Somatostatin Receptor Subtypes 2 and 5 Inhibit Corticotropin-Releasing Hormone-Stimulated Adrenocorticotropin Secretion from AtT-20 Cells

Abstract

Somatostatin (SRIH) regulates pituitary adrenocorticotropin (ACTH) secretion by interacting with a family of homologous G protein-coupled membrane receptors. The SRIH receptor subtypes (sst<sub>1</sub>–sst<sub>5</sub>) that control ACTH release remain unknown. Using novel, subtype-selective SRIH analogs, we have identified the SRIH receptor subtypes involved in regulating ACTH release from AtT-20 cells, a model for cell line pituitary corticotropes. Radioligand-binding studies with <sup>125</sup>I-SRIH-14 and <sup>125</sup>I-SRIH-28 showed that SRIH-14 and SRIH-28 recognized specific, high-affinity and saturable membrane-binding sites. Nonpeptidyl agonists with selectivity for the sst<sub>2</sub> (L-779,976; compound 2) or sst<sub>1</sub>/sst<sub>5</sub>) (L-817,818; compound 5) receptor subtypes potently displaced <sup>125</sup>I-SRIH-28 from AtT-20 cell membranes, while agonists selective for the sst<sub>1</sub> (L-779,591; compound 1), sst<sub>3</sub> (L-796,778; compound 3) or sst<sub>4</sub> (L-803,087; compound 4) subtypes were inactive. Tyr<sup>11</sup>-SRIH-14, compound 2 (sst<sub>2</sub>) or compound 5 (sst<sub>5</sub>) inhibited forskolin and corticotropin-releasing hormone (CRH)-induced increases in intracellular cAMP. Furthermore, the sst<sub>2</sub> and sst<sub>5</sub> agonists potently inhibited CRH-induced ACTH release from AtT-20 cells. These results provide the first evidence that sst<sub>2</sub> and sst<sub>5</sub> receptor subtypes, but not sst<sub>1</sub>, sst<sub>3</sub> or sst<sub>4</sub>, inhibit cAMP accumulation and regulate ACTH secretion in the AtT-20 cell model of the rodent corticotrope.