FLUORESCEIN TRANSPORT ACROSS THE HUMAN BLOOD RETINA BARRIER IN THE DIRECTION VITREOUS TO BLOOD - QUANTITATIVE ASSESSMENT IN-VIVO

Inward and outward movement of fluorescein across the human blood-reti na barrier was studied in five healthy volunteers, using a differentia l spectrofluorometry method that eliminates the contribution of fluore scein glucuronide to the total fluorescence in the vitreous and in pla sma. The inward permeability across the blood-retina barrier, which is presumed to be passive, and the diffusion coefficient in the vitreous for fluorescein was calculated from data obtained 1 h after intraveno us injection of fluorescein. The rate of elimination of fluorescein fr om the vitreous across the blood-retina barrier was estimated from dat a obtained 7 to 12 h after injection of nuorescein. The calculations w ere based upon the free plasma fluorescein decay curve and the prereti nal fluorescein gradient in the vitreous. The mean inward permeability of fluorescein was 1.39 x 10(-7) cm/sec (range: 0.70-2.06 x 10(-7) cm /sec), whereas the mean outward permeability was 1.51 x 10(-5) cm/sec (range: 1.14-1.73 x 10(-5) cm/sec). We have thus found that the moveme nt of fluorescein across the blood-retina barrier is highly asymmetric , the outward transport being more than 100 times faster than the pass ive inward leakage. This could indicate the presence of an active pump ing mechanism in the blood-retina barrier, responsible for fluorescein transport in the direction from the vitreous to the blood.