Identification of the insulin receptor of cerebral microvessels

Cerebral microvessels are known to possess receptors for insulin and have recently been shown to respond to physiological levels of this hormone. Scatchard analysis of binding data obtained with isolated cerebral microvessels gave curvilinear plots and showed that neonatal porcine cerebral microvessels have a greater number of insulin receptors per unit of protein than adult bovine cerebral microvessels. The high-affinity form of the insulin receptors of both neonatal porcine and adult bovine cerebral microvessels have similar binding constants (dissociation constant = 0.3 X 10(-9) M). Dissociation of 125I-insulin from cerebral microvessels was accelerated by the presence of unlabeled insulin in preparations from both neonatal pigs and adult cows. 125I-insulin was covalently cross-linked to its receptor in cerebral microvessels with disuccinimidyl suberate, and the hormone-receptor complex was isolated on sodium dodecyl sulfate-polyacrylamide gels. Under reducing conditions, 125I-insulin was found associated with a polypeptide with a molecular weight of 130,000, which is indistinguishable from the alpha-subunit of the liver insulin receptor. In contrast, nonvascular cerebral cortical tissue contained an insulin receptor with an alpha-subunit that was lower in molecular weight than the form isolated from cerebral cortical microvessels.