IDENTIFICATION AND DISTRIBUTION OF INSULIN-RECEPTORS ON CULTURED BOVINE BRAIN MICROVESSEL ENDOTHELIAL-CELLS - POSSIBLE FUNCTION IN INSULIN PROCESSING IN THE BLOOD-BRAIN-BARRIER

The binding of I-125-insulin to primary cultures of bovine brain micro vessel endothlial cells was examined. Insulin binding was both time an d temperature dependent and inhibited by excess unlabeled insulin. Fur thermore, the specific binding of insulin was polarized to the apical side of the cell monolayers. Upon binding, the labeled insulin was int ernalized, with approximately 70% resistant to acid wash over a 90-min period. The inhibition of insulin internalization observed with cell monolayers exposed to either phenylarsine oxide or unlabeled insulin s uggests a receptor-mediated endocytic process. Furthermore, the abilit y of chloroquine to reduce the metabolism of insulin indicates a signi ficant portion of the peptide is processed through a lysosomal pathway . In contrast to the fluid-phase endocytosis marker, Lucifer yellow, a s much as 65% of internalized insulin undergoes apical to basolateral trancytosis in brain microvessel endothelial cells. While most of the effluxed insulin was degraded, as assessed by trichloroacetic acid pre cipitation, the results of the present study suggest insulin receptors within the brain microvasculature may be involved in the processing a nd transport of bloodborne insulin. (C) 1994 Wiley-Liss, Inc.