Competition for adenyl cyclase coupled (3H)-prostacyclin binding sites with prostaglandin E2 in rat peritoneal macrophages.

Prostaglandins regulate macrophage function by their action on membrane-associated adenyl cyclase. In order to define more directly macrophage-prostaglandin interactions, a binding assay has been developed for macrophage receptors using (3H)-PGI2 as ligand. (3H)-PGI2 binding was specific, saturable and reversible. Moreover, specific binding showed to be enriched in a membrane-enriched fraction of the cells. The assay conditions ensured stability of (3H)-PGI2 during incubations and should exclude intracellular accumulation of the ligand in macrophages. Unlabelled PGE2 and PGI2 competed for (3H)-PGI2 specific binding in both macrophages and membrane preparations. PGE2 showed to be more potent in this respect than PGI2, a phenomena which was also observed for prostaglandin activation of cAMP production in macrophages. The data suggest an interaction at receptor level of endogenously released PGE2 and PGI2 by peritoneal macrophages in vivo and provide support for a previously proposed mechanism of action of low concentrations of PGE2, counteracting stimulation of cAMP production by PGI2 in macrophages.