The effect of indomethacin on dilator prostanoid receptor-mediated cAMP formation was investigated using primary cultures of vascular smooth muscle cells from the newborn pig cerebral microvessels. Cerebral microvascular smooth muscle cells responded to dilator prostanoids(iloprost> PGE) by increasin cAMP formation and release into the media(EC= 2 x 10 M and 2 x 1O-M for iloprost and PGE2, respectively). Indomethacin inhibited iloprost-and PGE2-evoked increases in cAMP formation (lC= iO M) and release(lC= 10_6 M) by microvascular smooth muscle cells (maximal inhibition 80-90%), whereas isoproterenol-induced CAMP formation was only slightly attenuated at the highest concentration of indomethacin used (1 0 M). Aspirin was much less effective in inhibiting dilator prostanoid-induced cAMP formation and re-lease by the cells. Direct analysis of prostacyclin receptor sites using HJiloprost as the ligand revealed saturable, high affinity