(5Z)-carbacyclin displays agonist-antagonist properties on prostacyclin-receptors in platelets and vascular myocytes

(5E)- and (5Z)-carbacyclin are chemically stable analogues of prostacyclin (PGI2), which mimic PGI2 actions. In particular, they inhibit platelet aggregation and relax vascular smooth muscle, through the activation of adenylate cyclase (AC) being, however, less potent than PGI2. The characteristics of AC activity modulation by the two isomeric carbacyclins in membranes of human platelets and of myocytes cultured from rabbit mesenteric artery have been investigated. In human platelet membranes, both carbacyclins stimulated AC activity with the same efficacy as PGE1 and PGI2; in addition, these two prostaglandins inhibited the aggregation of human and rabbit platelets to the same extent as PGE1. On the contrary, in myocytes (5Z)-carbacyclin fails to produce the same degree of stimulation of AC elicited by PGI2, (5E)-carbacyclin and PGE1, nor does it induce the maximal relaxation of rabbit mesenteric artery attained with the other prostaglandins. (5Z)-carbacyclin is also able to antagonize the activation of AC by PGE1, PGI2 or (5E)-carbacyclin, acting therefore as a partial agonist. In conclusion, (5Z)-carbacyclin is a full agonist at the platelet level both in human and rabbit, thus excluding possible interspecies differences, but it is a partial agonist on myocytes. Therefore, (5Z)-carbacyclin appears to discriminate between PGI2-receptors in the two target cells.