The uptake index method applied to studies on the blood‐retinal barrier I. A methodological study

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Abstract
The validity of the uptake index method for studies on passage through the blood-retinal barrier was evaluated. Data for the brain were also obtained for comparison. A rapid intracarotid injection of 0.2 ml buffered Ringer solution in rats did not result in a measurable increase in the permeability to EDTA for either the blood-retinal or the blood-brain barriers, while a markedly hypertonic solution caused an immediate osmotic breakdown of both barriers. For determinations of the retinal uptake index (RUI) tritiated water was a suitable reference substance. A more diffusible reference, such as 14C-ethanol, resulted in a lower net extraction, probably due to a more rapid wash-out by the choroidal blood flow. Clearance of both reference and test substances from the retina was considerably faster than from the brain. For this reason a 5 s interval between the intracarotid injection and the enucleation was used as routine for determinations of RUI. With this technique RUI for the transported monosaccharide 3-0-methyl-D-glucose was significantly larger than for L-glucose, 53.8 and 14.2%, respectively. The RUI for L-glucose was mainly due to L-glucose confined to extravascular choroidal fluid adhering to the retinal tissue samples. By prolonging the interval between injection and enucleation, and subtracting the activity due to recirculation, this extraretinal contamination could be practically eliminated. Apparently, the uptake index method can be a useful tool in studies on transport of metabolic substrates through the blood-retinal barrier.