Differential agonist activity of somatostatin and L-362855 at human recombinant sst4 receptors

1 The operational characteristics of somatostatin (SRIF) sst(4) receptors are poorly understood. In this study, we have characterized human recombinant sst4 receptors expressed in CHO cells (CHOsst(4)) by radioligand binding and microphysiometry. 2 Increasing concentrations SRIF or other SRIF receptor ligands inhibited specific [I-125]-Tyr(11)-SRIF binding in CHOsst(4) cell membranes with respective PIC50 values of SRIF (8.82), L-362855 (7.40), BIM-23027 (< 5.5) and MK-678 (< 5.5). 3 These ligands displayed agonist activity, producing concentration-dependent increases in rates of extracellular acidification (EAR) with pEC(50) values of SRIF (9.6) and L-362855 (8.0), respectively. BIM-23027 and MK-678 were at least 1000 times weaker than SRIF. The SRIF maximum was about 40% of that observed with L-362855. 4 In the presence of SRIF (0.1 - 1 nM), concentration-effect curves to L-362855 were displaced to the right with a progressive reduction in the L-362855 maximum. 5 When cells were only exposed to a single maximally effective concentration of SRIF or L-362855, there was no difference in the magnitude of the agonist-induced increase in EAR. However, a second agonist challenge, 30 min later showed that responses to SRIF but not L-362855 were markedly desensitized. 6 When concentration-effect curves to SRIF and L-362855 were obtained by combining data from cells exposed to only a single agonist concentration, SRIF (pEC(50) 9.2) was approximately 20 times more potent than L-362855 (pEC(50) 8.0) but the maxima were the same. Responses to both SRIF and L-362855 were abolished by pertussis toxin. 7 SRIF and L-362855-induced increases in EAR were inhibited by N-ethyl isopropyl amiloride (10 mu M) but were not modified by inhibitors of PKC (Go-6976), MAP kinase (PD-98059), tyrosine kinase (genistein) or tyrosine phosphatase (sodium orthovanadate). 8 The results suggest that SRIF-induced increases in EAR in CHOsst(4) cells involved activation of the Na+/H+ antiporter and were mediated via Gi/Go G proteins. Responses to SRIF, but not L-362855, were subject to marked desensitization which may be a consequence of differential activation of receptor-effector coupling pathways.