Estimation of the permeability of the blood-retinal barrier in normal individuals

The fluorescein kinetics in the vitreous was simulated with a computer to consider several factors such as permeability of the blood-retinal barrier, outward active transport, plasma fluorescein dynamics, diffusion of fluorescein in the vitreous, and fluorescein leakage from the blood-aqueous barrier. Kinetic vitreous fluorophotometry was performed in normal individuals to estimate the inward and outward permeability of the blood-retinal barrier based on the theory of the simulation model. The results of the simulation studies suggest that the fluorescein concentration in the posterior vitreous after intravenous administration is dependent mainly on the inward permeability and on the plasma concentration and that the outward permeability has little influence on the fluorescein kinetics at the early phase. In the pharmacokinetic analysis of the results of kinetic vitreous fluorophotometry, we obtained average values of 1.8 X 10(-5) cm/min and 5.6 X 10(-4) cm/min for the inward permeability and outward permeability coefficients, respectively. The diffusion coefficient of fluorescein in the vitreous was estimated at 7.9 X 10(-4) cm2/min on the average. The outward permeability of the blood-retinal barrier is approximately 31 times the inward permeability. This suggests that a facilitated process that transports fluorescein outward from the vitreous cavity exists in the blood-retinal barrier of human eyes.