Aqueous humor dynamics of the Brown-Norway rat

Abstract

PURPOSE. The study aimed to provide a quantitative description of aqueous humor dynamics in healthy rat eyes. METHODS. One eye of 26 anesthetized adult Brown-Norway rats was cannulated with a needle connected to a perfusion pump and pressure transducer. Pressure-flow data were measured in live and dead eyes by varying pump rate (constant-flow technique) or by modulating pump duty cycle to hold intraocular pressure (IOP) at set levels (modified constant-pressure technique). Data were fit by the Goldmann equation to estimate conventional outflow facility (C) and unconventional outflow rate (F un ). Parameter estimates were respectively checked by inserting a shunt of similar conductance into the eye and by varying eye hydration methodology. RESULTS. Rat IOP averaged 14.6 ± 1.9 mm Hg at rest. Pressure-flow data were repeatable and indistinguishable for the two perfusion techniques, yielding C = 0.023 ± 0.002 μL/min/mm Hg and F un = 0.096 ± 0.024 μL/min. C was similar for live and dead eyes and increased upon shunt insertion by an amount equal to shunt conductance, validating measurement accuracy. At 100% humidity F un dropped to 0.003 ± 0.030 μL/min. Physiological washout was not observed (−0.35 ± 0.65%/h), and trabecular anatomy looked normal. CONCLUSIONS. Rat aqueous humor dynamics are intermediate in magnitude compared to those in mice and humans, consistent with species differences in eye size. C does not change with time or death. Evaporation complicates measurement of F un even when eyes are not enucleated. Absence of washout is a notable finding seen only in mouse and human eyes to date.