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PGE₂-Mediated eNOS Induction in Prolonged Hypercapnia

¹ From the Departments of Pediatrics, Ophthalmology, and Pharmacology, Research Center, Hôpital Ste. Justine, Montreal, Québec; and the ² Department of Pharmacology and Therapeutics, McGill University, Montreal, Québec, Canada.

PURPOSE. Because prostaglandins (PGs) are implicated in acute hypercapnia-induced hyperemia, this study was conducted to test the hypothesis that prolonged hypercapnia may cause a sustained increase in retinal blood flow (RBF) through a PG-dependent induction of endothelial nitric oxide synthase (eNOS).

METHODS. Time-dependent RBF (microsphere technique), PGE₂, nitrite (NO₂^-), and NOS protein (reduced nicotinamide adenine dinucleotide phosphate [NADPH]-diaphorase staining) production were measured in hypercapnia (6% CO₂)-treated piglets. From the same species, PGE₂, eNOS mRNA, NOS protein, and vasomotor responses were measured in eyecup preparations, as were Ca²⁺ transients in neuroretinovascular endothelial cells.

RESULTS. Hypercapnia caused biphasic (at 0.5 hours and 6–8 hours) increases in RBF that were abolished with normalization of the pH. The early phase (0.5 hour) was associated with an increase in PGE₂ levels and the latter phase (6–8 hours) with an increase in NO₂^- and NOS protein. Inhibition of cyclooxygenase by diclofenac prevented the early and late increase in RBF. NOS inhibitor L-nitro-arginine prevented only the latter. Hypercapnic acidosis increased retinal PGE₂ levels and eNOS-dependent vasorelaxation ex vivo. The ex vivo time course of eNOS mRNA expression corresponded with the late-phase increase in RBF and was blocked by the transcription inhibitor actinomycin D and the receptor-operated Ca²⁺ channel blocker SK&F96365. In neuroretinovascular cells, acidosis increased Ca²⁺ transients, which were inhibited by SK&F96365, but not diclofenac.

CONCLUSIONS. This study discloses a previously unexplored mechanism for late retinal hyperemia during sustained hypercapnia that appears secondary to the induced expression of eNOS mediated by PGE₂.

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