Movement of carboxyfluorescein across retinal pigment epithelium-choroid.

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 microM. 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(-6) cm sec-1. Neither ouabain (1.0 microM), low external sodium concentration nor probenecid (100 microM) 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(-6) cm sec-1 at 75 microM. 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 microM), ouabain (1.0 microM), probenecid (100 microM), iodipamide (5.0 mM), hippurate (30 mM) or low external sodium concentration (25 mM). The net outward movement of CF became saturated as concentration was increased and a Lineweaver-Burk plot gave an apparent Km of 237 microM and Vmax of 1.8 nmol hr-1 cm-2. The results indicated that part of the outward movement 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.