Blood-retinal barrier permeability to carboxyfluorescein and fluorescein in monkeys

Lipid solubility is a major determinant of permeability across the blood-brain barrier, to which the blood-retinal barrier (BRB) has many similarities. Carboxyfluorescein is a dye with about 1/1000 the lipid solubility of fluorescein, but their molecular sizes and spectral characteristics are similar. We studied the importance of lipid solubility in BRB permeability by comparing the BRB permeabilities to these two dyes. Dye in the vitreous and plasma of four monkeys was measured by fluorophotometry. The estimated inward permeability coefficients (Pin) were 11 +/- 7.4 X 10(-6) cm/min (mean and SD) for carboxyfluorescein and 21 +/- 5.9 X 10(-6) cm/min for fluorescein. The ratio of the means was 1/1.9, far from the expected 1/1000. This finding suggests that the BRB does not function as a continuous lipid membrane and that other factors are more important determinants of permeability for these dyes than lipid solubility.