Specificity of the opioid affinity reagent, DALECK, in a set of isolated tissue assay systems

The C-terminal chloromethyl ketone derivative of D-Ala2-Leu5-enkephalin (DALECK) has previously been shown to act as an affinity reagent at opioid receptors. The specificity of this derivative in its reversible interaction with functional opioid receptors has been examined here in a set of four field-stimulated isolated tissue preparations; the mouse, rat and rabbit vas deferens and the guinea pig ileum. Agonist potencies relative to selective reference agonists and Schild analysis were used to elucidate the overall activity of DALECK when interacting reversibly with opiate receptors under normal physiological conditions in the isolated tissue preparations. Under these conditions the ligand shows a very strong mu-selectivity. Data obtained in the guinea pig ileum suggest that DALECK is more potent than Tyr-D-Ala-Gly-N(CH3)Phe-Gly-ol (DAGO) when acting through mu-receptors. In contrast in the mouse vas deferens DALECK is at least 70-fold less potent than the delta-ligand D-Thr2-Leu5-enkephalin-Thr (DTLET). DALECK shows little interaction with kappa-receptors.