High-salt diet increases sensitivity to NO and eNOS expression but not NO production in THALs

Fluorescent probes for the detection of reactive oxygen species in human spermatozoa

Reactive oxygen species (ROS) production is a by-product of mitochondrial activity and is necessary for the acquisition of the capacitated state, a requirement for functional spermatozoa. However, an increase in oxidative stress, due to an abnormal production of ROS, has been shown to be related to loss of sperm function, highlighting the importance of an accurate detection of sperm ROS, given the specific nature of this cell. In this work, we tested a variety of commercially available fluorescent probes to detect ROS and reactive nitrogen species (RNS) in human sperm, to define their specificity. Using both flow cytometry (FC) and fluorescence microscopy (FM), we confirmed that MitoSOX™ Red and dihydroethidium (DHE) detect superoxide anion (as determined using antimycin A as a positive control), while DAF-2A detects reactive nitrogen species (namely, nitric oxide). For the first time, we also report that RedoxSensor™ Red CC-1, CellROX^® Orange Reagent, and MitoPY1 seem to be mostly sensitive to hydrogen peroxide, but not superoxide. Furthermore, mean fluorescence intensity (and not percentage of labeled cells) is the main parameter that can be reproducibly monitored using this type of methodology.

cGMP induces degradation of NKCC2 in the thick ascending limb via the ubiquitin-proteasomal system

The thick ascending limb of Henle's loop (TAL) reabsorbs NaCl via the apical Na⁺-K⁺-2Cl^- cotransporter (NKCC2). NKCC2 activity is regulated by surface NKCC2 levels. The second messenger cGMP decreases NKCC2 activity by decreasing surface NKCC2 levels. We found that surface NKCC2 undergoes constitutive degradation. Therefore, we hypothesized that cGMP decreases NKCC2 levels by increasing NKCC2 ubiquitination and proteasomal degradation. We measured surface NKCC2 levels by biotinylation of surface proteins, immunoprecipitation of NKCC2, and ubiquitin in TALs. First, we found that inhibition of proteasomal degradation blunts the cGMP-dependent decrease in surface NKCC2 levels [vehicle: 100%, db-cGMP (500 µM): 70.3 ± 9.8%, MG132 (20 µM): 97.7 ± 5.0%, and db-cGMP + MG132: 103.3 ± 3.4%, n = 5, P < 0.05]. We then found that cGMP decreased the internalized NKCC2 pool and that this effect was prevented by inhibition of the proteasome but not the lysosome. Finally, we found that NKCC2 is constitutively ubiquitinated in TALs and that cGMP enhances the rate of NKCC2 ubiquitination [vehicle: 59 ± 14% and db-cGMP (500 µM): 111 ± 25%, n = 5, P < 0.05]. We conclude that NKCC2 is constitutively ubiquitinated and that cGMP stimulates NKCC2 ubiquitination and proteasomal degradation. Our data suggest that the cGMP-induced NKCC2 ubiquitination and degradation may contribute to the cGMP-induced decrease of the NKCC2-dependent NaCl reabsorption in TALs.

Thick Ascending Limb Sodium Transport in the Pathogenesis of Hypertension

The thick ascending limb plays a key role in maintaining water and electrolyte balance. The importance of this segment in regulating blood pressure is evidenced by the effect of loop diuretics or local genetic defects on this parameter. Hormones and factors produced by thick ascending limbs have both autocrine and paracrine effects, which can extend prohypertensive signaling to other structures of the nephron. In this review, we discuss the role of the thick ascending limb in the development of hypertension, not as a sole participant, but one that works within the rich biological context of the renal medulla. We first provide an overview of the basic physiology of the segment and the anatomical considerations necessary to understand its relationship with other renal structures. We explore the physiopathological changes in thick ascending limbs occurring in both genetic and induced animal models of hypertension. We then discuss the racial differences and genetic defects that affect blood pressure in humans through changes in thick ascending limb transport rates. Throughout the text, we scrutinize methodologies and discuss the limitations of research techniques that, when overlooked, can lead investigators to make erroneous conclusions. Thus, in addition to advancing an understanding of the basic mechanisms of physiology, the ultimate goal of this work is to understand our research tools, to make better use of them, and to contextualize research data. Future advances in renal hypertension research will require not only collection of new experimental data, but also integration of our current knowledge.

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.

Statins reverse renal inflammation and endothelial dysfunction induced by chronic high salt intake

High salt intake (HS) is a risk factor for cardiovascular and kidney disease. Indeed, HS may promote blood-pressure-independent tissue injury via inflammatory factors. The lipid-lowering 3-hydroxy 3-methylglutaryl-coenzyme A (HMG-CoA) reductase inhibitors exert beneficial lipid-independent effects, reducing the expression and synthesis of inflammatory factors. We hypothesized that HS impairs kidney structure and function in the absence of hypertension, and these changes are reversed by atorvastatin. Four groups of rats were treated for 6 wk in metabolic cages with their diets: normal salt (NS); HS, NS plus atorvastatin and HS plus atorvastatin. We measured basal and final body weight, urinary sodium and protein excretion (UProtV), and systolic blood pressure (SBP). At the end of the experimental period, cholesterolemia, creatinine clearance, renal vascular reactivity, glomerular volume, cortical and glomerular endothelial nitric oxide synthase (eNOS), and transforming growth factor (TGF)-β1 expression were measured. We found no differences in SBP, body weight, and cholesterolemia. HS rats had increased creatinine clearence, UProtV, and glomerular volume at the end of the study. Acetylcholine-induced vasodilatation decreased by 40.4% in HS rats ( P < 0.05). HS decreased cortical and glomerular eNOS and caused mild glomerular sclerosis, interstitial mononuclear cell infiltration, and increased cortical expression of TGF-β1. All of these salt-induced changes were reversed by atorvastatin. We conclude that long-term HS induces inflammatory and hemodynamic changes in the kidney that are independent of SBP. Atorvastatin corrected all, suggesting that the nitric oxide-oxidative stress balance plays a significant role in the earlier stages of salt induced kidney damage.

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.

Regulation of blood pressure and salt homeostasis by endothelin

Endothelin (ET) peptides and their receptors are intimately involved in the physiological control of systemic blood pressure and body Na homeostasis, exerting these effects through alterations in a host of circulating and local factors. Hormonal systems affected by ET include natriuretic peptides, aldosterone, catecholamines, and angiotensin. ET also directly regulates cardiac output, central and peripheral nervous system activity, renal Na and water excretion, systemic vascular resistance, and venous capacitance. ET regulation of these systems is often complex, sometimes involving opposing actions depending on which receptor isoform is activated, which cells are affected, and what other prevailing factors exist. A detailed understanding of this system is important; disordered regulation of the ET system is strongly associated with hypertension and dysregulated extracellular fluid volume homeostasis. In addition, ET receptor antagonists are being increasingly used for the treatment of a variety of diseases; while demonstrating benefit, these agents also have adverse effects on fluid retention that may substantially limit their clinical utility. This review provides a detailed analysis of how the ET system is involved in the control of blood pressure and Na homeostasis, focusing primarily on physiological regulation with some discussion of the role of the ET system in hypertension.

High salt intake delayed angiotensin II-induced hypertension in mice with a genetic variant of NADPH oxidase

BACKGROUND

gp91(PHOX), a catalytic subunit of NAD(P)H oxidase, is involved in angiotensin II (Ang II)-induced superoxide (O₂⁻) generation. This study was designed to examine the hypothesis that an enhancement in O₂⁻ generation due to elevated Ang II induces salt-sensitivity, which contributes to the development of hypertension.

METHODS

Assessment of blood pressure and renal excretory responses to Ang II infusion (2.2 ng·min/g) for 2 weeks via osmotic minipump was made in knockout (KO; n = 20) mice lacking the gene for gp91(PHOX) which were fed on either normal-salt (NS; 0.04% NaCl) or high-salt (HS, 4% NaCl) diet and compared these responses with those in wild-type (WT; n = 23) mice.

RESULTS

Ang II induced increase in systolic blood pressure (SBP) was started within the 4th day in all groups except in HS fed KO mice in which SBP increased after the 10(th) day of Ang II infusion. The increases in SBP were lower in KO than WT mice at the end of 2-week infusion period. In Ang II + HS fed KO mice, the urinary excretion rate of nitrite/nitrate (U(NOx)V) markedly increased but 8-isoprostane excretion rate remained unchanged. These findings indicate that an increase in nitric oxide (NO) with a lack of O₂⁻ formation was involved in the delayed hypertension in Ang II + HS fed KO mice.

CONCLUSION

These data suggest that an enhanced O₂⁻ activity and its interaction with NO contribute to the early developmental phase of Ang II-induced salt-sensitive hypertension.American Journal of Hypertension (2011). doi:10.1038/ajh.2010.173.

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.

Hemodynamic parameters during normal and hypertensive pregnancy in rats: evaluation of renal salt and water transporters

OBJECTIVE

To determine whether alterations in extracellular volume expansion observed during normal and hypertensive pregnancy run in parallel to changes in the mRNA expression of renal transporters.

METHODS

Wistar rats were divided into four groups: control (C, n = 5); pregnancy (P, n = 5); N(omega)-nitro-l-arginine methyl ester (L-NAME; 50 mg/kg/d)-treated control (H, n = 6); and pregnant rats (HP, n = 6). Hemodynamic studies were performed on day 14 of pregnancy, at which time we also analyzed of the sodium transporters (NHE3, Na/K/2Cl and Na/Cl), potassium channel (ROMK2) and water channel (AQP2).

RESULTS

As expected, P rats presented high cardiac output (CO) and normal blood pressure (BP), whereas H rats presented lower CO and elevated BP. A significant (threefold) increase in total vascular resistance and a decrease in stroke volume were observed in the HP group. Hypertension resulting from nitric oxide (NO) synthesis inhibition blunted systemic hemodynamic adaptations during pregnancy. Compared with C rats, mRNA expression of ROMK2 in P rats was lower, whereas that of AQP2 was higher. Expression of AQP2 was significantly higher in H than in C or HP groups. Expression of BSC and NHE3 was lower in the HP than in the P group. The NO inhibition also provoked renal transporter alterations in HP.

CONCLUSIONS

Our results suggest that tubule transporter variants may mediate the hemodynamic adaptations seen during pregnancy, although we cannot rule out the hypothesis that other factors are also mediating hemodynamic changes.

Time-course of changes to nitric oxide signaling pathways in form-deprivation myopia in guinea pigs

The aim of this study was to investigate the time-course change of nitric oxide synthase (NOS) activity and cyclic GMP (cGMP) concentration in the posterior retina, choroid and sclera after differing periods of form-deprivation in guinea pigs. Three groups of guinea pigs were subjected to monocular FD for 7, 14 or 21 days. NOS activity and cGMP concentrations in ocular tissues of FD eyes and control eyes were analyzed by radioimmunoassay. The presence of NOS isoforms was detected by immunohistochemistry. Guinea pigs presented with considerable myopia after 14 days of FD. Retinal NOS activity in the FD group was lower than in the control group after 7 days of FD and was higher than in the control group after 14 and 21 days of FD. The choroidal and scleral NOS activities in the FD groups were higher than in the control groups after 21 days. The cGMP concentrations in the FD groups were higher than in the control groups at 21 days of the retinal, choroidal, and scleral tissues. Furthermore, the retinal cGMP concentration in the FD group was also significantly elevated at 14 days relative to the control group. We detected expression of three NOS isoforms in guinea pig ocular tissues. Our main observations were a change in NOS activity and an up-regulation in cGMP concentrations in posterior ocular tissues during the development of myopia. The function of elevated NOS activity may be mediated by cGMP.

Influence of salt on subcellular localization of nitric oxide synthase activity and expression in the renal inner medulla

1. The aims of this study were: (i) to characterize the subcellular localization of nitric oxide synthase (NOS) 1 and NOS3 activity and expression within the cytosolic, plasma membrane and intracellular membrane subcellular fractions of the renal inner medulla of rats; and (ii) to determine whether NOS1 and NOS3 activity and expression in subcellular fractions of the renal inner medulla are regulated by dietary salt intake. Although the NOS system is important in maintaining Na(+) and water homeostasis, the identity of the NOS isoform that is sensitive to dietary Na(+) remains unclear. In addition, subcellular localization of both NOS1 and NOS3 has been shown to regulate enzymatic activity and influence the ability of NOS to produce nitric oxide (NO). 2. Renal inner medullae were dissected from male Sprague-Dawley rats and separated into cytosolic, plasma membrane and intracellular membrane fractions for measurement of NOS activity and western blot analysis. 3. On a normal-salt diet, NOS activity and NOS1 and NOS3 protein expression were present in all three subcellular fractions, although total NOS activity was enriched in the intracellular membrane fraction. In response to a high-salt diet, urinary nitrate/nitrite (NO(x)) increased. Despite an increase in NO(x) excretion, total NOS activity in the renal inner medullary homogenate was decreased. There were no detectable differences in NOS activity in the subcellular fractions. Expression of NOS1 protein was decreased in the cytoplasmic and plasma membrane fractions, although maintained in the intracellular membrane fraction, in response to high salt. Expression of NOS3 protein was unaffected by high salt. 4. In conclusion, we hypothesize that NOS1 localization in the intracellular membrane is important in increasing NO production to aid Na(+) and water homeostasis.

Mathematical model of nitric oxide convection and diffusion in a renal medullary vas rectum

In this study, the generation, convection, diffusion, and consumption of nitric oxide (NO) in and around a single renal medullary descending or ascending vas rectum in rat were modeled using CFD. The vascular lumen (with a core RBC-rich layer and a parietal layer), the endothelium, the pericytes and the interstitium were represented as concentric cylinders. We accounted for the generation of NO by vascular endothelial cells, and that by the epithelial cells of medullary thick ascending limbs (mTALs) and inner medullary collecting ducts (IMCDs), the latter via interstitial boundary conditions. Luminal velocity profiles were obtained by modeling blood flow dynamics. Our results suggest that convection (i.e., blood flow per se) does not significantly affect NO concentrations along the cortico-medullary axis, because the latter are mostly determined by the rate of NO production and that of NO consumption by hemoglobin. However, the shear stress-mediated effects of blood flow on NO generation rates, and therefore NO concentrations, were predicted to be important. Finally, we found that unless epithelial NO generation rates (per unit tubular surface area) are at least 10 times lower than endothelium NO generation rates, NO production by mTALs and IMCDs affects vascular NO concentrations, with possible consequences for medullary blood flow distribution.

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.

Regulation of thick ascending limb transport: role of nitric oxide

Nitric oxide (NO) plays a role in many physiological and pathophysiological processes. In the kidney, NO reduces renal vascular resistance, increases glomerular filtration rate, alters renin release, and inhibits transport along the nephron. The thick ascending limb is responsible for absorbing 20-30% of the filtered load of NaCl, much of the bicarbonate that escapes the proximal nephron, and a significant fraction of the divalent cations reclaimed from the forming urine. Additionally, this nephron segment plays a role in K+ homeostasis. This article will review recent advances in our understanding of the role NO plays in regulating the transport processes of the thick ascending limb. NO has been shown to inhibit NaCl absorption primarily by reducing Na+-K+-2Cl- cotransport activity. NO also inhibits bicarbonate absorption by reducing Na+/H+ exchange activity. It has also been reported to enhance luminal K+ channel activity and thus is likely to alter K+ secretion. The source of NO may be vascular structures such as the afferent arteriole or vasa recta, or the thick ascending limb itself. NO is produced by NO synthase 3 in this segment, and several factors that regulate its activity both acutely and chronically have recently been identified. Although the effects of NO on thick ascending limb transport have received a great deal of attention recently, its effects on divalent ion absorption and many other issues remain unexplored.

A high-salt diet dissociates NO synthase-3 expression and NO production by the thick ascending limb

NO produced by endothelial NO synthase (NOS3) decreases sodium transport by the thick ascending limb (THAL). We found previously that 7 days of high salt (HS) increased THAL-NOS3 expression but not NO production. NOS3 phosphorylation regulates enzyme activity. We hypothesized that HS acutely increases NOS3 expression and NO production, and, over time, changes in NOS3 phosphorylation dissociate NO production from expression. NOS3 expression increased by 71+/-13%, 127+/-24%, and 69+/-16% at days 1, 3, and 7 of HS, respectively. At days 14 and 28, expression was back to normal salt. After 1 day of HS, NO production in response to 250 micromol/L L-arginine was elevated by 146% and, by day 3, returned to normal salt. Similar increases were found in response to endothelin-1. Inhibitors of NOS1/2 did not blunt the salt-induced increase in NO. Phosphorylation at Thr495, an inhibitory site, decreased by 39+/-8% at day 1 of HS and then increased by 116+/-18% at day 3. Phosphorylation at Ser633 and Ser1177 (stimulatory sites) decreased by &25% at day 1 and remained depressed at day 3. Superoxide production increased by 71% at day 1, decreased by 57% at day 3, and decreased by 55% at day 7. The NOS inhibitor L-NG-nitroarginine methyl ester did not alter superoxide levels at any time point. The addition of reduced nicotinamide-adenine dinucleotide phosphate and tetrahydrobiopterin had no effect on NO release after 3 days of HS. We conclude the following: (1) HS transiently increases NO production and NOS3 expression; (2) NOS3 expression and NO production are dissociated by HS; and (3) changes in phosphorylation explain how THAL NOS3 activity and expression are dissociated by HS.

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.

Endothelin stimulates endothelial nitric oxide synthase expression in the thick ascending limb

Endothelin-1 (ET-1) acutely inhibits NaCl reabsorption by the thick ascending limb (THAL) by activating the ET(B) receptor, stimulating endothelial nitric oxide synthase (eNOS), and releasing nitric oxide (NO). In nonrenal tissue, chronic exposure to ET-1 stimulates eNOS expression via the ET(B) receptor and activation of phosphatidylinositol 3-kinase (PI3K). We hypothesized that ET-1 increases eNOS expression in the THAL by binding to ET(B) receptors and stimulating PI3K. In primary cultures of medullary THALs treated for 24 h, eNOS expression increased by 36 +/- 18% with 0.01 nM ET-1, 123 +/- 30% with 0.1 nM (P < 0.05; n = 5), and 71 +/- 30% with 1 nM, whereas 10 nM had no effect. BQ-788, a selective ET(B) receptor antagonist, completely blocked stimulation of eNOS expression caused by 0.1 nM ET-1 (12 +/- 25 vs. 120 +/- 40% for ET-1 alone; P < 0.05; n = 5). BQ-123, a selective ET(A) receptor antagonist, did not affect the increase in eNOS caused by 0.1 nM ET-1. Sarafotoxin c (S6c; 0.1 microM), a selective ET(B) receptor agonist, increased eNOS expression by 77 +/- 30% (P < 0.05; n = 6). Wortmannin (0.01 microM), a PI3K inhibitor, completely blocked the stimulatory effect of 0.1 microM S6c (77 +/- 30 vs. -28 +/- 9%; P < 0.05; n = 6). To test whether the increase in eNOS expression heightens activity, we measured NO release in response to simultaneous treatment with l-arginine, ionomycin, and clonidine using a NO-sensitive electrode. NO release by control cells was 337 +/- 61 and 690 +/- 126 pA in ET-1-treated cells (P < 0.05; n = 5). Taken together, these data suggest that ET-1 stimulates THAL eNOS, activating ET(B) receptors and PI3K and thereby increasing NO production.

Inhibition of Na-K-ATPase in thick ascending limbs by NO depends on O2- and is diminished by a high-salt diet

A high-salt diet enhances nitric oxide (NO)-induced inhibition of transport in the thick ascending limb (THAL). Long exposures to NO inhibit Na-K-ATPase in cultured cells. We hypothesized that NO inhibits THAL Na-K-ATPase after long exposures and a high-salt diet would augment this effect. Rats drank either tap water or 1% NaCl for 7-10 days. Na-K-ATPase activity was assessed by measuring ouabain-sensitive ATP hydrolysis by THAL suspensions. After 2 h, spermine NONOate (SPM; 5 microM) reduced Na-K-ATPase activity from 0.44 +/- 0.03 to 0.30 +/- 0.04 nmol P(i).microg protein(-1).min(-1) in THALs from rats on a normal diet (P < 0.03). Nitroglycerin also reduced Na-K-ATPase activity (P < 0.04). After 20 min, SPM had no effect (change -0.07 +/- 0.05 nmol P(i).microg protein(-1).min(-1)). When rats were fed high salt, SPM did not inhibit Na-K-ATPase after 120 min. To investigate whether ONOO(-) formed by NO reacting with O(2)(-) was involved, we measured O(2)(-) production. THALs from rats on normal and high salt produced 35.8 +/- 0.3 and 23.7 +/- 0.8 nmol O(2)(-).min(-1).mg protein(-1), respectively (P < 0.01). Because O(2)(-) production differed, we studied the effects of the O(2)(-) scavenger tempol. In the presence of 50 microM tempol, SPM did not inhibit Na-K-ATPase after 120 min (0.50 +/- 0.05 vs. 0.52 +/- 0.07 nmol P(i).microg protein(-1).min(-1)). Propyl gallate, another O(2)(-) scavenger, also prevented SPM-induced inhibition of Na-K-ATPase activity. SPM inhibited pump activity in tubules from rats on high salt when O(2)(-) levels were increased with xanthine oxidase and hypoxanthine. We concluded that NO inhibits Na-K-ATPase after long exposures when rats are on a normal diet and this inhibition depends on O(2)(-). NO donors do not inhibit Na-K-ATPase in THALs from rats on high salt due to decreased O(2)(-) production.

Luminal flow induces eNOS activation and translocation in the rat thick ascending limb. II. Role of PI3-kinase and Hsp90

Endothelial nitric oxide synthase (eNOS) regulates NaCl absorption by the thick ascending limb of the loop of Henle (THAL). We found that augmenting luminal flow induces eNOS activation and translocation to the apical membrane of THALs (Ortiz PA, Hong NJ, and Garvin JL. Am J Physiol Renal Physiol 287: F274-F280, 2004). In other cells, eNOS activation by shear stress is mediated by phosphatidylinositol 3-OH kinase (PI3)-kinase. We hypothesized that luminal flow induces eNOS activation via PI3-kinase. Pretreatment of THALs with wortmannin, a PI3-kinase inhibitor, significantly reduced flow-induced nitric oxide (NO) release by 75% (from 53.6 +/- 6 to 13.2 +/- 5.7 pA/mm). Increasing luminal flow from 0 to 20 nl/min induced eNOS translocation to the apical membrane, whereas in the presence of wortmannin eNOS translocation was prevented (basolateral = 32 +/- 2%, middle = 38 +/- 1%, apical = 30 +/- 1%, n = 5, not significant vs. no flow). We next studied which PI3-kinase product mediates eNOS translocation. Addition of PI(3,4,5)P(3) (5 microM) in the absence of flow increased NO levels (P < 0.05) and induced eNOS translocation to the apical membrane (from 40 +/- 4 to 60 +/- 2% of total eNOS, n = 6, P < 0.05). Incubation with PI(3,4)P(2) or PI(4,5)P(2) did not change eNOS localization. We next tested whether heat shock protein (Hsp)90 is involved in eNOS translocation. The Hsp90 inhibitor geldanamycin blocked flow-induced eNOS translocation to the apical membrane (n = 6). Flow also induced translocation of Hsp90 to the apical membrane (from 35 +/- 2 to 57 +/- 2%; P < 0.05) in a PI3-kinase-dependent manner. We conclude that luminal flow induces eNOS translocation and activation in the THAL via PI3-kinase and that Hsp90 is involved in eNOS translocation to the apical membrane.

Trafficking and activation of eNOS in epithelial cells

In mammalian cells, formation of nitric oxide (NO) is catalysed by a family of enzymes termed NO synthases (NOS). There are three isoforms of this enzyme, NOS I, II and III. NOS III was originally cloned and identified in endothelial cells; thus this isoform is commonly called endothelial NOS (eNOS). The physiological role of NO produced by eNOS has been documented in most organs, including the brain, lung, cardiovascular system, kidney, liver, gastrointestinal tract and reproductive organs. The bioavailability of NO in these tissues is determined by the balance between its rate of production and degradation. The rate of NO production by eNOS is ultimately dependent on the activity of the enzyme. In the past years, co- and post-translational modifications such as myristoylation, palmitoylation, phosphorylation, protein-protein interactions and subcellular localization have been shown to play an important role in determining eNOS activity. In order to maintain specificity, the production of most signalling molecules occurs in an organized spatial and temporal pattern. Spatial localization of eNOS has been shown to be regulated by different mechanisms that control its targeting from the Golgi apparatus to the plasma membrane, correct compartmentalization within the membrane, and internalization from the plasma membrane to the cytoplasm after activation. Thus, regulated localization and trafficking of eNOS may be essential in regulating enzyme activity and maintaining the spatial and temporal organization of NO signalling in different cell types.

Gene transfer of eNOS to the thick ascending limb of eNOS-KO mice restores the effects of L-arginine on NaCl absorption

The thick ascending limb of the loop of Henle (THAL) plays an essential role in the regulation of sodium and water homeostasis by the kidney. l-Arginine, the substrate for nitric oxide synthase (NOS), decreases NaCl absorption by THALs. We hypothesized that eNOS produces the NO that regulates THAL NaCl transport and that selective expression of eNOS in the THAL of eNOS knockout(-/-) mice would restore the effects of l-arginine on NaCl absorption. eNOS-/- mice were anesthetized, the left kidney was exposed, and the renal interstitium was injected with recombinant adenoviral vectors that expressed green fluorescent protein (GFP) or eNOS driven by the promoter of the Na/K/2Cl cotransporter Ad-NKCC2GFP and Ad-NKCC2eNOS, respectively. In Ad-NKCC2eNOS-transduced kidneys, eNOS expression was detected 7 days after injection but was absent in Ad-NKCC2GFP-transduced kidneys. In THALs from eNOS-/- mice transduced with Ad-NKCC2eNOS, adding L-arginine increased DAF-2DA fluorescence, a measure of NO production, by 9.1+/-1.1% (P<0.05; n=5), but not in THALs transduced with Ad-NKCC2GFP. In THALs from eNOS-/- mice transduced with Ad-NKCC2eNOS, Cl absorption averaged 85.9+/-11.8 pmol/min per millimeter. Adding l-arginine (1 mmol/L) to the bath decreased Cl absorption to 59.7+/-11.0 pmol/min per millimeter (P<0.05; n=6). In THALs transduced with Ad-NKCC2GFP, Cl absorption averaged 96.0+/-21.0 pmol/min per millimeter. Adding L-arginine to the bath did not significantly affect Cl absorption (100.6+/-20.6 pmol/min per millimeter; n=4). We concluded that gene transfer of eNOS to the THAL of eNOS-/- mice restores L-arginine-induced inhibition of NaCl transport and NO production. These data indicate that eNOS is essential for the regulation of THAL NaCl transport by NO.