Central nervous system monoamine neurotransmitter turnover in primary and obesity-related human hypertension

The Role of Central Nervous System Mechanisms in Resistant Hypertension

Arterial hypertension remains a primary global health problem with significant impact on cardiovascular morbidity and mortality. The low rate of hypertension control and failure to achieve target blood pressure levels particularly among high-risk patients with resistant hypertension has triggered renewed interest in unravelling the underlying mechanisms to implement therapeutic approaches for better patient management. Here, we summarize the crucial role of neurogenic mechanisms in drug-resistant hypertension, with a specific focus on central control of blood pressure, the factors involved in central integration of afferent signalling to increase sympathetic drive in resistant hypertension, and briefly review recently introduced interventional strategies distinctively targeting sympathetic activation.

Predictors of blood pressure response: Obesity is associated with a less pronounced treatment response after renal denervation

OBJECTIVES

The purpose of this study was to identify predictors of BP response.

BACKGROUND

Catheter-based renal denervation (RDN) causes significant blood pressure (BP) reductions in a large number of patients with resistant hypertension.

METHODS

One hundred one consecutive patients with resistant hypertension who underwent RDN with the Symplicity™ catheter were included in this retrospective study. Primary endpoint was the change in office systolic BP after 6 months. Uni- and multivariate logistic regression analyses were performed to detect baseline predictors of a significant BP response 6 months after RDN (age, gender, office and ambulatory BP, renal function, body mass index [BMI], diabetes mellitus, antihypertensive medication, number of ablations).

RESULTS

The procedure was technically uneventful in all patients. Mean BP at baseline was 166.6/90.2 ± 22.5/16.4 mmHg and decreased by -14.7 (P < 0.0001)/-5.3 (P < 0.001) ± 22.8/14.1 mm Hg at 6-month follow-up. Similarly, paired analysis of 24-hr-ambulatory BP measurement (n = 71) showed a significant reduction of mean systolic BP by 6.8 ± 14.4 mm Hg (P < 0.0002). Upon univariate analyses, a higher baseline office systolic BP (P < 0.0001) and lower BMI (P = 0.014) were identified as significant predictors of the magnitude of BP response after 6 months. Importantly, on multivariate analysis, baseline office systolic BP (standardized ß = -0.46; r = -0.47; P < 0.0001) and BMI (standardized ß = 0.21; r = 0.95; P = 0.019) remained significant.

CONCLUSION

Blood pressure reductions after RDN were more pronounced in patients with higher baseline blood pressure and lower BMI. These findings may have implications regarding patient selection for renal denervation. © 2015 Wiley Periodicals, Inc.

The complex interaction between overweight, hypertension, and sympathetic overactivity

There is ample evidence in the epidemiological and clinical literature that hypertension and overweight are closely and causally interrelated. Sympathetic nervous system (SNS) overactivity has been well documented in both hypertension and overweight, but it is not clear whether this is a coincidental finding or whether the association reflects a mechanistic role of SNS in these two interrelated clinical conditions. Whereas in this review we focus on the evidence for a primary role of SNS in the development of hypertension and overweight, it is clear that the process can be initiated from other starting points such as primary overeating or sleep apnea. After overweight evolves, hormones secreted by fat cells further accelerate SNS overactivity, weight gain, and blood pressure increase. The main thesis of this article is that regardless of where the process started, the same clinical picture of hypertension, overweight, and SNS overactivity will emerge. There is good evidence that in genetically prone individuals, prolonged SNS stimulation elicits a down regulation of beta-adrenergic receptors. This in turn decreases the ability to dissipate calories and diminishes the beta-adrenoceptor-mediated vasodilatation. We hypothesize that beta-adrenoceptor downregulation is the linchpin in the association of SNS with overweight and hypertension.

Blood Pressure Control at Rest and during Exercise in Obese Children and Adults

The hemodynamic responses to exercise have been studied to a great extent over the past decades, and an exaggerated blood pressure response during an acute exercise bout has been considered as an indicator of cardiovascular risk. Obesity is a major factor influencing the blood pressure response to exercise since evidence indicates that the arterial pressure response to exercise is exacerbated in obese compared with lean adults. Signs of augmented responses (such as an exaggerated blood pressure response) to physical exertion appear early in life (from the prepubertal years) in obese individuals. Understanding the mechanisms that drive the altered hemodynamic responses during exercise in obese individuals and prevent the progression to hypertension is vitally important. This paper focuses on the evidence linking obesity with alterations of the autonomic nervous system and discusses the potential mechanisms and consequences of the altered sympathetic nervous system behavior in obese individuals at rest and during exercise. Furthermore, this paper presents the alterations in the reflex regulatory mechanisms ("exercise pressor reflex" and baroreflex) in obese children and adults and addresses the effects of training on obesity-related disturbances.

The sympathetic nervous system and blood pressure in humans: implications for hypertension

A neurogenic component to primary hypertension (hypertension) is now well established. Along with raised vasomotor tone and increased cardiac output, the chronic activation of the sympathetic nervous system in hypertension has a diverse range of pathophysiological consequences independent of any increase in blood pressure. This review provides a perspective on the actions and interactions of angiotensin II, inflammation and vascular dysfunction/brain hypoperfusion in the pathogenesis and progression of neurogenic hypertension. The optimisation of current treatment strategies and the exciting recent developments in the therapeutic targeting of the sympathetic nervous system to control hypertension (for example, catheter-based renal denervation and carotid baroreceptor stimulation) will be outlined.

Therapeutic strategies for targeting excessive central sympathetic activation in human hypertension

The pathogenesis of hypertension and its mode of progression are complex, multifactoral and incompletely understood. However, there is accumulating evidence from humans and animal models of hypertension indicating that excessive central sympathetic nerve activity (SNA) plays a pathogenic role in triggering and sustaining the essential hypertensive state (the so-called 'neuroadrenergic hypothesis'). Importantly, augmented central sympathetic outflow has also been implicated in the initiation and progression of a plethora of pathophysiological processes independent of any increase in blood pressure, such as left ventricular hypertrophy and cardiac arrhythmias. Thus, the sympathetic nervous system constitutes an important putative drug target in hypertension. However, traditional pharmacological approaches for the management of essential hypertension appear ineffective in reducing central sympathetic outflow. Recently, several new and promising therapeutic strategies targeting neurogenic hypertension have been developed. The present report will provide a brief update of this topic with a particular emphasis on human studies examining the efficacy of novel pharmacological approaches (central sympatholytics and statins), lifestyle modification (aerobic exercise training, weight loss and stress reduction) and surgical intervention (renal denervation, chronic carotid baroreflex stimulation and deep brain stimulation) in reducing excessive central sympathetic activation in hypertension.

Differing pattern of sympathoexcitation in normal-weight and obesity-related hypertension

Hypertension in normal-weight and obese individuals is characterized by activation of the sympathetic nervous system. Measurement of spillover of the sympathetic transmitter, norepinephrine, to plasma indicates that the regional pattern of sympathetic activation in the 2 "variants" of essential hypertension differs, excluding the heart in obesity-related hypertension. Whether sympathetic nerve firing characteristics also differ is unknown. We studied multiunit and single fiber sympathetic nerve firing properties in patients with normal-weight hypertension and obesity-related hypertension, comparing these with nerve characteristics in normal-weight and obese people with normal blood pressure. Both normal-weight hypertensive (n=10) and obese hypertensive (n=14) patients had increased total multiunit muscle sympathetic nerve activity compared with the normal-weight (n=11) and obese (n=11) people with normal blood pressure (65+/-4 versus 47+/-6 bursts per 100 heartbeats, P<0.01 in the normal-weight groups and 68+/-4 versus 53+/-3 bursts per 100 beats, P<0.01 in the obese groups). Sympathetic activation in normal-weight hypertension was characterized by increased firing rate of single vasoconstrictor fibers (70+/-8 versus 28+/-3 spikes per 100 beats; P<0.001), increased firing probability per heartbeat (39+/-3% versus 20+/-3%; P<0.001), and higher incidence of multiple spikes per heartbeat (30+/-4% versus 17+/-4%; P<0.05). Sympathetic activation in obesity-related hypertension differed, involving recruitment of previously silent fibers, which fired at a normal rate. The pattern of sympathetic activation in normal-weight and obesity-related hypertension differs in terms of both the firing characteristics of individual sympathetic fibers and the sympathetic outflows involved. The underlying central nervous system mechanism and the adverse consequences of the 2 modes of sympathetic activation may differ.

Chronic sympathetic activation: consequence and cause of age-associated obesity?

Primary aging in adult humans is associated with a progressive, tonic activation of the peripheral sympathetic nervous system (SNS). The purpose of this SNS activation and its physiological impact are, however, unknown. We hypothesize that the chronic stimulation of the SNS with aging is driven in part by a progressive accumulation of body fat. This "error" is sensed by the central nervous system via increases in adiposity-sensitive humoral signals (e.g., leptin, insulin) that cross the blood-brain barrier, activate subcortical areas involved in the regulation of energy balance (e.g., ventromedial hypothalamus), and stimulate SNS outflow to peripheral tissues. The SNS activation is intended to increase beta-adrenergic thermogenesis in order to expend excess energy as heat rather than by storage of fat. Recent evidence, however, indicates that these adjustments are not effective in augmenting energy expenditure with aging. Indeed, older sedentary adults demonstrate reduced, not increased, beta-adrenergic stimulation of metabolic rate because of reduced tissue responsiveness, presumably mediated by SNS-induced impairment of beta-adrenergic signaling. As a result, age-associated SNS activation, initiated as a consequence of accumulating adiposity with the intent of preventing further fat storage, ironically, may in time evolve into a potential mechanism contributing to the development of obesity with aging.

The role of the sympathetic nervous system in linking obesity with hypertension in white versus black Americans

Several previous studies confirmed that obesity is a major risk factor for the development of cardiovascular diseases, including hypertension. A large number of clinical studies considered the role of the sympathetic nervous system in linking obesity with hypertension, and there is substantial evidence that human obesity is characterized by defects in sympathetic cardiovascular control. The association of obesity with hypertension has been well documented in most racial, ethnic, and socioeconomic groups. Ethnicity may be an important factor to consider since sympathetic nervous system activity, and the propensity for obesity and hypertension, all differ substantially among different populations. Obesity is actually accompanied by increased sympathetic nerve discharge to skeletal muscles, a main site for energy expenditure. Adiposity-related sympathetic overactivity is a compensatory mechanism to burn fat and decrease weight gain, but at the cost of increased sympathetic discharge to the peripheral vasculature, which could predispose to hypertension. Thus, sympathetic nervous system activity is important in the development and maintenance of obesity-related hypertension in different racial and ethnic populations, including white and black Americans.

Contrasting clinical properties and exercise responses in obese and lean hypertensive patients

OBJECTIVES

We sought to test whether the differences in activity of the renin-angiotensin and sympathetic nervous systems at rest or during exercise can explain the differing cardiovascular properties and outcomes of lean and obese hypertensive patients.

BACKGROUND

Although lean hypertensive patients have fewer metabolic abnormalities than obese hypertensive patients, paradoxically they appear to have a poorer cardiovascular prognosis.

METHODS

To evaluate the heightened risks in lean hypertensive patients, this study compared metabolic, neuroendocrine and cardiovascular characteristics at rest and during a standardized treadmill protocol in obese (body mass index [BMI] = 32.5 +/- 0.3 kg/m2, n = 55) and lean (BMI = 24.3 +/- 0.2 kg/m2, n = 66) hypertensive patients. Normotensive obese (n = 21) and lean (n = 55) volunteers served as control subjects.

RESULTS

Compared with the lean normotensive subjects, the lean and obese hypertensive patients had greater left ventricular mass index (LVMI) values, but on multivariate analysis, LVMI correlated with plasma renin activity (p < 0.001) and plasma norepinephrine (PNE) (p < 0.01) in the lean but not the obese hypertensive patients. Arterial compliance (stroke volume/pulse pressure ratio) was reduced in the lean hypertensive patients, in whom it correlated (p = 0.033) with PNE. The PNE rose less (22%) in the obese than in the lean (55%) hypertensive patients in response to standing (p < 0.05). Likewise, during treadmill exercise, there were lesser increases in renin (65% vs. 145%, p < 0.01) and epinephrine (200% vs. 500%, p < 0.05) in the obese hypertensive patients. These changes were also less in obese patients than in lean control subjects, indicating attenuated neurohormonal responses to stress in obesity.

CONCLUSIONS

Compared with obese hypertensive patients, cardiovascular properties in lean hypertensive patients are more dependent on catecholamines and the renin system. The different neuroendocrine responses to dynamic stimuli in lean and obese patients also might help to explain the disparity in their cardiovascular outcomes.

Overweight and hypertension : a 2-way street?

Cross-sectionally, higher weight is associated with higher blood pressure levels; prospectively, baseline weight and weight gain predict higher blood pressure. The loss of weight is frequently associated with a decrease in blood pressure. These findings suggest that weight gain may pathophysiologically contribute to blood pressure elevation. In this review, we present data to indicate that the reverse is also true; persons of equal weight who had higher initial blood pressures gain more weight in the future. We also propose a plausible hypothesis to explain this reverse relationship. Both the blood pressure elevation and the gain of weight may reflect a primary increase in sympathetic tone. It is well known that in a milieu of increased sympathetic tone, the beta-adrenergic responsiveness decreases. Sympathetic overactivity and decreased cardiovascular beta-adrenergic responsiveness have been described in hypertension. beta-Adrenergic receptors mediate increases in energy expenditure. If these metabolic receptors were downregulated in hypertension, the ability of hypertensive patients to dissipate calories would decrease and they would gain more weight. The possible relationship of decreased beta-adrenergic responsiveness to weight in hypertension can be experimentally tested. Such research may contribute to an explanation of why patients with hypertension can rarely lose weight. An understanding of this pathophysiological relationship may open new avenues for therapeutic interventions.

Obesity, hypertension, and sympathetic nervous system activity

Several epidemiologic studies have shown that obesity represents an independent risk factor for the development of cardiovascular diseases, including hypertension, myocardial ischemic disease, and cardiac arrhythmias. One of the most appealing concepts in obesity-related hypertension is that a specific etiology can be identified. There is now substantial evidence that human obesity is characterized by abnormalities in sympathetic cardiovascular control. The application of sensitive techniques to assess sympathetic nervous system (SNS) activity in humans, including catecholamine levels, norepinephrine (NE) spillover techniques, and microneurography have furthered this concept. Catecholamine levels in obesity have been conflicting, with high, normal, and low levels reported. However, studies examining weight loss have found that the fall in blood pressure (BP) was highly correlated with reductions in plasma NE. Examination of NE spillover in obesity has shown regional overactivity in the kidneys. High renal SNS activity could lead to sodium retention and abnormal glomerular hemodynamics that could raise BP. Microneurography, which determines muscle sympathetic outflow, has shown consistent elevation in obesity, but no difference between normotensive and hypertensive obesity. However, the hyperinsulinemia of obesity may act in concert with the SNS to elevate BP, as the combination of the two seems to produce vascular constriction. Leptin also has several cardiovascular actions that may contribute to BP regulation. Epidemiologic studies also found that SNS activity predicts hypertension in obese subjects.