Characterization of somatostatin specific binding in plasma cell membranes of human placenta

Somatostatin is a known inhibitor of hormone secretion and of nutrient transport. Because somatostatin-like immunoreactivity has been detected in amniotic fluid and the placenta has both hormone secretory and nutrient transport functions, we investigated the possible existence of somatostatin receptors on placenta cell membranes. Binding of 125I-Tyr1- and 125I-Tyr11-somatostatin (5-21%) to solubilized placenta cell membranes was observed. Binding was time-, temperature-, and pH-dependent and occurred maximally with incubation at concentrations of 25 micrograms of membrane protein. Displacement of binding of 125I-Tyr1 and Tyr11 somatostatin by cold cyclic and linear somatostatin and somatostatin analogs Ala-5, Ala-8, and Ala-11 was observed. Scatchard analysis of data revealed high capacity of (Ro 0.44 mol/micrograms X 10(-12) but low affinity (Kd 1.8 M X 10(-7) binding sites similar to that reported in other tissues. Binding was not reversible under our experimental conditions. The significance of this low affinity binding of somatostatin to placenta cell membranes remains to be determined.