Epithelial Na channels and short-term renal response to salt deprivation

Severe Salt-Losing Syndrome and Hyperkalemia Induced by Adult Nephron-Specific Knockout of the Epithelial Sodium Channel α-Subunit

Systemic pseudohypoaldosteronism type 1 (PHA-1) is a severe salt-losing syndrome caused by loss-of-function mutations of the amiloride-sensitive epithelial sodium channel (ENaC) and characterized by neonatal life-threatening hypovolemia and hyperkalemia. The very high plasma aldosterone levels detected under hypovolemic or hyperkalemic challenge can lead to increased or decreased sodium reabsorption, respectively, through the Na(+)/Cl(-) cotransporter (NCC). However, the role of ENaC deficiency remains incompletely defined, because constitutive inactivation of individual ENaC subunits is neonatally lethal in mice. We generated adult inducible nephron-specific αENaC-knockout mice (Scnn1a(Pax8/LC1)) that exhibit hyperkalemia and body weight loss when kept on a regular-salt diet, thus mimicking PHA-1. Compared with control mice fed a regular-salt diet, knockout mice fed a regular-salt diet exhibited downregulated expression and phosphorylation of NCC protein, despite high plasma aldosterone levels. In knockout mice fed a high-sodium and reduced-potassium diet (rescue diet), although plasma aldosterone levels remained significantly increased, NCC expression returned to control levels, and body weight, plasma and urinary electrolyte concentrations, and excretion normalized. Finally, shift to a regular diet after the rescue diet reinstated the symptoms of severe PHA-1 syndrome and significantly reduced NCC phosphorylation. In conclusion, lack of ENaC-mediated sodium transport along the nephron cannot be compensated for by other sodium channels and/or transporters, only by a high-sodium and reduced-potassium diet. We further conclude that hyperkalemia becomes the determining factor in regulating NCC activity, regardless of sodium loss, in the ENaC-mediated salt-losing PHA-1 phenotype.

Acute effects of aldosterone on the epithelial Na channel in rat kidney

The acute effects of aldosterone administration on epithelial Na channels (ENaC) in rat kidney were examined using electrophysiology and immunodetection. Animals received a single injection of aldosterone (20 μg/kg body wt), which reduced Na excretion over the next 3 h. Channel activity was assessed in principal cells of cortical collecting ducts as amiloride-sensitive whole cell clamp current (INa). INa averaged 100 pA/cell, 20-30% of that reported for the same preparation under conditions of chronic stimulation. INa was negligible in control animals that did not receive hormone. The acute physiological response correlated with changes in ENaC processing and trafficking. These effects included increases in the cleaved forms of α-ENaC and γ-ENaC, assessed by Western blot, and increases in the surface expression of β-ENaC and γ-ENaC measured after surface protein biotinylation. These changes were qualitatively and quantitatively similar to those of chronic stimulation. This suggests that altered trafficking to or from the apical membrane is an early response to the hormone and that later increases in channel activity require stimulation of channels residing at the surface.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

Chronic angiotensin II infusion drives extensive aldosterone-independent epithelial Na+ channel activation

The inability of mineralocorticoid receptor (MR) blockade to reduce hypertension associated with high angiotensin (Ang) II suggests direct actions of Ang II to regulate tubular sodium reabsorption via the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. We used freshly isolated aldosterone-sensitive distal nephron from mice to delineate the synergism and primacy between aldosterone and Ang II in controlling functional ENaC activity. Inhibition of MR specifically prevented the increased number of functionally active ENaC, but not ENaC open probability elicited by a low sodium diet. In contrast, we found no functional role of glucocorticoid receptors in the regulation of ENaC activity by dietary salt intake. Simultaneous inhibition of MR and Ang II type 1 receptors ameliorated the enhanced ENaC activity caused by low dietary salt intake and produced significantly greater natriuresis than either inhibitor alone. Chronic systemic Ang II infusion induced more than 2 times greater increase in ENaC activity than observed during dietary sodium restriction. Importantly, ENaC activity remained greatly above control levels during maximal MR inhibition. We conclude that during variations in dietary salt intake both aldosterone and Ang II contribute complementarily to the regulation of ENaC activity in the aldosterone-sensitive distal nephron. In contrast, in the setting of Ang II-dependent hypertension, ENaC activity is upregulated well above the physiological range and is not effectively suppressed by inhibition of the aldosterone-MR axis. This provides a mechanistic explanation for the resistance to MR inhibition that occurs in hypertensive subjects having elevated intrarenal Ang II levels.

Aldosterone-independent regulation of the epithelial Na+ channel (ENaC) by vasopressin in adrenalectomized mice

The epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN) is under negative-feedback regulation by the renin-angiotensin-aldosterone system in protection of sodium balance and blood pressure. We test here whether aldosterone is necessary and sufficient for ENaC expression and activity in the ASDN. Surprisingly, ENaC expression and activity are robust in adrenalectomized (Adx) mice. Exogenous mineralocorticoid increases ENaC activity equally well in control and Adx mice. Plasma [AVP] is significantly elevated in Adx vs. control mice. Vasopressin (AVP) stimulates ENaC. Inhibition of the V(2) AVP receptor represses ENaC activity in Adx mice. The absence of aldosterone combined with elevated AVP release compromises normal feedback regulation of ENaC in Adx mice in response to changes in sodium intake. These results demonstrate that aldosterone is sufficient but not necessary for ENaC activity in the ASDN. Aldosterone-independent stimulation by AVP shifts the role of ENaC in the ASDN from protecting Na(+) balance to promoting water reabsorption. This stimulation of ENaC likely contributes to the hyponatremia of adrenal insufficiency.

Potassium excretion during antinatriuresis: perspective from a distal nephron model

Renal excretion of Na(+) and K(+) must be regulated independently within the distal nephron, but is complicated by the fact that changing excretion of one solute requires adjustments in the transport of both. It is long known that hypovolemia increases Na(+) reabsorption while impairing K(+) excretion, even when distal Na(+) delivery is little changed. Renewed interest in this micropuncture observation came with identification of the molecular defects underlying familial hyperkalemic hypertension (FHH), which also increases distal Na(+) reabsorption and impairs K(+) excretion. In this work, a mathematical model of the distal nephron (Weinstein AM. Am J Physiol Renal Physiol 295: F1353-F1364, 2008), including the distal convoluted tubule (DCT), connecting segment (CNT), and collecting duct (CD), is used to examine renal K(+) excretion during antinatriuresis. Within the model, Na(+) avidity is represented as the modulation of DCT NaCl reabsorption, and the K(+) secretion signal is an aldosterone-like effect on principal cells of the CNT and CD. The first model prediction is that changes in DCT NaCl reabsorption are not mediated by NaCl cotransporter density alone, but require additional adjustments of both peritubular Na-K-ATPase and KCl cotransport. A second observation is that the CNT response to increased DCT Na(+) reabsorption should not only stabilize CD K(+) delivery but also compensate for the compromise of K(+) excretion downstream, as low Na(+) delivery increases CD K(+) reabsorption. Such anticipatory regulation is seen with the aldosterone response of hypovolemia, while the FHH phenotype manifests enhanced DCT NaCl transport but a blunted aldosterone effect. The model emphasizes the need for two distinct signals to the distal nephron, regulating Na(+) excretion and K(+) excretion, in contrast to a single switch apportioning NaCl reabsorption and Na(+)-for-K(+) exchange.

Circadian regulation of renal function

Urinary excretion of water and all major electrolytes exhibit robust circadian oscillations. The 24-h periodicity has been well documented for several important determinants of urine formation, including renal blood flow, glomerular filtration, tubular reabsorption, and tubular secretion. Disturbance of the renal circadian rhythms is increasingly recognized as a risk factor for hypertension, polyuria, and other diseases and may contribute to renal fibrosis. The origin of these rhythms has been attributed to the reactive response of the kidney to circadian changes in volume and/or in the composition of extracellular fluids that are entrained by rest/activity and feeding/fasting cycles. However, numerous studies have shown that most of the renal excretory rhythms persist for long periods of time, even in the absence of periodic environmental cues. These observations led to the hypothesis of the existence of a self-sustained mechanism, enabling the kidney to anticipate various predictable circadian challenges to homeostasis. The molecular basis of this mechanism remained unknown until the recent discovery of the mammalian circadian clock made of a system of autoregulatory transcriptional/translational feedback loops, which have been found in all tissues studied, including the kidney. Here, we present a review of the growing evidence showing the involvement of the molecular clock in the generation of renal excretory rhythms.

Surface expression of sodium channels and transporters in rat kidney: effects of dietary sodium

The abundance of Na transport proteins in the luminal membrane of the rat kidney was assessed using in situ biotinylation and immunoblotting. When animals were fed an Na-deficient diet for 1 wk, the amounts of epithelial Na channel (ENaC) beta-subunit (beta-ENaC) and gamma-subunit (gamma-ENaC) and Na-Cl cotransporter (NCC) protein in the surface fraction increased relative to controls by 1.9-, 3.5-, and 1.5-fold, respectively. The amounts of the luminal Na/H exchanger (NHE3) and the luminal Na-K-2Cl cotransporter (NKCC2) did not change significantly. The increases in ENaC subunits were mimicked by administration of aldosterone for 1 wk, but the increase in NCC was not. When the animals were fed a high-Na (5% NaCl) diet for 1 wk, the surface expression of beta-ENaC increased by 50%, whereas that of the other membrane proteins did not change, relative to controls. The biochemical parameter most strongly affected by dietary Na was the abundance of the 65-kDa cleaved form of gamma-ENaC at the surface. This increased by 8.5-fold with Na depletion and decreased by 40% with Na loading. The overall 14-fold change reflected regulation of the total abundance of the subunit as well as the fraction of the subunit protein in the cleaved form. We conclude that cleavage of gamma-ENaC and its expression at the apical surface play a major role in the regulation of renal Na reabsorption.

K+ secretion in the rat kidney: Na+ channel-dependent and -independent mechanisms

Renal Na(+) and K(+) excretion was measured in rats with varying dietary K(+) intake. The requirement for channel-mediated distal nephron Na(+) reabsorption was assessed by infusing the animals with the K(+)-sparing diuretic amiloride via osmotic minipumps. At infusion rates of 2 nmol/min, the concentration of amiloride in the urine was 38 microM, corresponding to concentrations of 9-23 microM in the distal tubular fluid, sufficient to block >98% of Na(+) transport through apical Na(+) channels (ENaC). With a control K(+) intake (0.6% KCl), amiloride reduced K(+) excretion rates (U(K)V) from 0.85 +/- 0.15 to 0.05 +/- 0.01 micromol/min during the first 2 h of infusion, suggesting that distal nephron K(+) secretion was completely dependent on the activity of Na(+) channels. When K(+) intake was increased by feeding overnight with a diet containing 10% KCl, amiloride reduced U(K)V from 7.5 +/- 0.7 to 1.3 +/- 0.1 micromol/min despite an increased plasma K(+) of 9 mM, again suggesting a major but not exclusive role for the Na(+) channel-dependent pathway of K(+) secretion. The maximal measured rates of amiloride-sensitive K(+) excretion correspond well with estimates based on apical K(+) channel activity in distal nephron segments. However, when the animals were adapted to the high-K(+) diet for 7-9 days, the diuretic decreased U(K)V less, from 6.1 +/- 0.6 to 3.0 +/- 0.8 micromol/min, indicating an increasing fraction of K(+) excretion that was independent of Na(+) channels. This indicates the upregulation of a Na(+) channel-independent mechanism for secreting K(+).

Regulated sodium transport in the renal connecting tubule (CNT) via the epithelial sodium channel (ENaC)

The aldosterone-sensitive distal nephron (ASDN) includes the late distal convoluted tubule 2, the connecting tubule (CNT) and the collecting duct. The appropriate regulation of sodium (Na(+)) absorption in the ASDN is essential to precisely match urinary Na(+) excretion to dietary Na(+) intake whilst taking extra-renal Na(+) losses into account. There is increasing evidence that Na(+) transport in the CNT is of particular importance for the maintenance of body Na(+) balance and for the long-term control of extra-cellular fluid volume and arterial blood pressure. Na(+) transport in the CNT critically depends on the activity and abundance of the amiloride-sensitive epithelial sodium channel (ENaC) in the luminal membrane of the CNT cells. As a rate-limiting step for transepithelial Na(+) transport, ENaC is the main target of hormones (e.g. aldosterone, angiotensin II, vasopressin and insulin/insulin-like growth factor 1) to adjust transepithelial Na(+) transport in this tubular segment. In this review, we highlight the structural and functional properties of the CNT that contribute to the high Na(+) transport capacity of this segment. Moreover, we discuss some aspects of the complex pathways and molecular mechanisms involved in ENaC regulation by hormones, kinases, proteases and associated proteins that control its function. Whilst cultured cells and heterologous expression systems have greatly advanced our knowledge about some of these regulatory mechanisms, future studies will have to determine the relative importance of the various pathways in the native tubule and in particular in the CNT.

Preferential assembly of epithelial sodium channel (ENaC) subunits in Xenopus oocytes: role of furin-mediated endogenous proteolysis

The epithelial sodium channel (ENaC) is preferentially assembled into heteromeric alphabetagamma complexes. The alpha and gamma (not beta) subunits undergo proteolytic cleavage by endogenous furin-like activity correlating with increased ENaC function. We identified full-length subunits and their fragments at the cell surface, as well as in the intracellular pool, for all homo- and heteromeric combinations (alpha, beta, gamma, alphabeta, alphagamma, betagamma, and alphabetagamma). We assayed corresponding channel function as amiloride-sensitive sodium transport (I(Na)). We varied furin-mediated proteolysis by mutating the P1 site in alpha and/or gamma subunit furin consensus cleavage sites (alpha(mut) and gamma(mut)). Our findings were as follows. (i) The beta subunit alone is not transported to the cell surface nor cleaved upon assembly with the alpha and/or gamma subunits. (ii) The alpha subunit alone (or in combination with beta and/or gamma) is efficiently transported to the cell surface; a surface-expressed 65-kDa alpha ENaC fragment is undetected in alpha(mut)betagamma, and I(Na) is decreased by 60%. (iii) The gamma subunit alone does not appear at the cell surface; gamma co-expressed with alpha reaches the surface but is not detectably cleaved; and gamma in alphabetagamma complexes appears mainly as a 76-kDa species in the surface pool. Although basal I(Na) of alphabetagamma(mut) was similar to alphabetagamma, gamma(mut) was not detectably cleaved at the cell surface. Thus, furin-mediated cleavage is not essential for participation of alpha and gamma in alphabetagamma heteromers. Basal I(Na) is reduced by preventing furin-mediated cleavage of the alpha, but not gamma, subunits. Residual current in the absence of furin-mediated proteolysis may be due to non-furin endogenous proteases.

Chronic furosemide or hydrochlorothiazide administration increases H+-ATPase B1 subunit abundance in rat kidney

Furosemide administration stimulates distal acidification. This has been attributed to the increased lumen-negative voltage in the distal nephron, but the aspect of regulatory mechanisms of H+-ATPase has not been clear. The purpose of this study is to investigate whether chronic administration of diuretics alters the expression of H+-ATPase and whether electrogenic Na+ reabsorption is involved in this process. A 7-day infusion of furosemide or hydrochlorothiazide (HCTZ) lowered urine pH significantly. However, this effect of furosemide-induced distal acidification was not changed with amiloride-blocking electrogenic Na+ reabsorption. On immunoblotting, a polyclonal antibody against the H+-ATPase B1 subunit recognized a specific ∼56-kDa band in membrane fractions from the kidney. The protein abundance of H+-ATPase was significantly increased by furosemide and HCTZ infusion in both the cortex and outer medulla. Furosemide plus amiloride administration also increased the H+-ATPase protein abundance significantly. However, no definite subcellular redistribution of H+-ATPase was observed by furosemide ± amiloride infusion with immunohistochemistry. Chronic furosemide ± amiloride administration induced a translocation of pendrin to the apical membrane, while total protein abundance was not increased. The mRNA expression of H+-ATPase was not altered by furosemide ± amiloride infusion. We conclude that chronic administration of diuretics enhances distal acidification by increasing the abundance of H+-ATPase irrespective of electrogenic Na+ reabsorption. This upregulation of H+-ATPase in the intercalated cells may be the result of tubular hypertrophy by diuretics.

Sex and body-type interactions in the regulation of renal sodium transporter levels, urinary excretion, and activity in lean and obese zucker rats

BACKGROUND

Female humans and rodents are relatively protected against the development of hypertension and renal disease. Whether this protection is modified during insulin resistance and obesity, however, is not known.

OBJECTIVE

Because renal sodium reabsorption has a central role in determining blood pressure, we hypothesized that lean female rats would bave reduced renal expression, activity, and urinary excretion of 8 major sodium transporters/channels.

METHODS

Lean and obese, male and female Zucker rats (n = 4-8 per group) were fed progressively higher levels of dietary NaCl over a period of 54 days. Urinary excretion of renal sodium transport proteins was determined for 3 different dietary levels (0.04%, 0.4%, and 4%) of NaCl. With the high-NaCl diet, natriuretic responses to benzamil, furosemide, and thiazide were used as in vivo markers for activity of the epithelial sodium channel (ENaC), the bumetanide-sensitive Na-K-2C1 cotransporter (NKCC2), and the thiazide-sensitive NaCl cotransporter (NCC), respectively.

RESULTS

Female rats (of both body types) had lower plasma renin activity and insulin levels than their male counterparts. Likewise, immunoblotting revealed female rats had increased whole kidney abundance of NCC and of the alpha, beta, and gamma subunits of ENaC, as well as decreased abundance of the type 3 sodium hydrogen exchanger (NHE3), type 2 sodium phosphate cotransporter (NaPi-2), and alpha-1 sodium-potassium-adenosine triphosphatase (Na-K-ATPase), compared with males. Obese rats had reduced levels of NKCC2, NHE3, and gamma-ENaC, but higher levels of NaPi-2 and NCC. Urine excretion of sodium transporters in lean female rats was nearly undetectable, whereas obese rats of both sexes excreted markedly more NKCC2 and NCC, which agreed with greater natriuretic responses to thiazide and furosemide.

CONCLUSIONS

Obese female rats are similar to lean female rats with regard to the sex-distinct pattern of renal sodium transporters. However, obese female rats are more like obese male rats with regard to increased natriuretic response tofurosemide and thiazide, and to urine excretion of several transporters including NCC. Our results suggest that, with obesity, there is some loss of the protective female advantage.

Regulation of blood pressure, the epithelial sodium channel (ENaC), and other key renal sodium transporters by chronic insulin infusion in rats

Hyperinsulinemia is associated with hypertension. Dysregulation of renal distal tubule sodium reabsorption may play a role. We evaluated the regulation of the epithelial sodium channel (ENaC) and the thiazide-sensitive Na-Cl cotransporter (NCC) during chronic hyperinsulinemia in rats and correlated these changes to blood pressure as determined by radiotelemetry. Male Sprague-Dawley rats (∼270 g) underwent one of the following three treatments for 4 wk ( n = 6/group): 1) control; 2) insulin-infused plus 20% dextrose in drinking water; or 3) glucose water-drinking (20% dextrose in water). Mean arterial pressures were increased by insulin and glucose (mmHg at 3 wk): 98 ± 1 (control), 107 ± 2 (insulin), and 109 ± 3 (glucose), P < 0.01. Insulin (but not glucose) increased natriuretic response to benzamil (ENaC inhibitor) and hydrochlorothiazide (NCC inhibitor) on average by 125 and 60%, respectively, relative to control rats, suggesting increased activity of these reabsorptive pathways. Neither insulin nor glucose affected the renal protein abundances of NCC or the ENaC subunits (α, β, and γ) in kidney cortex, outer medulla, or inner medulla in a major way, as determined by immunoblotting. However, insulin and to some extent glucose increased apical localization of these subunits in cortical collecting duct principal cells, as determined by immunoperoxidase labeling. In addition, insulin decreased cortical “with no lysine” kinase (WNK4) abundance (by 16% relative to control), which may have increased NCC activity. Overall, insulin infusion increased blood pressure, and NCC and ENaC activity in rats. Increased apical targeting of ENaC and decreased WNK4 expression may be involved.

Regulation of the renal thiazide-sensitive Na-Cl cotransporter, blood pressure, and natriuresis in obese Zucker rats treated with rosiglitazone

Previously, we showed an increase in protein abundance of the renal thiazide-sensitive Na-Cl cotransporter (NCC) in young, prediabetic, obese Zucker rats relative to lean age mates (Bickel CA, Verbalis JF, Knepper MA, and Ecelbarger CA. Am J Physiol Renal Physiol 281: F639–F648, 2001). To test whether this increase correlated with increased thiazide sensitivity (NCC activity) and blood pressure, and could be modified by insulin-sensitizing agents, we treated lean and obese Zucker rats (9 wk old) with either a control diet or this diet supplemented with 3 mg/kg body wt rosiglitazone (RGZ), a peroxisomal proliferator-activated receptor subtype γ agonist and potent insulin-sensitizing agent, for 12 wk ( n = 9/group). The rise in blood pressure, measured continuously by radiotelemetry, was significantly blunted in the RGZ-treated obese rats. Similarly, blood glucose and urinary albumin were markedly decreased in these rats. RGZ-treated rats whether lean or obese excreted a NaCl load faster but excreted less sodium in response to hydrochlorothiazide, applied as a novel in vivo measure of NCC activity. Obese rats had increased renal protein abundance and urinary excretion of NCC; however, this was not significantly reduced by RGZ (densitometry in cortex homogenate − %lean control): 100 ± 9, 93 ± 4, 124 ± 9, and 141 ± 14 for lean control, lean RGZ, obese control, and obese RGZ, respectively. Subcellular localization, as evaluated by confocal microscopy and immunoblotting following differential centrifugation, of NCC was not different between rat groups. Overall, RGZ reduced blood pressure and thiazide sensitivity; however, the mechanism(s) did not seem to involve a decrease in NCC protein abundance or cellular location. Decreased NCC activity may have contributed to the maintenance of normotension in RGZ-treated obese rats.

Bradykinin regulates cyclooxygenase-2 in rat renal thick ascending limb cells

Cyclooxygenase-2 (COX-2) is constitutively expressed in a subset of thick ascending limb cells in the cortex and medulla and increases when the renin-angiotensin and kallikrein-kinin systems are activated. Although the contribution of angiotensin II to the regulation of COX-2 is known, the effects of bradykinin on COX-2 expression have not been determined in this nephron segment. We evaluated expression of B2 bradykinin receptors in thick ascending limb cells containing COX-2 and the effect of bradykinin on COX-2 expression in primary cultured medullary thick ascending cells. The presence of B2 receptors was studied in renal sections by immunohistochemistry with antibodies against B2, COX-2, and Tamm-Horsfall glycoprotein. B2 receptors were detected on the apical and basolateral portion of the thick ascending cells. These cells also contained COX-2, suggesting that COX-2 expression may be regulated via B2 receptor. Incubation of cultured medullary thick ascending cells with bradykinin (10(-7) to 10(-5) mol/L) induced a significant increase on COX-2 protein expression. Maximal expression of COX-2 was observed 4 hours after exposure to bradykinin (10(-7) mol/L), effect abolished by a B2 receptor antagonist (HOE-140; 10(-6) mol/L). Prostaglandin E2 production increased when these cells were challenged with bradykinin for 4 hours, indicating that COX-2 was enzymatically active. We have demonstrated (1) the presence of B2 receptors in thick ascending limb cells expressing COX-2 and (2) the stimulatory effect of bradykinin on COX-2 protein expression, via B2 receptors, in cultured medullary thick ascending cells. We suggest that bradykinin can affect ion transport in the thick ascending limb via a COX-2-mediated mechanism.

The epithelial sodium channel: from molecule to disease

Genetic analysis has demonstrated that Na absorption in the aldosterone-sensitive distal nephron (ASDN) critically determines extracellular blood volume and blood pressure variations. The epithelial sodium channel (ENaC) represents the main transport pathway for Na+ absorption in the ASDN, in particular in the connecting tubule (CNT), which shows the highest capacity for ENaC-mediated Na+ absorption. Gain-of-function mutations of ENaC causing hypertension target an intracellular proline-rich sequence involved in the control of ENaC activity at the cell surface. In animal models, these ENaC mutations exacerbate Na+ transport in response to aldosterone, an effect that likely plays an important role in the development of volume expansion and hypertension. Recent studies of the functional consequences of mutations in genes controlling Na+ absorption in the ASDN provide a new understanding of the molecular and cellular mechanisms underlying the pathogenesis of salt-sensitive hypertension.

B-type natriuretic peptide exerts broad functional opposition to transforming growth factor-beta in primary human cardiac fibroblasts: fibrosis, myofibroblast conversion, proliferation, and inflammation

The natriuretic peptides, including human B-type natriuretic peptide (BNP), have been implicated in the regulation of cardiac remodeling. Because transforming growth factor-beta (TGF-beta) is associated with profibrotic processes in heart failure, we tested whether BNP could inhibit TGF-beta-induced effects on primary human cardiac fibroblasts. BNP inhibited TGF-beta-induced cell proliferation as well as the production of collagen 1 and fibronectin proteins as measured by Western blot analysis. cDNA microarray analysis was performed on RNA from cardiac fibroblasts incubated in the presence or absence of TGF-beta and BNP for 24 and 48 hours. TGF-beta, but not BNP, treatment resulted in a significant change in the RNA profile. BNP treatment resulted in a remarkable reduction in TGF-beta effects; 88% and 85% of all TGF-beta-regulated mRNAs were affected at 24 and 48 hours, respectively. BNP opposed TGF-beta-regulated genes related to fibrosis (collagen 1, fibronectin, CTGF, PAI-1, and TIMP3), myofibroblast conversion (alpha-smooth muscle actin 2 and nonmuscle myosin heavy chain), proliferation (PDGFA, IGF1, FGF18, and IGFBP10), and inflammation (COX2, IL6, TNFalpha-induced protein 6, and TNF superfamily, member 4). Lastly, BNP stimulated the extracellular signal-related kinase pathway via cyclic guanosine monophosphate-dependent protein kinase signaling, and two mitogen-activated protein kinase kinase inhibitors, U0126 and PD98059, reversed BNP inhibition of TGF-beta-induced collagen-1 expression. These findings demonstrate that BNP has a direct effect on cardiac fibroblasts to inhibit fibrotic responses via extracellular signal-related kinase signaling, suggesting that BNP functions as an antifibrotic factor in the heart to prevent cardiac remodeling in pathological conditions.

Interleukin-1beta, transforming growth factor-beta1, and bradykinin attenuate cyclic AMP production by human pulmonary artery smooth muscle cells in response to prostacyclin analogues and prostaglandin E2 by cyclooxygenase-2 induction and downregulation of adenylyl cyclase isoforms 1, 2, and 4

Increased levels of inflammatory cytokines contribute to the pathophysiology of pulmonary hypertension. Prostacyclin (PGI2) analogues, which relax pulmonary vessels mainly through cAMP elevation, have a major therapeutic role. In this study, we show that prolonged incubation with bradykinin (BK), interleukin-1beta (IL-1beta), and transforming growth factor-beta1 (TGF-beta1) markedly impairs cAMP accumulation in human pulmonary artery smooth muscle cells in response to short-term incubation with prostaglandin E2 (PGE2) and the PGI2 analogues iloprost and carbaprostacyclin. A similar reduction in cAMP accumulation in response to a direct adenylyl cyclase activator, forskolin, suggested that the effect was attributable to downregulation of adenylyl cyclase. Reverse transcriptase-polymerase chain reaction studies showed downregulation of adenylyl cyclase isoforms 1, 2, and 4. The effect of IL-1beta, BK, and TGF-beta1 on cAMP levels was abrogated by the selective COX-2 inhibitor NS398. Furthermore, it was mimicked by prolonged incubation with the COX-2 product PGE2 and PGI2 analogues or the COX substrate arachidonic acid, suggesting that it was mediated by endogenous prostanoids produced by COX-2. Consistent with this, IL-1beta, BK, and TGF-beta1 all induced COX-2 and PGE2 release. These results show that BK, IL-1beta, and TGF-beta1 downregulate adenylyl cyclase in human pulmonary artery smooth muscle cells via COX-2 induction and prostanoid release. This suggests a novel mechanism whereby mediators and cytokines produced in pulmonary hypertension may impair the therapeutic effects of prostacyclin analogues such as iloprost and carbaprostacyclin.

Na channels in the rat connecting tubule

Epithelial Na channels were investigated using patch-clamp techniques in connecting tubule (CNT) segments isolated from rat kidney. Cell-attached patches with Li+ in the patch pipette contained channels with conductances for inward currents of 13-16 pS and slow opening and closing kinetics, similar to properties of Na channels in the cortical collecting tubule (CCT). Macroscopic amiloride-sensitive currents (INa) were also observed under whole cell clamp conditions. These currents were undetectable in cells from control rats but were large when the animals were infused with aldosterone (1,380+/-340 pA/cell at a holding potential of -100 mV) or fed a high-K diet (670+/-260 pA/cell) for 1 wk. Under both of these conditions, currents in cells of the CNT were two- to fourfold larger than currents in cells of the CCT of the same animals. In aldosterone-treated animals, currents in cells of the initial collecting tubule (iCT) were intermediate, such that the relative magnitude of INa was as follows: CNT > iCT > CCT. Quantitative analysis of the results suggests that the maximal capacity of the aggregate population of CNTs to reabsorb Na could be as high as 18 micromol/min, or approximately 10% of the filtered load of Na. This capacity is approximately 10 times higher than that of the CCT.

Differential effects of selective cyclooxygenase-2 inhibitors on endothelial function in salt-induced hypertension

BACKGROUND

In view of the ongoing controversy about potential differences in cardiovascular safety of selective cyclooxygenase (COX)-2 inhibitors (coxibs), we compared the effects of 2 different coxibs and a traditional NSAID on endothelial dysfunction, a well-established surrogate of cardiovascular disease, in salt-induced hypertension.

METHODS AND RESULTS

Salt-sensitive (DS) and salt-resistant (DR) Dahl rats were fed a high-sodium diet (4% NaCl) for 56 days. From days 35 to 56, diclofenac (6 mg x kg(-1) x d(-1); DS-diclofenac), rofecoxib (2 mg x kg(-1) x d(-1); DS-rofecoxib), celecoxib (25 mg x kg(-1) x d(-1); DS-celecoxib) or placebo (DS-placebo) was added to the chow. Blood pressure increased with sodium diet in the DS groups, which was more pronounced after diclofenac and rofecoxib treatment (P<0.005 versus DS-placebo) but was slightly decreased by celecoxib (P<0.001 versus DS-placebo). Sodium diet markedly reduced NO-mediated endothelium-dependent relaxations to acetylcholine (10-10-10-5 mol/L) in aortic rings of untreated hypertensive rats (P<0.005 versus DR-placebo). Relaxation to acetylcholine improved after celecoxib (P<0.005 versus DS-placebo and DS-rofecoxib) but remained unchanged after rofecoxib and diclofenac treatment. Vasoconstriction after nitric oxide synthase inhibition, indicating basal NO release, with N(omega)-nitro-L-arginine methyl ester (10-4 mol/L) was blunted in DS rats (P<0.05 versus DR-placebo), normalized by celecoxib, but not affected by rofecoxib or diclofenac. Indicators of oxidative stress, 8-isoprostane levels, were elevated in untreated DS rats on 4% NaCl (6.55+/-0.58 versus 3.65+/-1.05 ng/mL, P<0.05) and normalized by celecoxib only (4.29+/-0.58 ng/mL).

CONCLUSIONS

These data show that celecoxib but not rofecoxib or diclofenac improves endothelial dysfunction and reduces oxidative stress, thus pointing to differential effects of coxibs in salt-induced hypertension.

Collecting duct-specific gene inactivation of alphaENaC in the mouse kidney does not impair sodium and potassium balance

Aldosterone controls the final sodium reabsorption and potassium secretion in the kidney by regulating the activity of the epithelial sodium channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN). ASDN consists of the last portion of the distal convoluted tubule (late DCT), the connecting tubule (CNT), and the collecting duct (CD) (i.e., the cortical CD [CCD] and the medullary CD [MCD]). It has been proposed that the control of sodium transport in the CCD is essential for achieving sodium and potassium balance. We have tested this hypothesis by inactivating the alpha subunit of ENaC in the CD but leaving ENaC expression in the late DCT and CNT intact. Under salt restriction or under aldosterone infusion, whole-cell voltage clamp of principal cells of CCD showed no detectable ENaC activity, whereas large amiloride-sensitive currents were observed in control littermates. The animals survive well and are able to maintain sodium and potassium balance, even when challenged by salt restriction, water deprivation, or potassium loading. We conclude that the expression of ENaC in the CD is not a prerequisite for achieving sodium and potassium balance in mice. This stresses the importance of more proximal nephron segments (late DCT/CNT) to achieve sodium and potassium balance.

Sodium and calcium transport pathways along the mammalian distal nephron: from rabbit to human

The final adjustment of renal sodium and calcium excretion is achieved by the distal nephron, in which transepithelial ion transport is under control of various hormones, tubular fluid composition, and flow rate. Acquired or inherited diseases leading to deranged renal sodium and calcium balance have been linked to dysfunction of the distal nephron. Diuretic drugs elicit their effects on sodium balance by specifically inhibiting sodium transport proteins in the apical plasma membrane of distal nephron segments. The identification of the major apical sodium transport proteins allows study of their precise distribution pattern along the distal nephron and helps address their cellular and molecular regulation under various physiological and pathophysiological settings. This review focuses on the topological arrangement of sodium and calcium transport proteins along the cortical distal nephron and on some aspects of their functional regulation. The availability of data on the distribution of transporters in various species points to the strengths, as well as to the limitations, of animal models for the extrapolation to humans.