[125I]Tyr10-cortistatin14 labels all five somatostatin receptors

The recently cloned rat preprocortistatin, which shows homology to the preprosomatostatin peptide, is thought to be enzymatically cleaved to cortistatin14 (CST14) similarly to somatostatin14 (SRIF14). High structural similarity of cortistatin14 compared to SRIF14 suggested binding properties to somatostatin receptors similar to SRIF14. In the present study, we expressed stably the five human somatostatin receptor subtypes (hsst1-hsst5) in CCL39 cells (Chinese hamster lung fibroblast cells). The receptors were labelled with an iodinated analogue of CST14 ([125I]Tyr10)-cortistatin14, [125I]Tyr10-CST) to establish the pharmacological profile of hsst1-hsst5 sites labelled with [125I]Tyr10-CST. In parallel, [Leu8,D-Trp22,125I-Tyr25]-SRIF28 ([125I]LTT-SRIF28) was used as a control at the five recombinant SRIF receptors stably expressed in CCL39 cells. High affinity [125I]Tyr10-CST binding could be demonstrated to all five recombinant somatostatin receptor subtypes. The pKd (-log mol/l) and Bmax values (fmol/mg) for hsst1-5 receptors were: 10.02+/-0.04, 220+/-30; 9.45+/-0.09, 340+/-70; 10.06+/-0.11, 340+/-50; 9.67+/-0.14, 340+/-110 and 10.33+/-0.03, 5630+/-1330, respectively. The pharmacological profiles determined with [125I]Tyr10-CST and [125I]LTT-SRIF28 were very similar at every receptor studied. These data suggest that cortistatin and somatostatin have similar high affinity for SRIF receptors. None of the receptors showed marked selectivity for either CST14/CST17 or the somatostatins. In conclusion, the data show that cortistatin and somatostatin have very similar high affinity to all five recombinant somatostatin receptors. It remains to be seen whether there are specific receptors which bind only somatostatins or cortistatins.