Rat somatostatin receptor type 1 couples to G proteins and inhibition of cyclic AMP accumulation

The pharmacology, signal transduction, and coupling to G proteins of the rat somatostatin (SRIF) receptor (SSTR)1 have been characterized in transfected Chinese hamster ovary (CHO) (K1 strain) cells. The expressed receptor exhibited saturable, high affinity binding of several radioiodinated SRIF analogues. Three different radioligands were used to determine the pharmacological properties of this SSTR subtype. [125I-Tyr11]SRIF-14 (125I-S-14), [Leu8,D-Trp22,125I-Tyr25]SRIF-28 (125I-S-28), and cyclo(D-Trp-Lys-Abu-Phe-MeAla-125I-Tyr) (125I-peptide C) displayed the following rank order of affinity (Kd) for the SSTR1 subtype: 125I-S-14 > or = 125I-S-28 > 125I-peptide C. Competition of 125I-S-14 with S-14, S-28, or peptide C displayed the same rank order of potency. Chemical cross-linking of specifically bound 125I-S-28 to membranes from CHO cells expressing the receptor indicated that the molecular weight of the SSTR1 expressed in CHO cells is approximately 70,000, suggesting that it is heavily glycosylated. Previous reports have suggested that the human SSTR1 [Mol. Pharmacol. 42:28-34 (1992)] couples poorly to G proteins. The coupling of the rat SSTR1 to G proteins was demonstrated by three independent methods. (a) Binding of 125I-S-14 to the SSTR1 subtype was inhibited in a dose-dependent fashion by incubation of membranes with guanosine-5'-O-(3-thio)triphosphate. (b) Treatment of cells with pertussis toxin decreased binding by 80%. (c) Immunoprecipitation of 125I-S-14 binding was observed with antiserum specific for Gi alpha 1,2, but not with antiserum specific for Gs alpha, in membranes from transfected cells. In CHO cells transfected with the SSTR1 cDNA, SRIF inhibited forskolin-stimulated cAMP accumulation by up to 50%, in a dose-dependent fashion (ED50 = 1.1 nM). Pertussis toxin treatment decreased both the efficacy and the potency of the SRIF-mediated inhibition of cAMP accumulation (from 50% to 22%), compared with control untreated cells. These data suggest that the rat SSTR1 inhibits cAMP accumulation by coupling to pertussis toxin-sensitive G proteins.