Inhibition of Ns-stimulated human platelet adenylate cyclase by forskolin

Y Watanabe and KH Jakobs

The diterpene, forskolin, increases basal adenylate cyclase activity in membranes of human platelets to more than 20-fold with an EC50 of about 5 microM. However, when the platelet adenylate cyclase was activated via the stimulatory coupling component, Ns, e.g., by the hormone, prostaglandin E1, or the stable GTP analog, guanosine 5'-[gamma- thio]triphosphate, added in combination with a protease, forskolin was able to inhibit the enzyme. The inhibition was half-maximal and maximal (40-50% inhibition) at 0.01 and 0.1 microM forskolin, respectively, and occurred without apparent lag phase. At a maximally inhibitory concentration, forskolin largely reduced the apparent affinity of the Ns-stimulated platelet adenylate cyclase for its substrate MgATP in a noncompetitive manner, which resulted in a pronounced inhibition by forskolin at low substrate concentrations and a further increase in activity at high MgATP concentrations. Treatment of intact platelets or platelet membranes with agents known to interfere with Ni-mediated adenylate cyclase inhibition did not diminish but even increased the forskolin-induced inhibition of the adenylate cyclase. However, inhibition of the prostaglandin E1-stimulated adenylate cyclase by forskolin and the inhibitory hormonal agents, thrombin and epinephrine, were not additive at maximally inhibitory concentrations. Furthermore, increasing concentrations of Mg2+ and Mn2+ reduced (Mg2+) or even reversed (Mn2+) the forskolin-induced inhibition. The data indicate that forskolin apparently has two distinct effects on the platelet adenylate cyclase, namely inhibition and stimulation. The data furthermore suggest that the adenylate cyclase inhibition by forskolin is not mediated by the inhibitory guanine nucleotide-binding protein Ni, but may be due to an action of the diterpene at the adenylate cyclase catalytic moiety, particularly when activated by Ns, or a closely related membrane component.

Volume 29, Issue 3, pp. 258-263, 03/01/1986
Copyright © 1986 by American Society for Pharmacology and Experimental Therapeutics