These findings suggest that the inhibitory action of forskolin on MG transport is unrelated to its well established ability to activate adenylate cyclase and that human platelets possess a hexose transport which is unresponsive to cyclic AMP.