Diffuse cytoplasmic staining of retinal capillary endothelium.

The distribution of several hemeprotein tracers in retinal capillaries of Wistar-Furth rats was studied by electron microscopic cytochemistry after incubation in 3,3'-diaminobenzidine. Diffuse cytoplasmic reaction product was frequently observed in the endothelial cells after intravenous injection of horseradish peroxidase (HRP) or lactoperoxidase (LP), or after perfusion of HRP. Occasionally, pericytes were also diffusely stained. In contrast, injection of microperoxidase, catalase, or hemoglobin did not cause diffuse staining. The diffuse staining was similar with HRP types II, VI, and VIII, and at concentrations of 2.5, 5, and 10 mg/100 g body weight. Despite the staining, the blood-retinal barrier remained intact. The findings indicate that HRP and LP are capable of causing diffuse nonspecific staining of retinal capillary endothelial cells, even at relatively low concentrations. Since these tracers are frequently used in studies of the blood-retinal barrier, the results of such studies should be interpreted with caution.