Ontogeny and Characterization of Basic Somatomedin Receptors in Rat Placenta*

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Abstract
The binding of [125I]basic somatomedin (B-SM) (insulin-like growth factor-I/SM-C) in rat placenta was investigated. Low but specific [125I]B-SM binding to placenta was detected in rats from the end of the 2nd wk (14 days) of gestation. The binding increased until .apprx. 17 days of gestation and then reached a plateau. Binding characteristics did not alter during gestation. In contrast to previous observations [125.LAMBDA.]B-SM binding to the tissue was completed 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 Scatcard 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 MW > 330 K[kilodaltons] and 140 K in the presence of reduction and > 330 K in the absence of the reductant. Detailed analysis of the receptor in term placenta subsequent to 5-13% gradient gel containing sodium dodecyl sulfate revealed 2 more labeled components (MW 250 K and 36 K) irrespective of the reductant. The intensity of the 250 K and 36 K was low compared to 140 K and 330 K components. Apparently, 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.

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