Physiology of renal sodium transport

Creatine kinase activity is associated with blood pressure

BACKGROUND

We previously hypothesized that high activity of creatine kinase, the central regulatory enzyme of energy metabolism, facilitates the development of high blood pressure. Creatine kinase rapidly provides adenosine triphosphate to highly energy-demanding processes, including cardiovascular contraction, and antagonizes nitric oxide-mediated functions. Relatively high activity of the enzyme, particularly in resistance arteries, is thought to enhance pressor responses and increase blood pressure. Tissue creatine kinase activity is reported to be high in black people, a population subgroup with greater hypertension risk; the proposed effects of high creatine kinase activity, however, are not "race dependent." We therefore assessed whether creatine kinase is associated with blood pressure in a multiethnic population.

METHODS AND RESULTS

We analyzed a stratified random sample of the population of Amsterdam, The Netherlands, consisting of 1444 citizens (503 white European, 292 South Asian, 580 black, and 69 of other ethnicity) aged 34 to 60 years. We used linear regression analysis to investigate the association between blood pressure and normal serum creatine kinase after rest, as a substitute measure of tissue activity. Creatine kinase was independently associated with blood pressure, with an increase in systolic and diastolic pressure, respectively, of 8.0 (95% CI, 3.3 to 12.7) and 4.7 (95% CI, 1.9 to 7.5) mm Hg per log creatine kinase increase after adjustment for age, sex, body mass index, and ethnicity.

CONCLUSIONS

Creatine kinase is associated with blood pressure. Further studies are needed to explore the nature of this association, including how variation in cardiovascular creatine kinase activity may affect pressor responses.

Transient activation and delayed inhibition of Na+,K+,Cl- cotransport in ATP-treated C11-MDCK cells involve distinct P2Y receptor subtypes and signaling mechanisms

In C11-MDCK cells, which resemble intercalated cells from collecting ducts of the canine kidney, P2Y agonists promote transient activation of the Na+,K+,Cl- cotransporter (NKCC), followed by its sustained inhibition. We designed this study to identify P2Y receptor subtypes involved in dual regulation of this carrier. Real time polymerase chain reaction analysis demonstrated that C11-MDCK cells express abundant P2Y1 and P2Y2 mRNA compared with that of other P2Y receptor subtypes. The rank order of potency of agents (ATP approximately UTP >> 2-(methylthio)-ATP (2MeSATP); adenosine 5'-[beta-thio]diphosphate (ADPbetaS) inactive) indicated that P2Y2 rather than P2Y1 receptors mediate a 3-4-fold activation of NKCC within the first 5-10 min of nucleotide addition. NKCC activation in ATP-treated cells was abolished by the intracellular calcium chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, calmodulin (CaM) antagonists trifluoroperazine and W-7, and KN-62, an inhibitor of Ca2+/CaM-dependent protein kinase II. By contrast with the transient activation, 30-min incubation with nucleotides produced up to 4-5-fold inhibition of NKCC, and this inhibition exhibited a rank order of potency (2MeSATP > ADPbetaS > ATP >> UTP) typical of P2Y1 receptors. Unlike the early response, delayed inhibition of NKCC occurred in 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid-loaded cells and was completely abolished by the P2Y1 antagonists MRS2179 and MRS2500. Transient activation and delayed inhibition of NKCC in C11 cell monolayers were observed after the addition of ATP to mucosal and serosal solutions, respectively. NKCC inhibition triggered by basolateral application of ADPbetaS was abolished by MRS2500. Our results thus show that transient activation and delayed inhibition of NKCC in ATP-treated C11-MDCK cells is mediated by Ca2+/CaM-dependent protein kinase II- and Ca2+-independent signaling triggered by apical P2Y2 and basolateral P2Y1 receptors, respectively.

ClC-5: a chloride channel with multiple roles in renal tubular albumin uptake

ClC-5 is a chloride (Cl(-)) channel expressed in renal tubules and is critical for normal tubular function. Loss of function nonsense or missense mutations in ClC-5 are associated with Dent's disease, a condition in which patients present with low molecular weight (LMW) proteinuria (including albuminuria), hypercalciuria and nephrolithiasis. Several key studies in ClC-5 knockout mice have shown that the proteinuria results from defective tubular reabsorption of proteins. ClC-5 is typically regarded as an intracellular Cl(-) channel and thus the defect in this receptor-mediated uptake pathway was initially attributed to the failure of the early endosomes to acidify correctly. ClC-5 was postulated to play a key role in transporting the Cl(-) ions required to compensate for the movement of H(+) during endosomal acidification. However, more recent studies suggest additional roles for ClC-5 in the endocytosis of albumin. ClC-5 is now known to be expressed at low levels at the cell surface and appears to be a key component in the assembly of the macromolecular complex involved in protein endocytosis. Furthermore, mutations in ClC-5 affect the trafficking of v-H(+)-ATPase and result in decreased expression of the albumin receptor megalin/cubulin. Thus, the expression of ClC-5 at the cell surface as well as its presence in endosomes appears to be essential for normal protein uptake by the renal proximal tubule.

Thick ascending tubular cells in the loop of Henle: Regulation of electrolyte homeostasis

Renal medullary tubular cells in the loop of Henle have crucial importance for the regulation of homeostasis of the extracellular fluid. These cells receive limited amount of blood and oxygen, and are also constantly challenged by the hypertonic environment. The medullary tubular cells in the last part of the loop of Henle have one of the highest known contents of mitochondria of all mammalian cells, reflecting their need for oxidative metabolism in order to sustain high ATP production for active transepithelial electrolyte transport. The commonly used diureticum furosemide targets one of the transporters present in these tubular cells with resulting diuresis. Several pathological states are associated with altered function of the medullary tubular cells, and the nephrotoxic substances tacrolimus and cyclosporine act on these cells. The specific Tamm-Horsfall glycoprotein is produced by medullary tubular cells. Alteration in the urinary excretion of this protein is used as marker of tubular damage.

Drug combinations and impaired renal function -- the 'triple whammy'

BACKGROUND

Recent studies have identified the 'triple whammy' in which combinations of diuretics, nonsteroidal anti-inflammatory drugs (NSAIDs), ACE inhibitors (ACEI) and/or angiotensin receptor antagonists (ARA) may impair renal function.

METHODS

We performed a cross-sectional study of patients admitted to a general medical ward of a teaching hospital. Age, sex, disease status and prior consumption of the 'target' drugs, diuretics, NSAIDs (including aspirin), ACEI and ARA were correlated with creatinine and creatinine clearance on admission.

RESULTS

Three hundred and one patients (48% male) were included, 135 were on no prior target drugs, 87 on one, 60 on two and 19 on three such drugs. There was a significant (P < 0.01) correlation between both creatinine and creatinine clearance with male sex, age and number of target drugs. Multivariate analysis confirmed these associations but did not support associations between renal function and heart failure or total number of diagnoses. Increasing doses of diuretics, possibly because in many cases this included two drugs, but not the other drugs, were significantly (P < 0.001) associated with impaired renal function. For the other three drug groups patients on doses of any drug at lower than the defined daily dose (DDD) did not have significantly different creatinine or creatinine clearance from those on doses at or above the DDD.

CONCLUSION

Taking two or more of the identified drugs was associated with significant renal impairment but did not correlate with heart failure or other diseases for which the drugs might have been prescribed. Care is necessary to balance the demonstrated advantages of these medications against the risk of inducing renal failure.

Sodium retention in cirrhotic rats is associated with increased renal abundance of sodium transporter proteins

BACKGROUND

Liver cirrhosis with ascites is associated with a decrease in renal sodium excretion and therefore sodium retention.

METHODS

In this paper, we utilize transporter-specific antibodies to address the hypothesis that dysregulation of one or more sodium transporters or channels is associated with sodium chloride (NaCl) retention in a rat model of cirrhosis induced by repeated exposure to carbon tetrachloride. Age-matched controls and cirrhotic rats were pair fed to ensure identical NaCl and water intake for 4 days prior to euthanasia for quantitative immunoblotting studies.

RESULTS AND CONCLUSION

The rats manifested marked extracellular fluid volume expansion with massive ascites. Plasma aldosterone levels were markedly elevated. Analysis of immunoblots revealed marked increases in the abundances of both of the major aldosterone-sensitive apical transport proteins of the renal tubule, namely the thiazide-sensitive NaCl cotransporter NCC and the epithelial sodium channel alpha subunit (alpha-ENaC). These results are consistent with an important role for hyperaldosteronism in the pathogenesis of sodium retention and ascites formation in cirrhosis. In addition, we observed a large decrease in cortical NHE3 abundance (proximal tubule) and a large increase in NKCC2 abundance (thick ascending limb), potentially shifting premacula densa sodium absorption from proximal tubule to loop of Henle (which powers urinary concentration and dilution).

Functional sodium magnetic resonance imaging of the intact rat kidney

BACKGROUND

Renal fluid homeostasis depends to a large extent on the sodium concentration gradient along the corticomedullary axis. The spatial distribution and extent of this gradient were previously determined by invasive methods, which yielded a range of results. We demonstrate here the capacity of sodium magnetic resonance imaging (MRI) to quantify non-invasively renal sodium distribution in the intact kidney.

METHODS

Sodium MRI was applied to study normal, diuretic, and obstructed rat kidneys in vivo. The images were recorded at 4.7 Tesla using a 3-dimensional gradient echo sequence, with high spatial and temporal resolution. The tissue sodium concentration (TSC) was obtained by taking into account the measured nuclear relaxation rates and MRI visibility relative to a reference saline solution.

RESULTS

The corticomedullary sodium gradient increased linearly from the cortex to the inner medulla by approximately 31 mmol/L/mm, from a TSC of approximately 60 mmol/L to approximately 360 mmol/L. Furosemide induced a 50% reduction in the inner-medulla sodium and a 25% increase in the cortical sodium. The kinetics of these changes was related to the specific site and mechanism of the loop diuretic. Distinct profiles of the sodium gradient were observed in acute obstructed kidneys, as well as spontaneously obstructed kidneys. The changes in the sodium gradient correlated with the extent of damage and the residual function of the kidneys.

CONCLUSION

Quantitative assessment of the renal corticomedullary sodium gradient by high resolution sodium MRI may help verify new aspects of the kidney concentrating mechanism and serve as a non-invasive diagnostic method of renal function.

Molecular variants of the sodium/hydrogen exchanger type 3 gene and essential hypertension

OBJECTIVES

The objectives of this study were to identify polymorphic variants within the gene coding for the sodium/hydrogen exchanger type 3 (NHE3) and to examine their relationship with hypertension and biochemical indices of sodium balance.

DESIGN AND METHODS

Case-control comparisons on a total of 691 subjects of which 399 (68% with essential hypertension) were of African or Afro-Caribbean origin (blacks) and 292 (50% with essential hypertension) were of Caucasian origin (whites).

RESULTS

Eight exons of the C terminus of the NHE3 gene were screened systematically. A total of six variants were identified: (G1579A, G1709A, G1867A, C1945T, A2041G and C2405T). Further analyses in relation to essential hypertension and phenotypic characteristics were confined to the more frequent A2041G and the C2405T polymorphisms. The genotype frequencies of the A2041G polymorphism were significantly different between the whites and blacks, with the A allele being more frequent in the white population (0.43 for the whites and 0.14 for the blacks, respectively; P < 0.001). In contrast, there was no significant difference in the C2405T polymorphism between whites and blacks (C allele frequency: 0.86 for the whites and 0.88 for the blacks, respectively). In both the white and the black groups, there were no significant associations between these variants and essential hypertension (P > 0.05) or with serum electrolytes, creatinine or plasma renin activity (PRA) (ANOVA P > 0.05).

CONCLUSIONS

These results suggest a high degree of structural conservation of the NHE3 gene; however, the lack of association between these polymorphisms and blood pressure status does not necessarily eliminate the participation of this important sodium/hydrogen exchanger in the pathophysiology of essential hypertension, as we cannot exclude the existence of functionally important genetic variants in other sequences within the NEH3 gene.

MOLECULAR CHANGES IN PROXIMAL TUBULE FUNCTION IN DIABETES MELLITUS

Diabetic kidney disease is initially associated with hypertension and increased urinary albumin excretion. The hypertension is mediated by enhanced volume expansion due to enhanced salt and water retention by the kidney. The increased urinary albumin is not only due to increased glomerular leak, but also to a decrease in albumin reabsorption by the proximal tubule. The precise molecular mechanisms underlying these two phenomena and whether there is any link between the increase in Na(+) retention and proteinuria remain unresolved. There is significant evidence to suggest that increased Na(+) retention by the proximal tubule Na(+)/H(+) exchanger isoform 3 (NHE3) can play a role in some forms of hypertension. Increased NHE3 activity in models of diabetes mellitus may explain, in part, the enhanced salt retention observed in patients with diabetic kidney disease. The NHE3 also plays a role in receptor-mediated albumin uptake in the proximal tubule. The uptake of albumin requires the assembly of a macromolecular complex that is thought to include the megalin/cubulin receptor, NHE3, the vacuolar type H(+)-ATPase (v-H(+)-ATPase), the Cl(-) channel ClC-5 and interactions with the actin cytoskeleton. The NHE3 seems to exist in two functionally distinct membrane domains, one involved with Na(+) reabsorption and the other involved in albumin uptake. The present review focuses on the evidence derived from in vivo studies, as well as complementary studies in cell culture models, for a dual role of NHE3 in both Na(+) retention and albumin uptake. We suggest a possible mechanism by which disruption of the proximal tubule albumin uptake mechanism in diabetes mellitus may lead to both increased Na(+) retention and proteinuria.

Sex differences in the alterations of Na(+), K(+)-ATPase following ischaemia-reperfusion injury in the rat kidney

Postischaemic acute renal failure (ARF) is influenced by sex. Na(+), K(+)-ATPase (NKA) plays a crucial role in the pathogenesis of postischaemic ARF. We tested the impact of sex on mRNA, protein expression, cellular distribution and enzyme activity of NKA following renal ischaemia-reperfusion (I-R) injury. The left renal pedicle of uninephrectomized female (F) and male (M) Wistar rats was clamped for 55 min followed by 2 h (T2) and 16 h (T16) of reperfusion. Uninephrectomized, sham-operated F and M rats served as controls (n= 6 per group). Blood urea nitrogen, serum creatinine and renal histology were evaluated to detect the severity of postischaemic ARF. mRNA expression of NKA alpha1 and beta1 subunits were detected by RT-PCR. The effect of I-R on cellular distribution was compared by Triton X-100 extraction. Cellular proteins were divided into Triton-insoluble and Triton-soluble fractions and assessed by Western blot. NKA enzyme activity was also determined. After the ischaemic insult blood urea nitrogen and serum creatinine were higher and renal histology showed more rapid progression in M versus F (P < 0.05). mRNA expression of the NKA alpha1 subunit decreased in I-R groups versus controls, but was higher in F versus M both in control and I-R groups (P < 0.05). However, protein levels of the NKA alpha1 subunit in total tissue homogenate did not differ in controls, but were higher in F versus M in I-R groups (P < 0.05). Triton X-100 extractability was lower in F versus M at T16 (P < 0.05). NKA enzyme activity was the same in controls, but was higher in F versus M in I-R groups (T2: 14.9 +/- 2.3 versus 9.15 +/- 2.21 U) (T16: 11.7 +/- 4.1 versus 5.65 +/- 2.3 U; P < 0.05). mRNA and protein expression of the NKA beta1 subunit did not differ between F and M in any of the protocol. We concluded that NKA is more protected from the detrimental effects of postischaemic injury in females. Higher mRNA and protein expression of the NKA alpha1 subunit and higher enzyme activity might be additional contributing factors to the improved postischaemic renal function of female rats.

Differential gene expression in the kidney of sickle cell transgenic mice: upregulated genes

The S+S-Antilles transgenic mouse used in this study has renal defects similar to those seen in sickle cell anemia patients: congested glomeruli, medullary fibrosis, renal enlargement, vasoocclusion, and a urine concentrating defect. We used gene expression microarrays to identify genes highly up-regulated in the kidneys of these mice and validated their expression by real-time PCR. Kidney hypoxia, as demonstrated by the presence of deoxyhemoglobin, was detected by blood oxygen dependent magnetic resonance imaging (BOLD-MRI). Some of the up-regulated genes included cytochrome P450 4a14, glutathione-S-transferase alpha-1, mitochondrial hydroxymethylglutaryl CoA synthase, cytokine inducible SH-2 containing protein, retinol dehydrogenase type III, arginase II, glycolate oxidase, Na/K ATPase, renin-1, and alkaline phosphatase 2. An increase in enzyme activity was also demonstrated for one of the up-regulated genes (arginase II). These genes can be integrated into several different pathophysiological processes: a hypoxia cascade, a replacement cascade, or an ameliorating cascade, one or all of which may explain the phenotype of this disease. We conclude that microarray technology is a powerful tool to identify genes involved in renal disease in sickle cell anemia and that the identification of various metabolic pathways may open new avenues for therapeutic interventions.

Aldosterone stimulates surface expression of NHE3 in renal proximal brush borders

The mineralocorticoid aldosterone is one of the major regulators of extracellular volume and blood pressure. It acts by enhancing Na(+) reabsorption across tight epithelia such as renal collecting ducts and colon. In addition, it has been shown that aldosterone stimulates NaCl and volume reabsorption in renal proximal tubules by an unknown mechanism. To test the hypothesis that the application of aldosterone results in greater activity of the apical Na(+)/H(+) exchanger-3 (NHE3), we investigated the effect of aldosterone on amiloride-sensitive, proximal tubular volume reabsorption and proximal tubular NHE3 abundance in adrenalectomized rats. Aldosterone at physiological concentrations (dosage 36 microg/100 g b.w. per day) increased NHE3-dependent proximal tubular volume reabsorption and the abundance of NHE3 in brush borders without changing the total amount of NHE3 in cortical homogenates. These results indicate that renal proximal tubular NHE3 is a target for aldosterone-mediated regulation resulting in increased Na(+) reabsorption and thus extracellular volume and blood pressure. Further studies are required to determine the precise mechanism of action, especially whether the action of aldosterone on proximal tubular function is direct or indirect.

Dopamine and the kidney: a role in hypertension?

PURPOSE OF REVIEW

Defective transduction of the dopamine receptor signal in the kidney has been shown to be important in the pathogenesis of hypertension This review will discuss the genetic mechanism for the defective renal dopaminergic function and the interaction with other gene variant products in the pathogenesis of salt sensitivity and essential hypertension.

RECENT FINDINGS

Single nucleotide polymorphisms of G protein-coupled receptor kinase type 4 (GRK4) phosphorylate, desensitize, and diminish the inhibitory action of D receptors on sodium transport in the kidney. Inhibition of GRK4 expression normalizes renal proximal tubule D receptor function in humans and rodents and ameliorates the hypertension in genetically hypertensive rats. Expression of the GRK4 variant, GRK4gammaA142V, produces hypertension and impairs the natriuretic effect of D receptor stimulation in mice. In humans, GRK4 single nucleotide polymorphisms are associated with essential hypertension, particularly salt sensitive hypertension. The prediction of the hypertensive phenotype is most accurate when elements of the renin-angiotensin system and GRK4 are included in the analysis.

SUMMARY

GRK4 single nucleotide polymorphisms, by preventing the natriuretic function of the dopaminergic system and by allowing the antinatriuretic function of angiotensin II type 1 receptors to predominate, may be responsible for salt sensitivity. Hypertension develops with additional perturbations caused by the variants of other genes (e.g., alpha-adducin, angiotensin converting enzyme, angiotensinogen, angiotensin II type 1 receptor, aldosterone synthase, 11beta-hydroxysteroid dehydrogenase type 2), the quantitative interaction of which may vary depending upon the genetic background.

Up-regulation of early growth response gene-1 via the CXCR3 receptor induces reactive oxygen species and inhibits Na+/K+-ATPase activity in an immortalized human proximal tubule cell line

The CXCR3 chemokine receptor, a member of the CXCR family, has been linked to a pathological role in autoimmune disease, inflammatory disease, allograft rejection, and ischemia. In the kidney, expression of the CXCR3 receptor and its ligands is up-regulated in states of glomerulonephritis and in allograft rejection, but little is known about the expression and functional role the CXCR3 receptor might play. Here, we study the function of the CXCR3 chemokine receptor in an immortalized human proximal tubular cell line (IHKE-1). Stimulation of the CXCR3 receptor by its selective agonist monokine induced by IFN-gamma leads via a Ca(2+)-dependent mechanism to an up-regulation of early growth response gene (EGR)-1. Overexpression of EGR-1 induces down-regulation of copper-zinc superoxide dismutase and manganese superoxide dismutase and stimulates the generation of reactive oxygen species (ROS) via the NADH/NADPH-oxidase system. EGR-1 overexpression or treatment with monokine induced by IFN-gamma resulted in a ROS-dependent inhibition of basolateral Na(+)/K(+)-ATPase activity, compromising sodium transport in these cells. Thus, activation of the CXCR3 receptor in proximal tubular cells might disturb natriuresis during inflammatory and ischemic kidney disease via EGR-1-mediated imbalance of ROS.

A-type natriuretic peptide receptor in the spontaneously hypertensive rat kidney

Renal NPR-A binding characteristics was examined in SHR. Renal ANP binding sites of NPR-A showed a lower maximal binding capacity and higher affinity in SHR than in WKY at all intrarenal sites. Despite the lower B(max) in SHR, both ANP(1-28) and ANP(5-25) stimulate similar or greater cGMP production in isolated glomeruli. Studies on guanylate cyclase from glomerular and papillary membranes have reported an increased basal and stimulated guanylate cyclase activity in SHR. The present study provides further evidences for altered NPR-A receptors in SHR kidney, which might act as a negative feedback in response to hypertension.

Molecular aspects of acute inhibition of Na(+)-H(+) exchanger NHE3 by A(2)-adenosine receptor agonists

Adenosine regulates Na(+) homeostasis by its acute effects on renal Na(+) transport. We have shown in heterologously transfected A6/C1 cells (renal cell line from Xenopus laevis) that adenosine-induced natriuresis may be effected partly via A(2) adenosine receptor-mediated inactivation of the renal brush border membrane Na(+)-H(+) exchanger NHE3. In this study we utilized A6/C1 cells stably expressing wild-type as well as mutated forms of NHE3 to assess the molecular mechanism underlying A(2)-dependent control of NHE3 function. Cell surface biotinylation combined with immunoprecipitation revealed that NHE3 is targeted exclusively to the apical domain and that the endogenous Xenopus NHE is located entirely on the basolateral side of A6/C1 transfectants. Stimulation of A(2)-adenosine receptors located on the basolateral side for 15 min with CPA (N6-cyclopentyladenosine) acutely decreased NHE3 activity (microspectrofluorimety). This effect was mimicked by 8-bromo-cAMP and entirely blocked by pharmacological inhibition of PKA (with H89) or singular substitution of two PKA target sites (serine 552 and serine 605) on NHE3. Downregulation of NHE3 activity by CPA was attributable to a reduction of NHE3 intrinsic transport activity without change in surface NHE3 protein at 15 min. At 30 min, the decrease in transport activity was associated with a decrease in apical membrane NHE3 antigen. In conclusion, two highly conserved target serine sites on NHE3 determine NHE3 modulation upon A(2)-receptor activation and NHE3 inactivation by adenosine proceeds via two phases with distinct mechanisms.

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.

Spatially distributed alternative splice variants of the renal Na-K-Cl cotransporter exhibit dramatically different affinities for the transported ions

Three splice variants of the renal Na-K-Cl cotransporter (NKCC2 F, A, and B) are spatially distributed along the thick ascending limb of the mammalian kidney. To test whether NKCC2 splice variants differ in ion transport characteristics we expressed cDNAs encoding rabbit NKCC2 F, A, and B in Xenopus oocytes and determined the ion dependence of bumetanide-sensitive (86)Rb influx. The three splice variants of NKCC2 showed dramatic differences in their kinetic behavior. The medullary variant F exhibited 3-4-fold lower affinity than variants A and B for Na(+) and K(+). Chloride affinities also markedly distinguish the three variants (K(m)F = 111.3, K(m)A = 44.7, and K(m)B = 8.9 mm Cl(-)). Thus, the kinetic properties of the NKCC2 splice variants are consistent with the spatial distribution of the variants along the thick ascending limb as they are involved in reabsorbing Na(+), K(+), and Cl(-) from a progressively diluted fluid in the tubule lumen. Variant B also showed an anomalous inhibition of rubidium influx at high extracellular Na(+) concentrations, possibly important in its highly specialized role in the macula densa. The adaptation of the kinetic characteristics of the NKCC2 variants to the luminal concentrations of substrate represents an excellent example of functional specialization and diversity that can be achieved through alternative mRNA splicing.

Luminal P2Y2 receptor-mediated inhibition of Na+ absorption in isolated perfused mouse CCD

Extracellular nucleotides regulate renal transport. A luminal P2Y2 receptor in mouse cortical collecting duct (CCD) principal cells has been demonstrated elsewhere. Herein the effects of adenosine triphosphate (ATP) and uridine triphosphate (UTP) on electrogenic Na+ absorption in perfused CCD of mice kept on a low-NaCl diet were investigated. Simultaneously, transepithelial voltage (V(te)), transepithelial resistance (R(te)), and fura-2 [Ca2+]i fluorescence were measured. Baseline parameters were V(te), -16.5 +/- 1.2 mV; R(te), 80.8 +/- 7.1 Omega cm2; and equivalent short-circuit current (I(sc)), -261.0 +/- 25.1 microA/cm2 (n = 45). Amiloride (10 microM) almost completely inhibited I(sc) to -3.9 +/- 3.8 microA/cm2 (n = 10). Luminal ATP (100 microM) reduced V(te) from -16.5 +/- 2.1 to -12.5 +/- 1.93 and increased R(te) from 113.1 +/- 16.2 to 123.8 +/- 16.7 Omega cm2, which resulted in a 31.7% inhibition of amiloride-sensitive I(sc) (n = 12). Similarly, luminal UTP reversibly reduced V(te) from -22.0 +/- 2.1 to -13.6 +/- 2.1 mV and increased R(te) from 48.4 +/- 5.3 to 59.2 +/- 7.1 Omega cm2, which resulted in 49.1% inhibition of Na+ absorption (n = 6). In parallel, luminal ATP and UTP elevated [Ca2+]i in CCD, increasing the fura-2 ratio by 2.7 +/- 0.7 and 4.0 +/- 1.2, respectively. Basolateral ATP and UTP (100 microM) also inhibited amiloride-sensitive I(sc) by 21.8 (n = 14) and 20.1% (n = 8), respectively. Inhibition of luminal nucleotide-induced [Ca2+]i increase by Ca2+ store depletion with cyclopiazonic acid (3 microM) did not affect nucleotide-mediated inhibition of Na+ transport (n = 7). No evidence indicated the activation of a luminal Ca2+-activated Cl- conductance, a phenomenon previously shown in M-1 CCD cells (J Physiol 524: 77-99, 2000). In essence, these data indicate that luminal ATP and UTP, most likely via P2Y2 receptors, mediate inhibition of amiloride-sensitive I(sc) in perfused mouse CCD. This inhibition appears to occurs independently of an increase of cytosolic Ca2+.

Furosemide stimulates macula densa cyclooxygenase-2 expression in rats

BACKGROUND

During a low salt intake, maintenance of renal blood flow and renin secretion depends on intact formation of prostaglandins. In the juxtaglomerular apparatus, the inducible isoform of cyclooxygenase, cyclooxygenase-2 (COX-2), is restricted to the macula densa and the cortical thick ascending limb of Henle (cTALH) cells, and is inversely regulated by dietary salt intake. This study aimed to elucidate whether the effect of NaCl on macula densa COX-2 expression is mediated by transepithelial transport of NaCl.

METHODS

To this end, male Sprague-Dawley rats received subcutaneous infusions of the loop diuretic furosemide (12 mg/day) or were fed with the diuretic hydrochlorothiazide (30 mg/kg day) for seven days each. To compensate for their salt and water loss, the animals had free access to normal water and to salt water (0.9% NaCl, 0.1% KCl). COX-2 expression in kidney cortex was assessed by immunohistochemical staining and by semiquantitative ribonuclease protection assay for COX-2 mRNA.

RESULTS

After six days of furosemide infusion to salt-substituted rats, there was no change of extracellular volume. Furosemide led to a fivefold and threefold increase of plasma renin activity and renocortical renin mRNA level, respectively. In parallel, there was a threefold increase of renocortical COX-2 abundance, while the COX-1 mRNA level remained unchanged. Moreover, the percentage of juxtaglomerular apparatuses immunopositive for COX-2 increased threefold in response to furosemide compared with vehicle-infused animals. Hydrochlorothiazide treatment increased plasma renin activity twofold but did not change kidney cortical renin mRNA, COX-2 mRNA, or COX-2 immunoreactivity.

CONCLUSION

Our findings suggest that inhibition of salt transport in the loop of Henle, but not in the distal tubule, causes a selective stimulation of COX-2 expression in the macula densa region. This up-regulation may be of relevance for macula densa signaling, which links tubular salt transport rate with glomerular filtration rate and renin secretion.

Is greater tissue activity of creatine kinase the genetic factor increasing hypertension risk in black people of sub-Saharan African descent?

We postulate that the genetic factor increasing the propensity of black people of sub-Saharan African descent to develop high blood pressure is the relatively high activity of creatine kinase, predominantly in vascular and cardiac muscle tissue. Such greater activity of creatine kinase has been reported in skeletal muscle of black untrained subjects has been reported to be almost twice the activity found in white subjects. Creatine kinase, a key enzyme of cellular energy metabolism, increases the capacity of the cell to function under high demands. The enzyme regulates, buffers and transports, via phosphocreatine and creatine, energy produced by glycolysis and oxidative phosphorylation to sites of energy consumption such as myofibrils and membrane ion pumps. At these cellular locations, it is involved in the contraction process and active trans- membranous transport by readily providing the ATP needed for these processes. In addition, creatine kinase is increasingly reported to be involved in trophic responses. Furthermore, by using H+ and ADP to synthesize ATP, creatine kinase prevents acidification of the cell, providing relative protection against the effects of ischaemia. Greater creatine kinase activity in cardiovascular muscle and other tissues with high energy demands could increase cardiovascular contractile reserve, enhance trophic responses and increase renal tubular ability to retain salt. This could facilitate the development of arterial hypertension under chronic provocative circumstances, with higher mean blood pressures, more left ventricular hypertrophy and relatively fewer ischaemic events. Therefore, greater cellular activity of creatine kinase might explain the greater hypertension risk and the clinical characteristics of hypertensive disease observed in the black population.