Genetic heterogeneity of familial hyperkalaemic hypertension

Learning Physiology From Inherited Kidney Disorders

The identification of genes causing inherited kidney diseases yielded crucial insights in the molecular basis of disease and improved our understanding of physiological processes that operate in the kidney. Monogenic kidney disorders are caused by mutations in genes coding for a large variety of proteins including receptors, channels and transporters, enzymes, transcription factors, and structural components, operating in specialized cell types that perform highly regulated homeostatic functions. Common variants in some of these genes are also associated with complex traits, as evidenced by genome-wide association studies in the general population. In this review, we discuss how the molecular genetics of inherited disorders affecting different tubular segments of the nephron improved our understanding of various transport processes and of their involvement in homeostasis, while providing novel therapeutic targets. These include inherited disorders causing a dysfunction of the proximal tubule (renal Fanconi syndrome), with emphasis on epithelial differentiation and receptor-mediated endocytosis, or affecting the reabsorption of glucose, the handling of uric acid, and the reabsorption of sodium, calcium, and magnesium along the kidney tubule.

Genetic mechanisms of human hypertension and their implications for blood pressure physiology

Hypertension, or elevated blood pressure, constitutes a major public health burden that affects more than 1 billion people worldwide and contributes to ~9 million deaths annually. Hereditary factors are thought to contribute to up to 50% of interindividual blood pressure variability. Blood pressure in the general population approximately shows a normal distribution and is thought to be a polygenic trait. In rare cases, early-onset hypertension or hypotension are inherited as Mendelian traits. The identification of the underlying Mendelian genes and variants has contributed to our understanding of the physiology of blood pressure regulation, emphasizing renal salt handling and the renin angiotensin aldosterone system as players in the determination of blood pressure. Genome-wide association studies (GWAS) have revealed more than 100 variants that are associated with blood pressure, typically with small effect sizes, which cumulatively explain ~3.5% of blood pressure trait variability. Several GWAS associations point to a role of the vasculature in the pathogenesis of hypertension. Despite these advances, the majority of the genetic contributors to blood pressure regulation are currently unknown; whether large-scale exome or genome sequencing studies will unravel these factors remains to be determined.

Gordon Syndrome: a continuing story

Gordon Syndrome (GS) is a rare familial hypertension syndrome with a characteristic hyperkalaemia which distinguishes it from other syndromic forms of hypertension that typically cause hypokalaemia. Patients with GS respond to aggressive salt-restriction or relatively small doses of thiazide diuretics, which suggests that activation of the thiazide-sensitive Na/Cl cotransporter (NCC) in the distal nephron is to blame. However, the mechanism has proved to be complex. In 2001, mutations in genes encoding two serine/threonine kinases, WNK1 and WNK4, were identified as causing GS. However, it took several years to appreciate that these kinases operated in a cascade with downstream serine/threonine kinases (SPAK and OSR1) actually phosphorylating and activating NCC and the closely related cotransporters NKCC1 and NKCC2. The hyperkalaemia in GS arises from an independent action of WNK1/WNK4 to reduce cell-surface expression of ROMK, the secretory K-channel in the collecting ducts. However, mutations in WNK1/4 are present in a small minority of GS families, and further genes have emerged (CUL3 and KLHL3) that code for Cullin-3 (a scaffold protein in an ubiquitin-E3 ligase) and an adaptor protein, Kelch3, respectively. These new players regulate the ubiquitination and proteasomal degradation of WNK kinases, thereby adding to the complex picture we now have of NCC regulation in the distal nephron.

Detection of mutations in KLHL3 and CUL3 in families with FHHt (familial hyperkalaemic hypertension or Gordon's syndrome)

The study of families with rare inherited forms of hypo- and hyper-tension has been one of the most successful strategies to probe the molecular pathophysiology of blood pressure control and has revealed dysregulation of distal nephron Na+ reabsorption to be a common mechanism. FHHt (familial hyperkalaemic hypertension; also known as Gordon's syndrome) is a salt-dependent form of hypertension caused by mutations in the regulators of the thiazide-sensitive Na+-Cl- co-transporter NCC [also known as SLC12A3 (solute carrier family 12 member 3)] and is effectively treated by thiazide diuretics and/or dietary salt restriction. Variation in at least four genes can cause FHHt, including WNK1 [With No lysine (=K) 1] and WNK4, KLHL3 (kelch-like family member 3), and CUL3 (cullin 3). In the present study we have identified novel disease-causing variants in CUL3 and KLHL3 segregating in 63% of the pedigrees with previously unexplained FHHt, confirming the importance of these recently described FHHt genes. We have demonstrated conclusively, in two unrelated affected individuals, that rare intronic variants in CUL3 cause the skipping of exon 9 as has been proposed previously. KLHL3 variants all occur in kelch-repeat domains and so probably disrupt WNK complex binding. We have found no evidence of any plausible disease-causing variants within SLC4A8 (an alternative thiazide-sensitive sodium transporter) in this population. The results of the present study support the existing evidence that the CUL3 and KLHL3 gene products are physiologically important regulators of thiazide-sensitive distal nephron NaCl reabsorption, and hence potentially interesting novel anti-hypertensive drug targets. As a third of our non-WNK FHHt families do not have plausible CUL3 or KLHL3 variants, there are probably additional, as yet undiscovered, regulators of the thiazide-sensitive pathways.

Regulation of the renal Na+-Cl- cotransporter by phosphorylation and ubiquitylation

The activity of the renal thiazide-sensitive NaCl cotransporter (NCC) in the distal convoluted tubule plays a key role in defining arterial blood pressure levels. Increased or decreased activity of the NCC is associated with arterial hypertension or hypotension, respectively. Thus it is of major interest to understand the activity of NCC using in vivo models. Phosphorylation of certain residues of the amino-terminal domain of NCC has been shown to be associated with its activation. The development of phospho-specific antibodies against these sites provides a powerful tool that is helping to increase our understanding of the molecular physiology of NCC. Additionally, NCC expression in the plasma membrane is modulated by ubiquitylation, which represents another major mechanism for regulating protein activity. This work presents a review of our current knowledge of the regulation of NCC activity by phosphorylation and ubiquitylation.

Low renin hypertension

Low renin hypertension is an important and often underdiagnosed cause of hypertension. It may be associated with high aldosterone levels as in Conn's syndrome or low aldosterone levels as in Liddle syndrome, and syndrome of apparent mineralocorticoid excess, glucocorticoid remediable hypertension etc. Some forms of essential hypertension are also associated with low renin levels. Hypokalemia may be an important finding in low renin hypertension. The aldosterone to renin ratio helps in correct diagnosis. The treatment varies with etiology hence an accurate diagnosis is essential. Aldosterone antagonists play an important role in medical management of some varieties of low renin hypertension.

Syndromes of impaired ion handling in the distal nephron: pseudohypoaldosteronism and familial hyperkalemic hypertension

The distal nephron, which is the site of the micro-regulation of water absorption and ion handling in the kidneys, is under the control of aldosterone. Impairment of the mineralocorticoid signal transduction pathway results in resistance to the action of aldosterone and of mineralocorticoids in general. Herein, we review two syndromes in which ion handling in the distal nephron is impaired: pseudohypoaldosteronism (PHA) and familial hyperkalemic hypertension (FHH). PHA is a rare inherited syndrome characterized by mineralocorticoid resistance, which leads to salt loss, hypotension, hyperkalemia and metabolic acidosis. There are two types of this syndrome: a renal (autosomal dominant) type due to mutations of the mineralocorticoid receptor (MR), and a systemic (autosomal recessive) type due to mutations of the epithelial sodium channel (ENaC). There is also a transient form of PHA, which may be due to urinary tract infections, obstructive uropathy or several medications. FHH is a rare autosomal dominant syndrome, characterized by salt retention, hypertension, hyperkalemia and metabolic acidosis. In FHH, mutations of WNK (with-no-lysine kinase) 4 and 1 alter the activity of several ion transportation systems in the distal nephron. The study of the pathophysiology of PHA and FHH greatly elucidated our understanding of the renin-angiotensin-aldosterone system function and ion handling in the distal nephron. The physiological role of the distal nephron and the pathophysiology of diseases in which the renal tubule is implicated may hence be better understood and, based on this understanding, new drugs can be developed.

SPAK and WNK kinases: a new target for blood pressure treatment?

PURPOSE OF REVIEW

The regulation of sodium reabsorption by the distal kidney is fundamental to blood pressure control. The clinical success of thiazide diuretics as antihypertensive drugs underscores the importance of its target, the thiazide-sensitive sodium/chloride cotransporter (NCC), in this process. However, thiazides are often ineffective as monotherapy and have significant side-effects. An understanding of NCC regulation at a molecular level may allow the design of better tolerated and more effective antihypertensive agents. The aim of this review is to provide an overview of the recent developments in the regulation of NCC, highlighting a potential new therapeutic target for the treatment of hypertension.

RECENT FINDINGS

It has been appreciated for several years that WNK kinases affect NCC expression, following the discovery that mutations in WNK genes cause Gordon syndrome (pseudohypoaldosteronism type II), although the precise molecular mechanisms were unclear. What has emerged from further in-vitro work is a WNK signalling cascade with the STE20 kinases SPAK and OSR1 as the 'missing' intermediate kinases that are activated by WNKs. Confirmation that this WNK-SPAK cascade operates in vivo comes from work on the phenotype of a kinase-dead SPAK knockin mouse. This mouse is markedly hypotensive, salt wasting, and almost all of its NCC protein in the distal kidney is dephosphorylated. Finally, a genome-wide association study has identified an intronic SPAK polymorphism that associates with human blood pressure.

SUMMARY

SPAK is a recently identified regulator of NCC and, hence, sodium reabsorption in the distal nephron. SPAK gene variants may also be important players in essential hypertension. If the phenotype of the kinase-dead SPAK mouse mimics pharmacological inhibition of this kinase, then SPAK is a potentially very interesting new antihypertensive drug target.

Familial or genetic primary aldosteronism and Gordon syndrome

Salt-sensitive forms of hypertension have received considerable renewed attention in recent years. This article focuses on 2 main forms of salt-sensitive hypertension (familial or genetic primary aldosteronism [PA] and Gordon syndrome) and the current state of knowledge regarding their genetic bases. The glucocorticoid-remediable form of familial PA (familial hyperaldosteronism type I) is dealt with only briefly because it is covered in depth elsewhere.

A Single Amino Acid Substitution Makes WNK4 Susceptible to SB 203580 and SB 202190

Regulation of the SLC12 family of membrane transporters including NCCT involves a scaffold of interacting proteins including the STE 20 kinase SPAK and the WNK kinases, WNK 1 and WNK 4, which are mutated in the hypertensive syndrome of pseudohypoaldosteronism type 2 (PHAII). WNK4 regulates NCCT by affecting forward trafficking to the surface membrane. Studies in Xenopus using kinase dead WNK4 site mutants have produced inconsistent results with regard to the necessity of kinase function for NCCT regulation. Dynamic inhibition of WNK4 by small molecules may bring clarity to this issue however WNK4 is naturally resistant to commercial MAP kinase inhibitors owing to steric constraints prohibiting entry of small molecules to the active site. Using an approach similar to that used in p38 and ERK, we show that a single substitution in WNK4 (T261G) dramatically enhances its susceptibility to the inhibitors SB 202190 and SB 203580.

The thiazide-sensitive Na+-Cl- cotransporter: molecular biology, functional properties, and regulation by WNKs

The thiazide-sensitive Na+-Cl(-) cotransporter is the major salt reabsorption pathway in the distal convoluted tubule, which is located just after the macula densa at the beginning of the aldosterone-sensitive nephron. This cotransporter was identified at the molecular level in the early 1990s by the pioneering work of Steven C. Hebert and coworkers, opening the molecular area, not only for the Na+-Cl(-) cotransporter but also for the family of electroneutral cation-coupled chloride cotransporters that includes the loop diuretic-sensitive Na+-K+-2Cl(-) cotransporter of the thick ascending limb of Henle's loop. This work honoring the memory of Steve Hebert presents a brief review of our current knowledge about salt and water homeostasis generated as a consequence of cloning the cotransporter, with particular emphasis on the molecular biology, physiological properties, human disease due to decreased or increased activity of the cotransporter, and regulation of the cotransporter by a family of serine/threonine kinases known as WNK. Thus one of the legacies of Steve Hebert is a better understanding of salt and water homeostasis.

Heritable forms of hypertension

Among the causes of secondary hypertension are a group of disorders with a Mendelian inheritance pattern. Recent advances in molecular biology have unveiled the pathogenesis of hypertension in many of these conditions. Remarkably, the mechanism in every case has proved to be upregulation of sodium (Na) reabsorption in the distal nephron, with accompanying expansion of extracellular volume. In one group, the mutations involve the Na-transport machinery in distal tubule cells themselves: the distal convoluted tubule (DCT) cell and the principal cell of the collecting duct. Examples include Liddle's syndrome, with an activating mutation of epithelial Na channel (ENaC); two types of Gordon's syndrome, with mutations in two regulatory kinases [with no lysine (K) serine/threonine protein kinases (WNK)1 or WNK4]; and apparent mineralocorticoid excess (AME), with an inactivating mutation in the glucocorticoid-metabolizing 11beta-hydroxysteroid dehydrogenase type 2 enzyme (11HD2). In another group, abnormal adrenal steroid production leads to inappropriate stimulation of the mineralocorticoid receptor (MR) in the distal nephron. The pathophysiology may involve inappropriate production of aldosterone [in glucocorticoid-remediable aldosteronism (GRA) and familial hyperaldosteronism type II (FH II)], of cortisol (in familial glucocorticoid resistance), or of other steroid metabolites (in congenital adrenal hyperplasia and GRA). In contrast to earlier beliefs, hypertension in many of the inherited disorders may be mild, and electrolyte and acid-base abnormalities are often not present. Monogenic hypertension should therefore enter the differential diagnosis of any child or adolescent with hypertension. Plasma renin activity (PRA) is the appropriate screening tool for all types of inherited hypertension.

WNK kinases, renal ion transport and hypertension

Two members of a recently discovered family of protein kinases are the cause of an inherited disease known as pseudohypoaldosteronism type II (PHAII). These patients exhibit arterial hypertension together with hyperkalemia and metabolic acidosis. This is a mirror image of Gitelman disease that is due to inactivating mutations of the SLC12A3 gene that encodes the thiazide-sensitive Na(+):Cl(-) cotransporter. The uncovered genes causing PHAII encode for serine/threonine kinases known as WNK1 and WNK4. Physiological and biochemical studies have revealed that WNK1 and WNK4 modulate the activity of several transport pathways of the aldosterone-sensitive distal nephron, thus increasing our understanding of how diverse renal ion transport proteins are coordinated to regulate normal blood pressure levels. Observations discussed in the present work place WNK1 and WNK4 as genes involved in the genesis of essential hypertension and as potential targets for the development of antihypertensive drugs.

Diagnosis and treatment of low-renin hypertension

Plasma renin levels can be used to classify hypertension. A significant proportion of hypertensive individuals display a low-renin profile and thus low-renin hypertension (LRH) requires appropriate diagnosis and treatment. LRH includes essential, secondary and genetic forms, the most common of which are low-renin essential hypertension and primary aldosteronism. Several studies have investigated the relationship between PRA status and clinical response to different antihypertensive therapies. The present review will discuss the differential diagnosis of LRH subtypes and the most appropriate treatment options based on the pathophysiological background of this condition.

A new kindred with pseudohypoaldosteronism type II and a novel mutation (564D>H) in the acidic motif of the WNK4 gene

We identified a new kindred with the familial syndrome of hypertension and hyperkalemia (pseudohypoaldosteronism type II or Gordon's syndrome) containing an affected father and son. Mutation analysis confirmed a single heterozygous G to C substitution within exon 7 (1690G>C) that causes a missense mutation within the acidic motif of WNK4 (564D>H). We confirmed the function of this novel mutation by coexpressing it in Xenopus oocytes with either the NaCl cotransporter (NCCT) or the inwardly rectifying K-channel (ROMK). Wild-type WNK4 inhibits 22Na+ flux in Xenopus oocytes expressing NCCT by approximately 90% (P<0.001), whereas the 564D>H mutant had no significantly inhibitory effect on flux through NCCT. In oocytes expressing ROMK, wild-type WNK4 produced >50% inhibition of steady-state current through ROMK at a +20-mV holding potential (P<0.001). The 564D>H mutant produced further inhibition with steady-state currents to some 60% to 70% of those seen with the wild-type WNK4. Using fluorescent-tagged NCCT (enhanced cyan fluorescent protein-NCCT) and ROMK (enhanced green fluorescent protein-ROMK) to quantify the expression of the proteins in the oocyte membrane, it appears that the functional effects of the 564D>H mutation can be explained by alteration in the surface expression of NCCT and ROMK. Compared with wild-type WNK4, WNK4 564D>H causes increased cell surface expression of NCCT but reduced expression of ROMK. This work confirms that the novel missense mutation in WNK4, 564D>H, is functionally active and highlights further how switching charge on a single residue in the acid motif of WNK4 affects its interaction with the thiazide-sensitive target NCCT and the potassium channel ROMK.

Role of WNK kinases in regulating tubular salt and potassium transport and in the development of hypertension

A recently discovered family of protein kinases is responsible for an autosomal-dominant disease known as Gordon's syndrome or pseudohypoaldosteronism type II (PHA-II) that features hyperkalemia and hyperchloremic metabolic acidosis, accompanied by hypertension and hypercalciuria. Four genes have been described in this kinase family, which has been named WNK, due to the absence of a key lysine in kinase subdomain II (with no K kinases). Two of these genes, WNK1 and WNK4 located in human chromosomes 12 and 17, respectively, are responsible for PHA-II. Immunohystochemical analysis revealed that WNK1 and WNK4 are predominantly expressed in the distal convoluted tubule and collecting duct. The physiological studies have shown that WNK4 downregulates the activity of ion transport pathways expressed in these nephron segments, such as the apical thiazide-sensitive Na+-Cl−cotransporter and apical secretory K+channel ROMK, as well as upregulates paracellular chloride transport and phosphorylation of tight junction proteins such as claudins. In addition, WNK4 downregulates other Cl−influx pathways such as the basolateral Na+-K+-2Cl−cotransporter and Cl−/HCO3−exchanger. WNK4 mutations behave as a loss of function for the Na+-Cl−cotransporter and a gain of function when it comes to ROMK and claudins. These dual effects of WNK4 mutations fit with proposed mechanisms for developing electrolyte abnormalities and hypertension in PHA-II and point to WNK4 as a multifunctional regulator of diverse ion transporters.

WNK1 et WNK4, nouveaux acteurs de l’homéostasie hydrosodée

Arterial hypertension is a complex trait influenced by a variety of environmental and genetic factors. Several approaches can be used to identify its susceptibility genes : one is to study rare monogenic forms of hypertension, like familial hyperkalemic hypertension (FHH). Also known as pseudohypoaldosteronism type 2 or Gordon syndrome, FHH is characterized by hypertension, hyperkalemia despite normal renal glomerular filtration rate, abnormalities which are particularly sensitive to thiazide diuretics. Mild hyperchloremia, metabolic acidosis, and suppressed plasma renin activity are associated findings. Despite its phenotypic and genetic heterogeneity, mutations in two related genes, WNK1 and WNK4, were recently identified. These genes belong to a newly identified family of serine-threonine (with no lysine [K]) kinases. Both are highly expressed in the kidney and in a variety of epithelia involved in chloride transport. It has thus been postulated that these two kinases could be implicated in a new pathway of ionic transport regulation. Several studies have very recently confirmed this hypothesis in vitro, in Xenopus oocytes or kidney cell lines. They have shown that, in the renal distal tubule, WNK4 inhibits sodium reabsorption and potassium secretion, via inhibition of NCC (thiazide-sensitive Na+-Cl- cotransporter) and K+ channel ROMK activity, respectively. Interestingly, FHH mutations have opposite effects : while they lead to loss of NCC inhibition, they increase ROMK inhibition. Moreover, they also increase paracellular permeability to chloride of MDCK cells. WNK4 also inhibits apical and basal chloride transporters present in extra-renal epithelia, such as CFEX and Na+-K+-2 Cl-, respectively. It is also interesting to note that the WNK4-mediated negative regulation of NCC activity is in turn inhibited by WNK1. By its role on several transporters, WNK4 appears as a putative key regulator of ionic transport and blood pressure.

A case of hypertension and hyperkalaemia

A52-year-old man was referred urgently to hospital by his GP because he had hyperkalaemia (7.3 mmol/litre; normal range 3.5–5.0 mmol/litre). Two months previously he was diagnosed as having diabetes mellitus when he was admitted to hospital with diabetic ketoacidosis. At that time his plasma sodium was 126 mmol/litre (normal range 133–147 mmol/litre), potassium 2.7 mmol/litre, urea 18 mmol/litre (normal range 2.5–7.5 mmol/litre) and creatinine 159 μmmol/litre (normal range 60–120 μmol/litre). Bendrofluazide, nifedipine and atenolol were stopped. Six weeks later he felt unwell, visited his doctor, and was found to be hypertensive. He was started on lisinopril 5 mg once daily. Ten days later he was still hypertensive, and lisinopril was increased to 10 mg once daily and routine plasma urea and electrolytes were measured.

His past medical history includes a Colles' fracture in 1992. At this time, hypertension and hyperkalaemia were noted and investigated. He was diagnosed as having Gordon's syndrome. Three of his daughters and two of his granddaughters were also found to have the syndrome. His father had been hypertensive and died suddenly at the age of 40 years. His uncle had also died suddenly at the age of 33 years.

On examination his height was 1.60 m, body mass index (BMI) was 30 kg/m2 and blood pressure was 168/75 mmHg. Heart, chest, abdominal, and neurological examination were normal. The electrocardiogram did not show manifestations of hyperkalaemia.

The hyperkalaemia was treated with insulin/glucose infusion. Lisinopril was stopped and bendrofluazide was started with normalization of plasma potassium level and blood pressure.

Pseudo-hypoaldostéronismes

TWO FORMS: Pseudohypoaldosteronisms (PHA) are characterized by end-organ resistance to aldosterone inducing hyperkalemia and hyperaldosteronism. There are two forms of PHA classified according to the level of blood pressure with either hypotension (Type 1 PHA or PHA 1) or hypertension (Type 2 PHA or PHA 2). PHA 1: The association with hypotension and high renin level (PHA 1) is responsible for type 4 tubular acidosis and should suggest congenital or acquired excessive salt loss. Acquired forms are associated with salt wasting of urinary (nephropathy) or digestive (colon resection + ileostomy) origin. Congenital neonatal forms are either sporadic or autosomal dominant or recessive. Sporadic or autosomal dominant forms are caused by mutations in the mineralocorticoid receptor gene and generally remit with age. Autosomal recessive forms are caused by mutations in the gene encoding the amiloride-sensitive sodium channel and are clinically more severe with pulmonary symptoms. PHA 2: The association of hyperkalemia/hyperaldosteronism with high blood pressure should suggest PHA 2 or Gordon's syndrome, still called familial hyperkalemic hypertension. This form of low-renin hypertension is caused by mutations in the WNK genes (WNK 1 for PHA 2C and WNK 4 for PHA 2B), but other genes located on different loci are also involved. These WNK kinases constitute a new signalisation pathway that would regulate blood pressure and homeostasy of Na+, K+, H+ and Cl- ions.

Phenotypic and genetic heterogeneity of familial hyperkalaemic hypertension (Gordon syndrome)

1. Familial hyperkalaemic hypertension (FHH), also called pseudohypoaldosteronism type II (PHA2) or Gordon syndrome, is a rare Mendelian-form of low-renin hypertension. The first cases of FHH were reported approximately 30 years ago and they described the peculiar biochemical abnormalities (i.e. hyperkalaemia and hyperchloraemic acidosis despite a normal glomerular filtration rate). 2. Since then, more than 90 single cases and families have been reported in the literature. These various reports show marked differences in phenotype. 3. Our group has now collected 14 unrelated pedigrees originating from different parts of France and Europe. We confirm the large variations in the age of discovery and in the severity of the biochemical abnormalities from one individual to another and from one family to another one. 4. Blood pressure levels have no significant relationship with hyperkalaemia or hyperchloraemia, but there is a positive relationship with age, as in the normal population. 5. Analyses of clinical features and Mendelian segregation in our families demonstrate autosomal-dominant inheritance, as expected from the literature. 6. Efforts have been made in the past years to unravel the gene responsible for the disease. Until now, a primary responsibility of the gene encoding the thiazide-sensitive Na-Cl cotransporter (SLC12A3) has been excluded in PHA2 families. Three loci have been identified on chromosomes 1 (PHA2A), 17 (PHA2B) and 12 (PHA2C). 7. More recently, analysis of three additional pedigrees, including 10 affected subjects, with over 25 members allowed us to demonstrate further genetic heterogeneity and the existence of at least a fourth locus. 8. The genetic heterogeneity of this syndrome, and thus the variety of molecular defects, suggests the role of either several new components of the same pathway, multiple aldosterone- regulated effectors or direct or indirect partners of the Na-Cl cotransporter.