PGE(2)-mediated eNOS induction in prolonged hypercapnia.
OBJECTIVE
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), PGE(2), nitrite (NO(2)(-)), and NOS protein (reduced nicotinamide adenine dinucleotide phosphate [NADPH]-diaphorase staining) production were measured in hypercapnia (6% CO(2))-treated piglets. From the same species, PGE(2), eNOS mRNA, NOS protein, and vasomotor responses were measured in eyecup preparations, as were Ca(2+) 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 PGE(2) levels and the latter phase (6-8 hours) with an increase in NO(2)(-) 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 PGE(2) 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 Ca(2+) channel blocker SK&F96365. In neuroretinovascular cells, acidosis increased Ca(2+) 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 PGE(2).
Checchin
Daniella
D
Department of Pediatrics, Research Center, Hôpital Ste. Justine, 3175 Côte Ste. Catherine, Montreal, Québec H3T 1C5, Canada.
Hou
Xin
X
Hardy
Pierre
P
Abran
Daniel
D
Najarian
Taline
T
Beauchamp
Martin H
MH
Bernier
Sylvie G
SG
Gobeil
Fernand
F
Quiniou
Christiane
C
Varma
Daya R
DR
Chemtob
Sylvain
S
D000818:Animals; D001783:Blood Flow Velocity; D002118:Calcium / Q000378:metabolism; D002121:Calcium Channel Blockers / Q000494:pharmacology; D016861:Cyclooxygenase Inhibitors / Q000494:pharmacology; D003609:Dactinomycin / Q000494:pharmacology; D015232:Dinoprostone / Q000494:pharmacology*; D004790:Enzyme Induction / Q000187:drug effects; D006935:Hypercapnia / Q000201:enzymology* / Q000503:physiopathology; D006940:Hyperemia / Q000201:enzymology* / Q000503:physiopathology; D009252:NADPH Dehydrogenase / Q000378:metabolism; D019001:Nitric Oxide Synthase / Q000096:biosynthesis* / Q000235:genetics; D052250:Nitric Oxide Synthase Type III; D009573:Nitrites / Q000378:metabolism; D012333:RNA, Messenger / Q000378:metabolism; D011863:Radioimmunoassay; D012171:Retinal Vessels / Q000187:drug effects* / Q000201:enzymology / Q000503:physiopathology; D013552:Swine; D014664:Vasodilation
D002121:Calcium Channel Blockers; D016861:Cyclooxygenase Inhibitors; D009573:Nitrites; D012333:RNA, Messenger; D003609:Dactinomycin; D019001:Nitric Oxide Synthase; D052250:Nitric Oxide Synthase Type III; D009252:NADPH Dehydrogenase; D015232:Dinoprostone; D002118:Calcium