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) receptors, respectively, have been used to characterize these receptor types using radioligand binding assays as well as measurements of changes 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, SRIF-28, [D-Trp(8)]-SRIF and CGP 23996 all displaced [I-125]-[Tyr(11)]-SRIF binding with high potency (IC50 values of 0.43-1.27 nM) whilst seglitide, BIM-23027, BIM-23056 and L-362855 were either weak inhibitors of binding or were ineffective. 4 In contrast MK-678 (seglitide) and BIM-23027 were the most potent inhibitors of [I-125]-[Tyr(11)]-SRIF binding 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 increases in extracellular acidification rates in Ltk(-) cells expressing sst(2) receptors but not in Ltk(-) cells expressing sst, receptors. The 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 acidification rates in Ltk(-) cells expressing sst(2) receptors correlated well with their relative potencies in inhibiting [I-125]-[Tyr(11)]-SRIF binding (r = 0.94). 6 The increase in extracellular acidification produced by SRIF was markedly inhibited by pretreatment of cells with pertussis toxin (100 ng ml(-1)) indicating the involvement of pertussis 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) receptors nor did it inhibit forskolin stimulated increases in cyclic AMP levels in either cell type. 8 The results from the present study describe the operational characteristics of human sst(2) receptors expressed in Ltk(-) cells where receptor activation causes increases in extracellular acidification rates. This receptor is coupled to a pertussis toxin-sensitive G protein. In contrast, activation of sst(1) receptors, at a similar transfection density, did not cause increases in extracellular acidification rates.