Mechanism of permissive prostacyclin action in cerebrovascular smooth muscle.

Prostacyclin permissively allows increased cAMP and cerebral vasodilation to hypercapnia in piglets. The prostacyclin receptor (IP) is coupled to phospholipase C (PLC) in piglet cerebral microvascular smooth muscle cells (SMC). We hypothesize that inhibition of PLC blocks the permissive action of IP receptor agonist, iloprost, and direct activation of PKC substitutes for the IP receptor agonist in SMC. SMC cAMP production was measured at normal pHi/pHo and with reduced pHi/pHo in the absence and presence of iloprost (100 pM). Half of the cells were pretreated with U73122, the PLC inhibitor, which decreased the basal IP3 and blocked the increase in IP3 caused by iloprost. Without iloprost, decreasing pHi/pHo increased cAMP production (40%). With iloprost, the cAMP response to acidosis increased to over 80%. U73122 prevented accentuation of the cAMP response by iloprost. Phorbol myristate acetate augmented the response to acidosis similarly to iloprost. These data suggest IP agonists augment the cAMP response to acidosis via coupling through PLC to activate PKC.