Comparison of WNK4 and WNK1 kinase and inhibiting activities

WNK kinases are novel serine/threonine protein kinases. Mutations in two members of the WNK family, WNK1 and WNK4, cause familial hyperkalemic hypertension. These kinases regulate ion transport across diverse epithelia; WNK4 reduces activity of the Na-Cl cotransporter activity and the potassium channel, ROMK, by reducing their appearance at the plasma membrane. We examined the kinase activity of WNK1 and WNK4 in vitro. A glutathione S-transferase (GST) fusion protein of the WNK1 kinse domain phosphorylated itself and a substrate protein, as reported previously. A longer construct, containing the autoinhibitory domain, did not. A GST WNK4 kinase domain construct demonstrated no kinase activity, in vitro or in HEK 293 cells. WNK4 constructs that included a region homologous to the autoinhibitory domain of WNK1 inhibited WNK1 kinase activity. Inhibition by a short WNK4 segment, WNK4 (444-518), was greater than inhibition by WNK4 (444-563). Together, these results suggest that WNK4 must be activated by currently unknown factors to exhibit kinase activity and that WNK4 contains an inhibitory domain that can inhibit the kinase activity of WNK1.

Pathophysiology of functional mutations of the thiazide-sensitive Na-Cl cotransporter in Gitelman disease

Most of the missense mutations that have been described in the human SLC12A3 gene encoding the thiazide-sensitive Na(+)-Cl(-) cotransporter (TSC, NCC, or NCCT), as the cause of Gitelman disease, block TSC function by interfering with normal protein processing and glycosylation. However, some mutations exhibit considerable activity. To investigate the pathogenesis of Gitelman disease mediated by such mutations and to gain insights into structure-function relationships on the cotransporter, five functional disease mutations were introduced into mouse TSC cDNA, and their expression was determined in Xenopus laevis oocytes. Western blot analysis revealed immunoreactive bands in all mutant TSCs that were undistinguishable from wild-type TSC. The activity profile was: wild-type TSC (100%) > G627V (66%) > R935Q (36%) = V995M (32%) > G610S (12%) > A585V (6%). Ion transport kinetics in all mutant clones were similar to wild-type TSC, except in G627V, in which a small but significant increase in affinity for extracellular Cl(-) was observed. In addition, G627V and G610S exhibited a small increase in metolazone affinity. The surface expression of wild-type and mutant TSCs was performed by laser-scanning confocal microscopy. All mutants exhibited a significant reduction in surface expression compared with wild-type TSC, with a profile similar to that observed in functional expression analysis. Our data show that biochemical and functional properties of the mutant TSCs are similar to wild-type TSC but that the surface expression is reduced, suggesting that these mutations impair the insertion of a functional protein into the plasma membrane. The small increase in Cl(-) and thiazide affinity in G610S and G627V suggests that the beginning of the COOH-terminal domain could be implicated in defining kinetic properties.

Renal expression of sodium transporters and aquaporin-2 in hypothyroid rats

Hypothyroidism is associated with significant abnormalities in the renal handling of salt and water. To address the involvement of tubular transport proteins in these abnormalities, rats were rendered pharmacologically hypothyroid and the abundance of major tubular transport proteins was assessed by immunoblot and immunohistochemistry. Hypothyroidism resulted in a marked reduction in kidney size and creatinine clearance along with decreased or unchanged total kidney abundance of the transport proteins. Whereas the proximal tubular type 3 Na/H exchanger (NHE3) and type 2 Na-phosphate cotransporter (NaPi2) stood out by their disproportionately reduced abundance, the bumetanide-sensitive type 2 Na-K-2Cl cotransporter (NKCC2) and aquaporin-2 (AQP2) were unaltered in their total kidney abundance despite a markedly lower kidney mass. The latter proteins in fact showed enhanced immunostaining. Decreased NHE3 and NaPi2 expression was most likely due to a combination of triiodo-l-thyronine (T(3)) deficiency along with a reduced glomerular filtration rate. The increased abundance of NKCC2 and AQP2 may have been caused by an increased action of vasopressin since urinary excretion of this hormone was elevated. On the other hand, the thiazide-sensitive Na-Cl cotransporter; the alpha-, beta-, and gamma-subunits of the amiloride-sensitive epithelial Na channel; and the alpha(1)-subunit of Na-K-ATPase showed a moderate decrease in total kidney abundance that was largely proportional to the smaller kidney mass. Although the observed expression of transporters was associated with a balanced renal sodium handling, altered transporter abundance may become functionally relevant if the hypothyroid kidney is challenged by an additional destabilization of the milieu interieur that has previously been shown to result in an inadequate natriuresis and clinical symptoms.

WNK kinases regulate thiazide-sensitive Na-Cl cotransport

Pseudohypoaldosteronism type II (PHAII) is an autosomal dominant disorder of hyperkalemia and hypertension. Mutations in two members of the WNK kinase family, WNK1 and WNK4, cause the disease. WNK1 mutations are believed to increase WNK1 expression; the effect of WNK4 mutations remains unknown. The clinical phenotype of PHAII is opposite to Gitelman syndrome, a disease caused by dysfunction of the thiazide-sensitive Na-Cl cotransporter. We tested the hypothesis that WNK kinases regulate the mammalian thiazide-sensitive Na-Cl cotransporter (NCC). Mouse WNK4 was cloned and expressed in Xenopus oocytes with or without NCC. Coexpression with WNK4 suppressed NCC activity by more than 85%. This effect did not result from defects in NCC synthesis or processing, but was associated with an 85% reduction in NCC abundance at the plasma membrane. Unlike WNK4, WNK1 did not affect NCC activity directly. WNK1, however, completely prevented WNK4 inhibition of NCC. Some WNK4 mutations that cause PHAII retained NCC-inhibiting activity, but the Q562E WNK4 demonstrated diminished activity, suggesting that some PHAII mutations lead to loss of NCC inhibition. Gain-of-function WNK1 mutations would be expected to inhibit WNK4 activity, thereby activating NCC, contributing to the PHAII phenotype. Together, these results identify WNK kinases as a previously unrecognized sodium regulatory pathway of the distal nephron. This pathway likely contributes to normal and pathological blood pressure homeostasis.

Diuretic therapy and resistance in congestive heart failure

Treatment of congestive heart failure has changed dramatically during the past 20 years, but diuretic drugs remain an essential component. Diuretics are essential despite the fact that these drugs stimulate the renin-angiotensin-aldosterone (RAA) axis and lead to adaptive responses that may be counterproductive. In this paper, new diuretic drugs and new uses of older drugs are discussed. These approaches emphasize low-dose combination therapy and may prove superior to traditional approaches that rely exclusively on loop diuretics. Such approaches aim to prevent adverse compensatory processes that appear to result from chronic diuretic treatment. These include acute and chronic increases in plasma renin activity and stimulation of the sympathetic nervous system, both of which increase afterload and may tend to increase mortality. They also include adaptive changes in nephron structure and function resulting from diuretic-induced increases in distal sodium load and diuretic-induced neurohormonal stimulation. These adaptations blunt the effectiveness of diuretic therapy. Diuretic strategies that rely on combinations of diuretics are emphasized as a method to prevent resistance. If diuretic resistance does develop, higher-dose combination regimens, continuous diuretic infusions and mechanical ultrafiltration can be used to overcome diuretic adaptations and restore diuretic efficacy. The goal of reducing the extracellular fluid volume with the least stimulation of the RAA axis and minimal changes in nephron architecture can be achieved in many patients.

Interaction with grp58 increases activity of the thiazide-sensitive Na-Cl cotransporter

The thiazide-sensitive sodium-chloride cotransporter (NCC) is expressed by distal convoluted tubule cells of the mammalian kidney. We used yeast two-hybrid screening to identify that glucose-regulated protein 58 (grp58), a protein induced by glucose deprivation, binds to the COOH terminus of the NCC. Immunoprecipitation of rat kidney cortex homogenates using a guinea pig anti-NCC antibody confirmed that grp58 associates with the NCC in vivo. Northern blots indicated that grp58 is highly expressed in rat kidney cortex. Immunofluorescence showed that grp58 protein abundance in kidney is highest in epithelial cells of the distal nephron, where it colocalizes with NCC near the apical membrane. To determine whether this interaction has a functional significance, NCC and grp58 cRNA were coexpressed in Xenopus laevis oocytes. In oocytes overexpressing grp58, sodium uptake was increased compared with control. Because oocytes express endogenous grp58, antisense experiments were performed to evaluate whether endogenous grp58 affected NCC activity in oocytes. Sodium uptake was lower in oocytes injected with both antisense grp58 cRNA and sense NCC compared with sense NCC oocytes. Western blot analysis did not show any effect of grp58 expression on processing of the NCC. These data indicate a novel, functionally important interaction between grp58 and the NCC in rat kidney cortex.

Loop diuretic infusion increases thiazide-sensitive Na(+)/Cl(-)-cotransporter abundance: role of aldosterone

Chronic infusion of loop diuretics into animals induces structural and functional changes in the distal nephron. These changes include increases in the activity of the thiazide-sensitive Na(+)/Cl(-)-cotransporter (NCC). The NCC was recently demonstrated to be an aldosterone-induced protein. These experiments were designed to test the hypotheses that chronic loop diuretic infusion, with replacement of NaCl losses, increases NCC protein abundance and that this effect results, in part, from stimulation by aldosterone. Sprague-Dawley rats received vehicle (group 1), furosemide (22 mg/100 g body wt per d) (group 2), or furosemide plus spironolactone (22 and 20 mg/100 g body wt per d, respectively) (group 3). Urine output was higher for groups 2 and 3 than for group 1 (151 +/- 32, 149 +/- 24, and 12 +/- 4 ml, respectively; P < 0.0001). Immunoblot analysis of NCC protein demonstrated that loop diuretics increased NCC protein abundance by nearly 100% (from 2562 +/- 30 to 5248 +/- 151 arbitrary units, P < 0.01). Spironolactone decreased NCC protein abundance by 66% (to 3532 +/- 113 units), compared with the furosemide-treated group (P < 0.005). Northern blot analysis of NCC mRNA demonstrated no significant effect of furosemide (NCC/glyceraldehyde-3-phosphate dehydrogenase ratios: group 1, 0.6 +/- 0.12; group 2, 0.5 +/- 0.05; P > 0.05, NS) These results indicate that increased NCC activity during chronic loop diuretic infusion is associated with increases in NCC protein abundance. A portion of the furosemide effect can be prevented by blockade of mineralocorticoid receptors.

The distal convoluted tubule of rabbit kidney does not express a functional sodium channel

We sought to assess whether the distal convoluted tubule (DCT) segment of the rabbit nephron expresses a functional epithelial sodium channel. First, the transepithelial voltage (V(te), lumen vs. bath) was measured in isolated perfused DCT segments (assessed separately in the upstream half and the downstream half of the DCT). V(te) was zero and not affected by amiloride or barium in the upstream DCT. V(te) was sometimes negative in the downstream DCT and depolarized by amiloride and hyperpolarized by barium, suggesting inclusion of connecting tubule (CNT) cells. To determine expression of epithelial sodium channel (ENaC) mRNA subunits by the upstream DCT, rabbit alpha-, beta-, and gamma-ENaC cDNA fragments were cloned and primers were selected for single-nephron RT-PCR analysis. Although alpha-ENaC was expressed by the DCT, beta- and gamma-ENaC were not detected in the DCT. In contrast, the CNT, CCD, and outer medullary collecting duct (OMCD) expressed all three subunits. Nedd4 was also not detected in the DCT but was expressed by the CNT, CCD, and OMCD. When upstream DCT fragments were grown to confluent monolayers in primary culture, the epithelia exhibited negative voltages and high transepithelial resistances and expressed mRNA for all three ENaC subunits as well as for Nedd4. The absence of a negative voltage and failure to detect transcript for beta- and gamma-ENaC and Nedd4 in the native rabbit DCT suggest that the sodium channel is not a significant pathway for sodium absorption by this segment. The phenotype conversion observed when DCT cells are grown in culture does not rule out the possibility that there may be conditions in which the DCT in the intact kidney expresses sodium channel activity. The results are consistent with the notion that DCT sodium transport is predominantly, if not exclusively, electroneutral.

Divalent cation transport by the distal nephron: insights from Bartter's and Gitelman's syndromes

Elucidation of the gene defects responsible for many disorders of renal fluid and electrolyte homeostasis has provided new insights into normal and abnormal physiology. Identifying the causes of Gitelman's and Bartter's syndromes has greatly enhanced our understanding of ion transport by thick ascending limb and distal convoluted tubule cells. Despite this information, several phenotypic features of these diseases remain confusing, even in the face of molecular insight. Paramount among these are disorders of divalent cation homeostasis. Bartter's syndrome is caused by dysfunction of thick ascending limb cells. It is associated with calcium wasting, but magnesium wasting is usually mild. Loop diuretics, which inhibit ion transport by thick ascending limb cells, markedly increase urinary excretion of both calcium and magnesium. In contrast, Gitelman's syndrome is caused by dysfunction of the distal convoluted tubule. Hypocalciuria and hypomagnesemia are universal parts of this disorder. Yet although thiazide diuretics, which inhibit ion transport by distal convoluted tubule cells, reduce urinary calcium excretion, they have minimal effects on urinary magnesium excretion, when given acutely. This review proposes mechanisms that may account for the differences between the effects of diuretic drugs and the phenotypic features of Gitelman's and Bartter's syndromes. These mechanisms are based on recent insights from another inherited disease of ion transport, inherited magnesium wasting, and from a review of the chronic effects of diuretic drugs in animals and people.

Mammalian distal tubule: physiology, pathophysiology, and molecular anatomy

The distal tubule of the mammalian kidney, defined as the region between the macula densa and the collecting duct, is morphologically and functionally heterogeneous. This heterogeneity has stymied attempts to define functional properties of individual cell types and has led to controversy concerning mechanisms and regulation of ion transport. Recently, molecular techniques have been used to identify and localize ion transport pathways along the distal tubule and to identify human diseases that result from abnormal distal tubule function. Results of these studies have clarified the roles of individual distal cell types. They suggest that the basic molecular architecture of the distal nephron is surprisingly similar in mammalian species investigated to date. The results have also reemphasized the role played by the distal tubule in regulating urinary potassium excretion. They have clarified how both peptide and steroid hormones, including aldosterone and estrogen, regulate ion transport by distal convoluted tubule cells. Furthermore, they highlight the central role that the distal tubule plays in systemic calcium homeostasis. Disorders of distal nephron function, such as Gitelman's syndrome, nephrolithiasis, and adaptation to diuretic drug administration, emphasize the importance of this relatively short nephron segment to human physiology. This review integrates molecular and functional results to provide a contemporary picture of distal tubule function in mammals.

Sodium transport-related proteins in the mammalian distal nephron - distribution, ontogeny and functional aspects

The mammalian distal nephron plays a pivotal role in adjusting urinary sodium excretion. Successive portions of the renal tubule are formed to adapt to this function, and an axial heterogeneity of the distal segments has been defined. The specific transport properties of these epithelia are accomplished by the expression of proteins (cotransporters, exchangers, channels) governing the movement of ions on either cell side. Molecular cloning of these proteins has had a marked impact on the study of their localization and function in the healthy and diseased kidney. Electroneutral cation-chloride cotransporters [Na(K)CC] have been localized to the thick ascending limb and the distal convoluted tubule using specific probes. Proteins implicated in the function of aldosterone target cells, such as the epithelial Na(+) channel (ENaC), the mineralocorticoid receptor (MR) and 11beta-hydroxysteroid dehydrogenase type 2 (11HSD2), an enzyme that confers mineralocorticoid specificity, have been found in the terminal portion of the nephron and the collecting duct. A mineralocorticoid-sensitive component of thiazide-sensitive NaCl transport has been identified in the distal convoluted tubule. Analysis of the ontogeny of these proteins in the maturing kidney has provided a detailed picture of epithelial differentiation and morphological specialization of the renal tubule. The study of mutations of the proteins related with NaCl transport has led to the identification of the molecular causes of inherited human diseases associated with hypo- or hypertension, and the respective sites of an impaired ion transport could be mapped to the renal tubule.

Diuretic resistance: physiology and therapeutics

Diuretic drugs are usually effective treatment for edema when used judiciously. However, some patients become resistant to their effects. Adaptation to diuretic drugs and diuretic resistance may be caused by similar mechanisms. Diuretic adaptations can be classified as those that occur during diuretic action, those that cause sodium retention in the short term (causing 'post-diuretic NaCl retention'), and those that increase sodium retention chronically (the 'braking phenomenon'). Recent experimental work has indicated ways in which kidneys adapt to chronic diuretic treatment. First, nephron segments downstream from the site of diuretic action increase NaCl reabsorption during diuretic administration because delivered NaCl load is increased. Second, when diuretic concentrations in the tubule decline, the kidney tubules act to retain Na until the next dose of diuretic is administered. Third, the ability of the diuretic to increase renal NaCl excretion declines over time, an effect that results both from depletion of the extracellular fluid volume and from structural and functional changes of kidney tubules themselves. These adaptations all increase the rate of NaCl reabsorption and blunt the effectiveness of diuretic therapy. Many times, a second diuretic drug is effective treatment for diuretic resistance. Recent experimental results suggest that a second drug may act synergistically because it blocks the adaptive processes limiting the effectiveness of the first diuretic. Based on an understanding of the mechanisms of diuretic adaptation and resistance, treatment regimens can be designed to block specific adaptive mechanisms and improve diuretic effectiveness.

Defective processing and expression of thiazide-sensitive Na-Cl cotransporter as a cause of Gitelman's syndrome

Gitelman's syndrome is an autosomal recessive disorder of salt wasting and hypokalemia caused by mutations in the thiazide-sensitive Na-Cl cotransporter. To investigate the pathogenesis of Gitelman's syndrome, eight disease mutations were introduced into the mouse thiazide-sensitive Na-Cl cotransporter and studied by functional expression in Xenopus oocytes. Sodium uptake into oocytes that expressed the wild-type clone was more than sevenfold greater than uptake into control oocytes. Uptake into oocytes that expressed the mutated transporters was not different from control. Hydrochlorothiazide reduced Na uptake by oocytes expressing the wild-type gene to control values but had no effect on oocytes expressing the mutant clones. Western blots of oocytes injected with the wild-type clone showed bands representing glycosylated (125 kDa) and unglycosylated (110 kDa) forms of the transport protein. Immunoblot of oocytes expressing the mutated clones showed only the unglycosylated protein, indicating that protein processing was disrupted. Immunocytochemistry with an antibody against the transport protein showed intense membrane staining of oocytes expressing the wild-type protein. Membrane staining was completely absent from oocytes expressing mNCC(R948X); instead, diffuse cytoplasmic staining was evident. In summary, the results show that several mutations that cause Gitelman's syndrome are nonfunctional because the mutant thiazide-sensitive Na-Cl cotransporter is not processed normally, probably activating the "quality control" mechanism of the endoplasmic reticulum.

Developmental expression of sodium entry pathways in rat nephron

During the past several years, sites of expression of ion transport proteins in tubules from adult kidneys have been described and correlated with functional properties. Less information is available concerning sites of expression during tubule morphogenesis, although such expression patterns may be crucial to renal development. In the current studies, patterns of renal axial differentiation were defined by mapping the expression of sodium transport pathways during nephrogenesis in the rat. Combined in situ hybridization and immunohistochemistry were used to localize the Na-Pi cotransporter type 2 (NaPi2), the bumetanide-sensitive Na-K-2Cl cotransporter (NKCC2), the thiazide-sensitive Na-Cl cotransporter (NCC), the Na/Ca exchanger (NaCa), the epithelial sodium channel (rENaC), and 11beta-hydroxysteroid dehydrogenase (11HSD). The onset of expression of these proteins began in post-S-shape stages. NKCC2 was initially expressed at the macula densa region and later extended into the nascent ascending limb of the loop of Henle (TAL), whereas differentiation of the proximal tubular part of the loop of Henle showed a comparatively retarded onset when probed for NaPi2. The NCC was initially found at the distal end of the nascent distal convoluted tubule (DCT) and later extended toward the junction with the TAL. After a period of changing proportions, subsegmentation of the DCT into a proximal part expressing NCC alone and a distal part expressing NCC together with NaCa was evident. Strong coexpression of rENaC and 11HSD was observed in early nascent connecting tubule (CNT) and collecting ducts and later also in the distal portion of the DCT. Ontogeny of the expression of NCC, NaCa, 11HSD, and rENaC in the late distal convolutions indicates a heterogenous origin of the CNT. These data present a detailed analysis of the relations between the anatomic differentiation of the developing renal tubule and the expression of tubular transport proteins.

Rabbit distal convoluted tubule coexpresses NaCl cotransporter and 11 beta-hydroxysteroid dehydrogenase II mRNA

BACKGROUND

Although the renal cortical collecting duct (CCD) is a principal target for aldosterone, recent evidence suggests that salt transport by other nephron segments may also be regulated by aldosterone. Electroneutral and thiazide-sensitive NaCl cotransport by the distal convoluted tubule (DCT) of the rat is increased in animals deprived of dietary NaCl. We tested the hypothesis that the DCT of the rabbit is an aldosterone target tissue.

METHODS

The single-nephron reverse-transcriptase/polymerase chain reaction (RT-PCR) technique was used to determine mRNA expression of NaCl cotransporter and 11 beta-HSD 2 in dissected nephron segments. The rabbit NaCl cotransporter was first cloned and rabbit-specific primers selected. A micro-assay was developed to assess 11 beta-HSD 2 enzyme activity in 0.5 mm samples of the same nephron segments.

RESULTS

NaCl cotransporter was expressed in 0 of 6 proximal tubule (PT), 6 of 6 DCT and 3 of 6 CCD samples, while 11 beta-HSD was found in 0 of 7 PT, 7 of 7 DCT and 9 of 9 CCD samples. Corticosterone was converted to 11-dehydrocorticosterone at a high rate and to a similar extent by both the DCT and CCD, but not the PT.

CONCLUSIONS

We conclude that the DCT is a target tissue for the action of aldosterone. Axial heterogeneity of electroneutral (in DCT) and electrogenic (in CCD) Na transporters along the distal nephron may improve sodium recovery in low salt and volume states.

11Beta-hydroxysteroid dehydrogenase, mineralocorticoid receptor, and thiazide-sensitive Na-Cl cotransporter expression by distal tubules

Mineralocorticoid hormones regulate salt transport along the distal nephron by binding to intracellular receptors and activating gene transcription. Previous experiments showed that systemic aldosterone infusions stimulate thiazide-sensitive Na and Cl transport by distal convoluted tubule (DCT) cells; this effect could have been direct or secondary to systemic hormonal effects. Aldosterone target tissues express both mineralocorticoid receptors and the metabolic enzyme 11beta-hydroxysteroid dehydrogenase type 2. Mineralocorticoid receptors have been localized to the DCT in some experiments, but not in others. Expression of 11beta-hydroxysteroid dehydrogenase type 2 by DCT cells has not been investigated. The present experiments were designed to test the hypothesis that rat DCT cells are targets of aldosterone action. Patterns of mineralocorticoid receptor, 11beta-hydroxysteroid dehydrogenase, thiazide-sensitive Na-Cl cotransporter, and Na/Ca exchanger expression along the distal tubule were examined. A polyclonal antibody was generated to localize the thiazide-sensitive Na-Cl cotransporter. Thiazide-sensitive Na-Cl cotransporter and 11beta-hydroxysteroid dehydrogenase expression were examined using both in situ hybridization and immunocytochemistry; Na/Ca exchanger and mineralocorticoid receptor expression were examined by immunocytochemistry. The results indicate that 11beta-hydroxysteroid dehydrogenase is expressed by DCT cells, as well as connecting tubule cells and principal cells of the collecting duct; expression levels are low near the junction with the thick ascending limb and rise near the transition to the connecting tubule. Mineralocorticoid receptors are expressed by DCT cells, as well as along the thick ascending limb, connecting tubule, and collecting duct. The results indicate that components of the mineralocorticoid receptor system are expressed by DCT cells, suggesting that these cells are targets of aldosterone action.

Expression of the Na-K-2Cl cotransporter by macula densa and thick ascending limb cells of rat and rabbit nephron

Sodium and chloride transport by the macula densa and thick ascending limb of Henle's loop participates importantly in extracellular fluid volume homeostasis, urinary concentration and dilution, control of glomerular filtration, and control of renal hemodynamics. Transepithelial Na and Cl transport across the apical membrane of thick ascending limb (TALH) cells is mediated predominantly by a loop diuretic sensitive Na-K-2Cl cotransport pathway. The corresponding transport protein has recently been cloned. Functional studies suggest that the cotransporter is expressed by macula densa cells as well as by TALH cells. The current studies were designed to identify sites of Na-K-2Cl cotransporter expression along distal nephron in rabbit and rat. Non-isotopic high-resolution in situ hybridization, using an antisense probe for the apical form of the Na-K-2Cl cotransporter identified expression throughout the TALH, from the junction between inner and outer medulla to the transition to distal convoluted tubule. Expression by macula densa cells was confirmed by colocalization using markers specific for macula densa cells. First, Na-K-2Cl cotransporter mRNA was detected in macula densa cells that did not stain with anti-Tamm-Horsfall protein antibodies. Second, Na-K-2Cl cotransporter mRNA was detected in macula densa cells that show positive NADPH-diaphorase reaction, indicating high levels of constitutive nitric oxide synthase activity. In rat, levels of Na-K-2Cl cotransporter mRNA expression were similar in TALH and macula densa cells. In rabbit, expression levels were higher in macula densa cells than in surrounding TALH cells. The present data provide morphological support for a previously established functional concept that Na-K-2Cl cotransport at the TALH is accomplished by the expression of a well-defined cotransporter. At the macula densa, this transporter may establish a crucial link between tubular salt load and glomerular vascular regulation.

Adrenal steroids stimulate thiazide-sensitive NaCl transport by rat renal distal tubules

The current experiments were designed to test the hypothesis that adrenal steroids increase thiazide-sensitive Na and Cl transport by the mammalian renal distal convoluted tubule (DCT). Male Sprague-Dawley rats were adrenalectomized and received steroid hormones by osmotic pumps. Six groups of animals were studied as follows: group I, no hormones; group II, replacement levels of dexamethasone only; group III, replacement levels of aldosterone only; group IV, replacement levels of both hormones; group V; replacement levels of aldosterone and high levels of dexamethasone; and group VI, replacement levels of dexamethasone and high levels of aldosterone. Circulating levels of both hormones were found to be in the high physiological range when infused at the high rate. In vivo microperfusion of distal tubules was performed to determine rates of Na and Cl transport. Chlorothiazide was used to assess the magnitude of electroneutral Na-Cl cotransport. Both aldosterone and dexamethasone stimulated thiazide-sensitive Na and Cl transport by the distal tubule by more than fivefold. [3H]metolazone binding was measured to assess the number of thiazide-sensitive Na-Cl cotransporters in renal cortex. Each steroid also increased the number of [3H]metolazone binding sites in kidney cortex more than threefold. The results are consistent with the presence of both mineralocorticoid and glucocorticoid receptors in the mammalian DCT. Physiological changes in circulating levels of adrenal steroids may affect renal NaCl excretion in part by regulating the rate of electroneutral Na-Cl absorption by the DCT.

Expression of the thiazide-sensitive Na-Cl cotransporter in rat and human kidney

An electroneutral thiazide-sensitive Na-Cl cotransport pathway (TSC) has been localized functionally to the distal convoluted tubule (DCT), although the TSC has also been detected in the connecting tubule (CNT), the cortical collecting duct, and the medullary collecting tubule as well. The present experiments were designed to localize expression of message for the TSC in rat and human kidney. A riboprobe, generated from the mouse TSC, was used for in situ hybridization. Simultaneous immunocytochemistry, using antibodies to Tamm-Horsfall protein, band 3, and the Na+/Ca2+ exchanger, permitted delineation of specific nephron segments. In rat, message for the TSC was highly expressed in DCT cells but not elsewhere. The transition from thick ascending limb to DCT was abrupt, whereas the transition to CNT was gradual. In the more distal region of rat DCT (DCT-2), which contained few intercalated cells, both TSC message and Na+/Ca2+ exchanger immunoreactivity were present. Treatment of rats with furosemide for 5 days increased expression of TSC message within the DCT but did not induce its expression elsewhere. In humans, expression of TSC message was also highest in cells of the DCT. In humans, however, expression extended well into the CNT. These experiments indicate that the TSC is expressed predominantly by DCT cells in both rat and humans, although expression extends into the CNT cells in humans. They also show that the TSC and Na+/Ca2+ exchanger are coexpressed by a subpopulation of DCT cells near the junction with the CNT.

Expression of the thiazide-sensitive Na-Cl cotransporter by rabbit distal convoluted tubule cells

A thiazide-sensitive Na-Cl cotransporter contributes importantly to mammalian salt homeostasis by mediating Na-Cl transport along the renal distal tubule. Although it has been accepted that thiazide-sensitive Na-Cl cotransport occurs predominantly along the distal convoluted tubule in rats and mice, sites of expression in the rabbit have been controversial. A commonly accepted model of rabbit distal nephron transport pathways identifies the connecting tubule, not the distal convoluted tubule, as the predominant site of thiazide-sensitive Na-Cl cotransport. The thiazide-sensitive Na-Cl cotransporter has been cloned recently. The present experiments were designed to localize sites of thiazide-sensitive Na-Cl cotransporter mRNA expression along the rabbit distal nephron. Nonradioactive in situ hybridization with a thiazide-sensitive Na-Cl cotransporter probe was combined with immunocytochemistry with an antibody that recognizes distal convoluted tubule cells and with a Na+/Ca2+ exchanger antibody that recognizes only connecting tubule cells. The results indicate that thiazide-sensitive Na-Cl cotransporter mRNA is highly expressed by cells of the distal convoluted tubule and not by connecting tubule cells. Segments that stain with the Na+/Ca2+ exchanger antibody (connecting tubules) do not demonstrate thiazide-sensitive Na-Cl cotransporter mRNA expression. We conclude that the predominant site of thiazide-sensitive Na-Cl cotransporter mRNA expression in rabbit distal nephron is the distal convoluted tubule and that sites of mRNA expression of electroneutral Na and Cl transport are similar in rabbits, rats, and mice.

Diuretic drugs and the treatment of edema: from clinic to bench and back again

Despite wide variations in dietary NaCl intake, homeostatic mechanisms ensure that renal NaCl excretion matches intake at steady state. This does not imply, however, that extracellular fluid volume is maintained within narrow limits. In contrast with blood pressure, which appears to be tightly controlled, extracellular fluid volume varies significantly, even in normal individuals, when dietary NaCl intake changes. Cardiac, liver, or renal disease can perturb the relationship between NaCl intake and extracellular fluid volume and lead to symptomatic edema. All major classes of diuretic drugs in use today were developed between 1950 and 1970. These drugs were developed empirically, without knowledge of specific ion transport pathways, but experimental work during the past 15 years has shown that each major class of diuretic inhibits a specific ion transport protein in the kidney. These transport proteins have been characterized physiologically and the mechanisms by which each diuretic drug inhibits ion transport have been defined. Antibodies directed against these transport proteins have localized ion transport pathways to specific cell types along the nephron. Most recently, isoforms of each class of diuretic-sensitive Na transport pathway have been cloned. Ongoing experimental work is aimed at exploring relationships between families of transporters, determining the structural prerequisites for ion transport, and studying molecular mechanisms of transport regulation. Treatment of edema with diuretics is often straightforward, but can lead to adaptive changes in nephron structure and function. These adaptations can limit the effectiveness of diuretic drugs; maneuvers aimed at blocking these processes can be effective approaches to patients who are resistant to diuretic drugs.