enkephalin--ala(2)-mephe(4)-gly(5)- and 16-methylcyprenorphine

The antiarrhythmic activities of 16-methylcyprenorphine (M8008), nor-binaltorphimine (NBT) and naltrexone, which are relatively specific opioid receptor antagonists for delta, kappa and mu receptors, respectively, were examined during the 30 min following coronary artery occlusion in anaesthetised rats. The haemodynamic and electrocardiographic effects of the opioid receptor agonists [D-Ala2,D-Leu5]enkephalin (DADLE) (relatively selective for delta receptors), ICI-204448 (kappa) and glyol (mu) were also investigated over the 30-90 min post ligation period. When administered intravenously 5 min before ligation, M8008 (0.5 mg kg-1 and 2.5 mg kg-1) reduced the number of ventricular ectopic beats but had no effect on the incidence or duration of ventricular fibrillation. NBT and naltrexone were not antiarrhythmic at a dose of 0.5 mg kg-1 but at 2.5 mg kg-1 (a concentration at which both drugs block kappa receptors) the number of ventricular ectopic beats, the incidence of ventricular fibrillation and mortality were all reduced. All of the opioid receptor agonists caused a transient decrease in heart rate and in arterial blood pressure but none exhibited an arrhythmogenic effect. These studies suggest that the delta and kappa opioid receptor antagonists used may be antiarrhythmic as a result of blockade of the action of endogenously released peptides acting on these receptors or that they have a non-specific 'direct' antiarrhythmic action.

Topics: Animals; Arrhythmias, Cardiac; Coronary Disease; Coronary Vessels; Electrophysiology; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalin, Leucine-2-Alanine; Enkephalins; Hemodynamics; Male; Morphinans; Naltrexone; Narcotic Antagonists; Pyrrolidines; Rats; Rats, Inbred Strains; Receptors, Opioid; Receptors, Opioid, delta; Receptors, Opioid, kappa; Receptors, Opioid, mu

1. The irreversible opioid receptor antagonist beta-chlornaltrexamine (beta-CNA) has been shown previously to have agonist activity in the guinea-pig ileum preparation. However, the receptor type or types mediating this effect have not been established. 2. In this study, the agonism of beta-CNA was investigated by use of the competitive antagonist 16-methylcyprenorphine (RX8008M). Non-cumulative concentration-effect curves for beta-CNA were displaced in a non-parallel fashion indicating that the agonism was mediated by both mu- and kappa-receptors. 3. In principle, expression of agonism by an irreversible receptor antagonist could compromise its use in estimating agonist dissociation constants (pKAs) due to desensitization operating in addition to receptor inactivation. For kappa-receptors, this possibility was checked by use of ethylketocyclazocine (EKC) to mimic the agonist effects of beta-CNA and test whether subsequent EKC concentration-effect curves were displaced. For mu-receptors it was necessary to perform more involved experiments in which [D-Ala2, MePhe4, Gly-ol5]enkephalin (DAGOL) was used as a standard agonist and its pKA was estimated under different conditions of beta-CNA incubation. 4. These analyses indicated that neither the mu- nor the kappa-receptor-mediated agonism of beta-CNA was associated with appreciable receptor desensitization. In turn it was concluded that the usefulness of beta-CNA as a pharmacological tool for the estimation of mu- and kappa-opioid receptor agonist dissociation constants is not compromised by the agonist effects that the compound demonstrates at these receptors.

Topics: Animals; Cyclazocine; Drug Interactions; Electric Stimulation; Enkephalin, Ala(2)-MePhe(4)-Gly(5)-; Enkephalins; Ethylketocyclazocine; Guinea Pigs; Ileum; In Vitro Techniques; Male; Morphinans; Muscle, Smooth; Naltrexone; Narcotic Antagonists; Receptors, Opioid; Receptors, Opioid, kappa; Receptors, Opioid, mu