Somatostatin-14 and somatostatin-28 pretreatment down-regulate somatostatin-14 receptors and have biphasic effects on forskolin-stimulated cyclic adenosine, 3',5'-monophosphate synthesis and adrenocorticotropin secretion in mouse anterior pituitary tumor cells.

Abstract

Activation of somatostatin-14 (S-14) receptors on mouse AtT-20 pituitary tumor cells by S-14 or somatostatin-28 (S-28) inhibits forskolin-stimulated cAMP synthesis and ACTH secretion. In this study, the effects of prolonged exposure of cells to S-14 or S-28 was found to reduce, in a time- and concentration-dependent fashion, the density of S-14 receptors without affecting the affinity of these sites for [125I]Tyr11-S-14. This response was rapidly reversible after removal of peptide from incubation media. Additionally, S-14 and S-28 pretreatment also resulted in a time-dependent sensitizing effect on forskolin-stimulated cAMP formation and ACTH secretion which preceded S-14 receptor down-regulation. Enhancement of the forskolin response was concentration dependent, with maximal effects observed at 10(-8) M with either peptide. Higher pretreatment concentrations of S-14 resulted in an abolition of the enhanced biological response to forskolin; pretreatment with S-28 (10(-6) M) depressed forskolin- and (-)isoproterenol-induced cAMP formation below levels observed in nonpretreated cells. The enhancing effect of S-14 and S-28 required new protein synthesis, since it was partially blocked by cycloheximide; the depressor effect was independent of new protein synthesis. Both the enhanced and depressed forskolin responses after peptide pretreatment were reversible after withdrawal of S-14 or S-28; normalization of the forskolin response (cAMP formation and ACTH secretion) followed the return to control levels of S-14 receptor density. Pretreatment of cells with 10(-8) M or 10(-6) M S-28 increased and decreased, respectively, the ACTH secretory response to agonists which act in the absence of prior cAMP synthesis such as 8-bromo-cAMP, A-23187, and phorbol ester. The data suggest that S-14 receptor down-regulation is not causally associated with the sensitizing effects of S-14 and S-28 on adenylate cyclase and that the S-14 receptor may be also coupled to other effector systems which are involved in regulating the secretory function of AtT-20 cells.