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.