Leucine enkephalin antagonizes norepinephrine-induced 45Ca++ accumulation in rat atria

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.

Lateral hypothalamic area neurotransmission and neuromodulation of the specific cardiac effects of insular cortex stimulation

Microstimulation of the rat posterior insular cortex in phase with the ECG R wave elicits pure cardiac effects unaccompanied by changes in blood pressure or respiration. This technique has successfully demonstrated cardiac chronotropic organisation and arrhythmogenesis within the insula. Pathways exist linking the insular cortex with the lateral hypothalamic area (LHA). Similarly, the LHA has previously been shown to mediate the sympathetic and blood pressure effects of insular cortex stimulation. Therefore it was anticipated that the tachycardia elicited by insular phasic microstimulation would be responsive to LHA manipulations. Insular tachycardia sites in 28 chloralose-anesthetised male Wistar rats were phasically stimulated once with each cardiac cycle using 500 microA for 1 min before and after microinfusions (390 nl) into the LHA. The insular tachycardia response was abolished by LHA microinfusions of the synaptic blocker cobaltous chloride (4 mM). LHA microinjection of kynurenic acid (250 mM) attenuated insular tachycardia by 95%. Microinjection of naloxone (30 mM) similarly attenuated the tachycardia by 95%. Met-enkephalin (3.5 mM) was without effect on this response whereas Leu-enkephalin (3.5 mM) and neuropeptide Y (0.01 mM) (NPY) doubled the magnitude of the tachycardia. Dynorphin (0.12 mM), a specific kappa opioid receptor agonist, augmented the response to stimulation of insular tachycardia sites 8-fold. Consequently, it is suggested that the LHA contains an obligatory synapse mediating insular tachycardia and that glutamate is the likely neurotransmitter at this site. Neuromodulation of insular tachycardia may be effected by opiate kappa and NPY receptors, a finding of considerable clinical relevance.

Cardiovascular responses to kappa opioid agonists in intact and adrenal demedullated rats

The effects of three kappa opioid agonists namely, bremazocine, tifluadom and U-50,488H were studied on blood pressure and heart rate in urethane-anesthetized normal and bilateral adrenal demedullated rats. Bremazocine (0.2, 0.4 and 0.6 mg/kg i.v.) produced a dose-dependent decrease in heart rate, while only 0.4 mg/kg bremazocine produced marked hypotension. The effect appeared to be long lasting because even at 60 min following drug administration the decreases in both heart rate and blood pressure continued. Bilateral adrenal demedullation did not change bremazocine-induced fall in blood pressure but the bradycardia was partially blocked. Tifluadom (0.1-0.4 mg/kg i.v.) produced an initial arrest of heart beat followed by bradycardia which recovered in about 60 min. Except for a very transient fall soon after drug administration no significant effect was observed on blood pressure. In adrenal demedullated rats, tifluadom induced initial arrest of heart was not affected but the subsequent bradycardia was blocked. U-50,488H (0.2, 0.4 and 0.6 mg/kg i.v.) produced dose-dependent bradycardia and hypotension both of which were blocked following bilateral adrenal demedullation. Naltrexone methylbromide (MRZ 2663 BR), a quaternary opioid antagonist, injected 5 min prior to U-50,488H, blocked its cardiovascular effects. The results suggest that kappa opioid agonists given i.v. depress cardiovascular system and these effects are mediated through the adrenal medulla and peripheral opioid receptors. The differential effects of kappa opioid agonists on blood pressure and heart rate suggest that either the three kappa agents interact differentially at the kappa opioid receptors or the subtypes of receptors for the kappa opioid exist.

Proenkephalin-A derived peptides do not modulate cardiovascular effects of epinephrine on the isolated rat atrial preparations

The catecholamines and the opioid peptides are found to be co-localized in the adrenomedullary chromaffin cells. They are co-secreted from the chromaffin granules in response to various stimuli. The stress-induced released of epinephrine is known to exert its effect on the cardiovascular system resulting in the changes in heart rate and blood pressure. However, the role of the co-released proenkephalin-A derived peptides has not been extensively characterized. Previous work from several investigators suggested that the peptides modulate cardiac functions of the catecholamines. There is considerable conflicting results among these reports. Results from the isolated rat atrial preparation indicated that enkephalins attenuated the increase in atrial rate induced by norepinephrine through restriction of the calcium fluxes. Nonetheless, others reported insensitivity of the enkephalins in similar or different test systems. We further re-examined these discrepancies using the isolated rat atrial preparation to investigate the opioid peptide modulatory effect on the cardiovascular changes induced by exogenous epinephrine. Alterations in rate and force of contraction resulting from epinephrine and the peptides were both studied in parallel. The opioid peptides used in this study were [Met5]-enkephalin (ME), [Leu5]-enkephalin (LE), FMRFamide, [Met5]-enkephalyl-Arg6-Phe7 (MEAP), peptide E, and the non-selective opioid agonist, etorphine. We report here that none of the opioid peptides were effective in alleviating or attenuating the increase in heart rate and developed tension caused by epinephrine. The peptides did not affect the basal beating rate nor the force of contraction. Thus, the present results clearly demonstrate the insensitivity of the enkephalins in modulating the cardiac effects of epinephrine. They further indirectly support the prejunctional synaptic nerve endings as the potential peripheral site of action of the peptides.

Inhibitory action of opioid peptides on ouabain-sensitive Na+-K+ and Ca2+-dependent ATPase activities in bovine cardiac sarcolemma

The present study demonstrates that morphine (10(-6) and 10(-5) M), methionine-enkephalin or leucine-enkephalin (10(-10), 10(-8), and 10(-6) M) were able to inhibit significantly, in a dose-dependent manner, both the sarcolemmal Ca2+-dependent ATPase and the ouabain-sensitive Na+-K+ ATPase activities. The inhibitory action of these opioids on the two ATPases was not antagonized by preincubation with naloxone (10(-6) M). Naloxone alone (10(-8), 10(-6) and 10(-5) M) did not affect both the sarcolemmal Ca2+-dependent ATPase and the ouabain-sensitive Na+-K+ ATPase activities. Heat-denatured methionine-enkephalin (10(-6) M) or leucine-enkephalin (10(-6) M) also unaffected both the ATPases. The possibility is also discussed that opioid peptides may regulate myocardial contractility by modulating the movement of ions across the heart sarcolemma.

Effects of naloxone on the mechanical activity of isolated rat hearts perfused with morphine or opioid peptides

In isolated rat hearts, the infusion for 10 min of 10(-10), 10(-8) or 10(-6) M (-)naloxone affected the cardiac function by markedly increasing the coronary pressure and by reducing both the heart rate and the developed tension. A lower dose of (-)naloxone (10(-11) M) or a dose of 10(-6) M (+)naloxone, did not modify the cardiac function. Morphine (10(-6) or 10(-5) M) and 10(-10), 10(-8) or 10(-6) M methionine-enkephalin or leucine-enkephalin, both significantly reduced the coronary pressure of the isolated rat hearts, during the first 4-6 min of perfusion, but the coronary pressure progressively increased above the control value in the last 4 min of perfusion. Each opioid also influenced the mechanical activity of the isolated rat heart, by significantly lowering both the heart rate and the developed tension. (-)Naloxone, at all the doses tested, was only able to antagonise the hypotensive effect induced by the opioids on the coronary pressure and was ineffective in counteracting the negative inotropic and chronotropic effects produced by each opioid. The perfusion in the presence of (+)naloxone (even at a dose of 10(-6) M) did not affect the opioid-induced changes on both the coronary pressure and the mechanical performance of the isolated heart.

The effect of mu, delta, kappa and epsilon opioid receptor agonists on heart rate and blood pressure of the pithed rat

Opioid peptides and opioid receptors are found in the hearts of various species. Opioid peptides were also shown to modulate norepinephrine inducing changes in atrial rate, in vitro. Since we have recently shown a predominance of kappa and delta receptors in the rat atria, we found it of interest to study the role of highly selective opioid agonists on spontaneous and sympathetically stimulated heart rate. The pithed, artificially ventillated rat was used in these studies. D-Ala2-D-Leu5-enkephalin (DADL), was used as an delta-agonist, D-Ala2-MePhe4-Gly-ol5-enkephalin (DAGO) as a highly selective mu-agonist; Dynorphin A (1-17) as a kappa-agonist and beta-endorphin (beta-END) as a mixed epsilon-delta-mu agonist. Naloxone was used as an opiate antagonist. None of the above opioid peptides changed the basal blood pressure and heart rate at 1-100 nmol/kg except Dyn A-(1-17) which produced a brief depressor response (-15 +/- 2 mmHg, p less than 0.01). Stimulation of the spinal cord (50 v, 1 msec, 1 Hz, 30 sec) produced consistant pressor and cardiac accelerating responses. None of the opioid peptides studied blocked or enhanced the increase in blood pressure or heart rate produced by spinal cord stimulation. The depressor effect of the high dose of Dyn A-(1-17) was not blocked by naloxone. These results suggest that mu, delta or kappa opioid receptors in the rat heart have no role in the regulation of basal or sympathetically driven heart rate. Our data also suggest no role for these opioid receptors in modulation of basal arterial tone or norepinephrine-induced arteriolar constriction.

Reduced mechanical activity of perfused rat heart following morphine or enkephalin peptides administration

In the isolated and perfused rat heart, the addition of morphine, methionine-enkephalin or leucine-enkephalin to the coronary perfusate, significantly reduces the mechanical activity by negatively affecting both the heart rate and the developed tension. These effects are dose dependent and maximally evident with leucine-enkephalin. Furthermore all the opioids strongly reduce the activity of isoproterenol-stimulated hearts. The suggestion is made that opioid peptides directly influence the cardiac mechanical activity possibly by interacting with membrane-receptor systems.

Central and peripheral sites for cardiovascular actions of dynorphin-(1-13) in rats

Dynorphin-(1-13) (i.v.) induced bradycardia and hypotension in artificially ventilated anaesthetized rats. These effects were prevented by MRZ 2266 BS. The bradycardia was inhibited by bilateral vagotomy whereas the fall in blood pressure was not sensitive to bilateral vagotomy and chlorpheniramine treatment. In pithed rats, dynorphin-(1-13) reduced heart rate. This bradycardia was prevented by MRZ 2266 BS but not by tertatolol and cimetidine. It is suggested that the dynorphin-(1-13)-induced effects result from the stimulation of central and cardiac kappa-opiate receptors.

Proenkephalin- and prodynorphin- derived opioid peptides in guinea-pig heart

Consecutive high performance liquid chromatography (HPLC) fractionation and the mouse vas deferens assay were used to characterize opioid peptides in the guinea-pig heart. Atria were found to contain at least nine different opioid peptides derived from proenkephalin and prodynorphin. In ventricles at least seven different molecular species which may be derived only from prodynorphin were present. The total opioid activity in atria averaged 22 pmol and in ventricles 11 pmol [Met]enkephalin equivalents per g wet wt. The high content of [Leu]enkephalin in relation to [Met]enkephalin indicates the possibility of a dynorphinergic pathway of cardiac [Leu]enkephalin. Multiple cardiac opioid ligands and receptors, including kappa, may be functionally important in the peripheral control of cardiac performance and coronary circulation.

[Leu5]enkephalin inhibits norepinephrine-induced contraction of rat aorta

Leucine-enkephalin produces a dose related decrease in the contractile response of spirally cut strips of rat aorta to norepinephrine (NE). The maximal effect (55.0 +/- 1.0 percent reduction in developed tension) occurs at a leucine-enkephalin concentration of 10(-10) M. The attenuation of contractile response is reversed by naloxone.

Leucine-enkephalin increases norepinephrine-stimulated chronotropy and 45Ca++ uptake in guinea-pig atria

Norepinephrine (NE) (10(-5) M) increases the beating rate of isolated, spontaneously beating guinea-pig atria 78 +/- 3 beats per minute. Leucine-enkephalin (10(-7) M) increases the maximal chronotropic response to NE by 38%, i.e., by 30 beats per minute. In the presence of 10(-7) M leucine-enkephalin and 10(-7) M naloxone, the chronotropic response to NE is reduced to 38 +/- 3 beats per minute, a value observed in the presence of naloxone alone. Neither naloxone nor leucine enkephalin significantly altered the inotropic response to NE, or significantly altered the basal beating rate. Parallel effects were observed when 45Ca++ uptake by atrial tissue was examined. Incubation of atrial slices with 10(-5) M NE for 10 minutes minimally stimulated 45Ca++ uptake from 1.27 +/- 0.04 to 1.45 +/- 0.17 nmol/mg tissue. In the presence of 10(-7) M leucine-enkephalin, 45Ca++ uptake was increased to 1.95 +/- 0.14 nmol/mg tissue. 45Ca++ uptake was reduced to control values (1.19 +/- 0.09 nmol/mg tissue) in the presence of NE, leucine-enkephalin and naloxone (10(-7) M). The data are consistent with a leucine-enkephalin augmentation of NE-induced chronotropy in guinea-pig atria due to an enhancement of NE-dependent Ca++ accumulation. This effect of leucine-enkephalin is opposite that previously reported for rat atria, in which leucine-enkephalin inhibits both NE-induced positive chronotropy and Ca++ influx.