Effects of selective opioid receptor agonists and antagonists during myocardial ischaemia

Long-acting opioids and cardiovascular diseases: Help or hindrance!

Opioids are widely being used for chronic pain management, cough and diarrhea suppressants, anesthetic agents, and opioid de-addiction therapy. Opioid receptors, present in the central nervous system and peripheral tissues, are documented to regulate several cardiac functions through different signaling pathways. Long-acting opioids (LAO) have been successfully evaluated for their beneficial effects in various cardiovascular diseases viz. myocardial infarction, ischemic reperfusion injuries, atherosclerosis etc. However, on the other hand, several research studies pointed towards the harmful effects of LAOs which are mainly associated with QTc prolongation, torsade de pointes, ventricular arrhythmias, and cardiac arrest. This review shall familiarize readers with the benefits as well as the harmful effects of long-acting opioids in cardiovascular diseases. We have also provided an overview of cardiac opioid receptors, endogenous cardiac opioid peptides, and regulation of cardiovascular functions by central and cardiac opioid receptors.

Evidence for a dynorphin-mediated inner ear immune/inflammatory response and glutamate-induced neural excitotoxicity: an updated analysis

Acoustic overstimulation (AOS) is defined as the stressful overexposure to high-intensity sounds. AOS is a precipitating factor that leads to a glutamate (GLU)-induced Type I auditory neural excitotoxicity and an activation of an immune/inflammatory/oxidative stress response within the inner ear, often resulting in cochlear hearing loss. The dendrites of the Type I auditory neural neurons that innervate the inner hair cells (IHCs), and respond to the IHC release of the excitatory neurotransmitter GLU, are themselves directly innervated by the dynorphin (DYN)-bearing axon terminals of the descending brain stem lateral olivocochlear (LOC) system. DYNs are known to increase GLU availability, potentiate GLU excitotoxicity, and induce superoxide production. DYNs also increase the production of proinflammatory cytokines by modulating immune/inflammatory signal transduction pathways. Evidence is provided supporting the possibility that the GLU-mediated Type I auditory neural dendritic swelling, inflammation, excitotoxicity, and cochlear hearing loss that follow AOS may be part of a brain stem-activated, DYN-mediated cascade of inflammatory events subsequent to a LOC release of DYNs into the cochlea. In support of a DYN-mediated cascade of events are established investigations linking DYNs to the immune/inflammatory/excitotoxic response in other neural systems.

Activation of peripheral delta opioid receptors increases cardiac tolerance to arrhythmogenic effect of ischemia/reperfusion

OBJECTIVES

The objective of this study was to investigate the role of peripheral μ, δ1, δ2, and nociceptin opioid receptors agonists in the regulation of cardiac tolerance to the arrhythmogenic effect of ischemia/reperfusion in rats.

METHODS

Anesthetized open-chest male Wistar rats were subjected to either 45 minutes of left coronary artery occlusion (phase 1a 10 minutes and phase 2b 35 minutes) and 2 hours of reperfusion in Experiment 1 or 10 minutes of ischemia and 10 minutes of reperfusion in Experiment 2. In Experiment 1, saline or vehicle controls and the mu-specific opioids dermorphin-H (Derm-H) and ([d-Ala2, N-Me-Phe4, Gly-ol5] enkephalin (DAMAGO); the delta-1-specific opioid d-Pen2,5enkephalin (DPDPE); nociceptin; and the delta-2-specific opioids deltorphin-II (Delt-II), Delt-Dvariant (Delt-Dvar), and deltorphin-E (Delt-E) were infused 15 minutes prior to ischemia. In Experiment 2, DPDPE, Delt-D, Delt-Dvar, and Delt-E were infused at 15 minutes prior to ischemia. The universal opioid receptor antagonist naltrexone, the peripherally acting antagonist naloxone methiodide, the selective δ1 antagonist 7-benzylidene naltrexone maleate, and the specific δ2 antagonist naltriben mesylate were infused 25 minutes prior to ischemia.

RESULTS

In Experiment 1, pretreatment with the μ opioids Derm-H and DAMGO, DPDPE, and nociceptin at all doses tested did not reduce the incidence of ischemia-induced arrhythmias compared to controls during 45 minutes of ischemia. The δ2 opioids Delt-II (0.12 mg/kg), Delt-Dvar (0.3 mg/kg), and Delt-E (0.18 mg/kg) all demonstrated significant antiarrhythmic effects at the 150 nmol/kg dose compared to saline or vehicle controls. Nine of 19 animals treated with Delt-II were tolerant without ventricular arrhythmias to the arrhythmogenic effect of ischemia during the first 10 minutes of ischemia (phase 1a) and 11 of 19 were without ventricular arrhythmias during the following 35 minutes of ischemia (phase 1b). Delt-II also decreased the incidence of premature ventricular contractions and ventricular tachycardia by almost half during phase 1a. Delt-II did not affect the incidence of ventricular fibrillation (VF). Pretreatment with Delt-Dvar and Delt-E completely blocked the incidence of VF in phase 1b. Delt-E also decreased premature ventricular contractions by 50%, and the incidence of ventricular tachycardia decreased over twofold in phase 1b of ischemia. There was no enhanced tolerance by any of the delta-2 opioids to the arrhythmogenic effect of reperfusion after long-term ischemia. In Experiment 2, after 10 minutes of ischemia and 10 minutes of reperfusion, Delt-II (0.12 mg/kg) reduced the incidence of premature ventricular contractions and ventricular tachycardia compared to controls, and completely blocked the incidence of VF following 10 minutes of reperfusion. Delt-Dvar and Delt-E were without effect, as was DPDPE following 10 minutes of reperfusion. The antiarrhythmic effect of Delt-II during 10 minutes of ischemia and 10 minutes of reperfusion was completely blocked by the peripherally acting opioid receptor inhibitor naloxone methiodide and the selective delta-2 opioid receptor inhibitor naltriben mesylate, but not by the selective delta-1 inhibitor 7-benzylidene naltrexone maleate. The antagonists alone had no effect on arrhythmogenesis.

CONCLUSIONS

Peripheral delta-2 opioid receptor activation by Delt-II, Delt-Dvar, and Delt-E enhanced cardiac tolerance to the arrhythmogenic effects of ischemia.

The Effects of Opioid Peptides on Cardiovascular Function and Sympathetic Neurotransmission in Rats

The haemodynamic effects of three opioid receptor agonists, with some preferential activity on δ-, μ- and κ-receptors were investigated in anaesthetized and pithed rats, and effects on sympathetic neurotransmission were also investigated in pithed rats.

In anaesthetized rats, d-Ala2-d-Leu5-enkephalin (DADLE) (a predominantly δ-receptor agonist, 10 μg kg−1), glyol (μ, 0·5 mg kg−1) and R,S-N-C2-N-methyl-3,4,dichloro-phenylacetamido-2-C3-carboxy phenylethyl-pyrrolidine (ICI 204448) (·, 0·1 mg kg−1) by intravenous administration transiently decreased heart rate from 462 ± 12 to 432 ± 14, 460 ± 12 to 448 ± 13 and 460 ± 12 to 448 ± 11 beats min−1, respectively, and mean arterial blood pressure from 142 ± 6 to 111 ± 9, 141 ± 6 to 122 ± 5 and 148 ± 7 to 121 ± 6 mmHg, respectively. The effects of DADLE, but not those of glyol or ICI 204448, were blocked by M8008, a δ-receptor antagonist. In pithed rats, none of the opioid agonists had any significant effects on heart rate or mean arterial blood pressure; however, acetylcholine significantly reduced both heart rate and mean arterial blood pressure. All three opioid agonists reduced the positive chronotropic response to thoracic (C7-T2) spinal cord stimulation in pithed rats, by 17 ± 4,30 ± 2 and 20 ± 10% for DADLE, glyol and ICI 204448, respectively. This compared with a 48 ± 15% reduction with clonidine (5 μ kg−1). This effect of DADLE was almost abolished by M8008.

It is concluded that the haemodynamic effects of the opioid agonists studied are mediated via actions on the central nervous system and that a decrease in sympathetic neurotransmission may account for, at least in part, the bradycardia produced by opioid agonists in intact anaesthetized rats. It seems that the sympathetic nervous system is unlikely to be involved in the arrhythmogenic effects of opioid peptides.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Kappa opioid antagonists: past successes and future prospects

Antagonists of the kappa opioid receptor were initially investigated as pharmacological tools that would reverse the effects of kappa opioid receptor agonists. In the years following the discovery of the first selective kappa opioid antagonists, much information about their chemistry and pharmacology has been elicited and their potential therapeutic uses have been investigated. The review presents the current chemistry, ligand-based structure activity relationships, and pharmacology of the known nonpeptidic selective kappa opioid receptor antagonists. This manuscript endeavors to provide the reader with a useful reference of the investigations made to define the structure-activity relationships and pharmacology of selective kappa opioid receptor antagonists and their potential uses as pharmacological tools and as therapeutic agents in the treatment of disease states.

Effect of exogenous kappa-opioid receptor activation in rat model of myocardial infarction

The involvement of opioid receptor activation during ischemia-reperfusion is somewhat controversial. While it is generally accepted that activation of the delta-opioid receptor (DOR) is cardioprotective, and may indeed be an important mediator of ischemic preconditioning, the role of the kappa-opioid receptor (KOR) is less well understood. To this end, we examined three different KOR agonists and their effects upon infarct size and arrhythmia development. Male Sprague-Dawley rats were subjected to 30 minutes of occlusion followed by 90 minutes of reperfusion. Opioid receptor agonists were administered 10 minutes before the onset of ischemia, while the opioid antagonists were given 20 minutes before occlusion. Untreated rats exhibited an infarct size (IS/AAR%) of 52.4 +/- 2.7%. Pretreatment with the DOR agonist, BW373U86, limited infarct development to 37.2 +/- 1.8%, which was reversed by the selective DOR antagonist, BNTX. All three KOR agonists studied, U50,488, ICI 204,448, and BRL 52537 significantly reduced infarct size to levels comparable to that of BW373U86. The infarct-sparing effects of U50,488 and ICI 204,448 were abolished by the selective KOR antagonist, nor-BNI. Nor-BNI failed to inhibit the cardioprotective effects of BRL 52537. Furthermore, U50,488 and BRL 52537, but not ICI 204,448, significantly reduced the incidence of arrhythmias. These effects were not blocked by nor-BNI. These data demonstrate that KOR activation provides a similar degree of infarct size reduction as DOR activation. KOR agonists also reduced arrhythmogenesis; however, these responses appear to be independent of KOR activation.

Modulation of emesis by fentanyl and opioid receptor antagonists in Suncus murinus (house musk shrew)

The anti-emetic mechanism of action of fentanyl to inhibit nicotine (5 mg/kg, s.c.)-induced emesis was investigated in Suncus murinus. The anti-emetic action of fentanyl (40 microg/kg, s.c.) was antagonised by the opioid receptor antagonists naltrexone (1 mg/kg, s.c.), naloxone (1 mg/kg, s.c.), M8008 (16S-methylcyprenorphine; 1 mg/kg, s.c.) and MR 2266 (5,9-diethyl-2-(3-furylmethyl)2'-hydroxy-7,7-benzomorphan; 1 mg/kg) but not by naloxone methylbromide (1 mg/kg, s.c.), naloxone methyliodide (1 mg/kg, s.c.), naltrindole (1 mg/kg, s.c.), DIPPA (2-(3,4-dichlorophenyl)-N-methyl-N-[1S)-1-(3-isothiocyanatophenyl)-2-(1- pyrrolidinyl)-ethyl]acetamide; 3 mg/kg, i.p.) or naloxonazine (35 mg/kg, i.p.). This indicates an involvement of mu2-opioid receptors within the brain to mediate the anti-emetic effect of fentanyl. In other studies, naloxone 10-60 mg/kg, s.c. induced dose-related emesis but naltrexone was only emetic at 60 mg/kg, s.c. and naloxone methylbromide failed to induce emesis at doses up to 60 mg/kg, s.c. The emesis induced by a high dose of naloxone 60 mg/kg, s.c. was antagonized by CP-99,994 ((+)-(2S,3S)-3-(2-methoxybenzylamino)-2-phenylpiperidine; 3-30 mg/kg, i.p.), 8-OH-DPAT, ((+/-)-8-hydroxy-dipropylaminotetralin; 0.003-0.3 mg/kg, s.c.), buspirone (3 mg/kg, s.c.) and fluphenazine (1-3 mg/kg, i.p.) but not by naltrexone (1-30 mg/kg, s.c.), metoclopramide (0.3-3 mg/kg, i.p.), sulpiride (0.3-3 mg/kg, i.p.), domperidone (0.1-3 mg/kg, i.p.), ondansetron (0.3-3 mg/kg, i.p.), granisetron (0.3-3 mg/kg, i.p.), scopolamine (0.3-3 mg/kg, i.p.) or promethazine (0.3-3 mg/kg, i.p.). The data is discussed in relation to opioid receptor mechanisms moderating emesis and the identification of potential sites of drug action available to inhibit the emetic reflex.

Opioid antagonist modulation of ischaemia-induced ventricular arrhythmias: a peripheral mechanism

Ventricular arrhythmias are an important cause of death after myocardial ischaemia. Animal studies have generated conflicting data on the potentiating or attenuating effects of opioid agonists and antagonists on cardiac rhythm during acute myocardial ischaemia and coronary artery reperfusion. Whether these effects of opioid antagonists are mediated by central or peripheral nervous system mechanisms remains unclear. We examined (a) the effects of peripheral opioid receptor blockade on ischaemia-induced arrhythmia by using methylnaltrexone (MNTX), a novel quaternary derivative of naltrexone (NTX) that does not cross the blood-brain barrier, and (b) whether MNTX would modulate morphine effects during acute coronary artery ligation and reperfusion in the rabbit. The incidence and severity of cardiac arrhythmias were assessed during 40 min of coronary artery occlusion and reperfusion and summarised in an arrhythmia score (AS). MNTX reduced the incidence of ventricular fibrillation (VF) and arrhythmia score during coronary artery occlusion when compared with vehicle (p < 0.05). Naltrexone reduced the incidence of VF (p < 0.05). Although morphine alone had no significant effects, its coadministration blunted the antiarrhythmic properties of MNTX. The data suggest that blockade of opiate receptors outside the central nervous system may protect against ischaemia-induced arrhythmias.

Suppression of cAMP by phosphoinositol/Ca2+ pathway in the cardiac kappa-opioid receptor

To determine whether the phosphoinositol/Ca2+ pathway interacts with the adenylate cyclase/adenosine 3',5'-cyclic monophosphate (cAMP) pathway in the cardiac kappa-receptor, the effects of U-50488, a specific kappa-receptor agonist, on the intracellular Ca2+ concentration ([Ca2+]i) and forskolin-induced accumulation of cAMP in rat ventricular myocytes were determined after interference of the phosphoinositol/Ca2+ pathway. U-50488 suppressed the forskolin-induced accumulation of cAMP and elevated [Ca2+]i, which were blocked by norbinaltorphimine, a specific kappa-receptor antagonist, and pertussis toxin. The effects of U-50488 were qualitatively similar to those of A-23187, a Ca2+ ionophore, but opposite to those of 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA)-acetoxymethyl ester (AM), a [Ca2+]i chelator. Abolition of U-50488-induced elevation of [Ca2+]i by BAPTA-AM also abolished the effect of U-50488 on forskolin-induced accumulation of cAMP. Inhibition of the phospholipase C by specific inhibitors, U-73122 and neomycin, abolished the effects of U-50488 on both [Ca2+]i and forskolin-induced accumulation of cAMP. The results showed for the first time that kappa-receptor stimulation may suppress cAMP accumulation via activation of the phosphoinositol/Ca2+ pathway in the rat heart.

Increased formation of interstitial hydroxyl radical following myocardial ischemia: possible relationship to endogenous opioid peptides

The effects of myocardial ischemia and reperfusion on interstitial hydroxyl radical production, in the left ventricular myocardium of anesthetized cats, were investigated. Ringer's solution containing salicylic acid was perfused through an implanted microdialysis probe. Hydroxyl radical production was evaluated as the 2,3 and 2,5 dihydroxybenzoic acid (DHBA) concentrations in the microdialysates by an on-line high performance liquid chromatography system. Myocardial ischemia for 60 min, induced by ligation of the left anterior descending coronary artery, significantly increased both 2,3 and 2,5 DHBA levels when compared with the sham-operated cats. Naloxone (1 mg/kg, bolus, intravenous), an endogenous opioid peptide receptor antagonist, significantly suppressed the ischemia-induced production of hydroxyl radicals. Myocardial ischemia also induced cardiac arrhythmia. Naloxone reduced the severity of ischemia-induced arrhythmia, as observed by a significantly lower arrhythmia score (1.4 +/- 0.2 vs. 4.6 +/- 0.4 for control), and by diminished incidence of ventricular tachycardia (0/7 vs. 8/8 for control) and ventricular fibrillation (0/7 vs. 3/8 for control). Furthermore, perfusion of dynorphin (0.25 microgram, 2.5 micrograms and 25 micrograms), an endogenous opioid peptide receptor agonist, increased hydroxyl radical production. Our results suggest that, in anesthetized cats, myocardial ischemia can induce production of interstitial hydroxyl radical in left ventricular myocardium, and this production may involve the actions of released endogenous opioid peptides on their receptors.

Inhibition of [3H]-U69593 binding and the cardiac effects of U50, 488H by calcium channel blockers in the rat heart

1. The calcium channel blockers (CCBs), nifedipine, nicardipine, diltiazem and verapamil, were used to displace the binding of [3H]-U69593 ((5a, 7a,8b)-(+)-N-methyl-N-(7-[1-pyrrolidinyl]-1-oxaspiro[4,5] dec-8-yl)-benzeneacetamide), a specific kappa-opioid agonist, in the rat cardiac sarcolemma. The CCBs competed with the binding of [3H]-U69593 (4 nM) in a dose-dependent manner. The displacing potency of verapamil was 55 times greater than that of nifedipine. 2. The effects of two CCBs, verapamil and nifedipine, on the arrhythmogenic action of kappa-receptor stimulation by a specific kappa-receptor agonist, U50,488H (trans-(+/-(-3),4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl] cyclohexyl) benzeacetamide methanesulphonate), were also studied in the rat isolated perfused heart. U50,488H 80-800 nmol dose-dependently induced arrhythmias, which were completely abolished by a selective kappa-receptor antagonist, nor-BNI (nor-binaltorphimine, 17,17'-(dicyclopropylmethyl)-6,6',7,7'-6,6'-imino-7,7'-binorphinan -3,4',14, 14'-tetrol), at 100 nmol. The arrhythmogenic effect was also attenuated by both verapamil and nifedipine in a dose-dependent manner. The ED50 values for verapamil and nifedipine were 2.75 and 63.7 nmol, respectively. The antiarrhythmic potencies of these two CCBs were correlated to their displacing potencies and inversely related to their well known potencies in inhibiting transmembrane Ca2+ influx in the cardiac muscle. 3. Measurement of [Ca2+]i in the absence of free extracellular Ca2+ by a spectrofluorometric method, with fura-2 as Ca2+ indicator, showed that U50,488H 5 x 10(-5) M slowly increased [Ca2+]i in single ventricular myocytes and this effect was abolished by pretreatment with nor-BNI (5 microM), or ryanodine (5 microM). Verapamil 1 and 10 microM abolished the effect of U50,488H in 37.5% (3 out of 8) and 100% (12 out of 12) of the cells studied, respectively. On the other hand, nifedipine 10 and 100 microM had no effect at all. Neither verapamil nor nifedipine exerted any significant effect on the caffeine-induced Ca2+ transient. 4. The observations suggest that CCBs may inhibit the actions of kappa-receptor stimulation at the level of the kappa-receptor.

Decreased affinity of K-receptor binding during reperfusion following ischaemic preconditioning in the rat heart

The effects of ischaemic preconditioning with three cycles of ischaemia of 3 min and reperfusion of 5 min each cycle on ventricular fibrillation threshold (VFT) and ventricular fibrillation (VF), and binding properties of tritiated U69,593, a selective kappa opioid-receptor (k-receptor) agonist, during subsequent ischaemia and/or reperfusion were studied in the rat heart. It was found that ischaemic preconditioning significantly enhanced the VFT values during ischaemic and reperfusion. VF during the subsequent reperfusion period was also significantly reduced. The Kd of the [3H]U69,593 binding sites in the sarcolemma of the heart at 5 min of reperfusion was significantly increased following ischaemic preconditioning. The Bmax was, however, not altered after the preconditioning. The study provides evidence for the first time suggesting that the cardioprotective effects of ischaemic preconditioning may be related to a reduction in affinity of the K-receptor binding.

The roles of reactive oxygen species and endogenous opioid peptides in ischemia-induced arrhythmia of isolated rat hearts

Although the formation of oxygen-derived free radicals (or reactive oxygen species; ROS) and the release of endogenous opioid peptides (EOP) have been independently reported to be the major arrhythmogenic factors in ischemic hearts, possible relations between these two factors have seldom been investigated. Thus, we studied whether the ROS and EOP were related in the progression of ischemia-induced arrhythmias. Isolated rat hearts perfused in the Langendorff mode were treated with dynorphin A1-13 (kappa EOP receptor agonist), and/or allopurinol (xanthine oxidase inhibitor), before the onset of ischemia induced by ligating the left coronary arteries. Ischemic period lasted for 30 min, during which cardiac rhythms were recorded. At the end of ischemia, hearts were analyzed for the glutathione and ascorbate levels. Allopurinol (100 nmoles/heart) was effective in reducing the severity of arrhythmia (arrhythmia score: Mean +/- SEM 3.00 +/- 0.80 for allopurinol, 5.75 +/- 0.41 for placebo, p < 0.01), while dynorphin (10 micrograms/heart) potentiated the arrhythmia (6.71 +/- 0.52, p < 0.05 vs. placebo). Coadministration of allopurinol and dynorphin was capable of reducing arrhythmia (5.57 +/- 0.65) when compared with the administration of dynorphin alone (6.71 +/- 0.52, p < 0.05). Tissue oxidative stress was evaluated by the concentrations of glutathione (GSH) and ascorbate. Allopurinol did not significantly elevate tissue GSH concentrations (1.46 +/- 0.05 mumoles/g wet wt) in ischemic hearts, while dynorphin alone significantly decreased the GSH concentrations (0.96 +/- 0.08, p < 0.05) when compared with the placebo (1.32 +/- 0.03). The dynorphin-induced GSH decrease cannot be reversed by coadministration with allopurinol (0.90 +/- 0.104). Allopurinol significantly elevated tissue ascorbate levels (0.16 +/- 0.01) when compared with placebo (0.10 +/- 0.01, p < 0.05). Interestingly, dynorphin alone also elevated the tissue ascorbate concentrations (0.16 +/- 0.02). Coadministration of allopurinol and dynorphin further spiked the ascorbate levels (0.28 +/- 0.05, p < 0.01). In conclusion, the results suggested that ischemia-induced arrhythmia mechanisms might involve the formation of superoxide and other ROS, which were probably generated from the release of EOP (or EOP/EOP receptor interactions). Superoxide, the formation of which can be inhibited by allopurinol that exerted antiarrhythmic effect, was probably scavenged by ascorbate in myocardial ischemia. The ROS resulting from EOP/EOP receptor interactions were probably scavenged by glutathione system. Elevated ascorbate levels in dynorphin-treated hearts might result from the compensatory synthesis induced by decreased glutathione levels.

Electrophysiological effects of opioid receptor selective agonists on guinea-pig papillary muscle

The cardiac electrophysiological effects of DAMGO, DPDPE and U-50,488H which are selective opioid agonists for mu, delta and kappa receptors, respectively, were studied in the isolated guinea-pig right ventricular papillary muscles. Neither DAMGO (5.10(-6)-5.10(-5)M) nor DPDPE (5.10(-6)-5.10(-5)M) produced any significant effect on the action potential characteristics. However, U.-50,488H (10(-5),5.10(-5) M) caused a concentration-dependent reduction in the maximum rate of depolarization of phase 0 (Vmax) and in the action potential duration measured at 50% repolarization, APD50 and 90% repolarization, APD90. At 5.10(-5) M it also produced a decrease of action potential amplitude (APA). These results suggest that the reported electrophysiological effects of U-50,488H on the cardiac muscle action potential, may be, at least in part, explained by a direct cardiac action.

The electrophysiological effects of opioid receptor-selective antagonists on sheep Purkinje fibres

The cardiac electrophysiological effects of 16-methylcyprenorphine (M8008), nor-binaltorphimine (NBT) and naltrexone, which are relatively specific opioid antagonists for delta, kappa and mu receptors, respectively, were studied in paced (1.5 Hz) sheep Purkinje fibres in vitro. M8008 (1 ng ml-1-10 micrograms ml-1) caused a concentration-dependent reduction in the maximum rate of depolarisation of phase 0 (MRD) and in the action potential duration measured at 50% repolarisation, APD50. Neither NBT (10 ng ml-1-10 micrograms ml-1) nor naltrexone (1 ng ml-1-10 micrograms ml-1) produced any significant effect on the cardiac action potential. In the presence of a physiological salt solution modified to mimic some of the changes that occur during myocardial ischaemia (i.e. hypoxia, acidosis, hyperkalaemia), M8008 caused a more marked reduction in MRD and prolonged rather than shortened APD50. These results suggest that the reported antiarrhythmic activity of M8008, but not NBT or naltrexone, may be, at least in part, explained by a direct cardiac electrophysiological action.