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Regulation of the Cl−/[Formula: see text] exchanger pendrin has been suggested to explain the aldosterone paradox. A high-K+ diet has been proposed to downregulate a pendrin-mediated K+-sparing NaCl reabsorption pathway to maximize urinary K+ excretion. Here, we challenged the hypothesis, revealing that the accompanying anion, not K+, drives pendrin expression. Pendrin is downregulated with a high-KCl diet, preventing acidosis, and upregulated with an alkaline-rich high-K+ diet, preventing metabolic alkalosis. Pendrin regulation is prioritized for acid-base balance.

Pendrin is an apical Cl−/[Formula: see text] exchanger that provides renal K+-sparing NaCl absorption. The pendrin-null kidney has an inability to fully conserve K+ and limits renal K+ loss by downregulating the epithelial Na+ channel (ENaC). However, with Na+ restriction, the need to reduce ENaC for K+ balance conflicts with the need to stimulate ENaC for intravascular volume. Therefore, NaCl restriction stimulates ENaC less in pendrin-null mice than in wild-type mice, which mitigates their kaliuresis and hypokalemia but exacerbates volume contraction.

Pendrin is an intercalated cell Cl(−)/ [Formula: see text] exchanger thought to participate in K(+)-sparing NaCl absorption. However, its role in K(+) homeostasis has not been clearly defined. We hypothesized that pendrin-null mice will develop hypokalemia with dietary K(+) restriction. We further hypothesized that pendrin knockout (KO) mice mitigate urinary K(+) loss by downregulating the epithelial Na(+) channel (ENaC). Thus, we examined the role of ENaC in Na(+) and K(+) balance in pendrin KO and wild-type mice following dietary K(+) restriction. To do so, we examined the relationship between Na(+) and K(+) balance and ENaC subunit abundance in K(+)-restricted pendrin-null and wild-type mice that were NaCl restricted or replete. Following a NaCl-replete, K(+)-restricted diet, K(+) balance and serum K(+) were similar in both groups. However, following a Na(+), K(+), and Cl(−)-deficient diet, pendrin KO mice developed hypokalemia from increased K(+) excretion. The fall in serum K(+) observed in K(+)-restricted pendrin KO mice was enhanced with ENaC stimulation but eliminated with ENaC inhibition. The fall in serum K(+) observed in K(+)-restricted pendrin KO mice was enhanced with ENaC stimulation but eliminated with ENaC inhibition. However, reducing ENaC activity also reduced blood pressure and increased apparent intravascular volume contraction, since KO mice had lower serum Na(+), higher blood urea nitrogen and hemoglobin, greater weight loss, greater metabolic alkalosis, and greater NaCl excretion. We conclude that dietary Na(+) and K(+) restriction induces hypokalemia in pendrin KO mice. Pendrin-null mice limit renal K(+) loss by downregulating ENaC. However, this ENaC downregulation occurs at the expense of intravascular volume. NEW & NOTEWORTHY Pendrin is an apical Cl(−)/ [Formula: see text] exchanger that provides renal K(+)-sparing NaCl absorption. The pendrin-null kidney has an inability to fully conserve K(+) and limits renal K(+) loss by downregulating the epithelial Na(+) channel (ENaC). However, with Na(+) restriction, the need to reduce ENaC for K(+) balance conflicts with the need to stimulate ENaC for intravascular volume. Therefore, NaCl restriction stimulates ENaC less in pendrin-null mice than in wild-type mice, which mitigates their kaliuresis and hypokalemia but exacerbates volume contraction.

The WNK/SPAK “potassium-switch” pathway provides a homeostatic mechanism that activates the sodium chloride cotransporter, NCC, to limit urinary potassium excretion in dietary potassium deficiency at the expense of increasing salt-sensitivity and blood pressure. Although it is well understood that high dietary potassium turns off the switch, and decreases NCC activity, several reports suggest the counter anion in potassium-rich diets may have an influence on the pathway. The goal of the present study was to determine if the chronic response to a high K diet depends on the counter anion. Wild-type C57BL6J mice were randomized to control (1%K+), potassium chloride (KCl: 5%K+), or potassium bicarbonate (KHCO3: 5%K+) diets, and physiological, molecular, and imaging analyses were performed at 4 and 10 days of diet consumption. In contrast to the classic response to high KCl diet, characterized by reduced NCC phosphorylation and membrane localization, NCC phosphorylation and apical localization increased after 10 days on the high KHCO3 diet. This response was paralleled by increased phosphorylation of the downstream potassium-switch kinase, SPAK, which in turn phosphorylates NCC. Immunofluorescence imaging revealed WNK4 bodies were more numerous and larger in the high KHCO3 group compared to the control, consistent with activation of upstream WNK signaling. Urine excretion studies revealed mice on the high KHCO3 retain more Na+ than mice on control or KCl diets, paralleling enhanced NCC activation. Unlike mice on the high KCl diet, mice on the high KHCO3 diet develop and maintain metabolic alkalosis and do not exhibit profound hyperkalemia, coincident with more robust activation of ENaC and ROMK than with the high KCl diet. In summary, these data identify an unexpected NCC activation mechanism that may limit distal sodium delivery to prevent excessive potassium losses when alkalosis super activates potassium secretion in more distal segments. NIDDK, LeDucq Foundation This is the full abstract presented at the American Physiology Summit 2023 meeting and is only available in HTML format. There are no additional versions or additional content available for this abstract. Physiology was not involved in the peer review process.

This study shows that aquaporin-2 regulation is disrupted by a small fall in plasma potassium levels and the response is influenced by sexual dimorphism in renal potassium handling. The findings provided new insights into the mechanisms by which water balance is altered in dietary potassium deficiency and support defining the disorder as “potassium-dependent nephrogenic diabetes insipidus.”

Urinary K

The association between diabetes insipidus (DI) and chronic dietary K(+) deprivation is well known, but it remains uncertain how the disorder develops and whether it is influenced by the sexual dimorphism in K(+) handling. Here, we determined the plasma K(+) (P(K)) threshold for DI in male and female mice and ascertained if DI is initiated by polydipsia or by a central or nephrogenic defect. C57BL6J mice were randomized to a control diet or to graded reductions in dietary K(+) for 8 days, and kidney function and transporters involved in water balance were characterized. We found that male and female mice develop polyuria and secondary polydipsia. Altered water balance coincided with a decrease in aquaporin-2 (AQP2) phosphorylation and apical localization despite increased levels of the vasopressin surrogate marker copeptin. No change in the protein abundance of urea transporter-A1 was observed. The Na(+)-K(+)-2Cl(−) cotransporter decreased only in males. Desmopressin treatment failed to reverse water diuresis in K(+)-restricted mice. These findings indicate that even a small fall in P(K) is associated with nephrogenic DI (NDI), coincident with the development of altered AQP2 regulation, implicating low P(K) as a causal trigger of NDI. We found that P(K) decreased more in females, and, consequently, females were more prone to develop NDI. Together, these data indicate that AQP2 regulation is disrupted by a small decrease in P(K) and that the response is influenced by sexual dimorphism in K(+) handling. These findings provide new insights into the mechanisms linking water and K(+) balances and support defining the disorder as “potassium-dependent NDI.” NEW & NOTEWORTHY This study shows that aquaporin-2 regulation is disrupted by a small fall in plasma potassium levels and the response is influenced by sexual dimorphism in renal potassium handling. The findings provided new insights into the mechanisms by which water balance is altered in dietary potassium deficiency and support defining the disorder as “potassium-dependent nephrogenic diabetes insipidus.”

The stimulation of postprandial K+ clearance involves aldosterone-independent and -dependent mechanisms. In this context, serum- and glucocorticoid-induced kinase (SGK)1, a ubiquitously expressed kinase, is one of the primary aldosterone-induced proteins in the aldosterone-sensitive distal nephron. Germline inactivation of SGK1 suggests that this kinase is fundamental for K+ excretion under conditions of K+ load, but the specific role of renal SGK1 remains elusive. To avoid compensatory mechanisms that may occur during nephrogenesis, we used inducible, nephron-specific Sgk1 Pax8/ LC1 mice to assess the role of renal tubular SGK1 in K+ regulation. Under a standard diet, these animals exhibited normal K+ handling. When challenged by a high-K+ diet, they developed severe hyperkalemia accompanied by a defect in K+ excretion. Molecular analysis revealed reduced neural precursor cell expressed developmentally downregulated protein (NEDD)4-2 phosphorylation and total expression. γ-Epithelial Na+ channel (ENaC) expression and α/γENaC proteolytic processing were also decreased in mutant mice. Moreover, with no lysine kinase (WNK)1, which displayed in control mice punctuate staining in the distal convoluted tubule and diffuse distribution in the connecting tubule/cortical colleting duct, was diffused in the distal convoluted tubule and less expressed in the connecting tubule/collecting duct of Sgk Pax8/ LC1 mice. Moreover, Ste20-related proline/alanine-rich kinase phosphorylation, and Na+-Cl− cotransporter phosphorylation/apical localization were reduced in mutant mice. Consistent with the altered WNK1 expression, increased renal outer medullary K+ channel apical localization was observed. In conclusion, our data suggest that renal tubular SGK1 is important in the regulation of K+ excretion via the control of NEDD4-2, WNK1, and ENaC.

Dietary K+ load results in hyperkalemia, with consequent aldosterone release in order to stimulate K+ secretion in the distal nephron. The molecular mechanisms of this regulation are still not full...

Mutations in the CACNA1A gene, encoding the α1 subunit of the voltage-gated calcium channel Ca(V)2.1 (P/Q-type), have been associated with three neurological phenotypes: familial and sporadic hemiplegic migraine type 1 (FHM1, SHM1), episodic ataxia type 2 (EA2), and spinocerebellar ataxia type 6 (SCA6). We report a child with congenital ataxia, abnormal eye movements and developmental delay who presented severe attacks of hemiplegic migraine triggered by minor head traumas and associated with hemispheric swelling and seizures. Progressive cerebellar atrophy was also observed. Remission of the attacks was obtained with acetazolamide. A de novo 3 bp deletion was found in heterozygosity causing loss of a phenylalanine residue at position 1502, in one of the critical transmembrane domains of the protein contributing to the inner part of the pore. We characterized the electrophysiology of this mutant in a Xenopus oocyte in vitro system and showed that it causes gain of function of the channel. The mutant Ca(V)2.1 activates at lower voltage threshold than the wild type. These findings provide further evidence of this molecular mechanism as causative of FHM1 and expand the phenotypic spectrum of CACNA1A mutations with a child exhibiting severe SHM1 and non-episodic ataxia of congenital onset.

Mutations in the phosphoinositide phos- phatase myotubularin (MTM1) results in X-linked myo- tubular/centronuclear myopathy (XLMTM), character- ized by a severe decrease in muscle mass and strength in patients and murine models. However, the molecular mechanism involved in the muscle hypotrophy is un- clear. Here we show that the IGF1R/Akt pathway is affected in Mtm1-deficient murine muscles, character- ized by an increase in IGF1 receptor and Akt levels in both the presymptomatic and symptomatic phases. Moreover, up-regulation of atrogenes was observed in the presymptomatic phase of the myopathy, supporting overactivation of the ubiquitin-proteasome pathway. In parallel, the autophagy machinery was affected as indi- cated by the increase in the number of autophagosomes and of autophagy markers, such as LC3 and P62. However, phosphorylation of FOXO3a and mTOR were abnormal at late but not at early stages of the disease, suggesting that myotubularin acts both up- stream in the IGF1R/Akt pathway and downstream on the balance between the autophagy and ubiquitin-pro- teasome pathways in vivo. Adeno-associated virus-medi- ated delivery of Mtm1 into Mtm1-null muscles rescued muscle mass and normalized the expression levels of IGF1 receptor, the ubiquitin-proteasome pathway, and autophagy markers. These data support the hypothesis that the unbalanced regulation of the ubiquitin protea- some pathway and the autophagy machinery is a pri- mary cause of the XLMTM pathogenesis.—Al-Qusairi, L., Prokic, I., Amoasii, L., Kretz, C., Messaddeq, N., Mandel, J.-L., Laporte, J. Lack of myotubularin (MTM1) leads to muscle hypotrophy through unbal- anced regulation of the autophagy and ubiquitin-pro- teasome pathways. FASEB J. 27, 000 - 000 (2013). www.fasebj.org

Skeletal muscle contraction is triggered by the excitation-contraction (E-C) coupling machinery residing at the triad, a membrane structure formed by the juxtaposition of T-tubules and sarcoplasmic reticulum (SR) cisternae. The formation and maintenance of this structure is key for muscle function but is not well characterized. We have investigated the mechanisms leading to X-linked myotubular myopathy (XLMTM), a severe congenital disorder due to loss of function mutations in the MTM1 gene, encoding myotubularin, a phosphoinositide phosphatase thought to have a role in plasma membrane homeostasis and endocytosis. Using a mouse model of the disease, we report that Mtm1 -deficient muscle fibers have a decreased number of triads and abnormal longitudinally oriented T-tubules. In addition, SR Ca 2+ release elicited by voltage-clamp depolarizations is strongly depressed in myotubularin-deficient muscle fibers, with myoplasmic Ca 2+ removal and SR Ca 2+ content essentially unaffected. At the molecular level, Mtm1 -deficient myofibers exhibit a 3-fold reduction in type 1 ryanodine receptor (RyR1) protein level. These data reveal a critical role of myotubularin in the proper organization and function of the E-C coupling machinery and strongly suggest that defective RyR1-mediated SR Ca 2+ release is responsible for the failure of muscle function in myotubular myopathy.