Ontogeny and characterization of basic somatomedin receptors in rat placenta.

We investigated the binding of [125I]basic somatomedin (B-SM) (insulin-like growth factor-I/SM-C in rat placenta. Low but specific [125I]B-SM binding to placenta was detected in rats from the end of the second week (14 days) of gestation. The binding increased until about 17 days of gestation and then reached a plateau. Binding characteristics did not alter during gestation. In contrast to previous observations [125I]B-SM binding to the tissue was competed more effectively by B-SM than multiplication stimulating activity (a rat counterpart of IGF-II). Insulin competed for the binding at a higher concentration. The Scatchard plot derived from the binding competition data was curvilinear. Autoradiograms of affinity labeled B-SM receptors in placentas at different gestational age subsequent to sodium dodecyl sulfate-gel (7.5%) electrophoresis revealed labeled constituents with approximate mol wt greater than 330 K and 140 K in the presence of reductant and greater than 330 K in the absence of reductant. Detailed analysis of the receptor in term placenta subsequent to 5-13% gradient gel containing sodium dodecyl sulfate revealed two more labeled components (mol wt, 250 K and 36 K) irrespective of the reductant. However, the intensity of the 250 K and 36K was low compared to 140 K and 330 K components. These results suggest that B-SM receptor in rat placenta, similar to human placenta, binds preferentially to type I or 140 K subunit of the 330 K oligomeric SM receptor.