Insulin receptors in calf and human retinal blood vessels

A system for isolation of capillaries from the retina has been adapted to biochemical studies in vitro. Insulin receptors were identified in human and calf retinal blood vessels. Binding of 125I insulin by calf retinal blood vessels takes place through high affinity (low capacity) and low affinity (high capacity) receptor sites. Insulin binding is inhibited by glucagon, and cGMP and proinsulin. Dissociation of insulin receptor protein or lipid with Triton X-100 or phospholipase results in significant decreases in 125I insulin binding by retinal blood vessels. Dissociation of insulin bound occurs only at 0 degrees C which may be due to rapid internalization of insulin at higher temperatures. Activities of cAMP and cGMP phosphodiesterases and cyclase of retinal vessels were not significantly changed by incubation with insulin. At high concentrations in the media human growth hormone stimulates and then inhibits I125 insulin binding by retinal capillaries.