Binding to prostaglandin (PG) receptors and activation of adenyl cyclase by 20-isopropylidene-PGS in rabbit platelet membranes.

20-Isopropylidene-PGE1 (Isop-PGE1) was about 10 times more potent than PGE1 in inhibition of thrombin-induced aggregation of rabbit washed platelets. Likewise, 20-isopropylidene-17(R)-methyl-carbacyclin (CS-570), a stable PGI2 analogue, was more potent than carbacyclin in the anti-aggregatory activity. In order to define the platelet-prostaglandin interactions, a binding assay was done using platelet membranes with [3H]-PGE1 as a radioligand. Isop-PGE1 (IC50 = 0.18 microM) bound to the PG receptors more potently than PGE1 (IC50 = 2.1 microM). CS-570 (IC50 = 0.39 microM) was more potent than carbacyclin (IC50 = 1.9 microM). These indicate that introduction of an isopropylidene group to the carbon 20 of PGs increases the binding ability to the receptors. These PGE1 and PGI2 analogues activated platelet membrane adenyl cyclase and increased intracellular cAMP levels with the same potency series obtained in the binding experiments. All these results suggest that the binding to the receptors by these PGs is coupled to the activation of adenyl cyclase, followed by the increase in cAMP levels in platelets and the inhibition of platelet aggregation. Thus, the increased anti-aggregatory activity of 20-isop-PGs may be explained by their increased affinity for the PG receptors and stimulation of adenyl cyclase. 15-Epimeric-20-isopropylidene-PGE1 (15-Epi-isop-PGE1), which has an unnatural configuration of the 15-hydroxyl group, was much less potent than isop-PGE1 in the binding experiment and the other three investigations. This indicates that the configuration of the 15-hydroxyl group is important for the binding to the PG receptors and the consequent activities in platelets.