Effector coupling of somatostatin receptor subtypes on human endocrine tumors

Abstract

Effector coupling of somatostatin receptor subtypes sst 1 and sst 2 was examined in a reconstituted system. Forskolin-stimulated cyclic adenosine monophosphate (cAMP) formation was inhibited 66% by somatostatin (SRIF-14) in CHO cells expressing somatostatin receptor 1 (sst 1) (CHO-SR1), but not sst 2, in a dose-dependent manner with an ED 50 of 1 × 10 −9 mol/L SRIF-14. The inhibition was blocked by pertussis toxin (PTX), indicating that sst 1 is coupled to adenylyl cyclase via PTX-sensitive G i protein. In CHO cells, G iα2 and G iα3 mRNAs were detected. In adenylyl cyclase assays, 1 μmol/L SRIF-14 caused a 16% inhibition of forskolin-stimulated adenyly cyclase activity. Preincubation with G iα3, but not G i α1 G i α2 , antiserum blocked this inhibition. By contrast, sst 2 is coupled to adenylyl cyclase via G iα1. In cells expressing sst 2 with G iα1 (CHO-SR2G1), SRIF-14 significantly inhibited forskolin-stimulated cAMP formation by 53% and with an ED 50 at 4 × 10 −9 mmol/L SRIF-14, which was completely blocked by PTX; ED 50 values for sst 1 and sst 2 agree with the IC 50 values in binding assays. In CHO-SR1, the rank of potency of agonists affecting adenyl cyclase was SRIF-14 = SRIF-28 > RC 160 > SMS 201-995. In CHO-SR2G1, the rank was RC-160 > SRIF-14 = SRIF-28 > SMS201-995.