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

Electrical Diuretics: Dorsal Root Ganglion Stimulation to Increase Diuresis

BACKGROUND

Stimulation of diuresis is an essential component of heart failure treatment to reduce fluid overload. Over time, increasing doses of loop diuretics are required to achieve adequate urine output, and approximately 30% to 45% of patients develop diuretic resistance. We investigated the feasibility of affecting renal afferent sensory nerves by dorsal root ganglion neurostimulation as an alternative to medication to increase diuresis.

MATERIALS AND METHODS

Acute volume overload with an elevated and stable pulmonary capillary wedge pressure (PCWP) was induced by infusion of isotonic fluid in swine (N = 7). In each experiment, diuresis and blood electrolyte levels were measured during cycles of up to two hours (baseline, stimulation, poststimulation) through bladder catheterization. Efficacy was tested using bilateral dorsal root ganglion (bDRG) stimulation at the T11 and/or T12 vertebral levels.

RESULTS

An elevated, stable PCWP (15 ± 4 mm Hg, N = 7) was obtained after uploading. Under these conditions, average diuresis increased 20% to 205% compared with no stimulation. Side effects such as motor stimulation were mitigated by decreasing current or terminated spontaneously without intervention. There was no negative effect on acute kidney function because blood electrolyte concentrations remained stable. When stimulation was deactivated, urine output decreased significantly but did not return to baseline levels, suggesting a carry-over effect of up to two hours.

CONCLUSIONS

Electrical stimulation (bDRG) at T11 and/or T12 increased diuresis in an acute volume overload model. Side effects caused by unintended (motor) stimulation could be eliminated by reducing the electrical current while sustaining increased diuresis.

Neuroimmune interplay in kidney health and disease: Role of renal nerves

Renal nerves and their role in physiology and disease have been a topic of increasing interest in the past few decades. Renal inflammation contributes to many cardiorenal disease conditions, including hypertension, chronic kidney disease, and polycystic kidney disease. Much is known about the role of renal sympathetic nerves in physiology - they contribute to the regulation of sodium reabsorption, renin release, and renal vascular resistance. In contrast, far less is known about afferent, or "sensory," renal nerves, which convey signals from the kidney to the brain. While much remains unknown about these nerves in the context of normal physiology, even less is known about their contribution to disease states. Furthermore, it has become apparent that the crosstalk between renal nerves and the immune system may augment or modulate disease. Research from other fields, especially pain research, has provided critical insight into neuroimmune crosstalk. Sympathetic renal nerve activity may increase immune cell recruitment, but far less work has been done investigating the interplay between afferent renal nerves and the immune system. Evidence from other fields suggests that inflammation may augment afferent renal nerve activity. Furthermore, these nerves may exacerbate renal inflammation through the release of afferent-specific neurotransmitters.

Afferent neurons of the kidney with impaired firing pattern in inflammation - role of sodium currents?

Peripheral neurons with renal afferents exhibit a predominantly tonic firing pattern of higher frequency that is reduced to low frequencies (phasic firing pattern) in renal inflammation. We wanted to test the hypothesis that the reduction in firing activity during inflammation is due to high-activity tonic neurons switching from higher to low frequencies depending on altered sodium currents. We identified and cultivated afferent sensory neurons with renal projections from the dorsal root ganglia (Th11-L2). Cultivated neurons were incubated with the chemokine CXCL1 (1,5 nmol/ml) for 12 h. We characterized neurons as "tonic," i.e., sustained action potential (AP) firing, or "phasic," i.e., < 5 APs upon stimulation in the current clamp. Their membrane currents were investigated in a voltage clamp. Data analyzed: renal vs. non-renal and tonic vs. phasic neurons. Renal afferent neurons exposed to CXCL1 showed a decrease in tonic firing pattern (CXCL1: 35,6% vs. control: 57%, P < 0.05). Na+ and K+ currents were not different between control renal and non-renal DRG neurons. Phasic neurons exhibited higher Na+ and K+ currents than tonic resulting in shorter APs (3.7 ± 0.3 vs. 6.1 ± 0.6 ms, P < 0.01). In neurons incubated with CXCL1, Na+ and K+ peak current density increased in phasic (Na+: - 969 ± 47 vs. - 758 ± 47 nA/pF, P < 0.01; K+: 707 ± 22 vs. 558 ± 31 nA/pF, P < 0.01), but were unchanged in tonic neurons. Phasic neurons exposed to CXCL1 showed a broader range of Na+ currents ([- 365- - 1429 nA] vs. [- 412- - 4273 nA]; P < 0.05) similar to tonic neurons. After CXCL1 exposure, significant changes in phasic neurons were observed in sodium activation/inactivation as well as a wider distribution of Na+ currents characteristic of tonic neurons. These findings indicate a subgroup of tonic neurons besides mere tonic or phasic neurons exists able to exhibit a phasic activity pattern under pathological conditions.

Renal and hypothalamic inflammation in renovascular hypertension: role of afferent renal nerves

Renal denervation (RDN) is a potential therapy for drug-resistant hypertension. However, whether its effects are mediated by ablation of efferent or afferent renal nerves is not clear. Previous studies have implicated that renal inflammation and the sympathetic nervous system are driven by the activation of afferent and efferent renal nerves. RDN attenuated the renal inflammation and sympathetic activity in some animal models of hypertension. In the 2 kidney,1 clip (2K1C) model of renovascular hypertension, RDN also decreased sympathetic activity; however, mechanisms underlying renal and central inflammation are still unclear. We tested the hypothesis that the mechanisms by which total RDN (TRDN; efferent + afferent) and afferent-specific RDN (ARDN) reduce arterial pressure in 2K1C rats are the same. Male Sprague-Dawley rats were instrumented with telemeters to measure mean arterial pressure (MAP), and after 7 days, a clip was placed on the left renal artery. Rats underwent TRDN, ARDN, or sham surgery of the clipped kidney and MAP was measured for 6 wk. Weekly measurements of water intake (WI), urine output (UO), and urinary copeptin were conducted, and urine was analyzed for cytokines/chemokines. Neurogenic pressor activity (NPA) was assessed at the end of the protocol calculated by the depressor response after intraperitoneal injection of hexamethonium. Rats were euthanized and the hypothalamus and kidneys removed for measurement of cytokine content. MAP, NPA, WI, and urinary copeptin were significantly increased in 2K1C-sham rats, and these responses were abolished by both TRDN and ARDN. 2K1C-sham rats presented with renal and hypothalamic inflammation and these responses were largely mitigated by TRDN and ARDN. We conclude that RDN attenuates 2K1C hypertension primarily by ablation of afferent renal nerves which disrupts bidirectional renal neural-immune pathways.NEW & NOTEWORTHY Hypertension resulting from reduced perfusion of the kidney is dependent on renal sensory nerves, which are linked to inflammation in the kidney and hypothalamus. Afferent renal nerves are required for chronic increases in both water intake and vasopressin release observed following renal artery stenosis. Findings from this study suggest an important role of renal sensory nerves that has previously been underestimated in the pathogenesis of 2K1C hypertension.

Deletion of the Transient Receptor Potential Vanilloid 1 Channel Attenuates Sympathoexcitation and Hypertension and Improves Glomerular Filtration Rate in 2-Kidney-1-Clip Rats

BACKGROUND

Renal denervation lowers arterial blood pressure in both clinical populations and multiple experimental models of hypertension. This therapeutic effect is partly attributed to the removal of overactive renal sensory nerves. The TRPV1 (transient receptor potential vanilloid 1) channel is highly expressed in renal sensory nerves and detects changes in noxious and mechanosensitive stimuli, pH, and chemokines. However, the extent to which TRPV1 channels contribute to 2-kidney-1-clip (2K1C) renovascular hypertension has not been tested.

METHODS

We generated a novel Trpv1-/- (TRPV1 knockout) rat using CRISPR/Cas9 and 26-bp deletion in exon 3 and induced 2K1C hypertension.

RESULTS

The majority (85%) of rat renal sensory neurons retrogradely labeled from the kidney were TRPV1-positive. Trpv1-/- rats lacked TRPV1 immunofluorescence in the dorsal root ganglia, had a delayed tail-flick response to hot but not cold water, and lacked an afferent renal nerve activity response to intrarenal infusion of the TRPV1 agonist capsaicin. Interestingly, 2K1C hypertension was significantly attenuated in male Trpv1-/- versus wild-type rats. 2K1C hypertension significantly increased the depressor response to ganglionic blockade, total renal nerve activity (efferent and afferent), and afferent renal nerve activity in wild-type rats, but these responses were attenuated in male Trpv1-/- rats. 2K1C hypertension was attenuated in female rats with no differences between female strains. Finally, glomerular filtration rate was reduced by 2K1C in wild-type rats but improved in Trpv1-/- rats.

CONCLUSIONS

These findings suggest that renovascular hypertension requires activation of the TRPV1 channel to elevate renal afferent and sympathetic nerve activity, reduce glomerular filtration rate, and increase arterial blood pressure.

Angiotensin deficient FVB/N mice are normotensive

BACKGROUND AND PURPOSE

All previous rodent models lacking the peptide hormone angiotensin II (Ang II) were hypotensive. A mixed background strain with global deletion of the angiotensinogen gene was backcrossed to the FVB/N background (Agt-KO), a strain preferred for transgenic generation. Surprisingly, the resulting line turned out to be normotensive. Therefore, this study aimed to understand the unique blood pressure regulation of FVB/N mice without angiotensin peptides.

EXPERIMENTAL APPROACH

Acute and chronic recordings of blood pressure (BP) in freely-moving adult mice were performed to establish baseline BP. The pressure responses to sympatholytic and sympathomimetic as well as a nitric oxide inhibitor and donor compounds were used to quantify the neurogenic tone and endothelial function. The role of the renal nerves on baseline BP maintenance was tested by renal denervation. Finally, further phenotyping was done by gene expression analysis, histology and measurement of metabolites in plasma, urine and tissues.

KEY RESULTS

Baseline BP in adult FVB/N Agt-KO was unexpectedly unaltered. As compensatory mechanisms Agt-KO presented an increased sympathetic nerve activity and reduced endothelial nitric oxide production. However, FVB/N Agt-KO exhibited the renal morphological and physiological alterations previously found in mice lacking the production of Ang II including polyuria and hydronephrosis. The hypotensive effect of bilateral renal denervation was blunted in Agt-KO compared to wildtype FVB/N mice.

CONCLUSION AND IMPLICATIONS

We describe a germline Agt-KO line that challenges all previous knowledge on BP regulation in mice with deletion of the classical RAS. This line may represent a model of drug-resistant hypertension because it lacks hypotension.

A kidney-brain neural circuit drives progressive kidney damage and heart failure

Chronic kidney disease (CKD) and heart failure (HF) are highly prevalent, aggravate each other, and account for substantial mortality. However, the mechanisms underlying cardiorenal interaction and the role of kidney afferent nerves and their precise central pathway remain limited. Here, we combined virus tracing techniques with optogenetic techniques to map a polysynaptic central pathway linking kidney afferent nerves to subfornical organ (SFO) and thereby to paraventricular nucleus (PVN) and rostral ventrolateral medulla that modulates sympathetic outflow. This kidney-brain neural circuit was overactivated in mouse models of CKD or HF and subsequently enhanced the sympathetic discharge to both the kidney and the heart in each model. Interruption of the pathway by kidney deafferentation, selective deletion of angiotensin II type 1a receptor (AT1a) in SFO, or optogenetic silence of the kidney-SFO or SFO-PVN projection decreased the sympathetic discharge and lessened structural damage and dysfunction of both kidney and heart in models of CKD and HF. Thus, kidney afferent nerves activate a kidney-brain neural circuit in CKD and HF that drives the sympathetic nervous system to accelerate disease progression in both organs. These results demonstrate the crucial role of kidney afferent nerves and their central connections in engaging cardiorenal interactions under both physiological and disease conditions. This suggests novel therapies for CKD or HF targeting this kidney-brain neural circuit.

Impact of Selective Renal Afferent Denervation on Oxidative Stress and Vascular Remodeling in Spontaneously Hypertensive Rats

Oxidative stress and sustained sympathetic over-activity contribute to the pathogenesis of hypertension. Catheter-based renal denervation has been used as a strategy for treatment of resistant hypertension, which interrupts both afferent and efferent renal fibers. However, it is unknown whether selective renal afferent denervation (RAD) may play beneficial roles in attenuating oxidative stress and sympathetic activity in hypertension. This study investigated the impact of selective RAD on hypertension and vascular remodeling. Nine-week-old normotensive Wistar-Kyoto rats (WKY) and spontaneously hypertensive rats (SHR) were subjected to selective renal afferent denervation (RAD) with 33 mM of capsaicin for 15 min. Treatment with the vehicle of capsaicin was used as a control. The selective denervation was confirmed by the reduced calcitonin gene-related peptide expression and the undamaged renal sympathetic nerve activity response to the stimulation of adipose white tissue. Selective RAD reduced plasma norepinephrine levels, improved heart rate variability (HRV) and attenuated hypertension in SHR.It reduced NADPH oxidase (NOX) expression and activity, and superoxide production in the hypothalamic paraventricular nucleus (PVN), aorta and mesenteric artery of SHR. Moreover, the selective RAD attenuated the vascular remodeling of the aorta and mesenteric artery of SHR. These results indicate that selective removal of renal afferents attenuates sympathetic activity, oxidative stress, vascular remodeling and hypertension in SHR. The attenuated superoxide signaling in the PVN is involved in the attenuation of sympathetic activity in SHR, and the reduced sympathetic activity at least partially contributes to the attenuation of vascular oxidative stress and remodeling in the arteries of hypertensive rats.

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.

Renal sympathetic activation triggered by the rostral ventrolateral medulla is dependent of spinal cord AT1 receptors in Goldblatt hypertensive rats

Spinal cord neurons contribute to elevated sympathetic vasomotor activity in renovascular hypertension (2K1C), particularly, increased actions of angiotensin II. However, the origin of these spinal angiotensinergic inputs remains unclear. The present study aimed to investigate the role of spinal angiotensin II type 1 receptor (AT1) receptors in the sympathoexcitatory responses evoked by the activation of the rostral ventrolateral medulla (RVLM) in control and 2K1C Goldblatt rats. Hypertension was induced by clipping of the left renal artery. After 6 weeks, a catheter (PE-10) filled with losartan was inserted into the subarachnoid space and advanced to the T10-11 vertebral level in urethane-anesthetized rats. The effects of glutamate microinjection into the RVLM on blood pressure (BP), heart rate (HR), and renal and splanchnic sympathetic nerve activity (rSNA and sSNA, respectively) were evaluated in the presence or absence of spinal AT1 blockade. Tachycardic, pressor, and renal sympathoexcitatory effects caused by RVLM activation were significantly blunted by losartan in 2K1C rats, but not in control rats. However, no differences were found in the gene expression of angiotensin-converting enzyme, angiotensinogen, and renin in the spinal cord segments between the groups. In conclusion, acute sympathoexcitation induced by RVLM activation is dependent on the spinal AT1 receptor in Goldblatt, but not in control, rats. The involvement of other central cardiovascular nuclei in spinal angiotensinergic actions, as well as the source of angiotensin II, remains to be determined in the Goldblatt model.

Effects of Renal Denervation on the Enhanced Renal Vascular Responsiveness to Angiotensin II in High-Output Heart Failure: Angiotensin II Receptor Binding Assessment and Functional Studies in Ren-2 Transgenic Hypertensive Rats

Detailed mechanism(s) of the beneficial effects of renal denervation (RDN) on the course of heart failure (HF) remain unclear. The study aimed to evaluate renal vascular responsiveness to angiotensin II (ANG II) and to characterize ANG II type 1 (AT₁) and type 2 (AT₂) receptors in the kidney of Ren-2 transgenic rats (TGR), a model of ANG II-dependent hypertension. HF was induced by volume overload using aorto-caval fistula (ACF). The studies were performed two weeks after RDN (three weeks after the creation of ACF), i.e., when non-denervated ACF TGR enter the decompensation phase of HF whereas those after RDN are still in the compensation phase. We found that ACF TGR showed lower renal blood flow (RBF) and its exaggerated response to intrarenal ANG II (8 ng); RDN further augmented this responsiveness. We found that all ANG II receptors in the kidney cortex were of the AT₁ subtype. ANG II receptor binding characteristics in the renal cortex did not significantly differ between experimental groups, hence AT₁ alterations are not responsible for renal vascular hyperresponsiveness to ANG II in ACF TGR, denervated or not. In conclusion, maintained renal AT₁ receptor binding combined with elevated ANG II levels and renal vascular hyperresponsiveness to ANG II in ACF TGR influence renal hemodynamics and tubular reabsorption and lead to renal dysfunction in the high-output HF model. Since RDN did not attenuate the RBF decrease and enhanced renal vascular responsiveness to ANG II, the beneficial actions of RDN on HF-related mortality are probably not dominantly mediated by renal mechanism(s).

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.

Whole-Mount Kidney Clearing and Visualization Reveal Renal Sympathetic Hyperinnervation in Heart Failure Mice

Developing a three-dimensional (3D) visualization of the kidney at the whole-mount scale is challenging. In the present study, we optimized mouse whole-mount kidney clearing, which improved the transparency ratio to over 90% based on organ-specific perfusion (OSP)-clear, unobstructed brain imaging cocktails and computational analysis (CUBIC). The optimized OSP-CUBIC-compatible 3D immunostaining and imaging simultaneously visualized the high-resolution 3D structure of the whole-mount renal microvascular, glomerulus, and accompanying wrapped traveling sympathetic nerves in mice. A mouse model of pressure overload-induced heart failure (HF) was then established by minimally invasive transverse aortic constriction (MTAC). Further 3D quantification revealed renal sympathetic hyperinnervation (6.80 ± 1.04% vs. 3.73 ± 0.60%, P < 0.05) in mice with HF. In conclusion, this newly developed whole-organ tissue clearing and imaging system provides comprehensive information at the whole-mount scale and has great potential for kidney research. Our data suggest that renal sympathetic hyperinnervation is involved in HF associated with renal dysfunction.

Responsiveness of afferent renal nerve units in renovascular hypertension in rats

Previous data suggest that renal afferent nerve activity is increased in hypertension exerting sympathoexcitatory effects. Hence, we wanted to test the hypothesis that in renovascular hypertension, the activity of dorsal root ganglion (DRG) neurons with afferent projections from the kidneys is augmented depending on the degree of intrarenal inflammation. For comparison, a nonhypertensive model of mesangioproliferative nephritis was investigated. Renovascular hypertension (2-kidney, 1-clip [2K1C]) was induced by unilateral clipping of the left renal artery and mesangioproliferative glomerulonephritis (anti-Thy1.1) by IV injection of a 1.75-mg/kg BW OX-7 antibody. Neuronal labeling (dicarbocyanine dye [DiI]) in all rats allowed identification of renal afferent dorsal root ganglion (DRG) neurons. A current clamp was used to characterize neurons as tonic (sustained action potential [AP] firing) or phasic (1–4 AP) upon stimulation by current injection. All kidneys were investigated using standard morphological techniques. DRG neurons exhibited less often tonic response if in vivo axonal input from clipped kidneys was received (30.4% vs. 61.2% control, p < 0.05). However, if the nerves to the left clipped kidneys were cut 7 days prior to investigation, the number of tonic renal neurons completely recovered to well above control levels. Interestingly, electrophysiological properties of neurons that had in vivo axons from the right non-clipped kidneys were not distinguishable from controls. Renal DRG neurons from nephritic rats also showed less often tonic activity upon current injection (43.4% vs. 64.8% control, p < 0.05). Putative sympathoexcitatory and impaired sympathoinhibitory renal afferent nerve fibers probably contribute to increased sympathetic activity in 2K1C hypertension.

Antiproteinuric and Hyperkalemic Mechanisms Activated by Dual Versus Single Blockade of the RAS in Renovascular Hypertensive Rats

This study aimed to investigate the antiproteinuric and hyperkalemic mechanisms activated by dual renin-angiotensin system (RAS) blockade in renovascular hypertensive rats (2-kidney 1-clip model [2K-1C]). Six weeks after clipping the left renal artery or sham operation (2K), rats were treated with losartan, enalapril, or both drugs for two weeks. We found that 2K-1C rats displayed higher tail-cuff blood pressure (BP), increased non-clipped kidney Ang II concentration, and more pronounced urinary albumin excretion than 2K. BP was decreased by the treatment with either enalapril or losartan, and the combination of both drugs promoted an additional antihypertensive effect in 2K-1C rats. Renal Ang II content and albuminuria were reduced by either enalapril or losartan in monotherapy and restored to control levels by dual RAS blockade. Albuminuria in 2K-1C rats was accompanied by downregulation of the glomerular slit protein podocin, reduction of the endocytic receptors megalin and cubilin, and a marked decrease in the expression of the ClC-5 chloride channel, compared to 2K animals. Treatment with losartan and enalapril in monotherapy or combination increased the expression of podocin, cubilin, and ClC-5. However, only the combined therapy normalized podocin, cubilin, and ClC-5 protein abundance in the non-clipped kidney of 2K-1C rats. Renovascular hypertensive 2K-1C rats had a lower concentration of plasma potassium compared to 2K rats. Single RAS blockade normalized potassium plasma concentration, whereas 2K-1C rats treated with dual RAS blockade exhibited hyperkalemia. Hypokalemia in 2K-1C rats was accompanied by an increase in the cleaved activated forms of α-ENaC and γ-ENaC and the expression of β-ENaC. Combined RAS blockade but not monotherapy significantly reduced the expression of these ENaC subunits in 2K-1C rats. Indeed, double RAS blockade reduced the abundance of cleaved-α-ENaC to levels lower than those of 2K rats. Collectively, these results demonstrate that the antiproteinuric effect of dual RAS blockade in 2K-1C rats is associated with the restored abundance of podocin and cubilin, and ClC-5. Moreover, double RAS blockade-induced hyperkalemia may be due, at least partially, to an exaggerated downregulation of cleaved α-ENaC in the non-clipped kidney of renovascular hypertensive rats.

Dysregulation of the Excitatory Renal Reflex in the Sympathetic Activation of Spontaneously Hypertensive Rat

Excessive sympathetic activation plays crucial roles in the pathogenesis of hypertension. Chemical stimulation of renal afferents increases the sympathetic activity and blood pressure in normal rats. This study investigated the excitatory renal reflex (ERR) in the development of hypertension in the spontaneously hypertensive rat (SHR). Experiments were performed in the Wistar-Kyoto rat (WKY) and SHR aged at 4, 12, and 24 weeks under anesthesia. Renal infusion of capsaicin was used to stimulate renal afferents, and thus, to induce ERR. The ERR was evaluated by the changes in the contralateral renal sympathetic nerve activity and mean arterial pressure. At the age of 4 weeks, the early stage with a slight or moderate hypertension, the ERR was more enhanced in SHR compared with WKY. The pressor response was greater than the sympathetic activation response in the SHR. At the age of 12 weeks, the development stage with severe hypertension, there was no significant difference in the ERR between the WKY and SHR. At the age of 24 weeks, the later stage of hypertension with long-term several hypertensions, the ERR was more attenuated in the SHR compared with the WKY. On the other hand, the pressor response to sympathetic activation due to the ERR was smaller at the age of 12 and 24 weeks than those at the age of 4 weeks. These results indicate that ERR is enhanced in the early stage of hypertension, and attenuated in the later stage of hypertension in the SHR. Abnormal ERR is involved in the sympathetic activation and the development of hypertension.

Urinary DPP4 correlates with renal dysfunction, and DPP4 inhibition protects against the reduction in megalin and podocin expression in experimental CKD

This study investigated the molecular mechanisms underlying the antiproteinuric effect of DPP4 inhibition in 5/6 renal ablation rats and tested the hypothesis that the urinary activity of DPP4 correlates with chronic kidney disease (CKD) progression. Experiments were conducted in male Wistar rats who underwent 5/6 nephrectomy (Nx) or sham operation followed by 8 wk of treatment with the DPP4 inhibitor (DPP4i) sitagliptin or vehicle. Proteinuria increased progressively in Nx rats throughout the observation period. This increase was remarkably mitigated by sitagliptin. Higher levels of proteinuria in Nx rats compared to control rats were accompanied by higher urinary excretion of retinol-binding protein 4, a marker of tubular proteinuria, as well as higher urinary levels of podocin, a marker of glomerular proteinuria. Retinol-binding protein 4 and podocin were not detected in the urine of Nx + DPP4i rats. Tubular and glomerular proteinuria was associated with the reduced expression of megalin and podocin in the renal cortex of Nx rats. Sitagliptin treatment partially prevented this decrease. Besides, the angiotensin II renal content was significantly reduced in the Nx rats that received sitagliptin compared to vehicle-treated Nx rats. Interestingly, both urinary DPP4 activity and abundance increased progressively in Nx rats. Additionally, urinary DPP4 activity correlated positively with serum creatinine levels, proteinuria, and blood pressure. Collectively, these results suggest that DPP4 inhibition ameliorated both tubular and glomerular proteinuria and prevented the reduction of megalin and podocin expression in CKD rats. Furthermore, these findings suggest that urinary DPP4 activity may serve as a biomarker of renal disease and progression.

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.

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.

Renal Sensory Activity Regulates the γ-Aminobutyric Acidergic Inputs to the Paraventricular Nucleus of the Hypothalamus in Goldblatt Hypertension

Renal sensory activity is centrally integrated within brain nuclei involved in the control of cardiovascular function, suggesting that renal afferents regulate basal and reflex sympathetic vasomotor activity. Evidence has shown that renal deafferentation (DAx) evokes a hypotensive and sympathoinhibitory effect in experimental models of cardiovascular diseases; however, the underlying mechanisms involved in this phenomenon need to be clarified, especially those related to central aspects. We aimed to investigate the role of renal afferents in the control of γ-aminobutyric acid (GABA)ergic inputs to the paraventricular nucleus (PVN) of the hypothalamus in renovascular hypertensive (2K1C) rats and their influence in the regulation of cardiovascular function. Hypertension was induced by clipping the left renal artery. After 4 weeks, renal DAx was performed by exposing the left renal nerve to a 33 mM capsaicin solution for 15 min. After 2 weeks of DAx, microinjection of muscimol into the PVN was performed in order to evaluate the influence of GABAergic activity in the PVN and its contribution to the control of renal sympathetic nerve activity (rSNA) and blood pressure (BP). Muscimol microinjected into the PVN triggered a higher drop in BP and rSNA in the 2K1C rats and renal DAx mitigated these responses. These results suggest that renal afferents are involved in the GABAergic changes found in the PVN of 2K1C rats. Although the functional significance of this phenomenon needs to be clarified, it is reasonable to speculate that GABAergic alterations occur to mitigate microglia activation-induced sympathoexcitation in the PVN of 2K1C rats.

Interleukin-1β in hypothalamic paraventricular nucleus mediates excitatory renal reflex

Chemical stimulation of kidney causes sympathetic activation and pressor responses in rats. The excitatory renal reflex (ERR) is mediated by angiotensin type 1 receptor (AT₁R) and superoxide anions in hypothalamic paraventricular nucleus (PVN). The aim of this study is to determine whether interleukin-1β (IL-1β) in the PVN mediates the ERR, and whether the IL-1β production in the PVN is dependent on the AT₁R-superoxide anion signaling. Experiments were performed in adult rats under anesthesia. The ERR was induced by renal infusion of capsaicin, and evaluated by the responses of the contralateral renal sympathetic nerve activity (RSNA) and mean arterial pressure (MAP). Inhibition of IL-1β production with MCC950 in the PVN dose-dependently inhibited the capsaicin-induced ERR and sympathetic activation. The PVN microinjection of IL-1 receptor antagonist IL-1Ra or specific IL-1β antibody abolished the capsaicin-induced ERR, while IL-1β enhanced the ERR. Renal infusion of capsaicin promoted p65-NFκB phosphorylation and IL-1β production in the PVN, which were prevented by PVN microinjection of NADPH oxidase inhibitor apocynin or the superoxide anion scavenger tempol. The PVN microinjection of NFκB inhibitor BMS-345541 abolished the capsaicin induced-ERR and IL-1β production, but not the NADPH oxidase activation and superoxide anion production. Furthermore, capsaicin-induced p65-NFκB phosphorylation and IL-1β production in the PVN were prevented by AT₁R antagonist losartan, or angiotensin converting enzyme inhibitor captopril. These results indicate that capsaicin-induced ERR and sympathetic activation are mediated by IL-1β in the PVN. The IL-1β production in the PVN is dependent on the AT₁R-mediated superoxide anion generation and NFκB activation.

Role of spinal neurons in the maintenance of elevated sympathetic activity: a novel therapeutic target?

The control of sympathetic vasomotor activity involves a complex network within the brain and spinal circuits. An extensive range of studies has indicated that sympathoexcitation is a common feature in several cardiovascular diseases and that strategies to reduce sympathetic vasomotor overactivity in such conditions can be beneficial. In the present mini-review, we present evidence supporting the spinal cord as a potential therapeutic target to mitigate sympathetic vasomotor overactivity in cardiovascular diseases, focusing mainly on the actions of spinal angiotensin II on the control of sympathetic preganglionic neuronal activity.