Mesotocin binding to receptors in hen kidney plasma membranes.

Radioligand assays were performed to demonstrate the presence of a receptor for mesotocin (MT) in the membrane fractions of the kidney of the hen. Specific [125I]MT bindings were decreased by the presence of Mg2+ and Ca2+, increased by the presence of EDTA, increased during the first 4 h of incubation and then reached a plateau, and increased with the increase in the protein concentration from 2.5 to 20 micrograms. The membrane fraction showed binding specificity to [125I]MT. The Scatchard plot revealed a curvilinear profile that indicated the presence of two classes of binding sites: a high affinity site and a low affinity site. The equilibrium dissociation constant was 0.08 +/- 0.01 nM (mean +/- SEM; n = 5) in the high affinity site and 0.87 +/- 0.08 nM (n = 5) in the low affinity site. The maximum binding capacity of the high and low affinity sites was 42 +/- 4 and 129 +/- 6 fmol/mg protein, respectively. The results suggest the presence of two distinct MT receptors in the kidney of the hen.