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

Anatomophysiology of the Henle's Loop: Emphasis on the Thick Ascending Limb

The loop of Henle plays a variety of important physiological roles through the concerted actions of ion transport systems in both its apical and basolateral membranes. It is involved most notably in extracellular fluid volume and blood pressure regulation as well as Ca²⁺ , Mg²⁺ , and acid-base homeostasis because of its ability to reclaim a large fraction of the ultrafiltered solute load. This nephron segment is also involved in urinary concentration by energizing several of the steps that are required to generate a gradient of increasing osmolality from cortex to medulla. Another important role of the loop of Henle is to sustain a process known as tubuloglomerular feedback through the presence of specialized renal tubular cells that lie next to the juxtaglomerular arterioles. This article aims at describing these physiological roles and at discussing a number of the molecular mechanisms involved. It will also report on novel findings and uncertainties regarding the realization of certain processes and on the pathophysiological consequences of perturbed salt handling by the thick ascending limb of the loop of Henle. Since its discovery 150 years ago, the loop of Henle has remained in the spotlight and is now generating further interest because of its role in the renal-sparing effect of SGLT2 inhibitors. © 2022 American Physiological Society. Compr Physiol 12:1-21, 2022.

New drug targets for hypertension: A literature review

Hypertension is one of the most prevalent cardiovascular diseases worldwide. However, in the population of resistant hypertension, blood pressure is difficult to control effectively. Moreover, antihypertensive drugs may have adverse effect currently. Hence, new therapeutic targets and treatments are needed to uncovered and exploited to control hypertension and its comorbidities. In the past, classical drug targets, such as the aldosterone receptor, aldosterone synthase, and ACE2/angiotensin 1-7/Mas receptor axis, have been investigated. Recently, vaccines and drugs targeting the gastrointestinal microbiome, which represent drug classes, have also been investigated for the management of blood pressure. In this review, we summarized current knowledge on classical and new drug targets and discussed the potential utility of new drugs in the treatment of hypertension.

The Role of Endothelin and Endothelin Antagonists in Chronic Kidney Disease

Background

Endothelins (ET) are a family of peptides that act as potent vasoconstrictors and pro-fibrotic growth factors. ET-1 is integral to renal and cardiovascular pathophysiology and exerts effects via autocrine, paracrine and endocrine signaling pathways tied to regulation of aldosterone, catecholamines, and angiotensin. In the kidney, ET-1 is critical to maintaining renal perfusion and controls glomerular arteriole tone and hemodynamics. It is hypothesized that ET-1 influences the progression of chronic kidney disease (CKD), and the objective of this review is to discuss the pathophysiology, and role of ET and endothelin receptor antagonists (ERAs) in CKD.

Summary

The use of ERAs in hypertensive nephropathy has the potential to decrease proteinuria, and in diabetic nephropathy has the potential to restore glycocalyx thickness, also decreasing proteinuria. Focal segmental glomerular sclerosis has no specific Food and Drug Administration-approved therapy currently, however, ERAs show promise in decreasing proteinuria and slowing tissue damage. ET-1 is a potential biomarker for autosomal dominant polycystic kidney disease progression and so it is thought that ERAs may be of some therapeutic benefit.

Key Messages

Multiple studies have shown the utility of ERAs in CKD. These agents have shown to reduce blood pressure, proteinuria, and arterial stiffness. However, more clinical trials are needed, and the results of active or recently concluded studies are eagerly awaited.

Diurnal Regulation of Renal Electrolyte Excretion: The Role of Paracrine Factors

Many physiological processes, including most kidney-related functions, follow specific rhythms tied to a 24-h cycle. This is largely because circadian genes operate in virtually every cell type in the body. In addition, many noncanonical genes have intrinsic circadian rhythms, especially within the liver and kidney. This new level of complexity applies to the control of renal electrolyte excretion. Furthermore, there is growing evidence that paracrine and autocrine factors, especially the endothelin system, are regulated by clock genes. We have known for decades that excretion of electrolytes is dependent on time of day, which could play an important role in fluid volume balance and blood pressure control. Here, we review what is known about the interplay between paracrine and circadian control of electrolyte excretion. The hope is that recognition of paracrine and circadian factors can be considered more deeply in the future when integrating with well-established neuroendocrine control of excretion.

Identification of NFAT5 as a transcriptional regulator of the EDN1 gene in collecting duct

The inner medullary collecting duct (IMCD) produces very high levels of endothelin-1 (ET-1) that acts as an autocrine inhibitor of IMCD Na⁺ and water reabsorption. Recent studies suggest that IMCD ET-1 production is enhanced by extracellular hypertonicity as can occur during high salt intake. Although NFAT5 has been implicated in the IMCD ET-1 hypertonicity response, no studies in any cell type have identified NFAT5 as a transcriptional regulator of the EDN1 gene; the current study examined this using a mouse IMCD cell line (IMCD3). Media hypertonicity increased IMCD3 ET-1 mRNA in a dose- and time-dependent manner associated with increased NFAT5 nuclear localization. Knockdown of NFAT5 using small-interfering RNA or by CRISPR/Cas9-mediated targeting of exon 4 of the NFAT5 gene reduced the ET-1 hypertonicity response. Chromatin immunoprecipitation using an NFAT5 antibody pulled down ET-1 promoter regions containing NFAT5 consensus binding sequences. Transfected ET-1 promoter reporter constructs revealed maximal hypertonicity-induced reporter activity in the proximal 1-kb region; mutation of the two NFAT5 consensus-binding sites in this region abolished hypertonicity-induced reporter activity. The 1-kb ET-1 promoter-reporter construct lost hypertonicity responsiveness when transfected in CRISPR/Cas9-induced NFAT5-deficient cells. In summary, these findings represent the first description that NFAT5 is a direct transcriptional regulator of the EDN1 gene in IMCD cells and point to a potentially important mechanism by which body Na⁺ homeostasis is maintained.

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.

Angiotensin II-induced hypertension blunts thick ascending limb NO production by reducing NO synthase 3 expression and enhancing threonine 495 phosphorylation

Thick ascending limbs reabsorb 30% of the filtered NaCl load. Nitric oxide (NO) produced by NO synthase 3 (NOS3) inhibits NaCl transport by this segment. In contrast, chronic angiotensin II (ANG II) infusion increases net thick ascending limb transport. NOS3 activity is regulated by changes in expression and phosphorylation at threonine 495 (T495) and serine 1177 (S1177), inhibitory and stimulatory sites, respectively. We hypothesized that NO production by thick ascending limbs is impaired by chronic ANG II infusion, due to reduced NOS3 expression, increased phosphorylation of T495, and decreased phosphorylation of S1177. Rats were infused with 200 ng·kg(-1)·min(-1) ANG II or vehicle for 1 and 5 days. ANG II infusion for 5 days decreased NOS3 expression by 40 ± 12% (P < 0.007; n = 6) and increased T495 phosphorylation by 147 ± 26% (P < 0.008; n = 6). One-day ANG II infusion had no significant effect. NO production in response to endothelin-1 was blunted in thick ascending limbs from ANG II-infused animals [ANG II -0.01 ± 0.06 arbitrary fluorescence units (AFU)/min vs. 0.17 ± 0.02 AFU/min in controls; P < 0.01]. This was not due to reduced endothelin-1 receptor expression. Phosphatidylinositol 3,4,5-triphosphate (PIP3)-induced NO production was also reduced in ANG II-infused rats (ANG II -0.07 ± 0.06 vs. 0.13 ± 0.04 AFU/min in controls; P < 0.03), and this correlated with an impaired ability of PIP3 to increase S1177 phosphorylation. We conclude that in ANG II-induced hypertension NO production by thick ascending limbs is impaired due to decreased NOS3 expression and altered phosphorylation.

Angiotensin II-induced hypertension increases plasma membrane Na pump activity by enhancing Na entry in rat thick ascending limbs

Thick ascending limbs (TAL) reabsorb 30% of the filtered NaCl load. Na enters the cells via apical Na-K-2Cl cotransporters and Na/H exchangers and exits via basolateral Na pumps. Chronic angiotensin II (ANG II) infusion increases net TAL Na transport and Na apical entry; however, little is known about its effects on the basolateral Na pump. We hypothesized that in rat TALs Na pump activity is enhanced by ANG II-infusion, a model of ANG II-induced hypertension. Rats were infused with 200 ng·kg(-1)·min(-1) ANG II or vehicle for 7 days, and TAL suspensions were obtained. We studied plasma membrane Na pump activity by measuring changes in 1) intracellular Na (Nai) induced by ouabain; and 2) ouabain-sensitive oxygen consumption (QO2). We found that the ouabain-sensitive rise in Nai in TALs from ANG II-infused rats was 12.8 ± 0.4 arbitrary fluorescent units (AFU)·mg(-1)·min(-1) compared with only 9.9 ± 1.1 AFU·mg(-1)·min(-1) in controls (P < 0.024). Ouabain-sensitive oxygen consumption was 17 ± 5% (P < 0.043) greater in tubules from ANG II-treated than vehicle rats. ANG II infusion did not alter total Na pump expression, the number of Na pumps in the plasma membrane, or the affinity for Na. When furosemide (1.1 mg·kg(-1)·day(-1)) was coinfused with ANG II, no increase in plasma membrane Na pump activity was observed. We concluded that in ANG II-induced hypertension Na pump activity is increased in the plasma membrane of TALs and that this increase is caused by the chronically enhanced Na entry occurring in this model.

Blood pressure, artery size, and artery compliance parallel bone size and strength in mice with differing ece1 expression

The recombinant congenic mouse strains HcB-8 and HcB-23 differ in femoral shape, size, and strength, with HcB-8 femora being more gracile, more cylindrical, weaker, and having higher Young's modulus. In previous work, we mapped a robust, pleiotropic quantitative trait locus for these bone traits. Ece1, encoding endothelin converting enzyme 1, is a positional candidate gene for this locus, and was less expressed in HcB-8 bone. We hypothesized that the same genetic factors would impose analogous developmental trajectories on arteries to those in bones. Cardiovascular hemodynamics and biomechanics of carotids were measured in adult HcB-8 and HcB-23 mice. Biological differences in heart and arteries were examined at mRNA and protein levels. As in bone, Ece1 expression was higher in HcB-23 heart and arteries (p &lt; 0.05), and its expression was correlated with that of the endothelin B type receptor target Nos3, encoding endothelial nitric oxide synthase. HcB-8 mice had higher ambulatory blood pressure (p &lt; 0.005) than HcB-23 mice. Ex vivo, at identical pressures, HcB-8 carotid arteries had smaller diameters and lower compliance (p &lt; 0.05), but the same elastic modulus compared to HcB-23 carotid arteries. HcB-8 hearts were heavier than HcB-23 hearts (p &lt; 0.01). HcB-8 has both small, stiff bones and small, stiff arteries, lower expression of Ece1 and Nos3, associated in each case with less favorable function. These findings suggest that endothelin signaling could serve as a nexus for the convergence of skeletal and vascular modeling, providing a potential mechanism for the epidemiologic association between skeletal fragility and atherosclerosis.

Membrane-associated aquaporin-1 facilitates osmotically driven water flux across the basolateral membrane of the thick ascending limb

The thick ascending limb of the loop of Henle (TAL) reabsorbs ∼30% of filtered NaCl but is impermeable to water. The observation that little water traverses the TAL indicates an absence of water channels at the apical membrane. Yet TAL cells swell when peritubular osmolality decreases indicating that water channels must be present in the basolateral side. Consequently, we hypothesized that the water channel aquaporin-1 (AQP1) facilitates water flux across the basolateral membrane of TALs. Western blotting revealed AQP1 expression in microdissected rat and mouse TALs. Double immunofluorescence showed that 95 ± 2% of tubules positive for the TAL-specific marker Tamm-Horsfall protein were also positive for AQP1 (n = 6). RT-PCR was used to demonstrate presence of AQP1 mRNA and the TAL-specific marker NKCC2 in microdissected TALs. Cell surface biotinylation assays showed that 23 ± 3% of the total pool of AQP1 was present at the TAL basolateral membrane (n = 7). To assess the functional importance of AQP1 in the basolateral membrane, we measured the rate of cell swelling initiated by decreasing peritubular osmolality as an indicator of water flux in microdissected TALs. Water flux was decreased by ∼50% in Aqp1 knockout mice compared with wild-types (4.0 ± 0.8 vs. 8.9 ± 1.7 fluorescent U/s, P < 0.02; n = 7). Furthermore, arginine vasopressin increased TAL AQP1 expression by 135 ± 17% (glycosylated) and 41 ± 11% (nonglycosylated; P < 0.01; n =5). We conclude that 1) the TAL expresses AQP1, 2) ∼23% of the total pool of AQP1 is localized to the basolateral membrane, 3) AQP1 mediates a significant portion of basolateral water flux, and 4) AQP1 is upregulated in TALs of rats infused with dDAVP. AQP1 could play an important role in regulation of TAL cell volume during changes in interstitial osmolality, such as during a high-salt diet or water deprivation.

Tumor necrosis factor α decreases nitric oxide synthase type 3 expression primarily via Rho/Rho kinase in the thick ascending limb

Inappropriate Na(+) reabsorption by thick ascending limbs (THALs) induces hypertension. NO produced by NO synthase type 3 (NOS3) inhibits NaCl reabsorption by THALs. Tumor necrosis factor α (TNF-α) decreases NOS3 expression in endothelial cells and contributes to increases in blood pressure. However, the effects of TNF-α on THAL NOS3 and the signaling cascade are unknown. TNF-α activates several signaling pathways, including Rho/Rho kinase (ROCK), which is known to reduce NOS3 expression in endothelial cells. Therefore, we hypothesized that TNF-α decreases NOS3 expression via Rho/ROCK in rat THAL primary cultures. THAL cells were incubated with either vehicle or 1 nmol/L of TNF-α for 24 hours, and NOS3 expression was measured by Western blot. TNF-α decreased NOS3 expression by 51 ± 6% (P<0.002) and blunted stimulus-induced NO production. A 10-minute treatment with TNF-α stimulated RhoA activity by 60 ± 23% (P<0.04). Inhibition of Rho GTPase with 0.05 μg/mL of C3 exoenzyme blocked TNF-α-induced reductions in NOS3 expression by 30 ± 8% (P<0.02). Inhibition of ROCK with 10 μmol/L of H-1152 blocked TNF-α-induced decreases in NOS3 expression by 66 ± 15% (P<0.001). Simultaneous inhibition of Rho and ROCK had no additive effect. Myosin light chain kinase, NO, protein kinase C, mitogen-activated kinase kinase, c-Jun amino terminal kinases, and Rac-1 were also not involved in TNF-α-induced decreases in NOS3 expression. We conclude that TNF-α decreases NOS3 expression primarily via Rho/ROCK in rat THALs. These data suggest that some of the beneficial effects of ROCK inhibitors in hypertension could be attributed to the mitigation of TNF-α-induced reduction in NOS3 expression.

New concepts in endothelin control of sodium balance

1. Endothelin (ET)-1, which was originally found to be secreted by the vascular endothelium, is highly expressed in the kidney, particularly in the renal medulla. 2. Recent studies using genetic models have provided significant breakthroughs in the role of ET-1 in the kidney. For example, ET-1 in the medullary collecting duct physiologically regulates water and salt reabsorption, thereby controlling blood pressure. Surprisingly, to explain the blood pressure regulation both ET(A) and ET(B) receptors are necessary in collecting duct. In fact, we recently revealed that ET(A) receptor stimulation in the renal medulla was natriuretic and diuretic. 3. The expression and secretion of ET-1 in the renal medulla are regulated by multiple mechanisms, such as changes in osmolality, exaggerated renin-angiotensin system activity and hypoxia. The changes in the renal medullary ET system are likely to work as compensatory 'protective' natriuretic factors in response to high sodium exposure in the kidney. 4. In the present review, we focus on recent publications that describe our current knowledge of the functional role of renal medullary ET-1, including the recently characterized actions of ET(A) receptors, the second messenger systems, mechanisms of stimulating ET-1 production and how the ET system is involved in the development of hypertension.

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.

Endothelin-A receptor blockade slows the progression of renal injury in experimental renovascular disease

Endothelin (ET)-1, a potent renal vasoconstrictor with mitogenic properties, is upregulated by ischemia and has been shown to induce renal injury via the ET-A receptor. The potential role of ET-A blockade in chronic renovascular disease (RVD) has not, to our knowledge, been previously reported. We hypothesized that chronic ET-A receptor blockade would preserve renal hemodynamics and slow the progression of injury of the stenotic kidney in experimental RVD. Renal artery stenosis, a major cause of chronic RVD, was induced in 14 pigs and observed for 6 wk. In half of the pigs, chronic ET-A blockade was initiated (RVD+ET-A, 0.75 mg·kg(-1)·day(-1)) at the onset of RVD. Single-kidney renal blood flow, glomerular filtration rate, and perfusion were quantified in vivo after 6 wk using multidetector computer tomography. Renal microvascular density was quantified ex vivo using three-dimensional microcomputer tomography, and growth factors, inflammation, apoptosis, and fibrosis were determined in renal tissue. The degree of stenosis and increase in blood pressure were similar in RVD and RVD+ET-A pigs. Renal hemodynamics, function, and microvascular density were decreased in the stenotic kidney but preserved by ET-A blockade, accompanied by increased renal expression of vascular endothelial growth factor, hepatocyte growth factor, and downstream mediators such as phosphorilated-Akt, angiopoietins, and endothelial nitric oxide synthase. ET-A blockade also reduced renal apoptosis, inflammation, and glomerulosclerosis. This study shows that ET-A blockade slows the progression of renal injury in experimental RVD and preserves renal hemodynamics, function, and microvascular density in the stenotic kidney. These results support a role for ET-1/ET-A as a potential therapeutic target in chronic RVD.

Physiology of endothelin and the kidney

Since its discovery in 1988 as an endothelial cell-derived peptide that exerts the most potent vasoconstriction of any known endogenous compound, endothelin (ET) has emerged as an important regulator of renal physiology and pathophysiology. This review focuses on how the ET system impacts renal function in health; it is apparent that ET regulates multiple aspects of kidney function. These include modulation of glomerular filtration rate and renal blood flow, control of renin release, and regulation of transport of sodium, water, protons, and bicarbonate. These effects are exerted through ET interactions with almost every cell type in the kidney, including mesangial cells, podocytes, endothelium, vascular smooth muscle, every section of the nephron, and renal nerves. In addition, while not the subject of the current review, ET can also indirectly affect renal function through modulation of extrarenal systems, including the vasculature, nervous system, adrenal gland, circulating hormones, and the heart. As will become apparent, these pleiotropic effects of ET are of fundamental physiologic importance in the control of renal function in health. In addition, to help put these effects into perspective, we will also discuss, albeit to a relatively limited extent, how alterations in the ET system can contribute to hypertension and kidney disease.

Glycyrrhizin and glycyrrhetinic acid directly modulate rat cardiac performance

Root extract of liquorice is traditionally used to treat several diseases. Liquorice-derived constituents present several biological actions. In particular, glycyrrhizin and its aglycone, glycyrrhetinic acid, exhibit well-known cardiovascular properties. The aim of this research was to explore the direct cardiac activity of glycyrrhizin and glycyrrhetinic acid. The effects of synthetic glycyrrhizin and glycyrrhetinic acid were evaluated on the isolated and Langendorff perfused rat heart. The intracellular signaling involved in the effects of the two substances was analyzed on isolated and perfused heart and by Western blotting on cardiac extracts. Under basal conditions, both glycyrrhizin and glycyrrhetinic acid influenced cardiac contractility and relaxation. Glycyrrhizin induced significant positive inotropic and lusitropic effects starting from very low concentrations, while both inotropism and lusitropism were negatively affected by glycyrrhetinic acid. Both substances significantly increased heart rate. Analysis of the signal transduction mechanisms suggested that glycyrrhizin acts through the endothelin receptor type A/phospholipase C axis while glycyrrhetinic acid acts through endothelin receptor type B/Akt/nitric oxide synthase/nitric oxide axis. To our knowledge, these data reveal, for the first time, that both glycyrrhizin and glycyrrhetinic acid directly affect cardiac performance. Additional information on the physiological significance of these substances and their cardiac molecular targets may provide indication on their biomedical application.

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.

Angiotensin II stimulates thick ascending limb superoxide production via protein kinase C(α)-dependent NADPH oxidase activation

Angiotensin II (Ang II) stimulates thick ascending limb (TAL) O₂ production, but the receptor(s) and signaling mechanism(s)involved are unknown. The effect of Ang II on O₂. is generally attributed to the AT₁receptor. In some cells, Ang II stimulates protein kinase C (PKC), whose α isoform (PKCα) can activate NADPH oxidase. We hypothesized that in TALs, Ang II stimulates O₂. via AT₁and PKC α-dependent NADPH oxidase activation.In rat TALs, 1 nM Ang II stimulated O₂. from 0.760.17 to 1.97 0.21 nmol/min/mg (p < 0.001). An AT₁antagonist blocked the stimulatory effect of Ang II on O₂. (0.87 0.25 nmol/min/mg; p < 0.006), whereas an AT₂ antagonist had no effect (2.16 0.133 nmol/min/mg; p < 0.05 versus vehicle). Apocynin, an NADPH oxidase inhibitor, blocked Ang II-stimulated O₂by 90% (p <0.01). Ang II failed to stimulate O₂. in TALs from p47(phox) -/- mice (p < 0.02). Monitored by fluorescence resonance energy transfer, Ang II increased PKC activity from 0.02 0.03 to 0.13 0.02 arbitrary units (p < 0.03). A general PKC inhibitor, GF109203X, blocked the effect of Ang II on O₂(1.47 +/- .21 versus 2.72 +/- .47 nmol/min/mg with Ang II alone; p < 0.03). A PKCα- and ß-selective inhibitor, Gö6976, also blocked the stimulatory effect of Ang II on O₂. (0.59 +/- 0.15 versus 2.05 +/- 0.28 nmol/min/mg with Ang II alone; p < 0.001). To distinguish between PKC α and PKC ß, we used tubules expressing dominant-negative PKC α or -ß. In control TALs, Ang II stimulated O2. by 2.17 0.44 nmol/min/mg (p < 0.011). In tubules expressing dominant-negative PKC α, Ang II failed to stimulate O2. (change: -0.30 +/- 0.27 nmol/min/mg). In tubules expressing dominant-negative PKC ß1, Ang II stimulated O2. by 2.080.69 nmol/min/mg (p < 0.002). We conclude that Ang II stimulates TAL O₂production via activation of AT₁receptors and PKC α-dependent NADPH oxidase.

Angiotensin II stimulates thick ascending limb NO production via AT(2) receptors and Akt1-dependent nitric-oxide synthase 3 (NOS3) activation

Angiotensin II (Ang II) acutely stimulates thick ascending limb (TAL) NO via an unknown mechanism. In endothelial cells, activation of Ang II type 2 receptor (AT(2)) stimulates NO. Akt1 activates NOS3 by direct phosphorylation. We hypothesized that Ang II stimulates TAL NO production via AT(2)-mediated Akt1 activation, which phosphorylates NOS3 at serine 1177. We measured NO production by fluorescence microscopy. In isolated TALs, Ang II (100 nm) increased NO production by 1.1 +/- 0.2 fluorescence units/min (p < 0.01). Ang II increased cGMP accumulation by 4.9 +/- 1.3 fmol/microg (p < 0.01). Upon adding the AT(2) antagonist PD123319 (1 microm), Ang II failed to stimulate NO (0.1 +/- 0.1 fluorescence units/min; p < 0.001 versus Ang II); adding the AT(1) antagonist losartan (1 microm) resulted in Ang II stimulating NO by 0.9 +/- 0.1 fluorescence units/min. Akt inhibitor (5 microm) blocked Ang II-stimulated NO (-0.1 +/- 0.2 fluorescence units/min versus inhibitor alone). Phospho-Akt1 increased by 72% after 5 min (p < 0.006), returning to basal after 10 min. Phospho-Akt2 did not change after 5 min but increased by 115 and 163% after 10 and 15 min (p < 0.02). Phospho-Akt3 did not change. An AT(2) agonist increased pAkt1 by 78% (p < 0.02), PI3K inhibition blocked this effect. In TALs transduced with dominant negative Akt1, Ang II failed to stimulate NO (0.1 +/- 0.2 fluorescence units/min versus 1.2 +/- 0.2 for controls; p < 0.001). Ang II increased phospho-NOS3 at serine 1177 by 130% (p < 0.01) and 150% after 5 and 10 min (p < 0.02). Ang II increased phosphoNOS3 at serine 633 by 50% after 5 min (p < 0.01). Akt inhibition prevented NOS3 phosphorylation. We concluded that Ang II enhances TAL NO production via activation of AT(2) and Akt1-dependent phosphorylation of NOS3 at serines 1177 and 633.

Drug discovery for overcoming chronic kidney disease (CKD): the endothelin ET B receptor/nitric oxide system functions as a protective factor in CKD

Accelerated cardiovascular disease (CVD) is a frequent complication of renal disease. Chronic kidney disease (CKD) develops hypertension and dyslipidemia, which in turn can contribute to the progression of renal failure. There is general agreement that endothelin-1 (ET-1), which acts through the two subtypes of receptor ET(A) and ET(B), plays important physiological roles in the regulation of normal cardiovascular function and that excessive ET-1 production is linked to CVD and CKD. Although selective ET(A) or nonselective ET(A)/ET(B) receptor antagonisms have been recognized as a potential strategy for treatment of several cardiovascular disease, it remains unclear which of the antagonisms is suitable for the individuals with CKD because upregulation of the nitric oxide (NO) system via ET(B) receptor is responsible for renal function such as natriuresis, diuresis, and glomerular hemodynamics. Our findings clearly indicate that the blockade of ET receptors, in particular ET(A)-receptor antagonism, not only produces a potential renoprotective effect in CKD but also reduces the risk of CVD. In contrast, pharmacological blockade or genetic deficiency of ET(B) receptor seems to aggravate CKD and CVD in several experimental models of rats. Moreover, preliminary evidence in patients with CKD also suggests that both selective ET(A)- and nonselective ET(A)/ET(B)-receptor blockade decreases blood pressure but that selective ET(A) blockade has additional desirable effects on renal hemodynamics. Thus, at least in CKD, these findings support the notion that ET(B) receptor-mediated actions produce a renoprotective effect and that nonselective ET(A)/ET(B)-receptors blockade seem to offer no advantage over selective ET(A) antagonism, and if anything may potentially reduce the benefits.

Nitric oxide reduces flow-induced superoxide production via cGMP-dependent protein kinase in thick ascending limbs

We have shown that increased luminal flow induces O(2)(-) and nitric oxide (NO) production in thick ascending limbs (TALs). However, the interaction of flow-stimulated NO and O(2)(-) in TALs is unclear. We hypothesized that NO inhibits flow-induced O(2)(-) production in TALs via cGMP-dependent protein kinase (PKG). We measured flow-stimulated O(2)(-) production in rat TALs using dihydroethidium in the absence and presence of L-arginine (0.3 mM), the substrate for NO synthase. The addition of L-arginine reduced flow-induced net O(2)(-) production from 68 +/- 9 to 17 +/- 4 AU/s (P < 0.002). The addition of the NO synthase inhibitor N(G)-nitro-l-arginine methyl ester (L-NAME; 5 mM) in the presence of L-arginine stimulated production (L-arginine: 15 +/- 4 AU/s vs. L-arginine + L-NAME: 63 +/- 7 AU/s; P < 0.002). The guanylate cyclase inhibitor LY-83583 (10 microM) also enhanced flow-induced net O(2)(-) production in the presence of L-arginine (L-arginine: 7 +/- 4 AU/s vs. L-arginine + LY-83583: 53 +/- 7 AU/s; P < 0.01). In the presence of LY-83583, L-arginine only reduced flow-induced net O(2)(-) by 36% (LY-83583: 80 +/- 7 AU/s vs. LY-83583 + L-arginine: 51 +/- 3 AU/s; P < 0.006). The cGMP analog dibutyryl (db)-cGMP reduced flow-induced net O(2)(-) from 39 +/- 9 to 7 +/- 3 AU/s (P < 0.03). The PKG inhibitor KT-5823 (5 microM) partially restored flow-induced net O(2)(-) in the presence of L-arginine (L-arginine: 4 +/- 4 AU/s vs. L-arginine + KT-5823: 32 +/- 9 AU/s; P < 0.03) and db-cGMP (db-cGMP: 9 +/- 7 AU/s vs. db-cGMP + KT-5823: 54 +/- 5 AU/s; P < 0.01). Phosphodiesterase II inhibition had no effect on arginine-inhibited O(2)(-) production. We conclude that 1) NO reduces flow-stimulated O(2)(-) production, 2) this occurs primarily via the cGMP/PKG pathway, and 3) O(2)(-) scavenging by NO plays a minor role.

Angiotensin II decreases nitric oxide synthase 3 expression via nitric oxide and superoxide in the thick ascending limb

NO produced by NO synthase type 3 (NOS3) in medullary thick ascending limbs (mTHALs) inhibits Cl(-) reabsorption. Acutely, angiotensin II stimulates thick ascending limb NO production. In endothelial cells, NO inhibits NOS3 expression. Therefore, we hypothesized that angiotensin II decreases NOS3 expression via NO in mTHALs. After 24 hours, 10 and 100 nmol/L of angiotensin II decreased NOS3 expression by 23+/-9% (n=6; P<0.05) and 50+/-5% (n=7; P<0.001), respectively, in primary cultures of rat mTHALs. NO synthase inhibition by 4 mmol/L of N(G)-nitro-L-arginine methyl ester hydrochloride prevented angiotensin II from decreasing NOS3 expression (Delta=-5+/-8%; n=5). In the presence of N(G)-nitro-L-arginine methyl ester hydrochloride, the addition of exogenous NO (1 micromol/L spermine NONOate) restored the angiotensin II-induced decreases in NOS3 expression (-22+/-6%; n=7; P<0.013). In addition, NO scavenging with 10 micromol/L of carboxy-PTIO abolished the effect of angiotensin II in NOS3 expression (Delta=-1+/-8% versus carboxy-PTIO alone; n=6). Angiotensin II increases superoxide, and superoxide scavenges NO. Thus, we tested whether scavenging superoxide enhances the angiotensin II-induced reduction in NOS3 expression. Surprisingly, treatment with 100 micromol/L of Tempol, a superoxide dismutase mimetic, blocked the angiotensin II-induced decrease in NOS3 expression (Delta=-3+/-7%; n=6). This effect was not because of increased hydrogen peroxide. We concluded that angiotensin II-induced decreases in NOS3 expression in mTHALs require both NO and superoxide. Decreased NOS3 expression by angiotensin II in mTHALs could contribute to increased salt retention observed in angiotensin II-induced hypertension.

Endothelin-1 inhibits thick ascending limb transport via Akt-stimulated nitric oxide production

Endothelin-1 inhibits sodium reabsorption in the thick ascending limb (THAL) via stimulation of nitric oxide (NO) production. The mechanism whereby endothelin-1 stimulates THAL NO is unknown. We hypothesized that endothelin-1 stimulates THAL NO production by activating phosphatidylinositol 3-kinase (PI3K), stimulating Akt activity, and phosphorylating NOS3 at Ser1177. This enhances NO production and inhibits sodium transport. We measured 1) NO production by fluorescence microscopy using DAF2-DA, 2) Akt activity using a fluorescence resonance energy transfer-based Akt reporter, 3) phosphorylated NOS3 and Akt by Western blotting, and 4) NKCC2 activity by fluorescence microscopy. In isolated THAL, endothelin-1 (1 nmol/liter) increased NO production from 0.23 +/- 0.24 to 2.81 +/- 0.32 fluorescence units/min (p < 0.001; n = 5) but failed to stimulate NO production in THALs isolated from NOS3-/- mice. Wortmannin (150 nmol/liter), a PI3K inhibitor, reduced endothelin-1-stimulated NO by 83% (0.49 +/- 0.13 versus 3.31 +/- 0.49 fluorescence units/min for endothelin-1 alone; p < 0.006; n = 5). Endothelin-1 stimulated Akt activity by 0.16 +/- 0.02 arbitrary units as measured by fluorescence resonance energy transfer (p < 0.001; n = 5) and increased phosphorylation of Akt at Ser473 by 56 +/- 11% (p < 0.002; n = 7). Dominant-negative Akt blocked endothelin-1-induced NO by 60 +/- 8% (p < 0.001 versus control; n = 6), and an Akt inhibitor had a similar effect. Endothelin-1 increased phosphorylation of NOS3 at Ser1177 by 89 +/- 24% (p < 0.01; n = 7) but had no effect on Ser633. Endothelin-1 inhibited NKCC2 activity, an effect that was blocked by dominant-negative Akt and NOS inhibition. We conclude that endothelin-1 stimulates THAL NO production by activating PI3K, stimulating Akt activity, and phosphorylating NOS3 at Ser1177. This enhances NO production and inhibits sodium transport.

Collecting duct-derived endothelin regulates arterial pressure and Na excretion via nitric oxide

Mice with a collecting duct-specific deletion of endothelin-1 are hypertensive and have impaired Na excretion. Because endothelin-1 activates NO synthase (NOS) in the collecting duct, we hypothesized that impaired renal NO production in knockout mice exacerbates the hypertensive state. Control and knockout mice were treated chronically with N(G)-nitro-l-arginine methyl ester, and blood pressure (BP) and urinary nitrate/nitrite excretion were assessed. On a normal Na diet, knockout systolic BP was 18 mm Hg greater than in controls. N(G)-nitro-l-arginine methyl ester increased BP in control mice by 30 mm Hg and 10 mm Hg in collecting duct-specific deletion of endothelin-1 knockout mice, thereby abolishing the difference in systolic BP between the groups. A high-Na diet increased BP similarly in both groups. Urinary nitrate/nitrite excretion was lower in knockout mice than in controls on normal or high Na intake. In separate experiments, renal perfusion pressure was adjusted in anesthetized mice, and urinary nitrate/nitrite and Na excretion were determined. Similar elevations of BP increased urinary Na and nitrate/nitrite excretion in control mice but to a significantly lesser extent in knockout mice. Isoform-specific NOS activity and expression were determined in renal inner medulla homogenates from control and knockout mice. NOS1 and NOS3 activities were lower in knockout than in control mice given normal or high-Na diets. However, NOS1 or NOS3 protein expressions were similar in both groups on normal or high-Na intake. These data demonstrate that collecting duct-derived endothelin-1 is important in the following: (1) chronic N(G)-nitro-l-arginine methyl ester-induced hypertension; (2) full expression of pressure-dependent changes in sodium excretion; and (3) control of inner medullary NOS1 and NOS3 activity.

Endothelin and NOS1/nitric oxide signaling and regulation of sodium homeostasis

PURPOSE OF REVIEW

In general, the nitric oxide and endothelin signaling pathways in the kidney promote natriuresis. The basis for this statement will first be reviewed for each of these systems. Next, this review will outline the progression of data providing support for our hypothesis that an intra-renal mechanism of endothelin activation of ETB receptors stimulates NOS1 activity and nitric oxide production to promote sodium excretion.

RECENT FINDINGS

New information in recent years has provided considerable evidence that both nitric oxide and endothelin function to regulate sodium and water balance by the kidney. Furthermore, dysfunction of these pathways may play a role in salt-sensitivity and hypertension. While a strong picture has emerged to suggest these systems are important and powerful players in sodium homeostasis, many questions remain to be answered before we can apply these mechanisms to an understanding of clinical hypertension.

SUMMARY

Salt-sensitive hypertension contributes to the growing population of patients resistant to conventional antihypertensive therapy. Thus, a thorough understanding of the mechanisms related to the control of sodium excretion will allow a more focused approach for future therapeutic studies.

Endothelin(A) (ET(A)) and ET(B) receptor-mediated regulation of nitric oxide synthase 1 (NOS1) and NOS3 isoforms in the renal inner medulla

AIM

Our laboratory and others have shown that endothelin (ET)-1 directly stimulates nitric oxide (NO) production in inner medullary collecting duct (IMCD) cells. The goal of this study was to determine which NO synthase (NOS) isoforms in IMCD are sensitive to ET-1, and the role of ET(A) and ET(B) receptor activation in vivo and in vitro.

METHODS

NOS enzymatic activity and NOS isoform protein expression were examined in cultured IMCD-3 cells and isolated renal inner medulla. ET(B) receptor-deficient homozygous rats (sl/sl) have elevated levels of circulating ET-1 and lack a functional ET(B) signalling pathway in kidneys, and furthermore provides a unique model to study ET(A) receptor signalling in the renal inner medulla in vivo.

RESULTS

Incubation of IMCD-3 cells with exogenous ET-1 (50 nm) resulted in ET(A)-dependent increased NOS1 protein expression in IMCD-3 cells with no effect on NOS2 or NOS3 expression. ET(B) receptor antagonism has no effect on NOS expression in IMCD-3 cells. Consistent with in vitro results, cytosolic NOS1 protein expression was significantly greater in the renal inner medulla of sl/sl rats compared with heterozygous (sl/+) controls, with no alteration in NOS3 expression. In contrast to protein expression data, NOS1- and NOS3-specific enzymatic activities decreased in the cytosolic fraction from the renal inner medulla of sl/sl compared with sl/+.

CONCLUSION

These results provide evidence that both ET(A) and ET(B) receptors regulate NOS isoform activity in the renal inner medulla and specifically support the hypothesis that ET(A) receptor activation increases NOS1 expression.

Calcium regulation of endothelin-1 synthesis in rat inner medullary collecting duct

Collecting duct-derived endothelin-1 (ET-1) reduces blood pressure and inhibits Na and water reabsorption. Collecting duct ET-1 production is increased by volume expansion; however, the mechanism by which this occurs is unknown. We hypothesized that intracellular calcium, which is likely to be increased by volume expansion, regulates collecting duct ET-1 synthesis. Rat inner medullary collecting ducts (IMCD) were studied in primary culture. ET-1 release was decreased by 50–70% after chelation of intracellular calcium (BAPTA) or inhibition of CaM (W7) or CaMK (KN-93). These agents reduced ET-1 mRNA to a similar degree. CaM inhibition did not affect ET-1 mRNA stability. Transfection of IMCD with rat ET-1 promoter-luciferase constructs revealed maximal activity within 1.7 kb 5′ to the transcription start site; 5, 20, 35, and 90% of this activity were in the 0.08-, 0.37-, 1.0-, and 3.0-kb promoter regions, respectively. W7 markedly inhibited activity of the 3.0-kb but not 0.37- or 1.0-kb promoter regions. In contrast, W7 did not affect ET-1 release by rat aortic endothelial cells. Furthermore, transfected endothelial cells had maximal activity in the 0.37-kb region (as compared with the 1.7- and 3.0-kb regions), whereas W-7 had no effect on the activity of any of these promoter regions. In summary, IMCD ET-1 synthesis is regulated by calcium/CaM/CaMK-dependent pathways. The calcium/CaM-sensitive pathway is active in IMCD, but not endothelial cells. This suggests that IMCD-specific enhancer elements exist within the ET-1 promoter that confer unique calcium responsiveness.

Novel role of AQP-1 in NO-dependent vasorelaxation

Nitric oxide (NO) produced by endothelial cells diffuses to vascular smooth muscle cells to cause dilatation of the renal vasculature and other vessels. Although it is generally assumed that NO moves from cell to cell by free diffusion, we recently showed that aquaporin-1 (AQP-1) transports NO across cell membranes. AQP-1 is expressed in endothelial and vascular smooth muscle cells. We hypothesized that diffusion of NO into vascular smooth muscle cells and out of endothelial cells is facilitated by AQP-1, and transport of NO by AQP-1 is involved in endothelium-dependent relaxation. In intact aortic rings from AQP-1 −/− mice, vasorelaxation induced by acetylcholine (which increases endogenous NO) was reduced ( P < 0.0001 vs. control). No differences were found in the relaxation caused by intracellular delivery of NO or intracellular cGMP between strains. In endothelium-denuded aortic rings from AQP-1 −/− mice, the vasorelaxant capability of NO released in the extracellular space was reduced ( P < 0.0001 vs. control). Influx of NO (5 μM) into vascular smooth muscle cells was 0.17 ± 0.02 f.u./s for control and 0.07 ± 0.01 f.u./s for AQP-1 −/− mice, 62% lower ( P < 0.002). NO released by endothelial cells in response to 1 μM acetylcholine was 96.2 ± 17.7 pmol NO/mg for control and 41.9 ± 13.4 pmol NO/mg for AQP-1 −/− mice, 56% reduction ( P < 0.04). NOS3 expression was 1.33 ± 0.29 O.D. units for control and 3.84 ± 0.76 O.D. units for AQP-1 −/− mice, 188% increase ( P < 0.01). We conclude that 1) AQP-1 facilitates NO influx into vascular smooth muscle cells, 2) AQP-1 facilitates NO diffusion out of endothelial cells, and 3) transport of NO by AQP-1 is required for full expression of endothelium-dependent relaxation.

Role of PI3K and PKB/Akt in acute natriuretic and NO-mimetic effects of leptin

Apart from controlling energy balance, leptin, a peptide hormone secreted by white adipose tissue, is also involved in the regulation of cardiovascular function. Previous studies have documented that leptin stimulates natriuresis and nitric oxide (NO) production, but the mechanism of these effects is incompletely elucidated. We examined whether phosphoinositide 3-kinase (PI3K) and its downstream effector, protein kinase B/Akt are involved in acute natriuretic and NO-mimetic effects of leptin in anaesthetized rats. Leptin (1 mg/kg i.v.) induced a marked increase in natriuresis and this effect was abolished by pretreatment with either wortmannin (15 microg/kg) or LY294002 (0.6 mg/kg), two structurally different PI3K inhibitors. Moreover, leptin increased plasma concentration and urinary excretion of NO metabolites, nitrites+nitrates (NO(x)), and of NO second messenger, cyclic GMP. In addition, leptin increased NO(x) and cGMP in aortic tissue. The stimulatory effect of leptin on NO(x) and cGMP was prevented by PKB/Akt inhibitor, triciribine, but not by either wortmannin or LY294002. Triciribine had no effect on leptin-induced natriuresis. Leptin stimulated Akt phosphorylation at Ser(473) in aortic tissue but not in the kidney. These results suggest that leptin-induced natriuresis is mediated by PI3K but not Akt, whereas NO-mimetic effect of leptin results from PI3K-independent stimulation of Akt.

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.

Endothelin and nitric oxide mediate adaptation of the cortical collecting duct to metabolic acidosis

Endothelin (ET) and nitric oxide (NO) modulate ion transport in the kidney. In this study, we defined the function of ET receptor subtypes and the NO guanylate cyclase signaling pathway in mediating the adaptation of the rabbit cortical collecting duct (CCD) to metabolic acidosis. CCDs were perfused in vitro and incubated for 3 h at pH 6.8, and bicarbonate transport or cell pH was measured before and after acid incubation. Luminal chloride was reversibly removed to isolate H(+) and HCO(3)(-) secretory fluxes and to raise the pH of beta-intercalated cells. Acid incubation caused reversal of polarity of net HCO(3)(-) transport from secretion to absorption, comprised of a 40% increase in H(+) secretion and a 75% decrease in HCO(3)(-) secretion. The ET(B) receptor antagonist BQ-788, as well as the NO synthase inhibitor, N(G)-nitro-l-arginine methyl ester (l-NAME), attenuated the adaptive decrease in HCO(3)(-) secretion by 40%, but only BQ-788 inhibited the adaptive increase in H(+) secretion. There was no effect of inactive d-NAME or the ET(A) receptor antagonist BQ-123. Both BQ-788 and l-NAME inhibited the acid-induced inactivation (endocytosis) of the apical Cl(-)/HCO(3)(-) exchanger. The guanylate cyclase inhibitor LY-83583 and cGMP-dependent protein kinase inhibitor KT-5823 affected HCO(3)(-) transport similarly to l-NAME. These data indicate that signaling via the ET(B) receptor regulates the adaptation of the CCD to metabolic acidosis and that the NO guanylate cyclase component of ET(B) receptor signaling mediates downregulation of Cl(-)/HCO(3)(-) exchange and HCO(3)(-) secretion.

The endothelin system and its antagonism in chronic kidney disease

The incidence of chronic kidney disease (CKD) is increasing worldwide. Cardiovascular disease (CVD) is strongly associated with CKD and constitutes one of its major causes of morbidity and mortality. Treatments that slow the progression of CKD and improve the cardiovascular risk profile of patients with CKD are needed. The endothelins (ET) are a family of related peptides, of which ET-1 is the most powerful endogenous vasoconstrictor and the predominant isoform in the cardiovascular and renal systems. The ET system has been widely implicated in both CVD and CKD. ET-1 contributes to the pathogenesis and maintenance of hypertension and arterial stiffness and more novel cardiovascular risk factors such as oxidative stress and inflammation. Through these, ET also contributes to endothelial dysfunction and atherosclerosis. By reversal of these effects, ET antagonists may reduce cardiovascular risk. In particular relation to the kidney, antagonism of the ET system may be of benefit in improving renal hemodynamics and reducing proteinuria. ET likely also is involved in progression of renal disease, and data are emerging to suggest a synergistic role for ET receptor antagonists with angiotensin-converting enzyme inhibitors in slowing CKD progression.

Endothelin-1 stimulates NO production and inhibits cAMP accumulation in rat inner medullary collecting duct through independent pathways

Endothelin-1 (ET-1) inhibition of vasopressin (AVP)-stimulated cAMP accumulation in the collecting duct has been hypothesized to be mediated, at least in part, by nitric oxide (NO). To examine this, the effect of ET-1 on NO production by acutely isolated rat inner medullary collecting duct (IMCD) cell suspensions and the role of NO in mediating ET-1 effects on AVP-stimulated cAMP accumulation were studied. ET-1 dose dependently (first evident at 100 pM ET-1) increased IMCD NO production as determined by DAF-FM fluorescence. ET(B) receptor (BQ-788), but not ET(A) receptor (BQ-123), antagonism blocked this effect. Nonspecific NO synthase (NOS) inhibitors [N(G)-nitro-L-arginine methyl ester (L-NAME) or N(G)-monomethyl-L-arginine] or NOS-1 inhibitors (SMTC or VNIO) inhibited the ET-1 response, whereas NOS-2 or NOS-3 inhibitors (L-NAA or 1400W) were ineffective. ET-1 also increased cGMP accumulation. ET-1 caused a 35% reduction in AVP-stimulated cAMP levels; however, this response was not affected by L-NAME or SMTC. The addition of L-arginine, NADPH, tetrahydrobiopterin, or tempol (to reduce superoxide-dependent conversion of NO to peroxynitrate) did not affect the response. NO donors (SNAP or spermine NONOate), at concentrations that stimulated DAF-FM fluorescence and increased cGMP levels, did not alter AVP-stimulated cAMP accumulation in the IMCD cell suspensions. In conclusion, ET-1 stimulates IMCD NO production through activation of the ET(B) receptor and NOS-1. However, neither ET-1-mediated NO production nor NO donors inhibit AVP-stimulated cAMP accumulation, indicating that NO does not mediate ET-1 inhibition of cAMP production by the IMCD.

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.

Anatomically distinct activation of endothelin-3 and the L-arginine/nitric oxide pathway in the kidney with advanced aging

Aging is associated with spontaneous degenerative changes of renal function and structure. The aim of this study was to determine changes of the endothelin (ET) system and NO tissue bioactivity during the physiological aging process. Renal protein expression of ET-1 and ET-3, ETA, and ETB receptor mRNA expression, ET receptor binding and distribution, and tissue NO metabolite content were determined in adult, middle-aged, and senescent normotensive female Wistar rats. In senescent animals, medullary ET-3 content increased 3.4-fold (p<0.05 vs. adult), whereas aging did not affect ET-3 levels in the cortex. Local NO bioavailability, determined by NO metabolite tissue measurements, decreased in the cortex only. ET receptor binding capacity--predominantly due to ETB receptor binding--was lower in medulla than in cortex. Aging had no effect on ET-1 binding capacity or ET receptor distribution, whereas with advanced age gene expression of both receptors decreased. In conclusion, aging causes distinctive expressional changes of the renal endothelin system in otherwise healthy rats. The pronounced increase of endothelin-3 in the renal medulla is associated with preservation of local NO metabolite levels, changes not observed in the cortex. These findings could be important for pathologies and possibly therapy associated with renal aging.

Recent advances in the regulation of nitric oxide in the kidney

Nitric oxide (NO) plays important roles in the regulation of renal function and the long-term control of blood pressure. New roles of NO have been proposed recently in diabetes, nephrotoxicity, and pregnancy. NO derived from all 3 NOS isoforms contributes to the overall regulation of kidney function, and recent advances in our understanding of their regulation have been made lately. In this regard, substrate and cofactor availability play important roles in regulating nitric oxide synthase (NOS) activity not only by limiting enzyme activity but also by influencing the coupling of NOS with its cofactors, tetrahydrobiopterin and NADPH. Protein-protein interactions are now recognized to be important negative and positive regulators of NOS. Phosphorylation is another component of the mechanism whereby NOS is activated or deactivated. Increased NOS expression can also influence enzyme activity; however, the degree of expression does not always correlate with enzyme activity because increased NO levels can result in inhibition of NOS. Finally, other potential regulators of NOS such as endogenous L-arginine analogs may also be important. In this article, we summarize recent advances in the regulation of activity and expression of the NOS isoforms within the 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.