SUBTYPE-SPECIFIC SIGNALING MECHANISMS OF SOMATOSTATIN RECEPTORS SSTR1 AND SSTR2

Somatostatin regulates diverse cellular effecters, including adenylyl cyclase, ion channels, and ion exchangers. We expressed two somatostatin receptor subtypes, SSTR1 and SSTR2, stably in mouse fibroblast Ltk(-) cells and transiently in human embryonic kidney HEK293 cells to investigate subtype-specific pharmacological and functional properties. The effects of GTP gamma S and pertussis toxin on [I-125-Tyr(11)]somatostatin-14 binding indicated that SSTR2 may couple exclusively to pertussis toxin sensitive G proteins, whereas SSTR1 may couple to both pertussis-sensitive and -insensitive G proteins. When expressed either stably or transiently, both receptor subtypes mediated somatostatin inhibition of cAMP accumulation by a pertussis toxin-sensitive mechanism. In contrast, only SSTR1 mediated somatostatin inhibition of Na+-H+ exchange activity, and this action was insensitive to pertussis toxin. We generated two chimeric receptors by replacing sequential residues of SSTR2 with cognate sequences of SSTR1 to identify molecular determinants unique to SSTR1 that may confer coupling to the exchanger. SSTCR4 included a SSTR1 segment encompassing determinants within the fifth and sixth hydrophobic domains and the entire third cytoplasmic loop, while SSTCRS5 contained a SSTR1 segment spanning the second through sixth hydrophobic domains, including both second and third cytoplasmic loops. Although both chimeric receptors mediated somatostatin inhibition of cAMP accumulation, only SSTCR5 mediated the inhibition of Na+-H+ exchange activity, and this effect was pertussis-insensitive. These findings demonstrate both pharmacological and functional differences between SSTR1 and SSTR2. The ability of SSTR1 to selectively attenuate Na+-H+ exchange activity requires determinants outside the third cytoplasmic domain.