Somatostatin Receptors on Pituitary Somatotrophs, Thyrotrophs, and Lactotrophs: Pharmacological Evidence for Loose Coupling to Adenylate Cyclase

Abstract

Pharmacological characterization of somatostatin (SRIF) receptors located on somatotrophs, thyrotrophs, and lactotrophs was attempted by measuring the effects of 14 structural agonists of somatostatin (SRIF) on the inhibition of basal and GRF-stimulated GH and basal and TRH-stimulated PRL and TSH secretion. We also checked the abilities of the analogs to displace [125I]N-Tyr-SRIF binding to pituitary cell membranes and their potency to inhibit adenylate cyclase activity. There was a very good correlation (r = 0.975) between the displacement of [125I]N-Tyr-SRIF and the inhibition of adenylate cyclase activity by the analogs. The effects of the analogs on secretion of the three hormones followed the same rank order of potency. However, the active analogs displayed 2-6 times lower affinities in inhibiting PRL than GH or TSH secretions. The shift in affinity was even more pronounced in the case of the lower potency of the analogs as inhibitors of adenylate cyclase activity compared to hormone secretions. Pretreatment of the cells with pertussis toxin (100 ng/ml; 24 h) blocked SRIF inhibition of basal and GRF-stimulated adenylate cyclase activity and decreased by 83% [125I]N-Tyr-SRIF binding. It also blocked the ability of SRIF to inhibit GRF-induced GH and TRH-induced PRL and TSH secretion. However, pertussis toxin also increased GRF stimulation of GH secretion and decreased TRH stimulation of both TSH and PRL secretion. We conclude from our data that SRIF-binding sites located on the three target cells of the adenohypophysis are of a single class. These binding sites are negatively coupled to adenylate cyclase, but the inhibition of hormone secretions by SRIF cannot be explained solely through adenylate cyclase inhibition. Another mechanism of transduction must be involved in the actions of SRIF on its three pituitary target cells.

Two types of voltage‐dependent calcium current in rat somatotrophs are reduced by somatostatin.

“…However, the post-receptor events in this cell type are only partially understood so far. Somatostatin has, to a certain extent, an inhibitory CALCIUM CURRENTS AND SOMATOSTATIN4 action on cyclic AMP production (Sheppard, Spence & Kraicer, 1979), but additional mechanisms, such as Ca2+ mobilization, have also been reported (Dorflinger & Schonbrunn, 1983;Sheppard, Moore & Kraicer, 1985;Epelbaum, Enjalbert, Krantic, Musset, Bertrand, Rasolonjanahary, Shu & Kordon, 1987). We have previously shown that somatostatin increased membrane potassium conductance, leading to the hyperpolarization of the membrane potential.…”

G(o)‐2 protein mediates the reduction in Ca2+ currents by somatostatin in cultured ovine somatotrophs.

“…It has been suggested that the G. protein mediates the effect of SRIF on Ca2+ channels in GH3 cell lines (Kleuss et al 1991 . These G proteins are presumably involved in various effects of SRIF, which include a reduction in cAMP levels (Epelbaum et al 1987), an increase in K+ currents (Chen, Israel & Vincent, 1989;Chen, Zhang, Vincent & Israel, 1990b) and a decrease in Ca2+ currents (Chen et al 1990a). A G protein-dependent and cGMP-dependent protein kinasemediated decrease in Ca2+ currents by SRIF has also been reported in neurones (Meriney, Gray & Pilar, 1994), adding complexity to the mechanism of action of SRIF.…”

Epidermal Growth Factor Triggers an Original, Caspase-independent Pituitary Cell Death with Heterogeneous Phenotype

“…At the end of the treatment period, the medium was removed and replaced by fresh, serum-free medium. The accumulation of secreted PRL and GH in the medium was then measured over a 1-h period by subsequent standard GH-or PRL-RIAs (Epelbaum et al, 1987).…”

“…At the end of the treatment period, the medium was removed and replaced by fresh, serum-free medium. The accumulation of secreted PRL and GH in the medium was then measured over a 1-h period by subsequent standard GH-or PRL-RIAs (Epelbaum et al, 1987).The constructs used for cell transfection were obtained from P. Friesen (University of Wisconsin, Madison, WI) for pBabe-p35 or after the insertion of the cloned 1855-base pair cDNA fragment of Bcl-2 into pcDNA-3.1 vector (Hockenbery et al, 1993). The transient GH4C1 cell transfection was performed by using TransFast reagent (Promega, Madison, WI).…”