ESTIMATE OF THE SOLUTE PERMEABILITY COEFFICIENTS AND REFLECTION COEFFICIENTS OF THE CILIARY EPITHELIUM FROM AQUEOUS-PLASMA CONCENTRATION RATIOS

Rabbits were infused i.v. with radioactively labeled solutions of urea, sucrose and inulin for either 10 min or 8 h. In all experiments posterior chamber aqueous and plasma solute concentrations were determined. In some experiments lens, vitreous and anterior chamber aqueous were also taken for analysis. After 10 min of infusion, there is a significant loss of solute from the posterior chamber into the lens or vitreous. When compared to the posterior chamber aqueous the lens and vitreous concentrations were 5-9% and 16-28%, respectively, for the 3 solutes. After 8 h of perfusion with urea the lens and vitreous concentrations were only 60-70% of that in the posterior chamber, even after correction for the water content of the lens. The data were subjected to an analysis utilizing nonequilibrium thermodynamics. Equations were developed to allow estimates of .omega.RT (solute permeability coefficient) and .sigma. (reflection coefficient) of the ciliary epithelium. The results of this analysis indicate at the value of .sigma. for inulin is 0.83, for sucrose is 0.67, and for urea is 0.12. The solute permeability coefficients are 1.96, 0.03 and 0.004 .mu.l/min for urea, sucrose and inulin, respectively. A comparison of the reflection coefficients to the molecular radius of the solutes suggests a membrane pore diameter of about 1 nm. These findings are compatible with previous suggestions regarding the role of ultrafiltration in aqueous humor formation.