Differential gene regulation of renal salt entry pathways by salt load in the distal nephron of the rat

Dysregulation of renal aquaporins and epithelial sodium channel in lithium-induced nephrogenic diabetes insipidus

Lithium is used commonly to treat bipolar mood disorders. In addition to its primary therapeutic effects in the central nervous system lithium has a number of side effects in the kidney. The side effects include nephrogenic diabetes insipidus with polyuria, mild sodium wasting, and changes in acid/base balance. These functional changes are associated with marked structural changes in collecting duct cell composition and morphology, likely contributing to the functional changes. Over the past few years, investigations of lithium-induced renal changes have provided novel insight into the molecular mechanisms that are responsible for the disturbances in water, sodium, and acid/base metabolism. This includes dysregulation of renal aquaporins, epithelial sodium channel, and acid/base transporters. This review focuses on these issues with the aim to present this in context with clinically relevant features.

Dietary salt enhances benzamil-sensitive component of myogenic constriction in mesenteric arteries

Recent work from our laboratory indicates that epithelial Na(+) channel (ENaC) function plays an important role in modulating myogenic vascular reactivity. Increases in dietary sodium are known to affect vascular reactivity. Although previous studies have demonstrated that dietary salt intake regulates ENaC expression and activity in epithelial tissue, the importance of dietary salt on ENaC expression in vascular smooth muscle cells (VSMCs) and its role in myogenic constriction is unknown. Therefore, the goal of the present study was to determine whether dietary salt modulates ENaC expression and function in myogenic vasoconstriction. To accomplish this goal, we examined ENaC expression in freshly dispersed VSMCs and pressure-induced vasoconstrictor responses in isolated mesenteric resistance arteries from normotensive Sprague-Dawley rats fed a normal-salt (NS; 0.4% NaCl) or high-salt (HS; 8% NaCl for 2 wk) diet. VSMCs from the mesenteric arteries of NS-fed animals express alpha-, beta-, and gamma-ENaC. The HS diet reduced whole cell alpha- and gamma-ENaC and induced a pronounced translocation of beta-ENaC from intracellular regions toward the VSMC membrane (approximately 336 nm). Associated with this change in expression was a change in the importance of ENaC in pressure-induced constriction. Pressure-induced constriction in NS-fed animals was insensitive to ENaC inhibition with 1 microM benzamil, suggesting that ENaC proteins do not contribute to myogenic constriction in mesenteric arteries under NS intake. In contrast, ENaC inhibition blocked pressure-induced constriction in HS-fed animals. These data suggest that dietary sodium regulates ENaC expression and the quantitative importance of the vascular ENaC signaling pathway contributing to myogenic constriction.

Proinflammatory cytokines cause down-regulation of renal chloride entry pathways during sepsis

OBJECTIVE

Sepsis is the most important trigger for acute renal failure, with tubular dysfunction and collapse in urine concentration. As chloride plays a major role in the urinary concentrating mechanisms, we aimed to investigate the regulation of renal chloride entry pathways, such as kidney-specific chloride channel 1, kidney-specific chloride channel 2, Barttin, thiazide-sensitive Na+-Cl- cotransporter, renal outer medullary potassium channel, and Na+/K+-adenosine triphosphatase during sepsis.

DESIGN

Prospective animal trial.

SETTING

Laboratory of the Department of Anesthesiology.

SUBJECTS

Male C57/BL6 and B6129SF2/J mice and mice deficient for tumor necrosis factor-alpha, interleukin-1-receptor-1, interferon-gamma, or interleukin-6.

INTERVENTIONS

Mice were injected with lipopolysaccharide (LPS) or proinflammatory cytokines. Hemodynamic and renal variables, cytokine concentrations, and expression of renal chloride-reabsorbing systems were investigated. Experiments with cytokine knockout mice, renal artery-clipped mice, and mice treated with glucocorticoids, low-dose LPS, and sodium nitroprusside were performed.

MEASUREMENTS AND MAIN RESULTS

LPS-injected mice presented with decreased blood pressure and glomerular filtration rate, increased fractional chloride excretion, and depressed expression of renal chloride transporters/channels. Similar alterations were observed after application of tumor necrosis factor-alpha, interleukin-1beta, interferon-gamma, or interleukin-6. LPS-induced down-regulation of chloride transporters/channels was not affected in cytokine knockout mice. Glucocorticoid treatment inhibited LPS-induced increase of cytokine concentrations, diminished LPS-induced renal dysfunction, and attenuated the down-regulation of renal chloride transporters/channels. Injection of low-dose LPS increased renal tissue cytokines and down-regulated chloride entry pathways without arterial hypotension, indicating that renal ischemia due to systemic hypotension does not mediate down-regulation of renal chloride transporters/channels. In addition, renal ischemia induced by renal artery clipping or sodium nitroprusside administration did not influence chloride transporter/channel expression.

CONCLUSIONS

Our results demonstrate down-regulation of renal chloride transporters/channels during sepsis, which is probably mediated by proinflammatory cytokines and accounts for the development of LPS-induced tubular dysfunction. Our findings contribute to the understanding, on one hand, the failure of single-anticytokine strategies and, on the other hand, the beneficial effects of glucocorticoids in the therapy of septic patients.

Regulation of renal sodium transporters during severe inflammation

Sepsis-associated acute renal failure is characterized by decreased GFR and tubular dysfunction. The pathogenesis of endotoxemic tubular dysfunction with failure in urine concentration and increased fractional sodium excretion is poorly understood. This study investigated the regulation of renal sodium transporters during severe inflammation in vivo and in vitro. Injection of high-dosage LPS reduced BP and GFR, increased fractional sodium excretion, and strongly decreased the expression of Na(+)/H(+)-exchanger, renal outer medullary potassium channel, Na(+)-K(+)-2Cl(-) co-transporter, epithelial sodium channel, and Na(+)/K(+)-ATPase in mice. Also, injection of TNF-alpha, IL-1beta, or IFN-gamma decreased renal function and expression of renal sodium transporters. LPS-induced downregulation of sodium transporters was not affected in cytokine-knockout mice. However, supplementary glucocorticoid treatment, which inhibited LPS-induced increase of tissue cytokine concentrations, attenuated LPS-induced renal dysfunction and downregulation of tubular sodium transporters. Injection of low-dosage LPS increased renal tissue cytokines and downregulated renal sodium transporters without arterial hypotension. In vitro, in cortical collecting duct cells, cytokines also decreased expression of renal outer medullary potassium channel, epithelial sodium channel, and Na(+)/K(+)-ATPase. Renal hypoperfusion by renal artery clipping did not influence renal sodium transporter expression, in contrast to renal ischemia-reperfusion injury, which depressed transporter expression. These findings demonstrate downregulation of renal sodium transporters that likely accounts for tubular dysfunction during inflammation. These data suggest that alteration of renal sodium transporters during LPS-induced acute renal failure is mediated by cytokines rather than renal ischemia. However, in a complex in vivo model of severe inflammation, the possible presence and influence of renal hypoperfusion and reperfusion on the expression of renal sodium transporters cannot be completely excluded.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Guanylin and uroguanylin induce natriuresis in mice lacking guanylyl cyclase-C receptor

BACKGROUND

Guanylin (GN) and uroguanylin (UGN) are intestinally derived peptide hormones that are similar in structure and activity to the diarrhea-causing Escherichia coli heat-stable enterotoxins (STa). These secretagogues have been shown to affect fluid, Na+, K+, and Cl- transport in both the intestine and kidney, presumably by intracellular cyclic guanosine monophosphate (cGMP)-dependent signal transduction. However, the in vivo consequences of GN, UGN, and STa on renal function and their mechanism of action have yet to be rigorously tested.

METHODS

We hypothesized that intravenous administration of GN, UGN, or STa would cause an increase in natriuresis in wild-type mice via cGMP and guanylyl cyclase-C (GC-C, Gucy2c), the only known receptor for these peptide-hormones, and that the peptide-induced natriuresis would be blunted in genetically altered mice devoid of GC-C receptors (GC-C(-/-) null).

RESULTS

In wild-type mice using a modified renal clearance model, GN, UGN, and STa elicited significant natriuresis, kaliuresis, and diuresis as well as increased urinary cGMP levels in a time- and dose-dependent fashion. Absolute and fractional urinary sodium excretion levels were greatest approximately 40 minutes following a bolus infusion with pharmacologic doses of these peptides. Unexpectedly, GC-C(-/-) null mice also responded to the GN peptides similarly to that observed in wild-type mice. Glomerular filtration rate (GFR), blood pressure, and plasma cGMP in the mice (wild-type or GC-C(-/-) null) did not significantly vary between the vehicle- and peptide-treatment groups. The effects of UGN may also influence long-term renal function due to down-regulation of the Na+/K+ ATPase gamma-subunit and the Cl- channel ClC-K2 by 60% and 75%, respectively, as assessed by differential display polymerase chain reaction (PCR) (DD-PCR) and Northern blot analysis of kidney mRNA from mice treated with UGN.

CONCLUSION

GN, UGN, and STa act on the mouse kidney, in part, through a cGMP-dependent, GC-C-independent mechanism, causing significant natriuresis by renal tubular processes. UGN may have further long-term effects on the kidney by altering the expression of such transport-associated proteins as Na+/K+ ATPase and ClC-K2.

Increased blood pressure, aldosterone activity, and regional differences in renal ENaC protein during vasopressin escape

The syndrome of inappropriate antidiuretic hormone (SIADH) is associated with water retention and hyponatremia. The kidney adapts via a transient natriuresis and persistent diuresis, i.e., vasopressin escape. Previously, we showed an increase in the whole kidney abundance of aldosterone-sensitive proteins, the alpha- and gamma (70-kDa-band)-subunits of the epithelial Na(+) channel (ENaC), and the thiazide-sensitive Na-Cl cotransporter (NCC) in our rat model of SIADH. Here we examine mean arterial pressure via radiotelemetry, aldosterone activity, and cortical vs. medullary ENaC subunit and 11beta-hydroxysteroid dehydrogenase type 2 (11beta-HSD-2) protein abundances in escape. Eighteen male Sprague-Dawley rats (300 g) were sham operated (n = 6) or infused with desmopressin (dDAVP; n = 12, a V(2) receptor-selective analog of AVP). After 4 days, one-half of the rats receiving dDAVP were switched to a liquid diet, i.e., water loaded (WL) for 5-7 additional days. The WL rats had a sustained increase in urine volume and blood pressure (122 vs. 104 mmHg, P < 0.03, at 7 days). Urine and plasma aldosterone levels were increased in the WL group to 844 and 1,658% of the dDAVP group, respectively. NCC and alpha- and gamma-ENaC (70-kDa band) were increased significantly in the WL group (relative to dDAVP), only in the cortex. Beta- and gamma-ENaC (85-kDa band) were increased significantly by dDAVP in cortex and medulla relative to control. 11beta-HSD-2 was increased by dDAVP in the cortex and not significantly affected by water loading. These changes may serve to attenuate Na(+) losses and ameliorate hyponatremia in vasopressin escape.

Sodium chloride regulation of the alpha epithelial amiloride-sensitive sodium channel (alphaENaC) gene requires syntheses of new protein(s)

The epithelial amiloride-sensitive sodium channel (ENaC) plays a central role in sodium homeostasis and blood pressure control. The molecular effect of high sodium intake on the ENaC gene is not well known. This study examined the effects of high salt (HS) intake on alphaENaC gene transcription in rat kidney. Rats were injected intraperitoneally with hypertonic (1.5M NaCl) or normal saline solution (three rats per group). The serum sodium concentration of rats injected with hypertonic saline increased significantly 30 min after injection (158 +/- 2 mM versus 140 +/- 1 mM for normal saline injected rats and 139 +/- 1 mM for uninjected rats). At 3 h after injection, serum sodium decreased (144 +/- 1 mM) but remained above the control values (139 +/- 1 mM for normal saline injected rats, 139 +/- 1 mM for uninjected rats). The serum aldosterone decreased 1.5 and 3 h after the hypertonic saline injection (217 +/- 10 and 139 +/- 23 pg/ml for hypertonic saline injected rats, 358 +/- 2 pg/ml for uninjected rats). The kidney cortex was dissected macroscopically and total RNA was isolated at 1.5 and 3 h after treatment. Semi-quantitative RT-PCR studies revealed that following hypertonic saline treatment, alphaENaC mRNA levels were dramatically downregulated, compared with controls, as early as 1.5h. Western blot analysis showed similar patterns of protein downregulation. Inhibition of protein synthesis by cycloheximide (CHX) blocked the sodium chloride-induced alphaENaC mRNA downregulation, 3h after treatment. This indicates that synthesis of new, uncharacterized protein(s) is required for sodium chloride-mediated inhibition of alphaENaC gene transcription.

Parallel down-regulation of chloride channel CLC-K1 and barttin mRNA in the thin ascending limb of the rat nephron by furosemide

In the past few years the pivotal role of kidney Cl(-)channels (ClC-K) channels in maintaining salt and water homeostasis in the kidney has been established. The aim of the present study was to investigate the influence of the loop diuretic furosemide on the gene expression of the kidney chloride channel ClC-K1 and its recently described functional subunit barttin. Male Sprague Dawley rats received the loop diuretic furosemide (12 mg/kg/day) for 6 days. Rats had free access to 0.9% NaCl, 0.1%KCl solution to prevent volume depletion. Localisation and regulation of ClC-K1 and barttin mRNA was analysed by RNase protection and in situ hybridisation. Nephron-specific regulation was investigated by microdissection and real-time PCR quantification. In furosemide-treated rats ClC-K1 mRNA decreased to half in the inner medulla. In the renal cortex and outer medulla ClC-K1 mRNA levels were weak and did not change. Under furosemide treatment barttin mRNA was regulated in parallel with ClC-K1 mRNA. A significant mRNA decrease occurred after furosemide treatment in inner medulla (0.50 fold), whereas cortical and outer medulla levels remained unaffected. (35)S in situ hybridisation confirmed the regulation and distribution seen in the RNase protection assay experiments. Microdissection of the inner medullary collecting duct and thin limb of Henle's loop followed by real-time PCR revealed that CLC-K1 and barttin mRNA regulation in inner medulla was limited to the thin limb; mRNA levels in collecting ducts were not affected by furosemide treatment. Our findings imply that during furosemide treatment selective down-regulation of ClC-K1 and barttin mRNAs in thin limb plays a role in maintaining salt and water homeostasis.

Potassium channels in epithelial transport

Epithelial cells in the kidney, gastrointestinal tract and exocrine glands are engaged in vectorial transport of salt and nutrients. In these tissues, K(+) channels play an important role for the stabilization of membrane voltage and maintenance of the driving force for electrogenic transport. Luminal K(+) channels represent an exit pathway for the excretion of K(+) in secreted fluid, urine and faeces, thereby effecting body K(+) homeostasis. Indeed, the expression and function of several luminal K(+) channels is modulated by hormones regulating water, Na(+), and K(+) metabolism. In addition to net transport of K(+) in the serosal (or apical) direction, K(+) channels can be coupled functionally to K(+)-transporting ATPases such as the basolateral Na(+)/K(+) ATPase or the luminal H(+)/K(+) ATPase. These ATPases export Na(+) or H(+) and take up K(+), which is then recycled via K(+) channels. This review gives a short overview on the molecular identity of epithelial K(+) channels and summarizes the different mechanisms of K(+) channel function during transport in epithelial cells.

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.

Sodium transporter abundance profiling in kidney: effect of spironolactone

Renal tubule profiling studies were carried out to investigate the long-term effects of administration of spironolactone, a mineralocorticoid receptor antagonist, on abundances of the major Na transporter and Na channel proteins along the rat renal tubule. Oral administration of spironolactone for 7 days to NaCl-restricted rats did not significantly alter abundances of Na transporters expressed proximal to the macula densa, while substantially decreasing the abundances of the thiazide-sensitive Na-Cl cotransporter (NCC), the alpha-subunit of the amiloride-sensitive epithelial Na channel (ENaC), and the 70-kDa form of the gamma-subunit of ENaC. A dependency of NCC expression on aldosterone was confirmed by showing increased NCC expression in response to aldosterone infusion in adrenalectomized rats. Immunoperoxidase labeling of ENaC in renal cortex confirmed that dietary NaCl restriction causes a redistribution of ENaC to the apical domain of connecting tubule cells and showed that high-dose spironolactone administration does not block this apical redistribution. In contrast, spironolactone completely blocked the increase in alpha-ENaC abundance in response to dietary NaCl restriction. We conclude that the protein abundances of NCC, alpha-ENaC, and the 70-kDa form of gamma-ENaC are regulated via the classical mineralocorticoid receptor, but the subcellular redistribution of ENaC in response to dietary NaCl restriction is not prevented by blockade of the mineralocorticoid receptor.

Time course of renal Na-K-ATPase, NHE3, NKCC2, NCC, and ENaC abundance changes with dietary NaCl restriction

We have used peptide-directed antibodies to each major renal Na transporter and channel proteins to screen renal homogenates for changes in Na transporter protein expression after initiation of dietary NaCl restriction. After equilibration on a NaCl-replete diet (2.0 meq. 200 g body wt(-1). day(-1)), rats were switched to a NaCl-deficient diet (0.02 meq. 200 g body wt(-1). day(-1)). Na excretion fell to 25% of baseline levels on day 1, followed by a further decrease <4% of baseline levels on day 3, of NaCl restriction. The decreased Na excretion at day 1 occurred despite the absence of a significant increase in plasma aldosterone level or in the abundance of any of the major renal Na transporters. However, after a 1-day lag, plasma aldosterone levels increased in association with increases in abundances of three aldosterone-regulated Na transporter proteins: the thiazide-sensitive Na-Cl cotransporter (NCC), the alpha-subunit of the amiloride-sensitive epithelial Na channel (alpha-ENaC), and the 70-kDa form of gamma-ENaC. RNase protection assays of transporter mRNA levels revealed an increase in renal alpha-ENaC mRNA coincident with the increase in alpha-ENaC protein abundance. However, there was no change in NCC mRNA abundance, suggesting that the increase in NCC protein in response to dietary NaCl restriction was not a result of altered gene transcription. These results point to early regulatory processes that decrease renal Na excretion without an increase in the abundance of any Na transporter, followed by a late aldosterone-dependent response associated with upregulation of NCC and ENaC.

Human cortical distal nephron: distribution of electrolyte and water transport pathways

The exact distributions of the different salt transport systems along the human cortical distal nephron are unknown. Immunohistochemistry was performed on serial cryostat sections of healthy parts of tumor nephrectomized human kidneys to study the distributions in the distal convolution of the thiazide-sensitive Na-Cl cotransporter (NCC), the beta subunit of the amiloride-sensitive epithelial Na channel (ENaC), the vasopressin-sensitive water channel aquaporin 2 (AQP2), and aquaporin 3 (AQP3), the H(+) ATPase, the Na-Ca exchanger (NCX), plasma membrane calcium-ATPase, and calbindin-D28k (CaBP). The entire human distal convolution and the cortical collecting duct (CCD) display calbindin-D28k, although in variable amounts. Approximately 30% of the distal convolution profiles reveal NCC, characterizing the distal convoluted tubule. NCC overlaps with ENaC in a short portion at the end of the distal convoluted tubule. ENaC is displayed all along the connecting tubule (70% of the distal convolution) and the CCD. The major part of the connecting tubule and the CCD coexpress aquaporin 2 with ENaC. Intercalated cells, undetected in the first 20% of the distal convolution, were interspersed among the segment-specific cells of the remainder of the distal convolution, and of the CCD. The basolateral calcium extruding proteins, Na-Ca exchanger (NCX), and the plasma membrane Ca(2+)-ATPase were found all along the distal convolution, and, in contrast to other species, along the CCD, although in varying amounts. The knowledge regarding the precise distribution patterns of transport proteins in the human distal nephron and the knowledge regarding the differences from that in laboratory animals may be helpful for diagnostic purposes and may also help refine the therapeutic management of electrolyte disorders.

Nephron specific regulation of chloride channel CLC-K2 mRNA in the rat

BACKGROUND

This study investigated the influence of salt intake on the nephron specific gene expression of the kidney chloride channel CLC-K2. To this end, male Sprague-Dawley rats were fed a low (0.02% wt/wt), normal (0.6% wt/wt), or high salt (8% wt/wt) diet for ten days, or they received the loop diuretic furosemide (12 mg/kg/day) for six days.

METHODS

Expression and regulation of messenger RNA for CLC-K2 was demonstrated by RNase protection assay and in situ hybridization in kidney cortex, outer medulla and inner medulla. Tubular localization and regulation were determined precisely by reverse transcription-polymerase chain reaction (RT-PCR) and real time PCR of microdissected nephron segments.

RESULTS

In situ hybridization analysis and RNase protection assay of the total kidney revealed a down-regulation of CLC-K2 mRNA in the high salt diet rats and an up-regulation of CLC-K2 mRNA in furosemide treated rats, which were restricted to the outer medulla. Microdissection of collagenase treated kidney revealed CLC-K2 mRNA expression in the outer medullary thick ascending limb (mTAL), cortical thick ascending limb (cTAL), distal convoluted tubule (DCT), connecting tubule and cortical collecting duct (CNT/CCD), and outer medullary collecting duct (OMCD), whereas no signals were detected in proximal convoluted and straight tubules (PCT and PST), descending thin limb from the outer medulla (dTL), descending and ascending thin limb from the inner medulla (TL), inner medullary collecting duct (IMCD) and glomeruli (glom). Using RT-PCR and real time PCR, the changing levels of CLC-K2 mRNA after furosemide treatment or high salt diet were restricted to the mTAL, whereas CLC-K2 mRNA levels in cTAL and OMCD were not changed in furosemide or high salt rats compared to time paired controls.

CONCLUSIONS

Given that CLC-K2 expressed in the thick ascending limb of Henle's loop is responsible for net chloride reabsorption in this part of the nephron, our findings suggest that in states of surplus salt and in states of severe salt deprivation, selective regulation of CLC-K2 mRNA plays a role in the adaptation of the kidney to different salt loads.