Selective abolition of Mayer waves in conscious endotoxemic rats.

The relation between vascular sympathetic tone and the amplitude of arterial pressure (AP) Mayer waves was examined by analyzing 60-min recordings of AP and renal sympathetic nerve activity (RSNA) obtained in conscious rats before and after lipopolysaccharide administration, which results in strong sympathoexcitation. Mayer waves completely disappeared together with accompanying oscillations of RSNA. Meanwhile, the gain of the sympathetic baroreceptor reflex was increased, thus suggesting that abolition of Mayer waves resulted from the previously reported reduction of vascular reactivity to α-adrenoceptor stimulation. In conclusion, the amplitude of Mayer waves cannot be used indiscriminately as an index of vascular sympathetic tone.

Age-dependent ventricular arrhythmias risk, structural and molecular remodeling in systemic arterial hypertension.

INTRODUCTION: The left ventricular hypertrophy (LVH)-ventricular arrhythmias relationship associated with arterial hypertension and aging remains controversial. We aimed to assess the age-dependency of ventricular arrhythmias in spontaneously hypertensive rats (SHRs) and the corresponding ventricular structural and molecular remodeling. MATERIALS AND METHODS: Ventricular arrhythmias were quantified using 24-h radiotelemetry ECG monitoring in eight SHRs and four Wistar-Kyoto (WKY) rats at 14 (young), 24 (adult), and 48 (aging) weeks of age. Left ventricular histology and mRNA expressions of 89 proarrhythmogenic genes were assessed in six additional groups (n=4 each) of young, adult, and aging SHRs and WKYs. RESULTS: Regardless of their age, SHRs presented more premature ventricular contractions (PVCs) than age-matched WKYs (p<0.01). The arrhythmogenicity peak occurred in adult SHRs; ventricular tachycardias only occurred in adult SHRs. Among the SHRs, LV thickness, interstitial fibrosis, and the number of deregulated genes increased with age. Kcnj11 expression was deregulated in adult, but not in young or aging SHRs. DISCUSSION: This study confirms the presence of higher ventricular ectopy in SHRs than in age-matched WKYs. LVH appeared to be an adaptive, antiarrhythmic process. Myocardial energetic changes with advancing age, as reflected by Kcnj11 expression changes, could underlie this age-dependency of ventricular arrhythmias.

Renal sympathetic nerve activity and vascular reactivity to phenylephrine after lipopolysaccharide administration in conscious rats.

It has been proposed that sympathoexcitation is responsible for vascular desensitization to α1-adrenoceptor stimulation during lipopolysaccharide (LPS)-induced systemic inflammation. The present study tested this hypothesis by examining the effects of sympatho-deactivation with the α2-adrenoceptor agonist, dexmedetomidine, on mean arterial pressure (MAP), renal sympathetic nerve activity (RSNA), and vascular reactivity to phenylephrine in conscious rats with cardiac autonomic blockade (methylatropine and atenolol) following LPS administration. In male, adult Sprague-Dawley rats (n = 5 per group), RSNA and MAP were continuously recorded over 1-h periods, before and after LPS administration (20 mg/kg iv), and finally after infusion of either saline or dexmedetomidine (5 µg/kg, then 5 µg/kg/h iv). A full dose-response curve to phenylephrine was constructed under each condition. After pooling data from both groups of rats (n = 10), LPS significantly (P = 0.005) decreased MAP (from 115 ± 1 to 107 ± 2 mmHg), increased RSNA (to 403 ± 46% of baseline values) and induced 4 to 5-fold increases in the half-maximal effective dose (ED50) of phenylephrine (from 1.02 ± 0.09 to 4.76 ± 0.51 µg/kg). During saline infusion, RSNA progressively decreased while vascular reactivity did not improve. Treatment with dexmedetomidine decreased MAP, returned RSNA to near pre-endotoxemic levels, but only partially restored vascular reactivity to phenylephrine (ED50 was still threefold increased as compared with baseline values). These findings indicate that only part of the decrease in vascular reactivity to α1-adrenoceptor stimulation during endotoxemia can be accounted for by sympathetic activation, at least on a short-term basis.

Assessment of cardiac autonomic tone in conscious rats.

Cardiac autonomic tone can be assessed either by estimating separately vagal and sympathetic tones or by evaluating the net effect of their interaction, the so-called sympathovagal balance (SVB). To compare the most commonly used methods in rats, telemetric recordings of the electrocardiogram were performed in normotensive WKY rats, and in groups of spontaneously hypertensive (SHR) rats that were either untreated or chronically treated with the cholinesterase inhibitor, pyridostigmine, to enhance vagal tone. Cardiac autonomic blockers were administered alone and in combination, so that heart rate (HR) could be measured (1) under resting conditions, (2) with either autonomic branch blocked, and (3) with both branches blocked (which provided intrinsic HR, iHR). SVB was assessed as the ratio of resting HR to iHR. This calculation pointed to a sympathetic predominance in untreated SHRs and even more so in WKY rats, and to a marked vagal predominance in pyridostigmine-treated SHRs. By contrast, the ratio between low and high frequency components (LF/HF) of RR interval spectra did not significantly differ between the groups. Each autonomic tone was quantified as the HR change induced by its selective blocker or as the difference between iHR and HR after blockade of its counterpart. Both pharmacological methods indicated vagal enhancement in treated SHRs, but provided opposite results in terms of vagal vs. sympathetic predominance. These data seriously question the use of the LF/HF ratio as an index of SVB, and the possibility to reliably estimate vagal and sympathetic tones separately through current pharmacological approaches in conscious rats.

Pyridostigmine enhances atrial tachyarrhythmias in aging spontaneously hypertensive rats.

This study examined whether chronic administration of pyridostigmine, a reversible cholinesterase inhibitor, would exacerbate episodes of spontaneous atrial tachyarrhythmia (AT) in conscious, aging, spontaneously hypertensive rats (SHRs). Telemetric recordings of electrocardiogram (ECG, n = 5) and ECG/arterial pressure (n = 3) were performed in male 49-week old SHRs. After a 1-week period of continuous recording under baseline conditions, rats were implanted with osmotic minipumps that delivered pyridostigmine (15 mg/kg/day subcutaneously) for either 1 (n = 8) or 3 (n = 5) weeks. In the latter case, sympathovagal balance was assessed during the last infusion week by measuring heart rate (HR) changes in response to administration of cardiac autonomic blockers. An additional 1-week recording was performed after explantation of minipumps. Significant (P = 0.02) reductions in HR with no consistent changes in arterial pressure were observed. Frequency and duration of AT episodes were increased by pyridostigmine (0.01 ≤ P ≤ 0.07). This increase was sustained across the 3-week treatment period and reversible after cessation of treatment. Autonomic blockade revealed that intrinsic HR was above (P = 0.04) resting HR, pointing to a shift of sympathovagal balance towards vagal predominance. However, the respiratory-related component of HR variability (high-frequency power of RR interval) was lowered (P = 0.01) by pyridostigmine treatment, indicating reduced vagal modulation of HR. The results are consistent with a pathogenic role of the parasympathetic nervous system in the aging SHR model, and raise the possibility that sustained vagal activation may facilitate atrial arrhythmias.

Long-standing arterial hypertension is associated with Pitx2 down-regulation in a rat model of spontaneous atrial tachyarrhythmias.

AIMS: The timecourse of left atrial Pitx2 down-regulation in the setting of atrial tachyarrhythmias remains unknown. Accordingly, we aimed to assess the age dependency of left atrial Pitx2 expression in an experimental model of spontaneous atrial tachyarrhythmias in rats. METHODS AND RESULTS: Atrial sampling was performed in three groups (n = 4 each) of young (14-week-old), adult (24-week-old), and ageing (48-week-old) spontaneously hypertensive rats (SHRs), in which we previously demonstrated the age dependency of spontaneous atrial tachyarrhythmias, and three groups (n = 4 each) of age-matched normotensive Wistar-Kyoto (WKY) rats. mRNA expression of Pitx2 was studied using real-time polymerase chain reaction. Ageing SHRs presented significantly lower left atrial Pitx2 expressions compared with age-matched WKY rats (P = 0.02), while no significant difference was observed between young or adult SHRs and age-matched WKY rats (both P > 0.05). Among SHRs, Pitx2 expressions showed a progressive, age-dependent decrease (34.9 ± 6.7 in young SHRs, 17.1 ± 3.6 in adult SHRs, and 10.7 ± 1.7 in ageing SHRs, P = 0.04) and were significantly negatively correlated with both age (Spearman r = -0.86, P < 0.01) and heart weight (Spearman r = -0.76, P < 0.01). CONCLUSION: The present study suggests the presence of age-dependent left atrial Pitx2 down-regulation in SHRs. The strong negative correlation between left atrial Pitx2 expression and heart weight among SHRs may indicate a link between long-standing arterial hypertension and Pitx2-related atrial arrhythmogenicity.

A new pyrroline compound selective for I1-imidazoline receptors improves metabolic syndrome in rats.

Symptoms of the metabolic syndrome (MetS), such as insulin resistance, obesity, and hypertension, have been associated with sympathetic hyperactivity. In addition, the adiponectin pathway has interesting therapeutic potentials in MetS. Our purpose was to investigate how targeting both the sympathetic nervous system and the adipose tissue (adiponectin secretion) with a drug selective for nonadrenergic I1-imidazoline receptors (I1Rs) may represent a new concept in MetS pharmacotherapy. LNP599 [3-chloro-2-methyl-phenyl)-(4-methyl-4,5-dihydro-3H-pyrrol-2-yl)-amine hydrochloride], a new pyrroline derivative, displaced the specific [(125)I]para-iodoclonidine binding to I1R with nanomolar affinity and had no significant affinity for a large set of receptors, transporters, and enzymes. In addition, it can cross the blood-brain barrier and has good intestinal absorption, permitting oral as well as intravenous delivery. The presence of I1Rs was demonstrated in 3T3-L1 adipocytes; LNP599 had a specific stimulatory action on adiponectin secretion in adipocytes. Short-term administration of LNP599 (10 mg/kg i.v.) in anesthetized Sprague-Dawley rats markedly decreased sympathetic activity, causing hypotension and bradycardia. Long-term treatment of spontaneously hypertensive heart failure rats with LNP599 (20 mg/kg PO) had favorable effects on blood pressure, body weight, insulin resistance, glucose tolerance, and lipid profile, and it increased plasma adiponectin. The pyrroline derivative, which inhibits sympathetic activity and stimulates adiponectin secretion, has beneficial effects on all the MetS abnormalities. The use of one single drug with both actions may constitute an innovative strategy for the management of MetS.

Unprovoked atrial tachyarrhythmias in aging spontaneously hypertensive rats: the role of the autonomic nervous system.

Experimental models of unprovoked atrial tachyarrhythmias (AT) in conscious, ambulatory animals are lacking. We hypothesized that the aging, spontaneously hypertensive rat (SHR) may provide such a model. Baseline ECG recordings were acquired with radiotelemetry in eight young (14-wk-old) and eight aging (55-wk-old) SHRs and in two groups of four age-matched Wistar-Kyoto (WKY) rats. Quantification of AT and heart rate variability (HRV) analysis were performed based on 24-h ECG recordings in unrestrained rats. All animals were submitted to an emotional stress protocol (air-jet). In SHRs, carbamylcholine injections were also performed. Spontaneous AT episodes were observed in all eight aging SHRs (median, 91.5; range, 4-444 episodes/24 h), but not in young SHRs or WKY rats. HRV analysis demonstrated significantly decreased low frequency components in aging SHRs compared with age-matched WKY rats (P < 0.01) and decreased low/high frequency ratios in both young (P < 0.01) and aging (P = 0.01) SHRs compared with normotensive controls. In aging SHRs, emotional stress significantly reduced the number of arrhythmic events, whereas carbamylcholine triggered AT and significantly increased atrial electrical instability. This study reports the occurrence of unprovoked episodes of atrial arrhythmia in hypertensive rats, and their increased incidence with aging. Our results suggest that autonomic imbalance with relative vagal hyperactivity may be responsible for the increased atrial arrhythmogenicity observed in this model. We also provide evidence that, in this model, the sympatho-vagal imbalance preceded the occurrence of arrhythmia. These results indicate that aging SHRs may provide valuable insight into the understanding of atrial arrhythmias.

Effect of chronic cervical ganglionectomy on the spontaneous variability of internal carotid blood flow in the conscious rat.

The role of sympathetic innervation in the control of spontaneous fluctuations of cerebral blood flow is still poorly understood. In conscious, unrestrained rats, blood flow velocity (pulsed Doppler) was measured in both internal carotid arteries 1 week after either excision of the right superior cervical ganglion (n = 8) or sham surgery (n = 6). Using Fourier-based techniques, spectral power of each carotid blood flow (CBF) was computed over the whole recording period (246 min), which was segmented into nine consecutive 27.3 min periods. Variability of CBF (spectral power) was ∼40% higher (P < 0.02) on the denervated than on the intact side at frequencies <1 Hz. Coherence between left and right CBFs was similar in the two groups of rats, except in the 0.01-0.1 Hz frequency range where it was lower (P < 0.05) in rats with unilateral sympathectomy (0.54 ± 0.03) than in intact rats (0.74 ± 0.06). In this frequency range, mathematically removing the influence of arterial pressure had little effect on coherence between CBFs in both groups of rats, so that coherence remained significantly lower in rats with unilateral sympathectomy (0.52 ± 0.03) than in intact rats (0.70 ± 0.06). This study indicates that sympathetic innervation has an overall buffering influence on CBF variability. This modulatory role is especially important in a frequency range corresponding to slow fluctuations of CBF (lasting from 10 to 100 s), which are essentially unrelated to fluctuations of arterial pressure.

Role of the sympathetic nervous system in cerebrovascular responses to air-jet stress in rats.

This study examined the role of sympathetic nerves in the control of cerebral hemodynamics during air-jet stress. In adult male Sprague-Dawley rats, blood flow velocity (pulsed Doppler) was measured in both internal carotid arteries 1 week after excision of one superior cervical ganglion. Blood pressure (BP) and carotid blood flows (CBFs) were simultaneously recorded during exposure to air-jet stress. In 5 out of 13 rats, stress was applied after ß(2)-adrenoceptor blockade with ICI 118551 (0.4 mg/kg, then 0.2 mg/kg/h, i.v). Stress evoked an immediate rise in BP, CBFs, and vascular conductances. Vasodilatation was much larger on the denervated side than on the intact side (mean ± SEM: 78 ± 7 versus 19 ± 4%; P < 0.02) and lasted about 10 s. Thereafter, blood flows returned to or near normal and showed parallel variations while BP remained elevated. There was, therefore, a net vasoconstriction on both sides. In ICI 118551-treated rats, the initial vasodilatation was not significantly reduced on the denervated side (64 ± 4%), but the subsequent vasoconstriction was enhanced (P < 0.05) on both sides. In conclusion, air-jet stress evokes an immediate, short-lasting vasodilatation through a mechanism unrelated to ß(2)-adrenoceptor stimulation. Sympathetic nerves powerfully limit this phenomenon, and thus contribute to protect the cerebral circulation from stress-induced BP surges.

Time-frequency analysis of the baroreflex control of renal sympathetic nerve activity in the rat.

Cross-spectral analysis using the short-time Fourier transform (STFT) allows estimating the transfer function between spontaneous fluctuations of arterial pressure (AP) and renal sympathetic nerve activity (RSNA) at the heart rate (HR) frequency, which provides an index of sympathetic baroreflex sensitivity (sBRS) in rats. The method, however, cannot reliably compute more than one value per min. The goal of the present study was to achieve a better temporal resolution by using advanced methods. The first method is the continuous wavelet transform (CWT) using the Morlet wavelet. The second method is based on the empirical mode decomposition (EMD), a method that decomposes a signal into a sum of oscillating components. Using both methods, the transfer function was estimated over periods of 10s. The two methods, together with STFT, were applied to AP and RSNA signals that were simultaneously recorded in conscious, freely behaving rats (n=10) during 1h. When considering 1-h mean sBRS values obtained in each rat, both methods showed a strong correlation with STFT (R=0.96 and 0.91 for CWT and EMD, respectively, both P<0.001). In each rat, sBRS values obtained by the CWT and EMD methods were tightly correlated (R=0.93±0.01, n=294±13, P<0.001). With both methods, high-frequency variations of sBRS (0.0083-0.5Hz) accounted for about 40% of its overall variability. In urethane-anaesthetized rats (n=9), sBRS variability computed by either method was reduced by about two-thirds (P<0.001). Improving temporal resolution of sBRS computation reveals that in rats, sBRS exhibits fast, short-lasting fluctuations. These fluctuations largely depend on the state of vigilance.

Cardiovascular effects of a novel synthetic analogue of naturally occurring piperamides.

Cardiovascular responses to intravenous administration of a piperamide analogue, LASSBio 365, were investigated in anesthetized rats. LASSBio 365 [62.5-1000 microg/kg, intravenously (IV)] has potent cardiovascular effects that include hypotension and bradycardia, accompanied by a brief pressor effect and apnea. Bilateral vagotomy or atropine injection (2 mg/kg, IV) completely abolished the bradycardia. A drop in blood pressure was abolished in bivagotomized rats. However, it was only inhibited in atropine-treated rats. The apnea was inhibited by both treatments. The Bezold-Jarisch reflex (ie, hypotension, bradycardia, and apnea) induced by LASSBio 365 is altered neither by 5-HT3 antagonist (tropisetron, 0.1 mg/kg, intraarterially) nor by the P2x antagonist (PPADS, 8.6 mg/kg, IV). The pressor component was affected neither by any of these interventions nor by the 5-HT2 antagonist (ritanserin, 0.5 mg/kg, i.a.). In capsaicin-pretreated rats (50 mg/kg, subcutaneously), all responses evoked by LASSBio 365 were abolished, including the pressor effect, which was inhibited. The data show that LASSBio 365 evokes the Bezold-Jarish reflex, neither via serotonergic receptors nor purinergic receptors but perhaps via the vanilloid pathway.

Very high frequency components of renal sympathetic nerve activity in conscious rats.

The aim of this study was to examine whether multifibrenal sympathetic nerve activity (RSNA) of conscious rats contains frequency components of biological interest at frequencies above 25Hz. RSNA was recorded in 10 conscious Sprague-Dawley rats under baseline conditions and during infusion of vasoactive drugs that reflexly altered the mean RSNA level. The RSNA signal was band-pass filtered (300-3000Hz) before being sampled at 10,000Hz. The analytic envelope of this raw signal was then extracted using the Hilbert transform, and 132-s periods were submitted to Fourier transform analysis. Spectral power was computed from 0 to 25Hz and from 25 to 3000Hz (P(25-3000)). P(25-3000) was reduced by about 80% after either ganglionic blockade or euthanasia, which indicated that it was of biological origin and derived from the activity of postganglionic sympathetic neurons. After subtraction of post-mortem spectral power, basal P(25-3000) contributed 59.8+/-2.4% of total power. P(25-3000) was strongly barosensitive and thus, accounted for a major part of the reflex changes in total power. In each of the 10 rats, P(25-3000) was linearly correlated with the mean RSNA level (0.984+/-0.003) and even more so with the spectral power in the 0-25Hz frequency range (0.994+/-0.001). In conclusion, the RSNA of conscious rats contains very high frequency components that account for about 60% of the total spectral power and are modulated by the baroreceptor reflex. A reasonable approximation of this power can be obtained by computing spectra up to 25Hz.

Analysis of sympathetic neural discharge in rats and humans.

Neural signals convey information through two different modalities: intensity and discharge pattern. The intensity code is based on the number of action potentials per unit time, which is then easily translated into neurotransmitter release. This kind of information may be assessed simply by counting the number of spikes or bursts over a time unit. However, the discharge pattern is a further, efficient means of neural information transfer. Rhythmic patterns (i.e. oscillations) can support highly structured, temporal codes based on correlation and synchronization. It is therefore clear that applying frequency domain analysis to sympathetic activity recorded in animals and humans may provide additional information about the neural control of the circulation. Over the last century, data obtained by the analysis of sympathetic activity in experimental animals, and recently also in humans, have provided fundamental contributions to our understanding of the physiological mechanisms involved in the neural control of circulation, as well as how these are altered in cardiovascular and non-cardiovascular diseases. The aim of this paper is to address some aspects related to the recording, analysis and interpretation of sympathetic activity in rats and humans, with special emphasis on analysis in the frequency domain.

Baroreflex control of lumbar and renal sympathetic nerve activity in conscious rats.

This study compared the baroreflex control of lumbar and renal sympathetic nerve activity (SNA) in conscious rats. Arterial pressure (AP) and lumbar and renal SNA were simultaneously recorded in six freely behaving rats. Pharmacological estimates of lumbar and renal sympathetic baroreflex sensitivity (BRS) were obtained by means of the sequential intravenous administration of sodium nitroprusside and phenylephrine. Sympathetic BRS was significantly (P < 0.05) lower for lumbar [3.0 +/- 0.4 normalized units (NU)/mmHg] than for renal (7.6 +/- 0.6 NU/mmHg) SNA. During a 219-min baseline period, spontaneous lumbar and renal BRS were continuously assessed by computing the gain of the transfer function relating AP and SNA at heart rate frequency over consecutive 61.4-s periods. The transfer gain was considered only when coherence between AP and SNA significantly differed from zero, which was verified in 99 +/- 1 and 96 +/- 3% of cases for lumbar and renal SNA, respectively. When averaged over the entire baseline period, spontaneous BRS was significantly (P < 0.05) lower for lumbar (1.3 +/- 0.2 NU/mmHg) than for renal (2.3 +/- 0.3 NU/mmHg) SNA. For both SNAs, spontaneous BRS showed marked fluctuations (variation coefficients were 26 +/- 2 and 28 +/- 2% for lumbar and renal SNA, respectively). These fluctuations were positively correlated in five of six rats (R = 0.44 +/- 0.06; n = 204 +/- 8; P < 0.0001). We conclude that in conscious rats, the baroreflex control of lumbar and renal SNA shows quantitative differences but is modulated in a mostly coordinated way.