[I-125] [TYR(3)]OCTREOTIDE LABELS HUMAN SOMATOSTATIN SST(2) AND SST(5) RECEPTORS

Human somatostatin (somatotropin release inhibiting factor = SRIF) rec eptor subtypes sst(2) and sst(5) were stably expressed in Chinese hams ter lung fibroblast (CCL39) cells. [I-125][Tyr(3)]octreotide labelled with high affinity and in a saturable manner both sst(2) (pK(d) = 9.89 +/- 0.02, B-max = 210 +/- 10 fmol/mg, n = 3) and sst(5) sites (pK(d) = 9.64 +/- 0.04, B-max = 920 +/- 170 fmol/mg, n = 3). The pharmacologi cal profile of sst(2) sites established in CCL39 cells using SRIF and various peptide analogues was very similar to that described previousl y in CHO cells and in human cortex: SRIF14 = SRIF28 greater than or eq ual to seglitide > BIM 23014 = RC160 > octreotide > CGP 23996 greater than or equal to L362,855 > BIM 23052 > L361,301 = cortistatin(14) > B IM 23030 > BIM 23056 > cycloantagonist SA, However, peptides classical ly perceived as sst, receptor selective (e.g., seglitide, octreotide, vapreotide) showed also high affinity for human sst, receptors labelle d with [I-125[Tyr(3)]octreotide: SRIF28 > seglitide > SRIF14 > L361,30 1 = octreotide > cortistatin(14) = BIM 23014 = BIM 23052 > L362,855 = RC160 > CGP 23996 > BIM 23056 > cycloantagonist SA > BIM 23030, Furthe r radioligand binding studies were performed with [Leu(8),D-Trp(22),I- 125-Tyr(25)]SRIF28 ([I-125]LTT-SRIF28) acid [I-125]CGP 23996, At sst(2 ) receptors, Bmax values determined with [I-125][Tyr(3)]octreotide, [I -125]LTT-SRIF28 and [I-125]CGP 23996 were in the same range (180-370 f mol/mg). 5'-Guanylyl-imidodiphosphate (GppNHp) displaced all three rad ioligands to the same extent (85%) and the pharmacological profiles we re superimposable. By contrast, at sst(5) receptors B-max values were very different: [I-125][Tpr(3)]octreotide (920 fmol/g), [I-125]CGP 239 96 (3530 fmol/mg) and [I-125]LTT-SRIF28 (6950 fmol/mg). GppNHp affecte d [I-125][Tyr(3)]octreotide more than [I-125]CGP 23996 binding, wherea s [I-125]LTT-SRIF28 was much less affected, in addition, the affinity values determined in competition experiments at sst(5) receptors, vari ed markedly; whereas SRIF14, cortistatin(14) and SRIF28 showed 2-, 4- and 8-fold differences in affinity at sst(5) receptors labelled with [ I-125][Tyr(3)]octreotide and [I-125]LTT-SRIF28 compounds such as RC160 , L363,301, L362,855, octreotide or CGP 23996 showed between 42- and 1 23-fold lower affinity when sst, sites were labelled with [I-125]LTT-S RIF28. The present data suggest caution to be used when comparing affi nity profiles determined in binding studies using different radioligan ds. In addition, the present results suggest that effects produced by octreotide and related short chain SRIF analogues on hormone release, modulation of tumour growth and central effects may be mediated by eit her sst(2) and/or sst(5) receptors. (C) 1998 Elsevier Science B.V. All rights reserved.