Insulin Receptors in Human and Animal Placental Tissue

Abstract

The existence of polypeptide hormone receptors in the human placenta was evaluated by studying the specific binding of 125-I-labeled insulin, human growth hormone (hGH), human pro lac tin (hPRL) and glucagon to a defined placental membrane fraction. Only insulin showed specific binding to placental membranes. The binding of 125-I-insulin was time and temperature dependent. Its dissociation from the membrane was first order with a half time, at 24° C, of twenty minutes. Specific binding was readily observed at a concentration of 5 × 10−11 (7.5 μU./ml.). Inhibition of 125-I-insulin binding by unlabeled insulin was 30 per cent at 10−9 M, and >90 per cent at 10−9 M. Desalanine insulin was equally effective in inhibiting the binding of 125-I-insulin. Proinsulin was about twenty times less effective, and desoctapeptide insulin was even less effective. Structurally unrelated polypeptide hormones were without significant inhibitory effect. Binding sites of relatively high affinity (K1 = 4.2 × 108 M−1) and low capacity could be distinguished from those of lower affinity (K2 = 0.7 × 108M−1) and higher capacity. Insulin degrading activity was shown to be present in the placental membranes. Under standard binding assay conditions, less than 20 per cent of the 125-I-insulin present was degraded. The 125-I-insulin could be eluted from the membranes and appeared intact by several criteria. Specific binding was augmented by Ca++ and other divalent cations but was unaffected by high sodium chloride concentrations. Binding was greatly reduced by pretreatment of membranes with proteolytic enzymes. Incubation with phospholipase C, RNase and neur-aminidase had relatively little effect on binding. Insulin binding was unaffected by maternal diabetes but was reduced in membranes from early gestational placentas. There was considerable species variation in insulin binding, with the placental membranes of the guinea pig and monkey having the highest and those of the rat having the lowest binding. The characteristics of the insulin binding sites in the human placenta are similar to those in established insulin target tissues.