somatostatin--n-tyr(1)- and Pituitary-Neoplasms

The GH4C1 pituitary cell line contains specific plasma membrane receptors for the inhibitory neuropeptide somatostatin (SRIF). Unlike other peptides which bind to cell surface receptors on these cells, SRIF is not rapidly internalized via receptor-mediated endocytosis. Here we examined the effects of chronic SRIF pretreatment on the subsequent ability of GH4C1 cells to bind and respond to this hormone. Treatment of cells with 100 nM SRIF increased [125I-Tyr1]SRIF binding to a maximum value of 220% of control after 20 h. Scatchard analysis demonstrated that the number, but not the affinity, of the receptors was altered. The effect of SRIF was dose-dependent (ED50 = 2.3 +/- 0.4 nM), was not mimicked by an inactive analog, and was specific for the SRIF receptor. Furthermore, pretreatment of cells with other agents, which mimic SRIF's action to decrease intracellular cAMP and free Ca2+ concentrations, did not mimic the SRIF-induced increase in receptor number. Thus, occupancy of the SRIF receptor was required for SRIF receptor up-regulation. Inhibition of protein synthesis with cycloheximide did not prevent the SRIF-induced increase in receptors, consistent with an effect of SRIF to either reduce receptor degradation or cause slow redistribution of preexisting receptors to the plasma membrane. In contrast to the effects on receptor binding, pretreating cells with SRIF did not alter either basal cAMP levels or the potency of SRIF to inhibit cAMP accumulation (ED50 = 0.5 +/- 0.2 nM). However, the maximum cAMP produced by stimulators of adenylyl cyclase was increased. The observation that chronic SRIF exposure did not cause homologous desensitization in GH4C1 cells and increased rather than decreased SRIF receptor number is consistent with the fact that this neuropeptide is not rapidly internalized by receptor-mediated endocytosis.

Topics: Animals; Colforsin; Cycloheximide; Dose-Response Relationship, Drug; Pituitary Neoplasms; Rats; Receptors, Neurotransmitter; Receptors, Somatostatin; Somatostatin; Time Factors; Tumor Cells, Cultured; Vasoactive Intestinal Peptide

Addition of somatostatin (SRIF) inhibits corticotropin-releasing factor- and forskolin-stimulated cyclic AMP accumulation and adrenocorticotropin hormone secretion from mouse anterior pituitary tumor cells (AtT-20/D16-16). However, prior exposure of these cells to SRIF reduced the potency of SRIF to inhibit both corticotropin-releasing factor- and forskolin-stimulated cyclic AMP accumulation and adrenocorticotropin hormone release. This SRIF desensitization is time- and concentration-dependent and reversible. Cross-desensitization to SRIF analogs also occurred whereas SRIF pretreatment did not affect the inhibition by SRIF of 8-bromo-cyclic AMP-stimulated adrenocorticotropin hormone release or did it affect basal cyclic AMP levels, protein content or phosphodiesterase activity. These data indicate that SRIF can regulate the sensitivity of its own receptor and that SRIF desensitization may involve either a down-regulation of SRIF receptors or an uncoupling of these inhibitory receptors from adenylate cyclase.

Topics: 8-Bromo Cyclic Adenosine Monophosphate; Adrenocorticotropic Hormone; Animals; Colforsin; Corticotropin-Releasing Hormone; Cyclic AMP; Diterpenes; Dose-Response Relationship, Drug; Mice; Pituitary Neoplasms; Receptors, Cell Surface; Receptors, Somatostatin; Somatostatin; Somatostatin-28