Basic somatomedin (B-SM) receptors in human term placenta explants.

Binding of basic somatomedin (B-SM) and effect on alpha-aminoisobutyrate (AIB) uptake was studied in explants of term human placenta. B-SM demonstrated specific binding to the cultured explants. The binding was weakly inhibited by insulin. No significant degradation of the hormone was observed during incubation. The affinity cross-linking of [125I]B-SM binding to the explant membrane revealed a binding subunit of Mr 140 K. Addition of B-SM to the serum free medium stimulated the placental explant AIB uptake in a dose dependent manner. The incubated media containing FBS had less SM activity than the pre-incubated medium. The serum free incubated media did not contain measurable SM activity. The above findings that placental explants bind and have biological response to B-SM but do not produce B-SM suggest that SM has an endocrine rather than a paracrine action on human placenta.