Renal Tubular Transport Research Articles

Functional significance of channels and transporters expressed in the inner ear and kidney

A number of ion channels and transporters are expressed in both the inner ear and kidney. In the inner ear, K(+) cycling and endolymphatic K(+), Na(+), Ca(2+), and pH homeostasis are critical for normal organ function. Ion channels and transporters involved in K(+) cycling include K(+) channels, Na(+)-2Cl(-)-K(+) cotransporter, Na(+)/K(+)-ATPase, Cl(-) channels, connexins, and K(+)/Cl(-) cotransporters. Furthermore, endolymphatic Na(+) and Ca(2+) homeostasis depends on Ca(2+)-ATPase, Ca(2+) channels, Na(+) channels, and a purinergic receptor channel. Endolymphatic pH homeostasis involves H(+)-ATPase and Cl(-)/HCO(3)(-) exchangers including pendrin. Defective connexins (GJB2 and GJB6), pendrin (SLC26A4), K(+) channels (KCNJ10, KCNQ1, KCNE1, and KCNMA1), Na(+)-2Cl(-)-K(+) cotransporter (SLC12A2), K(+)/Cl(-) cotransporters (KCC3 and KCC4), Cl(-) channels (BSND and CLCNKA + CLCNKB), and H(+)-ATPase (ATP6V1B1 and ATPV0A4) cause hearing loss. All these channels and transporters are also expressed in the kidney and support renal tubular transport or signaling. The hearing loss may thus be paralleled by various renal phenotypes including a subtle decrease of proximal Na(+)-coupled transport (KCNE1/KCNQ1), impaired K(+) secretion (KCNMA1), limited HCO(3)(-) elimination (SLC26A4), NaCl wasting (BSND and CLCNKB), renal tubular acidosis (ATP6V1B1, ATPV0A4, and KCC4), or impaired urinary concentration (CLCNKA). Thus, defects of channels and transporters expressed in the kidney and inner ear result in simultaneous dysfunctions of these seemingly unrelated organs.

Reduction in non-glomerular renal clearance of the caffeine metabolite 1-methylxanthine by probenecid

Urinary caffeine metabolic ratios used to quantify the activity of numerous drug-metabolizing enzymes are an established component of cocktail approaches for metabolic phenotyping. Because in vitro evidence suggests that 1-methylxanthine (1-MX), a major caffeine metabolite, is actively secreted into urine by organic anion transporters (hOATs), coadministration of renal hOAT inhibitors like probenecid may impair these procedures. In a randomized, placebo-controlled, double-blind crossover design, single oral doses of 300 mg caffeine with oral coadministration of placebo or 500 mg probenecid 3 times daily for 2 days were administered to 7 healthy men. The plasma and urine concentrations of caffeine and its major metabolites 1,7-dimethylxanthine (1,7-DMX) and 1-MX were determined by high-performance liquid chromatography. Coadministration of probenecid resulted in a 34% reduction of the renal clearance of 1-MX (mean +/- SD 190 +/- 42 versus 290 +/- 83 ml min(-1), 95% CI on difference 0.2, 200, p = 0.04) with a 41% reduction in its estimated non-glomerular clearance. The renal clearances of caffeine and 1,7-DMX and the area under the plasma concentration-time curves of all substances were not significantly changed. 1-MX undergoes renal tubular secretion which is substantially reduced by probenecid, possibly due to inhibition of renal hOATs. This inhibition may explain the influence of probenecid on urinary caffeine metabolic ratios and, thus, its impact on the assessment of enzyme activities. It also suggests that 1-MX might serve as a model substrate for the renal tubular transport of organic anions.

Organ specific underexpression renal of Na+-dependent B0AT1 in the SHR correlates positively with overexpression of NHE3 and salt intake

The present study examines the renal and intestinal expression of Na(+)-dependent amino acid transporter B(0)AT1 during the development of hypertension in the spontaneous hypertensive rats (SHR) and its normotensive control (Wistar-Kyoto rat; WKY), and evaluates whether the expression of renal B(0)AT1 correlates with changes in the expression of Na(+) transporters, type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-K(+)-ATPase, known to occur in the SHR. The effect of high salt (HS) intake on the expression of renal and intestinal B(0)AT1 transcript abundance was also evaluated. For this purpose, the cloning of rat homolog of B(0)AT1 was performed. Rat B(0)AT1 shows high sequence homology to the mouse ortholog. Renal B(0)AT1 transcript abundance was lower in SHR than WKY at both 4 and 12 weeks of age. No significant differences between strains were observed in terms of intestinal expression of B(0)AT1. The decreased B(0)AT1 expression in SHR kidney was accompanied with an increase in NHE3 expression, suggesting an impaired Na(+) uptake. HS intake decreased renal B(0)AT1 mRNA in SHR and WKY at 4 weeks of age. In 12-week-old SHR, HS intake increased renal B(0)AT1 transcript abundance. Intestinal B(0)AT1 transcript was significantly increased by HS intake, though the effect was considerably more pronounced in the SHR. It is concluded, that underexpression of B(0)AT1 in the SHR kidney is organ specific, precedes the onset of hypertension and correlates negatively with the renal tubular transport of Na(+). The regulation of B(0)AT1 gene transcription appears to be under the influence of Na(+) delivery, being organ specific.

Cytochrome P450 Eicosanoids are Activators of Peroxisome Proliferator-Activated Receptor α

Cytochrome P450 (P450) eicosanoids regulate vascular tone, renal tubular transport, cellular proliferation, and inflammation. Both the CYP4A omega-hydroxylases, which catalyze 20-hydroxyeicosatetraenoic acid (20-HETE) formation, and soluble epoxide hydrolase (sEH), which catalyzes epoxyeicosatrienoic acid (EET) degradation to the dihydroxyeicosatrienoic acids (DHETs), are induced upon activation of peroxisome proliferator-activated receptor alpha (PPARalpha) by fatty acids and fibrates. In contrast, the CYP2C epoxygenases, which are responsible for EET formation, are repressed after fibrate treatment. We show here that P450 eicosanoids can bind to and activate PPARalpha and result in the modulation of PPARalpha target gene expression. In transactivation assays, 14,15-DHET, 11,2-EET, and 20-HETE were potent activators of PPARalpha. Gel shift assays showed that EETs, DHETs, and 20-HETE induced PPARalpha-specific binding to its cognate response element. Expression of apolipoprotein A-I was decreased 70% by 20-HETE, whereas apolipoprotein A-II expression was increased up to 3-fold by 11,12-EET, 14,15-DHET, and 20-HETE. In addition, P450 eicosanoids induced CYP4A1, sEH, and CYP2C11 expression, suggesting that they can regulate their own levels. Given that P450 eicosanoids have multiple cardiovascular effects, pharmacological modulation of their formation and/or degradation may yield therapeutic benefits.

Specificity and Regulation of Renal Sulfate Transporters

Sulfate is essential for normal cellular function. The kidney plays a major role in sulfate homeostasis. Sulfate is freely filtered and then undergoes net reabsorption in the proximal tubule. The apical membrane Na(+)/sulfate cotransporter NaS1 (SLC13A1) has a major role in mediating proximal tubule sulfate reabsorption, as demonstrated by the findings of hyposulfatemia and hypersulfaturia in Nas1-null mice. The anion exchanger SAT1 (SLC26A1), the founding member of the SLC26 sulfate transporter family, mediates sulfate exit across the basolateral membrane to complete the process of transtubular sulfate reabsorption. Another member of this family, CFEX (SLC26A6), is present at the apical membrane of proximal tubular cells. It also can transport sulfate by anion exchange, which probably mediates backflux of sulfate into the lumen. Knockout mouse studies have demonstrated a major role of CFEX as an apical membrane Cl(-)/oxalate exchanger that contributes to NaCl reabsorption in the proximal tubule. Several additional SLC26 family members mediate sulfate transport and show some level of renal expression (e.g., SLC26A2, SLC26A7, SLC26A11). Their roles in mediating renal tubular sulfate transport are presently unknown. This paper reviews current data available on the function and regulation of three sulfate transporters (NaS1, SAT1, and CFEX) and their physiological roles in the kidney.

Renal Transport of Adefovir, Cidofovir, and Tenofovir by SLC22A Family Members (hOAT1, hOAT3, and hOCT2)

The nephrotoxicity of the nucleotide antivirals adefovir, cidofovir and tenofovir is considered to depend on the renal tubular transport of them. Although it is known that the antivirals are substrates of the human renal organic anion transporter hOAT1 (SLC22A6), there is no information available on other organic ion transporters. The aim of the present study was to investigate whether the other renal organic anion transporter hOAT3 (SLC22A8) and organic cation transporter hOCT2 (SLC22A2) transport the antivirals. Uptake experiments were performed using HEK293 cells transfected with cDNA of the organic ion transporters. The uptake of adefovir, cidofovir and tenofovir in monolayers stably expressing hOAT3 increased time-dependently, compared with control. Probenecid, a typical inhibitor of organic anion transporters, completely inhibited their transport. The amounts of the antivirals taken up by hOAT3 were much lower than those by hOAT1. The transient expression of hOCT2 did not increase uptake of the antivirals. These results indicate that adefovir, cidofovir and tenofovir are substrates of hOAT3 as well as hOAT1, but that quantitatively hOAT1 is the major renal transporter for these drugs.

Regulation of renal glucose transporters during severe inflammation

Severe sepsis is accompanied by acute renal failure (ARF) with renal tubular dysfunction and glucosuria. In this study, we aimed to determine the regulation of renal tubular glucose transporters during severe experimental inflammation. Male C57BL/6J mice were injected with LPS or proinflammatory cytokines, and renal perfusion, glomerular filtration rate (GFR), fractional glucose excretion, and expression of tubular glucose transporters were determined. We found a decreased plasma glucose concentration with impaired renal tissue perfusion and GFR and increased fractional glucose excretion associated with decreased expression of SGLT2, SGLT3, and GLUT2 after LPS injection. Similar alterations were observed after application of TNF-alpha, IL-1beta, IL-6, or IFN-gamma. To clarify the role of proinflammatory cytokines, we performed LPS injections in knockout mice with deficiencies for TNF-alpha, IL-1 receptor type 1, IFN-gamma, or IL-6 as well as LPS injections in glucocorticoid-treated wild-type mice. LPS-induced alterations of glucose transporters also were present in single-cytokine knockout mice. In contrast, glucocorticoid treatment clearly attenuated LPS-induced changes in renal glucose transporter expression and improved GFR and fractional glucose excretion. LPS-induced decrease of renal perfusion was not improved by glucocorticoids, indicating a minor role of ischemia in the development of septic renal dysfunction. Our results demonstrate modifications of tubular glucose transporters during severe inflammation that are probably mediated by proinflammatory cytokines and account for the development of ARF with increased fractional glucose excretion. In addition, our findings provide an explanation why single anti-cytokine strategies fail in the therapy of septic patients and contribute to an understanding of the beneficial effects of glucocorticoids on septic renal dysfunction.

Abnormal Circadian Rhythm of Diuresis or Nocturnal Polyuria in a Subgroup of Children With Enuresis and Hypercalciuria is Related to Increased Sodium Retention During Daytime

In a subgroup of children with enuresis an increase in nighttime water and solute excretion has been documented. To investigate if modifications in renal function are involved in nocturnal enuresis, we assessed circadian variation in natriuresis and tubular sodium handling in polyuric hypercalciuric children. A total of 10 children with proved hypercalciuria and nocturnal polyuria and 10 age matched controls were included in the study. A 24-hour urine collection was performed in 8 sampling periods for measurement of urinary sodium excretion. Segmental tubular sodium transport was investigated during a daytime oral water load test and calculated according to standardized clearance methodology. The children with enuresis showed a marked increase in the fractional excretion of sodium during the night (0.93% +/- 0.36%), while daytime sodium excretion was decreased (0.84% +/- 0.23%). Analysis of segmental tubular sodium transport revealed decreased delivery of sodium to distal tubule (C(H2O) + C(Na) = 10.7 ml/100 ml glomerular filtration rate), indicating increased proximal tubular sodium reabsorption but also stimulation of distal sodium reabsorption as demonstrated by increased fractional distal sodium reabsorption (92.9% +/- 2.2%, controls 90.5% +/- 2.9%). Increased distal reabsorption was associated with increased fractional potassium excretion (17.5% +/- 2.7%, controls 13.6% +/- 6.4%), indicating increased distal tubular sodium/potassium exchange. No intrinsic defect in renal tubular sodium transport was found, but during the day increased sodium reabsorption in proximal and distal tubules was observed, suggesting extrarenal factors to be involved in altered circadian variation in solute and water excretion by the kidney.

Glucose Transporters in Normal and Diabetic Kidneys

In the last 12 years, data has accumulated supporting an important role for hyperglycemia in the development of diabetic nephropathy, and in the same period identification and characterization of renal glucose transporters has expanded rapidly. This new knowledge concerning glucose transporters is now being used to determine the roles they may play in diabetic kidney disease. Recent studies of renal glucose transporters have characterized their responses to diabetes, and their potential roles in the diabetic kidney. Glucose transporters have been shown to be rate-limiting for mesangial cell glucose uptake, glucose metabolism and extracellular matrix (ECM) production by these cells. Furthermore, increased renal GLUT1 has been proposed to play an important role in the development of diabetic glomerulosclerosis. Recent data suggest increased GLUT8 in podocytes may contribute to diabetic glomerular disease as well. Renal tubular glucose transporter expression is also altered in diabetes, responding to the increased need for glucose reabsorption . However, further investigation is required to determine the potential roles of these glucose transporters in the interstitial kidney disease associated with diabetes.

Detection of maleate-induced Fanconi syndrome by decreasing accumulation of125I-3-iodo-a-methyl-L-tyrosine in the proximal tubule segment-1 region of renal cortex in mice: A trial of separate evaluation of reabsorption

Fanconi syndrome is a renal dysfunction characterized by various combinations of renal tubular transport dysfunction involving amino acids, glucose, protein and other substances. Most reabsorption of amino acids occurs in proximal renal tubule segment 1 (S1). The present study evaluated the possibility of early detection of drug-induced Fanconi syndrome, based on decreased renal accumulation of 125I-3-iodo-alpha-methyl-L-tyrosine (125I-IMT), an amino acid transport marker, in the S1 region of renal cortex. The present experimental model used maleate (MAL)-induced Fanconi syndrome in mice. Results were compared between 125I-IMT and 3 other clinical renal radiopharmaceuticals: 99mTc-2,3-dimercaptosuccinic acid (99mTc-DMSA); 99mTc-mercaptoacetylglycylglycylglycine (99mTc-MAG3); and 99mTc-diethylenetriaminepentaacetic acid (99mTc-DTPA). Male ddY mice (age, 6 weeks; body weight, 25 g) were used to create a Fanconi model of renal dysfunction. A single dose of maleate disodium salt was administered by intraperitoneal injection (6 mmol/kg). Hematoxylin and eosin (HE) staining of the renal cortex, renal autoradiography and measurement of renal radioactivity of labeled compounds were performed at 30, 60, 90 and 120 min after MAL injection. At 5 min after injection of labeled compounds (18.5 kBq for accumulation experiment, 670 kBq for autoradiography), animals were sacrificed by ether overdose and kidneys were removed. For the accumulation experiment, radioactivity was measured using a well-type scintillation counter. For autoradiography, 20-microm sections of frozen kidney were used with Bio-Imaging Analyzer. At 30 min after MAL injection, HE staining showed pyknosis in some proximal tubule cells. At that time, accumulations of 125I-IMT and 99mTc-DMSA in the S1 region were approximately 67% and 55% of control levels (p < 0.005). MAL increased accumulation of 99mTc-DTPA in the S1 region, but had no effect on accumulation of 99mTc-MAG3 in the S 1 region. Decreased accumulation of 123I-IMT in the S1 region appears to represent a useful marker for detection of MAL-induced Fanconi syndrome. In future, we plan to assess the efficacy of using 125I-IMT to monitor renal dysfunction induced by nephrotoxic clinical drugs.

The use of mammalian cortical kidney slices for the study of tubule secretion: a pioneering step toward understanding organic anion transport

This essay examines the historical significance of an APS classic paper that is freely available on line: Cross RJ and Taggart JV. Renal tubular transport: accumulation of p-aminohippurate by rabbit kidney slices. Am J Physiol 161: 181–190, 1950 ( http://ajplegacy.physiology.org/cgi/reprint/161/1/181 ).

Urine sediment from a Chihuahua

A 6-year-old, intact male Chihuahua was presented with stranguria and painful urination of 5 days duration. Cystine crystals were observed in low numbers in unstained urine sediment preparations, and a diagnosis of cystinuria was made. Uroliths were removed surgically from the urethra and the bladder, and mineral analysis indicated the stones were composed of 100% cystine. Cystinuria results from an inherited defect in renal tubular transport of cystine that affects many breeds and has been found as an autosomal recessive trait in Newfoundlands. Accurate identification of cystine crystals in urine is an important means of diagnosing cystinuria.

Investigation of the pharmacokinetic and pharmacodynamic interactions between memantine and glyburide/metformin in healthy young subjects: A single-center, multiple-dose, open-label study

Background: The high prevalence rates of both 1026 Alzheimer's disease (AD) and type 2 diabetes mellitus in the elderly population suggest that concomitant pharmacotherapy is likely. Given the renal tubular transport and extensive urinary excretion of memantine and metformin, it was of interest to assess the pharmacokinetic and pharmacodynamic interaction with glyburide/metformin. Objective: The primary goal of this study was to determine1026 whether an in vivo pharmacokinetic or pharmacodynamic interaction exists between memantine (an uncompetitive, moderate-affinity, N-methyl- d-aspartate receptor antagonist with fast blocking/unblocking kinetics that is available in the United States for moderate to severe AD) and glyburide/metformin (a combination pharmacotherapy formulation approved for glycemic control in patients with type 2 diabetes mellitus). Methods: In this single-center, multiple-dose, open-label1026 study, healthy adult subjects received a single oral dose of memantine hydrochloride (20 mg) on day 1. After a 14-day washout period, subjects were orally administered 1.25-mg glyburide/250-mg metformin BID with food for 6 days. On day 21, subjects were coad-ministered memantine (20 mg) and glyburide/metformin with food. Assessments included determination of pharmacokinetic parameters for memantine and the antidiabetic agents when administered alone and in combination, pharmacodynamic measurements of blood glucose levels, and analyses of tolerability. Results: The study population consisted of 24 subjectResults:026 (13 women, 11 men; 79.2% white) with a mean (SD) age of 26.1 (5.6) years and a mean (SD) weight of 69.5 (11.3) kg. Twenty-one subjects completed the study: 2 discontinued due to adverse events judged unrelated to study medication, and 1 withdrew consent. No significant pharmacokinetic or pharmacodynamic interactions were observed between memantine and glyburide/metformin. Adverse events included dizziness (41.7% of patients) with memantine administration and gastrointestinal effects (nausea, 9.1 %; vomiting, 9.1%; abdominal cramps, 13.6%) with glyburide/metformin administration. Conclusions: No pharmacokinetic interactions between1026 memantine and glyburide/metformin were detected in this study of healthy young volunteers. Memantme had no effect on the pharmacodynamic activities of glyburide and metformin, and the drug combination was well tolerated in this population.

Altered AT 1 Receptor Regulation of ETB Receptors in Renal Proximal Tubule Cells of Spontaneously Hypertensive Rats

The renin-angiotensin and endothelin systems regulate blood pressure, in part, by affecting renal tubular sodium transport. In rodents, ETB receptors decrease proximal tubular reabsorption, whereas AT1 receptors produce the opposite effect. We hypothesize that ETB and AT1 receptors interact at the receptor level, and that the interaction is altered in spontaneously hypertensive rats (SHRs). In immortalized renal proximal tubule (RPT) cells from Wistar-Kyoto (WKY) rats, angiotensin II, via AT1 receptors, increased ETB receptor protein in a time- and concentration-dependent manner. In contrast, in SHR RPT cells, angiotensin II (10(-8) M/24 hours) had no effect on ETB receptor protein. AT1/ETB receptors colocalized and co-immunoprecipitated in both rat strains but long-term angiotensin II (10(-8) M/24 hours) treatment increased AT1/ETB co-immunoprecipitation in WKY but not in SHR cells. Short-term angiotensin II (10(-8) M/15 minutes) treatment decreased ETB receptor phosphorylation in both WKY and SHR cells, and increased ETB receptors in RPT cell surface membranes of RPT cells in WKY but not SHRs. Basal cell surface membrane ETB receptor expression was also higher in WKY than in SHRs. We conclude that AT1 receptors regulate ETB receptors by receptor interaction and modulation of receptor expression. The altered AT1 receptor regulation of ETB receptors in SHRs may play a role in the pathogenesis of hypertension.

ACUTE EFFECTS OF CANDESARTAN ON RAT RENAL HAEMODYNAMICS AND PROXIMAL TUBULAR REABSORPTION

1. The effects of the specific angiotensin II receptor type I (AT1) antagonist candesartan on renal proximal tubular sodium transport were studied using lithium clearance. The effects of candesartan on mean arterial blood pressure (MABP), renal plasma flow (RPF), glomerular filtration rate (GFR) and sodium and potassium excretion were also investigated. 2. Male Wistar rats were anaesthetized with Inactin (thiobutabarbital sodium; Sigma, St Louis, MO, USA). Clearance markers (8% polyfructosan, 1% para-aminohippuric acid and 4 mmol/l lithium chloride) were given into a jugular vein at the rate of 1.6 mL/h per 100 g bodyweight. Candesartan was given as bolus injection (0.01, 0.1, 0.2, 0.5 and 1.0 mg/kg) followed by 60 min continuous infusion at a rate of 0.5, 5, 10, 25 and 50 microg/min per kg, respectively. 3. The non-depressor dose of candesartan (0.01 mg/kg) did not alter RPF or GFR, whereas diuresis, natriuresis and kaliuresis were observed. The higher doses of candesartan reduced MABP, RPF and GFR, although diuresis, natriuresis and kaliuresis were still observed. 4. Renal tubular sodium and water reabsorption were inhibited after intravenous administration of candesartan independently of an alteration in arterial pressure. Lithium clearance data indicate that the site of inhibition was in the proximal nephron segment.

Renal tubular transport and the genetic basis of hypertensive disease

Several monogenic hypertensive disorders are caused by genetic mutations leading to the deranged function and/or regulation of renal tubular NaCl transport, such as mutations of the renal epithelial Na+ channel (ENaC) in Liddle syndrome, of the kinase WNK1 (with no K) in Gordon syndrome, and of the mineralocorticoid receptor, or of 11beta-hydroxysteroid dehydrogenase. Moreover, excessive formation of aldosterone in glucocorticoid-remediable hypertension leads to severe hypertension. Conversely, impaired function of the Na+,K+,2Cl- cotransporter (NKCC2), the renal outer medullary K+ channel (ROMK1), and the renal epithelial Cl- channel ClCKb/Barttin causes Bartter syndrome and defective Na+,Cl+ cotransporter (NCCT) Gitelman syndrome, salt-wasting disorders with hypotension. These monogenic disorders are rare, but illustrate the significance of renal tubular transport in blood pressure regulation. There is little doubt, however, that deranged renal salt reabsorption significantly contributes to essential hypertension polymorphisms of several genes participating in the regulation of renal Na+ transport have been shown to be associated with blood pressure and prevalence of hypertension. Two common genes will be discussed in more detail. The first encodes the renal Cl- channel ClCKb. A gain-of-function mutation of ClCKb, increasing channel activity by 7- to 20-fold is found in approximately 20% of unselected Caucasians and 40% of an unselected African population. The second common gene variant (prevalence, 3%-5% in unselected Caucasians), to be discussed in more detail, affects the serum and glucocorticoid inducible kinase SGK1, a kinase upregulated by mineralocorticoids and enhancing the activity of ENaC, ROMK, and Na+/K+ATPase. Both gene variants are associated with slightly increased blood pressure. SGK1 further stimulates the glucose transporter SGLT1, and the SGK1 gene variant correlates, in addition, with increased body mass index.

Gender differences in renal tubular taurocholate transport

The bile acids filtered through the glomeruli nearly completely escape urinary excretion due to an efficient tubular reabsorption process. Reabsorption is mediated by the sodium-dependent bile acid transporter ASBT, which is localized in the brush border membranes of proximal tubular cells. The purpose of the present study was to assess whether tubular taurocholate transport is regulated by sex hormones. Clearance studies and studies on proximal tubular cells freshly isolated from rat kidneys were performed. The studies with the isolated proximal tubular cells revealed a cell to bath 3H-taurocholate accumulation ratio of 5.63+/-0.28 in male and of 3.67+/-0.43 in female rats (p<0.01). This difference in cellular taurocholate uptake was corroborated by the clearance studies, which showed a 3H-taurocholate clearance of 133.9+/-28.1 in male rats and of 262.0+/-45.4 microl/min x 100 g b.w. in female rats (p<0.05). Testosterone treatment of female rats did not significantly alter the cell to bath 3H-taurocholate accumulation ratio. However, the cellular taurocholate accumulation significantly decreased, by 61.6+/-10.1%, following ethinylestradiol treatment of male rats. Ovariectomy, chemical castration of female rats with buserelin or treatment of female rats with the estrogen receptor antagonist ICI 182780 did not affect taurocholate uptake, but treatment of ovariectomized rats with ethinylestradiol decreased the taurocholate accumulation ratio by 53.7+/-15.8%. By determination of serum bile acids the possibility was excluded that this change was an indirect effect of cholestasis induced by ethinylestradiol. This study demonstrates gender differences in the renal handling of taurocholate in rats that may be related to an inhibitory effect of estrogens on taurocholate transport in proximal tubular cells. Since the ASBT protein content of the proximal tubular cells was found not to be different between male and female rats, a nongenomic mechanism may underly this estrogen effect.

CLCN5 mutation (R347X) associated with hypokalaemic metabolic alkalosis in a Turkish child: an unusual presentation of Dent's disease

Dent’s disease, an X-linked recessive tubular disorder, is characterized by low molecular weight proteinuria (LMWP) and nephrolithiasis associated with nephrocalcinosis and hypercalciuria. It is due to mutations that inactivate the renal voltage-gated chloride channel ClC-5 [1,2], which is encoded by a gene (CLCN5) located on chromosome Xp11.22. It is possible, however, that causative mutations were not identified in some patients with Dent’s disease [3–6]. Renal acidification abnormalities have not been a consistent feature of the phenotype, probably being secondary to long-standing hypercalciuria and nephrocalcinosis. Hypokalaemic metabolic alkalosis, however, has not been reported previously in Dent’s disease. Inherited disorders that manifest hypokalaemic metabolic alkalosis, such as the Bartter–Gitelman syndrome or the hyperprostaglandin E syndrome (also referred to as antenatal Bartter’s syndrome), are caused by the malfunction of renal tubular electrolyte transporters or ion channels. The hyperprostaglandin E syndrome is linked to the dysfunction of the sodium– potassium–chloride co-transporter (NKCC2) [7] or the renal outer medullary potassium channel (ROMK) [8]. The cardinal features of the syndrome are its antenatal onset—with polyhydramnios due to fetal polyuria, isothenuria and medullary nephrocalcinosis. When associated with sensorineural deafness or autosomal dominant hypocalcaemia, the hyperprostaglandin E syndrome is due to mutations in barttin, a b subunit of voltage-gated chloride channels [9], and the calcium-sensing receptor CaSR [10]. Bartter–Gitelman syndrome is linked to mutations in the basolateral chloride channel (ClC-Kb) [11] or in the sodium– chloride co-transporter (NCCT) [12]. The course of this disease is usually milder, mimicking chronic use of thiazides. ROMK, NKCC2 and NCCT mutations usually have uniform clinical presentations, whereas mutations in CLCNKB, encoding ClC-Kb, occasionally lead to phenotypic overlaps with the ROMK/ NKCC2 cohort. This study describes the first case of Dent’s disease due to a loss-of-function mutation in the CLCN5 gene, R347X, associated with a Bartter-like syndrome that is characterized by hypokalaemic metabolic alkalosis and secondary hyper-reninaemic hyperaldosteronism.

NaS i-1 and Sat-1: structure, function and transcriptional regulation of two genes encoding renal proximal tubular sulfate transporters

Sulfate (SO 4 2−) is an important anion regulating many metabolic and cellular processes. Maintenance of SO 4 2− homeostasis occurs in the renal proximal tubule via membrane transport proteins. Two SO 4 2− transporters that have been characterized and implicated in regulating serum SO 4 2− levels are: NaS i -1, a Na +–SO 4 2− cotransporter located at the brush border membrane and Sat-1, a SO 4 2−-anion exchanger located on the basolateral membranes of proximal tubular cells. Unlike Sat-1, for which very few studies have looked at regulation of its expression, NaS i -1 has been shown to be regulated by various hormones and dietary conditions in vivo. To study this further, NaS i -1 ( SLC13A1) and Sat-1 ( SLC26A1) gene structures were determined and recent studies have characterized their respective gene promoters. This review presents the current understanding of the transcriptional regulation of NaS i -1 and Sat-1, and describes possible pathogenetic implications which arise as a consequence of altered SO 4 2− homeostasis.

Single nucleotide polymorphisms in the CYP2J2 and CYP2C8 genes and the risk of hypertension

CYP2J2 and CYP2C8 metabolize arachidonic acid (AA) to cis-epoxyeicosatrienoic acids (EETs), which play a central role in regulating renal tubular fluid-electrolyte transport and vascular tone. We hypothesized that functionally relevant polymorphisms in the CYP2J2 or CYP2C8 genes influence hypertension risk. We examined associations between CYP2J2*7 (G-50 T promoter) and CYP2C8*3 (Arg139Lys and Lys399Arg, which are in 100% linkage disequilibrium) polymorphisms and hypertension in a biethnic population from Tennessee. CYP2J2*7 variant allele frequency was significantly higher in African-Americans versus Caucasians (14.1% versus 7.7%, P=0.01), irrespective of hypertension status. When analysed separately by race, the genotype distribution of the CYP2J2*7 variant allele was not significantly different among African-Americans with/without hypertension, but was significantly different among Caucasians with/without hypertension (P=0.03). Indeed, the odds ratio of having hypertension attributable to carrying the CYP2J2*7 variant allele adjusted for age, gender, body mass index and family history was 0.39 (95% confidence interval 0.17-0.89) among Caucasians, suggesting a protective effect. Additional subgroup analyses revealed a significantly lower CYP2J2*7 variant allele frequency in hypertensive versus normotensive Caucasian males (5.6% versus 12.5%, P=0.02) and in hypertensive versus normotensive Caucasians without a family history of hypertension (1.5% versus 11.0%, P=0.03). With respect to the CYP2C8*3 variant, genotype distribution and allele frequencies were similar between normotensive and hypertensive subjects. This study provides evidence for an association between CYP2J2*7 genotype and hypertension in Caucasian males and Caucasians without a family history of hypertension, but suggests no association between CYP2C8*3 genotype and hypertension. Confirmation of these findings in additional populations is warranted.