Effects of rat sinoaortic denervation on the vagal responsiveness and expression of muscarinic acetylcholine receptors

Acute renal denervation normalizes aortic function and decreases blood pressure in spontaneously hypertensive rats

Mechanisms involved in the acute responses to renal denervation (RDN) have yet to be fully understood. We assessed urinary volume, autonomic control and aorta vascular reactivity after acute RDN. Male normotensive Wistar rats and spontaneously hypertensive rats (SHR) were divided into normotensive + RDN (ND) or sham surgery (NS), and hypertensive + RDN (HD) or sham surgery (HS). Metabolic parameters and hemodynamic measurements were recorded 72h and 4 days after intervention, respectively. Aortic rings were studied 7 days post RDN in an isometric myograph. Concentration–response curves to phenylephrine, sodium nitroprusside and acetylcholine (10^–10–10⁻⁵ M) were performed. Two-way ANOVA was used for group comparisons and differences reported when p < 0.05. Results are presented as mean ± SEM. Urinary volume was 112% higher in HD vs. HS (HS = 14.94 ± 2.5 mL; HD = 31.69 ± 2.2 mL) and remained unchanged in normotensive rats. Systolic BP was lower in HD rats (HS = 201 ± 12 vs. HD = 172 ± 3 mmHg) without changes in normotensive group. HD group showed increased HF and LF modulation (HS = 5.8 ± 0.7 ms²vs. HD = 13.4 ± 1.4 ms²; HS = 3.5 ± 0.7 ms²vs. HD = 10.5 ± 1.7 ms², respectively). RDN normalized vascular reactivity in HD rats and increased phenylephrine response in ND rats. Acute fall in BP induced by RDN is associated with increased urinary volume, which in turn may also have contributed to functional changes of the aorta.

Patterns of cardiovascular variability after long-term sino-aortic denervation in unanesthetized adult rats

Baroreflex dysfunction is a diffuse chronic condition that is expected to be followed by a profound loss of organization of BP and HR variability. Nevertheless, long-term effects of baroreflex withdrawal are still debated. Aim of our work was to study BP and HR changes long term after sino-aortic denervation (SAD). Inter-beat-interval (IBI) and intra-arterial BP were recorded beat-by-beat in 43 Wistar-Kyoto rats (Controls, n = 33; SAD rats, n = 10). Power spectra were calculated in controls and in SAD rats within three days and at seven months from denervation. Compared to controls, chronic SAD rats showed 1) similar mean BP (control vs SAD: 95 ± 16 vs 87 ± 22 mmHg) and IBI (171 ± 22 vs 181 ± 15 ms) values, 2) dramatically higher values of BP variance (12 ± 2 vs 64 ± 2 mmHg2, p < 0.01) and of ultra- (ULF) and very-low-frequency (VLF) BP oscillations, 3) dramatically higher values of IBI variability (24 ± 2 vs 71 ± 4 ms2, p < 0.01) and of ULF-IBI oscillations that were synchronized with BP oscillations. Chronic SAD rats reveal a marked change in the pattern of cardiovascular variability characterized by the appearance of synchronized slower oscillations of BP and HR. The cardiovascular system, therefore, retains a high level of organization despite the absence of a reflex control mechanism.

The role of the baroreflex and parasympathetic nervous system in fructose-induced cardiac and metabolic alterations

It is well-established that baroreflex sensitivity is essential for blood pressure control, and also plays a key role in the modulation of disease-induced metabolic alterations. In order to investigate the role of the baroreflex in the cardiometabolic and inflammatory derangements promoted by fructose overload, Wistar rats underwent sinoaortic denervation (SAD) or sham surgery and were studied 90 days after receiving tap water (Den and Ctrl) or a 10% fructose solution (Fruc and Den-Fruc). All experimental groups showed marked and similar degree of baroreflex impairment compared to Ctrl. As expected, fructose overload effectively induced metabolic syndrome; however, when it was associated with SAD, several alterations were attenuated. While Fruc rats displayed increased sympathetic modulation and tone and reduced vagal modulation compared to Ctrl animals, Den-Fruc rats showed greater vagal tone and modulation when compared to the Fruc group. Moreover, the Den-Fruc group showed augmented expression of β1 adrenergic receptors and TNF/IL-10 ratio and reduction of β2 in the left ventricle. The increase in vagal function was correlated with improved insulin sensitivity (r² = 0.76), and decreased abdominal fat (r² = -0.78) and β2 receptors (r² = -0.85). Our results showed that: (1) chronic fructose overload induced severe baroreflex impairment, i.e. in a similar magnitude to that observed in SAD rats, which is accompanied by cardiometabolic dysfunctions; (2) the compensatory enhancement in parasympathetic function in SAD rats submitted to fructose intake may point out the possibility of use of approaches that improve vagal function as therapeutic target to attenuate fructose-induced cardiometabolic dysfunctions.

Molecular Mechanisms Linking Autonomic Dysfunction and Impaired Cardiac Contractility in Critical Illness

OBJECTIVES

Molecular mechanisms linking autonomic dysfunction with poorer clinical outcomes in critical illness remain unclear. We hypothesized that baroreflex dysfunction alone is sufficient to cause cardiac impairment through neurohormonal activation of (nicotinamide adenine dinucleotide phosphate oxidase dependent) oxidative stress resulting in increased expression of G-protein-coupled receptor kinase 2, a key negative regulator of cardiac function.

DESIGN

Laboratory/clinical investigations.

SETTING

University laboratory/medical centers.

SUBJECTS

Adult rats; wild-type/nicotinamide adenine dinucleotide phosphate oxidase subunit-2-deficient mice; elective surgical patients.

INTERVENTIONS

Cardiac performance was assessed by transthoracic echocardiography following experimental baroreflex dysfunction (sino-aortic denervation) in rats and mice. Immunoblots assessed G-protein-coupled receptor recycling proteins expression in rodent cardiomyocytes and patient mononuclear leukocytes. In surgical patients, heart rate recovery after cardiopulmonary exercise testing, time/frequency measures of parasympathetic variables were related to the presence/absence of baroreflex dysfunction (defined by spontaneous baroreflex sensitivity of <6 ms mm Hg). The associations of baroreflex dysfunction with intraoperative cardiac function and outcomes were assessed.

MEASUREMENTS AND MAIN RESULTS

Experimental baroreflex dysfunction in rats and mice resulted in impaired cardiac contractility and upregulation of G-protein-coupled receptor kinase 2 expression. In mice, genetic deficiency of gp91 nicotinamide adenine dinucleotide phosphate oxidase subunit-2 prevented upregulation of G-protein-coupled receptor kinase 2 expression in conditions of baroreflex dysfunction and preserved cardiac function. Baroreflex dysfunction was present in 81 of 249 patients (32.5%) and was characterized by lower parasympathetic tone and increased G-protein-coupled receptor kinase 2 expression in mononuclear leukocytes. Baroreflex dysfunction in patients was also associated with impaired intraoperative cardiac contractility. Critical illness and mortality were more frequent in surgical patients with baroreflex dysfunction (relative risk, 1.66 [95% CI, 1.16-2.39]; p = 0.006).

CONCLUSIONS

Reduced baroreflex sensitivity is associated with nicotinamide adenine dinucleotide phosphate oxidase subunit-2-mediated upregulation of G-protein-coupled receptor kinase 2 expression in cardiomyocytes and impaired cardiac contractility. Autonomic dysfunction predisposes patients to the development of critical illness and increases mortality.

Baroreceptor reflex control of heart rate in angiotensin type 1A receptor knockout mice

The baroreceptor reflex dampens the short-term fluctuations in blood pressure by feedback modulation of heart rate (HR) and vascular resistance. Impairment of this reflex has been observed in hypertension and heart failure. Angiotensin II, a blood borne hormone, acts via its type 1A receptor to attenuate the baroreceptor reflex and this reflex is reported to be dramatically altered in angiotensin type 1A receptor knockout mice. This study sought to further investigate changes in the arterial and cardiopulmonary baroreceptor reflex control of HR in angiotensin II type 1A receptor knocked out mice. In artificially ventilated, isoflurane anesthetized mice, the arterial and cardiopulmonary baroreceptor reflexes were activated via injection or slow infusions, respectively, of phenylephrine and sodium nitroprusside through the jugular vein. We observed no impairment of either the arterial or cardiopulmonary baroreceptor reflex control of HR in angiotensin type 1A receptor knockout mice.

Tachycardia, reduced vagal capacity, and age-dependent ventricular dysfunction arising from diminished expression of the presynaptic choline transporter

Healthy cardiovascular function relies on a balanced and responsive integration of noradrenergic and cholinergic innervation of the heart. High-affinity choline uptake by cholinergic terminals is pivotal for efficient ACh production and release. To date, the cardiovascular impact of diminished choline transporter (CHT) expression has not been directly examined, largely due to the transporter's inaccessibility in vivo. Here, we describe findings from cardiovascular experiments using transgenic mice that bear a CHT genetic deficiency. Whereas CHT knockout (CHT(-/-)) mice exhibit early postnatal lethality, CHT heterozygous (CHT(+/-)) mice survive, grow, and reproduce normally and exhibit normal spontaneous behaviors. However, the CHT(+/-) mouse heart displays significantly reduced levels of high-affinity choline uptake accompanied by significantly reduced levels of ACh. Telemeterized recordings of cardiovascular function in these mice revealed tachycardia and hypertension at rest. After treadmill exercise, CHT(+/-) mice exhibited slower heart rate recovery, consistent with a diminished cholinergic reserve, a contention validated through direct vagal nerve stimulation. Echocardiographic and histological experiments revealed an age-dependent decrease in fractional shortening, increased left ventricular dimensions, and increased ventricular fibrosis, consistent with ventricular dysfunction. These cardiovascular phenotypes of CHT(+/-) mice encourage an evaluation of humans bearing reduced CHT expression for their resiliency in maintaining proper heart function as well as risk for cardiovascular disease.