Association of Familial Hyperkalemia and Hypertension with Proximal Renal Tubular Acidosis and Epileptic Seizures.

INTRODUCTION: Familial hyperkalemic hypertension (FHHt) is an inherited disease characterized by hyperkalemia, hypertension, and hyperchloremic acidosis (HCA). The primary defect is a hyperactive sodium chloride co-transporter, expressed in the renal distal tubule. FHHt is caused by mutation in either WNK1, WNK4, KLHL3, or Cul3. The mechanism of HCA is not completely understood. METHODS: Clinical and genetic data were collected from the largest family with FHHt described in the literature. Urine ammonia was measured in 26 family members. Epilepsy was diagnosed clinically. RESULTS: Of the 85 family members, 44 are affected by the Q565E WNK4 mutation, and 28 are newly described. In genetically engineered mice, urinary ammonium was decreased. In our study, urine ammonium did not change. In 11 unaffected subjects, urine ammonia per creatinine was 8.013 ± 3.620 mm/mm, and in 15 subjects affected by FHHt, it was 8.990 ± 4.300 mm/mm (p = 0.546, not significant). Due to the large family size and prolonged follow-up, rare conditions can be identified. Indeed, two children have genetic generalized epilepsy and one child has migraine. The prevalence of epilepsy is 4.545% (2/44) much higher than in the general population (0.681%). This difference is statistically significant (χ2 with Yates correction = 5.127, p = 0.023). CONCLUSIONS: We provide further evidence that the origin of HCA in FHHt lies in the proximal renal tubule. The association of FHHt with epilepsy leads us to speculate that the raised serum K in susceptible subjects may cause a rise in CSF K, and extracellular cerebral K, leading to epilepsy.

Classification of pseudohypoaldosteronism type II as type IV renal tubular acidosis: results of a literature review.

Pseudohypoaldosteronism (PHA) type II (PHA2) is a genetic disorder that leads to volume overload and hyperkalemic metabolic acidosis. PHA2 and PHA type I (PHA1) have been considered to be genetic and pediatric counterparts to type IV renal tubular acidosis (RTA). Type IV RTA is frequently found in adults with chronic kidney disease and is characterized by hyperchloremic hyperkalemic acidosis with normal anion gap (AG). However, we recently observed that PHA1 was not always identical to type IV RTA. In this study, we focused on the acid-base balance in PHA2. Through a literature search published between 2008-2020, 46 molecularly diagnosed cases with PHA2 were identified (median age of 14 years). They comprised 11 sets of familial and 16 sporadic cases and the pathology was associated with mutations in WNK 4 (n = 1), KLHL3 (n = 17), and CUL3 (n = 9). The mean potassium (K+) level was 6.2 ± 0.9 mEq/L (n = 46, range 4.0-8.6 mEq/L), whereas that of chloride (Cl-) was 110 ± 3.5 mEq/L (n = 41, 100-119 mEq/L), with 28 of 41 cases identified as hyperchloremic. More than half of the cases (18/35) presented with metabolic acidosis. Although AG data was obtained only in 16 cases, all but one cases were within normal AG range. Both Cl- and HCO3- levels showed significant correlations with K+ levels, which suggested that the degree of hyperchloremia and acidosis reflect the clinical severity, and is closely related to the fundamental pathophysiology of PHA2. In conclusion, our study confirmed that PHA2 is compatible with type IV RTA based on laboratory findings.

Gordon syndrome caused by a CUL3 mutation in a patient with short stature in Korea: a case report.

Objectives: Gordon syndrome (GS), also known as pseudohypoaldosteronism type II, is a rare tubular disease characterized by hypertension, hyperkalemia, and metabolic acidosis. Its causative genes are CUL3, KLHL3, WNK1, and WNK4, and they are associated with varying severity of the disease. Herein, we report the first case of GS caused by a CUL3 mutation in a patient with short stature in Korea.Case presentation: A 7-year-old boy had hypertension, metabolic acidosis, and persistent hyperkalemia, which were initially detected during the evaluation of short stature. He was born small for gestational age at late preterm gestation. Laboratory test findings showed hyperkalemia with low trans-tubular potassium gradient, hyperchloremic metabolic acidosis with a normal anion gap, and low plasma renin levels. Genetic analysis revealed a heterozygous de novo mutation in the CUL3 gene (c.1377+1G > C in intron 9). Thus, a diagnosis of GS was made. The results of the endocrine function test (including growth hormone stimulation tests) were normal. After thiazide treatment, the patient's electrolyte levels were normalized. However, he presented with persistent hypertension and short stature.Conclusions: GS should be considered in children with short stature, hypertension, and hyperkalemia, and early treatment may reduce complications.

A rare case of persistent hyperkalaemia.

Hyperkalaemia is a common biochemical finding that can allude to preanalytical or truly pathological causes. Here, we present a case of a 41-year-old female patient who has regularly presented with incidences of isolated hyperkalaemia since 2012, with otherwise normal renal function and no other associated symptoms. Investigations into the patient's family history revealed similar biochemical findings in her brother and eldest son. Familial causes of hyperkalaemia were investigated and an eventual diagnosis of pseudo-hypoaldosteronism type 2C was established. This is a rare congenital renal tubular disorder - also known as Gordon syndrome - that can cause a characteristic triad of symptoms that include hyperkalaemia, metabolic acidosis and hypertension. The presence and severity of each of these symptoms is dependent upon the disease-causing mutation that occurs in WNK4, WNK1, CUL3 or KLHL3 genes. These mutations alter the regulation of sodium/chloride co-transporter (NCC) expression on the luminal membrane of the principal cells of the distal convoluted tubule, disrupting normal homeostatic regulation of electrolyte reabsorption and excretion. The resolution for treating this condition is the administration of a thiazide diuretic, which directly counteracts the effects of NCC co-transporter overexpression and consequently aims to resolve the symptoms that arise as a result of this aberrant signalling. The case described here uniquely presents an extremely rare pathogenic variant in the conserved acidic motif of WNK1 resulting in a clear electrolyte phenotype with no hypertension.

Familial pseudohyperkalemia induces significantly higher levels of extracellular potassium in early storage of red cell concentrates without affecting other standard measures of quality: A case control and allele frequency study.

BACKGROUND: Familial pseudohyperkalemia (FP) is characterized by an increased rate of potassium leakage in refrigerated red cells and is associated with the minor allele of the single nucleotide polymorphism rs148211042 (R723Q) in the ABCB6 gene. The study aims were to obtain the minor allele frequencies of ABCB6 variants and to measure supernatant potassium accumulation, and other red cell storage parameters, in red cell concentrates (RCC) from carriers of variant rs148211042 under standard blood bank conditions. STUDY DESIGN: Whole blood units were collected from 6 FP individuals and 11 controls and processed into RCC in additive solution. RCC were sampled and tested over cold storage for full blood count, extracellular potassium, glucose, lactate, microvesicle release, deformability, hemolysis, pH, adenosine triphosphate, and 2,3-diphosphoglycerate. RESULTS: Screening of genotyped cohorts identified that variant rs148211042 is present in 1 in 394 British citizens of European ancestry. FP RCC had significantly higher supernatant potassium at all time points from day 3 onwards (p < .001) and higher mean cell volume (p = .032) than controls. The initial rate of potassium release was higher in FP RCC; supernatant potassium reached 46.0 (23.8-57.6) mmol/L (mean [range]) by day 5, increasing to 68.9 (58.8-73.7) mmol/L by day 35. Other quality parameters were not significantly different between FP RCC and controls. CONCLUSION: These data suggest that if a blood donor has FP, reducing the RCC shelf-life to 5 days may be insufficient to reduce the risk of hyperkalemia in clinical scenarios such as neonatal large volume transfusion.

Aldosterone synthase (CYP11B2) deficiency among Palestinian infants: Three novel variants and genetic heterogeneity.

Aldosterone synthase deficiency (ASD) is a rare potentially life-threatening genetic disorder that usually presents during infancy due to pathogenic variants in the CYP11B2 gene. Knowledge about CYP11B2 variants in the Arab population is scarce. Here, we present and analyze five Palestinian patients and their different novel pathogenic variants. Data on clinical presentation, electrolytes, plasma renin activity, and steroid hormone levels of five patients diagnosed with ASD were summarized. Sequencing of the CYP11B2 gene exons was followed by evolutionary conservation analysis and structural modeling of the variants. All patients were from highly consanguineous Palestinian families. The patients presented at 1-4 months of age with recurrent vomiting, poor weight gain, hyponatremia, hyperkalemia, and low aldosterone levels. Genetic analysis of the CYP11B2 gene revealed three homozygous pathogenic variants: p.Ser344Profs*9, p.G452W in two patients from an extended family, and p.Q338stop. A previously described pathogenic variant was found in one patient: p.G288S. We described four different CYP11B2 gene pathogenic variants in a relatively small population. Our findings may contribute to the future early diagnosis and therapy for patients with ASD among Arab patients who present with failure to thrive and compatible electrolyte disturbances.

Understanding the molecular association between hyperkalemia and lung squamous cell carcinomas.

BACKGROUND: Previous studies indicated a strong association between hyperkalemia and lung squamous cell carcinomas (LSCC). However, the underlying mechanism is not fully understood so far. METHODS: Literature-based data mining was conducted to identify genes, molecule, and cell processes linked to both hyperkalemia and LSCC. Pathway analysis was performed to explore the interactive network, common-target network, and common-regulator network for both disorders. Then, a mega-analysis using 11 independent LSCC RNA expression datasets (358 LSCCs and 278 healthy controls) was performed to test the hypothesis that genes influencing hyperkalemia may also play roles in LSCC. RESULTS: There was a significant overlap between the genes implicated with both diseases (20 genes, p-value = 4.98e-15), which counts for 16% of all hyperkalemia genes (125 genes). Network analysis identified 12 molecules as common targets for hyperkalemia and LSCC, and 19 molecules as common regulators. Moreover, 19 molecules were identified within an interactive network, through which hyperkalemia and LSCC could exert influence on each other. In addition, meta-analysis identified one hyperkalemia promoter, SPP1, as a novel contributor for LSCC (LFC = 2.64; p-value = 2.81e-6). MLR analysis suggests geographical region as an influential factor for the expression levels of SPP1 in LSCC patients (p value = 0.036, 0.054). CONCLUSION: Our results showed that there was a common molecular basis for the pathology of both hyperkalemia and LSCC, and that genes promoting hyperkalemia might also play roles in the development of LSCC. However, this study did not suggest hypercalcemia as a casual factor for LSCC.

Effect of Angiotensin II on ENaC in the Distal Convoluted Tubule and in the Cortical Collecting Duct of Mineralocorticoid Receptor Deficient Mice.

Background Angiotensin II stimulates epithelial Na+ channel (ENaC) by aldosterone-independent mechanism. We now test the effect of angiotensin II on ENaC in the distal convoluted tubule (DCT) and cortical collecting duct (CCD) of wild-type (WT) and kidney-specific mineralocorticoid receptor knockout mice (KS-MR-KO). Methods and Results We used electrophysiological, immunoblotting and renal-clearance methods to examine the effect of angiotensin II on ENaC in KS-MR-KO and wild-type mice. High K+ intake stimulated ENaC in the late DCT/early connecting tubule (DCT2/CNT) and in the CCD whereas low sodium intake stimulated ENaC in the CCD but not in the DCT2/CNT. The deletion of MR abolished the stimulatory effect of high K+ and low sodium intake on ENaC, partially inhibited ENaC in DCT2/CNT but almost abolished ENaC activity in the CCD. Application of losartan inhibited ENaC only in DCT2/CNT of both wild-type and KS-MR-KO mice but not in the CCD. Angiotensin II infusion for 3 days has a larger stimulatory effect on ENaC in the DCT2/CNT than in the CCD. Three lines of evidence indicate that angiotensin II can stimulate ENaC by MR-independent mechanism: (1) angiotensin II perfusion augmented ENaC expression in KS-MR-KO mice; (2) angiotensin II stimulated ENaC in the DCT2/CNT but to a lesser degree in the CCD in KS-MR-KO mice; (3) angiotensin II infusion augmented benzamil-induced natriuresis, increased the renal K+ excretion and corrected hyperkalemia of KS-MR-KO mice. Conclusions Angiotensin II-induced stimulation of ENaC occurs mainly in the DCT2/CNT and to a lesser degree in the CCD and MR plays a dominant role in determining ENaC activity in the CCD but to a lesser degree in the DCT2/CNT.

Calcineurin dephosphorylates Kelch-like 3, reversing phosphorylation by angiotensin II and regulating renal electrolyte handling.

Calcineurin is a calcium/calmodulin-regulated phosphatase known for its role in activation of T cells following engagement of the T cell receptor. Calcineurin inhibitors (CNIs) are widely used as immunosuppressive agents; common adverse effects of CNIs are hypertension and hyperkalemia. While previous studies have implicated activation of the Na-Cl cotransporter (NCC) in the renal distal convoluted tubule (DCT) in this toxicity, the molecular mechanism of this effect is unknown. The renal effects of CNIs mimic the hypertension and hyperkalemia that result from germ-line mutations in with-no-lysine (WNK) kinases and the Kelch-like 3 (KLHL3)-CUL3 ubiquitin ligase complex. WNK4 is an activator of NCC and is degraded by binding to KLHL3 followed by WNK4's ubiquitylation and proteasomal degradation. This binding is prevented by phosphorylation of KLHL3 at serine 433 (KLHL3S433-P) via protein kinase C, resulting in increased WNK4 levels and increased NCC activity. Mechanisms mediating KLHL3S433-P dephosphorylation have heretofore been unknown. We now demonstrate that calcineurin expressed in DCT is a potent KLHL3S433-P phosphatase. In mammalian cells, the calcium ionophore ionomycin, a calcineurin activator, reduces KLHL3S433-P levels, and this effect is reversed by the calcineurin inhibitor tacrolimus and by siRNA-mediated knockdown of calcineurin. In vivo, tacrolimus increases levels of KLHL3S433-P, resulting in increased levels of WNK4, phosphorylated SPAK, and NCC. Moreover, tacrolimus attenuates KLHL3-mediated WNK4 ubiquitylation and degradation, while this effect is absent in KLHL3 with S433A substitution. Additionally, increased extracellular K+ induced calcineurin-dependent dephosphorylation of KLHL3S433-P These findings demonstrate that KLHL3S433-P is a calcineurin substrate and implicate increased KLHL3 phosphorylation in tacrolimus-induced pathologies.

[A case of hyperkalemic periodic paralysis presenting progressive myopathy with tubular aggregates].

A 33-year-old man admitted to our hospital for the evaluation of progressive muscular atrophy of his left lower leg. From his childhood, he had suffered from transient attacks of limb paralysis and myalgia lasting about 1 hour. At age 30, the muscle weakness and atrophy of his left lower leg emerged and progressed gradually. Muscle MR images showed atrophy and fat replacement in left lower leg, and muscle biopsy revealed tubular aggregates (TA). Genetic analysis showed heterozygous c.2111C>T/p.T704M missense mutation of SCN4A gene, which causes hyperkalemic periodic paralysis (HyperPP). Although HyperPP is rare, it is quite critical for clinicians to recognize that the patients of HyperPP often present progressive myopathy. We emphasize the importance of paying attention to progressive myopathy and discuss the pathological mechanism of myopathy through this case report.

Potassium conservation is impaired in mice with reduced renal expression of Kir4.1.

To better understand the role of the inward-rectifying K channel Kir4.1 (KCNJ10) in the distal nephron, we initially studied a global Kir4.1 knockout mouse (gKO), which demonstrated the hypokalemia and hypomagnesemia seen in SeSAME/EAST syndrome and was associated with reduced Na/Cl cotransporter (NCC) expression. Lethality by ~3 wk, however, limits the usefulness of this model, so we developed a kidney-specific Kir4.1 "knockdown" mouse (ksKD) using a cadherin 16 promoter and Cre-loxP methodology. These mice appeared normal and survived to adulthood. Kir4.1 protein expression was decreased ~50% vs. wild-type (WT) mice by immunoblotting, and immunofluorescence showed moderately reduced Kir4.1 staining in distal convoluted tubule that was minimal or absent in connecting tubule and cortical collecting duct. Under control conditions, the ksKD mice showed metabolic alkalosis and relative hypercalcemia but were normokalemic and mildly hypermagnesemic despite decreased NCC expression. In addition, the mice had a severe urinary concentrating defect associated with hypernatremia, enlarged kidneys with tubulocystic dilations, and reduced aquaporin-3 expression. On a K/Mg-free diet for 1 wk, however, ksKD mice showed marked hypokalemia (serum K: 1.5 ± 0.1 vs. 3.0 ± 0.1 mEq/l for WT), which was associated with renal K wasting (transtubular K gradient: 11.4 ± 0.8 vs. 1.6 ± 0.4 in WT). Phosphorylated-NCC expression increased in WT but not ksKD mice on the K/Mg-free diet, suggesting that loss of NCC adaptation underlies the hypokalemia. In conclusion, even modest reduction in Kir4.1 expression results in impaired K conservation, which appears to be mediated by reduced expression of activated NCC.

Role of WNK4 and kidney-specific WNK1 in mediating the effect of high dietary K+ intake on ROMK channel in the distal convoluted tubule.

With-no-lysine kinase 4 (WNK4) and kidney-specific (KS)-WNK1 regulate ROMK (Kir1.1) channels in a variety of cell models. We now explore the role of WNK4 and KS-WNK1 in regulating ROMK in the native distal convoluted tubule (DCT)/connecting tubule (CNT) by measuring tertiapin-Q (TPNQ; ROMK inhibitor)-sensitive K+ currents with whole cell recording. TPNQ-sensitive K+ currents in DCT2/CNT of KS- WNK1-/- and WNK4-/- mice were significantly smaller than that of WT mice. In contrast, the basolateral K+ channels (a Kir4.1/5.1 heterotetramer) in the DCT were not inhibited. Moreover, WNK4-/- mice were hypokalemic, while KS- WNK1-/- mice had normal plasma K+ levels. High K+ (HK) intake significantly increased TPNQ-sensitive K+ currents in DCT2/CNT of WT and WNK4-/- mice but not in KS- WNK1-/- mice. However, TPNQ-sensitive K+ currents in the cortical collecting duct (CCD) were normal not only under control conditions but also significantly increased in response to HK in KS- WNK1-/- mice. This suggests that the deletion of KS-WNK1-induced inhibition of ROMK occurs only in the DCT2/CNT. Renal clearance study further demonstrated that the deletion of KS-WNK1 did not affect the renal ability of K+ excretion under control conditions and during increasing K+ intake. Also, HK intake did not cause hyperkalemia in KS- WNK1-/- mice. We conclude that KS-WNK1 but not WNK4 is required for HK intake-induced stimulation of ROMK activity in DCT2/CNT. However, KS-WNK1 is not essential for HK-induced stimulation of ROMK in the CCD, and the lack of KS-WNK1 does not affect net renal K+ excretion.

Mechanism of Hyperkalemia-Induced Metabolic Acidosis.

Background Hyperkalemia in association with metabolic acidosis that are out of proportion to changes in glomerular filtration rate defines type 4 renal tubular acidosis (RTA), the most common RTA observed, but the molecular mechanisms underlying the associated metabolic acidosis are incompletely understood. We sought to determine whether hyperkalemia directly causes metabolic acidosis and, if so, the mechanisms through which this occurs.Methods We studied a genetic model of hyperkalemia that results from early distal convoluted tubule (DCT)-specific overexpression of constitutively active Ste20/SPS1-related proline-alanine-rich kinase (DCT-CA-SPAK).Results DCT-CA-SPAK mice developed hyperkalemia in association with metabolic acidosis and suppressed ammonia excretion; however, titratable acid excretion and urine pH were unchanged compared with those in wild-type mice. Abnormal ammonia excretion in DCT-CA-SPAK mice associated with decreased proximal tubule expression of the ammonia-generating enzymes phosphate-dependent glutaminase and phosphoenolpyruvate carboxykinase and overexpression of the ammonia-recycling enzyme glutamine synthetase. These mice also had decreased expression of the ammonia transporter family member Rhcg and decreased apical polarization of H+-ATPase in the inner stripe of the outer medullary collecting duct. Correcting the hyperkalemia by treatment with hydrochlorothiazide corrected the metabolic acidosis, increased ammonia excretion, and normalized ammoniagenic enzyme and Rhcg expression in DCT-CA-SPAK mice. In wild-type mice, induction of hyperkalemia by administration of the epithelial sodium channel blocker benzamil caused hyperkalemia and suppressed ammonia excretion.Conclusions Hyperkalemia decreases proximal tubule ammonia generation and collecting duct ammonia transport, leading to impaired ammonia excretion that causes metabolic acidosis.

Pseudohypoaldosteronism Type II: A Young Girl Presented with Hypertension, Hyperkalemia and Metabolic Acidosis.

Pseudohypoaldosteronism (PHA) type II is an extremely rare disorder which presents with hypertension, hyperkalemia, and normal anion gap metabolic acidosis. PHA II is also known as familial hyperkalemic hypertension, Gordon syndrome, and chloride shunt syndrome. PHA II is an autosomal dominant disorder and is caused by mutation in WNK1, WNK4, CULLIN3, KLHL3, OSR, SPAK gene. The expression of these proteins is limited to the distal convoluted tube and collecting duct of the kidney. PHA II usually responds to salt restriction and thiazide diuretics. We are reporting here a case of 16-year girl who presented with generalised fatigue and shortness of breath, and blood pressure (BP) of 220/110 mmHg. Laboratory investigation showed hyperkalemia, normal anion gap metabolic acidosis, and hypercalciuria. Workup for secondary causes of hypertension was negative. She responded to thiazide diuretics and her BP is well controlled, and acidosis and hyperkalemia are corrected.

Plasma Potassium Determines NCC Abundance in Adult Kidney-Specific γENaC Knockout.

The amiloride-sensitive epithelial sodium channel (ENaC) and the thiazide-sensitive sodium chloride cotransporter (NCC) are key regulators of sodium and potassium and colocalize in the late distal convoluted tubule of the kidney. Loss of the αENaC subunit leads to a perinatal lethal phenotype characterized by sodium loss and hyperkalemia resembling the human syndrome pseudohypoaldosteronism type 1 (PHA-I). In adulthood, inducible nephron-specific deletion of αENaC in mice mimics the lethal phenotype observed in neonates, and as in humans, this phenotype is prevented by a high sodium (HNa+)/low potassium (LK+) rescue diet. Rescue reflects activation of NCC, which is suppressed at baseline by elevated plasma potassium concentration. In this study, we investigated the role of the γENaC subunit in the PHA-I phenotype. Nephron-specific γENaC knockout mice also presented with salt-wasting syndrome and severe hyperkalemia. Unlike mice lacking αENaC or ßΕΝaC, an HNa+/LK+ diet did not normalize plasma potassium (K+) concentration or increase NCC activation. However, when K+ was eliminated from the diet at the time that γENaC was deleted, plasma K+ concentration and NCC activity remained normal, and progressive weight loss was prevented. Loss of the late distal convoluted tubule, as well as overall reduced ßENaC subunit expression, may be responsible for the more severe hyperkalemia. We conclude that plasma K+ concentration becomes the determining and limiting factor in regulating NCC activity, regardless of Na+ balance in γENaC-deficient mice.

ENaC and ROMK activity are inhibited in the DCT2/CNT of TgWnk4PHAII mice.

Mice transgenic for genomic segments harboring PHAII (pseudohypoaldosteronism type II) mutant Wnk4 (with-No-Lysine kinase 4) (TgWnk4PHAII) have hyperkalemia which is currently believed to be the result of high activity of Na-Cl cotransporter (NCC). This leads to decreasing Na+ delivery to the distal nephron segment including late distal convoluted tubule (DCT) and connecting tubule (CNT). Since epithelial Na+ channel (ENaC) and renal outer medullary K+ channel (ROMK or Kir4.1) are expressed in the late DCT and play an important role in mediating K+ secretion, the aim of the present study is to test whether ROMK and ENaC activity in the DCT/CNT are also compromised in the mice expressing PHAII mutant Wnk4. Western blot analysis shows that the expression of ßENaC and γENaC subunits but not αENaC subunit was lower in TgWnk4PHAII mice than that in wild-type (WT) and TgWnk4WT mice. Patch-clamp experiments detected amiloride-sensitive Na+ currents and TPNQ-sensitive K+ currents in DCT2/CNT, suggesting the activity of ENaC and ROMK. However, both Na+ and ROMK currents in DCT2/CNT of TgWnk4PHAII mice were significantly smaller than those in WT and TgWnk4WT mice. In contrast, the basolateral K+ currents in the DCT were similar among three groups, despite higher NCC expression in TgWnk4PHAII mice than those of WT and TgWnk4WTmice. An increase in dietary K+ intake significantly increased both ENaC and ROMK currents in the DCT2/CNT of all three groups. However, high-K+ (HK) intake-induced stimulation of Na+ and K+ currents was smaller in TgWnk4PHAII mice than those in WT and TgWnk4WT mice. We conclude that ENaC and ROMK channel activity in DCT2/CNT are inhibited in TgWnk4PHAII mice and that Wnk4PHAII-induced inhibition of ENaC and ROMK may contribute to the suppression of K+ secretion in the DCT2/CNT in addition to increased NCC activity.

The renal TRPV4 channel is essential for adaptation to increased dietary potassium.

To maintain potassium homeostasis, kidneys exert flow-dependent potassium secretion to facilitate kaliuresis in response to elevated dietary potassium intake. This process involves stimulation of calcium-activated large conductance maxi-K (BK) channels in the distal nephron, namely the connecting tubule and the collecting duct. Recent evidence suggests that the TRPV4 channel is a critical determinant of flow-dependent intracellular calcium elevations in these segments of the renal tubule. Here, we demonstrate that elevated dietary potassium intake (five percent potassium) increases renal TRPV4 mRNA and protein levels in an aldosterone-dependent manner and causes redistribution of the channel to the apical plasma membrane in native collecting duct cells. This, in turn, leads to augmented TRPV4-mediated flow-dependent calcium ion responses in freshly isolated split-opened collecting ducts from mice fed the high potassium diet. Genetic TRPV4 ablation greatly diminished BK channel activity in collecting duct cells pointing to a reduced capacity to excrete potassium. Consistently, elevated potassium intake induced hyperkalemia in TRPV4 knockout mice due to deficient renal potassium excretion. Thus, regulation of TRPV4 activity in the distal nephron by dietary potassium is an indispensable component of whole body potassium balance.

(Pro)Renin receptor regulates potassium homeostasis through a local mechanism.

(Pro)renin receptor (PRR) is highly expressed in the distal nephron, but it has an unclear functional implication. The present study was conducted to explore a potential role of renal PRR during high K+ (HK) loading. In normal Sprague-Dawley rats, a 1-wk HK intake increased renal expression of full-length PRR and urinary excretion of soluble PRR (sPRR). Administration of PRO20, a decoy peptide antagonist of PRR, in K+-loaded animals elevated plasma K+ level and decreased urinary K+ excretion, accompanied with suppressed urinary aldosterone excretion and intrarenal aldosterone levels. HK downregulated Na+-Cl- cotransporter (NCC) expression but upregulated CYP11B2 (cytochrome P-450, family 11, subfamily B, polypeptide 2), renal outer medullary K+ channel (ROMK), calcium-activated potassium channel subunit α1 (α-BK), α-Na+-K+-ATPase (α-NKA), and epithelial Na+ channel subunit ß (ß-ENaC), all of which were blunted by PRO20. After HK loading was completed, urinary, but not plasma renin, was upregulated, which was blunted by PRO20. The same experiments that were performed using adrenalectomized (ADX) rats yielded similar results. Interestingly, spironolactone treatment in HK-loaded ADX rats attenuated kaliuresis but promoted natriuresis, which was associated with the suppressed responses of ß-ENaC, α-NKA, ROMK, and α-BK protein expression. Taken together, we discovered a novel role of renal PRR in regulation of K+ homeostasis through a local mechanism involving intrarenal renin-angiotensin-aldosterone system and coordinated regulation of membrane Na+- and K+-transporting proteins.

New insights on hereditary erythrocyte membrane defects.

After the first proposed model of the red blood cell membrane skeleton 36 years ago, several additional proteins have been discovered during the intervening years, and their relationship with the pathogenesis of the related disorders have been somewhat defined. The knowledge of erythrocyte membrane structure is important because it represents the model for spectrin-based membrane skeletons in all cells and because defects in its structure underlie multiple hemolytic anemias. This review summarizes the main features of erythrocyte membrane disorders, dividing them into structural and altered permeability defects, focusing particularly on the most recent advances. New proteins involved in alterations of the red blood cell membrane permeability were recently described. The mechanoreceptor PIEZO1 is the largest ion channel identified to date, the fundamental regulator of erythrocyte volume homeostasis. Missense, gain-of-function mutations in the PIEZO1 gene have been identified in several families as causative of dehydrated hereditary stomatocytosis or xerocytosis. Similarly, the KCNN4 gene, codifying the so called Gardos channel, has been recently identified as a second causative gene of hereditary xerocytosis. Finally, ABCB6 missense mutations were identified in different pedigrees of familial pseudohyperkalemia. New genomic technologies have improved the quality and reduced the time of diagnosis of these diseases. Moreover, they are essential for the identification of the new causative genes. However, many questions remain to solve, and are currently objects of intensive studies.