Permeability of the isolated dog retinal pigment epithelium to carboxyfluorescein.

Outward (retina to choroid) and inward (choroid to retina) permeabilities of carboxyfluorescein and fluorescein in the isolated dog retinal pigment epithelium (RPE)-choroid were determined. Outward permeability was 9 and 47 times larger than inward permeability for carboxyfluorescein and fluorescein, respectively. The outward permeability of carboxyfluorescein was seven times lower than that of fluorescein, whereas there was no statistical difference between the inward permeabilities. Carboxyfluorescein is thus distinguished from fluorescein by its low affinity to the outwardly directed organic anion transport system. 10(-4) M probenecid caused greater than 98% inhibition of the outward transport of 6 X 10(-5) M carboxyfluorescein and 6 X 10(-6) M fluorescein.