PROBENECID INHIBITION OF THE OUTWARD TRANSPORT OF FLUORESCEIN ACROSS THE HUMAN BLOOD RETINA BARRIER

The effect of probenecid on the outward transport of fluorescein from vitreous to blood was studied in 13 insulin-dependent diabetic patient s with background retinopathy in a randomised double-masked placebo co ntrolled cross-over study. Fluorescein and fluorescein glucuronide wer e separated in the vitreous and in plasma by differential spectrofluor ometry. The data for fluorescein were analysed using a simplified math ematical model of the eye. The inward permeability was estimated from data obtained 1 h after injection and the outward transport from data obtained 7 h after injection. During placebo treatment the mean inward permeability was 3.75 x 10(-7) cm/sec and the mean outward permeabili ty was 8.25 x 10(-5) cm/sec. During probenecid treatment the mean inwa rd permeability was 3.34 x 10(-7) cm/sec and the mean outward permeabi lity was 1.44 x 10(-5) cm/sec. Thus, we found no significant change in inward permeability (p = 0.5879), whereas a significant decrease of 3 6% was found in the outward permeability of fluorescein (p = 0.0171). The demonstration that the outward permeability, which is more than 10 0-fold higher than the inward permeability in the healthy eye, is sign ificantly decreased by probenecid, demonstrates that active transport is involved in movement of fluorescein across the blood-retina barrier from the vitreous to the plasma.