Somatostatin (SRIH) regulates pituitary adrenocorticotropin (ACTH) secretion by interacting with a family of homologous G protein-coupled membrane receptors. The SRIH receptor subtypes (sst1–sst5) 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 125I-SRIH-14 and 125I-SRIH-28 showed that SRIH-14 and SRIH-28 recognized specific, high-affinity and saturable membrane-binding sites. Nonpeptidyl agonists with selectivity for the sst2 (L-779,976; compound 2) or sst1/sst5) (L-817,818; compound 5) receptor subtypes potently displaced 125I-SRIH-28 from AtT-20 cell membranes, while agonists selective for the sst1 (L-779,591; compound 1), sst3 (L-796,778; compound 3) or sst4 (L-803,087; compound 4) subtypes were inactive. Tyr11-SRIH-14, compound 2 (sst2) or compound 5 (sst5) inhibited forskolin and corticotropin-releasing hormone (CRH)-induced increases in intracellular cAMP. Furthermore, the sst2 and sst5 agonists potently inhibited CRH-induced ACTH release from AtT-20 cells. These results provide the first evidence that sst2 and sst5 receptor subtypes, but not sst1, sst3 or sst4, inhibit cAMP accumulation and regulate ACTH secretion in the AtT-20 cell model of the rodent corticotrope.
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