Bradycardia In Rats Research Articles

Disorders on cardiovascular parameters in rats and in human blood cells caused by Lachesis acrochorda snake venom

In Colombia, Lachesis acrochorda causes 2–3% of all snake envenomations. The accidents promote a high mortality rate (90%) due to blood and cardiovascular complications. Here, the effects of the snake venom of L. acrochorda (SVLa) were analyzed on human blood cells and on cardiovascular parameters of rats. SVLa induced blood coagulation, as measured by the prothrombin time test, but did not reduce the cell viability of neutrophils and platelets evaluated by the 3-(4,5-dimethylthiazol-2yl)-2,5-diphenyl tetrazolium bromide (MTT) reduction assay and by the lactate dehydrogenase (LDH) enzyme assay. In fact, SVLa increased the absorbance in tests made with platelets subjected to the MTT assay. SVLa induced platelet aggregation whose magnitude was comparable to that of the positive control adenosine diphosphate (ADP), and occurred earlier with increasing SVLa concentration. Acetylsalicylic acid (ASA, a cyclooxygenase inhibitor) or clopidogrel (an ADP receptor blocker) inhibited the aggregating effect of SVLa. Inhibition of SVLa-elicited platelet aggregation also resulted from the treatment with disodium ethylenediaminetetraacetate (Na2-EDTA; metalloproteinase inhibitor) and with 4-(2-aminoethyl) benzenesulfonyl fluoride hydrochloride (AEBSF, serine protease inhibitor). In isolated right atrium of rats, SVLa increased slightly, but significantly, the magnitude of the spontaneous contractions and, in isolated rat aorta, SVLa relaxed KCl- or phenylephrine-induced contractions. In vivo, SVLa induced hypotension and bradycardia in rats, with detection of hemorrhage in pulmonary and renal tissues. Altogether, under experimental conditions, SVLa induced blood coagulation, platelet aggregation, hypotension and bradycardia. Part of the effects presented here may be explained by the presence of snake venom metalloproteinases (SVMPs) and snake venom serine proteases (SVSPs), constituents of SVLa.

Polymer-mediated delivery of vaccines to treat opioid use disorders and to reduce opioid-induced toxicity

Vaccines offer a potential strategy to treat opioid use disorders (OUD) and to reduce the incidence of opioid-related overdoses. Vaccines induce opioid-specific polyclonal antibodies that selectively and effectively bind the target opioid and prevent its distribution across the blood-brain barrier. Because antibody-mediated reduction of drug distribution to the brain reduces drug-induced behavior and toxicity, vaccine efficacy depends on the quantity and quality of the antibody response. This study tested whether polymer-mediated delivery could improve vaccine efficacy against opioids as well as eliminate the need for booster injections normally required for a successful immunization. A series of novel biodegradable biocompatible thermogelling pentablock co-polymers were used to formulate a candidate vaccine against oxycodone in mice and rats. Polymer-based delivery of the anti-oxycodone vaccine was equally or more effective than administration in aluminum adjuvant in generating oxycodone-specific antibodies and in reducing oxycodone-induced effects and oxycodone distribution to the brain in mice and rats. The composition and release kinetics of the polymer formulations determined vaccine efficacy. Specifically, a formulation consisting of three simultaneous injections of the anti-oxycodone vaccine formulated in three different polymers with slow, intermediate, and fast release kinetics was more effective than an immunization regimen consisting of three sequential injections with the vaccine adsorbed on aluminum. The novel three-phased polymer vaccine formulation was effective in blocking oxycodone-induced antinociception, respiratory depression and bradycardia in rats.

Augmented fear bradycardia in rats with heart failure.

In congestive heart failure (CHF), while resting parasympathetic activity becomes reduced, parasympathetically-mediated responses to stressors have not been described. This study aimed to (1) elucidate the effect of CHF on fear bradycardia, a parasympathetically-mediated response, and (2) examine if brain oxidative stress of CHF mediates fear bradycardia. White noise sound (WNS) exposure to conscious rats induced freezing behavior and elicited bradycardia. WNS exposure-elicited bradycardia was greater in rats with CHF than in controls. Superoxide dismutase mimetics administered in the lateral/ventrolateral midbrain periaqueductal gray (l/vlPAG), a region that contributes to the generation of fear bradycardia, had no effect on the bradycardia response in control and CHF rats. Dihydroethidium staining in situ showed that superoxide generation in the l/vlPAG of CHF rats was increased as compared to controls. These results demonstrate that CHF leads to the augmentation of fear bradycardia. Moreover, oxidative stress in the l/vlPAG of CHF unlikely mediates the augmented fear bradycardia.

Ivabradine preserves dynamic sympathetic control of heart rate despite inducing significant bradycardia in rats.

Ivabradine is a selective bradycardic agent that inhibits hyperpolarization-activated cyclic nucleotide-gated (HCN) channels. HCN channels play a key role in mediating the positive chronotropic response to sympathetic nerve stimulation (SNS). We examined whether ivabradine would interfere with dynamic sympathetic control of heart rate (HR). The effect of intravenous ivabradine (2mg/kg, n = 7) or metoprolol (10mg/kg, n = 6) on the transfer function from SNS to HR was examined in anesthetized rats. Ivabradine preserved the asymptotic dynamic gain of the HR transfer function and nearly doubled the asymptotic dynamic gain of the transfer function from SNS to the R-R interval. In contrast, metoprolol abolished dynamic sympathetic control of HR. Preserved dynamic sympathetic control of HR, with coexisting bradycardia, may contribute to some of the beneficial effects of ivabradine previously reported in clinical application.

Sphingosine-1-Phosphate (S1P) Lyase Inhibition Causes Increased Cardiac S1P Levels and Bradycardia in Rats.

Inhibition of the sphingosine-1-phosphate (S1P)-catabolizing enzyme S1P lyase (S1PL) elevates the native ligand of S1P receptors and provides an alternative mechanism for immune suppression to synthetic S1P receptor agonists. S1PL inhibition is reported to preferentially elevate S1P in lymphoid organs. Tissue selectivity could potentially differentiate S1PL inhibitors from S1P receptor agonists, the use of which also results in bradycardia, atrioventricular block, and hypertension. But it is unknown if S1PL inhibition would also modulate cardiac S1P levels or cardiovascular function. The S1PL inhibitor 6-[(2R)-4-(4-benzyl-7-chlorophthalazin-1-yl)-2-methylpiperazin-1-yl]pyridine-3-carbonitrile was used to determine the relationship in rats between drug concentration, S1P levels in select tissues, and circulating lymphocytes. Repeated oral doses of the S1PL inhibitor fully depleted circulating lymphocytes after 3 to 4 days of treatment in rats. Full lymphopenia corresponded to increased levels of S1P of 100- to 1000-fold in lymph nodes, 3-fold in blood (but with no change in plasma), and 9-fold in cardiac tissue. Repeated oral dosing of the S1PL inhibitor in telemeterized, conscious rats resulted in significant bradycardia within 48 hours of drug treatment, comparable in magnitude to the bradycardia induced by 3 mg/kg fingolimod. These results suggest that S1PL inhibition modulates cardiac function and does not provide immune suppression with an improved cardiovascular safety profile over fingolimod in rats.

Fear‐induced bradycardia is abnormal in rats with heart failure (1132.3)

We examined autonomic responses to fear in conscious rats with heart failure (HF) after myocardial infarction and central neural mechanisms for the responses. Rats exposed to 5‐min white noise sound (90 dB) displayed freezing behavior (an index of fear), renal sympathoexcitation, and bradycardia. The bradycardia was parasympathetically mediated since atropine injected intravenously prevented the response in normal rats (N5). In HF rats (N7, fractional shortening<35%), the fear‐induced bradycardia was significantly (P<0.05) larger than that in normal rats (N9). Of note, in HF rats, arrhythmia appeared in association with the reduction in heart rate seen during fear. Moreover, in normal rats (N7), microinjection into the lateral/ventrolateral periaqueductal gray (l/vlPAG) of muscimol, a GABAA receptor agonist, significantly suppressed the bradycardia response to fear. Lastly, in HF rats (N6), microinjection into the l/vlPAG of Tempol, a superoxide dismutase mimetic, significantly suppressed the bradycardia response to fear. Taken together, these results suggest that fear stimulates the l/vlPAG and parasympathetically causes bradycardia in rats, and that brain oxidative stress in HF leads to dysfunction of the l/vlPAG, thereby exaggerating parasympathetic effects on heart rate and elevating risks for cardiovascular events during fear. Supported by Shimane Pref Grant for regional medicine.

Gas Chromatographic Mass Analysis and Further Pharmacological Actions of Cymbopogon proximus Essential Oil

The present study reports Gas chromatographic mass analysis (GC-MS) as well as important biological activities of Cymbopogon proximus essential oil. The chemical composition of the essential oil of Cymbopogon proximus was investigated by GC-MS. Furthermore, the effects of Cymbopogon proximus essential oil on the cardiac parasympathetic ganglia in rats, the intra-tracheal pressure in guinea-pigs and on carrageenan-induced inflammation in the rats paw, were studied. The GC-MS study led to the identification of 22 components with Piperitone representing (73.81%), Elemol (9.32%), alpha-Eudesmol (5.21%) and alpha-Terpineol (3.01%) of the oils composition. The percentage protective effect of the oil on the vagus-induced bradycardia in rats was 90.1±3.1%, which represents a significant protection. As for the effect of Cymbopogon oil on bronchoconstrictors-induced increase in intra-tracheal pressure in guinea-pigs, the oil antagonized the actions of 5-HT and histamine by 80±3.7 and 93±8.3%, respectively. Pharmacological investigations using Cymbopogon oil revealed its inherent ability to possess a bronchodilator activity mediated via blockade of both histamine and serotonin receptors. It possessed a significant ganglionic blocking action and a limited anti-inflammatory activity that seemed to involve blockade of histamine and serotonin receptors in the rats' paws.

Control of sympathetic and phrenic nerve activity by cholinergic mechanisms in the nucleus of the solitary tract (NTS)

Cholinergic receptors are present in the NTS and acetylcholine (ACH) microinjected into the medial NTS (mNTS) induces hypotension and bradycardia in rats (da Silva et al., 2008; AJP, 295, p R1774). In the present study, we tested the changes in the sympathetic and phrenic nerve activities produced by ACH injected alone or combined with atropine (ATR, muscarinic antagonist) into the mNTS or into the commissural NTS (cNTS). Decorticated arterially‐perfused in situ preparations of male Holtzman rats (60–90 g, n=5–6) were used to record thoracic sympathetic (tSNA) and phrenic nerve (PNA) activities. ACH (10 mM in 60 nL) was injected into the mNTS or cNTS before and 10 min after ATR (17 and 67 mM in 60 nL). Saline was injected as control. ACH into the mNTS reduced the tSNA (−66.1 ± 6.0 % vs. saline: −0.6 ± 3.3 % of baseline) and PNA frequency (−39.7 ± 2.7 % vs. saline: −2.4 ± 2.6 % of baseline), a response attenuated by ATR (67 mM) into the mNTS (tSNA: −29.7 ± 5.2 % and PNA: −12.8 ± 4.6 %, p<0.05). ACH into the cNTS increased PNA frequency (33.1 ± 6.8 %, vs. saline: −6.4 ± 2.7 %), without changing tSNA. ATR (17 mM) into the cNTS reduced the ACH‐induced increase in PNA frequency (8.6 ± 6.4 %, p<0.05). These data suggest that ACH acting on muscarinic receptors in the NTS may play an important role in the control of sympathetic and respiratory activities with distinct responses depending on the NTS level.Supported by: FAPESP, CNPq.

Neuropeptide Y Reverses Chronic Stress-induced Baroreflex Hypersensitivity in Rats

Chronic stress, as a risk factor for cardiovascular diseases, has been reported to result in elevated plasma neuropeptide Y (NPY) and be highly associated with abnormal cardiac autonomic function. This study aimed to explore the effect of NPY on the chronic stress-induced abnormal baroreceptor reflex sensitivity (BRS). Seven types of recognized stressors were used to develop chronic stress rat model. Subcutaneously implanting ALZET mini-osmotic pumps containing NPY were used to evaluate the action of NPY on the stressed male rats. We found that chronic stress showed no influence on baseline systolic blood pressure (SBP) and heart rate (HR), whereas NPY (85 µg for 30 days) could elevate baseline SBP and induce bradycardia in rats intervened by various stimuli. NPY pretreatment could preserve chronic stress-induced decreases in left ventricular systolic pressure (LVSP) and the maximum rate of change in left ventricular pressure in the isovolumic contraction period (+dp/dt_max) but has shown no effect on left ventricular end diastolic pressure (LVEDP) and the maximum rate of change in left ventricular pressure in the isovolumic relaxation period (-dp/dt_max). Notably, chronic stress led to baroreflex oversensitivity indicated by the elevated ratio of Δheart rate (HR)/ Δmean arterial blood pressure (MABP) in rats followed by vasoconstrictor (phenylephrine, PE) or vasodilator (sodium nitroprusside, SNP) administration, which was almost completely reversed by NPY pretreatment. The expressions of substance P (SP) and gamma aminobutyric acid A receptor (GABA_AR) in nucleus tractus solitarius were increased in chronic stress rats, which were counteracted by NPY pretreatment. We conclude that chronic stress-induced baroreflex hypersensitivity could be blocked by NPY pretreatment. Furthermore, the altered expressions of neurotransmitters and receptors in the brainstem might contribute to this process.

Dihydroergotoxine-induced bradycardia in rats.

Dihydroergotoxine (0.01-0.3 mg kg-1 i.v.) decreased heart rate in pentobarbitone-anaesthetized rats. The bradycardia was reduced but not blocked by pre-treatment with guanethidine, yohimbine, propranolol or pithing. It was not prevented by bivagotomy, atropine, sulpiride or haloperidol. Dihydroergotoxine failed to affect, either the bradycardia produced by electrical stimulation of the vagus, or the cardioacceleration induced by i.v. isoprenaline. The increase in heart rate elicited in pithed rats by electrical stimulation of the spinal cord was reduced by dihydroergotoxine; this effect being inhibited by yohimbine but not by sulpiride. In conclusion, the main mechanism by which dihydroergotoxine (i.v.) induces bradycardia in the rat involves stimulation of alpha 2-adrenoceptors located predominantly at the cardiac sympathetic nerve endings.

Epinephrine potentiates the analgesic and antidepressant effects of polyvinylpyrrolidone and cholecystokinin due to stimulation of afferents in the gastric mucosa

Treatment with epinephrine, polyvinylpyrrolidone, and cholecystokinin in the minimum effective doses produced maximum analgesic and antidepressant effects and caused bradycardia in rats. Administration of epinephrine in combination with polyvinylpyrrolidone or cholecystokinin in threshold doses (1/10-1/25 of the minimum effective dose) produced maximum analgesic and antidepressant effects, but did not cause bradycardia. The potentiating effect of epinephrine is related to stimulation of afferents in the gastric mucosa.

Effects of naltrexone on lipopolysaccharide-induced sepsis in rats

Naltrexone, an opioid antagonist, has been reported to possess an anti-inflammatory effect via blockade of opioid receptor. The aim of this study is to evaluate the protective effect of naltrexone on LPS-induced septic shock in rats. Sepsis was induced by administration of LPS (10 mg/kg, i.v.) in anesthetized rats. Results demonstrated that pretreatment with naltrexone (10 mg/kg, i.v.) significantly ameliorated hypotension and bradycardia of rats 6 h after LPS administration. In isolated blood vessel, study showed that pretreatment with naltrexone significantly improved norepinephrine-induced vasoconstriction and ACh-induced vasorelaxation in aorta of endotoxemic animals. Naltrexone significantly reduced the elevation of serum glutamate-oxalacetate transaminase and glutamate-pyruvate transaminase (as index of hepatic function) induced by LPS. The infiltration of polymorphonuclear neutrophils into liver 48 h after LPS treatment in mice was also reduced by naltrexone. On the other hand, naltrexone significantly decreased the levels of plasma TNF-alpha and inhibited overproduction of superoxide anions in aortic rings. However, naltrexone did not suppress the overproduction of NO (measured by its metabolites nitrite/nitrate in plasma) and iNOS expression in lungs induced by LPS. In in vitro study, naltrexone did not attenuate non-enzymatic iron-induced lipid peroxidation in rat brain homogenates. In conclusion, pretreatment with naltrexone significantly improved circulatory failure and hepatic dysfunction in sepsis. These effects were associated with reduction of TNF-alpha levels and superoxide anion formation, which may be attributed to antagonism of opioid receptors.

SODIUM CHANNEL BLOCKING AND ANTIARRHYTHMIC ACTIONS OF THE NOVEL ARYLPIPERAZINE RSD992

Bolus doses (4–128 μmol kg −1) and infusions (2–32 μmol kg −1 min −1) of the novel arylpiperazine drug RSD992 produced bradycardia in rats and guinea pigs but had minimal effect on ECG variables. RSD992 (2–32 μmol kg −1 min −1) increased threshold current ( I T) for induction of extra-systoles and induction of sustained ventricular fibrillation (VF T) and also increased the effective refractory period (ERP) and decreased the maximum following frequency (MFF) in rat and guinea pig hearts. RSD992 (32–512 μM) significantly increased PR and QRS intervals in isolated rat hearts subjected to conditions that mimic ischaemia (pH 6.4, K + 11 mM) but not in isolated hearts under normal perfusion conditions (pH 7.4, K + 3 mM). RSD992 (0.1–3.0 mM) reduced peak sodium current in rat cardiac (rNa v1.5) sodium channels more potently than neuronal (rNa v1.2a) sodium channels expressed in Xenopus oocytes. The voltage-dependence of sodium channel activation was unaffected whereas inactivation was shifted in a hyperpolarized direction thus suggesting RSD992 may preferentially interact with the inactive state of the sodium channel, a state usually associated with myocardial cell depolarization in ischaemic myocardium. RSD992 (2–24 μmol kg −1 min −1) decreased the incidence of ventricular arrhythmias and mortality in rats subject to coronary artery ligation. RSD992 exhibits frequency- and ischaemia-selective actions on myocardial sodium currents and antiarrhythmic actions in ischaemic rat myocardium.

Age-related peculiarities of the effect of alpha-adrenoreceptor blockade on cardiac function in rats.

Blockade of alpha(1)-adrenoreceptors with prazosin produces bradycardia. The degree of this bradycardia in rats depends on age. In adult (20-week-old) rats bradycardia is pronounced, in 3-week-old rats it is insignificant, and in 1-week-old rats bradycardia does not develop. Prazosin moderates bradycardia induced by vagal stimulation in rats of different age.

Volume loading inhibits baroreflex vagal bradycardia in rats

Volume loading tends to increase blood pressure. Resultant arterial baroreflexes, especially the heart rate component, would counteract renal compensation of volume expansion. In this study, we investigated effect of volume loading on baroreflex vagal bradycardia (BVB) induced by electrical stimulation of the aortic depressor nerves in chloralose/urethane-anesthetized, succinylcholine-immobilized, artificially ventilated rats. As a result, we confirmed that volume loading inhibits BVB. The inhibition was abolished following spinal cord transection at C 2. In contrast, volume loading did not affect the bradycardia induced by electrical stimulation of peripheral cut ends of the cervical vagus. These findings suggest that the inhibition of BVB by volume loading is mediated by afferent input which is conveyed in the sympathetic nerves and ascends in the spinal cord, and produced at a central site.

The role of nitric oxide in bradycardia of rats with obstructive cholestasis

Nitric oxide (NO) has an important role in controlling heart rate and contributes to the cholinergic antagonism of the positive chronotropic response to adrenergic stimulation. Based on evidence of NO overproduction in cholestasis and also on the existence of bradycardia in cholestatic subjects, this study aimed to evaluate the chronotropic effect of epinephrine in isolated atria of cholestatic rats and determine whether alterations in epinephrine-induced chronotropic responses of cholestatic rats are corrected after systemic inhibition of NO synthase (NOS) with NG-nitro-l-arginine (l-NNA). Male Sprague–Dawley rats were used. Cholestasis was induced by surgical ligation of the bile duct under general anesthesia and sham-operated animals were considered as control. The animals were divided into three groups, which received either l-arginine (200 mg/kg/day), l-NNA (10 mg/kg/day) or saline. One week after the operation, a lead II ECG was recorded from the animals, then spontaneously beating atria were isolated and chronotropic responses to epinephrine were evaluated in a standard oxygenated organ bath. The results showed that plasma γ-glutamyl transpeptidase and alanine aminotransferase activity was increased by bile-duct ligation, and that l-aginine treatment partially, but significantly, prevented the elevation of these markers of liver damage. The results showed that heart rate of cholestatic animals was significantly less than that of sham-operated control rats in vivo and this bradycardia was corrected with daily administration of l-NNA. The basal spontaneous beating rate of atria in cholestatic animals was not significantly different from that of sham-operated rats in vitro. Meanwhile, cholestasis induced a significant decrease in chronotropic effect of epinephrine. These effects were corrected by daily administration of l-NNA. Surprisingly l-arginine was as effective as l-NNA and increased the chronotropic effect of epinephrine in cholestatic rats but not in sham-operated animals. Systemic NOS inhibition corrected the decreased chronotropic response to adrenergic stimulation in cholestatic rats, and suggests an important role for NO in the pathophysiology of heart rate complications in cholestatic subjects. The opposite effect of chronic l-arginine administration in cholestasis and in control rats could be explained theoretically by an amelioration of cholestasis-induced liver damage by chronic l-arginine administration in bile duct-ligated rats.

Hemodynamic effects of tetrindol in alert normotensive mice and rats after blockade of nitric oxide synthesis.

Single intravenous injection of antidepressant tetrindol (1 and 10 mg/kg), a reversible monoamine oxidase A inhibitor, dose-dependently decreased heart rate and mean arterial pressure (in a concentration of 10 mg/kg) in alert NMRI mice and Sprague-Dawley rats. Nitric oxide synthase blockade with L-NAME attenuated tetrindol-induced bradycardia in rats and completely abolished this effect in mice.

Enhanced serotonin-mediated responses in the nucleus tractus solitarius of spontaneously hypertensive rats

Previous studies have demonstrated that injection of serotonin into the nucleus tractus solitarius (NTS) elicits hypotension and bradycardia in rats. The present study sought to further characterize this response and to examine the role of serotonergic mechanisms in the NTS in cardiovascular regulation in spontaneously hypertensive (SHR) rats. Injections of picomole amounts of serotonin into the NTS of chloralose-anesthetized normotensive Sprague–Dawley (S–D) or Wistar–Kyoto (WKY) rats produced hypotension and bradycardia that were eliminated by prior injection into the NTS of the selective 5HT 2 antagonist sarpogrelate. Bilateral injection of sarpogrelate did not alter blood pressure or reflex changes in heart rate in response to phenylephrine-induced increases in blood pressure or nitroprusside-induced decreases in blood pressure. In SHR rats, the depressor response produced by injection of serotonin into the NTS was markedly larger than in WKY rats, and was larger than depressor responses previously reported for other excitatory substances injected into the NTS. In SHR rats bilateral injection of sarpogrelate produced an increase in blood pressure, although it did not alter baroreceptor-evoked changes in heart rate. These results provide further support for the hypothesis that stimulation of 5HT 2 receptors in the NTS contributes to cardiovascular regulation independent of the baroreceptor reflex. Furthermore, this serotonergic system is altered in SHR rats, apparently acting tonically to reduce blood pressure.

Adaptation to periodic high-altitude hypoxia inhibits baroreflex vagal bradycardia in rats.

A 3-week course of adaptation to high-altitude hypoxia (4500 m above sea level) inhibited baroreflex vagal bradycardia induced by a rapid rise of systemic blood pressure in conscious rats. Bradycardic responses to electrical stimulation of peripheral end of the right vagus nerve and methacholine (M(2) muscarinic receptor agonist) in hypoxia-adapted rats did not differ from the control. It is concluded that hypoxia inhibits baroreflex vagal bradycardia by acting on a central element of the baroreceptor reflex arch.

Ozone-induced hypothermia and bradycardia in rats and guinea pigs in nose-only or whole-body inhalation systems

1. Ozone (O 3) inhalation induces a hypothermic response in rodents characterized primarily by core body temperature ( T CO) and heart rate (HR) decreases. 2. This preliminary study examined T CO, HR, and electrocardiographic changes in rats and guinea pigs during whole-body or nose-only exposure to 1 ppm 18O 3. 3. Rats exposed whole-body demonstrated an immediate hypothermic response, while that of the nose-only exposed rats was delayed ≈60 min; guinea pigs showed no hypothermic response to 18O 3. 4. 18O content in lavage fluid samples suggests that dose is greater in nose-only exposures than whole-body exposures, and that guinea pigs received higher doses than rats. 5. These initial findings suggest that dosimetric differences may result from differences in species responses and exposure scenarios.