Capillary permeability in cat choroid, studied with the single injection technique (II).

Transcapillary movements of 22Na, 51Cr-EDTA and 125I-myoglobin were studied in the cat choroid by means of the single injection technique, using labelled albumin as the reference tracer. By an external shunting of blood, recirculation of tracer was delayed and complete venous outflow curves became available for analysis. The initial extractions observed were 0.66, 0.35 and 0.08 respectively. The extractions decreased with time. Within 20--25 s the extracted fractions of the tracers had almost completely returned to the blood. The results indicate high permeability coefficients and small extravascular volumes of distribution. The retinal pigment epithelium seems to have a very low permeability to all three substances. Extraction of tritiated water at 20--25 s was about 0.30 probably due to rapid penetration of labelled water into the retina. Evidence for a stereo-specific transport system for glucose in the retinal pigment epithelium was found in experiments with labelled D-glucose and 3-O-methyl-D-glucose. The total amount of glucose delivered by the choroid was calculated from measurements of the a-v differences for unlabelled glucose.