MOLECULAR-CLONING AND FUNCTIONAL-CHARACTERIZATION OF A RAT SOMATOSTATIN SST(2(B)) RECEPTOR SPLICE VARIANT

1 The mouse somatostatin (SRIF) sst(2) receptor exists in two splice v ariants, sst(2(a)) and sst(2(b)), which differ in their intracellular carboxy-termini only. The murine sst(2(b)) receptor was reported to be less prone to agonist-induced desensitization as compared with the ss t(2(a)) receptor. To determine whether a sst(2(b)) splice variant with similar functional characteristics exists in the rat, we have isolate d a cDNA fragment from rat gastric mucosa encoding a sst(2(b)) recepto r and expressed the full-length protein in CHO-KI cells for functional characterization. 2 This study provides the first evidence for the oc currence in the rat of the sst(2(b)) receptor, which has a 15 amino ac id carboxy-terminus differing in composition to the 38 amino acid C-te rminus of the rat sst(2(a)) receptor. 3 In CHO-K1 cells expressing rat recombinant sst(2(a)) Or sst(2(b)) receptors, SRIF caused concentrati on-dependent increases in extracellular acidification rates (EAR) with pEC(50) values of 9.0 and 9.9, respectively. Pre-treatment with pertu ssis toxin (Ptx) caused a rightward displacement of the SRIF concentra tion-effect curves with pEC(50) values of 8.3 (sst(2(a))) and 8.4 (sst (2(b))).4 SRIF (3 pM-3 nM) also caused concentration-dependent inhibit ion of forskolin-stimulated cyclic AMP formation in CHO-sst(2(a)) cell s (pIC(50) 10.5) and CHO-sst(2(b)) cells (pIC(50) 10.4). The degree of inhibition was less with higher concentrations of SRIF resulting in b ell-shaped concentration-effect curves. Following pre-treatment with P tx, the inhibitory effect of SRIF was abolished and SRIF caused only i ncreases in cyclic AMP formation. 5 Both the SRIF-induced increases in EAR and inhibition of cyclic AMP formation were susceptible to agonis t-induced desensitization, but this was less apparent following pre-tr eatment with Ptx. 6 This demonstrates that the operational characteris tics of the recombinant rat sst(2(a)) and sst(2(b)) receptors are broa dly similar. Both isoforms couple to Ptx-sensitive as well as -insensi tive G proteins and are equally prone to agonist-induced desensitizati on.