Prostaglandin E(2) both stimulates and inhibits adenyl cyclase on platelets: Comparison of effects on cloned EP4 and EP3 prostaglandin receptor subtypes

The effects of prostaglandin E(2) (PGE(2)) on platelet cyclic AMP formation were examined and compared with effects on cloned prostaglandin receptors. PGE(2) gave a weak stimulation of adenyl cyclase in platelets compared with the PGI(2) analog Iloprost. In the presence of the adenyl cyclase stimulator forskolin, the response to PGE(2) was amplified in a synergistic manner. By contrast, in the presence of Iloprost, PGE(2) inhibited cyclic AMP formation. We postulate that the weak platelet response to PGE(2) is due to co-localization of a PGE(2) receptor that couples to stimulation of adenyl cyclase with the EP3 prostaglandin receptor that binds PGE(2) tightly and inhibits adenyl cyclase. In support of this postulate, we compared the responses obtained with platelets with those of cloned EP4 (stimulatory) and EP3 (inhibitory) prostaglandin receptor subtypes and show similar dose-response curves for stimulation and inhibition of cyclic AMP formation between platelets and cloned receptors.