Prostacyclin-dependent cyclic AMP formation in endothelial cells.

The effect of the stable prostacyclin (PGI2) mimetic iloprost on cyclic AMP levels was investigated in cultured porcine aortic endothelial cells. Iloprost (10(-10)-10(-5) mol/l) did not change cyclic AMP levels at passage 1 when endothelial cells were untreated but did so after inhibition of endogenous PGI2 formation by 48 or 72 h treatment with indomethacin or diclofenac (10(-5) mol/l). Iloprost increased cyclic AMP in a concentration-dependent manner and up to 6-fold above control when cells from passage 6 were used. In these cells, basal PGI2 generation was reduced to 20% of that at passage 1. Cyclic AMP stimulation by iloprost (10(-5) mol/l) in passage 6 cells was enhanced, reaching up to 11-fold the control level, when cells were cultured for 48 h in the presence of indomethacin or diclofenac (10(-5) mol/l). Cyclic AMP formation in LLC-PK1 cells, a kidney epithelial cell line without endogenous PGI2 biosynthesis, was markedly (25-fold above basal) stimulated by iloprost and unchanged by pretreatment with indomethacin and diclofenac. The data demonstrate that a continuous basal PGI2 generation occurs in porcine aortic endothelial cells that may be sufficient to completely desensitize PGI2-dependent adenylate cyclase activation, presumably at the receptor or GTP-binding protein level.