Renal denervation does not abolish sustained baroreflex-mediated reductions in arterial pressure

Chronic lowering of blood pressure by carotid baroreflex activation: mechanisms and potential for hypertension therapy

Recent technical advances have renewed interest in device-based therapy for the treatment of drug-resistant hypertension. Findings from recent clinical trials regarding the efficacy of electric stimulation of the carotid sinus for the treatment of resistant hypertension are reviewed here. The main goal of this article, however, is to summarize the preclinical studies that have provided insight into the mechanisms that account for the chronic blood pressure-lowering effects of carotid baroreflex activation. Some of the mechanisms identified were predictable and confirmed by experimentation. Others have been surprising and controversial, and resolution will require further investigation. Although feasibility studies have been promising, firm conclusions regarding the value of this device-based therapy for the treatment of resistant hypertension awaits the results of current multicenter trials.

Sustained suppression of sympathetic activity and arterial pressure during chronic activation of the carotid baroreflex

Following sinoaortic denervation, which eliminates arterial baroreceptor input into the brain, there are slowly developing adaptations that abolish initial sympathetic activation and hypertension. In comparison, electrical stimulation of the carotid sinus for 1 wk produces sustained reductions in sympathetic activity and arterial pressure. However, whether compensations occur subsequently to diminish these responses is unclear. Therefore, we determined whether there are important central and/or peripheral adaptations that diminish the sympathoinhibitory and blood pressure-lowering effects of more sustained carotid sinus stimulation. To this end, we measured whole body plasma norepinephrine spillover and alpha(1)-adrenergic vascular reactivity in six dogs over a 3-wk period of baroreflex activation. During the first week of baroreflex activation, there was an approximately 45% decrease in plasma norepinephrine spillover, along with reductions in mean arterial pressure and heart rate of approximately 20 mmHg and 15 beats/min, respectively; additionally, plasma renin activity did not increase. Most importantly, these responses during week 1 were largely sustained throughout the 3 wk of baroreflex activation. Acute pressor responses to alpha-adrenergic stimulation during ganglionic blockade were similar throughout the study, indicating no compensatory increases in adrenergic vascular reactivity. These findings indicate that the sympathoinhibition and lowering of blood pressure and heart rate induced by chronic activation of the carotid baroreflex are not diminished by adaptations in the brain and peripheral circulation. Furthermore, by providing evidence that baroreflexes have long-term effects on sympathetic activity and arterial pressure, they present a perspective that is opposite from studies of sinoaortic denervation.

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.

Chronic baroreflex activation by the Rheos system: an overview of results from European and North American feasibility studies

The baroreflex, whose role is well-known in short-term blood pressure regulation, has until recently been unexploited as a practical therapy for hypertension. Recent advancements in approach and technology embodied in the Rheos System have enabled chronic electrical activation of the baroreflex. Chronic results from feasibility studies indicate that Rheos Therapy has an acceptable safety profile and may lead to long-term control of pressure in drug-resistant hypertension patients. Other effects include significant reductions in left ventricular mass and left atrial size. The spectrum of therapeutic impact suggests that Rheos Therapy may improve long-term outcomes in drug-resistant hypertension and possibly benefit related populations. Larger-scale study in randomized, controlled trials are ongoing to verify chronic benefits.

Physiology of Kidney Renin

The protease renin is the key enzyme of the renin-angiotensin-aldosterone cascade, which is relevant under both physiological and pathophysiological settings. The kidney is the only organ capable of releasing enzymatically active renin. Although the characteristic juxtaglomerular position is the best known site of renin generation, renin-producing cells in the kidney can vary in number and localization. (Pro)renin gene transcription in these cells is controlled by a number of transcription factors, among which CREB is the best characterized. Pro-renin is stored in vesicles, activated to renin, and then released upon demand. The release of renin is under the control of the cAMP (stimulatory) and Ca2+(inhibitory) signaling pathways. Meanwhile, a great number of intrarenally generated or systemically acting factors have been identified that control the renin secretion directly at the level of renin-producing cells, by activating either of the signaling pathways mentioned above. The broad spectrum of biological actions of (pro)renin is mediated by receptors for (pro)renin, angiotensin II and angiotensin-( 1 – 7 ).

Mechanisms of blood pressure reduction by prolonged activation of the baroreflex

Recent technological advances have made the activation of the afferent limb of the baroreflex a viable therapeutic approach for lowering blood pressure. Experimental studies demonstrate sustained reductions in blood pressure in response to electrical baroreflex activation and initial results from clinical trials using device-based therapy for drug-resistant hypertension are promising. Although theoretically obvious at first glance, the mechanisms involved in the blood pressure lowering effect of baroreflex activation elude precise quantification, and experiments designed to investigate them invariably challenge preconceived notions and even dogmas. This paper is a brief overview of our current understanding of these mechanisms.

Acute and chronic electrical activation of baroreceptor afferents in awake and anesthetized subjects

Electrical stimulation of baroreceptor afferents was used in the 1960's in several species, including human beings, for the treatment of refractory hypertension. This approach bypasses the site of baroreceptor mechanosensory transduction. Chronic electrical stimulation of arterial baroreceptors, particularly of the carotid sinus nerve (Hering's nerve), was proposed as an ultimate effort to treat refractory hypertension and angina pectoris due to the limited nature of pharmacological therapy available at that time. Nevertheless, this approach was abandoned in the early 1970's due to technical limitations of implantable devices and to the development of better-tolerated antihypertensive medications. More recently, our laboratory developed the technique of electrical stimulation of the aortic depressor nerve in conscious rats, enabling access to hemodynamic responses without the undesirable effect of anesthesia. In addition, electrical stimulation of the aortic depressor nerve allows assessment of the hemodynamic responses and the sympathovagal balance of the heart in hypertensive rats, which exhibit a well-known decrease in baroreflex sensitivity, usually attributed to baroreceptor ending dysfunction. Recently, there has been renewed interest in using electrical stimulation of the carotid sinus, but not the carotid sinus nerve, to lower blood pressure in conscious hypertensive dogs as well as in hypertensive patients. Notably, previous undesirable technical outcomes associated with electrical stimulation of the carotid sinus nerve observed in the 1960's and 1970's have been overcome. Furthermore, promising data have been recently reported from clinical trials that evaluated the efficacy of carotid sinus stimulation in hypertensive patients with drug resistant hypertension.

Lowering of blood pressure during chronic suppression of central sympathetic outflow: insight from computer simulations

1. Chronic electrical stimulation of the carotid sinuses has provided unique insight into the mechanisms that cause sustained reductions in blood pressure during chronic suppression of central sympathetic outflow. 2. Because renal denervation does not abolish the sustained fall in arterial pressure in response to baroreflex activation, this observation has seemingly challenged the concept that the kidneys play a critical role in the long-term control of arterial pressure during chronic changes in sympathetic activity. The aim of the present study was to use computer simulations to provide a more comprehensive understanding of physiological mechanisms that mediate sustained reductions in arterial pressure during prolonged baroreflex-mediated suppression of central sympathetic outflow. 3. Physiological responses to baroreflex activation under different conditions were simulated by an established mathematical model of human physiology (QHP2008; see Supporting Information (Appendix S1) provided in the online version of this article and/or http://groups.google.com/group/modelingworkshop). The model closely reproduced empirical data, providing important validation of its accuracy. 4. The simulations indicated that baroreflex-mediated suppression of renal sympathetic nerve activity does chronically increase renal excretory function but that, in addition, hormonal and haemodynamic mechanisms also contribute to this natriuretic response. The contribution of these redundant natriuretic mechanisms to the chronic lowering of blood pressure is of increased importance when suppression of renal adrenergic activity is prevented, such as after renal denervation. Activation of these redundant natriuretic mechanisms occurs at the expense of excessive fluid retention. 5. More broadly, the present study illustrates the value of numerical simulations in elucidating physiological mechanisms that are not obvious intuitively and, in some cases, not readily testable in experimental studies.

Expression and function of intestinal hexose transporters after small intestinal denervation

BACKGROUND

The role of neural regulation in expression and function of intestinal hexose transporters is unknown. The aim of this study is to determine the role of intestinal innervation in gene expression and function of the membrane hexose transporters, SGLT1, GLUT2, and GLUT5 in the enterocyte. We hypothesize that denervation of the small intestine decreases expression of hexose transporters, which leads to decreased glucose absorption.

METHODS

Six groups of Lewis rats were studied (n = 6 each) as follows: control, 1 week after sham laparotomy, 1 and 8 weeks after syngeneic (no immune rejection) orthotopic small-bowel transplantation (SBT) (SBT1 and SBT8) to induce complete extrinsic denervation, and 1 and 8 weeks after selective disruption of intrinsic neural continuity to jejunoileum by gut transection and reanastomosis (T/A1 and T/A8). All tissue was harvested between 8 AM and 10 AM. In duodenum, jejunum, and ileum, mucosal messenger RNA (mRNA) levels were quantitated by real-time polymerase chain reaction (PCR), protein by Western blotting, and transporter-mediated glucose absorption using the everted sleeve technique.

RESULTS

Across the 6 groups, the relative gene expression of hexose transporter mRNA and protein levels were unchanged, and no difference in transporter-mediated glucose uptake was evident in any region. The glucose transporter affinity (K(m)) and functional transporter levels (V(max)) calculated for duodenum and jejunum showed no difference among the 6 groups.

CONCLUSION

Baseline regulation of hexose transporter function is not mediated tonically by intrinsic or extrinsic neural continuity to the jejunoileum.

Na(+)-ATPase in spontaneous hypertensive rats: possible AT(1) receptor target in the development of hypertension

Clinical and experimental data show an increase in sodium reabsorption on the proximal tubule (PT) in essential hypertension. It is well known that there is a link between essential hypertension and renal angiotensin II (Ang II). The present study was designed to examine ouabain-insensitive Na(+)-ATPase activity and its regulation by Ang II in spontaneously hypertensive rats (SHR). We observed that Na(+)-ATPase activity was enhanced in 14-week-old but not in 6-week-old SHR. The addition of Ang II from 10(-12) to 10(-6) mol/L decreased the enzyme activity in SHR to a level similar to that obtained in WKY. The Ang II inhibitory effect was completely reversed by a specific antagonist of AT(2) receptor, PD123319 (10(-8) mol/L) indicating that a system leading to activation of the enzyme in SHR is inhibited by AT(2)-mediated Ang II. Treatment of SHR with losartan for 10 weeks (weeks 4-14) prevents the increase in Na(+)-ATPase activity observed in 14-week-old SHR. These results indicate a correlation between AT(1) receptor activation in SHR and increased ouabain-insensitive Na(+)-ATPase activity. Our results open new possibilities towards our understanding of the pathophysiological mechanisms involved in the increased sodium reabsorption in PT found in essential hypertension.

Carotid baroreceptor stimulation as a therapeutic target in hypertension and other cardiovascular conditions

BACKGROUND

The role of the carotid baroreflex in blood pressure regulation has been known for a long time but its effects were thought to be short lived. Recent data indicate that stimulation of carotid baroreceptors may lower blood pressure not only for short periods of time, but also in the long run.

OBJECTIVE/METHODS

Recent advances in technology permitted the development of a new device (Rheos) that addresses problems with older devices. Several questions remain to be addressed before Rheos can be used widely, and several potential clinical applications remain to be clarified. This review examines these issues and comprehensively describes this therapeutic approach.

RESULTS/CONCLUSIONS

The carotid baroreceptor reflex is probably not completely in control of blood pressure. Baroreflexes are one of many control systems acting in concert.

Prolonged activation of the baroreflex decreases arterial pressure even during chronic adrenergic blockade

Previous studies suggest that prolonged electric activation of the baroreflex may reduce arterial pressure more than chronic blockade of alpha(1)- and beta(1,2)-adrenergic receptors. To determine whether central inhibition of sympathetic outflow has appreciable effects to chronically reduce arterial pressure by actions distinct from well-established mechanisms, we hypothesized that chronic baroreflex activation would lower arterial pressure substantially even during complete alpha(1)- and beta(1,2)-adrenergic receptor blockade. This hypothesis was tested in 6 dogs during adrenergic blockade (AB; 18 days) with and without electric activation of the carotid baroreflex (7 days). During chronic AB alone, there was a sustained decrease in the mean arterial pressure of 21+/-2 mm Hg (control: 95+/-4 mm Hg) and an approximately 3-fold increase in plasma norepinephrine concentration (control: 138+/-6 pg/mL), likely attributed to baroreceptor unloading. In comparison, during AB plus prolonged baroreflex activation, plasma norepinephrine concentration decreased to control levels, and mean arterial pressure fell an additional 10+/-1 mm Hg. Because of differences in plasma norepinephrine concentration, we also tested the acute blood pressure-lowering effects of MK-467, a peripherally acting alpha(2)-antagonist. After administration of MK-467, there was a significantly greater fall in arterial pressure during AB (15+/-3 mm Hg) than during AB plus prolonged baroreflex activation (7+/-3 mm Hg). These findings suggest that reflex-induced increases in sympathetic activity attenuate reductions in arterial pressure during chronic AB and that inhibition of central sympathetic outflow by prolonged baroreflex activation lowers arterial pressure further by previously undefined mechanisms, possibly by diminishing attendant activation of postjunctional alpha(2)-adrenergic receptors.

Normotensive sodium loading in conscious dogs: regulation of renin secretion during β-receptor blockade

Renin secretion is regulated in part by renal nerves operating through β1-receptors of the renal juxtaglomerular cells. Slow sodium loading may decrease plasma renin concentration (PRC) and cause natriuresis at constant mean arterial blood pressure (MAP) and glomerular filtration rate (GFR). We hypothesized that in this setting, renin secretion and renin-dependent sodium excretion are controlled by via the renal nerves and therefore are eliminated or reduced by blocking the action of norepinephrine on the juxtaglomerular cells with the β1-receptor antagonist metoprolol. This was tested in conscious dogs by infusion of NaCl (20 μmol·kg−1·min−1for 180 min, NaLoad) during regular or low-sodium diet (0.03 mmol·kg−1·day−1, LowNa) with and without metoprolol (2 mg/kg plus 0.9 mg·kg−1·h−1). Vasopressin V2receptors were blocked by Otsuka compound OPC31260 to facilitate clearance measurements. Body fluid volume was maintained by servocontrolled fluid infusion. Metoprolol per se did not affect MAP, heart rate, or sodium excretion significantly, but reduced PRC and ANG II by 30–40%, increased plasma atrial natriuretic peptide (ANP), and tripled potassium excretion. LowNa per se increased PRC (+53%), ANG II (+93%), and aldosterone (+660%), and shifted the vasopressin function curve to the left. NaLoad elevated plasma [Na+] by 4.5% and vasopressin by threefold, but MAP and plasma ANP remained unchanged. NaLoad decreased PRC by ∼30%, ANG II by ∼40%, and aldosterone by ∼60%, regardless of diet and metoprolol. The natriuretic response to NaLoad was augmented during metoprolol regardless of diet. In conclusion, PRC depended on dietary sodium and β1-adrenergic control as expected; however, the acute sodium-driven decrease in PRC at constant MAP and GFR was unaffected by β1-receptor blockade demonstrating that renin may be regulated without changes in MAP, GFR, or β1-mediated effects of norepinephrine. Low-sodium diet augments vasopressin secretion, whereas ANP secretion is reduced.

Rheos Baroreflex Hypertension Therapy System to treat resistant hypertension

Resistant hypertension has a high prevalence and is associated with high morbidity and mortality. The Rheos Baroreflex Hypertension Therapy System is an implantable device that offers a completely new approach to treating patients with resistant hypertension by electrically activating the carotid baroreflex. Preliminary results from current feasibility clinical trials have shown sustained decreases in blood pressure after 1 year. The pivotal trial for US FDA approval and market release is currently ongoing. This article profiles the Rheos System and evaluates the treatment of resistant hypertension in general.

Blood volume, blood pressure and total body sodium: internal signalling and output control

Total body sodium and arterial blood pressure (ABP) are mutually dependent variables regulated by complex control systems. This review addresses the role of ABP in the normal control of sodium excretion (NaEx), and the physiological control of renin secretion. NaEx is a pivotal determinant of ABP, and under experimental conditions, ABP is a powerful, independent controller of NaEx. Blood volume is a function of dietary salt intake; however, ABP is not, at least not in steady states. A transient increase in ABP after a step-up in sodium intake could provide a causal relationship between ABP and the regulation of NaEx via a hypothetical integrative control system. However, recent data show that subtle sodium loading (simulating salty meals) causes robust natriuresis without changes in ABP. Changes in ABP are not necessary for natriuresis. Normal sodium excretion is not regulated by pressure. Plasma renin is log-linearly related to salt intake, and normally, decreases in renin secretion are a precondition of natriuresis after increases in total body sodium. Renin secretion is controlled by renal ABP, renal nerve activity and the tubular chloride concentrations at the macula densa (MD). Renal nerve activity is related to blood volume, also at constant ABP, and elevates renin secretion by means of beta(1)-adrenoceptors. Recent results indicate that renal denervation reduces ABP and renin activity, and that sodium loading may decrease renin without changes in ABP, glomerular filtration rate or beta(1)-mediated nerve activity. The latter indicates an essential role of the MD mechanism and/or a fourth mediator of the physiological control of renin secretion.

Baroreflex stimulation: A novel treatment option for resistant hypertension

Hypertension is a major public health problem in both developing and developed countries. Despite the increasing awareness of hypertension and its implications among patients and the treating physicians, the prevalence of resistant hypertension remains high and is expected to increase. Many patients fail to reach their target blood pressure (BP) despite the wide availability of several antihypertensive agents and the continued recommendation of dietary and lifestyle modifications. Stimulation of the carotid sinus results in lowering of BP by initiating the baroreflex and, in so doing, reducing sympathetic tone and increasing renal excretory function, in part, by exerting inhibitory effects on renin secretion. Recent evidence from experimental studies suggests that the baroreflex may be more important in the setting of chronic hypertension than originally believed. In early-phase clinical trials that did not include control arms, implantation of a baroreflex stimulator yielded a sustained decrease in BP. An ongoing larger clinical trial with appropriate control arms is further exploring the safety and efficacy of the device. This article describes the history and potential mechanisms of action of this device including its extensive preclinical development and movement to human clinical trials.

Bionic cardiology: exploration into a wealth of controllable body parts in the cardiovascular system

Bionic cardiology is the medical science of exploring electronic control of the body, usually via the neural system. Mimicking or modifying biological regulation is a strategy used to combat diseases. Control of ventricular rate during atrial fibrillation by selective vagal stimulation, suppression of ischemia-related ventricular fibrillation by vagal stimulation, and reproduction of neurally commanded heart rate are some examples of bionic treatment for arrhythmia. Implantable radio-frequency-coupled on-demand carotid sinus stimulators succeeded in interrupting or preventing anginal attacks but were replaced later by coronary revascularization. Similar but fixed-intensity carotid sinus stimulators were used for hypertension but were also replaced by drugs. Recently, however, a self-powered implantable device has been reappraised for the treatment of drug-resistant hypertension. Closed-loop spinal cord stimulation has successfully treated severe orthostatic hypotension in a limited number of patients. Vagal nerve stimulation is effective in treating heart failure in animals, and a small-size clinical trial has just started. Simultaneous corrections of multiple hemodynamic abnormalities in an acute decompensated state are accomplished simply by quantifying fundamental cardiovascular parameters and controlling these parameters. Bionic cardiology will continue to promote the development of more sophisticated device-based therapies for otherwise untreatable diseases and will inspire more intricate applications in the twenty-first century.

Normotensive sodium loading in normal man: regulation of renin secretion during beta-receptor blockade

Saline administration may change renin-angiotensin-aldosterone system (RAAS) activity and sodium excretion at constant mean arterial pressure (MAP). We hypothesized that such responses are elicited mainly by renal sympathetic nerve activity by beta1-receptors (beta1-RSNA), and tested the hypothesis by studying RAAS and renal excretion during slow saline loading at constant plasma sodium concentration (Na+ loading; 12 micromol Na+.kg(-1).min(-1) for 4 h). Normal subjects were studied on low-sodium intake with and without beta1-adrenergic blockade by metoprolol. Metoprolol per se reduced RAAS activity as expected. Na+ loading decreased plasma renin concentration (PRC) by one-third, plasma ANG II by one-half, and plasma aldosterone by two-thirds (all P < 0.05); surprisingly, these changes were found without, as well as during, acute metoprolol administration. Concomitantly, sodium excretion increased indistinguishably with and without metoprolol (16 +/- 2 to 71 +/- 14 micromol/min; 13 +/- 2 to 55 +/- 13 micromol/min, respectively). Na+ loading did not increase plasma atrial natriuretic peptide, glomerular filtration rate (GFR by 51Cr-EDTA), MAP, or cardiac output (CO by impedance cardiography), but increased central venous pressure (CVP) by approximately 2.0 mmHg (P < 0.05). During Na+ loading, sodium excretion increased with CVP at an average slope of 7 micromol.min(-1).mmHg(-1). Concomitantly, plasma vasopressin decreased by 30-40% (P < 0.05). In conclusion, beta1-adrenoceptor blockade affects neither the acute saline-mediated deactivation of RAAS nor the associated natriuretic response, and the RAAS response to modest saline loading seems independent of changes in MAP, CO, GFR, beta1-mediated effects of norepinephrine, and ANP. Unexpectedly, the results do not allow assessment of the relative importance of RAAS-dependent and -independent regulation of renal sodium excretion. The results are compatible with the notion that at constant arterial pressure, a volume receptor elicited reduction in RSNA via receptors other than beta1-adrenoceptors, decreases renal tubular sodium reabsorption proximal to the macula densa leading to increased NaCl concentration at the macula densa, and subsequent inhibition of renin secretion.

A sympathetic view of the sympathetic nervous system and human blood pressure regulation

New ideas about the relative importance of the autonomic nervous system (and especially its sympathetic arm) in long-term blood pressure regulation are emerging. It is well known that mean arterial blood pressure is normally regulated in a fairly narrow range at rest and that blood pressure is also able to rise and fall 'appropriately' to meet the demands of various forms of mental, emotional and physical stress. By contrast, blood pressure varies widely when the autonomic nervous system is absent or when key mechanisms that govern it are destroyed. However, 24 h mean arterial pressure is still surprisingly normal under these conditions. Thus, the dominant idea has been that the kidney is the main long-term regulator of blood pressure and the autonomic nervous system is important in short-term regulation. However, this 'renocentric' scheme can be challenged by observations in humans showing that there is a high degree of individual variability in elements of the autonomic nervous system. Along these lines, the level of sympathetic outflow, the adrenergic responsiveness of blood vessels and individual haemodynamic patterns appear to exist in a complex, but appropriate, balance in normotension. Furthermore, evidence from animals and humans has now clearly shown that the sympathetic nervous system can play an important role in longer term blood pressure regulation in both normotension and hypertension. Finally, humans with high baseline sympathetic traffic might be at increased risk for hypertension if the 'balance' among factors deteriorates or is lost. In this context, the goal of this review is to encourage a comprehensive rethinking of the complexities related to long-term blood pressure regulation in humans and promote finer appreciation of physiological relationships among the autonomic nervous system, vascular function, ageing, metabolism and blood pressure.

Baroreflex stimulation in the treatment of hypertension

PURPOSE OF REVIEW

It is not uncommon for hypertension to be resistant to the effects of medical therapy, and this poses a significant risk of adverse cardiovascular events. Electrical stimulation of the carotid sinus is a novel treatment for hypertension, and has been shown to reduce blood pressure by activating the baroreflex and reducing sympathetic tone.

RECENT FINDINGS

Evidence suggests that the baroreceptors play a more important role in long-term blood pressure regulation than was once believed. It appears that the baroreflex attenuates chronic hypertension in large part by inhibiting renal sympathetic tone. Animal and human studies have demonstrated a safe and effective lowering of blood pressure with chronic electrical stimulation of the carotid sinus, and have generated enthusiasm for implantable carotid sinus stimulators in the treatment of hypertension.

SUMMARY

Electrical baroreflex stimulation appears safe and effective, and may represent a useful adjunct to medical therapy in patients with resistant hypertension.

Chronic baroreceptor activation enhances survival in dogs with pacing-induced heart failure

Much of the current pharmacological therapy for chronic heart failure targets neurohormonal activation. In spite of recent advances in drug therapy, the mortality rate for chronic heart failure remains high. Activation of the carotid baroreceptor (BR) reduces sympathetic outflow and augments vagal tone. We investigated the effect of chronic activation of the carotid BR on hemodynamic and neurohormonal parameters and on mortality in dogs with chronic heart failure. Fifteen dogs were instrumented to record hemodynamics. Electrodes were applied around the carotid sinuses to allow for activation of the BR. After 2 weeks of pacing (250 bpm), electrical carotid BR activation was initiated in 7 dogs and continued for the remainder of the study. The start of BR activation was used as a time reference point for the remaining 8 control dogs that did not receive BR activation. Survival was significantly greater for dogs undergoing carotid BR activation compared with control dogs (68.1+/-7.4 versus 37.3+/-3.2 days, respectively; P<0.01), although arterial pressure, resting heart rate, and left ventricular pressure were not different over time in BR-activated versus control dogs. Plasma norepinephrine was lower in dogs receiving BR activation therapy 31 days after the start of BR activation (401.9+/-151.5 versus 1121.9+/-389.1 pg/mL in dogs not receiving activation therapy; P<0.05). Plasma angiotensin II increased less in dogs receiving activation therapy (plasma angiotensin II increased by 157.4+/-58.6 pg/mL in control dogs versus 10.1+/-14.0 pg/mL in dogs receiving activation therapy; P<0.02). We conclude that chronic activation of the carotid BR improves survival and suppresses neurohormonal activation in chronic heart failure.

Baroreflex sensitivity inversely correlates with ambulatory blood pressure in healthy normotensive humans

Patients with hypertension have a blunted sensitivity of baroreflex control of heart period. In these patients, baroreflex sensitivity is positively related to heart rate variability and inversely related to blood pressure variability. We hypothesized that this relationship would also be evident in healthy normotensive subjects and that individuals with higher baroreflex sensitivity would have lower ambulatory 24-hour blood pressure. Twenty-four-hour ambulatory blood pressure and heart rate were recorded in 50 healthy, normotensive, nonobese individuals (31 women and 19 men). The baroreflex was assessed using sequential bolus administration of sodium nitroprusside and phenylephrine, and baroreflex sensitivity was calculated as the slope of the relation between systolic blood pressure and R-R interval during the resulting blood pressure transients. Baroreflex sensitivity was inversely correlated to 24-hour average mean arterial pressure (R=0.49; P<0.001) and positively related to daytime heart rate variability (R=0.33; P=0.02). In contrast, no relationship was found between baroreflex sensitivity and 24-hour heart rate or blood pressure variabilities. We conclude that the relationship between baroreflex sensitivity and daytime heart rate variability was similar to that reported previously in hypertensive subjects. Furthermore, the inverse relation between baroreflex sensitivity and mean arterial pressure supports the idea that the baroreflex may exert longer-term effects on blood pressure than thought previously.