[125I][Tyr3]octreotide labels human somatostatin sst2 and sst5 receptors

Human somatostatin (somatotropin release inhibiting factor = SRIF) receptor subtypes sst(2) and sst(5) were stably expressed in Chinese hamster 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 pharmacological profile of sst(2) sites established in CCL39 cells using SRIF and various peptide analogues was very similar to that described previously in CHO cells and in human cortex: SRIF14 = SRIF28 greater than or equal to seglitide > BIM 23014 = RC160 > octreotide > CGP 23996 greater than or equal to L362,855 > BIM 23052 > L361,301 = cortistatin(14) > BIM 23030 > BIM 23056 > cycloantagonist SA, However, peptides classically perceived as sst, receptor selective (e.g., seglitide, octreotide, vapreotide) showed also high affinity for human sst, receptors labelled with [I-125[Tyr(3)]octreotide: SRIF28 > seglitide > SRIF14 > L361,301 = octreotide > cortistatin(14) = BIM 23014 = BIM 23052 > L362,855 = RC160 > CGP 23996 > BIM 23056 > cycloantagonist SA > BIM 23030, Further 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 fmol/mg). 5'-Guanylyl-imidodiphosphate (GppNHp) displaced all three radioligands to the same extent (85%) and the pharmacological profiles were superimposable. By contrast, at sst(5) receptors B-max values were very different: [I-125][Tpr(3)]octreotide (920 fmol/g), [I-125]CGP 23996 (3530 fmol/mg) and [I-125]LTT-SRIF28 (6950 fmol/mg). GppNHp affected [I-125][Tyr(3)]octreotide more than [I-125]CGP 23996 binding, whereas [I-125]LTT-SRIF28 was much less affected, in addition, the affinity values determined in competition experiments at sst(5) receptors, varied 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 123-fold lower affinity when sst, sites were labelled with [I-125]LTT-SRIF28. The present data suggest caution to be used when comparing affinity profiles determined in binding studies using different radioligands. 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 either sst(2) and/or sst(5) receptors. (C) 1998 Elsevier Science B.V. All rights reserved.