Renal nerves in the maintenance of hypertension: a potential therapeutic target

Renal denervation in resistant hypertension: a review of clinical trials and future perspectives

A previous report using the National Health and Nutrition Examination Survey demonstrated an increase in the prevalence of resistant hypertension, which does not respond to traditional therapy and medication. Studies using various animal hypertensive models have demonstrated significant blood pressure (BP) reduction following renal artery denervation (RDN). Catheter-based RDN became available in clinical trials as a possible treatment option for resistant hypertension. Although first clinical trials of RDN have demonstrated the efficacy and safety of this treatment mortality for lowering BP in patients with resistant hypertension, the role of RDN has been questioned since the results of the Symplicity HTN-3 trial. Considering the ethnic differences demonstrated in the Symplicity HTN-Japan and Global Symplicity registry, by contrast, RDN might be an effective for resistant hypertension in Asian population. Here, we discuss RDN applications and technology, the old and new clinical evidence of RDN, patients' selection of RDN responder, and optimization of RDN procedure in this review. The available evidence demonstrates that RDN could be effective in carefully selected patients with resistant hypertension, paving the way for future research in this area.

Reduced Renal Mass, Salt-Sensitive Hypertension Is Resistant to Renal Denervation

Aim: Activation of the sympathetic nervous system is common in resistant hypertension (RHT) and also in chronic kidney disease (CKD), a prevalent condition among resistant hypertensives. However, renal nerve ablation lowers blood pressure (BP) only in some patients with RHT. The influence of loss of nephrons per se on the antihypertensive response to renal denervation (RDNx) is unclear and was the focus of this study.

Methods: Systemic hemodynamics and sympathetically mediated low frequency oscillations of systolic BP were determined continuously from telemetrically acquired BP recordings in rats before and after surgical excision of ∼80% of renal mass and subsequent RDNx.

Results: After reduction of renal mass, rats fed a high salt (HS) diet showed sustained increases in mean arterial pressure (108 ± 3 mmHg to 128 ± 2 mmHg) and suppression of estimated sympathetic activity (∼15%), responses that did not occur with HS before renal ablation. After denervation of the remnant kidney, arterial pressure fell (to 104 ± 4 mmHg), estimated sympathetic activity and heart rate (HR) increased concomitantly, but these changes gradually returned to pre-denervation levels over 2 weeks of follow up. Subsequently, sympathoinhibition with clonidine did not alter arterial pressure while significantly suppressing estimated sympathetic activity and HR.

Conclusion: These results indicate that RDNx does not chronically lower arterial pressure in this model of salt-sensitive hypertension associated with substantial nephron loss, but without ischemia and increased sympathetic activity, thus providing further insight into conditions likely to impact the antihypertensive response to renal-specific sympathoinhibition in subjects with CKD.

Renal denervation for the management of resistant hypertension

Renal sympathetic denervation (RSD) as a therapy for patients with resistant hypertension has attracted great interest. The majority of studies in this field have demonstrated impressive reductions in blood pressure (BP). However, these trials were not randomized or sham-controlled and hence, the findings may have been overinflated due to trial biases. SYMPLICITY HTN-3 was the first randomized controlled trial to use a blinded sham-control and ambulatory BP monitoring. A surprise to many was that this study was neutral. Possible reasons for this neutrality include the fact that RSD may not be effective at lowering BP in man, RSD was not performed adequately due to limited operator experience, patients’ adherence with their anti-hypertensive drugs may have changed during the trial period, and perhaps the intervention only works in certain subgroups that are yet to be identified. Future studies seeking to demonstrate efficacy of RSD should be designed as randomized blinded sham-controlled trials. The efficacy of RSD is in doubt, but many feel that its safety has been established through the thousands of patients in whom the procedure has been performed. Over 90% of these data, however, are for the Symplicity™ system and rarely extend beyond 12 months of follow-up. Long-term safety cannot be assumed with RSD and nor should it be assumed that if one catheter system is safe then all are. We hope that in the near future, with the benefit of well-designed clinical trials, the role of renal denervation in the management of hypertension will be established.

Renal denervation attenuates NADPH oxidase-mediated oxidative stress and hypertension in rats with hydronephrosis

Hydronephrosis is associated with the development of salt-sensitive hypertension. Studies have suggested that increased sympathetic nerve activity and oxidative stress play important roles in hypertension and the modulation of salt sensitivity. The present study primarily aimed to examine the role of renal sympathetic nerve activity in the development of hypertension in rats with hydronephrosis. In addition, we aimed to investigate if NADPH oxidase (NOX) function could be affected by renal denervation. Partial unilateral ureteral obstruction (PUUO) was created in 3-wk-old rats to induce hydronephrosis. Sham surgery or renal denervation was performed at the same time. Blood pressure was measured during normal, high-, and low-salt diets. The renal excretion pattern, NOX activity, and expression as well as components of the renin-angiotensin-aldosterone system were characterized after treatment with the normal salt diet. On the normal salt diet, rats in the PUUO group had elevated blood pressure compared with control rats (115 ± 3 vs. 87 ± 1 mmHg, P < 0.05) and displayed increased urine production and lower urine osmolality. The blood pressure change in response to salt loading (salt sensitivity) was more pronounced in the PUUO group compared with the control group (15 ± 2 vs. 5 ± 1 mmHg, P < 0.05). Renal denervation in PUUO rats attenuated both hypertension (97 ± 3 mmHg) and salt sensitivity (5 ± 1 mmHg, P < 0.05) and normalized the renal excretion pattern, whereas the degree of renal fibrosis and inflammation was not changed. NOX activity and expression as well as renin and ANG II type 1A receptor expression were increased in the renal cortex from PUUO rats and normalized by denervation. Plasma Na(+) and K(+) levels were elevated in PUUO rats and normalized after renal denervation. Finally, denervation in PUUO rats was also associated with reduced NOX expression, superoxide production, and fibrosis in the heart. In conclusion, renal denervation attenuates hypertension and restores the renal excretion pattern, which is associated with reduced renal NOX and components of the renin-angiotensin-aldosterone system. This study emphasizes a link between renal nerves, the development of hypertension, and modulation of NOX function.

Renal Denervation

Resistant hypertension, defined as failure to reach blood pressure (BP) goals despite treatment with ≥3 antihypertensive agents, one of which is a diuretic, bears a significant risk of cardiovascular complications. Strong evidence exists, implicating the overactivation of the sympathetic nervous system (SNS) in the pathogenesis of resistant hypertension through complex neurohormonal interactions. Renal denervation is a novel attractive option to achieve adequate blockade of the sympathetic system, with subsequent BP reductions in patients with resistant hypertension. Data have shown promising results regarding the efficacy of the procedure, maintaining a favorable safety profile. As such, the paradigm of resistant hypertension has expanded in other conditions involving a hyperadrenergic state such as the metabolic syndrome, heart failure, arrhythmias, sleep apnea, and renal failure. This review focuses on the pathophysiological rationale of modifying SNS tone and the evidence of the benefits of such intervention beyond BP control.

Evidence and consequences of the central role of the kidneys in the pathophysiology of sympathetic hyperactivity

Chronic elevation of the sympathetic nervous system has been identified as a major contributor to the complex pathophysiology of hypertension, states of volume overload – such as heart failure – and progressive kidney disease. It is also a strong determinant for clinical outcome. This review focuses on the central role of the kidneys in the pathogenesis of sympathetic hyperactivity. As a consequence, renal denervation may be an attractive option to treat sympathetic hyperactivity. The review will also focus on first results and the still remaining questions of this new treatment option.

Renal Sympathetic Denervation for Treatment of Hypertension

OPINION STATEMENT: Sympathetic nervous system activation of the heart, kidney and peripheral vasculature increases cardiac output, fluid retention and vascular resistance and plays an important role in acute and chronic BP elevation. Renal sympathetic denervation via a percutaneous radiofrequency catheter based approach is a safe and effective procedure that lowers BP in patients with resistant hypertension. Exploratory studies in patients with resistant hypertension and a variety of comorbidities, including insulin resistance/metabolic syndrome, obstructive sleep apnea and the polycystic ovary syndrome, have shown benefit of renal denervation in attenuating the severity of the comorbid conditions, as well as reducing BP. However, more studies are needed to further address the long term effects of renal denervation and its safety and effectiveness in other disease states such as congestive heart failure.

Left ventricular hypertrophy: major risk factor in patients with hypertension: update and practical clinical applications

Left ventricular hypertrophy is a maladaptive response to chronic pressure overload and an important risk factor for atrial fibrillation, diastolic heart failure, systolic heart failure, and sudden death in patients with hypertension. Since not all patients with hypertension develop left ventricular hypertrophy, there are clinical findings that should be kept in mind that may alert the physician to the presence of left ventricular hypertrophy so a more definitive evaluation can be performed using an echocardiogram or cardiovascular magnetic resonance. Controlling arterial pressure, sodium restriction, and weight loss independently facilitate the regression of left ventricular hypertrophy. Choice of antihypertensive agents may be important when treating a patient with hypertensive left ventricular hypertrophy. Angiotensin-converting enzyme inhibitors or angiotensin II receptor blockers followed by calcium channel antagonists most rapidly facilitate the regression of left ventricular hypertrophy. With the regression of left ventricular hypertrophy, diastolic function and coronary flow reserve usually improve, and cardiovascular risk decreases.

[Renal sympathetic denervation. A novel interventional treatment option for therapy-resistant arterial hypertension]

As much as one third of patients with arterial hypertension are treatment refractory as they do not reach sufficient blood pressure control despite antihypertensive combination therapy of significant duration. In patients with therapy-resistant hypertension, the kidneys play a central role as activator of the sympathetic nervous system. Sympathetic nervous activation via efferent nerve fibers lying in the adventitia of the renal arteries leads to increased Na(+) reabsorption, increased renin secretion and reduction of renal plasma flow. Via afferent sympathetic fibers in the dorsal root of the spinal chord, the kidneys induce a further augmentation of central sympathetic nervous activity. With the method of renal sympathetic denervation (RSD) an interventional minimally invasive procedure has become available to precisely ablate afferent and efferent sympathetic nervous fibers surrounding the renal artery. Via an ablation catheter with an electrode tip and a radiofrequency generator a series of 4-6 ablation sites are administered in both renal arteries leading to denervation of the sympathetic nerve fibers while keeping the renal artery intact. Recent studies showed a significant and continuous reduction of blood pressure of 25-30 mmHg systolic and 10-15 mmHg diastolic for at least 2 years. Concerning the role of elevated sympathetic nervous system activity in forms of hypertension associated with other disorders, further applications of the procedure appear possible, although these are of a rather speculative nature at this time. The current mainstay of therapy-refractive hypertension is RSD, which is well supported by recent clinical data.

Trichomonas vaginalis degrades nitric oxide and expresses a flavorubredoxin-like protein: a new pathogenic mechanism?

Besides possessing many physiological roles, nitric oxide (NO) produced by the immune system in infectious diseases has antimicrobial effects. Trichomoniasis, the most widespread non-viral sexually transmitted disease caused by the microaerophilic protist Trichomonas vaginalis, often evolves into a chronic infection, with the parasite able to survive in the microaerobic, NO-enriched vaginal environment. We relate this property to the finding that T. vaginalis degrades NO under anaerobic conditions, as assessed amperometrically. This activity, which is maximal (133 +/- 41 nmol NO/10(8) cells per minute at 20 degrees C) at low NO concentrations (< or = 1.2 microM), was found to be: (i) NADH dependent, (ii) cyanide insensitive and (iii) inhibited by O(2). These features are consistent with those of the Escherichia coli A-type flavoprotein (ATF), recently discovered to be endowed with NO reductase activity. Using antibodies against the ATF from E. coli, a protein band was immunodetected in the parasite grown in a standard medium. If confirmed, the expression of an ATF in eukaryotes suggests that the genes coding for ATFs were transferred during evolution from anaerobic Prokarya to pathogenic protists, to increase their fitness for the microaerobic, parasitic life style. Thus the demonstration of an ATF in T. vaginalis would appear relevant to both pathology and evolutionary biology. Interestingly, genomic analysis has recently demonstrated that Giardia intestinalis and other pathogenic protists have genes coding for ATFs.