A novel method of selective ablation of afferent renal nerves by periaxonal application of capsaicin

Renal sympathetic denervation for resistant hypertension: where do we stand after more than a decade

Despite the current availability of safe and efficient drugs for treating hypertension, a substantial number of patients are drug-resistant hypertensives. Aiming this condition, a relatively new approach named catheter-based renal denervation was developed. We have now a clinically relevant time window to review the efficacy of renal denervation for treating this form of hypertension. This short review addresses the physiological contribution of renal sympathetic nerves for blood pressure control and discusses the pros and cons of renal denervation procedure for the treatment of resistant hypertension.

Renal sympathetic nerve activity regulates cardiovascular energy expenditure in rats fed high salt

We recently reported that a 4% high-salt diet + saline for drinking (HS + saline) leads to a catabolic state, reduced heart rate, and suppression of cardiovascular energy expenditure in mice. We suggested that HS + saline reduces heart rate via the suppression of the sympathetic nervous system to compensate for the high salt intake-induced catabolic state. To test this hypothesis, we directly measured renal sympathetic nerve activity (RSNA) in conscious Sprague-Dawley (SD) rats using a radiotelemetry system. We confirmed that HS + saline induced a catabolic state. HS + saline decreased heart rate, while also reducing RSNA in SD rats. In contrast, Dahl salt-sensitive (DSS) rats exhibited no change in heart rate and increased RSNA during high salt intake. Renal denervation significantly decreased heart rate and attenuated the catabolic state independent of blood pressure in DSS rats fed HS + saline, suggesting that salt-sensitive animals were unable to decrease cardiovascular energy consumption due to abnormal renal sympathetic nerve activation during high salt intake. These findings support the hypothesis that RSNA mediates heart rate during high salt intake in SD rats. However, the insensitivity of heart rate and enhanced RSNA observed in DSS rats may be additional critical diagnostic factors for salt-sensitive hypertension. Renal denervation may benefit salt-sensitive hypertension by reducing its effects on catabolism and cardiovascular energy expenditure.

Afferent innervation of the ischemic kidney contributes to renal dysfunction in renovascular hypertensive rats

The ablation of renal nerves, by destroying both the sympathetic and afferent fibers, has been shown to be effective in lowering blood pressure in resistant hypertensive patients. However, experimental studies have reported that the removal of sympathetic fibers may lead to side effects, such as the impairment of compensatory cardiorenal responses during a hemodynamic challenge. In the present study, we evaluated the effects of the selective removal of renal afferent fibers on arterial hypertension, renal sympathetic nerve activity, and renal changes in a model of renovascular hypertension. After 4 weeks of clipping the left renal artery, afferent renal denervation (ARD) was performed by exposing the left renal nerve to a 33 mM capsaicin solution for 15 min. After 2 weeks of ARD, we found reduced MAP (~ 18%) and sympathoexcitation to both the ischemic and contralateral kidneys in the hypertensive group. Moreover, a reduction in reactive oxygen species was observed in the ischemic (76%) and contralateral (27%) kidneys in the 2K1C group. In addition, ARD normalized renal function markers and proteinuria and podocin in the contralateral kidney. Taken altogether, we show that the selective removal of afferent fibers is an effective method to reduce MAP and improve renal changes without compromising the function of renal sympathetic fibers in the 2K1C model. Renal afferent nerves may be a new target in neurogenic hypertension and renal dysfunction.

Renal Nerve Deafferentation Attenuates the Fall in GFR during Intravenous Infusion of Furosemide in Anesthetized Rats

INTRODUCTION

Furosemide reduces the glomerular filtration rate (GFR) and increases the renal vascular resistance (RVR) despite inhibiting tubuloglomerular feedback but increases proximal tubule pressure, renin release, and renal nerve activity.

OBJECTIVE

This study tested the hypothesis that the fall in GFR with furosemide is due to volume depletion or activation of angiotensin type 1 (AT1) receptors or renal nerves.

METHODS

Furosemide was infused for 60 min at 1.0 mg·kg-1·h-1 in groups of 5-8 anesthetized rats. Additional groups received intravenous volume replacement to prevent fluid and Na+ losses or volume replacement plus losartan or plus sham denervation or plus renal denervation or renal nerve deafferentation.

RESULTS

At 60 min of infusion, furosemide alone reduced the GFR (-37 ± 4%; p < 0.01). This fall was not prevented by volume replacement or pretreatment with losartan, although losartan moderated the increase in RVR with furosemide (+44 ± 3 vs. +82 ± 7%; p < 0.01). Whereas the GFR fell after furosemide in rats after sham procedure (-31 ± 2%), it was not changed significantly after prior renal deafferentation. Proximal tubule pressure increased significantly but returned towards baseline over 60 min of furosemide, while urine output remained elevated, and GFR and renal blood flow depressed.

CONCLUSIONS

The fall in GFR over 60 min of furosemide infusion is independent of volume depletion or activation of AT1 receptors but is largely dependent on renal afferent nerves.

Renal Inflammation in DOCA-Salt Hypertension

Recent reports indicate that, in addition to treating hypertension, renal denervation (RDN) also mitigates renal inflammation. However, because RDN decreases renal perfusion pressure, it is unclear whether these effects are because of the direct effects of RDN on inflammatory signaling or secondary to decreased arterial pressure (AP). Therefore, this study was conducted to elucidate the contribution of renal nerves to renal inflammation in the deoxycorticosterone (DOCA)-salt rat, a model in which RDN decreases AP and abolishes renal inflammation. In Experiment 1, we assessed the temporal changes in renal inflammation by measuring renal cytokines and AP in DOCA-salt rats. Uninephrectomized (1K) adult male Sprague Dawley rats that received surgical RDN or sham (Sham) were administered DOCA (100 mg, SC) and 0.9% saline for 21 days. AP was measured by radiotelemetry, and urinary cytokine excretion was measured repeatedly. In Experiment 2, the contribution of renal nerves in renal inflammation was assessed in a 2-kidney DOCA-salt rat to control for renal perfusion pressure. DOCA-salt treatment was administered after unilateral (U-)RDN. In Experiment 1, DOCA-salt-induced increases in AP and renal inflammation (assessed by urinary cytokines) were attenuated by RDN versus Sham. In Experiment 2, GRO/KC (growth-related oncogene/keratinocyte chemoattractant), MCP (monocyte chemoattractant protein)-1, and macrophage infiltration were lower in the denervated kidney versus the contralateral Sham kidney. No differences in T-cell infiltration were observed. Together, these data support the hypothesis that renal nerves mediate, in part, the development of renal inflammation in the DOCA-salt rat independent of hypertension. The mechanisms and cell-specificity mediating these effects require further investigation.

Tacrolimus restores the high- and low-pressure baroreflex control of renal sympathetic nerve activity in cisplatin-induced renal injury rats

NEW FINDINGS

What is the central question of this study? Does immunosuppression restore the baroreflex control of renal sympathetic nerve activity (RSNA) in an animal model of kidney injury? What is the main finding and its importance? Tacrolimus, a calcineurin inhibitor, restored the high- and low-pressure baroreflex control of RSNA following cisplatin-induced renal injury.

ABSTRACT

Cisplatin administration causes depression of renal haemodynamic and excretory function and is associated with renal sympatho-excitation and loss of baroreflex regulation of renal sympathetic nerve activity (RSNA). This study investigated whether administration of the immunosuppressant tacrolimus in this cisplatin-mediated renal injury model could restore, or the acute intra-renal infusion of tumour necrosis factor α (TNF-α) could blunt, the high- or low-pressure baroreflex control of RSNA. Groups of control and cisplatin-treated (5 mg kg-1 , i.p. on day 0) rats received either saline or tacrolimus (0.25 mg kg-1  day-1 , i.p.) for 7 days prior to study. Rats were anaesthetised and prepared for measurement of mean arterial pressure (MAP), heart rate (HR) and RSNA. Baroreflex gain curves were generated and the degree of renal sympatho-inhibition determined (area under the curve (AUC) reported as %RSNA min) during acute volume expansion. Intrarenal TNF-α infusion (0.3 µg kg-1  h-1 ) in control rats decreased baroreflex gain by 32% (P < 0.05) compared to intra-renal saline infusion. In the cisplatin group (MAP: 98 ± 14 mmHg; HR: 391 ± 24beats min-1 ), the baroreflex gain for RSNA was 39% (P < 0.05) lower than that for the control group (MAP: 91 ± 7 mmHg; HR: 382 ± 29 beats min-1 ). In cisplatin-treated rats given daily tacrolimus (MAP: 84 ± 12 mmHg; HR: 357 ± 30 beats min-1 ), the baroreflex gain and renal sympatho-inhibition (AUC, 2440 ± 1071 vs. 635 ± 498% min) were restored to normal values. These findings provide evidence for the view that cisplatin administration initiates an injury involving inflammation which may contribute to the deranged baroreflex regulation of RSNA. This phenomenon appears mediated in part via the renal innervation.

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.

Does glucagon-like peptide-1 induce diuresis and natriuresis by modulating afferent renal nerve activity?

Glucagon-like peptide-1 (GLP-1), an incretin hormone, has diuretic and natriuretic effects. The present study was designed to explore the possible underlying mechanisms for the diuretic and natriuretic effects of GLP-1 via renal nerves in rats. Immunohistochemistry revealed that GLP-1 receptors were avidly expressed in the pelvic wall, the wall being adjacent to afferent renal nerves immunoreactive to calcitonin gene-related peptide, which is the dominant neurotransmitter for renal afferents. GLP-1 (3 μM) infused into the left renal pelvis increased ipsilateral afferent renal nerve activity (110.0 ± 15.6% of basal value). Intravenous infusion of GLP-1 (1 µg·kg^-1·min^-1) for 30 min increased renal sympathetic nerve activity (RSNA). After the distal end of the renal nerve was cut to eliminate the afferent signal, the increase in efferent renal nerve activity during intravenous infusion of GLP-1 was diminished compared with the increase in total RSNA (17.0 ± 9.0% vs. 68.1 ± 20.0% of the basal value). Diuretic and natriuretic responses to intravenous infusion of GLP-1 were enhanced by total renal denervation (T-RDN) with acute surgical cutting of the renal nerves. Selective afferent renal nerve denervation (A-RDN) was performed by bilateral perivascular application of capsaicin on the renal nerves. Similar to T-RDN, A-RDN enhanced diuretic and natriuretic responses to GLP-1. Urine flow and Na⁺ excretion responses to GLP-1 were not significantly different between T-RDN and A-RDN groups. These results indicate that the diuretic and natriuretic effects of GLP-1 are partly governed via activation of afferent renal nerves by GLP-1 acting on sensory nerve fibers within the pelvis of the kidney.

Autonomic regulation of T-lymphocytes: Implications in cardiovascular disease

The nervous and immune systems both serve as essential assessors and regulators of physiological function. Recently, there has been a great interest in how the nervous and immune systems interact to modulate both physiological and pathological states. In particular, the autonomic nervous system has a direct line of communication with immune cells anatomically, and moreover, immune cells possess receptors for autonomic neurotransmitters. This circumstantial evidence is suggestive of a functional interplay between the two systems, and extensive research over the past few decades has demonstrated neurotransmitters such as the catecholamines (i.e. dopamine, norepinephrine, and epinephrine) and acetylcholine have potent immunomodulating properties. Furthermore, immune cells, particularly T-lymphocytes, have now been found to express the cellular machinery for both the synthesis and degradation of neurotransmitters, which suggests the ability for both autocrine and paracrine signaling from these cells independent of the nervous system. The details underlying the functional interplay of this complex network of neuroimmune communication are still unclear, but this crosstalk is suggestive of significant implications on the pathogenesis of a number of autonomic-dysregulated and inflammation-mediated diseases. In particular, it is widely accepted that numerous forms of cardiovascular diseases possess imbalanced autonomic tone as well as altered T-lymphocyte function, but a paucity of literature exists discussing the direct role of neurotransmitters in shaping the inflammatory microenvironment during the progression or therapeutic management of these diseases. This review seeks to provide a fundamental framework for this autonomic neuroimmune interaction within T-lymphocytes, as well as the implications this may have in cardiovascular diseases.

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.

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.

Relevance of Targeting the Distal Renal Artery and Branches with Radiofrequency Renal Denervation Approaches-A Secondary Analysis from a Hypertensive CKD Patient Cohort

We searched for an association between changes in blood pressure (BP) at 12 and 24 months after renal denervation (RDN) and the different patterns of ablation spots placement along the renal artery vasculature. We performed a post-hoc analysis of a 24-month follow-up evaluation of 30 patients who underwent RDN between 2011 and 2012 using our previous database. Patients who had (i) resistant hypertension, as meticulously described previously, and (ii) Chronic kidney disease (CKD) stages 2, 3 and 4. Correlations were assessed using the Pearson or Spearman correlation tests as appropriate. The mean change in systolic ambulatory BP monitoring (ABPM) compared to baseline was −19.4 ± 12.7 mmHg at the 12th (p < 0.0001) and −21.3 ± 14.1 mmHg at the 24th month (p < 0.0001). There was no correlation between the ABPM Systolic Blood Pressure (SBP)-lowering effect and the total number of ablated spots in renal arteries (17.7 ± 6.0) either at 12 (r = −0.3, p = 0.1542) or at 24 months (r = −0.2, p = 0.4009). However, correlations between systolic BP-lowering effect and the number of ablation spots performed in the distal segment and branches were significant at the 12 (r = −0.7, p < 0.0001) and 24 months (r = −0.8, p < 0.0001) follow-up. Our findings indicate a substantial correlation between the numbers of ablated sites in the distal segment and branches of renal arteries and the systolic BP-lowering effect in the long-term.

Is Aberrant Reno-Renal Reflex Control of Blood Pressure a Contributor to Chronic Intermittent Hypoxia-Induced Hypertension?

Renal sensory nerves are important in the regulation of body fluid and electrolyte homeostasis, and blood pressure. Activation of renal mechanoreceptor afferents triggers a negative feedback reno-renal reflex that leads to the inhibition of sympathetic nervous outflow. Conversely, activation of renal chemoreceptor afferents elicits reflex sympathoexcitation. Dysregulation of reno-renal reflexes by suppression of the inhibitory reflex and/or activation of the excitatory reflex impairs blood pressure control, predisposing to hypertension. Obstructive sleep apnoea syndrome (OSAS) is causally related to hypertension. Renal denervation in patients with OSAS or in experimental models of chronic intermittent hypoxia (CIH), a cardinal feature of OSAS due to recurrent apnoeas (pauses in breathing), results in a decrease in circulating norepinephrine levels and attenuation of hypertension. The mechanism of the beneficial effect of renal denervation on blood pressure control in models of CIH and OSAS is not fully understood, since renal denervation interrupts renal afferent signaling to the brain and sympathetic efferent signals to the kidneys. Herein, we consider the currently proposed mechanisms involved in the development of hypertension in CIH disease models with a focus on oxidative and inflammatory mediators in the kidneys and their potential influence on renal afferent control of blood pressure, with wider consideration of the evidence available from a variety of hypertension models. We draw focus to the potential contribution of aberrant renal afferent signaling in the development, maintenance and progression of high blood pressure, which may have relevance to CIH-induced hypertension.

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.

Treating Hypertension Using Renal Artery Denervation: Problems and Progress

Early reports of renal denervation as a therapy for hypertension generated intense interest in this approach to management of elevated blood pressures despite ongoing treatment. The publication of the large, sham-controlled randomized clinical trial of renal denervation, Symplicity HTN-3, failed to show superiority of renal denervation by radiofrequency energy ablation compared with a sham procedure similar to the procedure used for denervation but without the application of energy to the renal artery. This prompted consideration of confounding factors and rethinking about the protocol and the procedure itself. This review describes these confounders and the progress made to improve trial design in the field of renal artery denervation.

The role of kidney in the inter-organ coordination of endogenous glucose production during fasting

Objective

The respective contributions to endogenous glucose production (EGP) of the liver, kidney and intestine vary during fasting. We previously reported that the deficiency in either hepatic or intestinal gluconeogenesis modulates the repartition of EGP via glucagon secretion (humoral factor) and gut–brain–liver axis (neural factor), respectively. Considering renal gluconeogenesis reportedly accounted for approximately 50% of EGP during fasting, we examined whether a reduction in renal gluconeogenesis could promote alterations in the repartition of EGP in this situation.

Methods

We studied mice whose glucose-6-phosphatase (G6Pase) catalytic subunit (G6PC) is specifically knocked down in the kidneys (K-G6pc^-/- mice) during fasting. We also examined the additional effects of intestinal G6pc deletion, renal denervation and vitamin D administration on the altered glucose metabolism in K-G6pc^-/- mice.

Results

Compared with WT mice, K-G6pc^-/- mice exhibited (1) lower glycemia, (2) enhanced intestinal but not hepatic G6Pase activity, (3) enhanced hepatic glucokinase (GK encoded by Gck) activity, (4) increased hepatic glucose-6-phosphate and (5) hepatic glycogen spared from exhaustion during fasting. Increased hepatic Gck expression in the post-absorptive state could be dependent on the enhancement of insulin signal (AKT phosphorylation) in K-G6pc^-/- mice. In contrast, the increase in hepatic GK activity was not observed in mice with both kidney- and intestine-knockout (KI-G6pc^-/- mice). Hepatic Gck gene expression and hepatic AKT phosphorylation were reduced in KI-G6pc^-/- mice. Renal denervation by capsaicin did not induce any effect on glucose metabolism in K-G6pc^-/- mice. Plasma level of 1,25 (OH)₂ D₃, an active form of vitamin D, was decreased in K-G6pc^-/- mice. Interestingly, the administration of 1,25 (OH)₂ D₃ prevented the enhancement of intestinal gluconeogenesis and hepatic GK activity and blocked the accumulation of hepatic glycogen otherwise observed in K-G6pc^-/- mice during fasting.

Conclusions

A diminution in renal gluconeogenesis that is accompanied by a decrease in blood vitamin D promotes a novel repartition of EGP among glucose producing organs during fasting, featured by increased intestinal gluconeogenesis that leads to sparing glycogen stores in the liver. Our data suggest a possible involvement of a crosstalk between the kidneys and intestine (via the vitamin D system) and the intestine and liver (via a neural gut-brain axis), which might take place in the situations of deficient renal glucose production, such as chronic kidney disease.

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Reduced renal G6Pase activity promotes increased hepatic glycogen during fasting.

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Reduced renal G6Pase activity enhances intestinal but not hepatic G6Pase activity.

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Reduced renal G6Pase activity results in low vitamin D level.

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Vitamin D injection restores metabolism in mice with reduced renal G6Pase activity.

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.

Bilateral renal cryodenervation decreases arterial pressure and improves insulin sensitivity in fructose-fed Sprague-Dawley rats

Consumption of food high in fructose is prevalent in modern diets. One week of moderately high fructose intake combined with high salt diet has been shown to increase blood pressure and failed to suppress plasma renin activity (PRA). We tested the hypothesis that the hypertension and high PRA are consequences of elevated renal sympathetic nerve activity (RSNA). In protocol 1, we assessed RSNA by telemetry in conscious Sprague-Dawley rats given 20% fructose or 20% glucose in drinking water on a 0.4% NaCl diet (NS) for 1 wk and then transitioned to a 4% NaCl diet (HS). After an additional week, mean arterial pressure (MAP) and RSNA increased significantly in fructose-fed but not glucose-fed HS rats. In protocol 2, fructose (Fruc)- or glucose (Glu)-fed rats on NS or HS diet for 3 wk underwent sham denervation (shamDNX) or bilateral renal denervation using cryoablation (cryoDNX). MAP was higher in Fruc-HS rats compared with Glu-NS, Glu-HS, or Fruc-NS rats and decreased after cryoDNX ( P < 0.01). MAP did not change in Fruc-HS shamDNX rats. Renal norepinephrine content decreased by 85% in cryoDNX ( P < 0.01 vs. shamDNX). PRA significantly decreased after cryoDNX in both Fruc-NS and Fruc-HS rats. Nonfasting blood glucose levels were similar among the four groups. Glucose-to-insulin ratio significantly increased in Fruc-HS cryoDNX rats, consistent with greater insulin sensitivity. Taken together, these studies show that renal sympathoexcitation is, at least in part, responsible for salt-dependent increases in MAP, increased PRA, and decreased insulin sensitivity in rats fed a moderately high fructose diet for as little as 3 wk.

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 Denervation: Paradise Lost? Paradise Regained?

Renal denervation is a relatively recent concept whose initial promising results suffered a setback following the SYMPLICITY 3 trial, which did not show a significant blood pressure-lowering effect in comparison to sham. In this review article, we begin with the history including the physiological basis behind the concept of renal denervation. Furthermore, we review the literature in support of renal denervation, including the recently published SPYRAL HTN-OFF MED, which demonstrated significant blood pressure reduction in the absence of antihypertensive medication. We further touch upon the potential pitfalls and possible future directions of renal denervation.

Targeting neural reflex circuits in immunity to treat kidney disease

Neural pathways regulate immunity and inflammation via the inflammatory reflex and specific molecular targets can be modulated by stimulating neurons. Neuroimmunomodulation by nonpharmacological methods is emerging as a novel therapeutic strategy for inflammatory diseases, including kidney diseases and hypertension. Electrical stimulation of vagus neurons or treatment with pulsed ultrasound activates the cholinergic anti-inflammatory pathway (CAP) and protects mice from acute kidney injury (AKI). Direct innervation of the kidney, by afferent and efferent neurons, might have a role in modulating and responding to inflammation in various diseases, either locally or by providing feedback to regions of the central nervous system that are important in the inflammatory reflex pathway. Increased sympathetic drive to the kidney has a role in the pathogenesis of hypertension, and selective modulation of neuroimmune interactions in the kidney could potentially be more effective for lowering blood pressure and treating inflammatory kidney diseases than renal denervation. Use of optogenetic tools for selective stimulation of specific neurons has enabled the identification of neural circuits in the brain that modulate kidney function via activation of the CAP. In this Review we discuss evidence for a role of neural circuits in the control of renal inflammation as well as the therapeutic potential of targeting these circuits in the settings of AKI, kidney fibrosis and hypertension.

Cooperative Oxygen Sensing by the Kidney and Carotid Body in Blood Pressure Control

Oxygen sensing mechanisms are vital for homeostasis and survival. When oxygen levels are too low (hypoxia), blood flow has to be increased, metabolism reduced, or a combination of both, to counteract tissue damage. These adjustments are regulated by local, humoral, or neural reflex mechanisms. The kidney and the carotid body are both directly sensitive to falls in the partial pressure of oxygen and trigger reflex adjustments and thus act as oxygen sensors. We hypothesize a cooperative oxygen sensing function by both the kidney and carotid body to ensure maintenance of whole body blood flow and tissue oxygen homeostasis. Under pathological conditions of severe or prolonged tissue hypoxia, these sensors may become continuously excessively activated and increase perfusion pressure chronically. Consequently, persistence of their activity could become a driver for the development of hypertension and cardiovascular disease. Hypoxia-mediated renal and carotid body afferent signaling triggers unrestrained activation of the renin angiotensin-aldosterone system (RAAS). Renal and carotid body mediated responses in arterial pressure appear to be synergistic as interruption of either afferent source has a summative effect of reducing blood pressure in renovascular hypertension. We discuss that this cooperative oxygen sensing system can activate/sensitize their own afferent transduction mechanisms via interactions between the RAAS, hypoxia inducible factor and erythropoiesis pathways. This joint mechanism supports our view point that the development of cardiovascular disease involves afferent nerve activation.

Reno-Cerebral Reflex Activates the Renin-Angiotensin System, Promoting Oxidative Stress and Renal Damage After Ischemia-Reperfusion Injury

AIMS

A kidney-brain interaction has been described in acute kidney injury, but the mechanisms are uncertain. Since we recently described a reno-cerebral reflex, we tested the hypothesis that renal ischemia-reperfusion injury (IRI) activates a sympathetic reflex that interlinks the renal and cerebral renin-angiotensin axis to promote oxidative stress and progression of the injury.

RESULTS

Bilateral ischemia-reperfusion activated the intrarenal and cerebral, but not the circulating, renin-angiotensin system (RAS), increased sympathetic activity in the kidney and the cerebral sympathetic regulatory regions, and induced brain inflammation and kidney injury. Selective renal afferent denervation with capsaicin or renal denervation significantly attenuated IRI-induced activation of central RAS and brain inflammation. Central blockade of RAS or oxidative stress by intracerebroventricular (ICV) losartan or tempol reduced the renal ischemic injury score by 65% or 58%, respectively, and selective renal afferent denervation or reduction of sympathetic tone by ICV clonidine decreased the score by 42% or 52%, respectively (all p < 0.05). Ischemia-reperfusion-induced renal damage and dysfunction persisted after controlling blood pressure with hydralazine.

INNOVATION

This study uncovered a novel reflex pathway between ischemic kidney and the brain that sustains renal oxidative stress and local RAS activation to promote ongoing renal damage.

CONCLUSIONS

These data suggest that the renal and cerebral renin-angiotensin axes are interlinked by a reno-cerebral sympathetic reflex that is activated by ischemia-reperfusion, which contributes to ischemia-reperfusion-induced brain inflammation and worsening of the acute renal injury. Antioxid. Redox Signal. 27, 415-432.

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 sympathetic denervation for treatment of patients with heart failure: summary of the available evidence

Heart failure syndrome results from compensatory mechanisms that operate to restore - back to normal - the systemic perfusion pressure. Sympathetic overactivity plays a pivotal role in heart failure; norepinephrine contributes to maintenance of the systemic blood pressure and increasing preload. Cardiac norepinephrine spillover increases in patients with heart failure; norepinephrine exerts direct toxicity on cardiac myocytes resulting in a decrease of synthetic activity and/or viability. Importantly, cardiac norepinephrine spillover is a powerful predictor of mortality in patients with moderate to severe HF. This provided the rationale for trials that demonstrated survival benefit associated with the use of beta adrenergic blockers in heart failure with reduced ejection fraction. Nevertheless, the MOXCON trial demonstrated that rapid uptitration of moxonidine (inhibitor of central sympathetic outflow) in patients with heart failure was associated with excess mortality and morbidity, despite reduction of plasma norepinephrine. Interestingly, renal norepinephrine spillover was the only independent predictor of adverse outcome in patients with heart failure, in multivariable analysis. Recently, renal sympathetic denervation has emerged as a novel approach for control of blood pressure in patients with treatment-resistant hypertension. This article summarizes the available evidence for the effect of renal sympathetic denervation in the setting of heart failure. Key messages Experimental studies supported a beneficial effect of renal sympathetic denervation in heart failure with reduced ejection fraction. Clinical studies demonstrated improvement of symptoms, and left ventricular function. In heart failure and preserved ejection fraction, renal sympathetic denervation is associated with improvement of surrogate endpoints.

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.

Effect of capsaicin-sensitive sensory neurons on bone architecture and mechanical properties in the rat hindlimb suspension model

Background/Objective

The participation of sensory neural regulation in bone metabolism has been widely studied. However, the physiological role of sensory neural regulation in the functional adaptation to weight bearing is not clear. This study was conducted to investigate the effect of capsaicin-induced sensory neuron lesions on cancellous architecture properties in a hindlimb suspension (HLS) model.

Methods

Thirty-two female rats were randomly assigned to four groups. Groups b and d underwent systemic capsaicin treatment, whereas Groups a and c were treated with vehicle. Then, Groups c and d were subjected to HLS, whereas Groups a and b were allowed hindlimbs full loading. The proximal trabecular and mid-shaft cortical bone structure were evaluated via microcomputed tomography, and the biomechanical properties of the tibial mid-shaft were assessed using the four-point bending test.

Results

The trabecular bone volume was reduced by 40% and 50% in Groups b and c, respectively, and was also reduced significantly in Group d. Trabecular thickness and trabecular separation in Group b were not significantly different from those of Group a. The cortical bone area fraction showed no significant difference among all groups. Compared with Group a, the ultimate strength in Group b decreased by 20.3%, whereas it did not change significantly in Group c.

Conclusion

The results suggest that capsaicin-sensitive sensory neurons play an important role in bone modelling. The effect of capsaicin is similar to HLS. However, HLS has no add-on effect to capsaicin in the reduction of bone density and mechanical properties.

Translational potential of this article: This study gives clues to the function of sensory neurons in bone modelling.

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.

Renal sympathetic denervation attenuates hypertension and vascular remodeling in renovascular hypertensive rats

Li P, Huang P, Yang Y, Liu C, Lu Y, Wang F, Sun W, Kong X. Renal sympathetic denervation attenuates hypertension and vascular remodeling in renovascular hypertensive rats. J Appl Physiol 122: 121–129, 2017. First published October 14, 2016; doi: 10.1152/japplphysiol.01019.2015 .—Sympathetic activity is enhanced in patients with essential or secondary hypertension, as well as in various hypertensive animal models. Therapeutic targeting of sympathetic activation is considered an effective antihypertensive strategy. We hypothesized that renal sympathetic denervation (RSD) attenuates hypertension and improves vascular remodeling and renal disease in the 2-kidney, 1-clip (2K1C) rat model. Rats underwent 2K1C modeling or sham surgery; then rats underwent RSD or sham surgery 4 wk later, thus resulting in four groups (normotensive-sham, normotensive-RSD, 2K1C-sham, and 2K1C-RSD). Norepinephrine was measured by ELISA. Echocardiography was used to assess heart function. Fibrosis and apoptosis were assessed by Masson and TUNEL staining. Changes in mean arterial blood pressure in response to hexamethonium and plasma norepinephrine levels were used to evaluate basal sympathetic nerve activity. The 2K1C modeling success rate was 86.8%. RSD reversed the elevated systolic blood pressure induced by 2K1C, but had no effect on body weight. Compared with rats in the 2K1C-sham group, rats in the 2K1C-RSD group showed lower left ventricular mass/body weight ratio, interventricular septal thickness in diastole, left ventricular end-systolic diameter, and left ventricular posterior wall thickness in systole, whereas fractional shortening and ejection fraction were higher. Right kidney apoptosis and left kidney hypertrophy were not changed by RSD. Arterial fibrosis was lower in animals in the 2K1C-RSD group compared with those in the 2K1C-sham group. RSD reduced plasma norepinephrine and basal sympathetic activity in rats in the 2K1C-RSD group compared with rats in the 2K1C-sham group. These results suggest a possible clinical efficacy of RSD for renovascular hypertension.

NEW & NOTEWORTHY The effects of renal sympathetic denervation (RSD) on hypertension, cardiac function, vascular fibrosis, and renal apoptosis were studied in the 2K1C rat model. Results showed that RSD attenuated hypertension, improved vascular remodeling, and reduced vascular fibrosis through decreased sympathetic activity in the 2K1C rat model, but it did not change the kidney size, renal apoptosis, or renal caspase-3 expression. These results could suggest possible clinical efficacy of RSD for renovascular hypertension.

Sympathetic denervation facilitates L-type Ca2+ channel activation in renal but not in mesenteric resistance arteries

OBJECTIVES

Sympathetic denervation enhances agonist-induced vasoconstriction. This effect may involve altered function of signaling mechanisms such as Rho kinase (Rock) and L-type Ca channels downstream from vasoconstrictor receptors. We tested if enhanced Rock and L-type calcium channel activation contribute to exaggerated norepinephrine-induced vasoconstrictions in renal and mesenteric resistance arteries after sympathectomy.

METHODS

Rats underwent neonatal sympathectomy or sham sympathectomy. Resistance arteries were investigated by small vessel myography. Vascular Rock and L-type Ca channel expression as well as Rock activation were investigated by quantitative real-time PCR and Western blot. Vascular smooth muscle cell (VSMC) membrane potential was recorded with microelectrodes.

RESULTS

Sympathetic denervation enhanced norepinephrine sensitivity in renal and mesenteric arteries. Both, Rock inhibition or L-type Ca inhibition shifted the norepinephrine concentration-response curve to the right. This effect was more pronounced in renal than in mesenteric arteries from sympathectomized vs. sham-sympathectomized animals. The L-type Ca channel activator S-(-)-BayK8644 elicited strong vasoconstrictions only in renal arteries from sympathectomized rats. Rock activity and L-type Ca channel α-subunit expression were similar in renal arteries from sympathectomized and sham-sympathectomized animals. VSMC membrane potential was -57.5 ± 2.0 and -64.3 ± 0.3 mV (P < 0.01), respectively, in renal arteries from sympathectomized and from sham-sympathectomized rats. Depolarization enhanced and KATP channel activation abolished S-(-)-BayK8644-induced contractions in renal arteries from sympathectomized rats.

CONCLUSION

Sympathetic denervation enhances L-type Ca channel-dependent signaling in renal but not in mesenteric arteries. This effect may be partly explained by the decreased VSMC membrane potential in denervated renal arteries.

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.

Hypertension: a problem of organ blood flow supply-demand mismatch

This review introduces a new hypothesis that sympathetically mediated hypertensive diseases are caused, in the most part, by the activation of visceral afferent systems that are connected to neural circuits generating sympathetic activity. We consider how organ hypoperfusion and blood flow supply–demand mismatch might lead to both sensory hyper-reflexia and aberrant afferent tonicity. We discuss how this may drive sympatho-excitatory-positive feedback and extend across multiple organs initiating, or at least amplifying, sympathetic hyperactivity. The latter, in turn, compounds the challenge to sufficient organ blood flow through heightened vasoconstriction that both maintains and exacerbates hypertension.

Complex reinnervation pattern after unilateral renal denervation in rats

Renal denervation (DNX) is a treatment for resistant arterial hypertension. Efferent sympathetic nerves regrow, but reinnervation by renal afferent nerves has only recently been shown in the renal pelvis of rats after unilateral DNX. We examined intrarenal perivascular afferent and sympathetic efferent nerves after unilateral surgical DNX. Tyrosine hydroxylase (TH), CGRP, and smooth muscle actin were identified in kidney sections from 12 Sprague-Dawley rats, to distinguish afferents, efferents, and vasculature. DNX kidneys and nondenervated kidneys were examined 1, 4, and 12 wk after DNX. Tissue levels of CGRP and norepinephrine (NE) were measured with ELISA and mass spectrometry, respectively. DNX decreased TH and CGRP labeling by 90% and 95%, respectively (P < 0.05) within 1 wk. After 12 wk TH and CGRP labeling returned to baseline with a shift toward afferent innervation (P < 0.05). Nondenervated kidneys showed a doubling of both labels within 12 wk (P < 0.05). CGRP content decreased by 72% [3.2 ± 0.3 vs. 0.9 ± 0.2 ng/gkidney; P < 0.05] and NA by 78% [1.1 ± 0.1 vs. 0.2 ± 0.1 pmol/mgkidney; P < 0.05] 1 wk after DNX. After 12 wk, CGRP, but not NE, content in DNX kidneys was fully recovered, with no changes in the nondenervated kidneys. The use of phenol in the DNX procedure did not influence this result. We found morphological reinnervation and transmitter recovery of afferents within 12 wk after DNX. Despite morphological evidence of sympathetic regrowth, NE content did not fully recover. These results suggest a long-term net surplus of afferent influence on the DNX kidney may be contributing to the blood pressure lowering effect of DNX.

Impaired excitability of renal afferent innervation after exposure to the inflammatory chemokine CXCL1

Recently, we showed that renal afferent neurons exhibit a unique firing pattern, i.e., predominantly sustained firing, upon stimulation. Pathological conditions such as renal inflammation likely alter excitability of renal afferent neurons. Here, we tested whether the proinflammatory chemokine CXCL1 alters the firing pattern of renal afferent neurons. Rat dorsal root ganglion neurons (Th11-L2), retrogradely labeled with dicarbocyanine dye, were incubated with CXCL1 (20 h) or vehicle before patch-clamp recording. The firing pattern of neurons was characterized as tonic, i.e., sustained action potential (AP) firing, or phasic, i.e., <5 APs following current injection. Of the labeled renal afferents treated with vehicle, 58.9% exhibited a tonic firing pattern vs. 7.8%, in unlabeled, nonrenal neurons (P < 0.05). However, after exposure to CXCL1, significantly more phasic neurons were found among labeled renal neurons; hence the occurrence of tonic neurons with sustained firing upon electrical stimulation decreased (35.6 vs. 58.9%, P < 0.05). The firing frequency among tonic neurons was not statistically different between control and CXCL1-treated neurons. However, the lower firing frequency of phasic neurons was even further decreased with CXCL1 exposure [control: 1 AP/600 ms (1-2) vs. CXCL1: 1 AP/600 ms (1-1); P < 0.05; median (25th-75th percentile)]. Hence, CXCL1 shifted the firing pattern of renal afferents from a predominantly tonic to a more phasic firing pattern, suggesting that CXCL1 reduced the sensitivity of renal afferent units upon stimulation.

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

Clinical data suggest that renal denervation (RDNX) may be an effective treatment for human hypertension; however, it is unclear whether this therapeutic effect is due to ablation of afferent or efferent renal nerves. We have previously shown that RDNX lowers arterial pressure in hypertensive Dahl salt-sensitive (S) rats to a similar degree observed in clinical trials. In addition, we have recently developed a method for selective ablation of afferent renal nerves (renal-CAP). In the present study, we tested the hypothesis that the antihypertensive effect of RDNX in the Dahl S rat is due to ablation of afferent renal nerves by comparing the effect of complete RDNX to renal-CAP during two phases of hypertension in the Dahl S rat. In the early phase, rats underwent treatment after 3 wk of high-NaCl feeding when mean arterial pressure (MAP) was ∼ 140 mmHg. In the late phase, rats underwent treatment after 9 wk of high NaCl feeding, when MAP was ∼ 170 mmHg. RDNX reduced MAP ∼ 10 mmHg compared with sham surgery in both the early and late phase, whereas renal-CAP had no antihypertensive effect. These results suggest that, in the Dahl S rat, the antihypertensive effect of RDNX is not dependent on pretreatment arterial pressure, nor is it due to ablation of afferent renal nerves.

The role of the renal afferent and efferent nerve fibers in heart failure

Renal nerves contain afferent, sensory and efferent, sympathetic nerve fibers. In heart failure (HF) there is an increase in renal sympathetic nerve activity (RSNA), which can lead to renal vasoconstriction, increased renin release and sodium retention. These changes are thought to contribute to renal dysfunction, which is predictive of poor outcome in patients with HF. In contrast, the role of the renal afferent nerves remains largely unexplored in HF. This is somewhat surprising as there are multiple triggers in HF that have the potential to increase afferent nerve activity, including increased venous pressure and reduced kidney perfusion. Some of the few studies investigating renal afferents in HF have suggested that at least the sympatho-inhibitory reno-renal reflex is blunted. In experimentally induced HF, renal denervation, both surgical and catheter-based, has been associated with some improvements in renal and cardiac function. It remains unknown whether the effects are due to removal of the efferent renal nerve fibers or afferent renal nerve fibers, or a combination of both. Here, we review the effects of HF on renal efferent and afferent nerve function and critically assess the latest evidence supporting renal denervation as a potential treatment in HF.

The renal nerves in chronic heart failure: efferent and afferent mechanisms

The function of the renal nerves has been an area of scientific and medical interest for many years. The recent advent of a minimally invasive catheter-based method of renal denervation has renewed excitement in understanding the afferent and efferent actions of the renal nerves in multiple diseases. While hypertension has been the focus of much this work, less attention has been given to the role of the renal nerves in the development of chronic heart failure (CHF). Recent studies from our laboratory and those of others implicate an essential role for the renal nerves in the development and progression of CHF. Using a rabbit tachycardia model of CHF and surgical unilateral renal denervation, we provide evidence for both renal efferent and afferent mechanisms in the pathogenesis of CHF. Renal denervation prevented the decrease in renal blood flow observed in CHF while also preventing increases in Angiotensin-II receptor protein in the microvasculature of the renal cortex. Renal denervation in CHF also reduced physiological markers of autonomic dysfunction including an improvement in arterial baroreflex function, heart rate variability, and decreased resting cardiac sympathetic tone. Taken together, the renal sympathetic nerves are necessary in the pathogenesis of CHF via both efferent and afferent mechanisms. Additional investigation is warranted to fully understand the role of these nerves and their role as a therapeutic target in CHF.

(In)activity-related neuroplasticity in brainstem control of sympathetic outflow: unraveling underlying molecular, cellular, and anatomical mechanisms

More people die as a result of physical inactivity than any other preventable risk factor including smoking, high cholesterol, and obesity. Cardiovascular disease, the number one cause of death in the United States, tops the list of inactivity-related diseases. Nevertheless, the vast majority of Americans continue to make lifestyle choices that are creating a rapidly growing burden of epidemic size and impact on the United States healthcare system. It is imperative that we improve our understanding of the mechanisms by which physical inactivity increases the incidence of cardiovascular disease and how exercise can prevent or rescue the inactivity phenotype. The current review summarizes research on changes in the brain that contribute to inactivity-related cardiovascular disease. Specifically, we focus on changes in the rostral ventrolateral medulla (RVLM), a critical brain region for basal and reflex control of sympathetic activity. The RVLM is implicated in elevated sympathetic outflow associated with several cardiovascular diseases including hypertension and heart failure. We hypothesize that changes in the RVLM contribute to chronic cardiovascular disease related to physical inactivity. Data obtained from our translational rodent models of chronic, voluntary exercise and inactivity suggest that functional, anatomical, and molecular neuroplasticity enhances glutamatergic neurotransmission in the RVLM of sedentary animals. Collectively, the evidence presented here suggests that changes in the RVLM resulting from sedentary conditions are deleterious and contribute to cardiovascular diseases that have an increased prevalence in sedentary individuals. The mechanisms by which these changes occur over time and their impact are important areas for future study.