PurposeThe aim of this study was to test the hypothesis that nitric oxide (NO) mediates a pressure-dependent, negative feedback loop that maintains conventional outflow homeostasis and thus IOP. If true, holding pressure during ocular perfusions will result in uncontrolled production of NO, hyper-relaxation of the trabecular meshwork, and washout.MethodsPaired porcine eyes were perfused at constant pressure of 15 mm Hg. After 1 hour acclimatization, one eye was exchanged with N5-[imino(nitroamino)methyl]-L-ornithine, methyl ester, monohydrochloride (L-NAME) (50 µm) and the contralateral eye with DBG, and perfused for 3 hours. In a separate group, one eye was exchanged with DETA-NO (100 nM) and the other with DBG and perfused for 30 minutes. Changes in conventional outflow tissue function and morphology were monitored.ResultsControl eyes exhibited a washout rate of 15% (P= 0.0026), whereas eyes perfused with L-NAME showed a 10% decrease in outflow facility from baseline over 3 hours (P< 0.01); with nitrite levels in effluent positively correlating with time and facility. Compared with L-NAME–treated eyes, significant morphological changes in control eyes included increased distal vessel size, number of giant vacuoles, and juxtacanalicular tissue separation from the angular aqueous plexi (P< 0.05). For 30-minute perfusions, control eyes showed a washout rate of 11% (P= 0.075), whereas DETA-NO–treated eyes showed an increased washout rate of 33% from baseline (P< 0.005). Compared with control eyes, significant morphological changes in DETA-NO–treated eyes also included increased distal vessel size, number of giant vacuoles and juxtacanalicular tissue separation (P< 0.05).ConclusionsUncontrolled NO production is responsible for washout during perfusions of nonhuman eyes where pressure is clamped.