Effect of chronic angiotensin converting enzyme inhibition on sympathetic nerve traffic and baroreflex control of the circulation in essential hypertension

Hypothalamic paraventricular nucleus augments baroreflex sensitivity, role of angiotensin II

The hypothalamic paraventricular nucleus (PVN) is an important brain region involved in control of the cardiovascular system. Direct injection of angiotensin II (AngII) into the PVN produces a short or long pressor response. This study was performed in anesthetized rats to find whether the parvocellular part of the paraventricular nucleus (PVNp) affects the baroreflex. And if so, what is the effect of AngII injected into the PVNp on the baroreflex? Drugs were microinjected into the PVNp while blood pressure and heart rate were recorded continuously. We found that microinjection of AT1 and AT2 receptor antagonists into the PVNp region did not affect the baseline mean arterial pressure (MAP) and heart rate (HR) indicating that under normal conditions AngII may not provide tonic activity, at least in anaesthetized animals. Bilateral microinjections of a synaptic blocker (CoCl₂) into the PVNp attenuated the baroreflex gains in responses to loading and unloading of baroreceptors, indicating that PVNp is involved in the baroreflex rate component. Microinjection of AngII into the PVNp increased MAP and HR. However, AngII slightly attenuated the baroreflex rate component using its two receptors AT1 and AT2. Collectively, these findings suggest that the PVNp as a whole is involved in the baroreflex. But AngII attenuates the heart rate response of the baroreflex through AT1 and AT2 receptors.

Sympathomodulatory Effects of Antihypertensive Drug Treatment

BACKGROUND

An activation of sympathetic neural influences to the heart and peripheral circulation has been shown to represent a hallmark of the essential hypertensive state, adrenergic neural factors participating together with other variables at the development and progression of the high blood pressure state as well as of the hypertension-related target organ damage. This represents the rationale for employing in hypertension treatment drugs which combine the blood pressure-lowering properties with the modulatory effects on the sympathetic neural function.

METHODS AND RESULTS

Several studies published during the past 40 years have investigated the impact of antihypertensive drugs on the sympathetic target as assessed by indirect and direct approaches. In the present paper, the effects of different monotherapies or combination drug treatment used in hypertension to lower elevated blood pressure values on various adrenergic markers will be examined. This will be followed by a discussion of the (i) hemodynamic and nonhemodynamic consequences of employing antihypertensive drugs with sympathomodulatory or sympathoexcitatory properties and (ii) mechanisms potentially responsible for the adrenergic responses to a given antihypertensive drug. The final part of this review will address the questions still open related to the impact of antihypertensive drug treatment on sympathetic function. Two questions in particular will be examined, i.e., whether antihypertensive drugs with sympathomodulatory properties may be capable to fully restore a "normal" adrenergic drive and how far sympathetic activity should be reduced in hypertensive patients.

CONCLUSION

Future investigations aimed at answering these questions will be needed in order to improve cardiovascular protection in treated hypertensive patients.

Cardiac autonomic dysfunction in chronic stroke women is attenuated after submaximal exercise test, as evaluated by linear and nonlinear analysis

Background

We evaluated cardiac autonomic modulation in women with chronic ischemic stroke (at least 4 years post-stroke) at rest and in response to submaximal exercise test.

Methods

Fourteen post-stroke women (S group) and 10 healthy women (C group) participated in this study. Autonomic modulation (using linear and nonlinear analysis), blood pressure and metabolic variables at rest were evaluated immediately after the exercise test and during the recovery period (20 min). All participants underwent submaximal exercise test on cycle ergometer with gas analysis.

Results

At rest, the S group displayed higher lactate concentration, systolic (SBP) and diastolic blood pressure (DBP) values when compared to C group. Furthermore, the S group had lower heart rate variability (HRV) in time domain (SDNN: S = 30 ± 5 vs. 40 ± 8 ms; rMSSD: S = 14 ± 2 vs. C = 34 ± 3 ms), decreased high frequency band of pulse interval (S = 8.4 ± 2 vs. 33.1 ± 9 %) and 2V pattern of symbolic analysis (S = 17.3 ± 1 vs. 30 ± 3 %) (both indicators of cardiac vagal modulation) when compared to C group. Immediately after exercise, S group presented higher values of lactate, SBP, DBP and double product when compared to C group, as well as decreased heart rate recovery (HRR) measured at the first, second and third minutes. At recovery time, all HRV parameters in time and frequency domains improved in the S group; however, HF band remained lower when compared to C group.

Conclusions

After the exercise test, women with chronic stroke presented reduced heart rate variability, reduced cardiac vagal modulation, as well as reduced HRR, while displayed an improvement of heart rate variability and cardiac vagal modulation when compared to their baseline. These results reinforce the importance of a physically active lifestyle for cardiovascular autonomic disorders observed in chronic stroke women.

The role of the kidney and the sympathetic nervous system in hypertension

Nearly one-third of the world's population has hypertension. The human and societal impact of hypertension is enormous. Primary hypertension accounts for 95 % of cases of hypertension in adults. The pathogenesis of primary hypertension is complex. The kidney and the sympathetic nervous system play important roles in the development and maintenance of hypertension. This review discusses their respective roles, the interaction between the two, implications of sympathetic overactivity in kidney disease and therapeutic interventions that have been developed on the basis of this knowledge, especially modulation of the sympathetic nervous system.

Metabolic syndrome: a sympathetic disease?

Metabolic syndrome is associated with adverse health outcomes and is a growing problem worldwide. Although efforts to harmonise the definition of metabolic syndrome have helped to better understand the prevalence and the adverse outcomes associated with the disorder on a global scale, the mechanisms underpinning the metabolic changes that define it are incompletely understood. Accumulating evidence from laboratory and human studies suggests that activation of the sympathetic nervous system has an important role in metabolic syndrome. Indeed, treatment strategies commonly recommended for patients with metabolic syndrome, such as diet and exercise to induce weight loss, are associated with sympathetic inhibition. Pharmacological and device-based approaches to target activation of the sympathetic nervous system directly are available and have provided evidence to support the important part played by sympathetic regulation, particularly for blood pressure and glucose control. Preliminary evidence is encouraging, but whether therapeutically targeting sympathetic overactivity could help to prevent metabolic syndrome and attenuate its adverse outcomes remains to be determined.

Combining irbesartan and trichlormethiazide enhances blood pressure reduction via inhibition of sympathetic activity without adverse effects on metabolism in hypertensive rats with metabolic syndrome

Sympathoexcitation and oxidative stress in the brain have pivotal roles in hypertension with metabolic syndrome (MetS). Here, we examined whether oral administration of irbesartan (IRB) and trichlormethiazide (TCM) decreases blood pressure (BP) via inhibiting sympathetic activity through anti-oxidant effects in the brain of spontaneously hypertensive rats (SHR-cp). IRB/TCM treatment decreased BP more profoundly than IRB monotherapy. Urinary norepinephrine excretion and oxidative stress in the brain were decreased in both IRB and IRB/TCM groups without any adverse effect on the metabolic profile. These findings suggest that IRB/TCM profoundly decreases BP in SHR-cp by inhibiting sympathetic activity via anti-oxidant effects in the brain.

High muscle sympathetic nerve activity is associated with left ventricular dysfunction in treated hypertensive patients

BACKGROUND

The presence of asymptomatic left ventricular diastolic dysfunction (LVDD) in hypertensive patients can be associated with the development of cardiac events. The increase in sympathetic activity may be 1 of the mechanisms that predisposes to this outcome. In this study, we analyzed 2 hypotheses: (i) whether sympathetic activity is higher in the presence of LVDD, independent of blood pressure control and (ii) whether different classes of LVDD have a different effect on sympathetic activity.

METHODS

After analyzing left ventricular function using echo Doppler cardiography, 45 hypertensive patients receiving treatment were allocated into 3 groups: normal function (LV-NF, n = 15), impaired relaxation (LV-IR, n = 15), and pseudonormal or restrictive (LV-P/R, n = 15). An age-, sex-, and body mass index-matched control group of normotensive volunteers (N, n = 14) was included. Muscle sympathetic nerve activity (MSNA), heart rate, and systolic blood pressure variabilities and baroreflex sensitivity were evaluated while the patient was in a supine position.

RESULTS

Blood pressure and antihypertensive drug use were similar among the hypertensive groups. The LV-IR and LV-P/R groups had similar MSNA (33±1 and 32±1 bursts/min, respectively), which was significantly higher than that of the LV-NF and N groups (26±3 and 15±2 bursts/min, respectively). The LV-IR and LV-P/R groups had significantly higher LF-systolic blood pressure variability and significantly lower baroreflex sensitivity compared with the N group.

CONCLUSIONS

The presence of asymptomatic LVDD is associated with increased MSNA, independent of blood pressure control. The sympathetic hyperactivity associated with LVDD is similar in the different patterns of LVDD studied.

Cardiac sympathetic hyperinnervation in deoxycorticosterone acetate-salt hypertensive rats

Sympathetic activities are elevated in the central SNSs (sympathetic nervous systems) of hypertensive animals, but it is not known whether sympathetic innervation is also elevated in the heart. Sympathetic hyper-responsiveness in hypertension may result from oxidative stress. The aim of the present study was to investigate sympathetic hyperinnervation in DOCA (deoxycorticosterone acetate)-salt hypertensive rats with established hypertension. At 4 weeks after the start of DOCA-salt treatment and uninephrectomization, male Wistar rats were randomized into three groups for 8 weeks: vehicle, NAC (N-acetylcysteine) and triple therapy (hydralazine, hydrochlorothiazide and reserpine). DOCA-salt was associated with increased oxidant release. DOCA-salt produced concentric left ventricular hypertrophy and cardiomyocyte hypertrophy. Sympathetic hyperinnervation was observed in DOCA-salt rats, as assessed by myocardial noradrenaline levels, immunofluorescent analysis of tyrosine hydroxylase, growth-associated factor 43 and neurofilament and Western blotting and real-time quantitative RT-PCR (reverse transcription-PCR) of NGF (nerve growth factor). Arrhythmic scores during programmed stimulation in DOCA-salt rats were significantly higher than those in the control rats. Triple therapy, despite being effective on BP (blood pressure), offered neither attenuated cardiomyocyte hypertrophy nor anti-arrhythmia. The effects of DOCA-salt treatment on NGF expression, sympathetic hyperinnervation and arrhythmias were attenuated by NAC. Furthermore, the effects of NAC on NGF were abolished by administering BSO (L-buthionine sulfoximine), an inhibitor of glutamate-cysteine ligase. In conclusion, DOCA-salt treatment contributes to up-regulation of NGF proteins probably through a free radical-dependent pathway in a BP-independent manner. DOCA-salt rats treated with NAC attenuate sympathetic hyperinnervation and thus show a beneficial effect on arrhythmogenic response to programmed electrical stimulation.

Sympathetic mechanisms, organ damage, and antihypertensive treatment

Sympathetic activation characterizes essential hypertension, contributing to the development and progression of the high blood pressure state. Throughout the years, evidence has been accumulated to show that adrenergic overdrive also participates in the pathogenesis of the end-organ damage associated with hypertension, including cardiac hypertrophy, left ventricular diastolic dysfunction, and heart failure, as well as the vascular structural and functional alterations that frequently can be detected in large, medium-size, and small arteries. Adrenergic overdrive also participates in the renal insufficiency and failure that may accompany the clinical course of the hypertensive state. This paper reviews evidence collected over the past few years documenting the importance of neurogenic factors in the development and progression of end-organ damage. The therapeutic implications of this evidence are also highlighted.

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.

Angiotensin-converting enzyme inhibition reverses diet-induced obesity, insulin resistance and inflammation in C57BL/6J mice

AIM

Angiotensin-converting enzyme (ACE) inhibition can reduce the body weight of mice maintained on a high-fat diet. The current study examined the effect of the ACE inhibitor, captopril (CAP), on the reversal of diet-induced obesity (DIO), insulin resistance and inflammation in mice.

MATERIALS AND METHODS

DIO was produced in C57BL/6J male mice (n=30) by maintaining animals on a high-fat diet (w/w 21% fat) for 12 weeks. During the subsequent 12-week treatment period, the animals were allowed access to the high-fat diet and either water containing CAP (0.05 mg ml(-1)) or plain tap water (CON, control).

RESULTS

From the first week of treatment, food intake and body weight decreased in CAP-treated mice compared with CON mice. Both peripheral insulin sensitivity and hepatic insulin sensitivity were improved in CAP-treated mice compared with CON mice. CAP-treated mice had decreased absolute and relative liver and epididymal fat weights compared with CON mice. CAP-treated mice had higher plasma adiponectin and lower plasma leptin levels than CON mice. Relative to CON mice, CAP-treated mice had reduced adipose and skeletal muscle monocyte chemoattractant protein 1 (MCP-1), adipose interleukin-6 (IL-6), toll-like receptor 4 (TLR4) and uncoupling protein 2 (UCP2) mRNA expressions. Furthermore, CAP-treated mice had increased peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), long chain acyl-CoA dehydrogenase (LCAD), hormone sensitive lipase (HSL) and decreased lipoprotein lipase (LPL) mRNA expressions in the liver.

CONCLUSION

The results of the current study indicate that in mice with DIO, CAP treatment reduced food intake and body weight, improved insulin sensitivity and decreased the mRNA expression of markers of inflammation. Thus, CAP may be a viable treatment for obesity, insulin resistance and inflammation.

Brain angiotensin peptides regulate sympathetic tone and blood pressure

Brain angiotensin II (Ang II) induces tonic sympathoexcitatory effects through AT1 receptor stimulation of glutamatergic neurons and sympathoinhibitory effects via GABAergic neurons in the rostral ventrolateral medulla, the brainstem 'pressor area'. NADPH-derived superoxide production and reactive oxygen species signalling is critical in these actions, and AT2 receptors in the rostral ventrolateral medulla appear to mediate opposing effects on sympathetic outflow. In the hypothalamic paraventricular nucleus, Ang II has AT1 receptor-mediated sympathoexcitatory effects and enhances nitric oxide formation, which in turn inhibits the Ang II effects through a GABAergic mechanism. Ang II also decreases the tonic sympathoinhibitory effect of gamma amino butyric acid within the paraventricular nucleus. Angiotensin III and Angiotensin IV increase blood pressure via brain AT1 receptor stimulation. Angiotensin (1-7) influences cardiovascular function through a specific Mas-receptor. This review examines the evidence that brain angiotensin peptides, glutamate, gamma amino butyric acid and nitric oxide interact within the rostral ventrolateral medulla and paraventricular nucleus to control sympathetic tone and blood pressure.

Sympathetic neural activity in hypertension and related diseases

BACKGROUND

Several hemodynamic variables, such as blood pressure, vascular resistance, cardiac output, and heart rate, are regulated, among others, by sympathetic cardiovascular influences. This has led many years ago investigators to advance the hypothesis that alterations in the sympathetic modulation of the cardiovascular system may occur in hypertension and related disease.

METHODS

The role of the sympathetic nervous system as promoter and amplifier of the hypertensive state has been examined in a consistent number of studies carried out by making use of sophisticated and sensitive approaches to evaluate adrenergic function, such as the norepinephrine spillover technique and the recording of efferent postganglionic muscle sympathetic-nerve traffic.

RESULTS

The results of the above-mentioned investigations support the concept that adrenergic activation characterizes essential hypertension, correlating with the clinical severity of the disease. Furthermore, sympathetic cardiovascular influences may favor the hypertensive disease progression, by concurring with other hemodynamic and nonhemodynamic factors at the development of target organ damage. Finally, an adrenergic overdrive of pronounced degree also characterizes hypertension-related cardiovascular and metabolic disease. In several of these clinical conditions, the adrenergic overdrive plays a role in the disease's physiopathology and prognosis.

CONCLUSIONS

The data reviewed in this article provide evidence that sympathetic activation represents a hallmark of the essential hypertensive state. They further show that adrenergic neural factors may participate at the development and progression of the hypertensive state and its complications. This represents the rationale for the use of antihypertensive and, in more in general, cardiovascular drugs capable to exert sympatho-inhibitory effects.

Pregnancy and the endocrine regulation of the baroreceptor reflex

The purpose of this review is to delineate the general features of endocrine regulation of the baroreceptor reflex, as well as specific contributions during pregnancy. In contrast to the programmed changes in baroreflex function that occur in situations initiated by central command (e.g., exercise or stress), the complex endocrine milieu often associated with physiological and pathophysiological states can influence the central baroreflex neuronal circuitry via multiple sites and mechanisms, thereby producing varied changes in baroreflex function. During pregnancy, baroreflex gain is markedly attenuated, and at least two hormonal mechanisms contribute, each at different brain sites: increased levels of the neurosteroid 3alpha-hydroxy-dihydroprogesterone (3alpha-OH-DHP), acting in the rostral ventrolateral medulla (RVLM), and reduced actions of insulin in the forebrain. 3alpha-OH-DHP appears to potentiate baroreflex-independent GABAergic inhibition of premotor neurons in the RVLM, which decreases the range of sympathetic nerve activity that can be elicited by changes in arterial pressure. In contrast, reductions in the levels or actions of insulin in the brain blunt baroreflex efferent responses to increments or decrements in arterial pressure. Although plasma levels of angiotensin II are increased in pregnancy, this is not responsible for the reduction in baroreflex gain, although it may contribute to the increased level of sympathetic nerve activity in this condition. How these different hormonal effects are integrated within the brain, as well as possible interactions with additional potential neuromodulators that influence baroreflex function during pregnancy and other physiological and pathophysiological states, remains to be clearly delineated.

Sympathetic nervous system overactivity and its role in the development of cardiovascular disease

This review examines how the sympathetic nervous system plays a major role in the regulation of cardiovascular function over multiple time scales. This is achieved through differential regulation of sympathetic outflow to a variety of organs. This differential control is a product of the topographical organization of the central nervous system and a myriad of afferent inputs. Together this organization produces sympathetic responses tailored to match stimuli. The long-term control of sympathetic nerve activity (SNA) is an area of considerable interest and involves a variety of mediators acting in a quite distinct fashion. These mediators include arterial baroreflexes, angiotensin II, blood volume and osmolarity, and a host of humoral factors. A key feature of many cardiovascular diseases is increased SNA. However, rather than there being a generalized increase in SNA, it is organ specific, in particular to the heart and kidneys. These increases in regional SNA are associated with increased mortality. Understanding the regulation of organ-specific SNA is likely to offer new targets for drug therapy. There is a need for the research community to develop better animal models and technologies that reflect the disease progression seen in humans. A particular focus is required on models in which SNA is chronically elevated.

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.

Is kidney ischemia the central mechanism in parallel activation of the renin and sympathetic system?

In chronic kidney disease simultaneous activation of the renin-angiotensin and sympathetic systems occurs. Kidney ischemia seems to play a key role in the pathogenesis. This review firstly summarizes experimental and clinical evidence in chronic kidney disease supporting this idea and addresses the possibility that this mechanism is also relevant in some other disease conditions.

Diabetic Hypertensive Leptin Receptor–Deficient db/db Mice Develop Cardioregulatory Autonomic Dysfunction

Leptin receptor–deficient db/db mice develop human type 2 diabetes mellitus, hypertension, and obesity with disrupted circadian blood pressure (BP) rhythm. Whether leptin is the sole mechanism mediating autonomic imbalance and hypertension is unclear. To explore this notion further, we measured BP by radiotelemetry combined with fast Fourier transformation and assessed autonomic function pharmacologically before and after renin-angiotensin system blockade with enalapril. The resting period BP (117±3 versus 108±1.0 mm Hg) and heart rate (HR; 488±12 versus 436±8 bpm) were higher in db/db mice compared with db/+ mice. BP and HR amplitudes were lower in db/db mice compared with db/+ mice. BP response to trimetaphan (−43±5 versus −27±3 mm Hg) and HR response to metoprolol (−59±12 versus −5±4 bpm) were greater in db/db mice than in db/+ mice. The HR response to atropine was blunted in db/db mice (59±17 versus 144±24 bpm), as were baroreflex sensitivity and HR variability. Enalapril improved autonomic regulation in db/db mice. Stimulation of central α-2 adrenoreceptors enhanced both parasympathetic HR control and baroreflex sensitivity in db/db mice. We suggest that functional, rather than structural, α-2 adrenoceptor changes and the renin-angiotensin system are involved in the increased sympathetic and decreased parasympathetic tones in db/db mice. Our data suggest that db/db mice exhibit features found in humans with type 2 diabetic autonomic neuropathy and could serve as a model for this complication.

Eprosartan modulates the reflex activation of the sympathetic nervous system in sodium restricted healthy humans

WHAT IS ALREADY KNOWN ABOUT THIS SUBJECT

A sympatho-inhibitory effect of ACE-inhibitors and AT(1) receptor antagonists has been widely demonstrated in animal models, but in humans this effect tends only to be present during chronic treatment in conditions with pre-existing high levels of sympathetic activity. Sodium restriction increases renal sympathetic nerve activity and the activity of the renin-angiotensin system and may be a favourable condition to demonstrate sympatho-inhibition as a short-term effect of the AT(1) receptor antagonist eprosartan in healthy humans.

WHAT THIS STUDY ADDS

Results from our study indicate that during sodium restriction eprosartan has a small inhibitory effect on nonbaroreflex mediated activation of the sympathetic nervous system. During arterial baroreflex mediated activation of the sympathetic nervous system this effect is, however, completely overruled by an increased sensitivity of the arterial baroreflex. AIMS To test the hypothesis that eprosartan inhibits both nonbaroreflex and arterial baroreflex mediated activation of the sympathetic nervous system, assessed by renal tubular function, systemic haemodynamics and vasoactive hormones, in sodium restricted healthy humans.

METHODS

The effect of eprosartan on urinary sodium, lithium and water excretion, heart rate (HR), blood pressure and vasoactive hormones was measured before, during and after a cold pressor test (CPT) and sodium nitroprusside (SNP) infusion in a randomized, placebo controlled, double-blind, crossover study in 17 healthy subjects. Glomerular filtration rate and renal tubular function were determined by a continuous infusion clearance technique and vasoactive hormones by radioimmunoassays.

RESULTS

Eprosartan attenuated the impact of the CPT on HR (mean difference from placebo (95% confidence interval) (3.9 (0.7, 7.0) min(-1)) and mean arterial pressure (MAP) (4.7 (0.3, 9.2) mmHg), but no effect of eprosartan was observed on the impact of the CPT on renal tubular function. During a SNP induced reduction in MAP of 10 mmHg eprosartan decreased fractional excretions of sodium (0.46 (0.14, 0.76)%) and lithium (5.1 (2.5, 7.6)%) and tended to increase HR (4.1 (-0.26, 8.4) min(-1)) and plasma concentrations of norepinephrine (33.8 (-5.8, 72.1) pg ml(-1)). CONCLUSIONS; These findings suggest that during mild sodium restriction eprosartan has a small inhibitory effect on nonbaroreflex mediated activation of the sympathetic nervous system. During arterial baroreflex mediated activation of the sympathetic nervous system this effect is, however, completely overruled by an increased sensitivity of the arterial baroreflex.

Why is blood pressure so hard to control in patients with type 2 diabetes?

Resistance to antihypertensive drugs is common in hypertensive patients with type 2 diabetes. This is unfortunate because hypertension is one of the most important risk factors for development of cardiovascular events, and the goal blood pressure level is set lower in diabetic subjects than in nondiabetic subjects. Previous outcome trials in diabetic subjects have mainly focused on end points such as microalbuminuria or the incidence of cardiovascular events rather than on reduction of blood pressure; some reports, however, have suggested mechanisms for the drug resistance. These include several clinical conditions known to be associated with difficulty in reducing blood pressure specifically in diabetes mellitus: change in the renin-angiotensin system and chymase, volume overload, central sympathetic hyperactivity, sleep apnea, secondary hypertension, pseudoresistance (white coat hypertension), and poor compliance related to subclinical depression. In this review, the authors focus on the mechanisms of resistance to antihypertensive therapy (particularly for monotherapy with either angiotensin-converting enzyme inhibitors or angiotensin II antagonists) in the treatment of diabetic hypertension.

The effect of gender on the sympathetic nerve hyperactivity of essential hypertension

We planned to determine whether or not there is a difference in the level of muscle sympathetic nerve activity (MSNA) between hypertensive women and hypertensive men. Sympathetic activation of essential hypertension (EHT) has been associated with increased cardiovascular events, which are known to be less likely to occur in women than in men. Normal women have been reported to have less sympathetic nerve activity than men, but no reported data are available regarding gender differences in sympathetic activity in hypertensive subjects. We examined 36 patients with untreated and uncomplicated EHT comprising 18 women and 18 men, and 36 normal controls comprising 18 women and 18 men. MSNA was quantified as the mean frequency of single units and as multiunit bursts using the technique of microneurography. The hypertensive groups had greater sympathetic nerve activity than the control groups. Female hypertensives had lower (P<0.001) single unit hyperactivity (56+/-1.7 impulses/100 cardiac beats) than male hypertensives (72+/-1.7 impulses/100 cardiac beats). Normotensive females had lower (P<0.01) single unit activity (42+/-3.6 impulses/100 cardiac beats) than normotensive males (56+/-4.6 impulses/100 cardiac beats). Similar results were obtained for the frequency of multiunit burst activity. Hypertension in women is associated with a lower level of central sympathetic hyperactivity than in men. It is suggested that this may at least partly explain the observed lower hypertension-related cardiovascular events in women than in men. In addition, the findings may have implications for gender-specific management of hypertension.

Angiotensin-sympathetic system interactions in cardiovascular and metabolic disease

BACKGROUND

Blood pressure, as well as blood volume homeostasis, depends to a large extent on humoral influences stemming from the renin-angiotensin axis and the sympathetic nervous system. Evidence has been provided that a large part of this homeostatic modulation is effected by the complex interactions between the two systems.

OBJECTIVES

The present review will focus on three major issues. First it will examine the physiological and pathophysiological relevance of angiotensin-sympathetic crosstalk discussing possible sites, mechanisms and effects of the interaction. It will then address the clinical relevance of these inter-relationships by reviewing data collected in cardiovascular and non-cardiovascular diseases. Finally, the influences of angiotensin II on adrenergic function will be examined as possible targets of cardiovascular drug treatment.

CONCLUSIONS

By interrupting the influences of angiotensin II on sympathetic function, therapeutic interventions aimed at blocking the renin-angiotensin system exert favourable effects on the haemodynamic, metabolic and renal profile. This has important implications for the treatment of hypertension, congestive heart failure, renal insufficiency and metabolic syndrome.

Effect of angiotensin II receptor blockade on autonomic nervous system function in patients with essential hypertension

It has long been proposed that the renin-angiotensin system exerts a stimulatory influence on the sympathetic nervous system, including augmentation of central sympathetic outflow and presynaptic facilitation of norepinephrine release from sympathetic nerves. We tested this proposition in 19 patients with essential hypertension, evaluating whether the angiotensin receptor blockers (ARBs) eprosartan and losartan had identifiable antiadrenergic properties. This was done in a prospective, randomized, three-way placebo-controlled study of crossover design. Patients were randomized to 600 mg of eprosartan daily, 50 mg of losartan daily, or placebo. The treatment period was 4 wk, with 2-wk washout periods. Multiunit firing rates in efferent sympathetic nerves distributed to skeletal muscle vasculature (muscle sympathetic nerve activity, MSNA) were measured with microneurography, testing whether ARBs inhibit central sympathetic outflow. In parallel, isotope dilution methodology was used to measure whole body norepinephrine spillover to plasma. Mean blood pressure on placebo was 151/98 mmHg, with both ARBs causing reductions of approximately 11 mmHg systolic and 6 mmHg diastolic pressure, placebo corrected. Both MSNA [35 +/- 12 bursts/min (mean +/- SD) on placebo] and whole body norepinephrine spillover [366 +/- 247 ng/min] were unchanged by ARB administration, indicating that the ARBs did not materially inhibit central sympathetic outflow or act presynaptically to reduce norepinephrine release at existing rates of nerve firing. These findings contrast with the easily demonstrable reduction in sympathetic nervous activity produced by antihypertensive drugs of the imidazoline-binding class, which are known to act within the brain to inhibit sympathetic nervous outflow. We conclude that sympathetic nervous inhibition is not a major component of the blood pressure-lowering action of ARBs in essential hypertension.

Long-term exercise training and angiotensin-converting enzyme inhibition differentially enhance myocardial capillarization in the spontaneously hypertensive rat

OBJECTIVES

To investigate whether combined treatment with lisinopril, an angiotensin-converting enzyme (ACE) inhibitor and exercise training would have an additive effect in enhancing the capillary supply of the left ventricular (LV) myocardium in spontaneously hypertensive rats (SHR).

DESIGN

Twelve-week-old male SHR were divided into four groups (10-12 each): sedentary, sedentary treated with lisinopril (15-20 mg/kg per day by gavage), exercise trained, and exercise trained while treated with lisinopril. Exercise training consisted of 1 h a day/5 days a week of running on a treadmill.

METHODS

After 10 weeks of experimental protocols, capillary surface density and length density were sterologically determined in 1 mum thick LV tissue samples from perfuse-fixed hearts.

RESULTS

Lisinopril significantly reduced systolic blood pressure (SBP) and LV mass in the sedentary with lisinopril and exercise trained with lisinopril groups but did not affect the heart rate (HR). Exercise training did not reduce SBP or LV mass, but significantly reduced HR in the exercise trained and exercise trained with lisinopril groups. Lisinopril treatment (sedentary with lisinopril), exercise training (exercise) and their combination (exercise trained with lisinopril) significantly increased myocardial capillary surface area density by 26, 38 and 65% and length density by 38, 48 and 67%, respectively.

CONCLUSION

Lisinopril administration and exercise training independently enhanced myocardial capillarization through a reduction of myocardial mass and stimulation of angiogenesis, respectively. A combination of the two treatments enhanced myocardial capillarization more than either intervention alone. This may aid in the restoration of the normal nutritional status of cardiac myocytes compromised by the hypertrophic state of hypertension.

Persistent sympathetic activation during chronic antihypertensive therapy: a potential mechanism for long term morbidity?

Previous studies have demonstrated that antihypertensive treatment resets baroreflex control of heart rate (HR) and increases cardiac vagal baroreflex sensitivity. However, it is uncertain whether baroreflex control of muscle sympathetic nerve activity (MSNA) also resets after treatment. We tested the hypothesis that chronic antihypertensive therapy alters baroreflex regulation of MSNA in patients with untreated moderate hypertension. Seven newly diagnosed patients with systolic blood pressure (BP) of 159+/-5 mm Hg (mean+/-SE) and diastolic BP of 103+/-4 mm Hg were studied before and after 1 to 2 weeks ' and 3 months (chronic) of antihypertensive treatment with losartan-hydrochlorothiazide (Hyzaar). MSNA and hemodynamics were measured supine, during a Valsalva maneuver (VM), and at 70 degrees head-up tilt (HUT) for 10 minutes. Data were compared with those obtained in 7 age-matched healthy controls. We found that Hyzaar lowered mean BP acutely and chronically by 20+/-4 and 23+/-3 mm Hg (both P<0.01) but did not change HR. Supine MSNA increased by 43+/-11% and 34+/-11% after acute and chronic treatment (both P<0.01). However, MSNA responses to VM and HUT did not differ after treatment compared with before treatment, indicating unchanged reflex control. These data indicate that sympathetic neural activity was augmented substantially by antihypertensive treatment with Hyzaar, consistent with an ongoing baroreflex unloading, and did not return to baseline or "reset" after 3 months of therapy. We speculate that persistent and marked sympathetic activation by the baroreflex may be a potential mechanism for hypertension that is refractory to antihypertensive therapy and may provide a target mechanism for persistent morbidity despite adequate BP control.

Protective Effects of an Angiotensin II Receptor Blocker and a Long-Acting Calcium Channel Blocker against Cardiovascular Organ Injuries in Hypertensive Patients

The purpose of this study is to compare the long-term effects of an angiotensin II receptor blocker (ARB) and a long-acting calcium channel blocker (CCB) on left ventricular geometry, hypertensive renal injury and a circulating marker of collagen synthesis in hypertensive patients. Patients with essential hypertension (24 men and 19 women; age, 37-79 years) were treated with a long-acting CCB, amlodipine (AML; 2.5-7.5 mg once daily) for 6 months. Then, AML was switched to an ARB, candesartan (CS; 4-12 mg once daily), in 22 patients (CS group), while AML was continued in the remaining 21 patients for another 6 months (AML group). At the end of each treatment period, ambulatory blood pressure monitoring (ABPM), echocardiography and sampling of blood and urine were performed. The average office blood pressure during the latter period was comparably controlled in the AML and the CS groups (AML: 130 +/- 8/87 +/- 7 mmHg; CS: 133 +/- 11/ 88 +/- 7 mmHg), while the average systolic blood pressure of 24-h ABPM was significantly lower in the AML than in the CS group (127 +/- 9 vs. 133 +/- 14 mmHg, p<0.05). Consequently, the left ventricular mass index was significantly decreased in the AML group (102 +/- 18 to 92 +/- 12 g/m2, p<0.05), while the change was insignificant in the CS group (103 +/- 25 to 98 +/- 21 g/m2). On the other hand, plasma procollagen I C-terminal peptide (PICP), a marker of collagen synthesis, was lowered by CS (86 +/- 21 to 70 +/- 21 ng/ml, p<0.01), but was not significantly affected by AML (80 +/- 127 to 74 +/- 91 ng/ml). CS reduced urinary albumin excretion (57 +/- 123 to 26 +/- 33 mg/g creatinine, p<0.05), but AML did not bring about significant changes (85 +/- 27 to 73 +/- 19 mg/g creatinine). The results suggested that long-acting CCBs are effective in improving left ventricular hypertrophy by controlling 24-h blood pressure, while ARBs possess protective effects against cardiovascular fibrosis and renal injury beyond their antihypertensive effects.

Angiotensin converting enzyme inhibition for cardiac hypertrophy in patients with end-stage renal disease: what is the evidence?

Dialysis patients show a high prevalence of cardiovascular complications among which left ventricular hypertrophy is one of the most frequent and is independently predictive of mortality. A recent study indicates that partial regression of left ventricular hypertrophy improves mortality and reduces cardiovascular events in end-stage renal disease (ESRD) patients, suggesting the importance of targeting therapeutic strategies to reduce cardiac hypertrophy and improve the outcome in these patients. The pathogenesis of left ventricular hypertrophy in ESRD patients is multifactorial and includes hypertension, activation of the renin-angiotensin system, increased sympathetic activity, chronic volume overload, chronic anaemia and hyperparathyroidism. In this paper, we review the available experimental and clinical evidence showing the important contribution of the renin-angiotensin system as well as its interaction with the sympathetic nervous system in the pathogenesis of left ventricular hypertrophy in ESRD patients. Furthermore, we summarize the results of currently available clinical studies that examined the effects of angiotensin-converting enzyme inhibition or angiotensin receptor antagonism on left ventricular hypertrophy in ESRD patients, and review evidences that support the use of angiotensin-converting enzyme inhibitors or angiotensin receptor antagonists in the ESRD population.

Sympathetic neural activation in nondiabetic metabolic syndrome and its further augmentation by hypertension

Hypertension is a major cardiovascular risk factor in the metabolic syndrome (MS) in which the presence of insulin resistance, glucose intolerance, abnormal lipoprotein metabolism, and central obesity all confer an increased risk. Because essential hypertension (EHT), insulinemia, and visceral fat are associated with sympathetic hyperactivity, which is itself known to increase cardiovascular risk, the aim of this study was to see if MS is a state of sympathetic nerve hyperactivity and if the additional presence of EHT intensifies this hyperactivity. In 69 closely matched subjects, comprising hypertensive MS (MS+EHT, 18), normotensive MS (MS-EHT, 17), hypertensives without MS (EHT, 16), and normotensive controls without MS (NC, 18), we measured resting muscle sympathetic nerve activity (MSNA) as assessed from multiunit discharges and from single units with defined vasoconstrictor properties (s-MSNA). The s-MSNA in MS+EHT (76+/-3.1 impulses/100 beats) was greater (at least P<0.01) than in MS-EHT (62+/-3.2 impulses/100 beats) and in EHT (60+/-2.3 impulses/100 beats), and all these were significantly greater (at least P<0.01) than in NC (46+/-2.7 impulse/100 beats). The multi-unit MSNA followed a similar trend. These findings suggest that MS is a state of sympathetic nerve hyperactivity and that the additional presence of hypertension further intensifies this hyperactivity. The degree of sympathetic hyperactivity seen in this study could be argued at least partly to contribute to the higher cardiovascular risk and metabolic abnormalities seen in MS+EHT patients.

Impact of type 2 diabetes mellitus on sympathetic neural mechanisms in hypertension

BACKGROUND

Essential hypertension (EHT) is a major cardiovascular risk factor, and the additional presence of type 2 diabetes mellitus (DM2) increases this risk. However, although the sympathetic nerve hyperactivity of EHT is known to play a role in cardiovascular risk, the level of sympathetic nerve activity is known neither in DM2 nor in hypertensive type 2 diabetic patients (EHT+DM2). Therefore, we planned to quantify the vasoconstrictor sympathetic nerve activity in patients with EHT+DM2 and with DM2 relative to that in matched groups with EHT and normal blood pressure (NT).

METHODS AND RESULTS

In 68 closely matched subjects with EHT+DM2 (n=17), DM2 (n=17), EHT (n=17), and NT (n=17), we measured resting muscle sympathetic nerve activity as the mean frequency of multiunit bursts (MSNA) and of single units (s-MSNA) with defined vasoconstrictor properties. The s-MSNA in EHT+DM2 (97+/-3.8 impulses/100 beats) was greater (at least P<0.001) than in EHT (69+/-3.4 impulses/100 beats) and DM2 (78+/-4.1 impulses/100 beats), and all these were significantly greater (at least P<0.01) than in NT (53+/-3.3 impulses/100 beats) despite similar age and body mass index. The MSNA followed a similar trend. In addition, the level of insulin was also raised in EHT+DM2 (20.4+/-3.6 microU/mL) and DM2 (18.1+/-3.1 microU/mL; at least P<0.05) compared with HT or NT.

CONCLUSIONS

Patients with EHT+DM2, EHT, or DM2 had central sympathetic hyperactivity, although plasma insulin levels were raised only in EHT+DM2 and DM2. The combination of EHT and DM2 resulted in the greatest sympathetic hyperactivity and level of plasma insulin, and this hyperactivity could constitute a mechanism for the increased risks of this condition.

AT1-receptor blockade and sympathetic neurotransmission in cardiovascular disease

1. The present survey is dealing with the interactions between the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS) in various organs and tissues, with an emphasis on the angiotensin AT-receptors located at the sympathetic nerve endings. 2. Angiotensin II, the main effector of the RAAS is known to stimulate sympathetic nerve traffic and its sequelae in numerous organs and tissues, such as the central nervous system, the adrenal medulla, the sympathetic ganglia and the sympathetic nerve endings. These stimulatory effects are mediated by AT(1)-receptors and counteracted by AT(1)-receptor antagonists. 3. Sympatho-inhibition at the level of the sympathetic nerve ending appears to be a class effect of the AT(1)-receptor blockers, mediated by presynaptic AT(1)-receptors. With respect to the ratio pre-/postsynaptic AT(1)-receptor antagonism important quantitative differences between the various compounds were found. 4. Both the pre- and postjunctional receptors at the sympathetic nerve endings belong to the AT(1)-receptor population. However, the presynaptic receptors belong to the AT(1B)-subtype, whereas the postjunctional receptors probably belong to a different AT(1)-receptor subpopulation. 5. Sympatho-inhibition is a class effect of the AT(1)-receptor antagonists. In conditions in which the SNS plays a pathophysiological role, such as hypertension and congestive heart failure, this property may well be of therapeutic relevance.

The influence of one-year treatment by angiotensin converting enzyme inhibitor on baroreflex sensitivity and flow-mediated vasodilation of the brachial artery in essential hypertension--comparison with calcium channel blockers

BACKGROUND

Both baroreflex sensitivity and flow-mediated vasodilator function have been recognized to have prognostic significance in cardiovascular diseases. Long-term antihypertensive treatment effects on these parameters, however, remain unclear.

SUBJECTS AND METHODS

We examined the effects of long-term treatment by angiotensin converting enzyme inhibitors (ACEI) orcalcium channel blockers (CCB) on baroreflex and flow-mediated vasodilator function in patients with essential hypertension (EH). We recruited 36 patients aged 56 +/- 11 years, with systolic blood pressure > or = 160 mmHg and/or diastolic blood pressure > or = 95 mmHg. Patients were assigned either to treatment by long-acting ACEI (n = 12) or CCB (n = 24). All patients were followed for 12 months. Optimal BP was achieved by two optional increases in treatment: dose-doubling of the primary drug during the first three months and the addition of diuretics or beta-blockers thereafter. Target blood pressure was 140/90 mmHg or a fall > or = 20/10 mmHg. Baroreflex sensitivity was examined by spectral analysis of blood pressure and RR interval variabilities before treatment and after 3 and 12 months of treatment. The flow-mediated vasodilator function was determined before and 12 months after treatment by measuring the change in brachial artery diameter during increases in flow induced by reactive hyperemia.

RESULTS

Baseline blood pressures were similar between the ACEI and CCB groups (172 +/- 5/103 +/- 2 vs. 172 +/- 4/101 +/- 3 mmHg). Blood pressures after 3 and 12 months of treatment also did not differ between the ACEI and CCB groups (149 +/- 4/91 +/- 2 vs. 145 +/- 2/85 +/- 2 mmHg, and 133 +/- 5/84 +/- 2 vs. 133 +/- 2/81 +/- 2 mmHg, respectively). Baseline baroreflex sensitivity was similar between the groups (6.7 +/- 0.8 vs. 5.9 +/- 0.6 msec/mmHg). This parameter remained unchanged at three months but increased after 12 months of treatment in both the ACEI (9.5 +/- 1.6 msec/mmHg, p = 0.05) and CCB (9.1 +/- 1.2 msec/mmHg, p = 0.006) groups. Percent increases in brachial arterial diameter and flow during reactive hyperemia increased in the group treated with ACEI (12.4 +/- 3.5 vs. 25.8 +/- 6.3% and 618 +/- 72 vs. 953 +/- 166, p < 0.05 for both) but both parameters remained unchanged in the group treated with CCB.

CONCLUSION

These data suggest that long-term blood pressure control with modem antihypertensive drugs improves baroreflex function. Treatment with ACEI may be more favorable for flow-mediated vasodilator function than treatment with CCB.

Enalapril and losartan reduce sympathetic hyperactivity in patients with chronic renal failure

The aim of this study was to compare the effects on BP and sympathetic activity of chronic treatment with an angiotensin (Ang)-converting enzyme (ACE) inhibitor and an AngII receptor blocker in hypertensive patients with chronic renal failure (CRF). In ten stable hypertensive CRF patients (creatinine clearance, 46 +/- 17 ml/min per 1.73 m(2)), muscle sympathetic nerve activity (MSNA), plasma renin activity (PRA), baroreceptor sensitivity, and 24-h ambulatory BP were measured in the absence of antihypertensive drugs (except diuretics) after 6 wk of enalapril (10 mg orally) and after 6 wk of losartan (100 mg orally). The order of the three phases was randomized. Normovolemia was controlled with diuretics and confirmed with extracellular fluid volume measurements throughout the study. Both enalapril and losartan reduced MSNA (from 33 +/- 10 to 27 +/- 13 and 27 +/- 13 bursts/min, respectively; P < 0.05) and average 24-h BP (from 141 +/- 8/93 +/- 8 to 124 +/- 9/79 +/- 8 and 127 +/- 8/81 +/- 9 mmHg; P < 0.01). PRA was not different during the treatments. The change in BP and the change in MSNA during the treatments were correlated (r = 0.70 and r = 0.63, respectively; both P < 0.05). Baroreceptor sensitivity was not affected by the treatments. This is the first study to compare the effects of ACE inhibition and AngII blockade on MSNA. In hypertensive CRF patients, enalapril and losartan equally reduced BP and MSNA. Differences in modes of action of the two drugs did not result in differences in effects on MSNA, supporting the view that AngII-mediated mechanisms contribute importantly in the pathogenesis of sympathetic hyperactivity in these patients.

Time course of sympathetic neural hyperactivity after uncomplicated acute myocardial infarction

BACKGROUND

Little information is available on sympathetic activity after acute myocardial infarction (AMI), despite the belief that sympathetic drive is important in relation to morbidity and mortality. Indirect indices such as plasma catecholamines are transiently elevated after uncomplicated AMI, whereas other prognostically important autonomic indices may be affected longer. We planned to quantify central sympathetic output to the periphery after uncomplicated AMI and to investigate its progress over time.

METHODS AND RESULTS

After uncomplicated AMI, 13 patients had muscle sympathetic nerve activity (MSNA) assessed from multiunit discharges and from single units with defined vasoconstrictor properties (s-MSNA). Measurements were obtained 2 to 4 days after AMI and were repeated after 3 and 6 months. We also examined 3 matched control groups comprising normal subjects, patients with coronary artery disease, and hospitalized patients without AMI. MSNA and s-MSNA after AMI (84+/-4.6 bursts/100 beats and 95+/-5.8 impulses/100 beats) were unchanged at 3 months but decreased (P<0.01 and P<0.001) after 6 months (75+/-4.0 bursts/100 beats and 80+/-4.4 impulses/100 beats). These were still greater (at least P<0.01) than values in normal subjects, patients with coronary artery disease, and hospitalized patients without AMI (51+/-3.9 bursts/100 beats, 58+/-4.7 impulses/100 beats; 56+/-2.2 bursts/100 beats, 61+/-2.2 impulses/100 beats; and 55+/-3.6 bursts/100 beats, 61+/-3.3 impulses/100 beats, respectively). This sympathetic hyperactivity was inversely correlated to left ventricular ejection fraction but not to changes in blood pressure.

CONCLUSIONS

A protracted state of sympathetic hyperactivity was shown to occur after uncomplicated AMI. It is suggested that this hyperactivity may explain delayed cardiovascular morbidity and mortality and that it arises because of an impairment of reflexes from cardiac receptors.

Effects of selective angiotensin II receptor blockade on sympathetic nerve activity in primary hypertensive subjects

OBJECTIVE

The role of the renin-angiotensin system in the regulation of sympathetic nervous activity in human hypertension was evaluated in patients with moderate primary hypertension. For that purpose, the effects of selective angiotensin II (ANG II) receptor blockade by valsartan on sympathetic outflow to the muscle vascular bed and hemodynamic parameters were examined. Results were compared with the effects of the peripherally acting calcium antagonist amlodipine.

DESIGN

Eighteen hypertensive but otherwise healthy subjects were examined in a double-blind, placebo-controlled, cross-over protocol receiving either valsartan or amlodipine or placebo for 7 days in a randomized sequence. Treatment periods were separated by washout periods of 2 weeks.

METHODS

At the seventh day of treatment, blood pressure, heart rate, muscle sympathetic nerve activity (MSNA), norepinephrine, renin and angiotensin were measured during resting conditions. Additionally, parameters were measured after administration of negative pressure of -15 mmHg to the lower part of the body and after a cold pressor test.

RESULTS

Both antihypertensive drugs significantly decreased oscillometrically measured systolic blood pressure and diastolic blood pressure without any difference in effect. While valsartan did not affect the heart rate at rest, amlodipine increased it significantly. Likewise, MSNA was significantly enhanced by amlodipine but not by valsartan. Only ANG II receptor blockade increased renin and angiotensin levels.

CONCLUSIONS

Selective ANG II receptor blockade not only decreases blood pressure, but also shifts the baroreflex set-point for the initiation of counter-regulatory reflex responses of heart rate and blood pressure towards normal blood pressure levels. Thus, data suggest that ANG II plays a pathogenetic role in the elevation of the baroreflex set point in primary hypertensive subjects.