Effect of Systemic Nitric Oxide Synthase Inhibition on Optic Disc Oxygen Partial Pressure in Normoxia and in Hypercapnia

Ioannis K. Petropoulos, Jean-Antoine C. Pournaras, Alexandros N. Stangos, Constantin J. Pournaras
2009 Investigative Ophthalmology and Visual Science  
PURPOSE. To investigate the effect of systemic nitric oxide synthase (NOS) inhibition on optic disc oxygen partial pressure (PO 2 ) in normoxia and hypercapnia. METHODS. Intervascular optic disc PO 2 was measured in 12 anesthetized minipigs by using oxygen-sensitive microelectrodes placed Ͻ50 m from the optic disc. PO 2 was measured continuously during 10 minutes under normoxia, hyperoxia (100% O 2 ), carbogen breathing (95% O 2 , 5% CO 2 ), and hypercapnia (increased inhaled CO 2 ).
more » ... CO 2 ). Measurements were repeated after intravenous injection of N -nitro-L-arginine methyl ester (L-NAME) 100 mg/kg. Intravenous L-arginine 100 mg/kg was subsequently given to three animals. RESULTS. Before L-NAME injection, an increase was observed in optic disc PO 2 during hypercapnia (⌬PO 2 ϭ 3.2 Ϯ 1.7 mm Hg; 18%; P ϭ 0.001) and carbogen breathing (⌬PO 2 ϭ 12.8 Ϯ 5.1 mm Hg; 69%; P Ͻ 0.001). Optic disc PO 2 in normoxia remained stable for 30 minutes after L-NAME injection (4% decrease from baseline; P Ͼ 0.1), despite a 21% increase of mean arterial pressure. Optic disc PO 2 increase under hypercapnia was blunted after L-NAME injection (⌬PO 2 ϭ 0.6 Ϯ 1.1 mm Hg; 3%; P Ͼ 0.1), and this effect was reversible by L-arginine. Moreover, L-NAME reduced the response to carbogen by 29% (⌬PO 2 ϭ 9.1 Ϯ 4.4 mm Hg; 49%; P ϭ 0.01 versus before L-NAME). The response to hyperoxia was not affected. CONCLUSIONS. Whereas systemic NOS inhibition did not affect optic disc PO 2 in normoxia, a blunting effect was noted on the CO 2 -induced optic disc PO 2 increase. Nitric oxide appears to mediate the hypercapnic optic disc PO 2 increase. (Invest Ophthalmol Vis Sci.
doi:10.1167/iovs.08-2413 pmid:18676634 fatcat:rvkcoe6n35ac5kpo4hrh3zm6rm