Nitric oxide modulation of neurally induced proximal tubular fluid reabsorption in the rat

Anesthesia and the renal sympathetic nervous system in perioperative AKI

Approximately 7% of patients undergoing non-cardiac surgery with general anesthesia develop postoperative acute kidney injury (AKI). It is well-known that general anesthesia may have an impact on renal function and water balance regulation, but the mechanisms and potential differences between anesthetics are not yet completely clear. Recently published large animal studies have demonstrated that volatile (gas) anesthesia stimulates the renal sympathetic nervous system more than intravenous propofol anesthesia, resulting in decreased water and sodium excretion and reduced renal perfusion and oxygenation. Whether this is the case also in humans remains to be clarified. Increased renal sympathetic nerve activity may impair renal excretory function and oxygenation and induce structural injury in ischemic AKI models and could therefore be a contributing factor to AKI in the perioperative setting. This review summarizes anesthetic agents' effects on the renal sympathetic nervous system that may be important in the pathogenesis of perioperative AKI.

Role of the paraventricular nucleus in the reflex diuresis to pulmonary lymphatic obstruction in rabbits

The changes in urine flow and renal sympathetic nerve activity (RSNA) due to pulmonary lymphatic obstruction (PLO) were examined in anesthetized, artificially ventilated New Zealand white rabbits. PLO was produced by pressurizing an isolated pouch created in the right external jugular vein at the points of entry of the right lymphatic ducts. During this maneuver, urine flow increased from 8.5 ± 0.3 mL/10 min to 12 ± 0.5 mL/10 min (P < 0.0001) and RSNA increased from 24.0 ± 4 to 40.0 ± 5 μV·s (P < 0.0001). Bilateral lesioning of the paraventricular nucleus (PVN) of the hypothalamus or cervical vagotomy abolished these responses. PLO increased c-fos gene expression in the PVN. The increase in urine flow due to PLO was attenuated by muscimol and abolished by kynurenic acid microinjections into the PVN. The results show that (i) neurons in the PVN are an important relay site in the reflex arc, which is activated by PLO; and (ii) this activation is regulated by glutamatergic and partly by GABAergic input to the PVN.

Update on the Angiotensin converting enzyme 2-Angiotensin (1-7)-MAS receptor axis: fetal programing, sex differences, and intracellular pathways

The renin-angiotensin-system (RAS) constitutes an important hormonal system in the physiological regulation of blood pressure. Indeed, dysregulation of the RAS may lead to the development of cardiovascular pathologies including kidney injury. Moreover, the blockade of this system by the inhibition of angiotensin converting enzyme (ACE) or antagonism of the angiotensin type 1 receptor (AT1R) constitutes an effective therapeutic regimen. It is now apparent with the identification of multiple components of the RAS that the system is comprised of different angiotensin peptides with diverse biological actions mediated by distinct receptor subtypes. The classic RAS can be defined as the ACE-Ang II-AT1R axis that promotes vasoconstriction, sodium retention, and other mechanisms to maintain blood pressure, as well as increased oxidative stress, fibrosis, cellular growth, and inflammation in pathological conditions. In contrast, the non-classical RAS composed of the ACE2-Ang-(1-7)-Mas receptor axis generally opposes the actions of a stimulated Ang II-AT1R axis through an increase in nitric oxide and prostaglandins and mediates vasodilation, natriuresis, diuresis, and oxidative stress. Thus, a reduced tone of the Ang-(1-7) system may contribute to these pathologies as well. Moreover, the non-classical RAS components may contribute to the effects of therapeutic blockade of the classical system to reduce blood pressure and attenuate various indices of renal injury. The review considers recent studies on the ACE2-Ang-(1-7)-Mas receptor axis regarding the precursor for Ang-(1-7), the intracellular expression and sex differences of this system, as well as an emerging role of the Ang1-(1-7) pathway in fetal programing events and cardiovascular dysfunction.

Renal sympathetic denervation for the treatment of systemic hypertension

Systemic hypertension is a common cardiovascular problem that often cannot be fully treated with existing nonpharmacologic and pharmacologic measures. A catheter-based strategy which denervates the renal afferent and efferent autonomic nervous system has been developed for the treatment of drug-resistant hypertension. In early clinical trials, this procedure was shown to be both feasible and safe in reducing blood pressure in patients with uncontrolled hypertension receiving treatment with a minimum of 3 antihypertensive drugs. In addition, this procedure has been associated with decreased renin secretion, preservation of renal function, improved glucose tolerance, and a reduction in left ventricular hypertrophy. The long-term effects of this procedure (beyond 3 years) still need to be determined. A large 530-patient sham-controlled trial is now in progress.

Renal sympathetic denervation and systemic hypertension

Hypertension represents a major health problem, with an appalling annual toll. Despite the plethora of antihypertensive drugs, hypertension remains resistant in a considerable number of patients, thus creating the need for alternative strategies, including interventional approaches. Recently, renal sympathetic denervation (RSD) using a very elegant, state-of-the-art technique (percutaneous, catheter-based radiofrequency ablation) was shown to be beneficial in patients with resistant hypertension. The pathophysiology of kidney function justifies the use of RSD in the treatment of hypertension. Data from older studies have shown that sympathectomy has efficiently lowered blood pressure and prolonged the life expectancy of patients with hypertension, but at considerable cost. RSD is devoid of the adverse effects of sympathectomy because of its localized nature, is minimally invasive, and provides short procedural and recovery times. In conclusion, this review outlines the pathophysiologic background of RSD, describes the past and the present of this interventional approach, and considers several future potential applications.

Redox control of renal function and hypertension

Loss of redox homeostasis and formation of excessive free radicals play an important role in the pathogenesis of kidney disease and hypertension. Free radicals such as reactive oxygen species (ROS) are necessary in physiologic processes. However, loss of redox homeostasis contributes to proinflammatory and profibrotic pathways in the kidney, which in turn lead to reduced vascular compliance and proteinuria. The kidney is susceptible to the influence of various extracellular and intracellular cues, including the renin-angiotensin-aldosterone system (RAAS), hyperglycemia, lipid peroxidation, inflammatory cytokines, and growth factors. Redox control of kidney function is a dynamic process with reversible pro- and anti-free radical processes. The imbalance of redox homeostasis within the kidney is integral in hypertension and the progression of kidney disease. An emerging paradigm exists for renal redox contribution to hypertension.

Discoordinate regulation of renal nitric oxide synthase isoforms in ovariectomized mRen2. Lewis rats

Estrogen depletion markedly exacerbates hypertension in female congenic mRen2. Lewis rats, a model of tissue renin overexpression. Because estrogen influences nitric oxide synthase (NOS) and NO may exert differential effects on blood pressure, the present study investigated the functional expression of NOS isoforms in the kidney of ovariectomized (OVX) mRen2. Lewis rats. OVX-mRen2. Lewis exhibited an increase in systolic blood pressure (SBP) of 171 +/- 5 vs. 141 +/- 7 mmHg (P < 0.01) for intact littermates. Renal cortical mRNA and protein levels for endothelial NOS (eNOS) were reduced 50-60% (P < 0.05) and negatively correlated with blood pressure. In contrast, cortical neuronal NOS (nNOS) mRNA and protein levels increased 100 to 300% (P < 0.05). In the OVX kidney, nNOS immunostaining was more evident in the macula densa, cortical tubules, and the medullary collecting ducts compared with the intact group. To determine whether the increase in renal nNOS expression constitutes a compensatory response to the reduction in renal eNOS, we treated both intact and OVX mRen2. Lewis rats with the selective nNOS inhibitor L-VNIO from 11 to 15 wk of age. The nNOS inhibitor reduced blood pressure in the OVX group (185 +/- 3 vs. 151 +/- 8 mmHg, P < 0.05), but pressure was not altered in the intact group (146 +/- 4 vs. 151 +/- 4 mmHg). In summary, exacerbation of blood pressure in the OVX mRen2. Lewis rats was associated with the discoordinate regulation of renal NOS isoforms. Estrogen sensitivity in this congenic strain may involve the influence of NO through the regulation of both eNOS and nNOS.

Nitric oxide in the kidney: functions and regulation of synthesis

In the kidney nitric oxide (NO) has numerous important functions including the regulation of renal haemodynamics, maintenance of medullary perfusion, mediation of pressure-natriuresis, blunting of tubuloglomerular feedback, inhibition of tubular sodium reabsorption and modulation of renal sympathetic neural activity. The net effect of NO in the kidney is to promote natriuresis and diuresis. Significantly, deficient renal NO synthesis has been implicated in the pathogenesis of hypertension. All three isoforms of nitric oxide synthase (NOS), namely neuronal NOS (nNOS or NOS1), inducible NOS (iNOS or NOS2) and endothelial NOS (eNOS or NOS3) are reported to contribute to NO synthesis in the kidney. The regulation of NO synthesis in the kidney by NOSs is complex and incompletely understood. Historically, many studies of NOS regulation in the kidney have emphasized the role of variations in gene transcription and translation. It is increasingly appreciated, however, that the constitutive NOS isoforms (nNOS and eNOS) are also subject to rapid regulation by post-translational mechanisms such as Ca(2+) flux, serine/threonine phosphorylation and protein-protein interactions. Recent studies have emphasized the role of post-translational regulation of nNOS and eNOS in the regulation of NO synthesis in the kidney. In particular, a role for phosphorylation of nNOS and eNOS at both activating and inhibitory sites is emerging in the regulation of NO synthesis in the kidney. This review summarizes the roles of NO in renal physiology and discusses recent advances in the regulation of eNOS and nNOS in the kidney by post-translational mechanisms such as serine/threonine phosphorylation.

Nitric oxide inhibition and the impact on renal nerve-mediated antinatriuresis and antidiuresis in the anaesthetized rat

The contribution of nitric oxide (NO) to the antinatriuresis and antidiuresis caused by low-level electrical stimulation of the renal sympathetic nerves (RNS) was investigated in rats anaesthetized with chloralose-urethane. Groups of rats, n= 6, were given i.v. infusions of vehicle, l-NAME (10 microg kg(-1) min(-1)), 1400W (20 microg kg(-1) min(-1)), or S-methyl-thiocitrulline (SMTC) (20 microg kg(-1) min(-1)) to inhibit NO synthesis non-selectively or selectively to block the inducible or neuronal NOS isoforms (iNOS and nNOS, respectively). Following baseline measurements of blood pressure (BP), renal blood flow (RBF), glomerular filtration rate (GFR), urine flow (UV) and sodium excretion (U(Na)V), RNS was performed at 15 V, 2 ms duration with a frequency between 0.5 and 1.0 Hz. RNS did not cause measurable changes in BP, RBF or GFR in any of the groups. In untreated rats, RNS decreased UV and U(Na)V by 40-50% (both P < 0.01), but these excretory responses were prevented in l-NAME-treated rats. In the presence of 1400W i.v., RNS caused reversible reductions in both UV and U(Na)V of 40-50% (both P < 0.01), while in SMTC-treated rats, RNS caused an inconsistent fall in UV, but a significant reduction (P < 0.05) in U(Na)V of 21%. These data demonstrated that the renal nerve-mediated antinatriuresis and antidiuresis was dependent on the presence of NO, generated in part by nNOS. The findings suggest that NO importantly modulates the neural control of fluid reabsorption; the control may be facilitatory at a presynaptic level but inhibitory on tubular reabsorptive processes.

Role of adrenergic receptors in the reflex diuresis in rabbits during pulmonary lymphatic obstruction

The role of adrenergic receptors in the reflex diuresis in response to pulmonary lymphatic drainage was examined in anaesthetized, artificially ventilated New Zealand White rabbits. Pulmonary lymphatic drainage was obstructed by raising the pressure in a pouch created from the right external jugular vein. This pulmonary lymphatic obstruction results in a reflex increase in urine flow and sodium excretion. This reflex is abolished by renal denervation and by administration of L-NAME, a non-selective inhibitor of nitric oxide synthase. Also, infusion of the relatively selective neuronal nitric oxide synthase blocker, 7-nitroindazole sodium salt, into the renal medulla abolished the reflex diuresis. In this study the effects of adrenergic receptor antagonists on the reflex increase in urine were observed. Both ureters were cannulated in order to determine urine flow from both kidneys separately. Prazosin, an alpha1 adrenergic receptor antagonist, was infused into the renal medulla of the right kidney, while the left kidney acted as control. Administration of prazosin in this manner did not block the reflex diuresis in response to pulmonary lymphatic obstruction in either kidney. However, rauwolscine, an alpha2 adrenergic receptor antagonist, abolished the reflex increase in urine and sodium excretion in the ipsilateral kidney while preserving it in the contralateral kidney. These findings suggest that the increase in urine flow in rabbits caused by pulmonary lymphatic obstruction is dependent upon activation of alpha2 adrenergic receptors within the renal medulla.

Nitric oxide and renal nerves: Comparison of effects on renal circulation and sodium excretion in anesthetized rats

BACKGROUND

An array of vasoconstrictor and vasodilator agents have the potential to control intrarenal circulation; however, their relative functional importance is unclear. We compared here the importance of nitric oxide and renal nerves in studies involving sequential blockade of nitric oxide synthesis and renal denervation.

METHODS

In anesthetized rats, N-nitro-L-arginine methylester (L-NAME) was used for nonselective inhibition of nitric oxide synthesis and 7-nitroindazole (7-NI) for inactivation of neuronal isoform of nitric oxide synthase (nNOS). Acute unilateral renal denervation was performed noninvasively enabling observation of rapid changes. Renal cortical and medullary blood flow was determined by laser Doppler flowmetry.

RESULTS

L-NAME decreased medullary blood flow and cortical blood flow, by 22% to 24%, whereas after 7-NI, medullary blood flow decreased by 22% and cortical blood flow about 10% (all changes significant). In untreated rats denervation significantly increased cortical blood flow (10% to 15%) but not medullary blood flow. In rats treated with L-NAME denervation partly prevented the post-inhibitor decrease in cortical blood flow but not in medullary blood flow. After 7-NI treatment, the decrease in cortical blood flow and medullary blood flow did not occur or a partial restoration of flow was seen. The denervation natriuresis was intact under L-NAME but attenuated following 7-NI.

CONCLUSION

A reduction of medullary blood flow after 7-NI, similar as after L-NAME, suggests that nitric oxide generated by nNOS is mainly responsible for adequate perfusion of the medulla whereas activity of nNOS and other isoform(s) is required to maintain cortical blood flow. Renal denervation partly restored cortical blood flow reduced by nitric oxide blockade. For medullary blood flow, such restoration was seen only after inactivation of nNOS alone, suggesting an intrinsic interaction of this isoform and renal nerves.

Role of nitric oxide in the reflex diuresis in rabbits during pulmonary lymphatic obstruction

The role of nitric oxide in the reflex diuresis in response to pulmonary lymphatic drainage was examined in anaesthetized, artificially ventilated New Zealand White rabbits. Pulmonary lymphatic drainage was obstructed by raising the pressure in a pouch created from the right external jugular vein. Pulmonary lymphatic obstruction resulted in a significant increase in urine flow from an initial control value of 8.9 +/- 0.5 ml (10 min)(-1) to 12.1 +/- 0.6 ml (10 min)(-1) during lymphatic obstruction (mean +/-s.e.m.; n= 17, P < 0.001). This increase in urine flow was accompanied by a significant increase in the excretion of sodium. Additionally, renal blood flow remained unchanged during the increase in urine flow caused by lymphatic obstruction. Intravenous infusion of L-NAME, a non-selective inhibitor of nitric oxide synthase (NOS), abolished the reflex diuresis. Furthermore, intraperitoneal administration of the relatively selective neuronal NOS blocker, 7-nitroindazole also abolished the response. It was observed that infusion of a more soluble neuronal NOS blocker, 7-nitroindazole sodium salt (7-NINA), into the renal medulla also abolished the reflex diuresis. These findings suggest that the increase in urine flow in rabbits caused by pulmonary lymphatic obstruction is dependent upon the integrity of neuronal NOS activity within the renal medulla.

Interactions between nitric oxide and superoxide on the neural regulation of proximal fluid reabsorption in hypertensive rats

This study investigated the role of nitric oxide (NO) and superoxide anions in modulating the renal nerve-dependent increases in proximal tubular fluid reabsorption (Jva). Renal nerve stimulation at 0.75 and 1.0 Hz (15 V, 0.2 ms) in anaesthetized Wistar rats had no effect on glomerular filtration rate but decreased urine flow and sodium excretion in a frequency-related manner, reaching 39 and 49% at 1.0 Hz, respectively (P < 0.01) and increased Jva by 11 and 31% (P < 0.01). In the stroke prone spontaneously hypertensive rats (SHRSP), basal mean blood pressure was higher (123 +/- 2 versus 99 +/- 2 mmHg, P < 0.001), glomerular filtration rate, urine flow, sodium excretion and proximal tubular fluid reabsorption (Jva) were lower (all P < 0.001) than in the Wistar rats. Renal nerve stimulation in the SHRSP did not change glomerular filtration rate but decreased urine flow, and sodium excretion by 18 and 34% (P < 0.05) at 1.0 Hz which was less (P < 0.05) than that in the Wistar rats. Under these conditions, Jva was increased at 0.75 Hz by 27%, and to a comparable extent at 1.0 Hz, which was a pattern very different from the frequency related rises reported in the Wistar rats. In the SHRSP, intratubular Nomega-nitro-L-arginine methyl ester (L-NAME) had no effect on baseline Jva or the pattern of response to renal nerve stimulation which contrasted with earlier reports in the Wistar rat. Intraluminal superoxide dismutase (SOD) had no effect on basal Jva in the Wistar rats but increased it in the SHRSP (P < 0.05) while the pattern of change in Jva during nerve stimulation was unaltered in both rat strains. By contrast, in the SHRSP, intraluminal sodium nitroprusside (SNP) resulted in a frequency related increase in Jva comparable to that obtained in the vehicle treated Wistar rats. These data suggest that in the hypertensive rats, superoxide anion production is raised which depresses Jva and interacts with NO preventing a normal Jva response to renal nerve stimulation.

Chronic nitric oxide synthase inhibition exacerbates renal dysfunction in cirrhotic rats

The present study investigated sodium balance and renal tubular function in cirrhotic rats with chronic blockade of the nitric oxide (NO) system. Rats were treated with the nonselective NO synthase inhibitor NG-nitro-l-arginine methyl ester (l-NAME) starting on the day of common bile duct ligation (CBL). Three weeks of daily sodium balance studies showed that CBL rats developed sodium retention compared with sham-operated rats and that l-NAME treatment dose dependently deteriorated cumulative sodium balance by reducing urinary sodium excretion. Five weeks after CBL, renal clearance studies were performed, followed by Western blotting of the electroneutral type 3 sodium/proton exchanger (NHE3) and the Na-K-ATPase present in proximal tubules. Untreated CBL rats showed a decreased proximal reabsorption with a concomitant reduction of NHE3 and Na-K-ATPase levels, indicating that tubular segments distal to the proximal tubules were responsible for the increased sodium reabsorption. l-NAME-treated CBL rats showed an increased proximal reabsorption measured by the lithium clearance method and showed a marked increase in NHE3 and Na-K-ATPase protein levels. Our results show that chronic l-NAME treatment exacerbates the sodium retention found in CBL rats by a significant increase in proximal tubular reabsorption.

Cardiovascular and renal control in NOS-deficient mouse models

Nitric oxide (NO) plays an essential role in the maintenance of cardiovascular and renal homeostasis. Endogenous NO is produced by three different NO synthase (NOS) isoforms: endothelial NOS (eNOS), inducible NOS (iNOS), and neuronal NOS (nNOS). To investigate which NOS is responsible for NO production in different tissues, NOS knockout (-/-) mice have been generated for the three isoforms. This review focuses on the regulation of cardiovascular and renal function in relation to blood pressure homeostasis in the different NOS-/- mice. Although regulation of vascular tone and cardiac function in eNOS-/- has been extensively studied, far less is known about renal function in these mice. eNOS-/- mice are hypertensive, but the mechanism responsible for their high blood pressure is still not clear. Less is known about cardiovascular and renal control in nNOS-/- mice, probably because their blood pressure is normal. Recent data suggest that nNOS plays important roles in cardiac function, renal homeostasis, and regulation of vascular tone under certain conditions, but these are only now beginning to be studied. Inasmuch as iNOS is absent from the cardiovascular system under physiological conditions, it may become important to blood pressure regulation only during pathological conditions related to inflammatory processes. However, iNOS is constitutively expressed in the kidney, where its function is largely unknown. Overall, the study of NOS knockout mice has been very useful and produced many answers, but it has also raised new questions. The appearance of compensatory mechanisms suggests the importance of the different isoforms to specific processes, but it also complicates interpretation of the data. In addition, deletion of a single gene may have physiologically significant effects in addition to those being studied. Thus the presence or absence of a specific phenotype may not reflect the most important physiological function of the absent gene.

Neural control of renal function

The renal nerves are the communication link between the central nervous system and the kidney. In response to multiple peripheral and central inputs, efferent renal sympathetic nerve activity is altered so as to convey information to the major structural and functional components of the kidney, the vessels, glomeruli, and tubules, each of which is innervated. At the level of each of these individual components, information transfer occurs via interaction of the neurotransmitter released at the sympathetic nerve terminal-neuroeffector junction with specific postjunctional receptors coupled to defined intracellular signaling and effector systems. In response to normal physiological stimuli, changes in efferent renal sympathetic nerve activity contribute importantly to homeostatic regulation of renal blood flow, glomerular filtration rate, renal tubular epithelial cell solute and water transport, and hormonal release. Afferent input from sensory receptors located in the kidney participates in this reflex control system via renorenal reflexes that enable total renal function to be self-regulated and balanced between the two kidneys. In pathophysiological conditions, abnormal regulation of efferent renal sympathetic nerve activity contributes significantly to the associated abnormalities of renal function which, in turn, are of importance in the pathogenesis of the disease.