Movement of Carboxyfluorescein across Retinal Pigment Epithelium–Choroid

Abstract

The movement of carboxyfluorescein (CF) across the isolated retinal pigment epithelium (RPE)–choroid of the albino rabbit was studied using an Ussing chamber under short-circuit conditions with CF concentrations ranging from 15 to 300 μ M. The inward (from the choroid to vitreous side) permeability of the tissue to CF showed no significant change over the concentration range tested, averaging 2·2 x 10 −6cm sec −1. Neither ouabain (1·0 μ M), low external sodium concentration nor probenecid (100 μ M) had significant effect on the inward movement of CF. The outward (from the vitreous to the choroid side) permeability of the tissue to CF gradually decreased as the concentration increased (ANOVA, P<0·01; Scheffe's test, P<0·05), averaging 3·7 x 10 −6cm sec −1at 75 μ M. Further, it was always significantly greater than the inward permeability at the corresponding concentration (unpaired t-test, P<0·05−0·01). The outward movement of CF was significantly inhibited by 2,4-dinitrophenol (10 μ M), ouabain (1.0 μ M, probenecid (100 μ M), iodipamide (5·0 m M, hippurate (30 m M) or low external sodium concentration (25 m M). The results indicated that part of the outward movemetn of CF across the RPE-choroid depends on carrier mediated active transport, but to a much lesser extent than that of fluorescein. The inward movement of CF was thought to occur almost exclusively by passive diffusion through the paracellular spaces.