DIFFERENCES IN THE OPERATIONAL CHARACTERISTICS OF THE HUMAN RECOMBINANT SOMATOSTATIN RECEPTOR TYPES, SST(1) AND SST(2), IN MOUSE FIBROBLAST(LTK(-)) CELLS

1 The human recombinant somatostatin (SRIF) receptors, sst(1) and sst( 2), have been stably expressed in mouse fibroblast (Ltk(-)) cells. Two stable clones, LSSR1/20 and LSSR11/13, expressing sst(1) and sst(2) r eceptors, respectively, have been used to characterize these receptor types using radioligand binding assays as well as measurements of chan ges in extracellular acidification rates using microphysiometry. 2 [I- 125]-[Tyr(11)]-SRIF bound to sst(1) and sst(2) receptors expressed in Ltk(-) cells with high affinity, K-d values being 1.52 nM and 0.23 nM, respectively. 3 In Ltk(-) cells expressing sst(1) receptors, SRIF, SR IF-28, [D-Trp(8)]-SRIF and CGP 23996 all displaced [I-125]-[Tyr(11)]-S RIF binding with high potency (IC50 values of 0.43-1.27 nM) whilst seg litide, BIM-23027, BIM-23056 and L-362855 were either weak inhibitors of binding or were ineffective. 4 In contrast MK-678 (seglitide) and B IM-23027 were the most potent inhibitors of [I-125]-[Tyr(11)]-SRIF bin ding in Ltk(-) cells expressing sst(2) receptors with IC50 values of 0 .014 and 0.035 nM, respectively. 5 SRIF and a number of SRIF agonists, including seglitide and BIM-23027, caused concentration-dependent inc reases in extracellular acidification rates in Ltk(-) cells expressing sst(2) receptors but not in Ltk(-) cells expressing sst, receptors. T he maximum increase in acidification rate produced by SRIF was 11.3+/- 0.7% above baseline (0.1-0.28 pH unit min(-1)). The relative potencies of the SRIF agonists examined in causing increases in extracellular a cidification rates in Ltk(-) cells expressing sst(2) receptors correla ted well with their relative potencies in inhibiting [I-125]-[Tyr(11)] -SRIF binding (r = 0.94). 6 The increase in extracellular acidificatio n produced by SRIF was markedly inhibited by pretreatment of cells wit h pertussis toxin (100 ng ml(-1)) indicating the involvement of pertus sis toxin-sensitive G proteins. 7 SRIF (1 mu M) had no effect on basal cyclic AMP levels in Ltk(-) cells expressing sst(1) or sst(2) recepto rs nor did it inhibit forskolin stimulated increases in cyclic AMP lev els in either cell type. 8 The results from the present study describe the operational characteristics of human sst(2) receptors expressed i n Ltk(-) cells where receptor activation causes increases in extracell ular acidification rates. This receptor is coupled to a pertussis toxi n-sensitive G protein. In contrast, activation of sst(1) receptors, at a similar transfection density, did not cause increases in extracellu lar acidification rates.