Prolonged hypercapnia-evoked cerebral hyperemia via K+ channel- and prostaglandin E2-dependent endothelial nitric oxide synthase induction

Mechanisms for secondary sustained increase in cerebral blood flow (CBF) during prolonged hypercapnia are unknown. We show that induction of endothelial NO synthase (eNOS) by an increase in prostaglandins (PGs) contributes to the secondary CBF increase during hypercapnic acidosis. Ventilation of Digs with 6% CO2 (Paco(2 approximate to)65 mm Hg; pH approximate to 7.2) caused a approximate to 2.5-fold increase in CBF at 30 minutes, which declined to basal values at 3 hours and gradually rose again at 6 and 8 hours; the latter increase was associated with PG elevation, nitrite formation, eNOS mRNA expression, and in situ NO synthase (NOS) reactivity (NADPH-diaphorase staining). Subjecting free-floating brain sections to acidotic conditions increased eNOS expression, the time course of which was: similar to that of CBF increase. Treatment of Dib's with the cyclooxygenase inhibitor diclofenac or the NOS inhibitor N-omega-nitro-L-arginine blunted thc initial rise and prevented the secondary CBF increase during hypercapnic acidosis; neuronal NOS blockers 1-(2-trifluoromethylphenyl) imidazole and 3-bromo-7-nitroindazole were ineffective. Diclofenac abolished the hypercapnia-induced rise in cerebrovascular nitrite production, eNOS mRNA expression, and NADPH-diaphorase reactivity. Acidosis (pH approximate to 7.15, Pco(2 approximate to)40 mm Hg; 6 hours) produced similar increases in prostaglandin E-2 (PGE(2)) and eNOS mRNA levels in isolated brain microvessels and in NADPH-diaphorase reactivity of brain microvasculature; these changes were prevented by diclofenac, by the receptor-operated Ca2+ channel blocker SK&F96365, and by the K-ATP channel blocker glybenclamide. Acidosis increased Ca2+ transients in brain endothelial cells, which were blocked by glybenclamide and SK&F96365 but not by diclofenac, Increased PG-related eNOS mRNA and NO-dependent vasorelaxation to substance P was detected as well in rat brain exposed to 6 hours of hypercapnia, PGE, was the only major prostanoid that modulated brain eNOS expression during acidosis. Thus, in prolonged hypercapnic acidosis, the secondary CBF rise is closely associated with induction of eNOS expression; this seems to be mediated by PGE(2) generated by a K-ATP and Ca2+ channel-dependent process.