Differential role of afferent and efferent renal nerves in the maintenance of early- and late-phase Dahl S hypertension

From fat to filter: the effect of adipose tissue-derived signals on kidney function

Obesity is associated with severe consequences for the renal system, including chronic kidney disease, kidney failure and increased mortality. Obesity has both direct and indirect effects on kidney health through several mechanisms, including activation of the renin-angiotensin system, mechanical compression, inflammation, fibrosis, increased filtration barrier permeability and renal nerve activity. The expansion of adipose tissue through hypertrophy and hyperplasia can induce haemodynamic changes that promote glomerular hyperfiltration to compensate for the greater metabolic demands of the increased body weight. Adipose expansion is also associated with the release of adipokines and pro-inflammatory cytokines, hyperinsulinaemia and insulin resistance, which exert direct and indirect effects on kidney function via various mechanisms. Increased uptake of fatty acids by the kidney leads to alterations in lipid metabolism and lipotoxicity, also contributing to the pro-inflammatory and pro-fibrotic environment. The role of the adipose tissue-brain-kidney axis in the obesity-associated decline in renal function is sustained by studies showing that stimulation of adipose tissue sensory neurons by locally released factors increases renal sympathetic nerve activity. Conversely, pre-existent kidney disease can contribute to adipose dysfunction through the accumulation of uraemic toxins and hormonal changes. These findings highlight the importance of crosstalk between adipose tissue and the kidneys and provide insights into the mechanisms underlying the associations between obesity and kidney disease.

The road to renal denervation for hypertension and beyond (HF): two decades of failed, succeeded, and to be determined

Activation of the sympathetic nervous system has been attributed to the development of hypertension. Two established approaches for treating hypertension are pharmacotherapy and lifestyle changes. With an improved understanding of renal nerve anatomy and physiology, renal denervation has been proposed as an alternative treatment for hypertension. Specifically, it has been shown that the interruption of sympathetic nerves connecting the kidney and the sympathetic nervous system can reduce blood pressure. Here, we present a review on how renal denervation can help hypertension patients, specifically focusing on our novel understanding of renal nerve anatomy, denervation technique, and subsequent clinical trials, and how it may be used to treat other cardiovascular diseases like heart failure.

Renal nerves in physiology, pathophysiology and interoception

Sympathetic efferent renal nerves have key roles in the regulation of kidney function and blood pressure. Increased renal sympathetic nerve activity is thought to contribute to hypertension by promoting renal sodium retention, renin release and renal vasoconstriction. This hypothesis led to the development of catheter-based renal denervation (RDN) for the treatment of hypertension. Two RDN devices that ablate both efferent and afferent renal nerves received FDA approval for this indication in 2023. However, in animal models, selective ablation of afferent renal nerves resulted in comparable anti-hypertensive effects to ablation of efferent and afferent renal nerves and was associated with a reduction in sympathetic nerve activity. Selective afferent RDN also improved kidney function in a chronic kidney disease model. Notably, the beneficial effects of RDN extend beyond hypertension and chronic kidney disease to other clinical conditions that are associated with elevated sympathetic nerve activity, including heart failure and arrhythmia. These findings suggest that the kidney is an interoceptive organ, as increased renal sensory nerve activity modulates sympathetic activity to other organs. Future studies are needed to translate this knowledge into novel therapies for the treatment of hypertension and other cardiorenal diseases.

Renal interoception in health and disease

Catheter based renal denervation has recently been FDA approved for the treatment of hypertension. Traditionally, the anti-hypertensive effects of renal denervation have been attributed to the ablation of the efferent sympathetic renal nerves. In recent years the role of the afferent sensory renal nerves in the regulation of blood pressure has received increased attention. In addition, afferent renal denervation is associated with reductions in sympathetic nervous system activity. This suggests that reductions in sympathetic drive to organs other than the kidney may contribute to the non-renal beneficial effects observed in clinical trials of catheter based renal denervation. In this review we will provide an overview of the role of the afferent renal nerves in the regulation of renal function and the development of pathophysiologies, both renal and non-renal. We will also describe the central projections of the afferent renal nerves, to give context to the responses seen following their ablation and activation. Finally, we will discuss the emerging role of the kidney as an interoceptive organ. We will describe the potential role of the kidney in the regulation of interoceptive sensitivity and in this context, speculate on the possible pathological consequences of altered renal function.

Elevated renal afferent nerve activity in a rat model of endothelin B receptor deficiency

Renal nerves have been an attractive target for interventions aimed at lowering blood pressure; however, the specific roles of renal afferent (sensory) versus efferent sympathetic nerves in mediating hypertension are poorly characterized. A number of studies have suggested that a sympathoexcitatory signal conveyed by renal afferents elicits increases in blood pressure, whereas other studies identified sympathoinhibitory afferent pathways. These sympathoinhibitory pathways have been identified as protective against salt-sensitive increases in blood pressure through endothelin B (ETB) receptor activation. We hypothesized that ETB-deficient (ETB-def) rats, which are devoid of functional ETB receptors except in adrenergic tissues, lack appropriate sympathoinhibition and have lower renal afferent nerve activity following a high-salt diet compared with transgenic controls. We found that isolated renal pelvises from high salt-fed ETB-def animals lack a response to a physiological stimulus, prostaglandin E2, compared with transgenic controls but respond equally to a noxious stimulus, capsaicin. Surprisingly, we observed elevated renal afferent nerve activity in intact ETB-def rats compared with transgenic controls under both normal- and high-salt diets. ETB-def rats have been previously shown to have heightened global sympathetic tone, and we also observed higher total renal sympathetic nerve activity in ETB-def rats compared with transgenic controls under both normal- and high-salt diets. These data indicate that ETB receptors are integral mediators of the sympathoinhibitory renal afferent reflex (renorenal reflex), and, in a genetic rat model of ETB deficiency, the preponderance of sympathoexcitatory renal afferent nerve activity prevails and may contribute to hypertension.NEW & NOTEWORTHY Here, we found that endothelin B receptors are an important contributor to renal afferent nerve responsiveness to a high-salt diet. Rats lacking endothelin B receptors have increased afferent nerve activity that is not responsive to a high-salt diet.

Renal nerve stimulation identifies renal innervation and optimizes the strategy for renal denervation in canine

BACKGROUND

Renal denervation (RDN) was still performed without any intra-procedural method for nerve mapping. Whether renal nerve stimulation (RNS) is an efficient way to identify renal autonomic innervation and optimize the strategy for RDN remain to be worthy for further exploration.

METHODS

The characteristics of renal autonomic innervation at the sites with different blood pressure (BP) responses to RNS were explored. Then, dogs anatomically eligible for RDN were randomly assigned into elevated BP response ablation group, reduced BP response ablation group, and RNS-control group. The postoperative outcomes were measured at baseline and after 4 weeks follow-up.

RESULTS

The proportion of afferent sensory nerve was higher at elevated BP response sites (ERS) than reduced BP response sites (RRS) and non-response sites (NRS) (P = 0.012 and P = 0.004). Conversely, the proportion of parasympathetic nerve at RRS was the highest (RRS vs. ERS, P = 0.017; RRS vs. NRS, P = 0.023). More importantly, there was a significant correlation between systolic blood pressure changes and the area ratios of afferent sensory and parasympathetic nerve (R = 0.859; P < 0.001). In addition, ablation at BP-elevation sites can result in a significant decrease in BP and plasma norepinephrine (NE) after 4 weeks (P = 0.002; P = 0.008), while ablation at BP-reduction sites can lead to significant increases in BP and plasma NE (P = 0.016; P = 0.033).

CONCLUSIONS

RNS is an effective method to identify renal autonomic innervation. It could not only help to identify optimal target sites, but also avoid ablation of sympathetic-inhibitory areas during RDN.

Contribution of afferent renal nerve signals to acute and chronic blood pressure regulation in stroke-prone spontaneously hypertensive rats

The activation of sympathetic nervous system plays a critical role in the development of hypertension. The input from afferent renal nerves may affect central sympathetic outflow; however, its contribution to the development of hypertension remains unclear. We investigated the role of afferent renal nerves in acute and chronic blood pressure regulation using normotensive Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP). Acute chemical stimulation of afferent renal nerves elicited larger increases in blood pressure and renal sympathetic nerve activity in young 9-week-old SHRSP compared to WKY. Selective afferent renal denervation (ARDN) and conventional total renal denervation (TRDN) ablating both afferent and efferent nerves in young SHRSP revealed that only TRDN, but not ARDN, chronically attenuated blood pressure elevation. ARDN did not affect plasma renin activity or plasma angiotensin II levels, whereas TRDN decreased both. Neither TRDN nor ARDN affected central sympathetic outflow and systemic sympathetic activity determined by neuronal activity in the parvocellular region of hypothalamic paraventricular nucleus and rostral ventrolateral medulla and by plasma and urinary norepinephrine levels, respectively. Renal injury was not apparent in young SHRSP compared with WKY, suggesting that renal afferent input might not be activated in young SHRSP. In conclusion, the chronic input from afferent renal nerves does not contribute to the development of hypertension in SHRSP despite the increased blood pressure response to the acute stimulation of afferent renal nerves. Efferent renal nerves may be involved in the development of hypertension via activation of the renin-angiotensin system in SHRSP.

Cardiorenal Syndrome: The Role of Neural Connections Between the Heart and the Kidneys

The maintenance of cardiovascular homeostasis is highly dependent on tightly controlled interactions between the heart and the kidneys. Therefore, it is not surprising that a dysfunction in one organ affects the other. This interlinking relationship is aptly demonstrated in the cardiorenal syndrome. The characteristics of the cardiorenal syndrome state include alterations in neurohumoral drive, autonomic reflexes, and fluid balance. The evidence suggests that several factors contribute to these alterations. These may include peripheral and central nervous system abnormalities. However, accumulating evidence from animals with experimental models of congestive heart failure and renal dysfunction as well as humans with the cardiorenal syndrome suggests that alterations in neural pathways, from and to the kidneys and the heart, including the central nervous system are involved in regulating sympathetic outflow and may be critically important in the alterations in neurohumoral drive, autonomic reflexes, and fluid balance commonly observed in the cardiorenal syndrome. This review focuses on studies implicating neural pathways, particularly the afferent and efferent signals from the heart and the kidneys integrating at the level of the paraventricular nucleus in the hypothalamus to alter neurohumoral drive, autonomic pathways, and fluid balance. Further, it explores the potential mechanisms of action for the known beneficial use of various medications or potential novel therapeutic manipulations for the treatment of the cardiorenal syndrome. A comprehensive understanding of these mechanisms will enhance our ability to treat cardiorenal conditions and their cardiovascular complications more efficaciously and thoroughly.

Fetal Undernutrition Programming, Sympathetic Nerve Activity, and Arterial Hypertension Development

A wealth of evidence showed that low birth weight is associated with environmental disruption during gestation, triggering embryotic or fetal adaptations and increasing the susceptibility of progeny to non-communicable diseases, including metabolic and cardiovascular diseases, obesity, and arterial hypertension. In addition, dietary disturbance during pregnancy in animal models has highlighted mechanisms that involve the genesis of arterial hypertension, particularly severe maternal low-protein intake (LP). Functional studies demonstrated that maternal low-protein intake leads to the renal decrease of sodium excretion and the dysfunction of the renin-angiotensin-aldosterone system signaling of LP offspring. The antinatriuretic effect is accentuated by a reduced number of nephron units and glomerulosclerosis, which are critical in establishing arterial hypertension phenotype. Also, in this way, studies have shown that the overactivity of the central and peripheral sympathetic nervous system occurs due to reduced sensory (afferent) renal nerve activity. As a result of this reciprocal and abnormal renorenal reflex, there is an enhanced tubule sodium proximal sodium reabsorption, which, at least in part, contributes directly to arterial hypertension development in some of the programmed models. A recent study has observed that significant changes in adrenal medulla secretion could be involved in the pathophysiological process of increasing blood pressure. Thus, this review aims to compile studies that link the central and peripheral sympathetic system activity mechanisms on water and salt handle and blood pressure control in the maternal protein-restricted offspring. Besides, these pathophysiological mechanisms mainly may involve the modulation of neurokinins and catecholamines pathways.

Circadian Control of Sodium and Blood Pressure Regulation

The attention for the control of dietary risk factors involved in the development of hypertension, includes a large effort on dietary salt restrictions. Ample studies show the beneficial role of limiting dietary sodium as a lifestyle modification in the prevention and management of essential hypertension. Not until the past decade or so have studies more specifically investigated diurnal variations in renal electrolyte excretion, which led us to the hypothesis that timing of salt intake may impact cardiovascular health and blood pressure regulation. Cell autonomous molecular clocks as the name implies, function independently to maintain optimum functional rhythmicity in the face of environmental stressors such that cellular homeostasis is maintained at all times. Our understanding of mechanisms influencing diurnal patterns of sodium excretion and blood pressure has expanded with the discovery of the circadian clock genes. In this review, we discuss what is known about circadian regulation of renal sodium handling machinery and its influence on blood pressure regulation, with timing of sodium intake as a potential modulator of the kidney clock.

Renal denervation based on experimental rationale

Excessive activation of the sympathetic nervous system is one of the pathophysiological hallmarks of hypertension and heart failure. Within the central nervous system, the paraventricular nucleus (PVN) of the hypothalamus and the rostral ventrolateral medulla in the brain stem play critical roles in the regulation of sympathetic outflow to peripheral organs. Information from the peripheral circulation, including serum concentrations of sodium and angiotensin II, is conveyed to the PVN via adjacent structures with a weak blood-brain barrier. In addition, signals from baroreceptors, chemoreceptors and cardiopulmonary receptors as well as afferent input via the renal nerves are all integrated at the level of the PVN. The brain renin-angiotensin system and the balance between nitric oxide and reactive oxygen species in these brain areas also determine the final sympathetic outflow. Additionally, brain inflammatory responses have been shown to modulate these processes. Renal denervation interrupts both the afferent inputs from the kidney to the PVN and the efferent outputs from the PVN to the kidney, resulting in the suppression of sympathetic outflow and eliciting beneficial effects on both hypertension and heart failure.

Renal denervation does not affect hypertension or the renin-angiotensin system in a rodent model of juvenile-onset polycystic kidney disease: clinical implications

We examined the effect of total and afferent renal denervation (RDN) on hypertension and the renin-angiotensin system (RAS) in a rodent model of juvenile-onset polycystic kidney disease (PKD). Lewis Polycystic Kidney (LPK) and control rats received total, afferent or sham RDN by periaxonal application of phenol, capsaicin or normal saline, respectively, and were monitored for 4-weeks. Afferent RDN did not affect systolic blood pressure (SBP) determined by radiotelemetry in either strain (n = 19) while total RDN significantly reduced SBP in Lewis rats 4-weeks post-denervation (total vs. sham, 122 ± 1 vs. 130 ± 2 mmHg, P = 0.002, n = 25). Plasma and kidney renin content determined by radioimmunoassay were significantly lower in LPK vs. Lewis (plasma: 278.2 ± 6.7 vs. 376.5 ± 11.9 ng Ang I/ml/h; kidney: 260.1 ± 6.3 vs. 753.2 ± 37.9 ng Ang I/mg/h, P < 0.001, n = 26). These parameters were not affected by RDN. Intrarenal mRNA expression levels of renin, angiotensinogen, angiotensin-converting enzyme (ACE)2, and angiotensin II receptor type 1a were significantly lower, whereas ACE1 expression was significantly higher in the LPK vs. Lewis (all P < 0.05, n = 26). This pattern of intrarenal RAS expression was not changed by RDN. In conclusion, RDN does not affect hypertension or the RAS in the LPK model and indicates RDN might not be a suitable antihypertensive strategy for individuals with juvenile-onset PKD.

Effect of renal denervation on long-term outcomes in patients with resistant hypertension

Increasing studies strongly prove that renal denervation, a minimally invasive surgery, is a promising new non-drug treatment method that can effectively control blood pressure in patients with resistant hypertension, but the evaluation of the long-term blood pressure control effect of renal denervation for resistant hypertension is still lacking. Here, we critically review current long-term follow-up data about the use of renal denervation for RH to comprehensively evaluate the effectiveness of renal denervation for RH, and to provide practical guidance for practitioners who are establishing a renal denervation service. Limited by the current research, many problems need to be solved before renal denervation is applied to RH. In addition, ambulatory blood pressure should be the first choice for the evaluation of blood pressure. Finally, the continuous antihypertensive effect of renal denervation in different renal denervation systems also needs to be strictly compared.

Function of Renal Nerves in Kidney Physiology and Pathophysiology

Renal sympathetic (efferent) nerves play an important role in the regulation of renal function, including glomerular filtration, sodium reabsorption, and renin release. The kidney is also innervated by sensory (afferent) nerves that relay information to the brain to modulate sympathetic outflow. Hypertension and other cardiometabolic diseases are linked to overactivity of renal sympathetic and sensory nerves, but our mechanistic understanding of these relationships is limited. Clinical trials of catheter-based renal nerve ablation to treat hypertension have yielded promising results. Therefore, a greater understanding of how renal nerves control the kidney under physiological and pathophysiological conditions is needed. In this review, we provide an overview of the current knowledge of the anatomy of efferent and afferent renal nerves and their functions in normal and pathophysiological conditions. We also suggest further avenues of research for development of novel therapies targeting the renal nerves.

Renal Denervation and Celiac Ganglionectomy Decrease Mean Arterial Pressure Similarly in Genetically Hypertensive Schlager (BPH/2J) Mice

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Renal denervation (RDNX) lowers mean arterial pressure (MAP) in patients with resistant hypertension. Less well studied is the effect of celiac ganglionectomy (CGX), a procedure which involves the removal of the nerves innervating the splanchnic vascular bed. We hypothesized that RDNX and CGX would both lower MAP in genetically hypertensive Schlager (BPH/2J) mice through a reduction in sympathetic tone. Telemeters were implanted into the femoral artery in mice to monitor MAP before and after RDNX (n=5), CGX (n=6), or SHAM (n=6). MAP, systolic blood pressure, diastolic blood pressure, and heart rate were recorded for 14 days postoperatively. The MAP response to hexamethonium (10 mg/kg, IP) was measured on control day 3 and postoperative day 10 as a measure of global neurogenic pressor activity. The efficacy of denervation was assessed by measurement of tissue norepinephrine. Control MAP was similar among the 3 groups before surgical treatments (≈130 mm Hg). On postoperative day 14, MAP was significantly lower in RDNX (−11±2 mm Hg) and CGX (−11±1 mm Hg) groups compared with their predenervation values. This was not the case in SHAM mice (−5±3 mm Hg). The depressor response to hexamethonium in the RDNX group was significantly smaller on postoperative day 10 (−10±5 mm Hg) compared with baseline control (−25±10 mm Hg). This was not the case in mice in the SHAM (day 10; −28±5 mm Hg) or CGX (day 10; −34±7 mm Hg) group. In conclusion, both renal and splanchnic nerves contribute to hypertension in BPH/2J mice, but likely through different mechanisms.

The involvement of renal afferents in the maintenance of cardiorenal diseases

Elevated sympathetic vasomotor activity is a common feature of cardiorenal diseases. Therefore, the sympathetic nervous system is an important therapeutic target, particularly the fibers innervating the kidneys. In fact, renal denervation has been applied clinically and shown promising results in patients with hypertension and chronic kidney disease. However, the underlying mechanisms involved in the cardiorenal protection induced by renal denervation have not yet been fully clarified. This mini-review highlights historical and recent aspects related to the role of renal sensory fibers in the control of cardiorenal function under normal conditions and in experimental models of cardiovascular disease. Results have demonstrated that alterations in renal sensory function participate in the maintenance of elevated sympathetic vasomotor activity and cardiorenal changes; as such, renal sensory fibers may be a potential therapeutic target for the treatment of cardiorenal diseases. Although it has not yet been applied in clinical practice, selective afferent renal denervation may be promising, since such an approach maintains efferent activity and can provide more refined control of renal function compared with total renal denervation. However, more studies are needed to understand the mechanisms by which renal afferents partially contribute to such changes, in addition to the need to evaluate the safety and advantages of the approach for application in the clinical practice.

Contribution of the Renal Nerves to Hypertension in a Rabbit Model of Chronic Kidney Disease

Overactivity of the sympathetic nervous system and high blood pressure are implicated in the development and progression of chronic kidney disease (CKD) and independently predict cardiovascular events in end-stage renal disease. To assess the role of renal nerves, we determined whether renal denervation (RDN) altered the hypertension and sympathoexcitation associated with a rabbit model of CKD. The model involves glomerular layer lesioning and uninephrectomy, resulting in renal function reduced by one-third and diuresis. After 3-week CKD, blood pressure was 13±2 mm Hg higher than at baseline (P<0.001), and compared with sham control rabbits, renal sympathetic nerve activity was 1.2±0.5 normalized units greater (P=0.01). The depressor response to ganglion blockade was also +8.0±3 mm Hg greater, but total norepinephrine spillover was 8.7±3.7 ng/min lower (bothP<0.05). RDN CKD rabbits only increased blood pressure by 8.0±1.5 mm Hg. Renal sympathetic activity, the response to ganglion blockade and diuresis were similar to sham denervated rabbits (non-CKD). CKD rabbits had intact renal sympathetic baroreflex gain and range, as well as normal sympathetic responses to airjet stress. However, hypoxia-induced sympathoexcitation was reduced by −9±0.4 normalized units. RDN did not alter the sympathetic response to hypoxia or airjet stress. CKD increased oxidative stress markers Nox5 and MCP-1 (monocyte chemoattractant protein-1) in the kidney, but RDN had no effect on these measures. Thus, RDN is an effective treatment for hypertension in this model of CKD without further impairing renal function or altering the normal sympathetic reflex responses to various environmental stimuli.

Sympathetic Nervous System Contributions to Hypertension: Updates and Therapeutic Relevance

The sympathetic nervous system plays a pivotal role in the long-term regulation of arterial blood pressure through the ability of the central nervous system to integrate neurohumoral signals and differentially regulate sympathetic neural input to specific end organs. Part 1 of this review will discuss neural mechanisms of salt-sensitive hypertension, obesity-induced hypertension, and the ability of prior experiences to sensitize autonomic networks. Part 2 of this review focuses on new therapeutic advances to treat resistant hypertension including renal denervation and carotid baroactivation. Both advances lower arterial blood pressure by reducing sympathetic outflow. We discuss potential mechanisms and areas of future investigation to target the sympathetic nervous system.

Device-Based Neuromodulation for Resistant Hypertension Therapy

Despite availability of effective drugs for hypertension therapy, significant numbers of hypertensive patients fail to achieve recommended blood pressure levels on ≥3 antihypertensive drugs of different classes. These individuals have a high prevalence of adverse cardiovascular events and are defined as having resistant hypertension (RHT) although nonadherence to prescribed antihypertensive medications is common in patients with apparent RHT. Furthermore, apparent and true RHT often display increased sympathetic activity. Based on these findings, technology was developed to treat RHT by suppressing sympathetic activity with electrical stimulation of the carotid baroreflex and catheter-based renal denervation (RDN). Over the last 15 years, experimental and clinical studies have provided better understanding of the physiological mechanisms that account for blood pressure lowering with baroreflex activation and RDN and, in so doing, have provided insight into which patients in this heterogeneous hypertensive population are most likely to respond favorably to these device-based therapies. Experimental studies have also played a role in modifying device technology after early clinical trials failed to meet key endpoints for safety and efficacy. At the same time, these studies have exposed potential differences between baroreflex activation and RDN and common challenges that will likely impact antihypertensive treatment and clinical outcomes in patients with RHT. In this review, we emphasize physiological studies that provide mechanistic insights into blood pressure lowering with baroreflex activation and RDN in the context of progression of clinical studies, which are now at a critical point in determining their fate in RHT management.

TRPV1 (Transient Receptor Potential Vanilloid 1) Cardiac Spinal Afferents Contribute to Hypertension in Spontaneous Hypertensive Rat

Hypertension is associated with increased sympathetic activity. A component of this sympathoexcitation may be driven by increased signaling from sensory endings from the heart to the autonomic control areas in the brain. This pathway mediates the so-called cardiac sympathetic afferent reflex, which is also activated by coronary ischemia or other nociceptive stimuli in the heart. The cardiac sympathetic afferent reflex has been shown to be enhanced in the heart failure state and in renal hypertension. However, little is known about its role in the development or progression of hypertension or the phenotype of the sensory endings involved. To investigate this, we used the selective afferent neurotoxin, resiniferatoxin (RTX) to chronically abolish the cardiac sympathetic afferent reflex in 2 models of hypertension; the spontaneous hypertensive rats (SHRs) and AngII (angiotensin II) infusion (240 ng/kg per min). Blood pressure (BP) was measured in conscious animals for 2 to 8 weeks post-RTX. Epidural application of RTX to the T1-T4 spinal segments prevented the further BP increase in 8-week-old SHR and lowered BP in 16-week-old SHR. RTX did not affect BP in Wistar-Kyoto normotensive rats nor in AngII-infused rats. Epicardial application of RTX (50 µg/mL) in 4-week-old SHR prevented the BP increase whereas this treatment does not lower BP in 16-week-old SHR. When RTX was administered into the L2-L5 spinal segments of 16-week-old SHR, no change in BP was observed. These findings indicate that signaling via thoracic afferent nerve fibers may contribute to the hypertension phenotype in the SHR but not in the Ang II infusion model of hypertension.

Renal nerves and leukocyte infiltration in the kidney during salt-sensitive hypertension

Based on previous studies suggesting a role of renal nerves in renal inflammation, the present studies were performed to test the hypothesis that renal nerves mediate renal damage in Dahl salt-sensitive (SS) hypertension by increasing renal leukocyte infiltration. Experiments were performed in Dahl SS rats with bilateral renal denervation (RDN) and bilateral sham operation (n = 10 or 11 per group) and with unilateral RDN and contralateral sham operation (n = 10). After denervation, rats were switched from a low-salt 0.4% NaCl (LS) diet to a high-salt 4% NaCl (HS) diet and maintained on HS diet for 21 days. Bilateral RDN reduced the magnitude of hypertension assessed by radiotelemetry in Dahl SS rats compared with sham-operated rats (mean arterial pressure 140.9 ±4.8 mmHg and 159.7 ± 3.5 mmHg, respectively) and reduced proteinuria at day 21 of HS diet. However, assessment of renal leukocyte infiltration demonstrated no significant effect of bilateral RDN on the number of infiltrating leukocytes (RDN 3.6 ± 0.5 × 10⁶ vs. sham operated 4.3 ± 0.3 × 10⁶ CD45+ cells) or any of the subsets examined by flow cytometry. The unilateral RDN experiment showed no effect of RDN on the renal infiltration of leukocytes (RDN 6.5 ± 0.9 × 10⁶ vs. sham operated 6.1 ± 1.1 × 10⁶ CD45+ cells/kidney) or renal damage in RDN vs. sham-operated kidney after 21 days of HS diet. This work investigated the relationship between renal nerves and renal inflammation during Dahl SS hypertension. Contrary to our hypothesis, the results of this work suggest that immune cell infiltration in the kidney of Dahl SS rats is not mediated by the renal nerves.

Role of the afferent renal nerves in sodium homeostasis and blood pressure regulation in rats

New Findings

What is the central question of this study?

What are the differential roles of the mechanosensitive and chemosensitive afferent renal nerves in the reno‐renal reflex that promotes natriuresis, sympathoinhibition and normotension during acute and chronic challenges to sodium homeostasis?

What is the main finding and its importance?

The mechanosensitive afferent renal nerves contribute to an acute natriuretic sympathoinhibitory reno‐renal reflex that may be integrated within the paraventricular nucleus of the hypothalamus. Critically, the afferent renal nerves are required for the maintenance of salt resistance in Sprague–Dawley and Dahl salt‐resistant rats and attenuate the development of Dahl salt‐sensitive hypertension.

Abstract

These studies tested the hypothesis that in normotensive salt‐resistant rat phenotypes the mechanosensitive afferent renal nerve (ARN) reno‐renal reflex promotes natriuresis, sympathoinhibition and normotension during acute and chronic challenges to fluid and electrolyte homeostasis. Selective ARN ablation was conducted prior to (1) an acute isotonic volume expansion (VE) or 1 m NaCl infusion in Sprague–Dawley (SD) rats and (2) chronic high salt intake in SD, Dahl salt‐resistant (DSR), and Dahl salt‐sensitive (DSS) rats. ARN responsiveness following high salt intake was assessed ex vivo in response to noradrenaline and sodium concentration (SD, DSR and DSS) and via in vivo manipulation of renal pelvic pressure and sodium concentration (SD and DSS). ARN ablation attenuated the natriuretic and sympathoinhibitory responses to an acute VE [peak natriuresis (µeq min⁻¹) sham 52 ± 5 vs. ARN ablation 28 ± 3, P < 0.05], but not a hypertonic saline infusion in SD rats. High salt (HS) intake enhanced ARN reno‐renal reflex‐mediated natriuresis in response to direct increases in renal pelvic pressure (mechanoreceptor stimulus) in vivo and ARN responsiveness to noradrenaline ex vivo in SD, but not DSS, rats. In vivo and ex vivo ARN responsiveness to increased renal pelvic sodium concentration (chemoreceptor stimulus) was unaltered during HS intake. ARN ablation evoked sympathetically mediated salt‐sensitive hypertension in SD rats [MAP (mmHg): sham normal salt 102 ± 2 vs. sham HS 104 ± 2 vs. ARN ablation normal salt 103 ± 2 vs. ARN ablation HS 121 ± 2, P < 0.05] and DSR rats and exacerbated DSS hypertension. The mechanosensitive ARNs mediate an acute sympathoinhibitory natriuretic reflex and counter the development of salt‐sensitive hypertension.

New Approaches in the Management of Sudden Cardiac Death in Patients with Heart Failure-Targeting the Sympathetic Nervous System

Cardiovascular diseases (CVDs) have been considered the most predominant cause of death and one of the most critical public health issues worldwide. In the past two decades, cardiovascular (CV) mortality has declined in high-income countries owing to preventive measures that resulted in the reduced burden of coronary artery disease (CAD) and heart failure (HF). In spite of these promising results, CVDs are responsible for ~17 million deaths per year globally with ~25% of these attributable to sudden cardiac death (SCD). Pre-clinical data demonstrated that renal denervation (RDN) decreases sympathetic activation as evaluated by decreased renal catecholamine concentrations. RDN is successful in reducing ventricular arrhythmias (VAs) triggering and its outcome was not found inferior to metoprolol in rat myocardial infarction model. Registry clinical data also suggest an advantageous effect of RDN to prevent VAs in HF patients and electrical storm. An in-depth investigation of how RDN, a minimally invasive and safe method, reduces the burden of HF is urgently needed. Myocardial systolic dysfunction is correlated to neuro-hormonal overactivity as a compensatory mechanism to keep cardiac output in the face of declining cardiac function. Sympathetic nervous system (SNS) overactivity is supported by a rise in plasma noradrenaline (NA) and adrenaline levels, raised central sympathetic outflow, and increased organ-specific spillover of NA into plasma. Cardiac NA spillover in untreated HF individuals can reach ~50-fold higher levels compared to those of healthy individuals under maximal exercise conditions. Increased sympathetic outflow to the renal vascular bed can contribute to the anomalies of renal function commonly associated with HF and feed into a vicious cycle of elevated BP, the progression of renal disease and worsening HF. Increased sympathetic activity, amongst other factors, contribute to the progress of cardiac arrhythmias, which can lead to SCD due to sustained ventricular tachycardia. Targeted therapies to avoid these detrimental consequences comprise antiarrhythmic drugs, surgical resection, endocardial catheter ablation and use of the implantable electronic cardiac devices. Analogous NA agents have been reported for single photon-emission-computed-tomography (SPECT) scans usage, specially the 123I-metaiodobenzylguanidine (123I-MIBG). Currently, HF prognosis assessment has been improved by this tool. Nevertheless, this radiotracer is costly, which makes the use of this diagnostic method limited. Comparatively, positron-emission-tomography (PET) overshadows SPECT imaging, because of its increased spatial definition and broader reckonable methodologies. Numerous ANS radiotracers have been created for cardiac PET imaging. However, so far, [11C]-meta-hydroxyephedrine (HED) has been the most significant PET radiotracer used in the clinical scenario. Growing data has shown the usefulness of [11C]-HED in important clinical situations, such as predicting lethal arrhythmias, SCD, and all-cause of mortality in reduced ejection fraction HF patients. In this article, we discussed the role and relevance of novel tools targeting the SNS, such as the [11C]-HED PET cardiac imaging and RDN to manage patients under of SCD risk.

Renal sensory nerves increase sympathetic nerve activity and blood pressure in 2-kidney 1-clip hypertensive mice

Renal denervation lowers arterial blood pressure (ABP) in multiple clinical trials and some experimental models of hypertension. These antihypertensive effects have been attributed to the removal of renal afferent nerves. The purpose of the present study was to define the function, anatomy, and contribution of mouse renal sensory neurons to a renal nerve-dependent model of hypertension. First, electrical stimulation of mouse renal afferent nerves produced frequency-dependent increases in ABP that were eliminated by ganglionic blockade. Stimulus-triggered averaging revealed renal afferent stimulation significantly increased splanchnic, renal, and lumbar sympathetic nerve activity (SNA). Second, kidney injection of wheat germ agglutinin into male C57Bl6 mice (12-14 wk; Jackson Laboratories) produced ipsilateral labeling in the T11-L2 dorsal root ganglia. Next, 2-kidney 1-clip (2K1C) hypertension was produced in male C57Bl6 mice (12-14 wk; Jackson Laboratories) by placement of a 0.5-mm length of polytetrafluoroethylene tubing around the left renal artery. 2K1C mice displayed an elevated ABP measured via telemetry and a greater fall in mean ABP to ganglionic blockade at day 14 or 21 vs. day 0. Renal afferent discharge was significantly higher in 2K1C-clipped vs. 2K1C-unclipped or sham kidneys. In addition, 2K1C-clipped vs. 2K1C-unclipped or sham kidneys had lower renal mass and higher mRNA levels of several proinflammatory cytokines. Finally, both ipsilateral renal denervation (10% phenol) or selective denervation of renal afferent nerves (periaxonal application of 33 mM capsaicin) at time of clipping resulted in lower ABP of 2K1C mice at day 14 or 21. These findings suggest mouse renal sensory neurons are activated to increase SNA and ABP in 2K1C hypertension. NEW & NOTEWORTHY This study documents the function, anatomy, and contribution of mouse renal sensory nerves to neurogenic hypertension produced by renal stenosis. Activation of renal afferents increased sympathetic nerve activity and blood pressure. Renal afferent activity was elevated in hypertensive mice, and renal afferent denervation lowered blood pressure. Clinically, patients with renal stenosis have been excluded from clinical trials for renal denervation, but this study highlights the potential therapeutic efficacy to target renal nerves in these patients.

Specific Afferent Renal Denervation Prevents Reduction in Neuronal Nitric Oxide Synthase Within the Paraventricular Nucleus in Rats With Chronic Heart Failure

Renal denervation (RDN) has been shown to restore endogenous neuronal nitric oxide synthase (nNOS) in the paraventricular nucleus (PVN) and reduce sympathetic drive during chronic heart failure (CHF). The purpose of the present study was to assess the contribution of afferent renal nerves to the nNOS-mediated sympathetic outflow within the PVN in rats with CHF. CHF was induced in rats by ligation of the left coronary artery. Four weeks after surgery, selective afferent RDN (A-RDN) was performed by bilateral perivascular application of capsaicin on the renal arteries. Seven days after intervention, nNOS protein expression, nNOS immunostaining signaling, and diaphorase-positive stained cells were significantly decreased in the PVN of CHF rats, changes that were reversed by A-RDN. A-RDN reduced basal lumbar sympathetic nerve activity in rats with CHF (8.5%±0.5% versus 17.0%±1.2% of max). Microinjection of nNOS inhibitor L-NMMA (L-N^G-monomethyl arginine citrate) into the PVN produced a blunted increase in lumbar sympathetic nerve activity in rats with CHF. This response was significantly improved after A-RDN (Δ lumbar sympathetic nerve activity: 25.7%±2.4% versus 11.2%±0.9%). Resting afferent renal nerves activity was substantially increased in CHF compared with sham rats (56.3%±2.4% versus 33.0%±4.7%). These results suggest that intact afferent renal nerves contribute to the reduction of nNOS in the PVN. A-RDN restores nNOS and thus attenuates the sympathoexcitation. Also, resting afferent renal nerves activity is elevated in CHF rats, which may highlight a crucial neural mechanism arising from the kidney in the maintenance of enhanced sympathetic drive in CHF.

Renal nerves contribute to hypertension in Schlager BPH/2J mice

Schlager mice (BPH/2J) are hypertensive due to a greater contribution of the sympathetic nervous system (SNS) and renin-angiotensin system (RAS). The kidneys of BPH/2J are hyper-innervated suggesting renal nerves may contribute to the hypertension. We therefore determined the effect of bilateral renal denervation (RD) on hypertension in BPH/2J. Mean arterial pressure (MAP) was measured by radiotelemetry before and for 3 weeks after RD in BPH/2J and BPN/3J. The effects of pentolinium and enalaprilat were examined to determine the contribution of the SNS and RAS, respectively. After 3 weeks, MAP was -10.9 ± 2.1 mmHg lower in RD BPH/2J compared to baseline and -2.1 ± 2.2 mmHg in sham BPH/2J (P < 0.001, n = 8-10). RD had no effect in BPN/3J (P > 0.1). The depressor response to pentolinium was greater in BPH/2J than BPN/3J, but in both cases the response in RD mice was similar to sham. Enalaprilat decreased MAP more in RD BPH/2J compared to sham (-12 vs -3 mmHg, P < 0.001) but had no effect in BPN/3J. RD reduced renal noradrenaline in both strains but more so in BPH/2J. RD reduced renin mRNA and protein, but not plasma renin in BPH/2J to levels comparable with BPN/3J mice. We conclude that renal nerves contribute to hypertension in BPH mice as RD induced a sustained fall in MAP, which was associated with a reduction of intrarenal renin expression. The lack of inhibition of the depressor effects of pentolinium and enalaprilat by RD suggests that vasoconstrictor effects of the SNS or RAS are not involved.

Selective ablation of TRPV1 by intrathecal injection of resiniferatoxin in rats increases renal sympathoexcitatory responses and salt sensitivity

This study tested the hypothesis that selective ablation of transient receptor potential vanilloid type 1 (TRPV1)-positive nerve fibers by intrathecal injection of resiniferatoxin (RTX) enhances renal sympathoexcitatory responses and salt sensitivity. Intrathecal injection of RTX (1.8 μg/kg) to the levels of lower thoracic and upper lumbar spinal cord (T8-L3) increased mean arterial pressure (MAP) in rats fed a normal (NS, 1% NaCl) or high-sodium (HS, 8% NaCl) diet for 4 weeks compared to vehicle-treated rats (NS: 121 ± 2 vs. 111 ± 2; HS: 154 ± 2 vs. 134 ± 2 mm Hg, both P < 0.05), with a greater increase in HS compared to NS rats (9 ± 1% vs. 15 ± 1%, P < 0.05). TRPV1 contents were decreased in T8-L3 segments of spinal dorsal horn but not in corresponding dorsal root ganglia and the kidney following RTX treatment (P < 0.05). Selective activation of GABA-A receptors with intrathecal T8-L3 segment-injection of muscimol (3 nmol/kg) decreased renal sympathetic nerve activity and increased urinary excretion in both NS and HS rats, with a greater effect in RTX-treated compared to vehicle-treated rats (P < 0.05). Chronic activation of GABA-A receptors with muscimol (50 mg/kg/day × 2, p.o.) abolished RTX treatment-induced pressor effects in NS and HS rats. GAD65/67, a GABA synthetase, in the spinal cord was downregulated and tyrosine hydroxylase in the kidney upregulated in NS or HS rats treated with RTX (P < 0.05). Thus, selective ablation of TRPV1-positive central terminals of sensory neurons plays a prohypertensive role possibly via inhibition of spinal GABA system especially with HS intake, suggesting that activation of TRPV1 in central terminals of sensory neurons may convey an antihypertensive effect.

Targeted afferent renal denervation reduces arterial pressure but not renal inflammation in established DOCA-salt hypertension in the rat

Recent preclinical studies show renal denervation (RDNx) may be an effective treatment for hypertension; however, the mechanism remains unknown. We have recently reported total RDNx (TRDNx) and afferent-selective RDNx (ARDNx) similarly attenuated the development of deoxycorticosterone acetate (DOCA)-salt hypertension. Whereas TRDNx abolished renal inflammation, ARDNx had a minimal effect despite an identical antihypertensive effect. Although this study established that ARDNx attenuates the development of DOCA-salt hypertension, it is unknown whether this mechanism remains operative once hypertension is established. The current study tested the hypothesis that TRDNx and ARDNx would similarly decrease mean arterial pressure (MAP) in the DOCA-salt hypertensive rat, and only TRDNx would mitigate renal inflammation. After 21 days of DOCA-salt treatment, male Sprague-Dawley rats underwent TRDNx ( n = 16), ARDNx ( n = 16), or Sham ( n = 14) treatment and were monitored for 14 days. Compared with baseline, TRDNx and ARDNx decreased MAP similarly (TRDNx -14 ± 4 and ARDNx -15 ± 6 mmHg). After analysis of diurnal rhythm, rhythm-adjusted mean and amplitude of night/day cycle were also reduced in TRDNx and ARDNx groups compared with Sham. Notably, no change in renal inflammation, injury, or function was detected with either treatment. We conclude from these findings that: 1) RDNx mitigates established DOCA-salt hypertension; 2) the MAP responses to RDNx are primarily mediated by ablation of afferent renal nerves; and 3) renal nerves do not contribute to the maintenance of renal inflammation in DOCA-salt hypertension.

Modulation of renal sympathetic innervation: recent insights beyond blood pressure control

Renal afferent and efferent sympathetic nerves are involved in the regulation of blood pressure and have a pathophysiological role in hypertension. Additionally, several conditions that frequently coexist with hypertension, such as heart failure, obstructive sleep apnea, atrial fibrillation, renal dysfunction, and metabolic syndrome, demonstrate enhanced sympathetic activity. Renal denervation (RDN) is an approach to reduce renal and whole body sympathetic activation. Experimental models indicate that RDN has the potential to lower blood pressure and prevent cardio-renal remodeling in chronic diseases associated with enhanced sympathetic activation. Studies have shown that RDN can reduce blood pressure in drug-naïve hypertensive patients and in hypertensive patients under drug treatment. Beyond its effects on blood pressure, sympathetic modulation by RDN has been shown to have profound effects on cardiac electrophysiology and cardiac arrhythmogenesis. RDN can display anti-arrhythmic effects in a variety of animal models for atrial fibrillation and ventricular arrhythmias. The first non-randomized studies demonstrate that RDN may promote the maintenance of sinus rhythm following catheter ablation in patients with atrial fibrillation. Registry data point towards a beneficial effect of RDN to prevent ventricular arrhythmias in patients with heart failure and electrical storm. Further large randomized placebo-controlled trials are needed to confirm the antihypertensive and anti-arrhythmic effects of RDN. Here, we will review the current literature on anti-arrhythmic effects of RDN with the focus on atrial fibrillation and ventricular arrhythmias. We will discuss new insights from preclinical and clinical mechanistic studies and possible clinical implications of RDN.

Role of afferent and efferent renal nerves in the development of AngII-salt hypertension in rats

Hypertension is the leading modifiable risk factor for death worldwide, yet the causes remain unclear and treatment remains suboptimal. Catheter-based renal denervation (RDNX) is a promising new treatment for resistant hypertension, but the mechanisms underlying its antihypertensive effect remain unclear. We recently found that RDNX attenuates deoxycorticosterone acetate-salt hypertension and that this is dependent on ablation of afferent renal nerves and is associated with decreased renal inflammation. To determine if this is common to other models of salt-sensitive hypertension, rats underwent complete RDNX (n = 8), selective ablation of afferent renal nerves (n = 8), or sham denervation (n = 8). Mean arterial pressure (MAP) and heart rate were measure by telemetry and rats were housed in metabolic cages for measurement of sodium and water balance. Rats were then subjected to angiotensin II (AngII)-salt hypertension (10 ng/kg/min, intravenous + 4% NaCl diet) for 2 weeks. At the end of the study, renal T-cell infiltration was quantified by flow cytometry. AngII resulted in an increase in MAP of ~50 mmHg in all three groups with no between group differences, and a transient bradycardia that was blunted by selective ablation of afferent renal nerves. Sodium and water balance were unaffected by AngII-salt treatment and similar between groups. Lastly, AngII infusion was not associated with T-cell infiltration into the kidneys, and T-cell counts were unaffected by the denervation procedures. These results suggest that AngII-salt hypertension in the rat is not associated with renal inflammation and that neither afferent nor efferent renal nerves contribute to this model.

Renal Afferents

Purpose of Review

The etiology of hypertension, a critical public health issue affecting one in three US adults, involves the integration of the actions of multiple organ systems, including the renal sympathetic nerves. The renal sympathetic nerves, which are comprised of both afferent (sensory input) and efferent (sympathetic outflow) arms, have emerged as a major potential therapeutic target to treat hypertension and disease states exhibiting excess renal sympathetic activity.

Recent Findings

This review highlights recent advances in both clinical and basic science that have provided new insight into the distribution, function, and reinnervation of the renal sympathetic nerves, with a focus on the renal afferent nerves, in hypertension and hypertension-evoked disease states including salt-sensitive hypertension, obesity-induced hypertension, and chronic kidney disease.

Summary

Increased understanding of the differential role of the renal afferent versus efferent nerves in the pathophysiology of hypertension has the potential to identify novel targets and refine therapeutic interventions designed to treat hypertension.

The innervation of the kidney in renal injury and inflammation: a cause and consequence of deranged cardiovascular control

Extensive investigations have revealed that renal sympathetic nerves regulate renin secretion, tubular fluid reabsorption and renal haemodynamics which can impact on cardiovascular homoeostasis normally and in pathophysiological states. The significance of the renal afferent innervation and its role in determining the autonomic control of the cardiovascular system is uncertain. The transduction pathways at the renal afferent nerves have been shown to require pro-inflammatory mediators and TRPV1 channels. Reno-renal reflexes have been described, both inhibitory and excitatory, demonstrating that a neural link exists between kidneys and may determine the distribution of excretory and haemodynamic function between the two kidneys. The impact of renal afferent nerve activity on basal and reflex regulation of global sympathetic drive remains opaque. There is clinical and experimental evidence that in states of chronic kidney disease and renal injury, there is infiltration of T-helper cells with a sympatho-excitation and blunting of the high- and low-pressure baroreceptor reflexes regulating renal sympathetic nerve activity. The baroreceptor deficits are renal nerve-dependent as the dysregulation can be relieved by renal denervation. There is also experimental evidence that in obese states, there is a sympatho-excitation and disrupted baroreflex regulation of renal sympathetic nerve activity which is mediated by the renal innervation. This body of information provides an important basis for directing greater attention to the role of renal injury/inflammation causing an inappropriate activation of the renal afferent nerves as an important initiator of aberrant autonomic cardiovascular control.

Renal denervation attenuates hypertension but not salt sensitivity in ETB receptor-deficient rats

Hypertension is a prevalent pathology that increases risk for numerous cardiovascular diseases. Because the etiology of hypertension varies across patients, specific and effective therapeutic approaches are needed. The role of renal sympathetic nerves is established in numerous forms of hypertension, but their contribution to salt sensitivity and interaction with factors such as endothelin-1 are poorly understood. Rats deficient of functional ET_B receptors (ET_B-def) on all tissues except sympathetic nerves are hypertensive and exhibit salt-sensitive increases in blood pressure. We hypothesized that renal sympathetic nerves contribute to hypertension and salt sensitivity in ET_B-def rats. The hypothesis was tested through bilateral renal sympathetic nerve denervation and measuring blood pressure during normal salt (0.49% NaCl) and high-salt (4.0% NaCl) diets. Denervation reduced mean arterial pressure in ET_B-def rats compared with sham-operated controls by 12 ± 3 (SE) mmHg; however, denervation did not affect the increase in blood pressure after 2 wk of high-salt diet (+19 ± 3 vs. +16 ± 3 mmHg relative to normal salt diet; denervated vs. sham, respectively). Denervation reduced cardiac sympathetic-to-parasympathetic tone [low frequency-high frequency (LF/HF)] during normal salt diet and vasomotor LF/HF tone during high-salt diet in ET_B-def rats. We conclude that the renal sympathetic nerves contribute to the hypertension but not to salt sensitivity of ET_B-def rats.

Renal sodium handling and sodium sensitivity

The pathophysiology of hypertension, which affects over 1 billion individuals worldwide, involves the integration of the actions of multiple organ systems, including the kidney. The kidney, which governs sodium excretion via several mechanisms including pressure natriuresis and the actions of renal sodium transporters, is central to long term blood pressure regulation and the salt sensitivity of blood pressure. The impact of renal sodium handling and the salt sensitivity of blood pressure in health and hypertension is a critical public health issue owing to the excess of dietary salt consumed globally and the significant percentage of the global population exhibiting salt sensitivity. This review highlights recent advances that have provided new insight into the renal handling of sodium and the salt sensitivity of blood pressure, with a focus on genetic, inflammatory, dietary, sympathetic nervous system and oxidative stress mechanisms that influence renal sodium excretion. Increased understanding of the multiple integrated mechanisms that regulate the renal handling of sodium and the salt sensitivity of blood pressure has the potential to identify novel therapeutic targets and refine dietary guidelines designed to treat and prevent hypertension.

Renal Nerves and Long-Term Control of Arterial Pressure

The objective of this review is to provide an in-depth evaluation of how renal nerves regulate renal and cardiovascular function with a focus on long-term control of arterial pressure. We begin by reviewing the anatomy of renal nerves and then briefly discuss how the activity of renal nerves affects renal function. Current methods for measurement and quantification of efferent renal-nerve activity (ERNA) in animals and humans are discussed. Acute regulation of ERNA by classical neural reflexes as well and hormonal inputs to the brain is reviewed. The role of renal nerves in long-term control of arterial pressure in normotensive and hypertensive animals (and humans) is then reviewed with a focus on studies utilizing continuous long-term monitoring of arterial pressure. This includes a review of the effect of renal-nerve ablation on long-term control of arterial pressure in experimental animals as well as humans with drug-resistant hypertension. The extent to which changes in arterial pressure are due to ablation of renal afferent or efferent nerves are reviewed. We conclude by discussing the importance of renal nerves, relative to sympathetic activity to other vascular beds, in long-term control of arterial pressure and hypertension and propose directions for future research in this field. © 2017 American Physiological Society. Compr Physiol 7:263-320, 2017.

Resting Afferent Renal Nerve Discharge and Renal Inflammation: Elucidating the Role of Afferent and Efferent Renal Nerves in Deoxycorticosterone Acetate Salt Hypertension

Renal sympathetic denervation (RDNx) has emerged as a novel therapy for hypertension; however, the therapeutic mechanisms remain unclear. Efferent renal sympathetic nerve activity has recently been implicated in trafficking renal inflammatory immune cells and inflammatory chemokine and cytokine release. Several of these inflammatory mediators are known to activate or sensitize afferent nerves. This study aimed to elucidate the roles of efferent and afferent renal nerves in renal inflammation and hypertension in the deoxycorticosterone acetate (DOCA) salt rat model. Uninephrectomized male Sprague-Dawley rats (275-300 g) underwent afferent-selective RDNx (n=10), total RDNx (n=10), or Sham (n=10) and were instrumented for the measurement of mean arterial pressure and heart rate by radiotelemetry. Rats received 100-mg DOCA (SC) and 0.9% saline for 21 days. Resting afferent renal nerve activity in DOCA and vehicle animals was measured after the treatment protocol. Renal tissue inflammation was assessed by renal cytokine content and T-cell infiltration and activation. Resting afferent renal nerve activity, expressed as a percent of peak afferent nerve activity, was substantially increased in DOCA than in vehicle (35.8±4.4 versus 15.3±2.8 %Amax). The DOCA-Sham hypertension (132±12 mm Hg) was attenuated by ≈50% in both total RDNx (111±8 mm Hg) and afferent-selective RDNx (117±5 mm Hg) groups. Renal inflammation induced by DOCA salt was attenuated by total RDNx and unaffected by afferent-selective RDNx. These data suggest that afferent renal nerve activity may mediate the hypertensive response to DOCA salt, but inflammation may be mediated primarily by efferent renal sympathetic nerve activity. Also, resting afferent renal nerve activity is elevated in DOCA salt rats, which may highlight a crucial neural mechanism in the development and maintenance of hypertension.