A Direct In Vitro Demonstration of Insulin Binding to Isolated Brain Microvessels

Abstract

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]-Iodoinsulin 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.

Hyperinsulinemia Suppresses Glucose Utilization in Specific Brain Regions:In VivoStudies Using the Euglycemic Clamp in the Rat

“…Insulin binding sites have been demonstrated throughout the brain, with highest concentrations in olfactory bulb, cerebral cortex, and hypothalamus (25). Circulating insulin binds to specific nerve endings in the circumventricular organs (46), whereas autoradiographic studies of brain blood vessels show insulin binding to the luminal surface of blood vessels throughout the brain, particularly the neocortex and hypothalamus (48).…”

Active targeting of brain tumors using nanocarriers

“…In vitro and in vivo studies have shown the presence of a high concentration of insulin receptors on both animal [126,127] and human cerebral capillaries [32]. Using autoradiography, Duffy and Pardridge [128] demonstrated the transcytosis of insulin through the BBB after injection into the carotid artery of 1-month-old rabbits.…”

Insulin in the brain: There and back again

“…For this reason, BECs are exposed to both peripheral and CNS signals. The BECs and ependymal cells that comprise the BBB and blood-CSF barrier possess insulin-binding sites (Baskin, et al, 1986; Frank, Jankovic-Vokes, Pardridge, & Morris, 1985; Frank & Pardridge, 1981; Miller & Borchardt, 1991). Those binding sites at the BBB and probably those at the blood-CSF barrier perform two distinct functions: some of them act as transporters of insulin across the BBB and others act as classic receptor sites, affecting the function of the barrier cell by activating intracellular machinery (Figure 2).…”