The sympathetic nervous system in hypertension: assessment by blood pressure variability and ganglionic blockade

Pathophysiology of Hypertension: The Mosaic Theory and Beyond

Dr Irvine Page proposed the Mosaic Theory of Hypertension in the 1940s advocating that hypertension is the result of many factors that interact to raise blood pressure and cause end-organ damage. Over the years, Dr Page modified his paradigm, and new concepts regarding oxidative stress, inflammation, genetics, sodium homeostasis, and the microbiome have arisen that allow further refinements of the Mosaic Theory. A constant feature of this approach to understanding hypertension is that the various nodes are interdependent and that these almost certainly vary between experimental models and between individuals with hypertension. This review discusses these new concepts and provides an introduction to other reviews in this compendium of Circulation Research.

Contribution of the Renal Nerves to Hypertension in a Rabbit Model of Chronic Kidney Disease

Overactivity of the sympathetic nervous system and high blood pressure are implicated in the development and progression of chronic kidney disease (CKD) and independently predict cardiovascular events in end-stage renal disease. To assess the role of renal nerves, we determined whether renal denervation (RDN) altered the hypertension and sympathoexcitation associated with a rabbit model of CKD. The model involves glomerular layer lesioning and uninephrectomy, resulting in renal function reduced by one-third and diuresis. After 3-week CKD, blood pressure was 13±2 mm Hg higher than at baseline (P<0.001), and compared with sham control rabbits, renal sympathetic nerve activity was 1.2±0.5 normalized units greater (P=0.01). The depressor response to ganglion blockade was also +8.0±3 mm Hg greater, but total norepinephrine spillover was 8.7±3.7 ng/min lower (bothP<0.05). RDN CKD rabbits only increased blood pressure by 8.0±1.5 mm Hg. Renal sympathetic activity, the response to ganglion blockade and diuresis were similar to sham denervated rabbits (non-CKD). CKD rabbits had intact renal sympathetic baroreflex gain and range, as well as normal sympathetic responses to airjet stress. However, hypoxia-induced sympathoexcitation was reduced by −9±0.4 normalized units. RDN did not alter the sympathetic response to hypoxia or airjet stress. CKD increased oxidative stress markers Nox5 and MCP-1 (monocyte chemoattractant protein-1) in the kidney, but RDN had no effect on these measures. Thus, RDN is an effective treatment for hypertension in this model of CKD without further impairing renal function or altering the normal sympathetic reflex responses to various environmental stimuli.

Association between ambulatory blood pressure variability and frailty among older hypertensive patients

Frailty and hypertension often coexist and are increasingly prevalent with advancing age. Although hypertension is independently associated with frailty, whether high blood pressure variability affecting frailty is unclear. In this retrospective study, we consecutively enrolled elderly patients with essential hypertension undergoing 24-hour ABPM. The frailty was assessed by a 38-item frailty index. The parameters of blood pressure variability of ABPM, including ARV, coefficient of CV, SD, and weighed SD were calculated. Ordinal logistic regression was used to investigate the association between blood pressure variability and frailty. A total of 242 hypertensive patients were recruited and divided into the frail group, pre-frail group, and non-frail group. The overall magnitudes of BP variability, assessed by ARV, CV, SD, and weighed SD, were significantly greater in patients with frailty than those with pre-frailty and non-frailty. With adjustment for covariates, ARV of 24-hour, diurnal, and nocturnal SBP were independently associated with frailty (24 hours, OR: 2.48, 95% CI: 2.01-3.07; daytime, OR: 1.83, 95% CI: 1.60-2.10; nighttime, OR: 1.19, 95% CI: 1.12-1.27). The CV of 24-hour, diurnal, and nocturnal SBP was independently associated with frailty in the study (24 hours, OR: 1.2, 95% CI: 1.05-3.07; daytime, OR: 1.19, 95% CI: 1.05-1.34; nighttime, OR: 1.13, 95% CI: 1.03-1.24). For SD and weighed SD, only 24-hour systolic SD was independent risk factor associated with frailty (OR: 1.12, 95% CI: 1.01-1.23). The greater blood pressure variability of SBP, particular ARV and CV, were independent risk factors associated with higher-order frailty status. Longitudinal studies are needed to investigate the causality associations between hypertension and frailty.

Evaluation of choroidal thickness via enhanced depth-imaging optical coherence tomography in patients with systemic hypertension

Purpose:

The purpose was to evaluate choroidal thickness via spectral domain optical coherence tomography (SD-OCT) and to compare the data with those of 24-h blood pressure monitoring, elastic features of the aorta, and left ventricle systolic functions, in patients with systemic hypertension.

Materials and Methods:

This was a case-control, cross-sectional prospective study. A total of 116 patients with systemic hypertension, and 116 healthy controls over 45 years of age, were included. Subfoveal choroidal thickness (SFCT) was measured using a Heidelberg SD-OCT platform operating in the enhanced depth imaging mode. Patients were also subjected to 24-h ambulatory blood pressure monitoring (ABPM) and standard transthoracic echocardiography (STTE). Patients were divided into dippers and nondippers using ABPM data and those with or without left ventricular hypertrophy (LVH+ and LVH-) based on STTE data. The elastic parameters of the aorta, thus aortic strain (AoS), the beta index (BI), aortic distensibility (AoD), and the left ventricular mass index (LVMI), were calculated from STTE data.

Results:

No significant difference in SFCT was evident between patients and controls (P ≤ 0.611). However, a significant negative correlation was evident between age and SFCT in both groups (r = −0.66/−0.56, P ≤ 0.00). No significant SFCT difference was evident between the dipper and nondipper groups (P ≤ 0.67), or the LVH (+) and LVH (-) groups (P ≤ 0.84). No significant correlation was evident between SFCT and any of AoS, BI, AoD, or LVMI.

Discussion:

The choroid is affected by atrophic changes associated with aging. Even in the presence of comorbid risk factors including LVH and arterial stiffness, systemic hypertension did not affect SFCT.

Current Approaches to Quantifying Tonic and Reflex Autonomic Outflows Controlling Cardiovascular Function in Humans and Experimental Animals

The role of the autonomic nervous system in the pathophysiology of human and experimental models of cardiovascular disease is well established. In the recent years, there have been some rapid developments in the diagnostic approaches used to assess and monitor autonomic functions. Although most of these methods are devoted for research purposes in laboratory animals, many have still found their way to routine clinical practice. To name a few, direct long-term telemetry recording of sympathetic nerve activity (SNA) in rodents, single-unit SNA recording using microneurography in human subjects and spectral analysis of blood pressure and heart rate in both humans and animals have recently received an overwhelming attention. In this article, we therefore provide an overview of the methods and techniques used to assess tonic and reflex autonomic functions in humans and experimental animals, highlighting current advances available and procedure description, limitations and usefulness for diagnostic purposes.

Sympathetic overactivity prevails over the vascular amplifier phenomena in a chronic kidney disease rat model of hypertension

We examined whether increased sympathetic nerve activity (SNA) accounts for enhanced depressor responses to ganglionic blockade in the Lewis polycystic kidney (LPK) model of chronic kidney disease (CKD) or whether it reflects increased vascular responses to vasodilation (vascular amplifier). Under urethane anesthesia, depressor responses to ganglionic blockade (hexamethonium, 0.5–40 mg/kg i.v.), and direct vasodilation (sodium nitroprusside [SNP], 2.5–40 μg/kg i.v. and adenosine, 3–300 μg/kg i.v.) were compared in the LPK with normotensive Lewis and spontaneously hypertensive rats (SHR) (total n = 37). Hexamethonium (8 mg/kg) produced a greater depressor response in the LPK (−51 ± 3 mmHg) compared with Lewis (−31 ± 3 mmHg, P <0.05) but not SHR (−46 ± 3 mmHg). In LPK, the ratio of the hexamethonium/vasodilator MAP responses was greater when compared with Lewis (hexamethonium/SNP 1.34 ± 0.1 vs. 0.9 ± 0.09 and hexamethonium/adenosine: 2.28 ± 0.3 vs. 1.16 ± 0.1, both P <0.05) but not SHR. Results for systolic blood pressure (SBP) were comparable. The slope of the relationship between the fall in SBP induced by hexamethonium and normalized low frequency (LF_nu) power was also greater in the LPK (17.93 ± 3.26 mmHg/LF_nu) compared with Lewis (2.78 ± 0.59 mmHg/LF_nu, P =0.001) and SHR (3.36 ±0.72 mmHg/LF_nu, P =0.003). These results indicate that in the LPK, sympathetic activity predominates over any vascular amplifier effect, supporting increased sympathetic vasomotor tone as a major contributor to hypertension in this model of CKD.

We investigated whether enhanced depressor responses to the ganglionic blocker hexamethonium is due to sympathetic overactivity or exaggerated vascular responses to vasodilation (vascular amplifier) in the Lewis polycystic kidney (LPK) rodent model of chronic kidney disease (CKD) compared to Lewis and spontaneously hypertensive rats (SHR). The main finding of our study is that sympathetic overactivity predominates over vascular amplification in response to ganglionic blockade in the LPK, indicating that increased sympathetic vasomotor tone is a major contributor to the hypertension in this model of CKD.

Enhanced GRK2 expression and desensitization of betaAR vasodilatation in hypertensive patients

Increased levels of G protein coupled receptor kinase GRK2 appear to participate in hypertension presumably through the desensitization of beta adrenergic receptors (betaARs) that mediate vasodilatation. There are contrasting data on the occurrence of betaAR desensitization in the vasculature, we therefore investigated betaAR vasodilatation and desensitization in normotensives and in hypertensive humans. In blood lymphocytes, we assessed betaAR signaling and GRK2 expression and found betaAR signaling alterations and, consistent with desensitization, ncreased GRK2 levels in hypertensives. We studied in vivo vasodilatation to the betaAR agonist isoproterenol (ISO) injected in the brachia artery in control conditions and during the concomitant infusion of heparin, a known in vitro nonspecific GRK inhibitor. ISO induced a dose-dependent vasorelaxation that was attenuated in hypertensives indicating a loss of betaAR signaling. Intra-arterial infusion of heparin nhibited lymphocyte GRK2 activity and prevented desensitization of betaAR vasodilatation in normotensives. In hypertensives, heparin restored vasodilatation to ISO, to levels observed in normotensives. Our results suggest that betaAR desensitization does indeed occur at the vascular levels in vivo, and that heparin by acting as a GRK inhibitor prevents this in normotensives and restores impaired betaAR vasodilation in hypertensives. We conclude that desensitization participates to impaired betaAR vasodilation in hypertension.

Enhanced responses to ganglion blockade do not reflect sympathetic nervous system contribution to angiotensin II-induced hypertension

OBJECTIVE

We examined whether a specific increase in sympathetic nervous system (SNS) activity accounts for the enhanced depressor response to ganglion blockade in angiotensin II (AngII)-induced hypertension in rabbits or whether it reflects a general increased sensitivity of arterial pressure to vasodilatation.

METHODS

Rabbits were renal denervated or sham-operated and 2 weeks later AngII (50 ng/kg per min) infusion commenced. Mean arterial pressure (MAP) responses to ganglion blockade (pentolinium) and vasodilators nitroprusside and adenosine were measured 2-4 weeks later.

RESULTS

Basal MAP was 74 +/- 2 mmHg and maximum hypotensive responses to pentolinium, nitroprusside and adenosine were -17 +/- 2, -17 +/- 1 and -21 +/- 2 mmHg. AngII increased MAP similarly in intact and renal denervated rabbits (+25 +/- 4 mmHg and +31 +/- 4 mmHg, respectively). In intact rabbits, depressor responses to pentolinium were augmented by 75% during AngII infusion but responses to vasodilators also increased by 73-106% suggesting general augmentation of vascular reactivity rather than a specific increase in SNS neural activity. Consistent with this notion, total noradrenaline spillover was similar in normal and AngII-treated rabbits. In renal denervated rabbits, AngII enhanced depressor responses to vasodilators but not pentolinium, suggesting that sympathetic activity may be reduced by AngII hypertension when renal nerves are absent. In anaesthetized rabbits, methoxamine-induced decreases in hindlimb vascular conductance were greater in hypertensive than normotensive rabbits suggesting the presence of vascular hypertrophy of sufficient magnitude to explain increased responses to ganglion blockade and vasodilators.

CONCLUSION

Enhanced depressor responses to ganglion blockade in AngII hypertension do not reflect augmented SNS activity, but rather, augmented sympathetic vasoconstriction mediated by a vascular amplifier effect.