Distal potassium handling based on flow modulation of maxi-K channel activity

Biomarkers to Guide Medical Therapy in Primary Aldosteronism

Primary aldosteronism (PA) is an endocrinopathy characterized by dysregulated aldosterone production that occurs despite suppression of renin and angiotensin II, and that is non-suppressible by volume and sodium loading. The effectiveness of surgical adrenalectomy for patients with lateralizing PA is characterized by the attenuation of excess aldosterone production leading to blood pressure reduction, correction of hypokalemia, and increases in renin-biomarkers that collectively indicate a reversal of PA pathophysiology and restoration of normal physiology. Even though the vast majority of patients with PA will ultimately be treated medically rather than surgically, there is a lack of guidance on how to optimize medical therapy and on key metrics of success. Herein, we review the evidence justifying approaches to medical management of PA and biomarkers that reflect endocrine principles of restoring normal physiology. We review the current arsenal of medical therapies, including dietary sodium restriction, steroidal and nonsteroidal mineralocorticoid receptor antagonists, epithelial sodium channel inhibitors, and aldosterone synthase inhibitors. It is crucial that clinicians recognize that multimodal medical treatment for PA can be highly effective at reducing the risk for adverse cardiovascular and kidney outcomes when titrated with intention. The key biomarkers reflective of optimized medical therapy are unsurprisingly similar to the physiologic expectations following surgical adrenalectomy: control of blood pressure with the fewest number of antihypertensive agents, normalization of serum potassium without supplementation, and a rise in renin. Pragmatic approaches to achieve these objectives while mitigating adverse effects are reviewed.

Patil S. Potassium Homeostasis

The average potassium intake in the United States population ranges from 90 to 120 mEq/day. About 98% of the total body’s potassium is intracellular, and only 2% is present in the extracellular compartment. This distributional proportion is essential for cellular metabolic reactions and maintaining a gradient for resting membrane potential. A loss of this gradient results in hyper- or hypopolarization of the cell membrane, especially in cardiac muscles leading to life-threatening arrhythmias. Multiple mechanisms in human maintain homeostasis. Transient initial changes are due to transcellular shifts activating sodium-potassium ATPase pumps on the cell membrane. The kidneys essentially take part in excess potassium excretion, maintaining total body stores constant within normal range. Gastrointestinal secretion of potassium is insignificant in individuals with normal renal function, however plays an essential role in individuals with compromised renal function. So far, a classic feedback mechanism was thought to maintain potassium homeostasis; however, a recently recognized feedforward mechanism acting independently also helps preserve potassium homeostasis. Hence, potassium homeostasis is vital for humans to function at a normal level.

Urinary Extracellular Vesicles for Renal Tubular Transporters Expression in Patients With Gitelman Syndrome

Background: The utility of urinary extracellular vesicles (uEVs) to faithfully represent the changes of renal tubular protein expression remains unclear. We aimed to evaluate renal tubular sodium (Na⁺) or potassium (K⁺) associated transporters expression from uEVs and kidney tissues in patients with Gitelman syndrome (GS) caused by inactivating mutations in SLC12A3.

Methods: uEVs were isolated by ultracentrifugation from 10 genetically-confirmed GS patients. Membrane transporters including Na⁺-hydrogen exchanger 3 (NHE3), Na⁺/K⁺/2Cl⁻ cotransporter (NKCC2), NaCl cotransporter (NCC), phosphorylated NCC (p-NCC), epithelial Na⁺ channel β (ENaCβ), pendrin, renal outer medullary K1 channel (ROMK), and large-conductance, voltage-activated and Ca²⁺-sensitive K⁺ channel (Maxi-K) were examined by immunoblotting of uEVs and immunofluorescence of biopsied kidney tissues. Healthy and disease (bulimic patients) controls were also enrolled.

Results: Characterization of uEVs was confirmed by nanoparticle tracking analysis, transmission electron microscopy, and immunoblotting. Compared with healthy controls, uEVs from GS patients showed NCC and p-NCC abundance were markedly attenuated but NHE3, ENaCβ, and pendrin abundance significantly increased. ROMK and Maxi-K abundance were also significantly accentuated. Immunofluorescence of the representative kidney tissues from GS patients also demonstrated the similar findings to uEVs. uEVs from bulimic patients showed an increased abundance of NCC and p-NCC as well as NHE3, NKCC2, ENaCβ, pendrin, ROMK and Maxi-K, akin to that in immunofluorescence of their kidney tissues.

Conclusion: uEVs could be a non-invasive tool to diagnose and evaluate renal tubular transporter adaptation in patients with GS and may be applied to other renal tubular diseases.

Persistent severe hyperkalemia following surgical treatment of aldosterone-producing adenoma

Primary aldosteronism is one of the most common causes of secondary hypertension. This condition is characterized by autonomous hypersecretion of aldosterone which produces sodium retention and potassium excretion, resulting in high blood pressure and potential hypokalemia. Transient postoperative hyporeninemic hypoaldosteronism with an increased risk of hyperkalemia may occur in some patients. We report the case of a 63-year-old patient with persistent hypokalemia, periodic paralysis, and refractory hypertension who was diagnosed with primary hyperaldosteronism due to elevated aldosterone, undetectable plasmatic renin concentration, and the presence of a left adrenal mass. One month after the surgery, the patient was admitted with signs of severe hyperkalemia (8 mmol/L) and worsened renal function, thus requiring hemodialysis. Fluid resuscitation, loop diuretic, and sodium bicarbonate treatment decreased his potassium. Zona glomerulosa insufficiency was confirmed by hormonal tests which exposed low aldosterone–renin axis. The fludrocortisone treatment was initiated and maintained, with consequent potassium and creatinine stabilization. Old age, long duration of hypertension, impaired renal function, severe hypokalemia before surgery, and large size of the aldosterone-producing adenoma are important risk factors for serious potassium imbalance after removal of the adenoma. We have to consider monitoring the patients after surgery for primary hyperaldosteronism in order to prevent severe hyperkalemia; therefore, postoperative immediate follow-up (arterial pressure, potassium, and renal function) is mandatory.

Mechanisms and controversies in mutant Cul3-mediated familial hyperkalemic hypertension

Autosomal dominant mutations in cullin-3 ( Cul3) cause the most severe form of familial hyperkalemic hypertension (FHHt). Cul3 mutations cause skipping of exon 9, which results in an internal deletion of 57 amino acids from the CUL3 protein (CUL3-∆9). The precise mechanism by which this altered form of CUL3 causes FHHt is controversial. CUL3 is a member of the cullin-RING ubiquitin ligase family that mediates ubiquitination and thus degradation of cellular proteins, including with-no-lysine [K] kinases (WNKs). In CUL3-∆9-mediated FHHt, proteasomal degradation of WNKs is abrogated, leading to overactivation of the WNK targets sterile 20/SPS-1 related proline/alanine-rich kinase and oxidative stress-response kinase-1, which directly phosphorylate and activate the thiazide-sensitive Na⁺-Cl^- cotransporter. Several groups have suggested different mechanisms by which CUL3-∆9 causes FHHt. The majority of these are derived from in vitro data, but recently the Kurz group (Schumacher FR, Siew K, Zhang J, Johnson C, Wood N, Cleary SE, Al Maskari RS, Ferryman JT, Hardege I, Figg NL, Enchev R, Knebel A, O'Shaughnessy KM, Kurz T. EMBO Mol Med 7: 1285-1306, 2015) described the first mouse model of CUL3-∆9-mediated FHHt. Analysis of this model suggested that CUL3-∆9 is degraded in vivo, and thus Cul3 mutations cause FHHt by inducing haploinsufficiency. We recently directly tested this model but found that other dominant effects of CUL3-∆9 must contribute to the development of FHHt. In this review, we focus on our current knowledge of CUL3-∆9 action gained from in vitro and in vivo models that may help unravel this complex problem.

Mutant Cullin 3 causes familial hyperkalemic hypertension via dominant effects

Mutations in the ubiquitin ligase scaffold protein Cullin 3 (CUL3) cause the disease familial hyperkalemic hypertension (FHHt). In the kidney, mutant CUL3 (CUL3-Δ9) increases abundance of With-No-Lysine [K] Kinase 4 (WNK4), with excessive activation of the downstream Sterile 20 (STE20)/SPS-1-related proline/alanine-rich kinase (SPAK) increasing phosphorylation of the Na+-Cl- cotransporter (NCC). CUL3-Δ9 promotes its own degradation via autoubiquitination, leading to the hypothesis that Cul3 haploinsufficiency causes FHHt. To directly test this, we generated Cul3 heterozygous mice (CUL3-Het), and Cul3 heterozygotes also expressing CUL3-Δ9 (CUL3-Het/Δ9), using an inducible renal epithelial-specific system. Endogenous CUL3 was reduced to 50% in both models, and consistent with autoubiquitination, CUL3-Δ9 protein was undetectable in CUL3-Het/Δ9 kidneys unless primary renal epithelia cells were cultured. Abundances of WNK4 and phosphorylated NCC did not differ between control and CUL3-Het mice, but they were elevated in CUL3-Het/Δ9 mice, which also displayed higher plasma [K+] and blood pressure. Abundance of phosphorylated Na+-K+-2Cl- cotransporter (NKCC2) was also increased, which may contribute to the severity of CUL3-Δ9-mediated FHHt. WNK4 and SPAK localized to puncta in NCC-positive segments but not in NKCC2-positive segments, suggesting differential effects of CUL3-Δ9. These results indicate that Cul3 haploinsufficiency does not cause FHHt, but dominant effects of CUL3-Δ9 are required.

Severe hyperkalemia following adrenalectomy for aldosteronoma: prediction, pathogenesis and approach to clinical management- a case series

BACKGROUND

As the field of Primary Aldosteronism (PA) becomes ever expanded, diagnosis of PA is increasingly diagnosed by endocrinologists. With increased PA screening, many of the cases are now found in patients with complex co-morbidities in addition to their hypertension. Post adrenalectomy renal impairment with hyperkalemia is now increasingly seen in these complex patients, as evidenced by the numerous reports on this issue that have appeared within the past 3 years. We present a small case series to illustrate the breadth of the problem, along with a discussion about how such CKD/hyperkalemic events may be predicted.

CASE PRESENTATION

We present three cases of primary aldosteronism with long standing hypertension (more than 10 years) hypokalemia (2.0-3.0 mmol/l). Serum aldosterone was high with low renin activity leading to high aldosterone to renin ratio (ARR). They underwent abdominal CT scan revealing adrenal mass and adrenal vein sample confirmed lateralization. None of the patients had evidence of renal disease before surgery (as evident by normal eGFR and serum creatinine). Post adrenalectomy they had reduction in the blood pressure and became eukalemic. Serum aldosterone and renin activity were low leading to a low ARR. Case 1 developed hyperkalemia and increased serum creatinine 6 weeks post operatively which resolved with initiation of fludrocortisone and every attempt to discontinue fludrocortisone resulted in hyperkalemia and rising creatinine. Her hyperkalemia is under control with oral sodium bicarbonate. Case 2 developed hyperkalemia and increasing creatinine 2 months post operatively transiently requiring fludrocortisone and later on managed with furosemide for hyperkalemia. Case 3 developed renal impairment and hyperkalemia 2 weeks post operatively requiring fludrocortisone.

CONCLUSION

Post APA resection severe hyperkalemia may be a common entity and screening should be actively considered in high risk patients. Older age, longer duration of hypertension, impaired pre-op and post-op GFR and higher levels of pre-op aldosterone and are all risk factors which predict the likelihood of developing post-operative hyperkalemia. Fludrocortisone, sodium bicarbonate, loop diuretics and potassium binders can be used for treatment. Treatment choice should be tailored to patient characteristics including fluid status, blood pressure and serum creatinine. Potassium binders should be avoided in patients with history of recent abdominal surgery, opioid use and constipation. Serum electrolytes and creatinine should be monitored every 1-2 weeks after starting treatment to ensure an adequate response. Prolonged management may be necessary in some cases and at-risk patients should be counselled as to the meaning and importance of post-operative changes in measured renal function and potassium.

Expression of a Diverse Array of Ca2+-Activated K+ Channels (SK1/3, IK1, BK) that Functionally Couple to the Mechanosensitive TRPV4 Channel in the Collecting Duct System of Kidney

The voltage- and Ca²⁺-activated, large conductance K⁺ channel (BK, maxi-K) is expressed in the collecting duct system of kidney where it underlies flow- and Ca²⁺-dependent K⁺ excretion. To determine if other Ca²⁺-activated K⁺ channels (KCa) may participate in this process, mouse kidney and the K⁺-secreting mouse cortical collecting duct (CCD) cell line, mCCDcl1, were assessed for TRPV4 and KCa channel expression and cross-talk. qPCR mRNA analysis and immunocytochemical staining demonstrated TRPV4 and KCa expression in mCCDcl1 cells and kidney connecting tubule (CNT) and CCD. Three subfamilies of KCa channels were revealed: the high Ca²⁺-binding affinity small-conductance SK channels, SK1and SK3, the intermediate conductance channel, IK1, and the low Ca²⁺-binding affinity, BK channel (BKα subunit). Apparent expression levels varied in CNT/CCD where analysis of CCD principal cells (PC) and intercalated cells (IC) demonstrated differential staining: SK1:PC<IC, and SK3:PC>IC, IK1:PC>IC, BKα:PC = IC, and TRPV4:PC>IC. Patch clamp analysis and fluorescence Ca²⁺ imaging of mCCDcl1 cells demonstrated potent TRPV4-mediated Ca²⁺ entry and strong functional cross-talk between TRPV4 and KCa channels. TRPV4-mediated Ca²⁺ influx activated each KCa channel, as evidenced by selective inhibition of KCa channels, with each active KCa channel enhancing Ca²⁺ entry (due to membrane hyperpolarization). Transepithelial electrical resistance (TEER) analysis of confluent mCCDcl1 cells grown on permeable supports further demonstrated this cross-talk where TRPV4 activation induce a decrease in TEER which was partially restored upon selective inhibition of each KCa channel. It is concluded that SK1/SK3 and IK1 are highly expressed along with BKα in CNT and CCD and are closely coupled to TRPV4 activation as observed in mCCDcl1 cells. The data support a model in CNT/CCD segments where strong cross talk between TRPV4-mediated Ca²⁺ influx and each KCa channel leads to enhance Ca²⁺ entry which will support activation of the low Ca²⁺-binding affinity BK channel to promote BK-mediated K⁺ secretion.

Developmental changes in renal tubular transport-an overview

The adult kidney maintains a constant volume and composition of extracellular fluid despite changes in water and salt intake. The neonate is born with a kidney that has a small fraction of the glomerular filtration rate of the adult and immature tubules that function at a lower capacity than that of the mature animal. Nonetheless, the neonate is also able to maintain a constant extracellular fluid volume and composition. Postnatal renal tubular development was once thought to be due to an increase in the transporter abundance to meet the developmental increase in glomerular filtration rate. However, postnatal renal development of each nephron segment is quite complex. There are isoform changes of several transporters as well as developmental changes in signal transduction that affect the capacity of renal tubules to reabsorb solutes and water. This review will discuss neonatal tubular function with an emphasis on the differences that have been found between the neonate and adult. We will also discuss some of the factors that are responsible for the maturational changes in tubular transport that occur during postnatal renal development.

Roles of Akt and SGK1 in the Regulation of Renal Tubular Transport

A serine/threonine kinase Akt is a key mediator in various signaling pathways including regulation of renal tubular transport. In proximal tubules, Akt mediates insulin signaling via insulin receptor substrate 2 (IRS2) and stimulates sodium-bicarbonate cotransporter (NBCe1), resulting in increased sodium reabsorption. In insulin resistance, the IRS2 in kidney cortex is exceptionally preserved and may mediate the stimulatory effect of insulin on NBCe1 to cause hypertension in diabetes via sodium retention. Likewise, in distal convoluted tubules and cortical collecting ducts, insulin-induced Akt phosphorylation mediates several hormonal signals to enhance sodium-chloride cotransporter (NCC) and epithelial sodium channel (ENaC) activities, resulting in increased sodium reabsorption. Serum- and glucocorticoid-inducible kinase 1 (SGK1) mediates aldosterone signaling. Insulin can stimulate SGK1 to exert various effects on renal transporters. In renal cortical collecting ducts, SGK1 regulates the expression level of ENaC through inhibition of its degradation. In addition, SGK1 and Akt cooperatively regulate potassium secretion by renal outer medullary potassium channel (ROMK). Moreover, sodium-proton exchanger 3 (NHE3) in proximal tubules is possibly activated by SGK1. This review focuses on recent advances in understanding of the roles of Akt and SGK1 in the regulation of renal tubular transport.

Inward-rectifying potassium channelopathies: new insights into disorders of sodium and potassium homeostasis

Inward-rectifying potassium (Kir) channels allow more inward than outward potassium flux when channels are open in mammalian cells. At physiological resting membrane potentials, however, they predominantly mediate outward potassium flux and play important roles in regulating the resting membrane potential in diverse cell types and potassium secretion in the kidneys. Mutations of Kir channels cause human hereditary diseases collectively called Kir channelopathies, many of which are characterized by disorders of sodium and potassium homeostasis. Studies on these genetic Kir channelopathies have shed light on novel pathophysiological mechanisms, including renal sodium and potassium handling, potassium shifting in skeletal muscles, and aldosterone production in the adrenal glands. Here, we review several recent advances in Kir channels and their clinical implications in sodium and potassium homeostasis.

Emerging role of the calcium-activated, small conductance, SK3 K+ channel in distal tubule function: regulation by TRPV4

The Ca2+-activated, maxi-K (BK) K+ channel, with low Ca2+-binding affinity, is expressed in the distal tubule of the nephron and contributes to flow-dependent K+ secretion. In the present study we demonstrate that the Ca2+-activated, SK3 (KCa2.3) K+ channel, with high Ca2+-binding affinity, is also expressed in the mouse kidney (RT-PCR, immunoblots). Immunohistochemical evaluations using tubule specific markers demonstrate significant expression of SK3 in the distal tubule and the entire collecting duct system, including the connecting tubule (CNT) and cortical collecting duct (CCD). In CNT and CCD, main sites for K+ secretion, the highest levels of expression were along the apical (luminal) cell membranes, including for both principal cells (PCs) and intercalated cells (ICs), posturing the channel for Ca2+-dependent K+ secretion. Fluorescent assessment of cell membrane potential in native, split-opened CCD, demonstrated that selective activation of the Ca2+-permeable TRPV4 channel, thereby inducing Ca2+ influx and elevating intracellular Ca2+ levels, activated both the SK3 channel and the BK channel leading to hyperpolarization of the cell membrane. The hyperpolarization response was decreased to a similar extent by either inhibition of SK3 channel with the selective SK antagonist, apamin, or by inhibition of the BK channel with the selective antagonist, iberiotoxin (IbTX). Addition of both inhibitors produced a further depolarization, indicating cooperative effects of the two channels on Vm. It is concluded that SK3 is functionally expressed in the distal nephron and collecting ducts where induction of TRPV4-mediated Ca2+ influx, leading to elevated intracellular Ca2+ levels, activates this high Ca2+-affinity K+ channel. Further, with sites of expression localized to the apical cell membrane, especially in the CNT and CCD, SK3 is poised to be a key pathway for Ca2+-dependent regulation of membrane potential and K+ secretion.

Interacting influence of diuretics and diet on BK channel-regulated K homeostasis

Large conductance, Ca-activated K channels (BK) are abundantly located in cells of vasculature, glomerulus, and distal nephron, where they are involved in maintaining blood volume, blood pressure, and K homeostasis. In mesangial cells and smooth muscle cells of vessels, the BK-α pore associates with BK-β1 subunits and regulates contraction in a Ca-mediated feedback manner. The BK-β1 also resides in connecting tubule cells of the nephron. BK-β1 knockout mice (β1KO) exhibit fluid retention, hypertension, and compromised K handling. The BK-α/β4 resides in acid/base transporting intercalated cells (IC) of the distal nephron, where they mediate K secretion in mammals on a high K, alkaline diet. BK-α expression in IC is increased by a high K diet via aldosterone. The BK-β4 subunit and alkaline urine are necessary for the luminal expression and function of BK-α in mouse IC. In distal nephron cells, membrane BK-α expression is inhibited by WNK4 in in vitro expression systems, indicating a role in the hyperkalemic phenotype in patients with familial hyperkalemic hypertension type 2 (FHHt2). β1KO and BK-β4 knockout mice (β4KO) are hypertensive because of exaggerated epithelial Na channels (ENaC) mediated Na retention in an effort to secrete K via only renal outer medullary K channels (ROMK). BK hypertension is resistant to thiazides and furosemide, and would be more amenable to ENaC and aldosterone inhibiting drugs. Activators of BK-α/β1 or BK-α/β4 might be effective blood pressure lowering agents for a subset of hypertensive patients. Inhibitors of renal BK would effectively spare K in patients with Bartter Syndrome, a renal K wasting disease.

Flow-induced activation of TRPV5 and TRPV6 channels stimulates Ca(2+)-activated K(+) channel causing membrane hyperpolarization

TRPV5 and TRPV6 channels are expressed in distal renal tubules and play important roles in the transcellular Ca(2+) reabsorption in kidney. They are regulated by multiple intracellular factors including protein kinases A and C, membrane phospholipid PIP2, protons, and divalent ions Ca(2+) and Mg(2+). Here, we report that fluid flow that generates shear force within the physiological range of distal tubular fluid flow activated TRPV5 and TRPV6 channels expressed in HEK cells. Flow-induced activation of channel activity was reversible and did not desensitize over 2min. Fluid flow stimulated TRPV5 and 6-mediated Ca(2+) entry and increased intracellular Ca(2+) concentration. N-glycosylation-deficient TRPV5 channel was relatively insensitive to fluid flow. In cells coexpressing TRPV5 (or TRPV6) and Slo1-encoded maxi-K channels, fluid flow induced membrane hyperpolarization, which could be prevented by the maxi-K blocker iberiotoxin or TRPV5 and 6 blocker La(3+). In contrast, fluid flow did not cause membrane hyperpolarization in cells coexpressing ROMK1 and TRPV5 or 6 channel. These results reveal a new mechanism for the regulation of TRPV5 and TRPV6 channels. Activation of TRPV5 and TRPV6 by fluid flow may play a role in the regulation of flow-stimulated K(+) secretion via maxi-K channels in distal renal tubules and in the mechanism of pathogenesis of thiazide-induced hypocalciuria.

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.

Role of BK channels in hypertension and potassium secretion

PURPOSE OF REVIEW

This review summarizes recent studies of hypertension associated with a defect in renal K excretion due to genetic deletions of various components of the large, Ca-activated K channel (BK), and describes new evidence and theories regarding K secretory roles of BK in intercalated cells.

RECENT FINDINGS

Isolated perfused tubule methods have revealed the importance of BK in flow-induced K secretion. Subsequently, mice with genetically deleted BK subunits revealed the complexities of BK-mediated K secretion. Deletion of BKα results in extreme aldosteronism, hypertension, and an absence of flow-induced K secretion. Deletion of the BKβ1 ancillary subunit results in decreased handling of a K load, increased plasma K, mild aldosteronism and hypertension that is exacerbated by a high K diet. Deletion of BKβ4 (β4KO) leads to insufficient K handling, high plasma K, fluid retention, but with milder hypertension. Fluid retention in β4KO may be the result of insufficient flow-induced secretion of adenosine triphosphate (ATP), which normally inhibits epithelial Na channels (ENaCs).

SUMMARY

Classical physiological analysis of electrolyte handling in knockout mice has enlightened our understanding of the mechanism of handling K loads by renal K channels. Studies have focused on the different roles of BK-α/β1 and BK-α/β4 in the kidney. BKβ1 hypertension may be a 'three-hit' hypertension, involving a K secretory defect, elevated production of aldosterone, and increased vascular tone. The disorders observed in BK knockout mice have shed new insights on the importance of proper renal K handling for maintaining volume balance and blood pressure.

Small-molecule modulators of inward rectifier K+ channels: recent advances and future possibilities

Inward rectifier potassium (Kir) channels have been postulated as therapeutic targets for several common disorders including hypertension, cardiac arrhythmias and pain. With few exceptions, however, the small-molecule pharmacology of this family is limited to nonselective cardiovascular and neurologic drugs with off-target activity toward inward rectifiers. Consequently, the actual therapeutic potential and 'drugability' of most Kir channels has not yet been determined experimentally. The purpose of this review is to provide a comprehensive summary of publicly disclosed Kir channel small-molecule modulators and highlight recent targeted drug-discovery efforts toward Kir1.1 and Kir2.1. The review concludes with a brief speculation on how the field of Kir channel pharmacology will develop over the coming years and a discussion of the increasingly important role academic laboratories will play in this progress.

Activation of PI3-kinase stimulates endocytosis of ROMK via Akt1/SGK1-dependent phosphorylation of WNK1

WNK kinases stimulate endocytosis of ROMK channels to regulate renal K+ handling. Phosphatidylinositol 3-kinase (PI3K)-activating hormones, such as insulin and IGF 1, phosphorylate WNK1, but how this affects the regulation of ROMK abundance is unknown. Here, serum starvation of ROMK-transfected HEK cells led to an increase of ROMK current density; subsequent addition of insulin or IGF1 inhibited ROMK currents in a PI3K-dependent manner. Serum and insulin also increased phosphorylation of the downstream kinases Akt1 and SGK1 as well as WNK1. A biotinylation assay suggested that insulin and IGF1 inhibit ROMK by enhancing its endocytosis, a process that WNK1 may mediate. Knockdown of WNK1 with siRNA or expression of a phospho-deficient WNK1 mutant (T58A) both prevented insulin-induced inhibition of ROMK currents, suggesting that phosphorylation at Threonine-58 of WNK1 is important to mediate the inhibition of ROMK by PI3K-activating hormones or growth factors. In vitro and in vivo kinase assays supported the notion that Akt1 and SGK1 can phosphorylate WNK1 at this site, and we established that Akt1 and SGK1 synergistically inhibit ROMK through WNK1. We used dominant-negative intersectin and dynamin constructs to show that SGK1-mediated phosphorylation of WNK1 inhibits ROMK by promoting its endocytosis. Taken together, these results suggest that PI3K-activating hormones inhibit ROMK by enhancing its endocytosis via a mechanism that involves phosphorylation of WNK1 by Akt1 and SGK1.

IRBIT governs epithelial secretion in mice by antagonizing the WNK/SPAK kinase pathway

Fluid and HCO(3)(-) secretion are fundamental functions of epithelia and determine bodily fluid volume and ionic composition, among other things. Secretion of ductal fluid and HCO(3)(-) in secretory glands is fueled by Na(+)/HCO(3)(-) cotransport mediated by basolateral solute carrier family 4 member 4 (NBCe1-B) and by Cl(-)/HCO(3)(-) exchange mediated by luminal solute carrier family 26, member 6 (Slc26a6) and CFTR. However, the mechanisms governing ductal secretion are not known. Here, we have shown that pancreatic ductal secretion in mice is suppressed by silencing of the NBCe1-B/CFTR activator inositol-1,4,5-trisphosphate (IP(3)) receptor-binding protein released with IP(3) (IRBIT) and by inhibition of protein phosphatase 1 (PP1). In contrast, silencing the with-no-lysine (WNK) kinases and Ste20-related proline/alanine-rich kinase (SPAK) increased secretion. Molecular analysis revealed that the WNK kinases acted as scaffolds to recruit SPAK, which phosphorylated CFTR and NBCe1-B, reducing their cell surface expression. IRBIT opposed the effects of WNKs and SPAK by recruiting PP1 to the complex to dephosphorylate CFTR and NBCe1-B, restoring their cell surface expression, in addition to stimulating their activities. Silencing of SPAK and IRBIT in the same ducts rescued ductal secretion due to silencing of IRBIT alone. These findings stress the pivotal role of IRBIT in epithelial fluid and HCO(3)(-) secretion and provide a molecular mechanism by which IRBIT coordinates these processes. They also have implications for WNK/SPAK kinase-regulated processes involved in systemic fluid homeostasis, hypertension, and cystic fibrosis.

Recent advances in distal tubular potassium handling

It is well known that sodium reabsorption and aldosterone play important roles in potassium secretion by the aldosterone-sensitive distal nephron. Sodium- and aldosterone-independent mechanisms also exist. This review focuses on some recent studies that provide novel insights into the sodium- and aldosterone-independent potassium secretion by the aldosterone-sensitive distal nephron. In addition, we discuss a study reporting on the regulation of the mammalian potassium kidney channel ROMK by intracellular and extracellular magnesium, which may be important in the pathogenesis of persistent hypokalemia in patients with concomitant potassium and magnesium deficiency. We also discuss outstanding questions and propose working models for future investigation.

Generation and analysis of the thiazide-sensitive Na+ -Cl- cotransporter (Ncc/Slc12a3) Ser707X knockin mouse as a model of Gitelman syndrome

Gitelman syndrome (GS) is characterized by salt-losing hypotension, hypomagnesemia, hypokalemic metabolic alkalosis, and hypocalciuria. To better model human GS caused by a specific mutation in the thiazide-sensitive Na(+) -Cl(-) cotransporter (NCC) gene SLC12A3, we generated a nonsense Ncc Ser707X knockin mouse corresponding to human p.Ser710X (c.2135C>A), a recurrent mutation with severe phenotypes in Chinese GS patients. Compared with wild-type or heterozygous littermates, homozygous (Hom) knockin mice fully recapitulated the phenotype of human GS. The markedly reduced Ncc mRNA and virtually absent Ncc protein expression in kidneys of Hom mice was primarily due to nonsense-mediated mRNA decay (NMD) surveillance mechanisms. Expression of epithelial Na(+) channel (Enac), Ca(2+) channels (Trpv5 and Trpv6), and K(+) channels (Romk1 and maxi-K) were significantly increased. Late distal convoluted tubules (DCT) volume was increased and DCT cell ultrastructure appeared intact. High K(+) intake could not correct hypokalemia but caused a further increase in maxi-K but not Romk1 expression. Renal tissue from a patient with GS also showed the enhanced TRPV5 and ROMK1 expression in distal tubules. We suggest that the upregulation of TRPV5/6 and of ROMK1 and Maxi-K may contribute to hypocalciuria and hypokalemia in Ncc Ser707X knockin mice and human GS, respectively.