Recent in vivo autoradiographic studies have suggested that circulating insulin may bind to the capillary wall, i.e., the blood-brain barrier. In the present study the blood-brain barrier insulin receptor was examined directly by measuring [125I]-iodoinsulin binding to capillaries isolated from fresh bovine cerebral cortex. Microvessels were prepared by gentle hand homogenization and trapping on nylon mesh. The binding was rapid, specific, and reversible with one-half maximal binding attained in 7 min and maximal binding achieved in 45 min at room temperature. The high affinity site has an affinity constant of 2.3 ± 0.3 nM−1, and 50% displacement of [125I]-iodoinsulin occurred at approximately 9 ng/ml. [125I]-lodoinsulin was not displaced by excess thyrotropin, prolactin, or growth hormone, and proinsulin was much less potent than porcine insulin. These studies confirm the presence of a specific insulin receptor on brain microvessels. Although insulin does not cross the blood-brain barrier, the presence of an insulin receptor provides a possible mechanism by which blood-borne insulin can influence brain cell metabolism.

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