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

Endogenous Opioids and Their Role in Stem Cell Biology and Tissue Rescue

Opioids are considered the oldest drugs known by humans and have been used for sedation and pain relief for several centuries. Nowadays, endogenous opioid peptides are divided into four families: enkephalins, dynorphins, endorphins, and nociceptin/orphanin FQ. They exert their action through the opioid receptors (ORs), transmembrane proteins belonging to the super-family of G-protein-coupled receptors, and are expressed throughout the body; the receptors are the δ opioid receptor (DOR), μ opioid receptor (MOR), κ opioid receptor (KOR), and nociceptin/orphanin FQ receptor (NOP). Endogenous opioids are mainly studied in the central nervous system (CNS), but their role has been investigated in other organs, both in physiological and in pathological conditions. Here, we revise their role in stem cell (SC) biology, since these cells are a subject of great scientific interest due to their peculiar features and their involvement in cell-based therapies in regenerative medicine. In particular, we focus on endogenous opioids’ ability to modulate SC proliferation, stress response (to oxidative stress, starvation, or damage following ischemia–reperfusion), and differentiation towards different lineages, such as neurogenesis, vasculogenesis, and cardiogenesis.

Intracrine Endorphinergic Systems in Modulation of Myocardial Differentiation

A wide variety of peptides not only interact with the cell surface, but govern complex signaling from inside the cell. This has been referred to as an "intracrine" action, and the orchestrating molecules as "intracrines". Here, we review the intracrine action of dynorphin B, a bioactive end-product of the prodynorphin gene, on nuclear opioid receptors and nuclear protein kinase C signaling to stimulate the transcription of a gene program of cardiogenesis. The ability of intracrine dynorphin B to prime the transcription of its own coding gene in isolated nuclei is discussed as a feed-forward loop of gene expression amplification and synchronization. We describe the role of hyaluronan mixed esters of butyric and retinoic acids as synthetic intracrines, controlling prodynorphin gene expression, cardiogenesis, and cardiac repair. We also discuss the increase in prodynorphin gene transcription and intracellular dynorphin B afforded by electromagnetic fields in stem cells, as a mechanism of cardiogenic signaling and enhancement in the yield of stem cell-derived cardiomyocytes. We underline the possibility of using the diffusive features of physical energies to modulate intracrinergic systems without the needs of viral vector-mediated gene transfer technologies, and prompt the exploration of this hypothesis in the near future.

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.

Hypertensive state, independent of hypertrophy, exhibits an attenuated decrease in systolic function on cardiac kappa-opioid receptor stimulation

Opioids/opiates are commonly administered to alleviate pain, unload the heart, or decrease breathlessness in patients with advanced heart failure. As such, it is important to evaluate whether the myocardial opioidergic system is altered in cardiac disease. A hamster model of spontaneous hypertension was investigated before the development of hypertension (1 mo of age) and in the hypertensive state (10 mo of age) to evaluate the effect of prolonged hypertension on myocardial opioidergic activity. Plasma beta-endorphin was decreased before the development of hypertension and in the hypertensive state (P < 0.05). There was no change in cardiac beta-endorphin content at either time point. No differences were detected in cardiac or plasma dynorphin A, Met-enkephalin, or Leu-enkephalin, or in cardiac peptide expression of kappa- or delta-opioid receptors. mu-Opioid receptor was not detected in either model. To determine how hypertension affects myocardial opioid signaling, the ex vivo work-performing heart was used to assess the cardiac response to opioid administration in healthy hearts and those subjected to chronic hypertension. Agonists selective for the kappa- and delta-opioid receptors, but not mu-opioid receptors, induced a concentration-dependent decrease in cardiac function. The decrease in left ventricular systolic pressure on administration of the kappa-opioid receptor-selective agonist, U50488H, was attenuated in hearts from hamsters subjected to chronic, untreated hypertension (P < 0.05) compared with control. These results show that peripheral and myocardial opioid expression and signaling are altered in hypertension.

Naltrindole, a selective delta-opioid receptor antagonist, potentiates the lethal effects of cocaine by a central mechanism of action

The potentiation of the toxic and lethal effects of cocaine by the selective delta-opioid receptor antagonist naltrindole was explored in unrestrained, unanesthetized rats that received a continuous intravenous infusion of cocaine until death. The lethal dose of cocaine was lowered dose dependently in animals administered naltrindole intracisternally (3.0-30 microg), but not intravenously (30-300 microg). There was also a decrease in the lethal dose of cocaine following an injection of the nonselective opioid antagonist naltrexone, but not naloxone. However, the seizure-producing dose of cocaine was decreased dose dependently in rats that received naltrindole, regardless of the route of administration, naloxone, or naltrexone. In contrast, the effect of cocaine on heart rate was altered only by centrally administered naltrindole or intravenous naltrexone, with a dose of 30 microg naltrindole and 10 mg/kg naltrexone abolishing the bradycardic effect of cocaine. Despite this, neither naltrindole nor naltrexone changed the hypertensive effect of cocaine. Higher doses of naltrindole (100 microg i.c.) produced significant increases in heart rate and mean arterial pressure and were not tested in combination with cocaine. Because the lethal dose of cocaine was reduced only when naltrindole was administered intracisternally, the potentiation of the lethal effects of cocaine by naltrindole is through a central mechanism of action that may involve changes in cardiovascular function.

Stereospecific blocking effects of naloxone against hemodynamic compromise and ventricular dysfunction due to myocardial ischemia and reperfusion

Endogenous opioid peptides subserve regulatory roles in cardiovascular function and are released upon myocardial ischemia contributing to the development of ischemic arrhythmias and cardiogenic shock, which are reversed by the opioid antagonist naloxone. Since the hallmark of myocardial infarction is the impairment of hemodynamics and ventricular function, we evaluated further if blockade of opioids reverses the ischemia induced hemodynamic compromise, and if the effects are mediated by opioid receptors. Thirty-two mongrel dogs were anesthetized and artificially ventilated. Median thoracotomy was performed, the heart exposed, and the left anterior descending coronary artery isolated for subsequent occlusion and reperfusion. All cardiac parameters were recorded on an Electronics for Medicine recorder through the intracardiac catheters advanced from femoral vessels. Results indicate that naloxone significantly reversed the ischemic and reperfusion induced reduction in aortic, left ventricular and pulmonary arterial pressures, and left ventricular dp/dt. The inactive (+) stereoisomer of naloxone was without effect. These data demonstrate that opioids may have a role in the pathophysiology of myocardial infarction, mediated by opioid receptors, and provide new insight and strategies for the understanding and treatment of ischemic heart disease.

Age-dependent differences of ATP breakdown and ATP-catabolite release in ischemic and reperfused hearts

The hearts of young (6 months) and aged (24 months) rats, paced at a frequency of 300 bpm, were perfused by the Langendorff technique and subjected to: 20 min of equilibration perfusion, 30 min of global ischemia (95% reduction of the coronary flow) and 20 min of reperfusion. The control group was equilibrated for 20 min and then aerobically perfused for 50 min. After 20 min of stabilization, ATP and ADP levels and the adenine nucleotide pool were significantly higher in young than aged hearts (15% increase), but no modifications were found between the two age groups after 50 min of aerobic perfusion. Even the energy charge did not change under aerobic conditions. At the end of the ischemic period the levels of ATP and ADP decreased to a similar extent in young and aged hearts. After 20 min of reperfusion the myocardial level of ATP remained lower in comparison to the preischemic and control values in both age groups. At the end of the reperfusion there was a decrease in energy charge and creatine phosphate levels in the aged group in respect to the young group. The concentrations of adenosine, hypoxanthine and xanthine in coronary effluents did not change during ischemia and reperfusion irrespective of the age of the animals. On the contrary, the release of uric acid during ischemia and reperfusion was greater in aged than young hearts (90% increase). Moreover, the level of inosine in perfusates during the ischemic period was significantly lower in the 24-month-old group (30% decrease). These results are in accordance with the increased purine nucleoside phosphorylase activity and the decreased hypoxanthine phosphorybosyl-transferase activity found in the myocardium of the aged vs. young rats at the end of the reperfusion period. These data indicate that in the aged rat hearts, when exposed to ischemic and reperfusion conditions, there is a modification of purine breakdown which leads to a greater production of uric acid in respect to that found in young hearts.

Incorporation of [14C]hypoxanthine into cardiac adenine nucleotides: effect of aging and post-ischemic reperfusion

In order to investigate whether the 'hypoxanthine salvage' pathway of the cardiac muscle is modified with age, we aerobically perfused isolated hearts of 4-month- and 22-month-old male Wistar rats for 20 min with 0.18 microM [14C]hypoxanthine. A second group of hearts was subjected to a 30-min ischemic perfusion (95% reduction of the coronary flow), followed by 20 min of reperfusion. In this last 20 min, the perfusate contained the same concentration of [14C]hypoxanthine used under the aerobic condition. After 20 min of aerobic perfusion the myocardial levels of ATP were significantly lower (15%) in aged than young rat hearts, whilst no age-related differences were observed at the end of the reperfusion. In the young rats the incorporation of the isotope into ATP, ADP, and AMP was significantly higher (192%, 226%, and 300%, respectively), after 20 min of reperfusion with respect to the aerobic values. On the contrary, in the aged hearts, no significant change in the rate of [14C]-incorporation into ATP was observed during reperfusion, despite an increase of the [14C]-incorporation into ADP and AMP. Moreover, the content of each labeled adenine nucleotide was significantly higher in aged than young hearts at the end of the aerobic period, whereas the incorporation of the labeled hypoxanthine was not affected by age after 20 min of reperfusion. The release of uric acid into coronary effluents was greater (50%) in aged than young rats during the reperfusion period, but no age-dependent differences in the isotope incorporation into uric acid were observed. These data indicate that in the aged rat heart, perfused under aerobic conditions, there is an increased incorporation of hypoxanthine into ATP, although it does not further increase during postischemic reperfusion.

Effect of dynorphin A(1?13) on cardiomyocytes in culture: modulation of the response to increased extracellular calcium, but no effect on intrinsic cardiac contractile frequency or the response to isoproterenol or increased extracellular potassium

The purpose of this study was to determine whether the endogenous opioid peptide dynorphin A(1-13) has a direct effect on the heart or acts to modulate the cardiac chronotropic response to calcium, potassium, or beta-adrenergic receptor stimulation. Spontaneously contracting myocardial cell aggregates were prepared from 7-day-old chick embryos and were maintained in culture for 72 h before study. Dynorphin A(1-13), 10(-8) to 10(-6)M, did not alter spontaneous contractile frequency. Increases in [Ca2+]o spontaneously suppressed cardiac contractile frequency, and dynorphin A(1-13) significantly (p less than 0.05) enhanced this response. Nifedipine, 10(-8) M, antagonized the effect of increased [Ca2+]o on cardiac contractile frequency, but did not block the action of dynorphin A(1-13) to accentuate the effect of increasing [Ca2+]o. Dynorphin A(1-13) did not alter the significant (p less than 0.05) increase in contractile frequency produced by beta-adrenergic receptor stimulation by isoproterenol, or the suppression in contractile frequency produced by increases in extracellular potassium ([K+]o). These data indicate that dynorphin A(1-13) does not act directly on the cardiac myocyte to alter cardiac contractile frequency or alter the response to increases in [K+]o or to isoproterenol, but that dynorphin A(1-13) does modulate the response to increases in extracellular calcium.

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)

Nonopioid effects of beta-casomorphin-5 in guinea pig heart: alterations to the beta-adrenoceptor-G-protein complex and inhibition of myocardial responses to isoproterenol

The influence of beta-casomorphin-5 on the beta-adrenoceptor complex in guinea pig heart membranes was studied by means of binding studies, G-protein investigations and isolated heart preparations. In nanomolar concentrations beta-CM-5 induced an increase in receptor affinity towards the agonist isoproterenol whereas the antagonist affinity was reduced. The isoproterenol-stimulated increase in cardiac contractility, moreover, is reduced by 10 nM beta-CM-5. Furthermore, beta-CM-5 was found to inhibit the isoproterenol-induced GDP/GTP exchange as well as the [35S]GTP[S] binding to guinea pig heart membranes, indicating an involvement of G-proteins. These findings suggest that low concentrations of beta-CM-5 modulate the functional properties of the myocardial beta-adrenoceptor-G-protein complex, presumably resulting in its desensitization. The observed effects of beta-CM-5 are not prevented by naloxone and, therefore, are nonopioid in nature.

Morphine and (D-Ala2, NMe-Phe4, Gly-ol)-enkephalin increase the intracellular free calcium in isolated rat myocytes--effect of naloxone or pretreatment with morphine

The purpose of the present study was firstly to determine whether morphine and (D-Ala2, NMe-Phe4, Gly-ol)-enkephalin (DAGO), a highly selective mu-agonist, increased intracellular free calcium of rat myocytes and secondly to determine whether opioid receptors were involved. Two series of experiments were performed. In the first, the effect of morphine and DAGO on intracellular free calcium (Cai) of cultured isolated myocytes was studied with a spectrophotometric method using fura2-AM as the fluorescent Ca2+ indicator. In the second, the effect of morphine on Cai of isolated ventricular myocytes from rats which had received chronic daily injection of morphine for two weeks or myocytes which had been incubated in a solution with morphine for 12 hr was studied. It was found that both morphine at 100-250 microM and DAGO at 23-75 microM increased Cai dose-dependently and that the effect was significantly antagonized by naloxone at a concentration of 50 microM, which itself did not cause any significant alteration in Cai. Pretreatment with morphine also abolished the morphine-induced increase in Cai of isolated myocytes. The results suggest that morphine increases Cai by directly activating the cardiac receptors (most likely micro-receptors) on the membrane of ventricular myocytes.

Interaction between halothane and morphine on isolated heart muscle

The present study describes the effects of halothane on morphine activity in the isolated left atria of the rat. Concentration-response curves were obtained for the negative inotropic effects of morphine on electrically stimulated left atria. Morphine significantly decreased the contractile force, with an inhibitory concentration 16 (IC16) of 3.130.698 +/- 22.5 X 10(-9) M. The opiate agonist was more potent in reserpinized rats, causing a consistent negative inotropic action over a wide range (10(-8)-10(-4) M) or morphine concentrations. The IC16 of morphine was significantly (P less than 0.001) decreased in the presence of 1.5% v/v halothane. The administration of L-naloxone (3 X 10(-7)-10(-6) M) but not D-naloxone (10(-6) M) antagonized the inhibitory effects of morphine in the presence of halothane. These results demonstrate that halothane increases the potency of morphine on the isolated left atria and suggest that this effect is mediated by opioid receptors.

The neuroanatomy, neurophysiology, and neurochemistry of pain, stress, and analgesia in newborns and children

Beginning with a brief description of mature anatomic pathways and neurotransmitters in the "pain system," this article details their development in the human fetus, neonate, and child. Special emphasis is given to the basic mechanisms and physiologic effects of opioid analgesia. The clinical implications of these data are described, particularly with regard to the maintenance of cardiovascular stability and hormonal-metabolic homeostasis in newborns and children undergoing surgery or other forms of stress.

Self-administration of morphine contingent on heart rate in the rat

The tendency to associate a given response-reinforcement combination reflects the adaptive significance of the association. Biologically relevant reinforcement can be much more effective in modifying certain responses. For example, treatments that result in various types of illness readily condition aversions to novel flavors, but electric shock is relatively ineffective. While opioid self-administration contingent on lever pressing has been extensively studied, the potential for opioids to reinforce visceral responses remains to be determined. An approach to reinforcing changes in heart rate with drug infusions is described. Methods to control for unconditioned drug effects include reversing the direction of change in heart rate required for infusions and addition of a yoked control subject. In several instances, rats exposed to .1 mg/kg infusions of morphine sulfate contingent on tachycardia showed trends for elevated heart rate, with increased locomotor and grooming activity preceding infusions. Increases in heart rate were most pronounced during daytime, normally inactive periods.

Opioids stimulate sarcolemmal NAD(P)H-vanadate dehydrogenase activity

The present study demonstrates that the bovine cardiac sarcolemma possesses an NAD(P)H dehydrogenase activity which is able to oxidize both NADH and NAD(P)H in the presence of vanadate as an electron acceptor. The NADH dehydrogenase activity was significantly higher than the NAD(P)H dehydrogenase activity and both of them were almost completely inhibited by superoxide dismutase and atebrin and markedly reduced by the addition of the protonophore 2,4-dinitrophenol. The incubation of the sarcolemma in the presence of 10(-10), 10(-9), 10(-8) M methionine-enkephalin, a prevalent delta-opioid receptor agonist, or dynorphin A (1-17), a prevalent kappa-receptor agonist, produced a dose-dependent increase in the NAD(P)H dehydrogenase activity, with 10(-10) and 10(-9) M dynorphin A (1-17) more effective than the corresponding doses of methionine-enkephalin. The preincubation of the sarcolemma in the presence of superoxide-dismutase, atebrin or 2,4-dinitrophenol strongly inhibited the opioid-stimulated dehydrogenase activity. The stimulatory action elicited by 10(-8) M methionine-enkephalin or dynorphin A (1-17) was completely antagonized by 10(-8) M naloxone or Mr 1452, respectively, whilst 10(-8) M naloxone exerted only a partially antagonistic action against the effect produced by 10(-8) M dynorphin A (1-17), significantly more accentuated than the action of 10(-8) M Mr 1452 versus the same dose of methionine-enkephalin.

Distribution of neuropeptide-like immunoreactivity in intact and chronically decentralized middle cervical and stellate ganglia of dogs

Neuropeptide-like immunoreactivity to antisera raised against Leu- and Met-enkephalin, vasoactive intestinal peptide (VIP), neuropeptide Y (NPY) and substance P (SP) have been studied immunohistochemically in middle cervical and stellate ganglia of dogs. To investigate the relationship of the peptides to one another as well as to preganglionic and postganglionic neurons, intact and chronically decentralized middle cervical and stellate ganglia were studied. Ganglia were processed for immunohistochemistry in unoperated dogs and in dogs two weeks after unilateral ganglionic decentralization. The immunoreactivity for each peptide had a characteristic distribution in the ganglia. These distributions differed from one another and from the distribution of cardiac postganglionic sympathetic neurons. Camera lucida drawings of peptide distributions were made to compare different peptides and counts were made to determine the percentages of cells immunoreactive for a given peptide. The results demonstrated that enkephalin-like immunoreactivity in axons was present in both the stellate and middle cervical ganglia, but was heaviest in the caudal 2/3 of the stellate ganglia. Enkephalin-like immunoreactive fibers formed pericellular baskets around stellate ganglion neurons. VIP-like immunoreactive cell bodies and processes were distributed sparsely, but widely, in the stellate ganglia and to a lesser extent in the middle cervical ganglia. One of two commercial antisera to SP resulted in immunoreactive staining of cell bodies and processes in the stellate ganglia. SP-like immunoreactivity in neurons represented about 10% or less of the cells in the stellate ganglia. At least 80-85% of the neurons in the stellate and middle cervical ganglia were immunoreactive for NPY antisera. Decentralization eliminated enkephalin-like immunoreactive staining in the middle cervical and stellate ganglia, but not the VIP-, NPY- and SP-like immunoreactive staining of neurons in these ganglia. In summary, the enkephalin-like immunoreactive axons in the thoracic autonomic ganglia appear to be derived from extrinsic neurons, most likely from preganglionic spinal neurons. VIP-, SP- and NPY-like immunoreactivity were not significantly affected by decentralization. The results provide anatomical evidence for substrates related to neuropeptidergic synaptic mechanisms in thoracic autonomic ganglia.

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.

Method for stimulating the adrenergic system of an isolated perfused rat heart

A method is presented whereby isolated perfused rat hearts can be rapidly prepared for the stimulation of chronotropic activity by electrical pulses or exogenous noradrenaline. The mediation of the response by sympathetic nerves is demonstrated through modulation of the response by compounds with established pharmacological actions. Propranolol inhibits the increase in heart rate to both electrical stimulation and exogenous noradrenaline, whereas bretylium inhibits only electrically induced increases. Chronic pretreatment with 6-hydroxydopamine decreases the response to electrical stimulation but increases that to exogenous noradrenaline.

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.