Dietary sodium modulates the interaction between efferent and afferent renal nerve activity by altering activation of α2-adrenoceptors on renal sensory nerves

The Interaction of Central Nervous System and Acute Kidney Injury: Pathophysiology and Clinical Perspectives

Acute kidney injury (AKI) is a common disorder in critically ill hospitalized patients. Its main pathological feature is the activation of the sympathetic nervous system and the renin-angiotensin system (RAS). This disease shows a high fatality rate. The reason is that only renal replacement therapy and supportive care can reduce the impact of the disease, but those measures cannot significantly improve the mortality. This review focused on a generalization of the interaction between acute kidney injury and the central nervous system (CNS). It was found that the CNS further contributes to kidney injury by regulating sympathetic outflow and oxidative stress in response to activation of the RAS and increased pro-inflammatory factors. Experimental studies suggested that inhibiting sympathetic activity and RAS activation in the CNS and blocking oxidative stress could effectively reduce the damage caused by AKI. Therefore, it is of significant interest to specify the mechanism on how the CNS affects AKI, as we could use such mechanism as a target for clinical interventions to further reduce the mortality and improve the complications of AKI.

Systematic Review Registration: [www.ClinicalTrials.gov], identifier [registration number].

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

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

Renal denervation alleviates renal ischemic reperfusion injury-induced acute and chronic kidney injury in rats partly by modulating miRNAs

BACKGROUND

Renal denervation (RDN) has been used to promote kidney injury repair, whereas miRNAs have been found to be involved in the pathophysiology of renal injury. However, the miRNA alterations that occur after RDN and the related protective mechanisms remain to be determined.

METHODS

Renal ischemic reperfusion injury (IRI) rat model was established and RDN was performed. Animals were killed at 24 h and 2 weeks following the operation. Tyrosine hydroxylase (TH) levels, renal function, tubular cell apoptosis and histological sections were examined at 24 h, whereas renal fibrosis and capillary vessels were assessed at 2 weeks. Furthermore, the expression of miRNAs in the injured kidney was determined using micro-array and the target genes were analyzed.

RESULTS

We found that TH was eliminated and that renal function was improved in the denervation group at 24 h. RDN reduced tubular cell apoptosis and mitigated the histological lesion. Furthermore, an increase of capillary vessel density and reduction of renal fibrosis were observed after 2 weeks. Moreover, the numbers of miRNAs were up-regulated after RDN treatment, and the miRNAs targeted pro-angiogenic, anti-fibrotic and inflammatory pathways.

CONCLUSIONS

RDN is a reliable method for alleviating IRI-induced acute and chronic kidney injury, and modulating the miRNA-related pro-angiogenic, anti-fibrotic or inflammatory pathways involved in this process.

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.

Role of α2-Adrenoceptors in Hypertension: Focus on Renal Sympathetic Neurotransmitter Release, Inflammation, and Sodium Homeostasis

The kidney is extensively innervated by sympathetic nerves playing an important role in the regulation of blood pressure homeostasis. Sympathetic nerve activity is ultimately controlled by the central nervous system (CNS). Norepinephrine, the main sympathetic neurotransmitter, is released at prejunctional neuroeffector junctions in the kidney and modulates renin release, renal vascular resistance, sodium and water handling, and immune cell response. Under physiological conditions, renal sympathetic nerve activity (RSNA) is modulated by peripheral mechanisms such as the renorenal reflex, a complex interaction between efferent sympathetic nerves, central mechanism, and afferent sensory nerves. RSNA is increased in hypertension and, therefore, critical for the perpetuation of hypertension and the development of hypertensive kidney disease. Renal sympathetic neurotransmission is not only regulated by RSNA but also by prejunctional α2-adrenoceptors. Prejunctional α2-adrenoceptors serve as autoreceptors which, when activated by norepinephrine, inhibit the subsequent release of norepinephrine induced by a sympathetic nerve impulse. Deletion of α2-adrenoceptors aggravates hypertension ultimately by modulating renal pressor response and sodium handling. α2-adrenoceptors are also expressed in the vasculature, renal tubules, and immune cells and exert thereby effects related to vascular tone, sodium excretion, and inflammation. In the present review, we highlight the role of α2-adrenoceptors on renal sympathetic neurotransmission and its impact on hypertension. Moreover, we focus on physiological and pathophysiological functions mediated by non-adrenergic α2-adrenoceptors. In detail, we discuss the effects of sympathetic norepinephrine release and α2-adrenoceptor activation on renal sodium transporters, on renal vascular tone, and on immune cells in the context of hypertension and kidney disease.

Crosstalk between the nervous system and the kidney

Under physiological states, the nervous system and the kidneys communicate with each other to maintain normal body homeostasis. However, pathological states disrupt this interaction as seen in hypertension, and kidney damage can cause impaired renorenal reflex and sodium handling. In acute kidney injury (AKI) and chronic kidney disease (CKD), damaged kidneys can have a detrimental effect on the central nervous system. CKD is an independent risk factor for cerebrovascular disease and cognitive impairment, and many factors, including retention of uremic toxins and phosphate, have been proposed as CKD-specific factors responsible for structural and functional cerebral changes in patients with CKD. However, more studies are needed to determine the precise pathogenesis. Epidemiological studies have shown that AKI is associated with a subsequent risk for developing stroke and dementia. However, recent animal studies have shown that the renal nerve contributes to kidney inflammation and fibrosis, whereas activation of the cholinergic anti-inflammatory pathway, which involves the vagus nerve, the splenic nerve, and immune cells in the spleen, has a significant renoprotective effect. Therefore, elucidating mechanisms of communication between the nervous system and the kidney enables us not only to develop new strategies to ameliorate neurological conditions associated with kidney disease but also to design safe and effective clinical interventions for kidney disease, using the neural and neuroimmune control of kidney injury and disease.

Sympathetic regulation of NCC in norepinephrine-evoked salt-sensitive hypertension in Sprague-Dawley rats

Salt sensitivity of blood pressure is characterized by inappropriate sympathoexcitation and renal Na⁺ reabsorption during high salt intake. In salt-resistant animal models, exogenous norepinephrine (NE) infusion promotes salt-sensitive hypertension and prevents dietary Na⁺-evoked suppression of the Na⁺-Cl^- cotransporter (NCC). Studies of the adrenergic signaling pathways that modulate NCC activity during NE infusion have yielded conflicting results implicating α₁- and/or β-adrenoceptors and a downstream kinase network that phosphorylates and activates NCC, including with no lysine kinases (WNKs), STE20/SPS1-related proline-alanine-rich kinase (SPAK), and oxidative stress response 1 (OxSR1). In the present study, we used selective adrenoceptor antagonism in NE-infused male Sprague-Dawley rats to investigate the differential roles of α₁- and β-adrenoceptors in sympathetically mediated NCC regulation. NE infusion evoked salt-sensitive hypertension and prevented dietary Na⁺-evoked suppression of NCC mRNA, protein expression, phosphorylation, and in vivo activity. Impaired NCC suppression during high salt intake in NE-infused rats was paralleled by impaired suppression of WNK1 and OxSR1 expression and SPAK/OxSR1 phosphorylation and a failure to increase WNK4 expression. Antagonism of α₁-adrenoceptors before high salt intake or after the establishment of salt-sensitive hypertension restored dietary Na⁺-evoked suppression of NCC, resulted in downregulation of WNK4, SPAK, and OxSR1, and abolished the salt-sensitive component of hypertension. In contrast, β-adrenoceptor antagonism attenuated NE-evoked hypertension independently of dietary Na⁺ intake and did not restore high salt-evoked suppression of NCC. These findings suggest that a selective, reversible, α₁-adenoceptor-gated WNK/SPAK/OxSR1 NE-activated signaling pathway prevents dietary Na⁺-evoked NCC suppression, promoting the development and maintenance of salt-sensitive hypertension.

AKI and the Neuroimmune Axis

Neuroimmune interaction is an emerging concept, wherein the nervous system modulates the immune system and vice versa. This concept is gaining attention as a novel therapeutic target in various inflammatory diseases including acute kidney injury (AKI). Vagus nerve stimulation or treatment with pulsed ultrasound activates the cholinergic anti-inflammatory pathway to prevent AKI in mice. The kidneys are innervated by sympathetic efferent and sensory afferent neurons, and these neurons also may play a role in the modulation of inflammation in AKI. In this review, we discuss several neural circuits with respect to the control of renal inflammation and AKI as well as optogenetics as a novel tool for understanding these complex neural circuits.

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.

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.

Role of the Sympathetic Nervous System and Its Modulation in Renal Hypertension

The kidneys are densely innervated with renal efferent and afferent nerves to communicate with the central nervous system. Innervation of major structural components of the kidneys, such as blood vessels, tubules, the pelvis, and glomeruli, forms a bidirectional neural network to relay sensory and sympathetic signals to and from the brain. Renal efferent nerves regulate renal blood flow, glomerular filtration rate, tubular reabsorption of sodium and water, as well as release of renin and prostaglandins, all of which contribute to cardiovascular and renal regulation. Renal afferent nerves complete the feedback loop via central autonomic nuclei where the signals are integrated and modulate central sympathetic outflow; thus both types of nerves form integral parts of the self-regulated renorenal reflex loop. Renal sympathetic nerve activity (RSNA) is commonly increased in pathophysiological conditions such as hypertension and chronic- and end-stage renal disease. Increased RSNA raises blood pressure and can contribute to the deterioration of renal function. Attempts have been made to eliminate or interfere with this important link between the brain and the kidneys as a neuromodulatory treatment for these conditions. Catheter-based renal sympathetic denervation has been successfully applied in patients with resistant hypertension and was associated with significant falls in blood pressure and renal protection in most studies performed. The focus of this review is the neural contribution to the control of renal and cardiovascular hemodynamics and renal function in the setting of hypertension and chronic kidney disease, as well as the specific roles of renal efferent and afferent nerves in this scenario and their utility as a therapeutic target.

Role of renal sensory nerves in physiological and pathophysiological conditions

Whether activation of afferent renal nerves contributes to the regulation of arterial pressure and sodium balance has been long overlooked. In normotensive rats, activating renal mechanosensory nerves decrease efferent renal sympathetic nerve activity (ERSNA) and increase urinary sodium excretion, an inhibitory renorenal reflex. There is an interaction between efferent and afferent renal nerves, whereby increases in ERSNA increase afferent renal nerve activity (ARNA), leading to decreases in ERSNA by activation of the renorenal reflexes to maintain low ERSNA to minimize sodium retention. High-sodium diet enhances the responsiveness of the renal sensory nerves, while low dietary sodium reduces the responsiveness of the renal sensory nerves, thus producing physiologically appropriate responses to maintain sodium balance. Increased renal ANG II reduces the responsiveness of the renal sensory nerves in physiological and pathophysiological conditions, including hypertension, congestive heart failure, and ischemia-induced acute renal failure. Impairment of inhibitory renorenal reflexes in these pathological states would contribute to the hypertension and sodium retention. When the inhibitory renorenal reflexes are suppressed, excitatory reflexes may prevail. Renal denervation reduces arterial pressure in experimental hypertension and in treatment-resistant hypertensive patients. The fall in arterial pressure is associated with a fall in muscle sympathetic nerve activity, suggesting that increased ARNA contributes to increased arterial pressure in these patients. Although removal of both renal sympathetic and afferent renal sensory nerves most likely contributes to the arterial pressure reduction initially, additional mechanisms may be involved in long-term arterial pressure reduction since sympathetic and sensory nerves reinnervate renal tissue in a similar time-dependent fashion following renal denervation.

Renal sensory and sympathetic nerves reinnervate the kidney in a similar time-dependent fashion after renal denervation in rats

Efferent renal sympathetic nerves reinnervate the kidney after renal denervation in animals and humans. Therefore, the long-term reduction in arterial pressure following renal denervation in drug-resistant hypertensive patients has been attributed to lack of afferent renal sensory reinnervation. However, afferent sensory reinnervation of any organ, including the kidney, is an understudied question. Therefore, we analyzed the time course of sympathetic and sensory reinnervation at multiple time points (1, 4, and 5 days and 1, 2, 3, 4, 6, 9, and 12 wk) after renal denervation in normal Sprague-Dawley rats. Sympathetic and sensory innervation in the innervated and contralateral denervated kidney was determined as optical density (ImageJ) of the sympathetic and sensory nerves identified by immunohistochemistry using antibodies against markers for sympathetic nerves [neuropeptide Y (NPY) and tyrosine hydroxylase (TH)] and sensory nerves [substance P and calcitonin gene-related peptide (CGRP)]. In denervated kidneys, the optical density of NPY-immunoreactive (ir) fibers in the renal cortex and substance P-ir fibers in the pelvic wall was 6, 39, and 100% and 8, 47, and 100%, respectively, of that in the contralateral innervated kidney at 4 days, 4 wk, and 12 wk after denervation. Linear regression analysis of the optical density of the ratio of the denervated/innervated kidney versus time yielded similar intercept and slope values for NPY-ir, TH-ir, substance P-ir, and CGRP-ir fibers (all R(2) > 0.76). In conclusion, in normotensive rats, reinnervation of the renal sensory nerves occurs over the same time course as reinnervation of the renal sympathetic nerves, both being complete at 9 to 12 wk following renal denervation.

Distinct roles of central and peripheral prostaglandin E2 and EP subtypes in blood pressure regulation

Prostaglandin E(2) (PGE(2)) is a major prostanoid with a wide variety of biological activities. PGE(2) can influence blood pressure (BP) both positively and negatively. In particular, centrally administered PGE(2) induces hypertension whereas systemic administration of PGE(2) produces a hypotensive effect. These physiologically opposing effects are generated by the existence of multiple EP receptors, namely EP(1-4), which are G protein-coupled receptors with distinct signaling properties. This review highlights the distinct roles of PGE(2) in BP regulation and the involvement of specific EP receptor subtypes.

β-Adrenergic receptor stimulation increases surface NKCC2 expression in rat thick ascending limbs in a process inhibited by phosphodiesterase 4

The thick ascending limb of the loop of Henle (THAL) reabsorbs ∼30% of the filtered NaCl in a process mediated by the apical Na-K-2Cl cotransporter NKCC2. Stimulation of β-adrenergic receptors in the THAL enhances NaCl reabsorption and increases intracellular cAMP. We found that intracellular cAMP stimulates NKCC2 trafficking to the apical membrane via protein kinase A (PKA). Several cAMP-specific phosphodiesterases (PDE) have been identified in rat THALs, and PDE4 decreases cAMP generated by β-adrenergic stimulation in other cells. However, it is not known whether β-adrenergic receptors activation stimulates NKCC2 trafficking. Thus we hypothesized that β-adrenergic receptor stimulation enhances THAL apical membrane NKCC2 expression via the PKA pathway and PDE4 blunts this effect. THAL suspensions were obtained from Sprague-Dawley rats, and surface NKCC2 expression was measured by surface biotinylation and Western blot. Incubation of THALs with the β-adrenergic receptor agonist isoproterenol at 0.5 and 1.0 μM increased surface NKCC2 by 17 ± 1 and 29 ± 5% respectively (P < 0.05). Preventing cAMP degradation with 3-isobutyl-methylxanthine (IBMX; a nonselective phosphodiesterase inhibitor) enhanced isoproterenol-stimulated surface NKCC2 expression to 51 ± 7% (P < 0.05 vs. isoproterenol). The β-adrenergic receptor antagonist propranolol or the PKA inhibitor H-89 completely blocked isoproterenol + IBMX-induced increase on surface NKCC2, while propranolol or H-89 alone had no effect. Selective inhibition of PDE4 with rolipram (20 μM) potentiated the effect of isoproterenol on surface NKCC2 and increased cAMP levels. We concluded that β-adrenergic receptor stimulation enhances surface NKCC2 expression in the THALs via PKA and PDE4 blunts this effect.