DIFFERENTIAL AGONIST ACTIVITY OF SOMATOSTATIN AND L-362855 AT HUMAN RECOMBINANT SST(4) RECEPTORS

1 The operational characteristics of somatostatin (SRIF) sst(4) recept ors are poorly understood. In this study, we have characterized human recombinant sst4 receptors expressed in CHO cells (CHOsst(4)) by radio ligand 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 S RIF (8.82), L-362855 (7.40), BIM-23027 (< 5.5) and MK-678 (< 5.5). 3 T hese ligands displayed agonist activity, producing concentration-depen dent increases in rates of extracellular acidification (EAR) with pEC( 50) values of SRIF (9.6) and L-362855 (8.0), respectively. BIM-23027 a nd 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 SRI F (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 concentra tion of SRIF or L-362855, there was no difference in the magnitude of the agonist-induced increase in EAR. However, a second agonist challen ge, 30 min later showed that responses to SRIF but not L-362855 were m arkedly desensitized. 6 When concentration-effect curves to SRIF and L -362855 were obtained by combining data from cells exposed to only a s ingle agonist concentration, SRIF (pEC(50) 9.2) was approximately 20 t imes more potent than L-362855 (pEC(50) 8.0) but the maxima were the s ame. Responses to both SRIF and L-362855 were abolished by pertussis t oxin. 7 SRIF and L-362855-induced increases in EAR were inhibited by N -ethyl isopropyl amiloride (10 mu M) but were not modified by inhibito rs of PKC (Go-6976), MAP kinase (PD-98059), tyrosine kinase (genistein ) or tyrosine phosphatase (sodium orthovanadate). 8 The results sugges t that SRIF-induced increases in EAR in CHOsst(4) cells involved activ ation of the Na+/H+ antiporter and were mediated via Gi/Go G proteins. Responses to SRIF, but not L-362855, were subject to marked desensiti zation which may be a consequence of differential activation of recept or-effector coupling pathways.