Assessment of cardiac autonomic tone in conscious rats

Changes in cardiovascular autonomic control induced by chronic inhibition of acetylcholinesterase during pyridostigmine or donepezil treatment of spontaneously hypertensive rats

Chronic treatment with acetylcholinesterase inhibitors may be a promising therapeutic strategy for treatment of cardiovascular diseases. The aim of our study was to analyze the changes in blood pressure (BP) and heart rate (HR) during 14 days of treatment with two different acetylcholinesterase inhibitors - pyridostigmine (PYR) having only peripheral effects or donepezil (DON) with both peripheral and central effects. In addition, we studied their effects on the cardiovascular response to restraint stress and on sympathovagal control of HR in normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR). SHR were characterized by elevated BP and increased low-frequency component of systolic BP variability (LF-SBPV), but their cardiac vagal tone and HR variability (HRV) were reduced compared with WKY. Chronic treatment with either acetylcholinesterase inhibitor decreased HR and increased HRV in both strains. PYR treatment slightly decreased BP and LF-SBPV in the dark phase of the day. Neither drug significantly altered BP response to stress, but PYR attenuated HR increase during restraint stress. Regarding sympathovagal balance, acute methylatropine administration caused a greater increase of HR in WKY than in SHR. Chronic PYR or DON treatment enhanced HRV and HR response to methylatropine (vagal tone) in WKY, whereas PYR but not DON treatment potentiated HRV and vagal tone in SHR. In conclusion, vagal tone was lower in SHR compared with WKY, but was enhanced by chronic PYR treatment in both strains. Thus, chronic peripheral, but not central, acetylcholinesterase inhibition has major effects on HR and its variability in both normotensive and hypertensive rats.

Acute poisoning by chlorpyrifos differentially impacts survival and cardiorespiratory function in normotensive and hypertensive rats

Hypertension is the most important and well-known risk factor for cardiovascular disease (CVD). Recently, acute organophosphate (OP) poisoning has also been pointed as a CVD risk factor. Despite this evidence, no studies have contrasted the acute toxicosis and cardiovascular (CV) effects of OP poisoning under conditions of normotension and hypertension. In this work, adult male normotensive Wistar and Spontaneously Hypertensive rats (SHR) were intraperitoneally injected with saline or chlorpyrifos (CPF), an OP compound, monitored for acute toxicosis signs and 24-h survival. After poisoning, blood pressure, heart rate and ventilation were recorded, the Bezold-Jarisch Reflex (BJR), the Chemoreflex (CR) were chemically activated, as well as the cardiac autonomic tone (AUT) was assessed. Erythrocyte and brainstem acetylcholinesterase and plasmatic butyrylcholinesterase (BuChE) activities were measured as well as lipid peroxidation, advanced oxidation protein products (AOPP), nitrite/nitrate levels, expression of catalase, TNFα and angiotensin-I converting enzyme (ACE-1) within the brainstem. CPF induced a much more pronounced acute toxicosis and 33 % lethality in SHR. CPF poisoning impaired ventilation in SHR, the BJR reflex responses in Wistar rats, and the chemoreflex tachypneic response in both strains. CPF inhibited activity of cholinesterases in both strains, increased AOPP and nitrite/nitrate levels and expression of TNFα and ACE-1 in the brainstem of Wistar rats. Interestingly, SHR presented a reduced intrinsic BuChE activity, an important bioscavenger. Our findings show that, CPF at sublethal doses in normotensive rats lead to lethality and much more pronounced acute toxicity signs in the SHR. We also showed that cardiorespiratory reflexes were differentially impacted after CPF poisoning in both strains and that the cardiorespiratory disfunction seems to be associated with interference in cholinergic transmission, oxidative stress and inflammation. These results points to an increased susceptibility to acute toxicosis in hypertension, which may impose a significant risk to vulnerable populations.

An Integrative Literature Review of Heart Rate Variability Measures to Determine Autonomic Nervous System Responsiveness Using Pharmacological Manipulation

BACKGROUND

Heart rate variability (HRV) is defined as the difference in the timing of intervals between successive heartbeats and is used as a surrogate measure to the responsiveness of the autonomic nervous system. A review and synthesis of HRV as an indicator of autonomic nervous system responsiveness to pharmacologic stimulation/blockade of sympathetic and/or parasympathetic nervous system branches have not been completed.

PURPOSE

The aim of this integrative review is to synthesize research examining pharmacological modulation of the autonomic nervous system and the response of time domain, frequency domain, and nonlinear measures of HRV.

CONCLUSIONS

Sympathetic nervous system blockade resulted in a consistent decrease in the standard deviation of normal-normal interval metric across studies. Stimulation of the parasympathetic nervous system was associated with an increase in several time, frequency, and nonlinear HRV indices, whereas blockade of the parasympathetic nervous system led to a decrease in similar indices.

CLINICAL IMPLICATIONS

Recommendations to improve the reproducibility of future HRV research are provided for standardization of recording, analysis, and metric decisions and more thorough reporting of HRV indices in published studies. Alterations in autonomic nervous system input to the cardiovascular system are associated with an increased risk for adverse patient outcomes and increased mortality; therefore, understanding the influence of pharmacologic autonomic nervous system modulation on HRV indices and important considerations for reproducible HRV research design will inform future translational research on cardiovascular risk reduction.

Intermittent exposure to chlorpyrifos results in cardiac hypertrophy and oxidative stress in rats

Forbidden in some countries due to its proven toxicity to humans, chlorpyrifos (CPF) still stands as an organophosphate pesticide (OP) highly used worldwide. Cardiotoxicity assessment is an unmet need in pesticide regulation and should be deeply studied through different approaches to better inform and generate an appropriate regulatory response to OP use. In the present study, we used our 4-week intermittent OP exposure model in rats to address the CPF effects on cardiac morphology allied with cardiovascular functional and biomolecular evaluation. Rats were intermittently treated with CPF at doses of 7 mg/kg and 10 mg/kg or saline (i.p.) and assessed for cardiac morphology (cardiomyocyte diameter and collagen content), cardiopulmonary Bezold-Jarisch reflex (BJR) function, cardiac autonomic tone, left ventricle (LV) contractility, cardiac expression of NADPH oxidase (Nox2), catalase (CAT), superoxide dismutase 1 (SOD1), superoxide dismutase 2 (SOD2) and cardiac levels of advanced oxidation protein products (AOPP) and thiobarbituric acid reactive substances (TBARS). Plasma butyrylcholinesterase (BuChE) and brainstem acetylcholinesterase (AChE) were also measured. Intermittent exposure to CPF induced cardiac hypertrophy, increasing cardiomyocyte diameter and collagen content. An impairment of cardioinhibitory BJR responses and an increase in cardiac vagal tone were also observed in CPF-treated animals without changes in LV contractility. CPF exposure increased cardiac Nox-2, CAT, SOD1, and TBARS levels and inhibited plasma BuChE and brainstem AChE activities. Our data showed that intermittent exposure to CPF induces cardiac hypertrophy together with cardiovascular reflex impairment, imbalance of autonomic tone and oxidative stress, which may bring significant cardiovascular risk to individuals exposed to OP compounds seasonally.

Patterns of cardio-respiratory motor outputs during acute and subacute exposure to chlorpyrifos in an ex-vivo in situ preparation in rats

While a considerable body of literature has characterized the clinical features induced by organophosphate pesticides, the field lacks scrutiny into cardio-respiratory changes in different phases of poisoning. Herein, we evaluated the impact of chlorpyrifos (CPF) and its active metabolite chlorpyrifos-oxon (CPO) on the cardiorespiratory system during acute and subacute phases of poisoning using an in situ experimental rodent model. CPF (30 mg/kg) was injected intraperitoneally to rats beforehand (24 h) whereas CPO (15 mg/kg) was added into the perfusate reservoir to evaluate the effects on the motor outputs throughout the three phases of the respiratory cycle: inspiration, post-inspiration and late expiration. Phrenic, recurrent laryngeal (RLN) and thoracic sympathetic nerve activity (tSNA) were recorded. Heart rate was derived from the electrocardiogram (ECG) and the baro- and chemo-reflexes tested. CPF and CPO led to a time-dependent change in cardiorespiratory motor outputs. In the acute phase, the CPO induced bradypnea, transiently reduced the inspiratory time (TI), and increased the amplitude of phrenic. Post-inspiratory (PI) discharge recorded from the RLN was progressively reduced while tSNA was increased. CPO significantly depressed the chemoreflex but had no effect on baroreflex. During subacute phase, CPF prolongated T_I with no effect on respiratory rate. Both the RLN PI discharge, the chemoreflex and the baroreflex sympathetic gain were reduced. In addition, both CPF and CPO shifted the cardiac sympatho-vagal balance towards sympathetic dominance. Our data show that different phases of poisoning are associated with specific changes in the cardio-respiratory system and might therefore demand distinct approaches by health care providers.

A pharmacological characterization of electrocardiogram PR and QRS intervals in conscious telemetered rats

INTRODUCTION

The conscious telemetered rat is widely used as an early in vivo screening model for assessing the cardiovascular safety of novel pharmacological agents. The current study aimed to identify its utility in assessing electrocardiogram (ECG) PR and QRS interval changes.

METHOD

Male Han-Wistar rats (~250 g) were implanted with radio-telemetry devices for the recording of ECG and haemodynamic parameters. Animals (n = 4-8) were treated with single doses of calcium (nifedipine, diltiazem or verapamil; CCBs) or sodium channel blockers (quinidine or flecainide; SCBs) or their corresponding vehicles in an ascending dose design. Data was recorded continuously up to 24 h post-dose. Pharmacokinetic analysis of blood samples was performed to allow comparison of effects to published data in other species.

RESULTS

Of the CCBs, only diltiazem (300 mg/kg) prolonged the PR interval (49 ± 2 versus vehicle: 43 ± 1 ms), although this was not statistically significant (p = .11). QA interval decreased with nifedipine (30 ± 1 versus 24 ± 0 ms) and diltiazem (34 ± 1 versus 27 ± 1 ms) but increased with verapamil (30 ± 0 versus 37 ± 1 ms) demonstrating pharmacological activity of each agent. Both SCBs, caused statistically significant (p < .05) increases in both intervals - quinidine (100 mg/kg; PR: 50 ± 2 versus 43 ± 1 ms; QRS: 22 ± 2 versus 18 ± 1 ms) and flecainide (9 mg/kg; PR: 56 ± 1 versus 46 ± 1 ms; QRS: 27 ± 1 versus 21 ± 1 ms). Drug plasma exposure was confirmed in all animals.

DISCUSSION

At similar plasma concentrations to other species, the conscious telemetered rat demonstrates limited utility in assessing PR interval prolongation by CCBs, despite significant contractility effects being observed. However, results with SCBs demonstrate a potential application for evaluating drug-induced QRS prolongation.

Effect of anaesthetic and choice of neuromuscular blocker on vagal control of heart rate under laboratory animal experimental conditions

Neuromuscular-blocking agents are commonly used in laboratory animal research settings. Due to actions of cholinergic receptors at locations other than the motor end-plate, these agents have a strong propensity to modulate autonomic outflow and may therefore not be desirable in studies examining autonomic function. This study aimed to compare the effect of two non-depolarizing neuromuscular-blocking agents, pancuronium and cisatracurium, on blood pressure, heart rate and non-invasive indices of autonomic function (heart rate variability, systolic blood pressure variability and baroreflex sensitivity) under two different types of anaesthesia in Lewis rats. Pancuronium produced a profound vagolytic response characterized by tachycardia, reduction in heart rate variability and baroreflex sensitivity under urethane anaesthesia, and with minimal effect under isoflurane anaesthesia. Conversely, cisatracurium produced no evidence of vagolytic action under either urethane or isoflurane anaesthesia. Therefore, for studies interested in examining autonomic function, particularly baroreflex or vagal function, neuromuscular blockade would be best achieved using cisatracurium.

Comparison between spectral analysis and symbolic dynamics for heart rate variability analysis in the rat

Spectral analysis of heart rate (HR) has been widely used to assess the autonomic cardiovascular control. A nonlinear approach, known as symbolic analysis, has been reported to be very useful to assess the autonomic control of cardiovascular system in humans, but very few studies reported on the differences between these two approaches on experimental models. Two distinct approaches were used to elicit autonomic changes in conscious Wistar rats: (1) pharmacological blockade of cardiac autonomic receptors with atenolol (ATE, N = 9) or methylatropine (ATR, N = 9) and (2) mild changes in arterial pressure (AP) induced by phenylephrine (PHE, N = 9) or sodium nitroprusside (NPS, N = 9). Series of cardiac interval (CI) and systolic AP (SAP) were assessed using spectral analysis and symbolic dynamics. Results show that, for spectral analysis, the power in high frequency band of CI and the power in low frequency band of SAP are the most reliable indices of vagal and sympathetic modulation, respectively. For symbolic analysis, results point 0V% and 1V% to be related to sympathetic and 2UV% to vagal modulation. Interestingly, the incidence of 1V patterns, hitherto with unknown meaning, was revealed the best index of sympathetic modulation in the rat and should be accounted for in the future studies.

Complex and interacting influences of the autonomic nervous system on cardiac electrophysiology in conscious mice

Mice may now be the preferred animal model for biomedical research due to its anatomical, physiological, and genetic similarity to humans. However, little is known about accentuated antagonism of chronotropic and dromotropic properties in conscious mice. Accordingly, we describe the complex and interacting influence of the autonomic nervous system on cardiac electrophysiology in conscious mice. Specifically, we report the effects of single and combined cardiac autonomic blockade on measurements of pulse interval (heart rate), atrio-ventricular interval, sinus node recovery time (SNRT), SNRT corrected for spontaneous sinus cycle, and Wenckebach cycle length in conscious mice free of the confounding influences of anesthetics and surgical trauma. Autonomic influences were quantified as the change in parameter induced by its selective blocker (Sympathetic or Parasympathetic Effect) or as the difference between the intrinsic value and the value after a selective blocker (Sympathetic or Parasympathetic Tonus). Sympatho-Vagal Balance (SVB) was assessed as the ratio of control interval to intrinsic interval. SVB suggests slight parasympathetic dominance in the control of cardiac electrophysiology intervals. Furthermore, results documents a complex interaction between the sympathetic and parasympathetic divisions of the autonomic nervous system in the control of cardiac electrophysiology parameters. Specifically, the parasympathetic effect was greater than the parasympathetic tonus in the control of cardiac electrophysiology parameters. In contrast, the sympathetic effect was smaller than the sympathetic tonus in the control of cardiac electrophysiology parameters. Results have important implications because actions of pharmacological agents that alter the autonomic control of cardiac electrophysiology are transformed by these interacting mechanisms.