Specific Somatostatin Receptors on Human Pituitary Adenoma Cell Membranes

Specific somatostatin (SRIH) receptors on human pituitary adenoma cell membranes were characterized using [125I]Tyr11-SRIH as the radioligand. Specific binding of [125I] Tyr11-SRIH to adenoma cell membranes reached a steady state within 30 min at 25 C, and semilogarithmic analysis of the data revealed that the rate of the binding was linear at 25 C with a t½ of 13.2 min. Specific binding increased linearly with 5-160 fig plasma membrane protein. SRIH-14 and SRIH-28 inhibited [125I]Tyr11-SRIH binding to adenoma cell membranes with ID50S of 0.32 and 0.50 nM, respectively, while secretin, glucagon, gastrin, cholecystokinin-8, bombesin, TRH, LHRH, human GHreleasing factor-(l–44)-NH2, D-Ala2-met-enkephalin, γ-aminobutyric acid and taurine did not significantly inhibit binding. All of 13 GH-secreting adenomas investigated had specific and high affinity SRIH receptors, with a dissociation constant (Kd) of 0.80 ± 0.15 nM (mean ± SEM) and a maximal binding capacity (Bmax) of 234.2 ± 86.9 fmol/mg protein (mean ± SE...