DEVELOPMENT OF A SELECTIVE AGONIST AT THE SOMATOSTATIN RECEPTOR SUBTYPE SSTR1

Somatostatin (SRIF) induces its biological actions by interact ing wit h a family of five recently cloned receptors. SRIF receptor subtype, S STR1, has high affinity for SRIF, but no ligand has been available tha t selectively binds to this receptor. des-amino acid(1,2,5)[DTryptopha n(8), ropyl-4-aminomethyl-L-phenylalanine(9)]SRIF(des-AA (1,2,5)[DTrp( 8),IAmp(9)]SRIF inhibits the binding of [(125)ITyr(11)] SRIF to the cl oned human SSTR1 with an affinity of 1.8 +/- 0.7 nM, but does not bind to the other cloned SRIF receptors. des-AA(1,5)[(125)ITyr(2),DTrp(8), IAmp(9)]SRIF bound selectively, potently and saturably to SSTR1 with a Kd of 0.5 +/- 0.1 nM and a maximal binding density of 226 +/- 56 fmol /mg of protein. The binding of des-AA(1,5)[(125)ITyr(2),IAmp(9)]SRIF t o SSTR1 was potently inhibited by SRIF, [DTrp(8)]SRIF, des-AA(1,2,5)[D Trp(8),IAmp(9),DSer(13)]SRIF and SRIF 28 with K-i values of 0.7 +/- 0. 3, 0.2 +/- 0.2, 4.3 +/- 0.7 and 0.6 +/- 0.1 nM, respectively. SRIF ana logs that selectively bind to SSTR2 and SSTR5 were impotent in displac ing des-AA(1,5)[(125)ITyr(2),DTrp(8),IAmp(9)]SRIF from human SSTR1, de s-AA(1,5)[(125)ITyr(2),DTrp(8),IAmp(9)]SRIF binding to SSTR1 expressed in COS-7 cells was reduced by GTPgS, and this effect was prevented by pertussis toxin treatment. In contrast, the binding of [(125)ITyr(11) ] SRIF to SSTR1 was not affected by these treatments. These findings i ndicate that des-AA(1,5)[(125)ITyr(2),DTrp(8),IAmp(9)]SRIF may bind to SSTR1 in a different manner than SRIF. des-AA(1,2,5)[DTrp(8),IAmp(9)] SRIF and its tyrosine analog are the first ligands that selectively bi nd to SSTR1 with high affinity and should be useful in localizing and determining the functional properties of this receptor.