Angiotensin II blockade [corrected] enhances baroreflex control of sympathetic outflow in heart failure

Autonomic nervous system and cardiac neuro-signaling pathway modulation in cardiovascular disorders and Alzheimer's disease

The heart is a functional syncytium controlled by a delicate and sophisticated balance ensured by the tight coordination of its several cell subpopulations. Accordingly, cardiomyocytes together with the surrounding microenvironment participate in the heart tissue homeostasis. In the right atrium, the sinoatrial nodal cells regulate the cardiac impulse propagation through cardiomyocytes, thus ensuring the maintenance of the electric network in the heart tissue. Notably, the central nervous system (CNS) modulates the cardiac rhythm through the two limbs of the autonomic nervous system (ANS): the parasympathetic and sympathetic compartments. The autonomic nervous system exerts non-voluntary effects on different peripheral organs. The main neuromodulator of the Sympathetic Nervous System (SNS) is norepinephrine, while the principal neurotransmitter of the Parasympathetic Nervous System (PNS) is acetylcholine. Through these two main neurohormones, the ANS can gradually regulate cardiac, vascular, visceral, and glandular functions by turning on one of its two branches (adrenergic and/or cholinergic), which exert opposite effects on targeted organs. Besides these neuromodulators, the cardiac nervous system is ruled by specific neuropeptides (neurotrophic factors) that help to preserve innervation homeostasis through the myocardial layers (from epicardium to endocardium). Interestingly, the dysregulation of this neuro-signaling pathway may expose the cardiac tissue to severe disorders of different etiology and nature. Specifically, a maladaptive remodeling of the cardiac nervous system may culminate in a progressive loss of neurotrophins, thus leading to severe myocardial denervation, as observed in different cardiometabolic and neurodegenerative diseases (myocardial infarction, heart failure, Alzheimer's disease). This review analyzes the current knowledge on the pathophysiological processes involved in cardiac nervous system impairment from the perspectives of both cardiac disorders and a widely diffused and devastating neurodegenerative disorder, Alzheimer's disease, proposing a relationship between neurodegeneration, loss of neurotrophic factors, and cardiac nervous system impairment. This overview is conducive to a more comprehensive understanding of the process of cardiac neuro-signaling dysfunction, while bringing to light potential therapeutic scenarios to correct or delay the adverse cardiovascular remodeling, thus improving the cardiac prognosis and quality of life in patients with heart or neurodegenerative disorders.

Autonomic modulation and cardiac arrhythmias: old insights and novel strategies

The autonomic nervous system (ANS) plays a critical role in both health and states of cardiovascular disease. There has been a long-recognized role of the ANS in the pathogenesis of both atrial and ventricular arrhythmias (VAs). This historical understanding has been expanded in the context of evolving insights into the anatomy and physiology of the ANS, including dysfunction of the ANS in cardiovascular disease such as heart failure and myocardial infarction. An expanding armamentarium of therapeutic strategies-both invasive and non-invasive-have brought the potential of ANS modulation to contemporary clinical practice. Here, we summarize the integrative neuro-cardiac anatomy underlying the ANS, review the physiological rationale for autonomic modulation in atrial and VAs, highlight strategies for autonomic modulation, and finally frame future challenges and opportunities for ANS therapeutics.

Dysregulation of the Renin-Angiotensin System and the Vasopressinergic System Interactions in Cardiovascular Disorders

Purpose of Review

In many instances, the renin-angiotensin system (RAS) and the vasopressinergic system (VPS) are jointly activated by the same stimuli and engaged in the regulation of the same processes.

Recent Findings

Angiotensin II (Ang II) and arginine vasopressin (AVP), which are the main active compounds of the RAS and the VPS, interact at several levels. Firstly, Ang II, acting on AT1 receptors (AT1R), plays a significant role in the release of AVP from vasopressinergic neurons and AVP, stimulating V1a receptors (V1aR), regulates the release of renin in the kidney. Secondly, Ang II and AVP, acting on AT1R and V1aR, respectively, exert vasoconstriction, increase cardiac contractility, stimulate the sympathoadrenal system, and elevate blood pressure. At the same time, they act antagonistically in the regulation of blood pressure by baroreflex. Thirdly, the cooperative action of Ang II acting on AT1R and AVP stimulating both V1aR and V2 receptors in the kidney is necessary for the appropriate regulation of renal blood flow and the efficient resorption of sodium and water. Furthermore, both peptides enhance the release of aldosterone and potentiate its action in the renal tubules.

Summary

In this review, we (1) point attention to the role of the cooperative action of Ang II and AVP for the regulation of blood pressure and the water-electrolyte balance under physiological conditions, (2) present the subcellular mechanisms underlying interactions of these two peptides, and (3) provide evidence that dysregulation of the cooperative action of Ang II and AVP significantly contributes to the development of disturbances in the regulation of blood pressure and the water-electrolyte balance in cardiovascular diseases.

Impaired neuronal and vascular responses to angiotensin II in a rabbit congestive heart failure model

Congestive heart failure (CHF) is characterised by activation of the renin-angiotensin-aldosterone system (RAAS) and the sympathetic nervous system (SNS). Both systems are known to interact and to potentiate each other's activities. We recently demonstrated that angiotensin II (Ang II) enhances sympathetic nerve traffic via prejunctionally-located AT1-receptors. At present, little is known about the effects of Ang II at the level of the sympathetic neurones in CHF.

Accordingly, we investigated the effect of Ang II in the presence and absence of the AT1-receptor antagonist, eprosartan, on stimulation-induced nerve traffic in isolated thoracic aorta preparations obtained from rabbits suffering from experimentally-induced CHF. Control-preparations were obtained from age-matched animals. Sympathetic activity was assessed by a [3H]noradrenaline spill-over model. Additionally, Ang II constrictor responses were compared between CHF and control vessels in the presence and absence of eprosartan. Additionally, to study postjunctional facilitation, the effects of Ang II on postsynaptic α-adrenoceptor-mediated responses were studied using noradrenaline.

Stimulation-evoked SNS-neurotransmission was similar in both groups (CHF versus control). Ang II (0.1 nM—0.1 µM) caused a concentration-dependent increase of the stimulation-evoked sympathetic outflow in both groups, with a maximum at 10 nM (control [n=7], FR2/FR12.03±0.11 and CHF-preparations [n=7], FR2/FR11.71±0.07). The enhancement by Ang II was decreased in CHF-preparations compared with controls (p<0.05). Eprosartan concentration-dependently attenuated the Ang II-enhanced (10 nM) sympathetic outflow in both CHF- and control preparations. The sympathoinhibitory potency of eprosartan was similar in both groups (control pIC508.81±0.31; CHF 8.65±0.42).

Ang II (1 nM—0.3 µM) concentration-dependently increased the contractile force in control preparations (Emax21.64±3.86 mN, pD27.63±0.02, n=7). Eprosartan (1 nM—0.1 µM) influenced the Ang IIcontractions via a mixed form of antagonism. In CHF-preparations, Ang II caused impaired vascular contraction. The KCl-induced contraction was decreased in the CHF- compared with control preparations (13.02±0.64 mN versus 30.40±0.89 mN). The relative Ang II contraction (% of KCl) was also decreased (2.3% vs. 58.0%). Concentration-response curves to noradrenaline (%KCl) were similar (control pD26.93±0.05, Emax131.0±2.7; CHF pD27.00±0.05, Emax136.7±2.6) (p>0.05) and were not affected by Ang II.

We conclude that Ang II-enhanced sympathetic neurotransmission is mediated by the prejunctional AT1-receptor in both control and CHF-preparations. The decreased facilitation of SNS effects by Ang II may be explained by down-regulation or desensitisation of the neuronal AT1-receptor. Additionally, the aortic contractile capacity in heart failure rabbits appears to be decreased, probably as a result of heart failure-associated neuroendocrine and functional changes.

Arterial baroreceptor reflex control of renal sympathetic nerve activity following chronic myocardial infarction in male, female, and ovariectomized female rats

There is controversy regarding whether the arterial baroreflex control of renal sympathetic nerve activity (SNA) in heart failure is altered. We investigated the impact of sex and ovarian hormones on changes in the arterial baroreflex control of renal SNA following a chronic myocardial infarction (MI). Renal SNA and arterial pressure were recorded in chloralose-urethane anesthetized male, female, and ovariectomized female (OVX) Wistar rats 6-7 wk postsham or MI surgery. Animals were grouped according to MI size (sham, small and large MI). Ovary-intact females had a lower mortality rate post-MI (24%) compared with both males (38%) and OVX (50%) (P < 0.05). Males and OVX with large MI, but not small MI, displayed an impaired ability of the arterial baroreflex to inhibit renal SNA. As a result, the male large MI group (49 ± 6 vs. 84 ± 5% in male sham group) and OVX large MI group (37 ± 3 vs. 75 ± 5% in OVX sham group) displayed significantly reduced arterial baroreflex range of control of normalized renal SNA (P < 0.05). In ovary-intact females, arterial baroreflex control of normalized renal SNA was unchanged regardless of MI size. In males and OVX there was a significant, positive correlation between left ventricle (LV) ejection fraction and arterial baroreflex range of control of normalized renal SNA, but not absolute renal SNA, that was not evident in ovary-intact females. The current findings demonstrate that the arterial baroreflex control of renal SNA post-MI is preserved in ovary-intact females, and the state of left ventricular dysfunction significantly impacts on the changes in the arterial baroreflex post-MI.

Central integration and neural control of blood pressure during the cold pressor test: a comparison between hydrochlorothiazide and aliskiren

Individuals with hypertension and sympathetic overactivity are at risk for cardiovascular events. Renin inhibitors are new while thiazide diuretics are first-class drugs used for treatment of hypertension. The purpose of this study was to determine whether 6 months of treatment with aliskiren (ALSK) or hydrochlorothiazide (HCTZ) would alter blood pressure (BP) and muscle sympathetic nerve activity (MSNA) indices in older mild hypertensives during a cold pressor test (CPT). We hypothesized that the ALSK group would demonstrate a blunted response compared to HCTZ. Nineteen (9 men, 10 women) subjects performed a CPT pre- and post treatment where heart rate (HR), systolic BP (SBP) and diastolic BP (DBP), and MSNA were measured. Blood samples were withdrawn for assessment of renal-adrenal hormones. Both medications lowered ambulatory SBP and DBP (P < 0.05). Direct renin tended to be higher in the ALSK group after treatment (P = 0.081). Aldosterone was higher in the HCTZ group after treatment (P < 0.001). As expected, both groups showed increases in HR, SBP, DBP, and MSNA during the CPT (all P < 0.05). All cardiovascular and MSNA responses were similar pre- and post treatment in both groups (peak CPT SBP: 26 ± 10 vs. 17 ± 21 and 21 ± 20 vs. 29 ± 15 mmHg for pre vs. post for HCTZ and ALSK, respectively; peak CPT MSNA burst frequency: 13 ± 8 vs. 11 ± 11 and 11 ± 17 vs. 6 ± 13 bursts/min; all P > 0.05). Treatment with these antihypertensive medications lowered BP but was not successful in lowering the responsiveness to the CPT.

Regulation of the renal sympathetic nerves in heart failure

Heart failure (HF) is a serious debilitating condition with poor survival rates and an increasing level of prevalence. HF is associated with an increase in renal norepinephrine (NE) spillover, which is an independent predictor of mortality in HF patients. The excessive sympatho-excitation that is a hallmark of HF has long-term effects that contribute to disease progression. An increase in directly recorded renal sympathetic nerve activity (RSNA) has also been recorded in animal models of HF. This review will focus on the mechanisms controlling sympathetic nerve activity (SNA) to the kidney during normal conditions and alterations in these mechanisms during HF. In particular the roles of afferent reflexes and central mechanisms will be discussed.

Modulation of angiotensin II signaling following exercise training in heart failure

Sympathetic activation is a consistent finding in the chronic heart failure (CHF) state. Current therapy for CHF targets the renin-angiotensin II (ANG II) and adrenergic systems. Angiotensin converting enzyme (ACE) inhibitors and ANG II receptor blockers are standard treatments along with β-adrenergic blockade. However, the mortality and morbidity of this disease is still extremely high, even with good medical management. Exercise training (ExT) is currently being used in many centers as an adjunctive therapy for CHF. Clinical studies have shown that ExT is a safe, effective, and inexpensive way to improve quality of life, work capacity, and longevity in patients with CHF. This review discusses the potential neural interactions between ANG II and sympatho-excitation in CHF and the modulation of this interaction by ExT. We briefly review the current understanding of the modulation of the angiotensin type 1 receptor in sympatho-excitatory areas of the brain and in the periphery (i.e., in the carotid body and skeletal muscle). We discuss possible cellular mechanisms by which ExT may impact the sympatho-excitatory process by reducing oxidative stress, increasing nitric oxide. and reducing ANG II. We also discuss the potential role of ACE2 and Ang 1-7 in the sympathetic response to ExT. Fruitful areas of further investigation are the role and mechanisms by which pre-sympathetic neuronal metabolic activity in response to individual bouts of exercise regulate redox mechanisms and discharge at rest in CHF and other sympatho-excitatory states.

Temporal profile and mechanisms of the prompt sympathoexcitation following coronary ligation in Wistar rats

Our aim was to assess the timing and mechanisms of the sympathoexcitation that occurs immediately after coronary ligation. We recorded thoracic sympathetic (tSNA) and phrenic activities, heart rate (HR) and perfusion pressure in Wistar rats subjected to either ligation of the left anterior descending coronary artery (LAD) or Sham operated in the working heart-brainstem preparation. Thirty minutes after LAD ligation, tSNA had increased (basal: 2.5±0.2 µV, 30 min: 3.5±0.3 µV), being even higher at 60 min (5.2±0.5 µV, P<0.01); while no change was observed in Sham animals. HR increased significantly 45 min after LAD (P<0.01). Sixty minutes after LAD ligation, there was: (i) an augmented peripheral chemoreflex - greater sympathoexcitatory response (50, 45 and 27% of increase to 25, 50 and 75 µL injections of NaCN 0.03%, respectively, when compared to Sham, P<0.01); (ii) an elevated pressor response (32±1 versus 23±1 mmHg in Sham, P<0.01) and a reduced baroreflex sympathetic gain (1.3±0.1 versus Sham 2.0±0.1%.mmHg-1, P<0.01) to phenylephrine injection; (iii) an elevated cardiac sympathetic tone (ΔHR after atenolol: -108±8 versus -82±7 bpm in Sham, P<0.05). In contrast, no changes were observed in cardiac vagal tone and bradycardic response to both baroreflex and chemoreflex between LAD and Sham groups. The immediate sympathoexcitatory response in LAD rats was dependent on an excitatory spinal sympathetic cardiocardiac reflex, whereas at 3 h an angiotensin II type 1 receptor mechanism was essential since Losartan curbed the response by 34% relative to LAD rats administered saline (P<0.05). A spinal reflex appears key to the immediate sympathoexcitatory response after coronary ligation. Therefore, the sympathoexcitatory response seems to be maintained by an angiotensinergic mechanism and concomitant augmentation of sympathoexcitatory reflexes.

Chronic renin inhibition lowers blood pressure and reduces upright muscle sympathetic nerve activity in hypertensive seniors

Cardiovascular risk remains high in patients with hypertension even with adequate blood pressure (BP) control. One possible mechanism may be sympathetic activation via the baroreflex. We tested the hypothesis that chronic inhibition of renin reduces BP without sympathetic activation, but diuresis augments sympathetic activity in elderly hypertensives. Fourteen patients with stage-I hypertension (66 ± 5 (SD) years) were treated with a direct renin inhibitor, aliskiren (n = 7), or a diuretic, hydrochlorothiazide (n = 7), for 6 months. Muscle sympathetic nerve activity (MSNA), BP, direct renin and aldosterone were measured during supine and a graded head-up tilt (HUT; 5 min 30° and 20 min 60°), before and after treatment. Sympathetic baroreflex sensitivity (BRS) was assessed. Both groups had similar BP reductions after treatment (all P < 0.01), while MSNA responses were different between hydrochlorothiazide and aliskiren (P = 0.006 pre/post × drug). Both supine and upright MSNA became greater after hydrochlorothiazide treatment (supine, 72 ± 18 post vs. 64 ± 15 bursts (100 beats)(-1) pre; 60° HUT, 83 ± 10 vs. 78 ± 13 bursts (100 beats)(-1); P = 0.002). After aliskiren treatment, supine MSNA remained unchanged (69 ± 13 vs. 64 ± 8 bursts (100 beats)(-1)), but upright MSNA was lower (74 ± 15 vs. 85 ± 10 bursts (100 beats)(-1); P = 0.012 for pre/post × posture). Direct renin was greater after both treatments (both P < 0.05), while upright aldosterone was greater after hydrochlorothiazide only (P = 0.002). The change in upright MSNA by the treatment was correlated with the change of aldosterone (r = 0.74, P = 0.002). Upright sympathetic BRS remained unchanged after either treatment. Thus, chronic renin inhibition may reduce upright MSNA through suppressed renin activity, while diuresis may evoke sympathetic activation via the upregulated renin-angiotensin-aldosterone system, without changing intrinsic sympathetic baroreflex function in elderly hypertensive patients.

Central angiotensin type 1 receptor blockade decreases cardiac but not renal sympathetic nerve activity in heart failure

In heart failure (HF), cardiac sympathetic nerve activity (SNA; CSNA) is increased, which has detrimental effects on the heart and promotes arrhythmias and sudden death. There is evidence that the central renin-angiotensin system plays an important role in stimulating renal SNA in HF. Because SNA to individual organs is differentially controlled, we have investigated whether central angiotensin receptor blockade decreases CSNA in HF. We simultaneously recorded CSNA and renal SNA in conscious normal sheep and in sheep with HF induced by rapid ventricular pacing (ejection fraction: <40%). The effect of blockade of central angiotensin type 1 receptors by intracerebroventricular infusion of losartan (1 mg/h for 5 hours) on resting levels and baroreflex control of CSNA and renal SNA were determined. In addition, the levels of angiotensin receptors in central autonomic nuclei were determined using autoradiography. Sheep in HF had a large increase in CSNA (43±2 to 88±3 bursts per 100 heart beats; P<0.05) and heart rate, with no effect on renal SNA. In HF, central infusion of losartan for 5 hours significantly reduced the baseline levels of CSNA (to 69±5 bursts per 100 heart beats) and heart rate. Losartan had no effect in normal animals. In HF, angiotensin receptor levels were increased in the paraventricular nucleus and supraoptic nucleus but reduced in the area postrema and nucleus tractus solitarius. In summary, infusion of losartan reduced the elevated levels of CNSA in an ovine model of HF, indicating that central angiotensin receptors play a critical role in stimulating the increased sympathetic activity to the heart.

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.

Response of cardiac sympathetic nerve activity to intravenous irbesartan in heart failure

To determine the effect of irbesartan treatment on resting levels and arterial baroreflex control of cardiac sympathetic nerve activity (CSNA) in heart failure (HF), we studied conscious normal sheep and sheep with HF induced by rapid ventricular pacing for 8-10 wk (n = 7 per group). In HF, there is a large increase in CSNA that is detrimental to outcome. The causes of this increase in CSNA and the effect of angiotensin receptor blockers on CSNA in HF are unclear. CSNA, arterial blood pressure, heart rate (HR), and arterial baroreflex curves were recorded during a resting period and after 90 min of irbesartan infusion (12 mg.kg(-1).h(-1) iv). This dose of irbesartan abolished the pressor response to intravenous ANG II infusion but caused only a slight decrease in the pressor response to centrally administered ANG II. In HF, there was a large increase in CSNA (from 44 +/- 3 to 87 +/- 3 bursts/100 heartbeats). Irbesartan reduced arterial pressure in the normal and HF groups, but the usual baroreflex-mediated increases in CSNA and HR were prevented. This resulted from a significant leftward shift in the CSNA and HR baroreflex curves in both groups. Irbesartan also decreased the sensitivity of the arterial baroreflex control of CSNA. Short-term treatment with an angiotensin receptor blocker, at a dose that abolished the response to circulating, but not central, ANG II, prevented the reflex increase in CSNA in response to the drug-induced fall in arterial pressure.

Exercise training normalizes ACE and ACE2 in the brain of rabbits with pacing-induced heart failure

Exercise training (EX) normalizes sympathetic outflow and plasma ANG II in chronic heart failure (CHF). The central mechanisms by which EX reduces this sympathoexcitatory state are unclear, but EX may alter components of the brain renin-angiotensin system (RAS). Angiotensin-converting enzyme (ACE) may mediate an increase in sympathetic nerve activity (SNA). ACE2 metabolizes ANG II to ANG-(1-7), which may have antagonistic effects to ANG II. Little is known concerning the regulation of ACE and ACE2 in the brain and the effect of EX on these enzymes, especially in the CHF state. This study aimed to investigate the effects of EX on the regulation of ACE and ACE2 in the brain in an animal model of CHF. We hypothesized that the ratio of ACE to ACE2 would increase in CHF and would be reduced by EX. Experiments were performed on New Zealand White rabbits divided into the following groups: sham, sham + EX, CHF, and CHF + EX (n = 5 rabbits/group). The cortex, cerebellum, medulla, hypothalamus, paraventricular nucleus (PVN), nucleus tractus solitarii (NTS), and rostral ventrolateral medulla (RVLM) were analyzed. ACE protein and mRNA expression in the cerebellum, medulla, hypothalamus, PVN, NTS, and RVLM were significantly upregulated in CHF rabbits (ratio of ACE to GAPDH: 0.3 +/- 0.03 to 0.8 +/- 0.10 in the RVLM, P < 0.05). EX normalized this upregulation compared with CHF (0.8 +/- 0.1 to 0.4 +/- 0.1 in the RVLM). ACE2 protein and mRNA expression decreased in CHF (ratio of ACE2 to GAPDH: 0.3 +/- 0.02 to 0.1 +/- 0.01 in the RVLM). EX increased ACE2 expression compared with CHF (0.1 +/- 0.01 to 0.8 +/- 0.1 in the RVLM). ACE2 was present in the cytoplasm of neurons and ACE in endothelial cells. These data suggest that the activation of the central RAS in animals with CHF involves an imbalance of ACE and ACE2 in regions of the brain that regulate autonomic function and that EX can reverse this imbalance.

Regulation of central angiotensin type 1 receptors and sympathetic outflow in heart failure

Angiotensin type 1 receptors (AT(1)Rs) play a critical role in a variety of physiological functions and pathophysiological states. They have been strongly implicated in the modulation of sympathetic outflow in the brain. An understanding of the mechanisms by which AT(1)Rs are regulated in a variety of disease states that are characterized by sympathoexcitation is pivotal in development of new strategies for the treatment of these disorders. This review concentrates on several aspects of AT(1)R regulation in the setting of chronic heart failure (CHF). There is now good evidence that AT(1)R expression in neurons is mediated by activation of the transcription factor activator protein 1 (AP-1). This transcription factor and its component proteins are upregulated in the rostral ventrolateral medulla of animals with CHF. Because the increase in AT(1)R expression and transcription factor activation can be blocked by the AT(1)R antagonist losartan, a positive feedback mechanism of AT(1)R expression in CHF is suggested. Oxidative stress has also been implicated in the regulation of receptor expression. Recent data suggest that the newly discovered catabolic enzyme angiotensin-converting enzyme 2 (ACE2) may play a role in the modulation of AT(1)R expression by altering the balance between the octapeptide ANG II and ANG- (1-7). Finally, exercise training reduces both central oxidative stress and AT(1)R expression in animals with CHF. These data strongly suggest that multiple central and peripheral influences dynamically alter AT(1)R expression in CHF.

Central sympathetic overactivity: maladies and mechanisms

There is growing evidence to suggest that many disease states are accompanied by chronic elevations in sympathetic nerve activity. The present review will specifically focus on central sympathetic overactivity and highlight three main areas of interest: 1) the pathological consequences of excessive sympathetic nerve activity; 2) the potential role of centrally derived nitric oxide in the genesis of neural dysregulation in disease; and 3) the promise of several novel therapeutic strategies targeting central sympathetic overactivity. The findings from both animal and human studies will be discussed and integrated in an attempt to provide a concise update on current work and ideas in these important areas.

Basis for the preferential activation of cardiac sympathetic nerve activity in heart failure

In heart failure (HF), sympathetic nerve activity is increased. Measurements in HF patients of cardiac norepinephrine spillover, reflecting cardiac sympathetic nerve activity (CSNA), indicate that it is increased earlier and to a greater extent than sympathetic activity to other organs. This has important consequences because it worsens prognosis, provoking arrhythmias and sudden death. To elucidate the mechanisms responsible for the activation of CSNA in HF, we made simultaneous direct neural recordings of CSNA and renal SNA (RSNA) in two groups of conscious sheep: normal animals and animals in HF induced by chronic, rapid ventricular pacing. In normal animals, the level of activity, measured as burst incidence (bursts of pulse related activity/100 heart beats), was significantly lower for CSNA (30 +/- 5%) than for RSNA (94 +/- 2%). Furthermore, the resting level of CSNA, relative to its maximum achieved while baroreceptors were unloaded by reducing arterial pressure, was set at a much lower percentage than RSNA. In HF, burst incidence of CSNA increased from 30 to 91%, whereas burst incidence of RSNA remained unaltered at 95%. The sensitivity of the control of both CSNA and RSNA by the arterial baroreflex remained unchanged in HF. These data show that, in the normal state, the resting level of CSNA is set at a lower level than RSNA, but in HF, the resting levels of SNA to both organs are close to their maxima. This finding provides an explanation for the preferential increase in cardiac norepinephrine spillover observed in HF.

Role of oxidant stress on AT1 receptor expression in neurons of rabbits with heart failure and in cultured neurons

We have previously reported that the expression of Angiotensin II (Ang II) type 1 receptors (AT1R) was increased in the rostral ventrolateral medulla (RVLM) of rabbits with chronic heart failure (CHF) and in the RVLM of normal rabbits infused with intracerebroventricular (ICV) Ang II. The present study investigated whether oxidant stress plays a role in Ang II-induced AT1R upregulation and its relationship to the transcription factor activator protein 1 (AP1) in CHF rabbits and in the CATHa neuronal cell line. In CATHa cells, Ang II significantly increased AT1R mRNA by 123+/-11%, P<0.01; c-Jun mRNA by 90+/-20%, P<0.01; c-fos mRNA by 148+/-49%, P<0.01; NADPH oxidase activity by 126+/-43%, P<0.01 versus untreated cells. Tempol and Apocynin reversed the increased expression of AT1R mRNA, c-Jun mRNA, c-fos mRNA, and superoxide production induced by Ang II. We also examined the effect of ICV Tempol on the RVLM of CHF rabbits. Compared to vehicle treated CHF rabbits, Tempol significantly decreased AT1R protein expression (1.6+/-0.29 versus 0.88+/-0.16, P<0.05), phosphorylated Jnk protein (0.4+/-0.05 versus 0.2+/-0.04, P<0.05), cytosolic phosphorylated c-Jun (0.56+/-0.1 versus 0.36+/-0.05, P<0.05), and nuclear phosphorylated c-Jun (0.67+/-0.1 versus 0.3+/-0.08, P<0.01). Tempol also significantly decreased the AP-1-DNA binding activity in the RVLM of CHF rabbits compared to the vehicle group (9.14 x 10(3) versus 41.95 x 10(3) gray level P<0.01). These data suggest that Ang II induces AT1R upregulation at the transcriptional level by induction of oxidant stress and activation of AP1 in both cultured neuronal cells and in intact brain of rabbits. Antioxidant agents may be beneficial in CHF and other states where brain Ang II is elevated by decreasing AT1R expression through the Jnk and AP1 pathway.

Mechanisms of sympathetic activation in heart failure

1. Heart Failure (HF) is a serious, debilitating condition with poor survival rates and an increasing level of prevalence. A characteristic of HF is a compensatory neurohumoral activation that increases with the severity of the condition. 2. The increase in sympathetic activity may be beneficial initially, providing inotropic support to the heart and peripheral vasoconstriction, but in the longer term it promotes disease progression and worsens prognosis. This is particularly true for the increase in cardiac sympathetic nerve activity, as shown by the strong inverse correlation between cardiac noradrenaline spillover and prognosis and by the beneficial effect of beta-adrenoceptor antagonists. 3. Possible causes for the raised level of sympathetic activity in HF include altered neural reflexes, such as those from baroreceptors and chemoreceptors, raised levels of hormones, such as angiotensin II, acting on circumventricular organs, and changes in central mechanisms that may amplify the responses to these inputs. 4. The control of sympathetic activity to different organs is regionally heterogeneous, as demonstrated by a lack of concordance in burst patterns, different responses to reflexes, opposite responses of cardiac and renal sympathetic nerves to central angiotensin and organ-specific increases in sympathetic activity in HF. These observations indicate that, in HF, it is essential to study the factors causing sympathetic activation in individual outflows, in particular those that powerfully, and perhaps preferentially, increase cardiac sympathetic nerve activity.

Angiotensin II--nitric oxide interactions in the control of sympathetic outflow in heart failure

Activation of the sympathetic nervous system is a compensatory mechanism which initially provides support for the circulation in the face of a falling cardiac output. It has been recognized for some time that chronic elevation of sympathetic outflow with the consequent increase in plasma norepinephrine, is counterproductive to improving cardiac function. Indeed, therapeutic targeting to block excessive sympathetic activation in heart failure is becoming a more accepted modality. The mechanism(s) by which sympathetic excitation occurs in the heart failure state are not completely understood. Components of abnormal cardiovascular reflex regulation most likely contribute to this sympatho-excitation. However, central mechanisms which relate to the elaboration of angiotensin II (Ang II) and nitric oxide (NO) may also play an important role. Ang II has been shown to be a sympatho-excitatory peptide in the central nervous system while NO is sympatho-inhibitory. Recent studies have demonstrated that blockade of Ang II receptors of the AT(1) subtype augments arterial baroreflex control of sympathetic nerve activity in the heart failure state, thereby predisposing to a reduction in sympathetic tone. Ang II and NO interact to regulate sympathetic outflow. Blockade of NO production in normal conscious rabbits was only capable of increasing sympathetic outflow when accompanied by a background infusion of Ang II. Conversely, providing a source of NO to rabbits with heart failure reduced sympathetic nerve activity when accompanied by blockade of AT(1) receptors. Chronic heart failure is also associated with a decrease in NO synthesis in the brain as indicated by a reduction in the mRNA for the neuronal isoform (nNOS). Chronic blockade of Ang II receptors can up regulate nNOS expression. In addition, exercise training of rabbits with developing heart failure has been shown to reduce sympathetic tone, decrease plasma Ang II, improve arterial baroreflex function and increase nNOS expression in the central nervous system. This review summarizes a large number of studies which have concentrated on the mechanisms of sympatho-excitation in heart failure. It now seems clear that one mechanism which is important in regulating sympathetic outflow in this disease state depends upon a central interaction between Ang II and NO at the cellular and nuclear levels.

Cardiac sympathetic afferent reflexes in heart failure

Heart failure is characterized by an elevation in sympathetic tone. The mechanisms responsible for this sympatho-excitation of heart failure are not completely understood. Several studies from this laboratory have compared differences in the cardiac "sympathetic afferent" reflex between sham dogs and dogs with pacing-induced heart failure. We found 1) that the cardiac sympathetic afferent reflex is augmented in heart failure, 2) tonic cardiac sympathetic afferent inputs play an important role in the elevated sympathetic tone in heart failure, 3) cardiac sympathetic afferents are sensitized in heart failure and 4) the central gain of the cardiac sympathetic afferent reflex in heart failure is sensitized and that this sensitization may be related to augmented central Ang II and blunted NO mechanisms. These studies integrate into the regulation of sympathetic outflow in heart failure which is likely to be mediated by a variety of peripheral inputs modulated by central substances. If the cardiac sympathetic afferent reflex is one of the excitatory reflexes which contribute to sympathetic activation in heart failure, a comprehensive understanding of neuro-humoral regulation of this reflex may result in more definitives and rational therapy targeted to the sympathetic nervous system in this disease state.

Chemoreflex function in heart failure

Peripheral and central chemoreflexes are the dominant autonomic mechanisms regulating ventilatory patterns in response to changes in partial pressures of oxygen and carbon dioxide in arterial blood and exert powerful effects on neural circulatory control. Both reflex pathways are capable of eliciting increases in sympathetic nerve traffic and consequent increases in blood pressure. Chronic heart failure is accompanied by a sustained elevation in sympathetic nerve traffic, which is thought to be an important component in the pathophysiology and progression of the disease. The role of chemoreflex mechanisms in the control of sympathetic function during heart failure is an important topic for which there are many questions and few answers. This review summarizes available evidence documenting peripheral and central chemoreflex function in heart failure, possible mechanisms for their alteration, and their possible contribution to ventilatory, and circulatory abnormalities that occur in heart failure.

Exercise training improves endogenous nitric oxide mechanisms within the paraventricular nucleus in rats with heart failure

Previously, we have demonstrated that an altered endogenous nitric oxide (NO) mechanism within the paraventricular nucleus (PVN) contributes to increased renal sympathetic nerve activity (RSNA) in heart failure (HF) rats. The goal of this study was to examine the effect of exercise training (ExT) in improving the endogenous NO mechanism within the PVN involved in the regulation of RSNA in rats with HF. ExT significantly restored the decreased number of neuronal NO synthase (nNOS)-positive neurons in the PVN (129 +/- 17 vs. 99 +/- 6). nNOS mRNA expression and protein levels in the PVN were also significantly increased in HF-ExT rats compared with HF-sedentary rats. To examine the functional role of NO within the PVN, an inhibitor of NOS, N(G)-monomethyl-L-arginine, was microinjected into the PVN. Dose-dependent increases in RSNA, arterial blood pressure (BP), and heart rate (HR) were produced in all rats. There was a blunted increase in these parameters in HF rats compared with the sham-operated rats. ExT significantly augmented RSNA responses in rats with HF (33% vs. 20% at the highest dose), thus normalizing the responses. The NO donor sodium nitroprusside, microinjected into the PVN, produced dose-dependent decreases in RSNA, BP, and HR in both sham and HF rats. ExT significantly improved the blunted decrease in RSNA in HF rats (36% vs. 17% at the highest dose). In conclusion, our data indicate that ExT improves the altered NO mechanism within the PVN and restores NO-mediated changes in RSNA in rats with HF.

Sympathoexcitation by central ANG II: roles for AT1 receptor upregulation and NAD(P)H oxidase in RVLM

Chronic heart failure is often associated with sympathoexcitation and blunted arterial baroreflex function. These phenomena have been causally linked to elevated central ANG II mechanisms. Recent studies have shown that NAD(P)H oxidase-derived reactive oxygen species (ROS) are important mediators of ANG II signaling and therefore might play an essential role in these interactions. The aims of this study were to determine whether central subchronic infusion of ANG II in normal animals has effects on O2- production and expression of NAD(P)H oxidase subunits as well as ANG II type 1 (AT1) receptors in the rostral ventrolateral medulla (RVLM). Twenty-four male New Zealand White rabbits were divided into four groups and separately received a subchronic intracerebroventricular infusion of saline alone, ANG II alone, ANG II with losartan, and losartan alone for 1 wk. On day 7 of intracerebroventricular infusion, mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) values were recorded, and arterial baroreflex sensitivity was evaluated while animals were in the conscious state. We found that ANG II significantly increased baseline RSNA (161.9%; P < 0.05), mRNA and protein expression of AT1 receptors (mRNA, 66.7%; P < 0.05; protein, 85.1%; P < 0.05), NAD(P)H oxidase subunits (mRNA, 120.0-200.0%; P < 0.05; protein, 90.9-197.0%; P < 0.05), and O2- production (83.2%; P < 0.05) in the RVLM. In addition, impaired baroreflex control of HR (Gain(max) reduced by 48.2%; P < 0.05) and RSNA (Gain(max) reduced by 53.6%; P < 0.05) by ANG II was completely abolished by losartan. Losartan significantly decreased baseline RSNA (-49.5%; P < 0.05) and increased baroreflex control of HR (Gain(max) increased by 64.8%; P < 0.05) and RSNA (Gain(max) increased by 67.9%; P < 0.05), but had no significant effects on mRNA and protein expression of AT1 receptor and NAD(P)H oxidase subunits and O2- production in the RVLM. These data suggest that in normal rabbits, NAD(P)H oxidase-derived ROS play an important role in the modulation of sympathetic activity and arterial baroreflex function by subchronic central treatment of exogenous ANG II via AT1 receptors.

Interactions between the sympathetic nervous system and the RAAS in heart failure

Therapy for heart failure caused by left ventricular systolic dysfunction is based on interference with maladaptive activation of the sympathetic nervous system (SNS) and the renin-angiotensin-aldosterone system (RAAS). Agents that block beta-adrenergic receptors, decrease angiotensin-II formation, and antagonize the effects of angiotensin II and aldosterone have been shown to improve morbidity and mortality in this syndrome. Therefore, from a theoretical point of view, it would be desirable to actually diminish the degree of overactivity of these two homeostatic systems. There are compelling physiologic arguments and much experimental data to suggest that beta-adrenergic blockade may diminish activity of the RAAS. Conversely, angiotensin-converting enzyme inhibitors, angiotensin-II antagonists, and aldosterone antagonists may diminish activity of the SNS. Some clinical trials data may be interpreted in a fashion that suggests that part of the benefit of interfering with each system may relate to diminishing activity of the other. If true, combined therapy may lead to a virtuous cycle in which diminishing the adverse effects of each individual system is combined with reduced activity of the other. Such a cycle may be one factor underlying the impressive clinical results of recent neurohormonally based therapeutic trials and reinforces the need to look beyond acute hemodynamic effects of therapeutic agents when assessing their long-term impact in heart failure.

Cardiac sympathetic afferent stimulation impairs baroreflex control of renal sympathetic nerve activity in rats

It is well known that cardiac sympathetic afferent reflexes contribute to increases in sympathetic outflow and that sympathetic activity can antagonize arterial baroreflex function. In this study, we tested the hypothesis that in normal rats, chemical and electrical stimulation of cardiac sympathetic afferents results in a decrease in the arterial baroreflex function by increasing sympathetic nerve activity. Under alpha-chloralose (40 mg/kg) and urethane (800 mg/kg i.p.) anesthesia, renal sympathetic nerve activity, mean arterial pressure, and heart rate were recorded. The arterial baroreceptor reflex was evaluated by infusion of nitroglycerin (25 microg i.v.) and phenylephrine (10 microg i.v.). Left ventricular epicardial application of capsaicin (0.4 microg in 2 microl) blunted arterial baroreflex function by 46% (maximum slope 3.5 +/- 0.3 to 1.9 +/- 0.2%/mmHg, P < 0.01). When the central end of the left cardiac sympathetic nerve was electrically stimulated (7 V, 1 ms, 20 Hz), the sensitivity of the arterial baroreflex was similarly decreased by 42% (maximum slope 3.2 +/- 0.3 to 1.9 +/- 0.4%/mmHg; P < 0.05). Pretreatment with intracerebroventricular injection of losartan (500 nmol in 1 microl of artificial cerebrospinal fluid) completely prevented the impairment of arterial baroreflex function induced by electrical stimulation of the central end of the left cardiac sympathetic nerve (maximum slope 3.6 +/- 0.4 to 3.1 +/- 0.5%/mmHg). These results suggest that the both chemical and electrical stimulation of the cardiac sympathetic afferents reduces arterial baroreflex sensitivity and the impairment of arterial baroreflex function induced by cardiac sympathetic afferent stimulation is mediated by central angiotensin type 1 receptors.

Redox signaling in central neural regulation of cardiovascular function

One of the most prominent concepts to emerge in cardiovascular research over the past decade, especially in areas focused on angiotensin II (AngII), is that reactive oxygen species (ROS) are critical signaling molecules in a wide range of cellular processes. Many of the physiological effects of AngII are mediated by ROS, and alterations in AngII-mediated redox mechanisms are implicated in cardiovascular diseases such as hypertension and atherosclerosis. Although most investigations to date have focused on the vasculature as a key player, the nervous system has recently begun to gain attention in this field. Accumulating evidence suggests that ROS have important effects on central neural mechanisms involved in blood pressure regulation, volume homeostasis, and autonomic function, particularly those that involve AngII signaling. Furthermore, oxidant stress in the central nervous system is implicated in the neuro-dysregulation associated with some forms of hypertension and heart failure. The main objective of this review is to discuss the recent progress and prospects for this new field of central redox signaling in cardiovascular regulation, while also addressing the molecular tools that have spurred it forward.

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.

Intravenous angiotensin II does not affect dynamic baroreflex characteristics of the neural or peripheral arc

Although the elevation of angiotensin II (Ang II) associated with cardiovascular diseases has been considered to suppress the arterial baroreflex function, how Ang II affects dynamic arterial pressure (AP) regulation remains unknown. The aim of the present study was to elucidate the acute effects of Ang II on dynamic AP regulation by the arterial baroreflex. In seven anesthetized Japanese white rabbits, we randomly perturbed intra-carotid sinus pressure (CSP) according to a binary white noise sequence while recording renal sympathetic nerve activity (RSNA) and AP. We estimated the neural arc transfer function from CSP to RSNA and the peripheral arc transfer function from RSNA to AP before and after 30-min intravenous administration of Ang II (100 ng/kg/min). Ang II increased mean AP from 75.7 +/- 3.1 to 95.5 +/- 5.1 mmHg (p < 0.01), while it did not affect mean RSNA (from 5.9 +/- 1.3 to 5.7 +/- 1.2 a.u.). The neural arc transfer functions did not differ before or after Ang II administration (dynamic gain: -0.94 +/- 0.04 vs. -0.94 +/- 0.13, corner frequency: 0.06 +/- 0.01 vs.0.06 +/- 0.01 Hz, pure delay: 0.16 +/- 0.01 vs. 0.17 +/- 0.02 s). The peripheral arc transfer function did not differ before or after Ang II administration (dynamic gain: 1.18 +/- 0.05 vs. 1.06 +/- 0.11, natural frequency: 0.07 +/- 0.01 vs. 0.08 +/- 0.01 Hz, damping ratio: 1.19 +/- 0.06 vs. 1.24 +/- 0.19, pure delay: 0.83 +/- 0.06 vs. 0.78 +/- 0.05 s). Intravenous Ang II hardly affects the dynamic characteristics of neural and peripheral arc around the physiological operating pressure.

Decreased facilitation by angiotensin II of noradrenergic neurotransmission in isolated mesenteric artery of rabbits with chronic heart failure

Both in human and in experimental heart failure (HF), the renin-angiotensin system and the sympathetic nervous system are activated. In a previous study a facilitatory action of angiotensin II (Ang II) was shown in the rabbit mesenteric artery, which was mediated via prejunctionally located Ang II type 1 (AT ) receptors. Very little is known about the effects of Ang II on sympathetic neurotransmission at the peripheral level in congestive heart failure (CFH). Accordingly, in the isolated mesenteric arteries obtained from rabbits with experimentally induced CHF, as well as in age-matched control rabbits, the effect of Ang II on contractions provoked by electrical field stimulation was investigated in the presence and absence of the AT receptor antagonist eprosartan. Additionally, to investigate a possible postjunctional facilitation, the effects of Ang II on alpha-adrenoceptor-mediated responses were studied using noradrenaline (NA). Lastly, the vasoconstrictor effects of Ang II were compared between HF rabbits and controls, by constructing concentration-response curves to Ang II. In control rabbits, Ang II 0.5 n caused an enhancement of stimulation-induced responses by a factor 3.2 +/- 0.5, 2.4 +/- 0.3, and 1.5 +/- 0.08, at 1, 2, and 4 Hz, respectively ( < 0.05 at all frequencies compared with vehicle). In rabbits with HF, the enhancement by Ang II (0.5 n ) amounted to a factor 2.1 +/- 0.2, 1.7 +/- 0.1, and 1.2 +/- 0.04, at 1, 2, and 4 Hz, respectively ( < 0.05 compared with vehicle at all frequencies). Accordingly, the enhancing effect of Ang II was more pronounced in the control group compared with rabbits with HF ( < 0.05 at each frequency). Eprosartan (1 nM -0.1 microM) could inhibit the facilitatory effects of Ang II in arteries from HF as well as from control rabbits. Contractile responses to exogenous NA (3 n -0.1 m ) were the same in HF rabbits and controls, and they were unaltered in the presence of Ang II 0.5 n Ang II (0.1 nM -1 microM) caused a concentration-dependent increase in contractile force, which was the same in HF rabbits and controls. From these findings it can be concluded that in rabbits with CHF as well as in control animals, Ang II facilitates the stimulation-induced vasoconstrictor responses via prejunctionally located AT receptors. The facilitating effect was decreased in vessels obtained from rabbits with CHF, whereas responses to exogenous Ang II were unchanged. These findings may be explained by downregulation or uncoupling of the prejunctional AT receptor.

Effects of the AT1-receptor antagonist eprosartan on the progression of left ventricular dysfunction in dogs with heart failure

1. We examined the effects of eprosartan, an AT(1) receptor antagonist, on the progression of left ventricular (LV) dysfunction and remodelling in dogs with heart failure (HF) produced by intracoronary microembolizations (LV ejection fraction, EF 30 to 40%). 2. Dogs were randomized to 3 months of oral therapy with low-dose eprosartan (600 mg once daily, n=8), high-dose eprosartan (1200 mg once daily, n=8), or placebo (n=8). 3. In the placebo group, LV end-diastolic (EDV) and end-systolic (ESV) volumes increased after 3 months (68+/-7 vs 82+/-9 ml, P<0.004, 43+/-1 vs 58+/-7 ml, P<0.003, respectively), and EF decreased (37+/-1 vs 29+/-1%, P<0.001). In dogs treated with low-dose eprosartan, EF, EDV, and ESV remained unchanged over the course of therapy, whereas in dogs treated with high-dose eprosartan, EF increased (38+/-1 vs 42+/-1%, P<0.004) and ESV decreased (41+/-1 vs 37+/-1 ml, P<0.006), Eprosartan also decreased interstitial fibrosis and cardiomyocyte hypertrophy. 4. We conclude that eprosartan prevents progressive LV dysfunction and attenuates progressive LV remodelling in dogs with moderate HF and may be useful in treating patients with chronic HF.

Acute effect of endothelin AB antagonist on sympathetic outflow in conscious rats with heart failure

Although ET-1 antagonists have been beneficial in the treatment of heart failure (HF), their involvement in the effect on the sympathetic nervous system in HF remains unknown. The present study investigated the role of endogenous endothelin (ET) in the sympathetic nervous system in HF by observing the effect of ET AB antagonist (TAK-044) on renal sympathetic nerve activity (RSNA) in conscious rats with HF (n = 7). HF was induced by left coronary artery ligation and 6 weeks later, TAK-044 was intravenously administered in the conscious and freely moving rats. RSNA, mean arterial pressure (MAP) and heart rate were compared with rats with sham operations (sham; n = 7). MAP was significantly decreased in both groups; however, RSNA was significantly decreased only in the HF group at 5 min after administration, and this change continued until 10 min. There was also an effect of TAK-044 on the arterial baroreflex function indicated by the slope of RSNA to the changes in MAP during phenylephrine and nitroprusside injection in both groups. Compared with the sham group, the HF group showed impaired arterial baroreflex control of RSNA during phenylephrine injection, and intravenous administration of TAK-044 normalized this abnormality, whereas the function in the sham group was not changed. These data show that ET AB antagonist suppressed renal sympathetic activity in rats with HF, and improved arterial baroreflex function. The beneficial effect of endothelin antagonist on heart failure may involve improvement of the increased sympathoexcitation and impaired arterial baroreflex function in HF.

Microinjection of ANG II into paraventricular nucleus enhances cardiac sympathetic afferent reflex in rats

The aims of present study were to determine whether angiotensin II (ANG II) in the paraventricular nucleus (PVN) is involved in the central integration of the cardiac sympathetic afferent reflex and whether this effect is mediated by the ANG type 1 (AT(1)) receptor. While the animals were under alpha-chloralose and urethane anesthesia, mean arterial pressure, heart rate, and renal sympathetic nerve activity (RSNA) were recorded in sinoaortic-denervated and cervical-vagotomized rats. A cannula was inserted into the left PVN for microinjection of ANG II. The cardiac sympathetic afferent reflex was tested by electrical stimulation (5, 10, 20, and 30 Hz in 10 V and 1 ms) of the afferent cardiac sympathetic nerves or epicardial application of bradykinin (BK) (0.04 and 0.4 microg in 2 microl). Microinjection of ANG II (0.03, 0.3, and 3 nmol) into the PVN resulted in dose-related increases in the RSNA responses to electrical stimulation. The percent change of RSNA response to 20- and 30-Hz stimulation increased significantly at the highest dose of ANG II (3 nmol). The effects of ANG II were prevented by pretreatment with losartan (50 nmol) into the PVN. Microinjection of ANG II (0.3 nmol) into the PVN significantly enhanced the RSNA responses to epicardial application of BK, which was abolished by pretreatment with losartan (50 nmol) into the PVN. These results suggest that exogenous ANG II in the PVN augments the cardiac sympathetic afferent reflex evoked by both electrical stimulation of cardiac sympathetic afferent nerves and epicardial application of BK. These central effects of ANG II are mediated by AT(1) receptors.

Endogenous angiotensin II in the NTS contributes to sympathetic activation in rats with aortocaval shunt

Recent studies have suggested that the central nervous system is responsible for activation of sympathetic nerve activity (SNA) and the renin-angiotensin system in heart failure (HF). The aim of this study was to determine whether activation of the renin-angiotensin system within the nucleus of the solitary tract (NTS) plays a role in enhanced SNA in HF. High-output HF was induced by an aortocaval (A-V) shunt with some modifications in the rat. These rats exhibited a left ventricular dilatation and hemodynamic signs of high-output HF. Urinary catecholamine excretion and maximal renal SNA (RSNA) were greater in the A-V shunted rats than in the control rats. Microinjection of an angiotensin II type 1-receptor antagonist, CV11974, into the NTS was performed. The arterial pressure and RSNA were reduced by CV11974 to a greater degree in the A-V shunted rats than in the control rats. The expression of angiotensin-converting enzyme mRNA in the medulla was greater in the A-V shunted rats than in the control rats. These results suggest that activation of the renin-angiotensin system within the NTS contributes to an enhanced SNA in this model.

Peripheral and central interactions between the renin-angiotensin system and the renal sympathetic nerves in control of renal function

Increases in renal sympathetic nerve activity (RSNA) regulate the functions of the nephron, the vasculature, and the renin-containing juxtaglomerular granular cells. As increased activity of the renin-angiotensin system can also influence nephron and vascular function, it is important to understand the interactions between RSNA and the renin-angiotensin system in the control of renal function. These interactions can be intrarenal, that is, the direct (via specific innervation) and indirect (via angiotensin II) contributions of increased RSNA to the regulation of renal function. The effects of increased RSNA on renal function are attenuated when the activity of the renin-angiotensin system is suppressed or antagonized with angiotensin-converting enzyme inhibitors or angiotensin II-type AT1 receptor antagonists. The effects of intrarenal administration of angiotensin II are attenuated following renal denervation. These interactions can also be extrarenal, that is, in the central nervous system, wherein RSNA and its arterial baroreflex control are modulated by changes in activity of the renin-angiotensin system. In addition to the circumventricular organs, the permeable blood-brain barrier of which permits interactions with circulating angiotensin II, there are interactions at sites behind the blood-brain barrier that depend on the influence of local angiotensin II. The responses to central administration of angiotensin II type AT1 receptor antagonists, into the ventricular system or microinjected into the rostral ventrolateral medulla, are modulated by changes in activity of the renin-angiotensin system produced by physiological changes in dietary sodium intake. Similar modulation is observed in pathophysiological models wherein activity of both the renin-angiotensin and sympathetic nervous systems is increased (e.g., congestive heart failure). Thus, both renal and extrarenal sites of interaction between the renin-angiotensin system and RSNA are involved in influencing the neural control of renal function.

Chronic endothelin-1 blockade reduces sympathetic nerve activity in rabbits with heart failure

Endothelin-1 (ET-1) is elevated in chronic heart failure (CHF). In this study, we determined the effects of chronic ET-1 blockade on renal sympathetic nerve activity (RSNA) in conscious rabbits with pacing-induced CHF. Rabbits were chronically paced at 320–340 beats/min for 3–4 wk until clinical and hemodynamic signs of CHF were present. Resting RSNA and arterial baroreflex control of RSNA were determined. Responses were determined before and after the ET-1 antagonist L-754,142 (a combined ETA and ETB receptor antagonist, n = 5) was administered by osmotic minipump infusion (0.5 mg · kg−1 · h−1 for 48 h). In addition, five rabbits with CHF were treated with the specific ETA receptor antagonist BQ-123. Baseline RSNA (expressed as a percentage of the maximum nerve activity during sodium nitroprusside infusion) was significantly higher (58.3 ± 4.9 vs. 27.0 ± 1.0, P < 0.001), whereas baroreflex sensitivity was significantly lower in rabbits with CHF compared with control (3.09 ± 0.19 vs. 6.04 ± 0.73, P < 0.001). L-754,142 caused a time-dependent reduction in arterial pressure and RSNA in rabbits with CHF. In addition, BQ-123 caused a reduction in resting RSNA. For both compounds, RSNA returned to near control levels 24 h after removal of the minipump. These data suggest that ET-1 contributes to sympathoexcitation in the CHF state. Enhancement of arterial baroreflex sensitivity may further contribute to sympathoinhibition after ET-1 blockade in heart failure.

The interaction of angiotensin II and osmolality in the generation of sympathetic tone during changes in dietary salt intake. An hypothesis

At rest, sympathetic nerves exhibit tonic activity which contributes to arterial pressure maintenance. Significant evidence suggests that the absolute level of sympathetic tone is altered in a number of physiologic and pathophysiologic states. However, the mechanisms by which such changes in sympathetic tone occur are incompletely understood. The purpose of this review is to present evidence that humoral factors are essential in these changes and to detail specifically an hypothesis for the mechanisms that underlie the changes in sympathetic tone that are produced during increases or decreases in dietary salt intake. It is proposed that the net effect of changes in dietary salt on sympathetic activity is determined by the balance between simultaneous and parallel sympathoinhibitory and sympathoexcitatory humoral mechanisms. A key element of the sympathoinhibitory mechanism is the chronic sympathoexcitatory effects of angiotensin II (ANG II). When salt intake increases, ANG II levels fall, and the sympathoexcitatory actions of ANG II are lost. Simultaneously, a sympathoexcitatory pathway is triggered, possibly via increases in osmolality which activate osmoreceptors or sodium receptors. In normal individuals, the sympathoinhibitory effects of increased salt predominate, sympathetic activity decreases, and arterial pressure remains normal despite salt and water retention. However, in subjects with salt-sensitive hypertension, it appears that the sympathoexcitatory effects of salt predominate, possibly due to an inability to adequately suppress the levels or actions of ANG II. The net result, therefore, is an inappropriate increase in sympathetic activity during increased dietary salt which may contribute to the hypertensive process.

AT1 receptor block does not affect arterial baroreflex during pregnancy in rabbits

The role of ANG II in the arterial baroreflex control of renal sympathetic nerve activity (RSNA) in eight term-pregnant (P) and eight nonpregnant (NP) conscious rabbits was assessed using sequential intracerebroventricular and intravenous infusions of losartan, an AT1 receptor antagonist. The blood pressure (BP)-RSNA relationship was generated by sequential inflations of aortic and vena caval perivascular occluders. Pregnant rabbits exhibited a lower maximal RSNA reflex gain (-44%) that was primarily due to a reduction in the maximal sympathetic response to hypotension (P, 248 +/- 20% vs. NP, 357 +/- 41% of rest RSNA, P < 0.05). Intracerebroventricular losartan decreased resting BP in P (by 9 +/- 3 mmHg, P < 0.05) but not NP rabbits, and had no effect on the RSNA baroreflex in either group. Subsequent intravenous losartan decreased resting BP in NP and further decreased BP in P rabbits, but had no significant effect on the maximal RSNA reflex gain. ANG II may have an enhanced role in the tonic support of BP in pregnancy, but does not mediate the gestational depression in the arterial baroreflex control of RSNA in rabbits.

Renin-angiotensin system inhibition on noradrenergic nerve terminal function in pacing-induced heart failure

Chronic angiotensin-converting enzyme (ACE) inhibition has been shown to improve cardiac sympathetic nerve terminal function in heart failure. To determine whether similar effects could be produced by angiotensin II AT(1) receptor blockade, we administered the ACE inhibitor quinapril, angiotensin II AT(1) receptor blocker losartan, or both agents together, to rabbits with pacing-induced heart failure. Chronic rapid pacing produced left ventricular dilation and decline of fractional shortening, increased plasma norepinephrine (NE), and caused reductions of myocardial NE uptake activity, NE histofluorescence profile, and tyrosine hydroxylase immunostained profile. Administration of quinapril or losartan retarded the progression of left ventricular dysfunction and attenuated cardiac sympathetic nerve terminal abnormalities in heart failure. Quinapril and losartan together produced greater effects than either agent alone. The effect of renin-angiotensin system inhibition on improvement of left ventricular function and remodeling, however, was not sustained. Our results suggest that the effects of ACE inhibitors are mediated via the reduction of angiotensin II and that angiotensin II plays a pivotal role in modulating cardiac sympathetic nerve terminal function during development of heart failure. The combined effect of ACE inhibition and angiotensin II AT(1) receptor blockade on cardiac sympathetic nerve terminal dysfunction may contribute to the beneficial effects on cardiac function in heart failure.

Increased brain angiotensin receptor in rats with chronic high-output heart failure

BACKGROUND

The renin-angiotensin system (RAS) plays a key role in the pathophysiology of chronic heart failure (CHF). In rats, we reported that CHF enhances dipsogenic responses to centrally administered angiotensin I, and central inhibition of the angiotensin-converting enzyme (ACE) prevents cardiac hypertrophy in CHF. This suggests that the brain RAS is activated in CHF. To clarify the mechanism of the central RAS activation in CHF, we examined brain ACE and the angiotensin receptor (AT) among rats with CHF.

METHODS AND RESULTS

We created high-output heart failure in 22 male Sprague-Dawley rats by aortocaval shunt. Four weeks after surgery, we examined ACE mRNA by reverse transcriptase polymerase chain reaction (RT-PCR) and AT by binding autoradiography. ACE mRNA levels were not significantly increased in the subfornical organ (SFO), the hypothalamus, or in the lower brainstem of CHF rats (n = 5) compared with sham-operated rats (SHM) (n = 6). Binding densities for type 1 AT (AT1) in the SFO (P < .05), paraventricular hypothalamic nuclei (P < .05), and solitary tract nuclei (P < .05) were higher in rats with CHF (n = 5) than in SHM rats (n = 6). Thus, in rats with CHF, AT1 expression is increased in brain regions that are closely related to water intake, vasopressin release, and hemodynamic regulation.

CONCLUSIONS

The fact that AT1 expression was upregulated in important brain regions related to body fluid control in CHF rats indicates that the brain is a major site of RAS action in CHF rats and, therefore, a possible target site of ACE-inhibitors in the treatment of CHF.

ANG II and baroreflex function in rabbits with CHF and lesions of the area postrema

Blockade of the angiotensin II (ANG II) type 1 receptor (AT(1)) has been shown to restore baroreflex sensitivity in rats and rabbits with experimental chronic heart failure (CHF). Because the modulation of baroreflex function in response to ANG II is mediated in part by AT(1) receptors located in the area postrema, we hypothesized that lesions of the area postrema would prevent the enhancement in baroreflex function in response to AT(1)-receptor blockade in rabbits with pacing-induced CHF. Experiments were carried out on 24 male New Zealand White rabbits that were divided into sham (n = 12) and lesioned (n = 12) groups further divided into normal and CHF subgroups (n = 6 each). All rabbits were identically instrumented to measure cardiac external dimensions, central venous pressure, arterial pressure, heart rate (HR), and renal sympathetic nerve activity (RSNA). After 3-4 wk of pacing, baroreflex sensitivity (infusions of phenylephrine and nitroprusside) was evaluated before and after intravenous administration of the AT(1)-receptor antagonist L-158,809. Maximum baroreflex sensitivity in nonpaced rabbits was 5.4 +/- 0.7 beats. min(-1). mmHg(-1) and 5.2 +/- 0.5% of maximum/mmHg for HR and RSNA curves, respectively, and was not altered by L-158,809 in either intact or lesioned rabbits. In contrast, L-158,809 enhanced baroreflex sensitivity in intact rabbits with CHF (HR from 1.6 +/- 0.3 to 4.1 +/- 0.7 beats. min(-1). mmHg(-1), P < 0.001; RSNA from 2.3 +/- 0.2 to 4.9 +/- 0.4% of maximum/mmHg, P < 0.001). However, in CHF rabbits with area postrema lesions, L-158,809 failed to enhance baroreflex sensitivity. Interestingly, area postrema lesions did not normalize the baroreflex in CHF rabbits. From these data we conclude that the area postrema mediates the normalization of baroreflex sensitivity after AT(1) blockade in rabbits with CHF but does not modify resting baroreflex function.

The sympathetic nervous system and baroreflexes in hypertension and hypotension

Blood pressure and blood volume are closely regulated by the interrelated actions of the sympathetic nervous system (SNS) and the renin-angiotensin-aldosterone system (RAAS). Reflex vasoconstriction caused by parallel SNS and RAAS activation is modulated by two interactive negative feedback systems called baroreflex. The aortic-carotid baroreflex systems respond to momentary changes in systolic blood pressure, adjusting the degree of SNS-dependent peripheral vasoconstriction and cardiac output to allow maintenance of a relatively constant perfusion pressure. Cardiopulmonary baroreflexes respond to momentary changes in cardiac filling, adjusting the degree of peripheral venoconstriction and venous return to maintain cardiac preload and stroke volume. Under normal conditions, each baroreflex system exhibits a degree of tonic negative feedback so that it can alter SNS output immediately, providing counterregulatory increases or decreases in pressure or volume to maintain homeostasis. The SNS is inappropriately active in obesity and hypertension and plays a causal or permissive role in all forms of chronic hypertension. If the negative feedback control exerted by the baroreflexes over the SNS and renin-angiotensin-aldosterone system (RAAS) were perfect, chronic hypertension would not occur. Activity of the baroreflexes, however, is chronically altered by maladaptive changes such as cardiac and vascular fibrosis and hypertrophy. Long-term increases in SNS and RAAS activity also exert ongoing deleterious effects on the heart and vasculature by directly facilitating further cardiac hypertrophy and arterial stiffening. These effects appear to contribute to a vicious cycle of chronic hypertension and target organ damage. Other syndromes of abnormal blood pressure (BP) control, including orthostatic hypotension and baroreflex failure are examples of abnormal baroreflex activity and SNS control.

New analytic framework for understanding sympathetic baroreflex control of arterial pressure

The sympathetic baroreflex is an important feedback system in stabilization of arterial pressure. This system can be decomposed into the controlling element (mechanoneural arc) and the controlled element (neuromechanical arc). We hypothesized that the intersection of the two operational curves representing their respective functions on an equilibrium diagram should define the operating point of the arterial baroreflex. Both carotid sinuses were isolated in 16 halothane-anesthetized rats. The vagi and aortic depressor nerves were cut bilaterally. Carotid sinus pressure (CSP) was sequentially altered in 10-mmHg increments from 80 to 160 mmHg while sympathetic efferent nerve activity (SNA) and systemic arterial pressure (SAP) were recorded simultaneously under various hemorrhagic conditions. The mechanoneural arc was characterized by the response of SNA to CSP and the neuromechanical arc by the response of SAP to SNA. We parametrically analyzed the relationship between input and output for each arc using a four-parameter logistic equation model. In baseline states, the two arcs intersected each other at the point at which the instantaneous gain of each arc attained its maximum. Severe hemorrhage lowered the gain and offset of the neuromechanical arc and moved the operating point, whereas the mechanoneural arc remained unchanged. The operating points measured under the closed-loop conditions were indistinguishable from those estimated from the intersections of the two arc curves on the equilibrium diagram. The average root mean square errors of estimate for arterial pressure and SNA were 2 and 3%, respectively. Such an analytic approach could explain a mechanism for the determination of the operating point of the sympathetic baroreflex system and thus helps us integratively understand its function.

Angiotensin II and sympathoactivation in heart failure

Excessive activity of the sympathetic nervous system (SNS) contributes to the development and progression of the syndrome of congestive heart failure (CHF) in patients with decreased left ventricular function. The factors underlying chronic sympathoactivation are poorly understood, particularly in stable patients. This review summarizes both clinical and experimental data regarding the effects of angiotensin II (A-II) on the activity of the SNS. The focus is on both the direct effects of A-II on the SNS and an indirect effect medicated through alteration in function of the baroreflex. Available evidence is consistent with a potentially important effect of A-II on SNS activity, perhaps most likely via the baroreflex. Important issues regarding the direct effect of A-II on regional SNS activity, and on the physiological relevance of effects seen only at high plasma concentration of A-II remain to be fully elucidated.

Brain renin-angiotensin system and sympathetic hyperactivity in rats after myocardial infarction

Blockade of brain "ouabain" prevents the sympathetic hyperactivity and impairment of baroreflex function in rats with congestive heart failure (CHF). Because brain "ouabain" may act by activating the brain renin-angiotensin system (RAS), the aim of the present study was to assess whether chronic treatment with the AT1-receptor blocker losartan given centrally normalizes the sympathetic hyperactivity and impairment of baroreflex function in Wistar rats with CHF postmyocardial infarction (MI). After left coronary artery ligation (2 or 6 wk), rats received either intracerebroventricular losartan (1 mg. kg-1. day-1, CHF-Los) or vehicle (CHF-Veh) by osmotic minipumps. To assess possible peripheral effects of intracerebroventricular losartan, one set of CHF rats received the same rate of losartan subcutaneously. Sham-operated rats served as control. After 2 wk of treatment, mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) at rest and in response to air-jet stress and intracerebroventricular injection of the alpha2-adrenoceptor-agonist guanabenz were measured in conscious animals. Arterial baroreflex function was evaluated by ramp changes in MAP. Compared with sham groups, CHF-Veh groups showed impaired arterial baroreflex control of HR and RSNA, increased sympathoexcitatory and pressor responses to air-jet stress, and increased sympathoinhibitory and hypotensive responses to guanabenz. The latter is consistent with decreased activity in sympathoinhibitory pathways. Chronic intracerebroventricular infusion of losartan largely normalized these abnormalities. In CHF rats, the same rate of infusion of losartan subcutaneously was ineffective. In sham-operated rats, losartan intracerebroventricularly or subcutaneously did not affect sympathetic activity. We conclude that the chronic increase in sympathoexcitation, decrease in sympathoinhibition, and desensitized baroreflex function in CHF all appear to depend on the brain RAS, since this whole pattern of changes can be normalized by chronic central AT1-receptor blockade with losartan.