On the presence of opioid receptors in guinea-pig ventricular tissue

Mechanisms of QT prolongation by buprenorphine cannot be explained by direct hERG channel block

Buprenorphine is a μ-opioid receptor (MOR) partial agonist used to manage pain and addiction. QT_C prolongation that crosses the 10 msec threshold of regulatory concern was observed at a supratherapeutic dose in two thorough QT studies for the transdermal buprenorphine product BUTRANS^®. Because QT_C prolongation can be associated with Torsades de Pointes (TdP), a rare but potentially fatal ventricular arrhythmia, these results have led to further investigation of the electrophysiological effects of buprenorphine. Drug-induced QT_C prolongation and TdP are most commonly caused by acute inhibition of hERG current (I_hERG) that contribute to the repolarizing phase of the ventricular action potentials (APs). Concomitant inhibition of inward late Na⁺ (I_NaL) and/or L-type Ca²⁺ (I_CaL) current can offer some protection against proarrhythmia. Therefore, we characterized the effects of buprenorphine and its major metabolite norbuprenorphine on cardiac hERG, Ca²⁺, and Na⁺ ion channels, as well as cardiac APs. For comparison, methadone, a MOR agonist associated with QT_C prolongation and high TdP risk, and naltrexone and naloxone, two opioid receptor antagonists, were also studied. Whole cell recordings were performed at 37°C on cells stably expressing hERG, Ca_V1.2, and Na_V1.5 proteins. Microelectrode array (MEA) recordings were made on human induced pluripotent stem cell-derived cardiomyocytes (iPSC-CMs). The results showed that buprenorphine, norbuprenorphine, naltrexone, and naloxone had no effect on I_hERG, I_CaL, I_NaL, and peak Na⁺ current (I_NaP) at clinically relevant concentrations. In contrast, methadone inhibited I_hERG, I_CaL, and I_NaL. Experiments on iPSC-CMs showed a lack of effect for buprenorphine, norbuprenorphine, naltrexone, and naloxone, and delayed repolarization for methadone at clinically relevant concentrations. The mechanism of QT_C prolongation is opioid moiety-specific. This remains undefined for buprenorphine, while for methadone it involves direct hERG channel block. There is no evidence that buprenorphine use is associated with TdP. Whether this lack of TdP risk can be generalized to other drugs with QT_C prolongation not mediated by acute hERG channel block warrants further study.

Opioid receptor blockade prevents propofol-induced hypotension in rats

BACKGROUND

Propofol is an intravenous anesthetic that is widely used to anesthetize patients during neurosurgical procedures. Although propofol is considered to be an essential component of contemporary management of acute brain injury in the operating room and in critical care settings, propofol-induced hypotension (PIH) remains a frequent and undesirable side effect. After 3 decades of clinical use, multiple proposed causes of PIH, and conflicting experimental results, the mechanism of PIH is still a puzzle for neuroscience and anesthesiology. This study evaluated the role of opioid receptors in PIH.

METHODS

Pentobarbital-anesthetized rats were subjected to systemic or central pretreatment with naloxone followed by intravenous or central administration of propofol.

RESULTS

In the absence of pretreatment with naloxone, intravenous (7.5 mg/kg) and intracistenal propofol (10 µg) injection induced 45% and 35% reductions in the mean arterial pressure, respectively (P<0.05). Both systemic (5 mg/kg) and central (100 µg) pretreatment with naloxone prevented PIH without independently affecting mean arterial pressure.

CONCLUSIONS

This experiment in anesthetized rats indicates that central and peripheral opioid receptor blockade prevents PIH, suggesting that these receptors are involved in the cardiovascular alterations elicited by propofol administration.

The diverse molecular mechanisms responsible for the actions of opioids on the cardiovascular system

The actions of opioid agonist and antagonist drugs have not been well characterized in the heart and cardiovascular system. This stems from the limited role opioid receptors have been perceived to have in the regulation of the cardiovascular system. Instead, the focus of opioid receptor research, for many years, relates to the characterization of the actions of opioid drugs in analgesia associated with receptor activation in the CNS. However, recent studies suggest that opioid receptors have a role in the heart and cardiovascular system. While some of these actions may be mediated by activation of peripheral opioid receptors, others are not, and may result from direct or receptor-independent actions on cardiac tissue and the peripheral vascular system. This review will outline some of the diverse molecular mechanisms that may be responsible for the cardiovascular actions of opioids, and will characterize the role opioid receptors have in several cardiovascular pathophysiological disease states, including hypertension, heart failure, and ischaemic arrhythmogenesis. In many instances, it would appear that the effects of opioid agonists (and antagonists) in cardiovascular disease models may be mediated by opioid receptor-independent actions of these drugs.

Modulation of sympathetic actions on the heart by opioid receptor stimulation

The sympathetic nervous system, the most important extrinsic regulatory mechanism of the heart, is inhibited postsynaptically and presynaptically by opioid peptides produced in the heart via their respective receptors. The cardiac actions of beta-adrenergic receptor (beta-AR) stimulation are attenuated by activation of the opioid receptor (OR) with OR agonist at ineffective concentrations, implying cross-talk between the OR and beta-AR. This cross-talk results from inhibition of the Gs protein and adenylyl cyclase of the beta-AR pathway by the pertussis toxin-sensitive G protein of the opioid pathway. Alterations in cross-talk between these two receptors occur in pathological situations to meet bodily needs. In myocardial ischemia, when the sympathetic activity is increased, the inhibition of beta-AR stimulation by kappa-opioid stimulation is also enhanced, thus reducing the workload, oxygen consumption and cardiac injury. Whereas cardiac responsiveness to sympathetic discharges is also reduced after chronic hypoxia, the cross-talk between kappa-OR and beta-AR is reduced to prevent undue suppression of the sympathetic influence on the heart. On the other hand, impairment of the cross-talk may result in abnormality. A lack or a significant reduction in the inhibition of beta-AR stimulation by kappa-OR stimulation may lead to an excessive increase in cardiac activities, which contribute to the maintenance of high arterial blood pressure in spontaneously hypertensive rats. Other than opioid peptides, female sex hormone and adenosine also inhibit the sympathetic actions on the heart. In addition, sympathetic action is also inhibited presynaptically by kappa-opioid peptides via their receptor.

Cardiac opioids

Opioid peptides have long been considered as neuropeptides or neurotransmitters. The more recent discovery of these same peptides in non-neuronal tissue suggests that the peptides may have autocrine, paracrine, or endocrine functions as well. The opioid peptides, enkephalins, dynorphins, and endorphins, have been found in isolated cardiac myocytes and heart tissue. This review will cover the recent literature on opioid peptides in respect to cardiac distribution, biochemistry, and function.

A peripheral site of action for the attenuation of baroflex-mediated bradycardia by intravenous mu-opioid agonists

We previously reported that i.v. DAMGO (Tyr-D-Ala-Gly-NMePhe-Gly-ol), a selective mu-opioid agonist, causes an increase in blood pressure with no change in heart rate in unanesthetized sheep and subsequently demonstrated that DAMGO attenuates baroreflex-mediated bradycardia. To determine the site and mechanism by which mu-agonists inhibit baroreflex sensitivity, we have carried out further investigations by using DAMGO and another mu-agonist, DALDA (Tyr-D-Arg-Phe-Lys-NH2). The bradycardic response to norepinephrine (NE) was significantly blunted after i.v. DAMGO or DALDA in both nonpregnant and pregnant sheep. In contrast, the tachycardic response to sodium nitroprusside (SNP) remained unchanged in the presence of DAMGO or DALDA. In view of the highly restricted distribution of DALDA across the blood-brain barrier (BBB), we hypothesized that the blunting of reflex-mediated bradycardia by mu-opioid agonists can occur peripherally. Pretreatment with the quaternary opioid antagonist, naloxone methiodide (NM), completely blocked the attenuation of baroreflex sensitivity by DAMGO and DALDA in both nonpregnant and pregnant animals. These data suggest that in addition to central mechanisms, mu-opioid agonists can inhibit baroreflex sensitivity at a peripheral site, most likely by inhibiting vagal influence on heart-rate control rather than by acting directly at baroreceptors.

Baroreflex-mediated bradycardia is blunted by intravenous mu- but not kappa-opioid agonists

To assess the cardiovascular effects of systemically administered opioid agonists, changes in blood pressure and heart rate were observed after intravenous (i.v.) administration of U50,488H (trans-3,4-dichloro-N-[2-(1-pyrrolidinyl) cyclohexyl]benzeneacetamide), a selective kappa-opioid receptor agonist, and DAMGO (D-Ala2, N-Me-Phe4, Gly5-ol), a selective mu-opioid-receptor agonist. Intravenous administration of U50,488H (1.2 mg/kg) and DAMGO (0.3 mg/kg) to the awake sheep resulted in an immediate increase in blood pressure. The pressor response to U50,488H was accompanied by an increase in heart rate. In contrast, there was no accompanying change in heart rate in response to DAMGO. We hypothesized that the lack of a reflex bradycardia to the pressor responses of both the mu- and kappa-opioid-receptor agonists was due to a blunting of baroreflex-mediated bradycardia. The reflex bradycardia to norepinephrine (0.6 microg/kg/min) was significantly reduced in the presence of DAMGO but not U50,488H. In view of the lack of effect of U50,488H on the baroreflex, we further hypothesized that the tachycardia it elicited was due to an increase in sympathetic activity. Pretreatment with propranolol (0.1 mg/kg) completely blocked the tachycardia elicited by U50,488H. These data suggest that the lack of a reflex bradycardia to the pressor response of DAMGO is due to a blunting of baroreflex-mediated bradycardia. In contrast, the increase in heart rate caused by U50,488H is mediated by sympathetic activation of the heart.

Effects of mu-, delta- and kappa-opioid antagonists in atrial preparations from nonfailing and failing human hearts

1. We examined the effects of naloxone (preferentially mu-antagonist), naltrindole (selective delta-antagonist) or nor-binaltorphimine (nor-BNI, selective kappa-antagonist) on auricular myocardium tissue from nonfailing and failing human hearts. 2. The opioid antagonists used in this study induced inhibitory effects in auricular strips from failing and nonfailing human hearts. In addition, the maximal effect, the IC50, and the slope of concentration-response curves obtained with mu-, delta-, and kappa-opioid antagonists were similar in failing and nonfailing human heart tissues. 3. The kappa-antagonist was more effective than naltrindole or naloxone. Moreover, the IC50 for nor-BNI (0.25 +/- 0.01 x 10(-5) M) was lower than the IC50 for naloxone (26.5 +/- 5.0 x 10(-5) M) and naltrindole (13.8 +/- 2.0 x 10(-5) M). Similar results were obtained in auricular strips from failing human hearts. 4. Our results demonstrate that the failing heart does not modify the inhibitory cardiac effects obtained with selective opioid antagonists.

Effects of mu-, delta- and kappa-agonists on isolated right atria of the rat

The present study examined the effects of morphine, DAMGO, DPDPE and U-50, 488H on auricular rate on isolated right atria of the rat. All the opioid agonists tested induced a decrease of auricular rate. The maximal effect obtained with U-50,488H (75 +/- 8.3%) was significantly (p < 0.001) higher than that obtained with morphine (12 +/- 2.7%), DAMGO (8 +/- 0.6%) or DPDPE (11 +/- 1.8%). The inhibitory effects of U-50,488H were not antagonized by the presence of naloxone (10(-7) or 5 x 10(-7) M) or MR-2266 (10(-7) or 5 x 10 (-7) M). Moreover, U-50,488H did not change the auricular chronotropism in the presence of atropine (5 x 10(-7) M). In this case the maximal inhibitory effect was 79 +/- 6.7%, similar to that obtained with the kappa-agonist alone (75 +/- 8.3%). Propranolol (10(-8) or 5 x 10(-8) M) modified the inhibitory effect of U-50,488H. The maximal effect obtained by the kappa-agonist in presence of propranolol was 100 +/- 0 significantly (p < 0.01) higher than that obtained with U-50,488H alone. These results demonstrated that the depressant action of U-50,488H was not blocked in the presence of opioid receptor antagonists and probably does not involve opioid receptors. Furthermore, propranolol caused a dose-dependent potentiation of the effects of the kappa-agonist supporting the conclusion that it is not mediated by opioid receptors.

Interaction of U-50,488H and noradrenergic systems on isolated right atria

1. The present study examined the effect of U-50,488H on auricular rate on isolated right atria of the rat. 2. The negative chronotropic action induced by the kappa-agonist (U-50,488H) was potentiated by propranolol (10(-8) or 5 x 10(-8) M) or yohimbine (5 x 10(-7) or 10(-6) M) and in reserpinized rats (5 mg/kg i.p. 24 h before the experiments). 3. These results suggest that catecholaminergic mechanisms are involved in the cardiac depressant effect induced by U-50,488H.

Possible opioid receptor function changes in isolated atria of the spontaneously hypertensive rat

1. A comparison of the effects of various opioid peptides on the heart rates of self-paced right atria was made, as taken from spontaneously hypertensive (SHR), Wistar Kyoto (WKY) and Sprague-Dawley (SD) rats at 4, 8, 12 and 16 weeks of age. 2. Beta-endorphin, dynorphin, met-enkephalin, DAGO and DADLE slightly decreased the spontaneously beating rate of all rat strains and ages, at 0.1 microM. Leu-enkephalin at 0.2 microM increased the spontaneous beating rate of SHR atria, but not that of atria from normotensive strains. 3. SHR atria were somewhat more sensitive than WKY atria to norepinephrine (NE)-induced positive chronotropy, but the differences were not statistically significant. 4. In the presence of mu, delta or kappa opioid receptor agonists, SHR atrial sensitivity to NE-induced chronotropy was enhanced at all ages studied. By contrast, NE chronotropy was not significantly altered by the opioids in normotensive rat atria. 5. Based on the above results, all the three major opioid receptor subtypes (mu, delta and kappa) appear to be present in rat atria but the function of these receptors appears to be greater in SHR than in WKY and SD atria. 6. The results suggest a possible involvement of altered opioid responsiveness in atria during hypertension development in SHR but the nature of this involvement appears to be complex and is not readily understandable on the basis of the present study.

Silent ischemia: a hypothetical mechanism

Ischemia of visceral organs, especially the heart, is often a painful and potentially life-threatening condition. However, in at least 75% of all cases myocardial ischemia may be "silent" (i.e., without pain or sensation). Yet, the mechanisms responsible for silent ischemia are not well understood. As such, many different theories have been advanced to explain silent ischemia; however, none have been able to adequately explain all of the experimental and clinical findings. This paper proposes a hypothetical mechanism that may help to understand mechanisms of silent ischemia.

Effects of morphine on isolated right atria of the rat

1. The present study describes the effects of morphine in the absence or presence of naloxone or atropine in the isolated right atria of the rat. 2. Morphine significantly decreased the auricular chronotropism. 3. The maximal effect was 10 +/- 1.0%. 4. Similar results were obtained in reserpinized animals (13 +/- 0.2% maximum). 5. Naloxone (5 x 10(-7) or 5 x 10(-6) M) did not change the inhibitory effects induced by morphine. 6. The maximal effect obtained with morphine in the presence of atropine (5 x 10(-7) M) was 9 +/- 0.1% similar to that obtained with morphine alone. 7. These results suggest that opioid or vagal mechanisms may not be involved in the cardiac inhibitory effects induced by morphine.

Kappa and delta opioid receptor stimulation affects cardiac myocyte function and Ca2+ release from an intracellular pool in myocytes and neurons

We investigated the effects of mu, delta, and kappa opioid receptor stimulation on the contractile properties and cytosolic Ca2+ (Cai) of adult rat left ventricular myocytes. Cells were field-stimulated at 1 Hz in 1.5 mM bathing Ca2+ at 23 degrees C. The mu-agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (10(-5) M) had no effect on the twitch. The delta-agonists methionine enkephalin and leucine enkephalin (10(-10) to 10(-6) M) and the kappa-agonist (trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclo-hexyl]- benzeneacetamide)methanesulfonate hydrate (U-50,488H; 10(-7) to 2 x 10(-5) M) had a concentration-dependent negative inotropic action. The sustained decrease in twitch amplitude due to U-50,488H was preceded by a transient increase in contraction. The effects of delta- and kappa-receptor stimulation were antagonized by naloxone and (-)-N-(3-furyl-methyl)-alpha-normetazocine methanesulfonate, respectively. In myocytes loaded with the Ca2+ probe indo-1, the effects of leucine enkephalin (10(-8) M) and U-50,488H (10(-5) M) on the twitch were associated with similar directional changes in the Cai transient. Myofilament responsiveness to Ca2+ was assessed by the relation between twitch amplitude and systolic indo-1 transient. Leucine enkephalin (10(-8) M) had no effect, whereas U-50,488H (10(-5) M) increased myofilament responsiveness to Ca2+. We subsequently tested the hypothesis that delta and kappa opioid receptor stimulation may cause sarcoplasmic reticulum Ca2+ depletion. The sarcoplasmic reticulum Ca2+ content in myocytes and in a caffeine-sensitive intracellular Ca2+ store in neurons was probed in the absence of electrical stimulation via the rapid addition of a high concentration of caffeine from a patch pipette above the cell. U-50,488H and leucine enkephalin slowly increased Cai or caused Cai oscillations and eventually abolished the caffeine-triggered Cai transient. These effects occurred in both myocytes and neuroblastoma-2a cells. In cardiac myocyte suspensions U-50,488H and leucine enkephalin both caused a rapid and sustained increase in inositol 1,4,5-trisphosphate. Thus, delta and kappa but not mu opioids have a negative inotropic action due to a decreased Cai transient. The decreased twitch amplitude due to kappa-receptor stimulation is preceded by a transient increase in contractility, and it occurs despite an enhanced myofilament responsiveness to Ca2+. The effects of delta and kappa opioids appear coupled to phosphatidylinositol turnover and, at least in part, may be due to sarcoplasmic reticulum Ca2+ depletion.(ABSTRACT TRUNCATED AT 400 WORDS)

Evidence for involvement of catecholamines in the effect of morphine on ventricular automaticity in the rat

1. The present study examined the effects of morphine on ectopic automaticity induced by local injury in the isolated right ventricle of the rat. 2. Morphine (10(-7)-5 x 10(-5) M) induced a significant increase of ventricular rate similar to that produced by noradrenaline. The excitatory effect of morphine was not modified by the presence of naloxone (5 x 10(-5) M). The maximal effect obtained with morphine in the presence of naloxone was 60 +/- 7%, similar to that obtained with morphine alone (67 +/- 15%). The EC50 values for morphine in the absence (0.89 x 10(-7) M) and presence of naloxone (0.87 x 10(-7) M) were also similar. Apparently this effect is not mediated by postsynaptic opioid receptors. 3. The ventricular automaticity induced in isolated right ventricle of the rat was significantly decreased by the highest concentrations of naloxone (5 x 10(-5) and 10(-7) M). 4. Morphine (10(-9)-5 x 10(-5) M) did not significantly change ventricular automaticity in the presence of propranolol (5 x 10(-8) M) or in reserpinized rats (5 mg kg-1 i.p. 24 h before the experiments). The maximal increases induced by morphine in the presence of propranolol or in reserpinized rats were 5 +/- 0.8% and 16 +/- 14.7% respectively. These results were significantly different from the maximal increase obtained without propranolol or with non-reserpinized animals. It is possible that the effects of morphine on ventricular automaticity could be mediated by an indirect effect located presynaptically at the adrenergic nerve terminals through the release of catecholamines.

Interaction between morphine and noradrenaline on isolated heart muscle

1. We evaluated the interaction between morphine and noradrenaline on the right atria. 2. Noradrenaline significantly increased atrial contraction rate, with an effective concentration 16 (EC16) of 85 +/- 0.8 x 10(-9) M. 3. The EC16 of noradrenaline significantly decreased in the presence of morphine. 4. These results demonstrate that morphine increases the potency of noradrenaline on the isolated right atria.

Different effects of prostaglandins on adrenergic neurotransmission in atrial and ventricular preparations

1. The effects of prostaglandin E2 (PGE2) and iloprost on the cardiac response to adrenergic nerve stimulation in guinea-pig atrial and ventricular preparations have been studied. 2. In guinea-pig isolated atria both PGE2 (0.1-10 nM) and iloprost (0.1-3 microM) concentration-dependently reduced the cardiac response to adrenergic nerve stimulation. 3. The inhibition of cyclo-oxygenase by indomethacin and acetylsalicylic acid potentiated the response to nerve stimulation in the atrial preparations. 4. Arachidonic acid (1-10 microM) reduced the response to nerve stimulation in atria. This effect was prevented by indomethacin and acetylsalicylic acid. 5. In guinea-pig ventricles PGE2 and iloprost were found to be effective at higher concentrations than in atrial preparations: arachidonic acid, indomethacin or acetylsalicylic acid did not modify the cardiac response to adrenergic nerve stimulation. 6. These results suggest a different modulator role for endogenous prostaglandins in atrial and ventricular tissue.

Effects of opioid drugs on capsaicin-sensitive neurones in guinea-pig atria

Transmural nerve stimulation of isolated guinea-pig atria in the presence of atropine induced a biphasic positive inotropic effect but only a slow increase in contractility (NANC response) in atria obtained from 6-hydroxy-dopamine-pretreated animals. The latter effect disappeared after exposure of the preparations to capsaicin. The effects of some opioid peptides were investigated on NANC responses. [D-Ala2,D-Leu5]enkephalin (DADLE) and [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO, 0.1-10 microM) inhibited the cardiac response to transmural nerve stimulation in a dose-dependent and naloxone-sensitive manner. Dynorphin-(1-13) and morphine, at 10-fold higher concentrations (1-10 microM), reduced the response in a naloxone-sensitive manner. Naloxone alone however did not affect the response. Opioid peptides were not able to reduce the positive inotropic effect induced by calcitonin gene-related peptide (CGRP), or the increase in cardiac contractility produced by capsaicin. These results suggest that opioid receptors exert a modulatory role on peripheral terminals of capsaicin-sensitive sensory nerves.

Failure of opioids to affect excitation and contraction in isolated ventricular heart muscle

The opioid agonists morphine (selective for mu-receptors) and ethylketocyclazocine (selective for kappa-receptors), at concentrations evoking strong effects in neuronal structures, did not significantly affect the configuration of the intracellularly recorded action potential and the force of contraction in ventricular heart muscle isolated from guinea pigs, rabbits and man. These results suggest that any changes of heart functions in vivo in response to opioid-like drugs are probably not mediated postsynaptically at the myocardial cell membrane but rather presynaptically, influencing the release of noradrenaline and/or acetylcholine from the nerve terminals.

Indirect evidence for a role of prostaglandins as second messengers of the prejunctional effect of opioids in guinea-pig ventricular preparations

The cardiac response to adrenergic nerve stimulation was dose dependently reduced in a statistically significant manner by 1-10 microM dynorphin-(1-13) in isolated atria, and by 0.1-1 microM dynorphin-(1-13) in guinea-pig ventricular preparations. The inhibitory effect of dynorphin was maintained in atria that had been pretreated with two cyclooxygenase inhibitors at concentrations that induce an 80% inhibition of the enzyme, namely indomethacin 3 microM and acetylsalicylic acid 200 microM. The inhibitory effect of dynorphin disappeared in similarly pretreated ventricular preparations. These results suggest that, whilst the mediation of the effect of dynorphin is carried out mainly by specific opioid receptors in the atrial section, in the ventricular tissue it occurs through the endogenous prostanoid system.

Increase in the relative abundance of preproenkephalin A messenger RNA in the ventricles of cardiomyopathic hamsters

Preproenkephalin A messenger RNA was detected in hamster heart by Northern blot analysis using a human preproenkephalin A cDNA probe. Ventricular levels of this messenger were one order of magnitude lower than atrial levels, which were equivalent to brain levels. Furthermore, in the heart of cardiomyopathic hamsters, an animal model of cardiac hypertrophy and congestive heart failure, the relative abundance of the preproenkephalin A messenger RNA was found to increase three- to four-fold in ventricles while no change was seen in atria. These results support the hypothesis that the heart has the potential for locally synthesizing enkephalins and provide evidence that alterations in preproenkephalin A messenger RNA levels are associated with the development of cardiac hypertrophy and failure.

Development of tolerance to effects of morphine on cardiac sympathetic response

1. In isolated guinea-pig atria, morphine potentiated the response to sympathetic stimulation in a naloxone-insensitive, but calcium-sensitive way. The potentiating effect of morphine disappeared in the presence of desmethylimipramine; moreover morphine enhanced the dose-effect curve for exogenous noradrenaline. 2. Morphine was ineffective in guinea-pig atria obtained from animals implanted with morphine pellets for 4 days; on the other hand DADLE and dynorphin-(1-13) maintained their usual inhibitory effect on cardiac sympathetic response in the same kind of preparation. 3. In atria obtained from morphine-tolerant animals and maintained in vitro in the presence of morphine, removal of morphine from the bathing solution caused a new potentiating effect.