Pharmacologic evaluation of a cyclic somatotatin analog with antagonist activity at Mu opioid receptors in vitro

The cyclic somatostatin analog D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH₂ (CTP) was evaluated for agonist and opioid antagonist actions and receptor selectivity in two bioassays: electrically stimulated guinea pig isolated ileum (GPI) and mouse isolated vas deferens (MVD). CTP (100, 300, 1000 nM) produced concentration-dependent antagonism of the mu agonist [Me-PHe³,D-Pro⁴]morphiceptin (PL017) in both the GPI and MVD. Schild analysis of the interactions between CTP and PL017 indicated competitive antagonism between these peptides (Schild slope GPI -0.97 ± 0.16, Schild slope MVD -1.4 ± 0.4), and also suggested that the mu receptors in the two tissues are not different (pA₂ GPI 7.1 ± 0.17, pA₂ MVD 6.9 ± 0.16). The effects of the delta selective agonist [D-Pen²,D-Pen⁵]enkephalin in the MVD were not antagonized by CTP. Likewise, in the GPI, CTP did not antagonize the kappa agonist (trans-3-4-dichloro-N-methyl-N-(2-(1-pyrrolidinyl)cyclohexyl)benzeneacetamine (U50,488H). In comparison, naloxone antagonized both PL017 and U50,488H in the GPI, as well as [D-Pen²,D-Pen⁵]enkephalin and PL017 in the MVD. In the MVD, CTP also exerted weak somatostatin-like actions (35% maximal inhibition) that could not be demonstrated in somatostatin-tolerant tissues. It also showed inhibitory actions at very high concentrations (3000 and 10,000 nM) that were blocked by both naloxone and the delta antagonist N,N-diallyl-Tyr-AIB-AIB-Phe-Leu-OH (ICI 174,864). ICI 174,864 antagonized [D-Pen²,D-Pen⁵]enkephalin in the MVD, but did not affect PL017. These results indicate that CTP is a selective mu receptor antagonist in vitro. These findings also emphasize the importance of receptor-selective antagonists such as CTP in characterizing the receptor populations in a tissue and provide an additional approach for evaluating the receptor selectivity of putative receptor-selective agonists.