Effect of dietary salt on neuronal nitric oxide synthase in the inner medullary collecting duct

Low Nitric Oxide Bioavailability Increases Renin Production in the Collecting Duct

In the kidney, the stimulation of renin production by the collecting duct (CD-renin) contributes to the development of hypertension. The CD is a major nephron segment for the synthesis of nitric oxide (NO), and low NO bioavailability in the renal medulla is associated with hypertension. However, it is unknown whether NO regulates renin production in the CD. To test the hypothesis that low intrarenal NO levels stimulate the production of CD-renin, we first examined renin expression in the distal nephron segments of CD-eNOS deficient mice. In these mice, specific CD-renin immunoreactivity was increased compared to wild-type littermates; however, juxtaglomerular (JG) renin was not altered. To further assess the intracellular mechanisms involved, we then treated M-1 cells with either 1 mM L-NAME (L-arginine analog), an inhibitor of NO synthase activity, or 1 mM NONOate, a NO donor. Both treatments increased intracellular renin protein levels in M-1 cells. However, only the inhibition of NOS with L-NAME stimulated renin synthesis and secretion as reflected by the increase in Ren1C transcript and renin protein levels in the extracellular media, respectively. In addition, NONOate induced a fast mobilization of cGMP and intracellular renin accumulation. These response was partially prevented by guanylyl cyclase inhibition with ODQ (1H-[1,2,4] oxadiazolo[4,3-a]quinoxalin-1]. Accumulation of intracellular renin was blocked by protein kinase G (PKG) and protein kinase C (PKC) inhibitors. Our data indicate that low NO bioavailability increases CD-renin synthesis and secretion, which may contribute to the activation of intrarenal renin angiotensin system.

The macula densa prorenin receptor is essential in renin release and blood pressure control

The prorenin receptor (PRR) was originally proposed to be a member of the renin-angiotensin system (RAS); however, recent work questioned their association. The present paper describes a functional link between the PRR and RAS in the renal juxtaglomerular apparatus (JGA), a classic anatomical site of the RAS. PRR expression was found in the sensory cells of the JGA, the macula densa (MD), and immunohistochemistry-localized PRR to the MD basolateral cell membrane in mouse, rat, and human kidneys. MD cell PRR activation led to MAP kinase ERK1/2 signaling and stimulation of PGE₂ release, the classic pathway of MD-mediated renin release. Exogenous renin or prorenin added to the in vitro microperfused JGA-induced acute renin release, which was inhibited by removing the MD or by the administration of a PRR decoy peptide. To test the function of MD PRR in vivo, we established a new mouse model with inducible conditional knockout (cKO) of the PRR in MD cells based on neural nitric oxide synthase-driven Cre-lox recombination. Deletion of the MD PRR significantly reduced blood pressure and plasma renin. Challenging the RAS by low-salt diet + captopril treatment caused further significant reductions in blood pressure, renal renin, cyclooxygenase-2, and microsomal PGE synthase expression in cKO vs. wild-type mice. These results suggest that the MD PRR is essential in a novel JGA short-loop feedback mechanism, which is integrated within the classic MD mechanism to control renin synthesis and release and to maintain blood pressure.

Renal Collectrin Protects against Salt-Sensitive Hypertension and Is Downregulated by Angiotensin II

Collectrin, encoded by the Tmem27 gene, is a transmembrane glycoprotein with approximately 50% homology with angiotensin converting enzyme 2, but without a catalytic domain. Collectrin is most abundantly expressed in the kidney proximal tubule and collecting duct epithelia, where it has an important role in amino acid transport. Collectrin is also expressed in endothelial cells throughout the vasculature, where it regulates L-arginine uptake. We previously reported that global deletion of collectrin leads to endothelial dysfunction, augmented salt sensitivity, and hypertension. Here, we performed kidney crosstransplants between wild-type (WT) and collectrin knockout (Tmem27^Y/- ) mice to delineate the specific contribution of renal versus extrarenal collectrin on BP regulation and salt sensitivity. On a high-salt diet, WT mice with Tmem27^Y/- kidneys had the highest systolic BP and were the only group to exhibit glomerular mesangial hypercellularity. Additional studies showed that, on a high-salt diet, Tmem27^Y/- mice had lower renal blood flow, higher abundance of renal sodium-hydrogen antiporter 3, and lower lithium clearance than WT mice. In WT mice, administration of angiotensin II for 2 weeks downregulated collectrin expression in a type 1 angiotensin II receptor-dependent manner. This downregulation coincided with the onset of hypertension, such that WT and Tmem27^Y/- mice had similar levels of hypertension after 2 weeks of angiotensin II administration. Altogether, these data suggest that salt sensitivity is determined by intrarenal collectrin, and increasing the abundance or activity of collectrin may have therapeutic benefits in the treatment of hypertension and salt sensitivity.

Nitric oxide synthesis in the adult and developing kidney

Nitric oxide (NO) is synthesized within the adult and developing kidney and plays a critical role in the regulation of renal hemodynamics and tubule function. In the adult kidney, the regulation of NO synthesis is very cell type specific and subject to distinct control mechanisms of NO synthase (NOS) isoforms. Endothelial NOS (eNOS) is expressed in the endothelial cells of glomeruli, peritubular capillaries, and vascular bundles. Neuronal NOS (nNOS) is expressed in the tubular epithelial cells of the macula densa and inner medullary collecting duct. Furthermore, in the immature kidney, the expression of eNOS and nNOS shows unique patterns distinct from that is observed in the adult. This review will summarize the localization and presumable function of NOS isoforms in the adult and developing kidney.

The role of transforming growth factor β1 in the regulation of blood pressure

Although human association studies suggest a link between polymorphisms in the gene encoding transforming growth factor (TGF) β1 and differing blood pressure levels, a causative mechanism for this correlation remains elusive. Recently we have generated a series of mice with graded expression of TGFβ1, ranging from approximately 10% to 300% compared to normal. We have found that blood pressure and plasma volume are negatively regulated by TGFβ1. Of note, the 10% hypomorph exhibits primary aldosteronism and markedly impaired urinary excretion of water and electrolytes. We here review previous literature highlighting the importance of TGFβ signaling as a natriuretic system, which we postulate is a causative mechanism explaining how polymorphisms in TGFβ1 could influence blood pressure levels.

Distinct regulation of inner medullary collecting duct nitric oxide production from mice and rats

Nitric oxide (NO) and NO synthase 1 (NOS1) maintain sodium and water homeostasis. The NOS1α and NOS1β splice variants are expressed in the rat inner medulla, but only NOS1β is expressed in the mouse. Collecting duct NOS1 is necessary for blood pressure control. We hypothesized that NOS1 splice variant expression and NO production in the inner medullary collecting duct (IMCD) are regulated differently in mice and rats by high dietary sodium. Male C57blk/J6 mice and Sprague-Dawley rats were fed a 0.4% (normal salt; NS), or 4% (high salt; HS) NaCl diet for 2 or 7 days. Mean arterial pressure was not altered by HS, whereas urinary sodium excretion in mice and rats was increased significantly. Urinary excretion of nitrate/nitrite (NO(x)) and IMCD nitrite production were significantly greater in mice compared with rats on the HS diet. Western blotting indicated that only NOS1β and NOS3 were expressed in the mouse IMCD and that expression was unaffected by the HS diet at either time point. In contrast, NOS1α was detected in the IMCD of rats, in addition to NOS1β and NOS3. Feeding of the HS diet for 2 days increased NOS1α and NOS1β expression in the rat IMCD and 7 day feeding of the HS diet further increased NOS1β expression. Expression of NOS3 was unchanged by the HS diet at either time point. In conclusion, IMCD NO production in mice and rats is distinctly regulated under both NS and HS conditions, including expression of NOS1 splice variants.

Nitric oxide and the A and B of endothelin of sodium homeostasis

PURPOSE OF REVIEW

In recent years, renal collecting duct-specific endothelin-1 (ET1), endothelin A (ETA) and endothelin B (ETB) receptors as well as nitric oxide synthase 1 (NOS1) knockout mice have been developed with subsequent identification for an integral role in regulation of sodium water homeostasis and ultimately blood pressure. The focus of this review is to integrate these models and to propose a scheme for the control of sodium excretion by the collecting duct and the endothelin/ETB/NOS system.

RECENT FINDINGS

NOS1 splice variants are expressed in the kidney, especially in the collecting duct. Mice express predominantly NOS1β in the medulla, with NOS1α and NOS1β in the cortex, whereas rats express NOS1α and NOS1β in both the cortex and medulla. Novel transcription of collecting duct ET1 mediated by epithelial sodium channels, mitochondrial Na/Ca exchangers and glucocorticoids has been determined. ET1 via the ETB receptor increases nitric oxide production in both rat and mouse collecting ducts, suggesting that NOS1β is linked to ET1-dependent NOS activation in the kidney. As well, genetic deletion of NOS1 splice variants in the collecting duct results in a salt-sensitive hypertensive phenotype in mice, much like the collecting duct ET1 and collecting duct ETB knockout mice.

SUMMARY

In the collecting duct, the ET1/nitric oxide pathways are intimately linked, and deletion of collecting duct ET1, ETB receptor or NOS1β results in a salt-sensitive phenotype, which is at least partially dependent on dysregulation of sodium and water reabsorption.

ATP mediates flow-induced NO production in thick ascending limbs

Mechanical stimulation caused by increasing flow induces nucleotide release from many cells. Luminal flow and extracellular ATP stimulate production of nitric oxide (NO) in thick ascending limbs. However, the factors that mediate flow-induced NO production are unknown. We hypothesized that luminal flow stimulates thick ascending limb NO production via ATP. We measured NO in isolated, perfused rat thick ascending limbs using the fluorescent dye DAF FM. The rate of increase in dye fluorescence reflects NO accumulation. Increasing luminal flow from 0 to 20 nl/min stimulated NO production from 17 ± 16 to 130 ± 37 arbitrary units (AU)/min (P < 0.02). Increasing flow from 0 to 20 nl/min raised ATP release from 4 ± 1 to 21 ± 6 AU/min (P < 0.04). Hexokinase (10 U/ml) plus glucose, which consumes ATP, completely prevented the measured increase in ATP. Luminal flow did not increase NO production in the presence of luminal and basolateral hexokinase (10 U/ml). When flow was increased with the ATPase apyrase in both luminal and basolateral solutions (5 U/ml), NO levels did not change significantly. The P2 receptor antagonist suramin (300 μmol/l) reduced flow-induced NO production by 83 ± 25% (P < 0.03) when added to both and basolateral sides. Luminal hexokinase decreased flow-induced NO production from 205.6 ± 85.6 to 36.6 ± 118.6 AU/min (P < 0.02). Basolateral hexokinase also reduced flow-induced NO production. The P2X receptor-selective antagonist NF023 (200 μmol/l) prevented flow-induced NO production when added to the basolateral side but not the luminal side. We conclude that ATP mediates flow-induced NO production in the thick ascending limb likely via activation of P2Y receptors in the luminal and P2X receptors in the basolateral membrane.

Regulation of renal NaCl transport by nitric oxide, endothelin, and ATP: clinical implications

NaCl absorption along the nephron is regulated not just by humoral factors but also by factors that do not circulate or act on the cells where they are produced. Generally, nitric oxide (NO) inhibits NaCl absorption along the nephron. However, the effects of NO in the proximal tubule are controversial and may be biphasic. Similarly, the effects of endothelin on proximal tubule transport are biphasic. In more distal segments, endothelin inhibits NaCl absorption and may be mediated by NO. Adenosine triphosphate (ATP) inhibits sodium bicarbonate absorption in the proximal tubule, NaCl absorption in thick ascending limbs via NO, and water reabsorption in collecting ducts. Defects in the effects of NO, endothelin, and ATP increase blood pressure, especially in a NaCl-sensitive manner. In diabetes, disruption of NO-induced inhibition of transport may contribute to increased blood pressure and renal damage. However, our understanding of how NO, endothelin, and ATP work, and of their role in pathology, is rudimentary at best.

Luminal flow regulates NO and O2(-) along the nephron

Urinary flow is not constant but in fact highly variable, altering the mechanical forces (shear stress, stretch, and pressure) exerted on the epithelial cells of the nephron as well as solute delivery. Nitric oxide (NO) and superoxide (O(2)(-)) play important roles in various processes within the kidney. Reductions in NO and increases in O(2)(-) lead to abnormal NaCl and water absorption and hypertension. In the last few years, luminal flow has been shown to be a regulator of NO and O(2)(-) production along the nephron. Increases in luminal flow enhance fluid, Na, and bicarbonate transport in the proximal tubule. However, we know of no reports directly addressing flow regulation of NO and O(2)(-) in this segment. In the thick ascending limb, flow-stimulated NO and O(2)(-) formation has been extensively studied. Luminal flow stimulates NO production by nitric oxide synthase type 3 and its translocation to the apical membrane in medullary thick ascending limbs. These effects are mediated by flow-induced shear stress. In contrast, flow-induced stretch and NaCl delivery stimulate O(2)(-) production by NADPH oxidase in this segment. The interaction between flow-induced NO and O(2)(-) is complex and involves more than one simply scavenging the other. Flow-induced NO prevents flow from increasing O(2)(-) production via cGMP-dependent protein kinase in thick ascending limbs. In macula densa cells, shear stress increases NO production and this requires that the primary cilia be intact. The role of luminal flow in NO and O(2)(-) production in the distal tubule is not known. In cultured inner medullary collecting duct cells, shear stress enhances nitrite accumulation, a measure of NO production. Although much progress has been made on this subject in the last few years, there are still many unanswered questions.

Salt inactivates endothelial nitric oxide synthase in endothelial cells

There is a 1-4 mmol/L rise in plasma sodium concentrations in individuals with high salt intake and in patients with essential hypertension. In this study, we used 3 independent assays to determine whether such a small increase in sodium concentrations per se alters endothelial nitric oxide synthase (eNOS) function and contributes to hypertension. By directly measuring NOS activity in living bovine aortic endothelial cells, we demonstrated that a 5-mmol/L increase in salt concentration (from 137 to 142 mmol/L) caused a 25% decrease in NOS activity. Importantly, the decrease in NOS activity was in a salt concentration-dependent manner. The NOS activity was decreased by 25, 45, and 70%, with the increase of 5, 10, and 20 mmol/L of NaCl, respectively. Using Chinese hamster ovary cells stably expressing eNOS, we confirmed the inhibitory effects of salt on eNOS activity. The eNOS activity was unaffected in the presence of equal milliosmol of mannitol, which excludes an osmotic effect. Using an ex vivo aortic angiogenesis assay, we demonstrated that salt attenuated the nitric oxide (NO)-dependent proliferation of endothelial cells. By directly monitoring blood pressure changes in response to salt infusion, we found that in vivo infusion of salt induced an acute increase in blood pressure in a salt concentration-dependent manner. In conclusion, our findings demonstrated that eNOS is sensitive to changes in salt concentration. A 5-mmol/L rise in salt concentration, within the range observed in essential hypertension patients or in individuals with high salt intake, could significantly suppress eNOS activity. This salt-induced reduction in NO generation in endothelial cells may contribute to the development of hypertension.

Renal medullary ETB receptors produce diuresis and natriuresis via NOS1

Endothelin-1 (ET-1) plays an important role in the regulation of salt and water excretion in the kidney. Considerable in vitro evidence suggests that the renal medullary ET(B) receptor mediates ET-1-induced inhibition of electrolyte reabsorption by stimulating nitric oxide (NO) production. The present study was conducted to test the hypothesis that NO synthase 1 (NOS1) and protein kinase G (PKG) mediate the diuretic and natriuretic effects of ET(B) receptor stimulation in vivo. Infusion of the ET(B) receptor agonist sarafotoxin S6c (S6c: 0.45 microg x kg(-1) x h(-1)) in the renal medulla of anesthetized, male Sprague-Dawley rats markedly increased the urine flow (UV) and urinary sodium excretion (UNaV) by 67 and 120%, respectively. This was associated with an increase in medullary cGMP content but did not affect blood pressure. In addition, S6c-induced diuretic and natriuretic responses were absent in ET(B) receptor-deficient rats. Coinfusion of N(G)-propyl-l-arginine (10 microg x kg(-1) x h(-1)), a selective NOS1 inhibitor, suppressed S6c-induced increases in UV, UNaV, and medullary cGMP concentrations. Rp-8-Br-PET-cGMPS (10 microg x kg(-1) x h(-1)) or RQIKIWFQNRRMKWKK-LRK(5)H-amide (18 microg x kg(-1) x h(-1)), a PKG inhibitor, also inhibited S6c-induced increases in UV and UNaV. These results demonstrate that renal medullary ET(B) receptor activation induces diuretic and natriuretic responses through a NOS1, cGMP, and PKG pathway.

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.

The effect of L-arginine methyl ester on indices of free radical involvement in a rat model of experimental nephrocalcinosis

The aim of this study was to test the effect of L: -arginine methyl ester (L-Arg) on indices of free radical involvement in a rat model of experimental nephrocalcinosis. Twenty-eight Sprague-Dawley rats were randomized into four groups of seven. The first group (G1), the sham-control group received pure distilled drinking water. The second group (G2) received drinking water containing 0.7% ethylene glycol (EG) in distilled water for 3 weeks. The third group (G3) received drinking water containing 0.7% EG in distilled water for 3 weeks and L-Arg was administered for 3 weeks. The fourth group (G4) received drinking water containing 0.7% EG in distilled water for 3 weeks and L-NAME was administered for 3 weeks. Urine and aortic blood was collected to determine some parameters. The kidneys were also removed for histological examination. The increase in blood urea nitrogen, serum creatinine, K(+), Mg(2+ )and uric acid were mild in group 3 compared with the groups 2 and 4. The urinary concentrations of Na(+), K(+), Mg(2+) and uric acid were noticed to be similar among the groups. However, Ca(2+ )and oxalate excretion were significantly higher in groups 2, 3 and 4 than in group 1. The mean values of SOD, CAT and GSH-Px values were significantly increased in group 3 when compared to groups 2 and 4. Presence of aggregated urinary crystals was clearer in experimental groups compared to group 1. The tubular dilatation, epithelial degeneration and lymphocytic infiltration were significantly found in groups 2 and 4. Mild tissue damage was observed in L-Arg-pretreated rats. Under polarized light microscope intense crystals in the cortex and medulla were observed in the kidney of group 2 and 4 and moderate crystals were noticed in group 3. In conclusion, L-Arg supplementation may decrease free radicals and tubulary membrane injury in nephrocalcinosis due to infiltrating leukocytes and decreased antioxidant enzyme activities in rats fed with EG diet.

Nitric oxide stimulates cyclooxygenase-2 in cultured cTAL cells through a p38-dependent pathway

To examine the interaction of nitric oxide (NO) and cyclooxygenase (COX-2) and the signaling pathway involved, primary cultured rabbit cortical thick ascending limb (cTAL) were used. In these cells, immunoreactive COX-2 and vasodilatory prostaglandins were increased by a NO donor, S-nitros- N-acetylpenicillamine (SNAP; 2.5 ± 0.3-fold control, n = 6, P < 0.01). SNAP increased expression of phosphorylated p38 (pp38; 2.4 ± 0.3-fold control; n = 5; P < 0.01), which was inhibited by the p38 inhibitor SB-203580 (1.3 ± 0.1-fold control, n = 5, P < 0.01). SB-203580 inhibited SNAP-induced COX-2 expression [1.4 ± 0.2-fold control, n = 6, not significant (NS) vs. control] and levels of PGE2significantly. In cTAL cells transfected with a luciferase reporter driven by the wild-type mouse COX-2 promoter, SNAP stimulated luciferase activity, which was reversed by SB-203580 (control vs. SNAP vs. SNAP + SB-203580: 1.4 ± 0.2-, 8.3 ± 1.4-, and 0.4 ± 0.1-fold control, respectively, n = 4, P < 0.01). Electrophoretic mobility shift assay indicated that SNAP stimulated nuclear factor (NF)-κB binding activity in cTAL that was also inhibited by the p38 inhibitor. SNAP was not able to stimulate a mutant COX-2 promoter construct that is not activated by NF-κB (0.9 ± 0.1, 1.2 ± 0.1, and 1.0 ± 0.2 respectively, n = 4, NS). Low chloride increased COX-2 expression (2.7 ± 0.4-fold control, n = 6, P < 0.01) and pp38 expression (2.8 ± 0.3-fold; n = 5, P < 0.01), which were reversed by the specific NO synthase (NOS) inhibitor 7-nitroindazole. Administration of a low-salt diet increased immunoreactive COX-2 and neuronal NOS (nNOS) in the macula densa and surrounding cTAL of kidneys of wild-type mice but did not significantly elevate COX-2 expression in nNOS−/−mice. In summary, these studies indicate that, in cTAL, NO can increase COX-2 expression in cTAL and macula densa through p38-dependent signaling pathways via activation of NF-κB.

Oxidative stress and nitric oxide deficiency in the kidney: a critical link to hypertension?

There is growing evidence that oxidative stress contributes to hypertension. Oxidative stress can precede the development of hypertension. In almost all models of hypertension, there is oxidative stress that, if corrected, lowers BP, whereas creation of oxidative stress in normal animals can cause hypertension. There is overexpression of the p22(phox) and Nox-1 components of NADPH oxidase and reduced expression of extracellular superoxide dismutase (EC-SOD) in the kidneys of ANG II-infused rodents, whereas there is overexpression of p47(phox) and gp91(phox) and reduced expression of intracellular SOD with salt loading. Several mechanisms have been identified that can make oxidative stress self-sustaining. Reactive oxygen species (ROS) can enhance afferent arteriolar tone and reactivity both indirectly via potentiation of tubuloglomerular feedback and directly by microvascular mechanisms that diminish endothelium-derived relaxation factor/nitric oxide responses, generate a cyclooxygenase-2-dependent endothelial-derived contracting factor that activates thromboxane-prostanoid receptors, and enhance vascular smooth muscle cells reactivity. ROS can diminish the efficiency with which the kidney uses O(2) for Na(+) transport and thereby diminish the P(O(2)) within the kidney cortex. This may place a break on further ROS generation yet could further enhance vasculopathy and hypertension. There is a tight relationship between oxidative stress in the kidney and the development and maintenance of hypertension.

Phosphorylation of neuronal and endothelial nitric oxide synthase in the kidney with high and low salt diets

BACKGROUND

Renal nitric oxide (NO) synthesis increases with increasing salt intake, however, the mechanisms underlying this are poorly understood. We hypothesized that activating or inhibitory phosphorylation of neuronal and endothelial nitric oxide synthase (nNOS, eNOS) regulates renal NO production in response to altered dietary salt.

METHODS

Sprague-Dawley rats were fed low, normal or high salt diets for 12 h or 2 weeks, and kidney NOS phosphorylation was analyzed by Western blot using phosphopeptide antibodies against the sites nNOS-Ser(1412), nNOS-Ser(847), eNOS-Ser(1176) and eNOS-Thr(494).

RESULTS

At 12 h, total nNOS increased 1.4-fold (p < 0.01) in the high salt group and decreased by 26% (p < 0.05) in the low salt group. Changes in expression of phospho-nNOS at 12 h were accounted for by the changes in total nNOS. No change in total or phospho-eNOS was seen at 12 h. At 2 weeks, in the low salt group expression of total nNOS increased 1.8-fold (p < 0.05) whereas expression of nNOS phosphorylated at the inhibitory site Ser(847) increased 4.3-fold (p < 0.01). Total eNOS was increased 3-fold in the low salt group (p < 0.01), with parallel increases in eNOS phosphorylated at both activating and inhibitory sites (p < 0.05). In the 2-week high salt group no changes in NOS expression or phosphorylation were seen, despite the observed increased excretion of urinary NO metabolites.

CONCLUSION

In summary, changes in phospho-nNOS and phospho-eNOS expression occurred in parallel with changes in total expression, thus, the overall activating and inhibitory effects of nNOS and eNOS phosphorylation at the sites studied were not changed by altered dietary salt.

Expression of endothelial nitric oxide synthase in developing rat kidney

Endothelium-derived nitric oxide (NO) is synthesized within the developing kidney and may play a crucial role in the regulation of renal hemodynamics. The purpose of this study was to establish the expression and intrarenal localization of the NO-synthesizing enzyme endothelial NO synthase (eNOS) during kidney development. Rat kidneys from 14 ( E14)-, 16 ( E16)-, 18 ( E18)-, and 20-day-old ( E20) fetuses and 1 ( P1)-, 3 ( P3)-, 7 ( P7)-, 14 ( P14)-, and 21-day-old ( P21) pups were processed for immunocytochemical and immunoblot analysis. In fetal kidneys, expression of eNOS was first observed in the endothelial cells of the undifferentiated intrarenal capillary network at E14. At E16, strong eNOS immunoreactivity was observed in the endothelial cells of renal vesicles, S-shaped bodies (stage II glomeruli), and stage III glomeruli at the corticomedullary junction. At E18- 20, early-stage developing glomeruli located in the subcapsular region showed less strong eNOS immunoreactivity than those of E16. The eNOS-positive immature glomeruli were observed in the nephrogenic zone until 7 days after birth. In fetal kidneys, eNOS was also expressed in the medulla in the endothelial cells of the capillaries surrounding medullary collecting ducts. After birth, eNOS immunostaining gradually increased in the developing vascular bundles and peritubular capillaries in the medulla and was highest at P21. Surprisingly, eNOS was also expressed in proximal tubules, in the endocytic vacuolar apparatus, only at P1. The strong expression of eNOS in the early stages of developing glomeruli and vasculature suggests that eNOS may play a role in regulating renal hemodynamics of the immature kidney.

A high-salt diet stimulates thick ascending limb eNOS expression by raising medullary osmolality and increasing release of endothelin-1

A high-salt diet increases renal endothelin (ET) production and thick ascending limb (THAL) endothelial nitric oxide synthase (eNOS) expression. ET stimulates THAL eNOS expression via ET(B) receptors. The tonicity of the renal medulla is highly variable, and hyperosmolality stimulates ET-1 synthesis by endothelial cells. We hypothesized that a high-salt diet raises medullary osmolality, increases ET release by the THAL, and thus enhances eNOS expression. Seven days of high salt (1% NaCl in drinking water) increased eNOS expression in THALs by 125 +/- 31%. High salt increased outer medullary osmolality from 362 +/- 13 to 423 +/- 6 mosmol/kg H(2)O (P < 0.05). Bosentan, a dual-ET receptor antagonist, blocked the increase in THAL eNOS expression caused by high salt (2.66 +/- 0.44 absorbance units with bosentan vs. 5.15 +/- 0.67 for vehicle; P < 0.05). Conscious systolic blood pressure did not differ between the two groups. In primary cultures of medullary THALs, raising osmolality from 300 to 350 and 400 mosmol/kg H(2)O using NaCl increased eNOS expression by 39 +/- 11% (P < 0.05) and 71 +/- 16%, respectively (P < 0.05). In primary cultures of THALs, raising osmolality from 300 to 400 mosmol/kg H(2)O for 1 h increased ET-1 release from 62 +/- 7 to 113 +/- 2 pg/mg protein (P < 0.05). BQ-788, an ET(B) receptor antagonist (1 muM), blocked the stimulatory effect of 400 mosmol/kg H(2)O on eNOS expression (70 +/- 13% vs. -5 +/- 10%; paired difference, 74 +/- 15%; P < 0.05). BQ-788 alone had no significant effect. We concluded that high salt stimulates THAL eNOS expression by increasing outer medullary osmolality, ET-1 release by the THAL and ET(B) receptor activation. This may be an important regulatory mechanism of THAL NaCl absorption when dietary salt intake is increased.

Nitric oxide inhibits superoxide-stimulated urea permeability in the rat inner medullary collecting duct

The inner medullary collecting duct (IMCD) contains relatively high nitric oxide (NO) synthetic capacity, but the effect of NO on IMCD transport remains unclear. We determined the effect of NO on basal and vasopressin (AVP)-stimulated urea (Purea) and water (Pf) permeabilities in isolated, perfused rat IMCD. The NO donor S-nitroso-N-acetylpenicillamine (SNAP) increased cGMP production in IMCD, but neither SNAP (10(-4) M) nor 8-BrcGMP (10(-4) M), the cell-permeable analog of cGMP, affected basal or AVP-stimulated Purea. The free radical superoxide is produced by oxidases in the kidney and can interact with NO. To determine the effect of superoxide generation on transport, IMCDs were incubated with diethyldithiocarbamate (DETC; 10(-3) M), the inhibitor of superoxide dismutase (SOD). DETC significantly increased basal and AVP-stimulated Purea (control: 28.7 +/- 4.5 vs. DETC: 40.9 +/- 6.2 x 10(-5) cm/s; P < 0.001; n = 9). Preincubation of IMCD with SNAP or the SOD mimetic tempol completely inhibited DETC-stimulated Purea. DETC caused a significant increase in superoxide generation by IMCD, and this was blocked by SNAP. Incubation of IMCD with the NO synthase (NOS) substrate l-arginine blocked the stimulatory effect of DETC on Purea, and this was reversed by the neuronal NOS inhibitor 7-nitroindazole. In contrast, neither basal nor AVP-stimulated Pf was affected by NO donors or DETC. In summary, exogenous or endogenously produced NO does not affect basal urea transport in the IMCD but inhibits superoxide-stimulated Purea. In the inner medulla, superoxide generation by local oxidases may stimulate urea transport, and the role of endogenous NO may be to dampen this effect by decreasing superoxide levels.

Role of renal NO production in the regulation of medullary blood flow

The unique role of nitric oxide (NO) in the regulation of renal medullary function is supported by the evidence summarized in this review. The impact of reduced production of NO within the renal medulla on the delivery of blood to the medulla and on the long-term regulation of sodium excretion and blood pressure is described. It is evident that medullary NO production serves as an important counterregulatory factor to buffer vasoconstrictor hormone-induced reduction of medullary blood flow and tissue oxygen levels. When NO synthase (NOS) activity is reduced within the renal medulla, either pharmacologically or genetically [Dahl salt-sensitive (S) rats], a super sensitivity to vasoconstrictors develops with ensuing hypertension. Reduced NO production may also result from reduced cellular uptake of l-arginine in the medullary tissue, resulting in hypertension. It is concluded that NO production in the renal medulla plays a very important role in sodium and water homeostasis and the long-term control of arterial pressure.

Paradoxes of nitric oxide in the diabetic kidney

As an important modulator of renal function and morphology, the nitric oxide (NO) system has been extensively studied in the diabetic kidney. However, a number of studies in different experimental and clinical settings have produced often confusing data and contradictory findings. We have reviewed a wide spectrum of findings and issues that have amassed concerning the pathophysiology of the renal NO system in diabetes, pointed out the controversies, and attempted to find some explanation for these discrepancies. Severe diabetes with profound insulinopenia can be viewed as a state of generalized NO deficiency, including in the kidney. However, we have focused our hypotheses and conclusions on the events occurring during moderate glycemic control with some degree of treatment with exogenous insulin, representing more the clinically applicable state of diabetic nephropathy. Available evidence suggests that diabetes triggers mechanisms that in parallel enhance and suppress NO bioavailability in the kidney. We hypothesize that during the early phases of nephropathy, the balance between these two opposing forces is shifted toward NO. This plays a role in the development of characteristic hemodynamic changes and may contribute to consequent structural alterations in glomeruli. Both endothelial (eNOS) and neuronal NO synthase can contribute to altered NO production. These enzymes, particularly eNOS, can be activated by Ca(2+)-independent and alternative routes of activation that may be elusive in traditional methods of investigation. As the duration of exposure to the diabetic milieu increases, factors that suppress NO bioavailability gradually prevail. Increasing accumulations of advanced glycation end products may be one of the culprits in this process. In addition, this balance is continuously modified by actual metabolic control and the degree of insulinopenia.

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.

Arginine vasopressin modulates expression of neuronal NOS in rat renal medulla

Arginine vasopressin (AVP) plays a central role in water balance. In principal cells of the collecting duct system, AVP controls the expression of several genes, including aquaporin-2. Because nitric oxide (NO) participates in the regulation of water reabsorption by the collecting duct system, we analyzed the effect of AVP on the expression of NO synthase (NOS) isoforms in the kidney. Rats were either water restricted or water loaded to modify the circulating AVP levels, and expressions of NOS isoforms were assessed by Western blot analysis. In water-restricted rats, endothelial NOS (eNOS) expression increased in the outer medulla, and neuronal NOS (nNOS) expression rose in both the outer medulla and the papilla. Conversely, water loading induced a decrease in expression of nNOS in the outer medulla and papilla but did not alter eNOS expression. Oral administration of the specific V(2)-receptor antagonist SR-121463B decreased nNOS expression in the outer medulla and papilla but did not alter eNOS expression levels. Finally, the very low nNOS expression levels observed in AVP-deficient Brattleboro rats was restored by AVP infusion for 1 wk. Thus AVP specifically increases nNOS expression levels in the renal outer medulla and papilla. Because nNOS is specifically expressed in principal cells of the collecting duct system, the stimulation of nNOS expression by AVP may participate in the control of water reabsorption.

Inhibition of inducible nitric oxide synthase during high dietary salt intake

BACKGROUND

Previous studies indicate that nitric oxide (NO) is involved in the regulation of blood pressure (BP) and natriuresis in response to high sodium intake. We investigated the role of inducible NO synthase (iNOS) in response to an increased salt intake.

METHODS

Conscious, chronically catheterized rats were exposed to a high-salt (6%) diet for 14 days while receiving vehicle or aminoguanidine ([AG]; 250 mg/kg/24 h), which selectively inhibits iNOS. A group of rats on normal salt intake + AG were also studied.

RESULTS

Aminoguanidine had no impact on BP (120 +/- 2 v 116 +/- 1 mm Hg, control v day 14) or 24-h urinary nitrite and nitrate excretion (UNOxV), in rats on normal salt but prevented lipopolysaccharide-induced hypotension. High salt alone had no impact on BP (120 +/- 1 v 121 +/- 1 mm Hg), whereas UNaV (1.3 +/- 0.2 v 3.5 +/- 0.6 microeq/min, P < .001) and UNOxV increased with high salt intake. The natriuretic response persisted (1.5 +/- 0.2 v 4.3 +/- 0.8 microeq/min, P < .005), but the increase in UNOXV was prevented with chronic AG although BP fell slightly (121 +/- 1 v 115 +/- 1 mm Hg, P < .05). There was no change in plasma volume with high salt, and 24-h UNaV increased appropriately in the presence of AG. The in vitro NOS activity was not increased in kidney homogenates by high salt diet, nor was it affected by chronic AG treatment.

CONCLUSION

We conclude that NO from an iNOS source is not essential for the regulation of sodium excretion and BP in the presence of a high-salt diet in a normal rat.

Effects of aging and alterations in dietary sodium intake on total nitric oxide production

Animal studies suggest that nitric oxide (NO) deficiency is linked to salt-sensitive hypertension and that NO activity decreases during normal aging. This study investigates the impact of increasing age and manipulations in dietary salt intake on biochemical indices of the NO system in healthy humans. We measured NO(2) + NO(3) (NO(X); stable oxidation products of NO) and cyclic guanosine monophosphate (cGMP; major second messenger) in plasma and urine of 30 healthy subjects aged 22 to 77 years. Subjects were maintained on controlled low NO(X) and low-, normal-, or high-salt diets for 3 days. Salt sensitivity of blood pressure was seen only in the oldest subjects. Plasma renin activity was suppressed by a high salt intake in all age groups, and baseline values declined with advancing age. Neither age nor salt intake correlated with indices of NO activity over the third 24-hour period of controlled salt intake. In a subgroup of subjects aged 33 +/- 4 years challenged with ultrahigh sodium intake (400 mEq/24 h), again there was no increase in NO(2) + NO(3) or cGMP measures. In contrast to animal studies, there is no correlation in humans between either salt intake or age and total NO production and activity, indicated by NO(2) + NO(3) and cGMP measures. This does not preclude undetected alterations occurring in NO production and/or activity in strategic locations in the kidney and cardiovascular system. Limitations of blood and urine measurements of NO(2) + NO(3) and cGMP as indices of NO activity are discussed.

Role of neuronal nitric oxide synthase (NOS1) in the pathogenesis of renal hemodynamic changes in diabetes

Nitric oxide (NO) has been implicated in the pathogenesis of renal hemodynamic changes in diabetes mellitus. However, the contribution of nitric oxide synthase (NOS) isoforms to intrarenal production of NO in diabetes remains unknown. To explore the role of NOS1 in the control of renal hemodynamics in diabetes, we assessed renal responses to inhibition of NOS1 with S-methyl-L-thiocitrulline (SMTC; administered into the abdominal aorta) in moderately hyperglycemic streptozotocin-diabetic rats (D) and their nondiabetic (C) and normoglycemic diabetic counterparts. The contribution of other NOS isoforms was also evaluated by assessing the responses to nonspecific NOS inhibition [N(G)-nitro-L-arginine methyl ester (L-NAME)] in SMTC-treated diabetic rats. The number of NOS1-positive cells in macula densa of D and C kidneys was also evaluated by immunohistochemistry. D rats demonstrated elevated glomerular filtration rate (GFR) compared with C. SMTC (0.05 mg/kg) normalized GFR in D but had no effect in C. SMTC-induced reduction of renal plasma flow (RPF) was similar in C and D. Normoglycemic diabetic rats demonstrated blunted renal hemodynamic responses to NOS1 inhibition compared with hyperglycemic animals. Mean arterial pressure was stable in all groups. L-NAME induced a further decrease in RPF, but not in GFR, in D rats treated with SMTC. Immunohistochemistry revealed increased numbers of NOS1-positive cells in D. These observations suggest that NOS1-derived NO plays a major role in the pathogenesis of renal hemodynamic changes early in the course of diabetes. NOS1 appears to be the most important isoform in the generation of hemodynamically active NO in this condition.

Nitric oxide regulates renal cortical cyclooxygenase-2 expression

We have previously shown that cyclooxygenase-2 (COX-2) is localized to the cortical thick ascending limb of the loop of Henle (cTALH)/macula densa of the rat kidney, and expression increases in response to low-salt diet and/or angiotensin-converting enzyme (ACE) inhibition. Because of the localization of neuronal nitric oxide synthase (nNOS) to macula densa and surrounding cTALH, the present study investigated the role of nitric oxide (NO) in the regulation of COX-2 expression. For in vivo studies, rats were fed a normal diet, low-salt diet or low-salt diet combined with the ACE inhibitor captopril. In each group, one-half of them were treated with the nNOS inhibitors 7-nitroinidazole (7-NI) or S-methyl-thiocitrulline. Both of these NOS inhibitors inhibited increases in COX-2 mRNA and immunoreactive protein in response to low salt and low salt+captopril. For in vitro studies, COX-2 expression was studied in primary cultures of rabbit cTALH cells immunodisssected with Tamm-Horsfall antibody. Basal COX-2 immunoreactivity expression was stimulated by S-nitroso-N-acetyl-penicillamine (SNAP), an NO donor, and intracellular cGMP concentration. The cultured cells expressed immunoreactive nNOS, and 7-NI inhibited basal COX-2 immunoreactivity expression, which could be partially overcome by cGMP. In summary, these studies indicate that NO is a mediator of increased renal cortical COX-2 expression seen in volume depletion and suggest important interactions between the NO and COX-2 systems in the regulation of arteriolar tone and the renin-angiotensin system by the macula densa.

Ontogeny of neuronal nitric oxide synthase, NOS I, in the developing porcine kidney

To determine if the developing kidney differs from the adult in the expression of the neuronal nitric oxide synthase, NOS I, these experiments measured mRNA gene expression by RNase protection assay and protein content by Western blot of NOS I in piglets at ages newborn and 3, 7, 10, 14, and 21 days and adult pigs. Whole kidney NOS I mRNA was greatest at birth and decreased progressively during renal maturation to adult levels. NOS I protein content paralleled this developmental pattern. Cortical NOS I protein was equivalent in newborn and 14-day-old piglets and was greater at both ages than the adult. Medullary NOS I protein was relatively greater than cortical in both immature ages and decreased from a peak at birth to adult levels. We conclude the following. 1) During postnatal maturation, renal NOS I mRNA and protein content show a pattern that is developmentally regulated. 2) This developmental pattern of NOS I after birth may, in part, contribute to the enhanced functional role of NO during renal maturation.

Localization of protein inhibitor of neuronal nitric oxide synthase in rat kidney

We have recently demonstrated that in rats with 5/6 nephrectomy (5/6 Nx), renal cortical and inner medullary neuronal NOS (nNOS) expression is downregulated, associated with decreased urinary excretion of nitric oxide (NO) products. Recently, a novel 89-amino acid protein [protein inhibitor of nNOS (PIN)] was isolated from rat brain and shown to inhibit nNOS activity. The present studies localized PIN in the rat kidney and determined the effect of 5/6 Nx on PIN expression. By RT-PCR, PIN mRNA was detected in the kidney cortex and inner medulla. Immunohistochemistry revealed staining for PIN in glomerular and vasa rectae endothelial cells. PIN was also localized to the apical membranes of inner medullary collecting duct (IMCD) cells. Two weeks after 5/6 Nx, inner medullary PIN expression was significantly upregulated (sham, 0.18+/-0.07 vs. 5/6 Nx, 0.58+/-0.13 arbitrary units; n = 6, P<0.02), as determined by Western blotting. In summary, our data show that PIN, a specific inhibitor of nNOS activity, is expressed in the IMCD, a site of high nNOS expression in the kidney. PIN expression is upregulated in the inner medulla of 5/6 Nx rats. Inhibition of nNOS activity by PIN may have important implications for the regulation of NO synthesis in the IMCD of normal and remnant kidneys.