Insulin-Like Growth Factor Receptors in Rat Placental Membranes*

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Abstract
The nature and relative quantity of insulin-like growth factor (IGF) receptors in rat placental microsomal membranes was investigated by competitive binding studies and covalent cross-linking followed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis [SDS-PAGE]. Binding studies revealed that 100 .mu.g membrane protein specifically bound 26.5% of [125I]iodo-IGF II, 19.6% of [125I]iodomultiplication-stimulating activity III-2([125I]iodo MSA III-2) and 11.5% [125I]iodo-IGF-I. IGF-II was equipotent with MSA-III-2, and both were approximately twice as potent as IGF-I in competing with [125I]iodo-IGF-I for binding. IGF-I competed for the binding of [125I]iodo-MSA and [125I]iodo-IGF-II with only 5-20% the potency of unlabeled MSA and IGF-II. Insulin competed weakly with [125I]iodo-IGF-I for binding (achieving half-maximal displacement at 20 .mu.g/ml), but did not compete with [125I]iodo-MSA for binding. While IGF-I binds to both IGF-I and IGF-II receptors, the majority of IGF-I binding apparently is due to an interaction with IGF-II receptors. Studies using [125I]iodo-IGF-I covalently cross-linked to placental membrane receptors followed by SDS-PAGE confirmed that molecular species characteristic of the subunits of IGF-I receptors are present in rat placenta. Rat placenta, like human placenta, contains receptors for both types of IGF. Unlike human placenta, the majority of the receptors are of the IGF-II type.

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