Binding of somatomedin-A and -C, NSILA-S and insulin to human placental cell membranes.

Binding of labeled SM-A, SM-C, NSILA-S and insulin to human placental cell membranes was studied in trying to answer the question how many receptor populations were involved using one single organ system. The data suggest that all labeled SM-like substances bind to closely related if not identical receptor populations. The binding is reduced by very low concentrations of SM-like material and only by very high concentrations of insulin. In contrast, as already known from the literature, 125I-insulin binds mostly to a different receptor population, which is sensitive both to insulin and SM-like substances. Furthermore, the data indicate that 125I-SM-A and 125I-SM-C, in addition to binding to similar or identical receptors, also bind to separate receptor populations, suggesting that the labels are composed of more than one component.