An improved strategy for evaluating the extent of chronic arterial baroreceptor denervation in conscious rats

There is no index or criterion of aortic barodenervation, nor can we differentiate among rats that have suffered chronic sham, aortic or sino-aortic denervation. The objective of this study was to develop a procedure to generate at least one quantitative, reproducible and validated index that precisely evaluates the extent of chronic arterial barodenervation performed in conscious rats. Data from 79 conscious male Wistar rats of about 65-70 days of age with diverse extents of chronic arterial barodenervation and used in previous experiments were reanalyzed. The mean arterial pressure (MAP) and the heart rate (HR) of all rats were measured systematically before (over 1 h) and after three consecutive iv bolus injections of phenylephrine (PHE) and sodium nitroprusside (SNP). Four expressions of the effectiveness of barodenervation (MAP lability, PHE ratio, SNP ratio, and SNP-PHE slope) were assessed with linear fixed models, three-level average variance, average separation among levels, outlier box plot analysis, and overlapping graphic analysis. The analysis indicated that a) neither MAP lability nor SNP-PHE slope was affected by the level of chronic sodium intake; b) even though the Box-Cox transformations of both MAP lability [transformed lability index (TLI)] and SNP-PHE slope [transformed general sensitivity index (TGSI), {((3-(ΔHRSNP-ΔHRPHE/ΔMAPSNP-ΔMAPPHE))-0.4-1)/-0.04597}] could be two promising indexes, TGSI proved to be the best index; c) TLI and TGSI were not freely interchangeable indexes for this purpose. TGSI ranges that permit differentiation between sham (10.09 to 11.46), aortic (8.40 to 9.94) and sino-aortic (7.68 to 8.24) barodenervated conscious rats were defined.

Evidence that blood pressure remains under the control of arterial baroreceptors in renal hypertensive rats

The purpose of the present study was to determine the range of the influence of the baroreflex on blood pressure in chronic renal hypertensive rats. Supramaximal electrical stimulation of the aortic depressor nerve and section of the baroreceptor nerves (sinoaortic denervation) were used to obtain a global analysis of the baroreceptor-sympathetic reflex in normotensive control and in chronic (2 months) 1-kidney, 1-clip hypertensive rats. The fall in blood pressure produced by electrical baroreceptor stimulation was greater in renal hypertensive rats than in normotensive controls (right nerve: -47 ± 8 vs -23 ± 4 mmHg; left nerve: -51 ± 7 vs -30 ± 4 mmHg; and both right and left nerves: -50 ± 8 vs -30 ± 4 mmHg; P < 0.05). Furthermore, the increase in blood pressure level produced by baroreceptor denervation in chronic renal hypertensive rats was similar to that observed in control animals 2-5 h (control: 163 ± 5 vs 121 ± 1 mmHg; 1K-1C: 203 ± 7 vs 170 ± 5 mmHg; P < 0.05) and 24 h (control: 149 ± 3 vs 121 ± 1 mmHg; 1K-1C: 198 ± 8 vs 170 ± 5 mmHg; P < 0.05) after sinoaortic denervation. Taken together, these data indicate that the central and peripheral components of the baroreflex are acting efficiently at higher arterial pressure in renal hypertensive rats when the aortic nerve is maximally stimulated or the activity is abolished.

Partial baroreceptor dysfunction and low plasma nitric oxide bioavailability as determinants of salt-sensitive hypertension: a reverse translational rat study

This study determined whether clinical salt-sensitive hypertension (cSSHT) results from the interaction between partial arterial baroreceptor impairment and a high-sodium (HNa) diet. In three series (S-I, S-II, S-III), mean arterial pressure (MAP) of conscious male Wistar ChR003 rats was measured once before (pdMAP) and twice after either sham (SHM) or bilateral aortic denervation (AD), following 7 days on a low-sodium (LNa) diet (LNaMAP) and then 21 days on a HNa diet (HNaMAP). The roles of plasma nitric oxide bioavailability (pNOB), renal medullary superoxide anion production (RMSAP), and mRNA expression of NAD(P)H oxidase and superoxide dismutase were also assessed. In SHM (n=11) and AD (n=15) groups of S-I, LNaMAP-pdMAP was 10.5±2.1 vs 23±2.1 mmHg (P<0.001), and the salt-sensitivity index (SSi; HNaMAP−LNaMAP) was 6.0±1.9 vs 12.7±1.9 mmHg (P=0.03), respectively. In the SHM group, all rats were normotensive, and 36% were salt sensitive (SSi≥10 mmHg), whereas in the AD group ∼50% showed cSSHT. A 45% reduction in pNOB (P≤0.004) was observed in both groups in dietary transit. RMSAP increased in the AD group on both diets but more so on the HNa diet (S-II, P<0.03) than on the LNa diet (S-III, P<0.04). MAP modeling in rats without a renal hypertensive genotype indicated that the AD*HNa diet interaction (P=0.008) increases the likelihood of developing cSSHT. Translationally, these findings help to explain why subjects with clinical salt-sensitive normotension may transition to cSSHT.

Baroreflex control of sympathetic activity in experimental hypertension

The arterial baroreceptor reflex system is one of the most powerful and rapidly acting mechanisms for controlling arterial pressure. The purpose of the present review is to discuss data relating sympathetic activity to the baroreflex control of arterial pressure in two different experimental models: neurogenic hypertension by sinoaortic denervation (SAD) and high-renin hypertension by total aortic ligation between the renal arteries in the rat. SAD depresses baroreflex regulation of renal sympathetic activity in both the acute and chronic phases. However, increased sympathetic activity (100%) was found only in the acute phase of sinoaortic denervation. In the chronic phase of SAD average discharge normalized but the pattern of discharges was different from that found in controls. High-renin hypertensive rats showed overactivity of the renin angiotensin system and a great depression of the baroreflexes, comparable to the depression observed in chronic sinoaortic denervated rats. However, there were no differences in the average tonic sympathetic activity or changes in the pattern of discharges in high-renin rats. We suggest that the difference in the pattern of discharges may contribute to the increase in arterial pressure lability observed in chronic sinoaortic denervated rats.

Effect of baroreceptor denervation on the autonomic control of arterial pressure in conscious mice

This study evaluated the role of arterial baroreceptors in arterial pressure (AP) and pulse interval (PI) regulation in conscious C57BL mice. Male animals, implanted with catheters in a femoral artery and a jugular vein, were submitted to sino-aortic (SAD), aortic (Ao-X) or carotid sinus denervation (Ca-X), 5 daysprior to the experiments. After basal recording of AP, the lack of reflex bradycardia elicited by administration of phenylephrine was used to confirm the efficacy of SAD, and cardiac autonomic blockade with methylatropine and propranolol was performed. The AP and PI variability were calculated in the time and frequency domains (spectral analysis/fast Fourier transform) with the spectra quantified in low-(LF; 0.25-1Hz) and high-frequency bands (HF; 1-5Hz). Basal AP and AP variability were higher after SAD, Ao-X or Ca-X than in intact mice. Pulse interval was similar among the groups, whereas PI variability was lower after SAD. Atropine elicited a slight tachycardia in control mice but did not change PI after total or partial denervation. The bradycardia caused by propranolol was higher after SAD, Ao-X or Ca-X compared with intact mice. The increase in the variability of AP was accompanied by a marked increase in the LF and HF power of the AP spectra after baroreceptor denervation. The LF and HF power of the PI were reduced by SAD and by Ao-X or Ca-X. Therefore, both sino-aortic and partial baroreceptor denervation in mice elicits hypertension and a remarkable increase in AP variability and cardiac sympathetic tonus. Spectral analysis showed an important contribution of the baroreflex in the power of LF oscillations of the PI spectra. Both sets of baroreceptors seem to be equally important in the autonomic regulation of the cardiovascular system in mice.

Effects of chronic baroreceptor stimulation on the autonomic cardiovascular regulation in patients with drug-resistant arterial hypertension

In patients with drug-resistant hypertension, chronic electric stimulation of the carotid baroreflex is an investigational therapy for blood pressure reduction. We hypothesized that changes in cardiac autonomic regulation can be demonstrated in response to chronic baroreceptor stimulation, and we analyzed the correlation with blood pressure changes. Twenty-one patients with drug-resistant hypertension were prospectively included in a substudy of the Device Based Therapy in Hypertension Trial. Heart rate variability and heart rate turbulence were analyzed using 24-hour ECG. Recordings were obtained 1 month after device implantation with the stimulator off and after 3 months of chronic electric stimulation (stimulator on). Chronic baroreceptor stimulation decreased office blood pressure from 185+/-31/109+/-24 mm Hg to 154+/-23/95+/-16 mm Hg (P<0.0001/P=0.002). Mean heart rate decreased from 81+/-11 to 76+/-10 beats per minute(-1) (P=0.001). Heart rate variability frequency-domain parameters assessed using fast Fourier transformation (FFT; ratio of low frequency:high frequency: 2.78 versus 2.24 for off versus on; P<0.001) were significantly changed during stimulation of the carotid baroreceptor, and heart rate turbulence onset was significantly decreased (turbulence onset: -0.002 versus -0.015 for off versus on; P=0.004). In conclusion, chronic baroreceptor stimulation causes sustained changes in heart rate variability and heart rate turbulence that are consistent with inhibition of sympathetic activity and increase of parasympathetic activity in patients with drug-resistant systemic hypertension; these changes correlate with blood pressure reduction. Whether the autonomic modulation has favorable cardiovascular effects beyond blood pressure control should be investigated in further studies.

Chronic absence of baroreceptor inputs prevents training-induced cardiovascular adjustments in normotensive and spontaneously hypertensive rats

We investigate whether arterial baroreceptors mediate the training-induced blood pressure fall and resting bradycardia in hypertensive (SHR) and normotensive rats (WKY). Male SHR and WKY rats, submitted to sino-aortic denervation (SAD) or sham surgery (SHAM group), were allocated to training (T; 55% of maximal exercise capacity) or sedentary (S) protocols for 3 months. Rats were instrumented with arterial and venous catheters for haemodynamic measurements at rest (power spectral analysis) and baroreceptor testing. Kidney and skeletal muscles were processed for morphometric analysis of arterioles. Elevated mean arterial pressure (MAP) and heart rate (HR) in SHAM SHRS were accompanied by increased sympathetic variability and arteriolar wall/lumen ratio [+3.4-fold on low-frequency (LF) power and +70%, respectively, versus WKYS, P < 0.05]. Training caused significant HR (similar to 9% in WKY and SHR) and MAP reductions (-8% in the SHR), simultaneously with improvement of baroreceptor reflex control of HR (SHR and WKY), LF reduction (with a positive correlation between LF power and MAP levels in the SHR) and normalization of wall/lumen ratio of the skeletal muscle arterioles (SHR only). In contrast, SAD increased pressure variability in both strains of rats, causing reductions in MAP (-13%) and arteriolar wall/lumen ratio (-35%) only in the SHRS. Training effects were completely blocked by SAD in both strains; in addition, after SAD the resting MAP and HR and the wall/lumen ratio of skeletal muscle arterioles were higher in SHRT versus SHRS and similar to those of SHAM SHRS. The lack of training-induced effects in the chronic absence of baroreceptor inputs strongly suggests that baroreceptor signalling plays a decisive role in driving beneficial training-induced cardiovascular adjustments.

Reflex control of arterial pressure and heart rate in short-term streptozotocin diabetic rats

Impaired baroreflex sensitivity in diabetes is well described and has been attributed to autonomic diabetic neuropathy. In the present study conducted on acute (10-20 days) streptozotocin (STZ)-induced diabetic rats we examined: 1) cardiac baroreflex sensitivity, assessed by the slope of the linear regression between phenylephrine- or sodium nitroprusside-induced changes in arterial pressure and reflex changes in heart rate (HR) in conscious rats; 2) aortic baroreceptor function by means of the relationship between systolic arterial pressure and aortic depressor nerve (ADN) activity, in anesthetized rats, and 3) bradycardia produced by electrical stimulation of the vagus nerve or by the iv injection of methacholine in anesthetized animals. Reflex bradycardia (-1.4 ± 0.1 vs -1.7 ± 0.1 bpm/mmHg) and tachycardia (-2.1 ± 0.3 vs -3.0 ± 0.2 bpm/mmHg) were reduced in the diabetic group. The gain of the ADN activity relationship was similar in control (1.7 ± 0.1% max/mmHg) and diabetic (1.5 ± 0.1% max/mmHg) animals. The HR response to vagal nerve stimulation with 16, 32 and 64 Hz was 13, 16 and 14% higher, respectively, than the response of STZ-treated rats. The HR response to increasing doses of methacholine was also higher in the diabetic group compared to control animals. Our results confirm the baroreflex dysfunction detected in previous studies on short-term diabetic rats. Moreover, the normal baroreceptor function and the altered HR responses to vagal stimulation or methacholine injection suggest that the efferent limb of the baroreflex is mainly responsible for baroreflex dysfunction in this model of diabetes.

Acute and chronic electrical activation of baroreceptor afferents in awake and anesthetized subjects

Electrical stimulation of baroreceptor afferents was used in the 1960's in several species, including human beings, for the treatment of refractory hypertension. This approach bypasses the site of baroreceptor mechanosensory transduction. Chronic electrical stimulation of arterial baroreceptors, particularly of the carotid sinus nerve (Hering's nerve), was proposed as an ultimate effort to treat refractory hypertension and angina pectoris due to the limited nature of pharmacological therapy available at that time. Nevertheless, this approach was abandoned in the early 1970's due to technical limitations of implantable devices and to the development of better-tolerated antihypertensive medications. More recently, our laboratory developed the technique of electrical stimulation of the aortic depressor nerve in conscious rats, enabling access to hemodynamic responses without the undesirable effect of anesthesia. In addition, electrical stimulation of the aortic depressor nerve allows assessment of the hemodynamic responses and the sympathovagal balance of the heart in hypertensive rats, which exhibit a well-known decrease in baroreflex sensitivity, usually attributed to baroreceptor ending dysfunction. Recently, there has been renewed interest in using electrical stimulation of the carotid sinus, but not the carotid sinus nerve, to lower blood pressure in conscious hypertensive dogs as well as in hypertensive patients. Notably, previous undesirable technical outcomes associated with electrical stimulation of the carotid sinus nerve observed in the 1960's and 1970's have been overcome. Furthermore, promising data have been recently reported from clinical trials that evaluated the efficacy of carotid sinus stimulation in hypertensive patients with drug resistant hypertension.

Lesions of the commissural nucleus of the solitary tract reduce arterial pressure in spontaneously hypertensive rats

It has been suggested that increased sympathetic activity and arterial chemoreceptors are important for the high blood pressure in spontaneously hypertensive rats (SHR). Electrolytic lesions of the commissural nucleus of the solitary tract (commNTS) abolish (1) the cardiovascular responses to chemoreflex activation with potassium cyanide (KCN) in normotensive rats and (2) the hypertension that follows acute aortic baroreceptor denervation in rats. Therefore, in this study we investigated the effects of electrolytic lesions of the commNTS on basal mean arterial pressure (MAP), baroreflex, and chemoreflex in SHR and in normotensive control Wistar-Kyoto (WKY) and Wistar rats. CommNTS lesions elicited a dramatic fall in MAP to normal levels during the period of Study (from the first to fourth day following lesions) in SHR and almost no changes in WKY and Wistar rats. The pressor responses to chemoreflex activation with KCN tested in the days 1 and 4 after commNTS lesions were abolished in SHR and in normotensive strains. The reflex tachycardia induced by sodium nitroprusside was also attenuated in days 1 and 4 after commNTS lesions in SHR, WKY, and Wistar rats. The data suggest that the integrity of commNTS is important for the maintenance or high blood pressure in SHR and for the reflex responses dependent on sympathetic activation either in SHR or in normotensive strains.

CHANGES IN VASCULAR-RESISTANCE DURING CAROTID OCCLUSION IN NORMAL AND BARORECEPTOR-DENERVATED RATS

In the present study, we investigated changes in mesenteric, renal, and hindquarter vascular resistance during the pressor response produced by bilateral carotid occlusion (BCO) in conscious, freely moving normal and denervated (aortic, carotid, or both) rats. BCO was performed using special previously implanted cuffs. In control normal rats, the increase in mean arterial pressure (MAP) during early and late responses (37 +/- 4 and 21 +/- 2 mm Hg, respectively) was related to increased renal (125 +/- 12% and 45 +/- 10%) and mesenteric (38 +/- 13% and 41 +/- 5%) but not hindquarter (14 +/- 4% and 8 +/- 7%) vascular resistance. In aortic-denervated rats, the greater MAP increase in early and late responses (57 +/- 4 and 44 +/- 4 mm Hg, respectively) compared with normal rats was related to a marked increase in hindquarter (137 +/- 26% and 106 +/- 26%) and mesenteric (104 +/- 14% and 66 +/- 9%) vascular resistance. In carotid-denervated rats, MAP increase and change in vascular resistance were similar to those values observed in control rats. Sinoaortic-denervated rats showed a greater MAP increase (34 +/- 4 mm Hg) during late response and a reduced increase in renal vascular resistance (46 +/- 6%) during early response. The present results show that 1) the pressor response to BCO in normal rats is associated with an increase in renal and mesenteric vascular resistance, 2) the aortic baroreceptors buffer the increase in mesenteric and especially hindquarter vascular resistance during BCO, and 3) the reduced pressor response in late response is probably related to a reduced increase in renal vascular resistance during this component compared with the early response.

Commissural NTS lesions and cardiovascular responses in aortic baroreceptor-denervated rats

Both acute (1 day) lesions of the commissural nucleus of the solitary tract (commNTS) and aortic baroreceptor denervation increase pressor responses to bilateral common carotid occlusion (BCO) during a 60-second period in conscious rats. In this study, we investigated the following: (1) the effects of commNTS lesions on basal mean arterial pressure (MAP) and heart rate (HR) of aortic denervated (ADNx) rats; (2) the effects of acute commNTS lesions on pressor responses to BCO in ADNx rats; and (3) the effects of chronic (10 days) commNTS lesions on the pressor response to BCO. ADNx increased basal MAP and HR in sham-lesioned rats. Acute commNTS lesions abolished the MAP and HR increases observed in ADNx rats. Acute commNTS lesions increased the pressor responses to BCO in rats with intact- baroreceptor innervation but produced no additional change in the pressor response to BCO in ADNx rats. Chronic commNTS lesions did not change the pressor responses to BCO in rats with intact-baroreceptor innervation. The data show that acute commNTS lesions abolish the MAP increase produced by aortic baroreceptor denervation. They also suggest that acute commNTS lesions enhance the pressor response to BCO by partial withdrawal of aortic baroreceptor inputs into the NTS. Chronically, reorganization in the remaining aortic baroreceptor or in the baroreflex function as a whole might produce normalization of the cardiovascular responses to BCO.

Cardiac and pulmonary arterial remodeling after sinoaortic denervation in normotensive rats

Blood pressure variability (BPV) and baroreflex dysfunction may contribute to end-organ damage process. We investigated the effects of baroreceptor deficit (10 weeks after sinoaortic denervation - SAD) on hemodynamic alterations, cardiac and pulmonary remodeling. Cardiac function and morphology of male Wistar intact rats (C) and SAD rats (SAD) (n = 8/group) were assessed by echocardiography and collagen quantification. BP was directly recorded. Ventricular hypertrophy was quantified by the ratio of left ventricular weight (LVW) and right ventricular weight (RVW) to body weight (BW). BPV was quantified in the time and frequency domains. The atrial natriuretic peptide (ANP), alpha-skeletal actin (alpha-skelectal), collagen type I and type III genes mRNA expression were evaluated by RT-PCR. SAD did not change BP, but increased BPV (11 +/- 0.49 vs. 5 +/- 0.3 mm Hg). As expected, baroreflex was reduced in SAD. Pulmonary artery acceleration time was reduced in SAD. In addition, SAD impaired diastolic function in both LV (6.8 +/- 0.26 vs. 5.02 +/- 0.21 mm Hg) and RV (5.1 +/- 0.21 vs. 4.2 +/- 0.12 mm Hg). SAD increased LVW/BW in 9% and RVW/BW in 20%, and augmented total collagen (3.8-fold in LV, 2.7-fold in RV, and 3.35-fold in pulmonary artery). Also, SAD increased type I (similar to 6-fold) and III (similar to 5-fold) collagen gene expression. Denervation increased ANP expression in LV (75%), in RV (74%) and increased a-skelectal expression in LV (300%) and in RV (546%). Baroreflex function impairment by SAD, despite not changing BP, induced important adjustments in cardiac structure and pulmonary hypertension. These changes may indicate that isolated baroreflex dysfunction can modulate target tissue damage. (C) 2011 Elsevier B.V. All rights reserved.

Baroreceptor-mediated activation of sympathetic nerve activity to salivary glands

Salivary gland function is regulated by both the sympathetic and parasympathetic nervous systems. Previously we showed that the basal sympathetic outflow to the salivary glands (SNA(SG)) was higher in hypertensive compared to normotensive rats and that diabetes reduced SNA(SG) discharge at both strains. In the present study we sought to investigate how SNA(SG) might be modulated by acute changes in the arterial pressure and whether baroreceptors play a functional role upon this modulation. To this end, we measured blood pressure and SNA(SG) discharge in Wistar-Kyoto rats (WRY-intact) and in WRY submitted to sinoaortic denervation (WRY-SAD). We made the following three major observations: (i) in WRY-intact rats, baroreceptor loading in response to intravenous infusion of the phenylephrine evoked an increase in SNA(SG) spike frequency (81%, p<0.01) accompanying the increase mean arterial pressure ((sic)MAP: +77 +/- 14 mmHg); (ii) baroreceptor unloading with sodium nitroprusside infusion elicited a decrease in SNA(SG) spike frequency (17%, p<0.01) in parallel with the fall in arterial blood pressure ((sic)MAP: 30 3 mmHg) in WRY-intact rats; iii) in the WRY-SAD rats, phenylephrine-evoked rises in the arterial pressure ((sic)MAP: +56 +/- 6 mmHg) failed to produce significant changes in the SNA(SG) spike frequency. Taken together, these data show that SNA(SG) increases in parallel with pharmacological-induced pressor response in a baroreceptor dependent way in anaesthetised rats. Considering the key role of SNA(SG) in salivary secretion, this mechanism, which differs from the classic cardiac baroreflex feedback loop, strongly suggests that baroreceptor signalling plays a decisive role in the regulation of salivary gland function. (C) 2012 Elsevier Inc. All rights reserved.