A simple three-compartment model of anterior segment kinetics.

Diffusion of fluorescein between the plasma and anterior chamber has been considered a one-step process with transfer coefficient kd. However, after systemic administration, fluorescein enters the anterior chamber by a different kinetic than this simple model predicts. In this study we added to the model another compartment, the 'iris', between the plasma and anterior chamber. When fluorescein diffuses into the anterior chamber, it must cross two barriers, one between plasma and iris with coefficient ki.ip, and one between iris and anterior chamber with coefficient ki.ia. Fluorescein was measured in the anterior chamber after intravenous injection and was compared to fluorescein concentration calculated by using both kinetic models, the three-compartment model (iris, anterior chamber, and cornea) and the two-compartment model (anterior chamber and cornea). Transfer coefficients were determined by the method of least squares in 12 pigmented rabbits and two groups of six human subjects measured in two previous fluorophotometry studies. In all subjects except one, the three-compartment model matched the data better than the two-compartment model. In a second experiment, diffusion of fluorescein out of the anterior chamber was studied in 15 pigmented rabbits. Diffusional loss was determined from the relative clearances of rhodamine-labeled dextran, a large molecule that left only by bulk outflow, and fluorescein that was cleared by diffusion and outflow. The upper 95% confidence limit of diffusional clearance was less than 11% of total clearance in anesthetized rabbits.