Potassium and sodium transport along the loop of Henle: effects of altered dietary potassium intake

Role of calcineurin in regulating renal potassium (K+) excretion: Mechanisms of calcineurin inhibitor-induced hyperkalemia

Calcineurin, protein phosphatase 2B (PP2B) or protein phosphatase 3 (PP3), is a calcium-dependent serine/threonine protein phosphatase. Calcineurin is widely expressed in the kidney and regulates renal Na+ and K+ transport. In the thick ascending limb, calcineurin plays a role in inhibiting NKCC2 function by promoting the dephosphorylation of the cotransporter and an intracellular sorting receptor, called sorting-related-receptor-with-A-type repeats (SORLA), is involved in modulating the effect of calcineurin on NKCC2. Calcineurin also participates in regulating thiazide-sensitive NaCl-cotransporter (NCC) in the distal convoluted tubule. The mechanisms by which calcineurin regulates NCC include directly dephosphorylation of NCC, regulating Kelch-like-3/CUL3 E3 ubiquitin-ligase complex, which is responsible for WNK (with-no-lysin-kinases) ubiquitination, and inhibiting Kir4.1/Kir5.1, which determines NCC expression/activity. Finally, calcineurin is also involved in regulating ROMK (Kir1.1) channels in the cortical collecting duct and Cyp11 2 expression in adrenal zona glomerulosa. In summary, calcineurin is involved in the regulation of NKCC2, NCC, and inwardly rectifying K+ channels in the kidney, and it also plays a role in modulating aldosterone synthesis in adrenal gland, which regulates epithelial-Na+-channel expression/activity. Thus, application of calcineurin inhibitors (CNIs) is expected to abrupt calcineurin-mediated regulation of transepithelial Na+ and K+ transport in the kidney. Consequently, CNIs cause hypertension, compromise renal K+ excretion, and induce hyperkalemia.

miRNA-23a modulates sodium-hydrogen exchanger 1 expression: studies in medullary thick ascending limb of salt-induced hypertensive rats

BACKGROUND

The kidney is the main organ in the pathophysiology of essential hypertension. Although most bicarbonate reabsorption occurs in the proximal tubule, the medullary thick ascending limb (mTAL) of the nephron also maintains acid-base balance by contributing to 25% of bicarbonate reabsorption. A crucial element in this regulation is the sodium-hydrogen exchanger 1 (NHE1), a ubiquitous membrane protein controlling intracellular pH, where proton extrusion is driven by the inward sodium flux. MicroRNA (miRNA) expression of hypertensive patients significantly differs from that of normotensive subjects. The aim of this study was to determine the functional role of miRNA alterations at the mTAL level.

METHODS

By miRNA microarray analysis, we identified miRNA expression profiles in isolated mTALs from high sodium intake-induced hypertensive rats (HSD) versus their normotensive counterparts (NSD). In vitro validation was carried out in rat mTAL cells.

RESULTS

Five miRNAs involved in the onset of salt-sensitive hypertension were identified, including miR-23a, which was bioinformatically predicted to target NHE1 mRNA. Data demonstrated that miRNA-23a is downregulated in the mTAL of HSD rats while NHE1 is upregulated. Consistently, transfection of an miRNA-23a mimic in an mTAL cell line, using a viral vector, resulted in NHE1 downregulation.

CONCLUSION

NHE1, a protein involved in sodium reabsorption at the mTAL level and blood pressure regulation, is upregulated in our model. This was due to a downregulation of miRNA-23a. Expression levels of this miRNA are influenced by high sodium intake in the mTALs of rats. The downregulation of miRNA-23a in humans affected by essential hypertension corroborate our data and point to the potential role of miRNA-23a in the regulation of mTAL function following high salt intake.

Potassium homeostasis: sensors, mediators, and targets

Transmembrane potassium (K) gradients are key determinants of membrane potential that can modulate action potentials, control muscle contractility, and influence ion channel and transporter activity. Daily K intake is normally equal to the amount of K in the entire extracellular fluid (ECF) creating a critical challenge - how to maintain ECF [K] and membrane potential in a narrow range during feast and famine. Adaptations to maintain ECF [K] include sensing the K intake, sensing ECF [K] vs. desired set-point and activating mediators that regulate K distribution between ECF and ICF, and regulate renal K excretion. In this focused review, we discuss the basis of these adaptions, including (1) potential mechanisms for rapid feedforward signaling to kidney and muscle after a meal (before a rise in ECF [K]), (2) how skeletal muscles sense and respond to changes in ECF [K], (3) effects of K on aldosterone biosynthesis, and (4) how the kidney responds to changes in ECF [K] to modify K excretion. The concepts of sexual dimorphisms in renal K handling adaptation are introduced, and the molecular mechanisms that can account for the benefits of a K-rich diet to maintain cardiovascular health are discussed. Although the big picture of K homeostasis is becoming more clear, we also highlight significant pieces of the puzzle that remain to be solved, including knowledge gaps in our understanding of initiating signals, sensors and their connection to homeostatic adjustments of ECF [K].

Characterization of Potassium-Induced Natriuresis in Hypertensive Postmenopausal Women During Both Low and High Sodium Intake

BACKGROUND

Potassium-induced natriuresis may contribute to the beneficial effects of potassium on blood pressure but has not been well-characterized in human postmenopausal hypertension. We determined the time course and magnitude of potassium-induced natriuresis and kaliuresis compared with hydrochlorothiazide in 19 hypertensive Hispanic postmenopausal women. We also determined the modulating effects of sodium intake, sodium-sensitivity, and activity of the thiazide-sensitive NCC (sodium-chloride cotransporter).

METHODS

Sixteen-day inpatient confinement: 8 days low sodium followed by 8 days high sodium intake. During both periods, we determined sodium and potassium excretion following 35 mmol oral KCl versus 50 mg hydrochlorothiazide. We determined sodium-sensitivity as change in 24-hour systolic pressure from low to high sodium. We determined NCC activity by standard thiazide-sensitivity test.

RESULTS

Steady-state sodium intake was the key determinant of potassium-induced natriuresis. During low sodium intake, sodium excretion was low and did not increase following 35 mmol KCl indicating continued sodium conservation. Conversely, during high sodium intake, sodium excretion increased sharply following 35 mmol KCl to ≈37% of that produced by hydrochlorothiazide. Under both low and high sodium intake, 35 mmol potassium was mostly excreted within 5 hours, accompanied by a sodium load reflecting the steady-state sodium intake, consistent with independent regulation of sodium/potassium excretion in the human distal nephron.

CONCLUSIONS

Potassium-induced natriuresis was not greater in sodium-sensitive versus sodium-resistant hypertensives or hypertensives with higher versus lower basal NCC activity. We studied an acute KCl challenge. It remains to further characterize potassium-induced natriuresis during chronic potassium increase and when potassium is administered a complex potassium-containing meal.

Dietary potassium and the kidney: lifesaving physiology

Potassium often has a negative connotation in Nephrology as patients with chronic kidney disease (CKD) are prone to develop hyperkalaemia. Approaches to the management of chronic hyperkalaemia include a low potassium diet or potassium binders. Yet, emerging data indicate that dietary potassium may be beneficial for patients with CKD. Epidemiological studies have shown that a higher urinary potassium excretion (as proxy for higher dietary potassium intake) is associated with lower blood pressure (BP) and lower cardiovascular risk, as well as better kidney outcomes. Considering that the composition of our current diet is characterized by a high sodium and low potassium content, increasing dietary potassium may be equally important as reducing sodium. Recent studies have revealed that dietary potassium modulates the activity of the thiazide-sensitive sodium-chloride cotransporter in the distal convoluted tubule (DCT). The DCT acts as a potassium sensor to control the delivery of sodium to the collecting duct, the potassium-secreting portion of the kidney. Physiologically, this allows immediate kaliuresis after a potassium load, and conservation of potassium during potassium deficiency. Clinically, it provides a novel explanation for the inverse relationship between dietary potassium and BP. Moreover, increasing dietary potassium intake can exert BP-independent effects on the kidney by relieving the deleterious effects of a low potassium diet (inflammation, oxidative stress and fibrosis). The aim of this comprehensive review is to link physiology with clinical medicine by proposing that the same mechanisms that allow us to excrete an acute potassium load also protect us from hypertension, cardiovascular disease and CKD.

Regulation of the Renal NaCl Cotransporter and Its Role in Potassium Homeostasis

Daily dietary potassium (K+) intake may be as large as the extracellular K+ pool. To avoid acute hyperkalemia, rapid removal of K+ from the extracellular space is essential. This is achieved by translocating K+ into cells and increasing urinary K+ excretion. Emerging data now indicate that the renal thiazide-sensitive NaCl cotransporter (NCC) is critically involved in this homeostatic kaliuretic response. This suggests that the early distal convoluted tubule (DCT) is a K+ sensor that can modify sodium (Na+) delivery to downstream segments to promote or limit K+ secretion. K+ sensing is mediated by the basolateral K+ channels Kir4.1/5.1, a capacity that the DCT likely shares with other nephron segments. Thus, next to K+-induced aldosterone secretion, K+ sensing by renal epithelial cells represents a second feedback mechanism to control K+ balance. NCC’s role in K+ homeostasis has both physiological and pathophysiological implications. During hypovolemia, NCC activation by the renin-angiotensin system stimulates Na+ reabsorption while preventing K+ secretion. Conversely, NCC inactivation by high dietary K+ intake maximizes kaliuresis and limits Na+ retention, despite high aldosterone levels. NCC activation by a low-K+ diet contributes to salt-sensitive hypertension. K+-induced natriuresis through NCC offers a novel explanation for the antihypertensive effects of a high-K+ diet. A possible role for K+ in chronic kidney disease is also emerging, as epidemiological data reveal associations between higher urinary K+ excretion and improved renal outcomes. This comprehensive review will embed these novel insights on NCC regulation into existing concepts of K+ homeostasis in health and disease.

Effect of Dietary Sodium and Potassium Intake on the Mobilization of Bone Lead among Middle-Aged and Older Men: The Veterans Affairs Normative Aging Study

Bone is a major storage site as well as an endogenous source of lead in the human body. Dietary sodium and potassium intake may play a role in the mobilization of lead from bone to the circulation. We examined whether association between bone lead and urinary lead, a marker of mobilized lead in plasma, was modified by dietary intake of sodium and potassium among 318 men, aged 48–93 years, in the Veterans Affairs (VA) Normative Aging Study. Dietary sodium and potassium were assessed by flame photometry using 24-h urine samples, and a sodium-to-potassium ratio was calculated from the resulting measures. Patella and tibia bone lead concentrations were measured by K-shell-x-ray fluorescence. Urinary lead was measured by inductively coupled plasma mass spectroscopy in 24-h urine samples. Linear regression models were used to regress creatinine clearance-corrected urinary lead on bone lead, testing multiplicative interactions with tertiles of sodium, potassium, and sodium-to-potassium ratio, separately. After adjustment for age, body mass index, smoking, vitamin C intake, calcium, and total energy intake, participants in the highest tertile of sodium-to-potassium ratio showed 28.1% (95% CI: 12.5%, 45.9%) greater urinary lead per doubling increase in patella lead, whereas those in the second and lowest tertiles had 13.8% (95% CI: −1.7%, 31.7%) and 5.5% (95% CI: −8.0%, 21.0%) greater urinary lead, respectively (p-for-interaction = 0.04). No statistically significant effect modification by either sodium or potassium intake alone was observed. These findings suggest that relatively high intake of sodium relative to potassium may play an important role in the mobilization of lead from bone into the circulation.

Molecular mechanisms for the regulation of blood pressure by potassium

It has been well documented that the amount of potassium in the diet is associated with blood pressure levels in the population: the higher the potassium consumption, the lower the blood pressure and the cardiovascular mortality. In the last few years certain mechanisms for potassium regulation of salt reabsorption in the kidney have been elucidated at the molecular level. In this work we discuss the evidence demonstrating the relationship between potassium intake and blood pressure levels in human populations and in animal models, as well as the experimental data that reveal the effects of potassium on transepithelial Na⁺ reabsorption in different nephron segments. We also discuss the physiological relevance of K⁺-induced natriuresis, and finally, we focus on the molecular mechanisms by which extracellular potassium modulates the activity of the renal NaCl cotransporter, which is the mechanism that has been best dissected so far.

Renal potassium physiology: integration of the renal response to dietary potassium depletion

We summarize the current understanding of the physiology of the renal handling of potassium (K⁺), and present an integrative view of the renal response to K⁺ depletion caused by dietary K⁺ restriction. This renal response involves contributions from different nephron segments, and aims to diminish the rate of excretion of K⁺ as a result of: decreasing the rate of electrogenic (and increasing the rate of electroneutral) reabsorption of sodium in the aldosterone-sensitive distal nephron (ASDN), decreasing the abundance of renal outer medullary K⁺ channels in the luminal membrane of principal cells in the ASDN, decreasing the flow rate in the ASDN, and increasing the reabsorption of K⁺ in the cortical and medullary collecting ducts. The implications of this physiology for the association between K⁺ depletion and hypertension, and K⁺ depletion and formation of calcium kidney stones are discussed.

WNK-SPAK/OSR1 signaling: lessons learned from an insect renal epithelium

WNK [with no lysine (K)] kinases regulate renal epithelial ion transport to maintain homeostasis of electrolyte concentrations, extracellular volume, and blood pressure. The SLC12 cation-chloride cotransporters, including the sodium-potassium-2-chloride (NKCC) and sodium chloride cotransporters (NCC), are targets of WNK regulation via the intermediary kinases SPAK (Ste20-related proline/alanine-rich kinase) and OSR1 (oxidative stress response). The pathway is activated by low dietary potassium intake, resulting in increased phosphorylation and activity of NCC. Chloride regulates WNK kinases in vitro by binding to the active site and inhibiting autophosphorylation and has been proposed to modulate WNK activity in the distal convoluted tubule in response to low dietary potassium. WNK-SPAK/OSR1 regulation of NKCC-dependent ion transport is evolutionarily ancient, and it occurs in the Drosophila Malpighian (renal) tubule. Here, we review recent studies from the Drosophila tubule demonstrating cooperative roles for chloride and the scaffold protein Mo25 (mouse protein-25, also known as calcium-binding protein-39) in the regulation of WNK-SPAK/OSR1 signaling in a transporting renal epithelium. Insights gained from this genetically manipulable and physiologically accessible epithelium shed light on molecular mechanisms of regulation of the WNK-SPAK/OSR1 pathway, which is important in human health and disease.

Axial and cellular heterogeneity in electrolyte transport pathways along the thick ascending limb

The thick ascending limb (TAL) extends from the border of the inner medulla to the renal cortex, thus ascending through regions with wide differences in tissue solute and electrolyte concentrations. Structural and functional differences between TAL cells in the medulla (mTAL) and the cortex (cTAL) would therefore be useful to adapt TAL transport function to a changing external fluid composition. While mechanisms common to all TAL cells play a central role in the reclamation of about 25% of the NaCl filtered by the kidney, morphological features, Na⁺ / K⁺ -ATPase activity, NKCC2 splicing and phosphorylation do vary between segments and cells. The TAL contributes to K⁺ homeostasis and TAL cells with high or low basolateral K⁺ conductances have been identified which may be involved in K⁺ reabsorption and secretion respectively. Although transport rates for HCO3- do not differ between mTAL and cTAL, divergent axial and cellular expression of H⁺ transport proteins in TAL have been documented. The reabsorption of the divalent cations Ca²⁺ and Mg²⁺ is highest in cTAL and paralleled by differences in divalent cation permeability and the expression of select claudins. Morphologically, two cell types with different cell surface phenotypes have been described that still need to be linked to specific functional characteristics. The unique external environment and its change along the longitudinal axis require an axial functional heterogeneity for the TAL to optimally participate in conserving electrolyte homeostasis. Despite substantial progress in understanding TAL function, there are still considerable knowledge gaps that are just beginning to become bridged.

The importance of the thick ascending limb of Henle's loop in renal physiology and pathophysiology

The thick ascending limb (TAL) of Henle’s loop is a crucial segment for many tasks of the nephron. Indeed, the TAL is not only a mainstay for reabsorption of sodium (Na⁺), potassium (K⁺), and divalent cations such as calcium (Ca²⁺) and magnesium (Mg²⁺) from the luminal fluid, but also has an important role in urine concentration, overall acid–base homeostasis, and ammonia cycle. Transcellular Na⁺ transport along the TAL is a prerequisite for Na⁺, K⁺, Ca²⁺, Mg²⁺ homeostasis, and water reabsorption, the latter through its contribution in the generation of the cortico-medullar osmotic gradient. The role of this nephron site in acid–base balance, via bicarbonate reabsorption and acid secretion, is sometimes misunderstood by clinicians. This review describes in detail these functions, reporting in addition to the well-known molecular mechanisms, some novel findings from the current literature; moreover, the pathophysiology and the clinical relevance of primary or acquired conditions caused by TAL dysfunction are discussed. Knowing the physiology of the TAL is fundamental for clinicians, for a better understanding and management of rare and common conditions, such as tubulopathies, hypertension, and loop diuretics abuse.

Renal potassium handling in rats with subtotal nephrectomy: modeling and analysis

We sought to decipher the mechanisms underlying the kidney's response to changes in K⁺ load and intake, under physiological and pathophysiological conditions. To accomplish that goal, we applied a published computational model of epithelial transport along rat nephrons in a sham rat, an uninephrectomized (UNX) rat, and a 5/6-nephrectomized (5/6-NX) rat that also considers adaptations in glomerular filtration rate and tubular growth. Model simulations of an acute K⁺ load indicate that elevated expression levels and activities of Na⁺/K⁺-ATPase, epithelial sodium channels, large-conductance Ca²⁺-activated K⁺ channels, and renal outer medullary K⁺ channels, together with downregulation of sodium-chloride cotransporters (NCC), increase K⁺ secretion along the connecting tubule, resulting in a >6-fold increase in urinary K⁺ excretion in sham rats, which substantially exceeds the filtered K⁺ load. In the UNX and 5/6-NX models, the acute K⁺ load is predicted to increase K⁺ excretion, but at significantly reduced levels compared with sham. Acute K⁺ load is accompanied by natriuresis in sham rats. Model simulations suggest that the lesser natriuretic effect observed in the nephrectomized groups may be explained by impaired NCC downregulation in these kidneys. At a single-nephron level, a high K⁺ intake raises K⁺ secretion along the connecting tubule and reabsorption along the collecting duct in sham, and even more in UNX and 5/6-NX. However, the increased K⁺ secretion per tubule fails to sufficiently compensate for the reduction in nephron number, such that nephrectomized rats have an impaired ability to excrete an acute or chronic K⁺ load.

Immunolocalization of hyperpolarization-activated cationic HCN1 and HCN3 channels in the rat nephron: regulation of HCN3 by potassium diets

Hyperpolarization-activated cationic and cyclic nucleotide-gated channels (HCN) comprise four homologous subunits (HCN1-HCN4). HCN channels are found in excitable and non-excitable tissues in mammals. We have previously shown that HCN2 may transport ammonium (NH4 (+)), besides sodium (Na(+)), in the rat distal nephron. In the present work, we identified HCN1 and HCN3 in the proximal tubule (PT) and HCN3 in the thick ascending limb of Henle (TALH) of the rat kidney. Immunoblot assays detected HCN1 (130 kDa) and HCN3 (90 KDa) and their truncated proteins C-terminal HCN1 (93 KDa) and N-terminal HCN3 (65 KDa) in enriched plasma membranes from cortex (CX) and outer medulla (OM), as well as in brush-border membrane vesicles. Immunofluorescence assays confirmed apical localization of HCN1 and HCN3 in the PT. HCN3 was also found at the basolateral membrane of TALH. We evaluated chronic changes in mineral dietary on HCN3 protein abundance. Animals were fed with three different diets: sodium-deficient (SD) diet, potassium-deficient (KD) diet, and high-potassium (HK) diet. Up-regulation of HCN3 was observed in OM by KD and in CX and OM by HK; the opposite effect occurred with the N-terminal truncated HCN3 in CX (KD) and OM (HK). SD diet did not produce any change. Since HCN channels activate with membrane hyperpolarization, our results suggest that HCN channels may play a role in the Na(+)-K(+)-ATPase activity, contributing to Na(+), K(+), and acid-base homeostasis in the rat kidney.

A unifying mechanism for WNK kinase regulation of sodium-chloride cotransporter

Mammalian with-no-lysine [K] (WNK) kinases are a family of four serine-threonine protein kinases, WNK1-4. Mutations of WNK1 and WNK4 in humans cause pseudohypoaldosteronism type II (PHA2), an autosomal-dominant disease characterized by hypertension and hyperkalemia. Increased Na(+) reabsorption through Na(+)-Cl(-) cotransporter (NCC) in the distal convoluted tubule plays an important role in the pathogenesis of hypertension in patients with PHA2. However, how WNK1 and WNK4 regulate NCC and how mutations of WNKs cause activation of NCC have been controversial. Here, we review current state of literature supporting a compelling model that WNK1 and WNK4 both contribute to stimulation of NCC. The precise combined effects of WNK1 and WNK4 on NCC remain unclear but likely are positive rather than antagonistic. The recent discovery that WNK kinases may function as an intracellular chloride sensor adds a new dimension to the physiological role of WNK kinases. Intracellular chloride-dependent regulation of WNK's may underlie the mechanism of regulation of NCC by extracellular K(+). Definite answer yet will require future investigation by tubular perfusion in mice with altered WNK kinase expression.

Modulation of NCC activity by low and high K(+) intake: insights into the signaling pathways involved

Modulation of Na⁺-Cl⁻ cotransporter (NCC) activity is essential to adjust K⁺ excretion in the face of changes in dietary K⁺ intake. We used previously characterized genetic mouse models to assess the role of Ste20-related proline-alanine-rich kinase (SPAK) and with-no-lysine kinase (WNK)4 in the modulation of NCC by K⁺ diets. SPAK knockin and WNK4 knockout mice were placed on normal-, low-, or high-K⁺-citrate diets for 4 days. The low-K⁺ diet decreased and high-K⁺ diet increased plasma aldosterone levels, but both diets were associated with increased phosphorylation of NCC (phospho-NCC, Thr⁴⁴/Thr⁴⁸/Thr⁵³) and phosphorylation of SPAK/oxidative stress responsive kinase 1 (phospho-SPAK/OSR1, Ser³⁸³/Ser³²⁵). The effect of the low-K⁺ diet on SPAK phosphorylation persisted in WNK4 knockout and SPAK knockin mice, whereas the effects of ANG II on NCC and SPAK were lost in both mouse colonies. This suggests that for NCC activation by ANG II, integrity of the WNK4/SPAK pathway is required, whereas for the low-K⁺ diet, SPAK phosphorylation occurred despite the absence of WNK4, suggesting the involvement of another WNK (WNK1 or WNK3). Additionally, because NCC activation also occurred in SPAK knockin mice, it is possible that loss of SPAK was compensated by OSR1. The positive effect of the high-K⁺ diet was observed when the accompanying anion was citrate, whereas the high-KCl diet reduced NCC phosphorylation. However, the effect of the high-K⁺-citrate diet was aldosterone dependent, and neither metabolic alkalosis induced by bicarbonate, nor citrate administration in the absence of K⁺ increased NCC phosphorylation, suggesting that it was not due to citrate-induced metabolic alkalosis. Thus, the accompanying anion might modulate the NCC response to the high-K⁺ diet.

Kidney-specific WNK1 inhibits sodium reabsorption in the cortical thick ascending limb

Kidney-specific WNK1 (KS-WNK1) is a variant of full-length WNK1. Previous studies have reported that KS-WNK1 is predominantly expressed in the distal convoluted tubule (DCT) where it regulates sodium-chloride cotransporter. The role of KS-WNK1 in other nephron segments is less clear. Here, we measured the expression of KS-WNK1 transcript in microdissected renal tubules and found that KS-WNK1 was most abundant in the DCT, followed by cortical thick ascending limb (cTAL), connecting tubule, and cortical collecting duct. A high K(+) diet enhanced the expression of KS-WNK1 in the DCT and cTAL, selectively. It has been reported that a high-K diet suppresses Na(+) reabsorption in TAL. To understand the role of KS-WNK1 in Na(+) transport in cTAL and the regulation by dietary K(+), we examined Na(+) reabsorption using in vitro microperfusion in cTAL isolated from KS-WNK1-knockout mice and wild-type littermates fed either a control-K(+) or high-K(+) diet. Furosemide-sensitive Na(+) reabsorption in cTAL was higher in KS-WNK1-knockout (KO) mice than in wild-type. A high-K(+) diet inhibited Na(+) reabsorption in cTAL from wild-type mice, but the inhibition was eliminated in KS-WNK1-KO mice. We further examined the role of KS-WNK1 using transgenic mice that overexpress KS-WNK1. Na(+) reabsorption in cTAL was lower in transgenic than in wild-type mice. In whole animal clearance studies, a high-K(+) diet increased daily urine volume and urinary Na(+) and K(+) excretion in wild-type mice, which was blunted in KS-WNK1-KO mice. Thus KS-WNK1 inhibits Na(+) reabsorption in cTAL and mediates the inhibition of Na(+) reabsorption in the segment by a high-K diet.

Pathophysiology and management of hypokalemia: a clinical perspective

Potassium (K(+)) ions are the predominant intracellular cations. K(+) homeostasis depends on external balance (dietary intake [typically 100 mmol per day] versus excretion [95% via the kidney; 5% via the colon]) and internal balance (the distribution of K(+) between intracellular and extracellular fluid compartments). The uneven distribution of K(+) across cell membranes means that a mere 1% shift in its distribution can cause a 50% change in plasma K(+) concentration. Hormonal mechanisms (involving insulin, β-adrenergic agonists and aldosterone) modulate K(+) distribution by promoting rapid transfer of K(+) across the plasma membrane. Extrarenal K(+) losses from the body are usually small, but can be marked in individuals with chronic diarrhea, severe burns or prolonged sweating. Under normal circumstances, the kidney's distal nephron secretes K(+) and determines final urinary excretion. In patients with hypokalemia (plasma K(+) concentration <3.5 mmol/l), after the exclusion of extrarenal causes, alterations in sodium ion delivery to the distal nephron, mineralocorticoid status, or a specific inherited or acquired defect in distal nephron function (each of which affects distal nephron K(+) secretion), should be considered. Clinical management of hypokalemia should establish the underlying cause and alleviate the primary disorder. This Review aims to inform clinicians about the pathophysiology and appropriate treatment for hypokalemia.

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.

A mathematical model of rat ascending Henle limb. III. Tubular function

K+ plays a catalytic role in AHL Na+ reabsorption via Na+-K+-2Cl- cotransporter (NKCC2), recycling across luminal K+ channels, so that luminal K+ is not depleted. Based on models of the ascending Henle limb (AHL) epithelium, it has been hypothesized that NH4+ may also catalyze luminal Na+ uptake. This hypothesis requires that luminal NH4+ not be depleted, implying replenishment via either direct secretion of NH4+, or NH3 in parallel with a proton. In the present work, epithelial models of rat medullary and cortical AHL (Weinstein AM, Krahn TA. Am J Physiol Renal Physiol 298: F000-F000, 2009) are configured as tubules and examined in simulations of function in vitro and in vivo to assess the feasibility of a catalytic role for NH4+ in Na+ reabsorption. Modulation of Na+ transport is also examined by peritubular K+ concentration and by Bartter-type transport defects in NKCC2 (type 1), in luminal membrane K+ channels (type 2), and in peritubular Cl- channels (type 3). It is found that a catalytic role for NH4+, which is significant in magnitude (relative to K+), is quantitatively realistic, in terms of uptake via NKCC2, and in terms of luminal membrane ammonia backflux. Simulation of a 90% NKCC2 defect is predicted to double distal Na+ delivery; it is also predicted to increase distal acid delivery (principally as NH4+). With doubling of medullary K+, the model predicts a 30% increase in distal Na+ delivery, but in this case there is a decrease in AHL acidification. This effect of peritubular K+ on proton secretion appears to be akin to type 3 Bartter's pathophysiology, in which there is decreased peritubular HCO3- exit, cytosolic alkalinization, and a consequent decrease in luminal proton secretion by NHE3. One consequence of overlapping and redundant roles for K+ and NH4+, is a blunted impact of luminal membrane K+ permeability on overall Na+ reabsorption, so that type 2 Bartter pathophysiology is not well captured by the model.

Reducing extracellular pH sensitizes the acinar cell to secretagogue-induced pancreatitis responses in rats

BACKGROUND & AIMS

Protease activation within the pancreatic acinar cell is a key early event in acute pancreatitis and may require low pH intracellular compartments. Clinical studies suggest that acidosis may affect the risk for developing pancreatitis. We hypothesized that exposure to an acid load might sensitize the acinar cell to secretagogue-induced pancreatitis.

METHODS

Secretagogues (cerulein, carbachol, and bombesin) can induce protease activation in acinar cells at high (100 nmol/L, 1 mmol/L, and 10 micromol/L, respectively) but not at physiologically relevant concentrations. The effects of decreasing extracellular pH (pHe) in early secretagogue-induced pancreatitis (zymogen activation and injury) were examined in rats (1) in vitro with isolated acini and (2) in vivo with an acid challenge.

RESULTS

In acini, lowering pHe from 7.6 to 6.8 enhanced secretagogue-induced zymogen activation and injury, but did not affect secretion. For cerulein, this sensitization was seen over a range of concentrations (0.01-100.00 nmol/L). However, reduced pHe alone had no effect on zymogen activation, amylase secretion, or cell injury. We have reported that zymogen activation is mediated by the vacuolar ATPase (vATPase), a proton transporter. vATPase inhibition, using concanamycin (100 nmol/L), blocked the low pHe effects on zymogen activation. An acute acid load given in vivo enhanced cerulein-induced (50 microg/kg) trypsinogen activation and pancreatic edema.

CONCLUSION

These studies suggest that acid challenge sensitizes the pancreatic acinar cell to secretagogue-induced zymogen activation and injury and may increase the risk for the development and severity of acute pancreatitis.

Alterations of growth plate and abnormal insulin-like growth factor I metabolism in growth-retarded hypokalemic rats: effect of growth hormone treatment

Hypokalemic tubular disorders may lead to growth retardation which is resistant to growth hormone (GH) treatment. The mechanism of these alterations is unknown. Weaning female rats were grouped ( n = 10) in control, potassium-depleted (KD), KD treated with intraperitoneal GH at 3.3 mg·kg−1·day−1 during the last week (KDGH), and control pair-fed with KD (CPF). After 2 wk, KD rats were growth retarded compared with CPF rats, the osseous front advance (±SD) being 67.07 ± 10.44 and 81.56 ± 12.70 μm/day, respectively. GH treatment did not accelerate growth rate. The tibial growth plate of KD rats had marked morphological alterations: lower heights of growth cartilage (228.26 ± 23.58 μm), hypertrophic zone (123.68 ± 13.49 μm), and terminal chondrocytes (20.8 ± 2.39 μm) than normokalemic CPF (264.21 ± 21.77, 153.18 ± 15.80, and 24.21 ± 5.86 μm). GH administration normalized these changes except for the distal chondrocyte height. Quantitative PCR of insulin-like growth factor I (IGF-I), IGF-I receptor, and GH receptor genes in KD growth plates showed downregulation of IGF-I and upregulation of IGF-I receptor mRNAs, without changes in their distribution as analyzed by immunohistochemistry and in situ hybridization. GH did not further modify IGF-I mRNA expression. KD rats had normal hepatic IGF-I mRNA levels and low serum IGF-I values. GH increased liver IGF-I mRNA, but circulating IGF-I levels remained reduced. This study discloses the structural and molecular alterations induced by potassium depletion on the growth plate and shows that the lack of response to GH administration is associated with persistence of the disturbed process of chondrocyte hypertrophy and depressed mRNA expression of local IGF-I in the growth plate.

Dual effect of insulin-like growth factor on the apical 70-pS K channel in the thick ascending limb of rat kidney

We used the patch-clamp technique to study the effect of insulin-like growth factor I (IGF-I) on the apical 70-pS K channel in the isolated thick ascending limb (TAL) of the rat kidney. The isolated TAL was cut open to gain access to the apical membrane. Addition of 25 nM IGF-I stimulates the apical 70-pS K channel and increases channel activity, defined by the product of channel open probability and channel number, from 0.31 to 1.21. The stimulatory effect of IGF-I is not mediated by nitric oxide- or protein tyrosine phosphatase-dependent mechanisms, because inhibition of nitric oxide synthase or blocking protein tyrosine phosphatase did not abolish the stimulatory effect of IGF-I on the 70-pS K channel. In contrast, inhibition of mitogen-activated protein (MAP) kinase with PD-98059 or U0126 abolished the stimulatory effect of IGF-I. This suggests that MAP kinase is responsible for mediating the effect of IGF-I on the apical K channels. Moreover, the effect of IGF-I on the apical 70-pS K channel is biphasic because high concentrations (>200 nM) inhibit apical 70-pS K channels. Application of 400 nM IGF-I decreased channel activity from 1.45 to 0.2. The inhibitory effect of IGF-I is not blocked by calphostin C (an inhibitor of PKC), but inhibition of protein tyrosine kinase with herbimycin A abolished the IGF-induced inhibition. We conclude that IGF-I has a dual effect on the apical 70-pS K channel in the TAL: low concentrations of IGF-I stimulate, whereas high concentrations inhibit the channel activity. The stimulatory effect of IGF-I is mediated by a MAP kinase-dependent pathway, whereas the inhibitory effect is the result of stimulation of protein tyrosine kinase.

Regulation of renal K transport by dietary K intake

Extracellular K must be kept within a narrow concentration range for the normal function of neurons, skeletal muscle, and cardiac myocytes. Maintenance of normal plasma K is achieved by a dual mechanism that includes extrarenal factors such as insulin and beta-adrenergic agonists, which stimulate the movement of K from extracellular to intracellular fluid and modulate renal K excretion. Dietary K intake is an important factor for the regulation of K secretion: An increase in K intake stimulates secretion, whereas a decrease inhibits K secretion and enhances absorption. This effect of changes in dietary K intake on tubule K transport is mediated by aldosterone-dependent and -independent mechanisms. Recently, it has been demonstrated that the protein tyrosine kinase (PTK)-dependent signal transduction pathway is an important aldosterone-independent regulatory mechanism that mediates the effect of altered K intake on K secretion. A low-K intake stimulates PTK activity, which leads to increase in phosphorylation of cloned inwardly rectifying renal K (ROMK) channels, whereas a high-K intake has the opposite effect. Stimulation of tyrosine phosphorylation also suppresses K secretion in principal cell by facilitating the internalization of apical K channels in the collecting duct.

Inhibition of heme oxygenase decreases sodium and fluid absorption in the loop of Henle

We previously demonstrated that carbon monoxide (CO) stimulates the apical 70-pS K+ channel in the thick ascending limb (TAL) of the rat kidney (Liu HJ, Mount DB, Nasjletti A, and Wang WH. J Clin Invest 103: 963-970, 1999). Because the apical K+ channel plays a key role in K+ recycling, we tested the hypothesis that heme oxygenase (HO)-dependent metabolites of heme may affect Na+ transport in the TAL. We used in vivo microperfusion to study the effect of chromium mesoporphyrin (CrMP), an inhibitor of HO, on fluid absorption (Jv) and Na+ absorption (JNa) in the loop of Henle and renal clearance methods to examine the effect of CrMP on renal sodium excretion. Microperfusion experiments demonstrated that addition of CrMP to the loop of Henle decreased Jv by 13% and JNa by 20% in animals on normal rat chow and caused a decrease in Jv (39%) and JNa (40%) in rats on a high-K+ (HK) diet. The effect of CrMP is the result of inhibition of HO because addition of MgPP, an analog of CrMP that does not inhibit HO, had no effect on Jv. Western blot analysis showed that HO-2 is expressed in the kidney and that the level of HO-2 was significantly elevated in animals on a HK diet. Renal clearance studies demonstrated that the infusion of CrMP increased the excretion of urinary Na+ (ENa) and volume (UV) without changes in glomerular filtration rate. The effect of CrMP on ENa and UV was larger in HK rats than those kept on normal chow. We conclude that HK intake increases HO-2 expression in the kidney and that HO-dependent metabolites of heme, presumably CO, play a significant role in the regulation of Na+ transport in the loop of Henle.

Altered expression of renal NHE3, TSC, BSC-1, and ENaC subunits in potassium-depleted rats

The purpose of this study was to examine whether hypokalemia is associated with altered abundance of major renal Na+ transporters that may contribute to the development of urinary concentrating defects. We examined the changes in the abundance of the type 3 Na+/H+ exchanger (NHE3), Na+ - K+-ATPase, the bumetanide-sensitive Na+ - K+ - 2Cl- cotransporter (BSC-1), the thiazide-sensitive Na+ - Cl- cotransporter (TSC), and epithelial sodium channel (ENaC) subunits in kidneys of hypokalemic rats. Semiquantitative immunoblotting revealed that the abundance of BSC-1 (57%) and TSC (46%) were profoundly decreased in the inner stripe of the outer medulla (ISOM) and cortex/outer stripe of the outer medulla (OSOM), respectively. These findings were confirmed by immunohistochemistry. Moreover, total kidney abundance of all ENaC subunits was significantly reduced in response to the hypokalemia: alpha-subunit (61%), beta-subunit (41%), and gamma-subunit (60%), and this was confirmed by immunohistochemistry. In contrast, the renal abundance of NHE3 in hypokalemic rats was dramatically increased in cortex/OSOM (736%) and ISOM (210%). Downregulation of BSC-1, TSC, and ENaC may contribute to the urinary concentrating defect, whereas upregulation of NHE3 may be compensatory to prevent urinary Na+ loss and/or to maintain intracellular pH levels.

K depletion enhances the extracellular Ca2+-induced inhibition of the apical K channels in the mTAL of rat kidney

We have shown previously that raising extracellular Ca(2)+ inhibited the apical 70-pS K channel in the thick ascending limb (TAL; Wang, W.H., M. Lu, and S.C. Hebert. 1996. Am. J. Physiol. 270:C103-C111). We now used the patch-clamp technique to study the effect of increasing the extracellular Ca(2)+ on the 70-pS K channel in the mTAL from rats on a different K diet. Increasing the extracellular Ca(2)+ from 10 microM to 0.5, 1, and to 1.5 mM in the mTAL from rats on a K-deficient (KD) diet inhibited the channel activity by 30, 65, and 90%, respectively. In contrast, raising the extracellular Ca(2)+ to 1.5 mM had no significant effect on channel activity in the mTAL from animals on a high K (HK) diet and further increasing the extracellular Ca(2)+ to 2.5, 3.5, and 5.5 mM decreased the channel activity by 29, 55, and 90%, respectively. Inhibition of the cytochrome P450 monooxygenase completely abolished the effect of the extracellular Ca(2)+ on channel activity in the mTAL from rats on a different K diet. In contrast, blocking cyclooxygenase did not significantly alter the responsiveness of the 70-pS K channel to the extracellular Ca(2)+. Moreover, addition of sodium nitropruside, a nitric oxide (NO) donor, not only increased the channel activity, but also blunted the inhibitory effect of the extracellular Ca(2)+ on the 70-pS K channel and decreased 20-hydroxyeicosatetraenoic acid (20-HETE) concentration in the mTAL from rats on a KD diet. In contrast, inhibiting NOS with L-NAME enhanced the inhibitory effect of the extracellular Ca(2)+ on the channel activity and increased 20-HETE concentration in the mTAL from rats on a high K diet. Western blot has further shown that the expression of inducible NO synthase (iNOS) is significantly higher in the renal medulla from rats on an HK diet than that on a KD diet. Also, addition of S-nitroso-N-acetylpenicillamine abolished the inhibitory effect of arachidonic acid on channel activity in the mTAL, whereas it did not block the inhibitory effect of 20-HETE. We conclude that a low dietary K intake increases the sensitivity of the 70-pS K channel to the extracellular Ca(2)+, and that a decrease in NOS activity is involved in enhancing the inhibitory effect of the extracellular Ca(2)+ on channel activity in the mTAL during K depletion.

Hypokalemia induces renal injury and alterations in vasoactive mediators that favor salt sensitivity

We investigated the hypothesis that hypokalemia might induce renal injury via a mechanism that involves subtle renal injury and alterations in local vasoactive mediators that would favor sodium retention. To test this hypothesis, we conducted studies in rats with diet-induced K+ deficiency. We also determined whether rats with hypokalemic nephropathy show salt sensitivity. Twelve weeks of hypokalemia resulted in a decrease in creatinine clearance, tubulointerstitial injury with macrophage infiltration, interstitial collagen type III deposition, and an increase in osteopontin expression (a tubular marker of injury). The renal injury was greatest in the outer medulla with radiation into the cortex, suggestive of an ischemic etiology. Consistent with this hypothesis, we found an increased uptake of a hypoxia marker, pimonidazole, in the cortex. The intrarenal injury was associated with increased cortical angiontensin-converting enzyme (ACE) expression and continued cortical angiotensin II generation despite systemic suppression of the renin-angiotensin system, an increase in renal endothelin-1, a decrease in renal kallikrein, and a decrease in urinary nitrite/nitrates and prostaglandin E(2) excretion. At 12 wk, hypokalemic rats were placed on a normal-K+ diet with either high (4%)- or low (0.01%)-NaCl content. Despite correction of hypokalemia and normalization of renal function, previously hypokalemic rats showed an elevated blood pressure in response to a high-salt diet compared with normokalemic controls. Hypokalemia is associated with alterations in vasoactive mediators that favor intrarenal vasoconstriction and an ischemic pattern of renal injury. These alterations may predispose the animals to salt-sensitive hypertension that manifests despite normalization of the serum K+.

Expression of rat thick limb Na/H exchangers in potassium depletion and chronic metabolic acidosis

BACKGROUND

Regulation of renal transporter expression has been shown to support adaptation of transporter activities in several chronic situations. Basolateral and apical Na/H exchangers (NHE) in medullary thick ascending limb (MTAL) are involved in NH4+ and HCO3+ absorption, respectively. The NH4+ absorption rate in Henle's loop is increased in chronic metabolic acidosis (CMA) and potassium depletion (KD), which may be secondary to the increased NH4+ concentration in luminal fluid and/or to an increased NH4+ absorptive capacity of MTAL. HCO3- absorptive capacity in Henle's loop is increased in CMA and decreased in metabolic alkalosis, but is unchanged in KD despite the presence of metabolic alkalosis. The present study compared the effects of NH4Cl-induced CMA and KD on the expression of basolateral NHE-1 and the effect of KD on the expression of apical NHE-3 in MTAL.

METHODS

NHE-1 and NHE-3 mRNAs and proteins were assessed by a competitive reverse transcription-polymerase chain reaction (RT-PCR) method and semiquantitative immunoblots, respectively, in MTAL-purified suspensions from rats with CMA and KD.

RESULTS

NHE-1 protein abundance was similarly increased (approximately 90%) at two and five weeks of KD, while NHE-1 mRNA amount in MTAL cells was increased at two weeks of KD and returned to normal values by five weeks of KD. In contrast, NHE-1 mRNA and protein abundance did not change in CMA. NHE-3 protein abundance remained unchanged in both two and five weeks of KD, while NHE-3 mRNA was unchanged by two weeks of KD and reduced by approximately 50% at five weeks of KD.

CONCLUSIONS

The results suggest the following: (1) in KD, where the increased NH4+ concentration of luminal fluid that favors NH4+ absorption is counterbalanced by a decrease in BSC1 expression and activity, the increased NHE-1 expression may support an increased MTAL NH4+ absorptive capacity in CMA, NHE-1 expression is not specifically regulated and remains unchanged, suggesting that the increase in NH4+ concentration in luminal fluid is the main determinant of increased NH4+ absorption in MTAL. (2) In KD, NHE-3 expression did not decrease despite the presence of metabolic alkalosis, in agreement with the unchanged HCO3- absorptive capacity of Henle's loop.

Role of 20-HETE in mediating the effect of dietary K intake on the apical K channels in the mTAL

We have used the patch-clamp technique to study the effect of dietary K intake on the apical K channels in the medullary thick ascending limb (mTAL) of rat kidneys. The channel activity, defined by the number of channels in a patch and the open probability (NPo), of the 30- and 70-pS K channels, was 0.18 and 0.11, respectively, in the mTAL from rats on a K-deficient diet. In contrast, NPo of the 30- and 70-pS K channels increased to 0.60 and 0.80, respectively, in the tubules from animals on a high-K diet. The concentration of 20-hydroxyeicosatetraenoic acid (20-HETE) measured with gas chromatography-mass spectrometry was 0.8 pg/microg protein in the mTAL from rats on a high-K diet and increased significantly to 4.6 pg/microg protein in the tubules from rats on a K-deficient diet. Addition of N-methylsulfonyl-12,12-dibromododec-11-enamide (DDMS) or 17-octadecynoic acid (17-ODYA), agents that inhibit the formation of 20-HETE, had no significant effect on the activity of the 30-pS K channels. However, DDMS/17-ODYA significantly increased the activity of the apical 70-pS K channel from 0.11 to 0.91 in the mTAL from rats on a K-deficient diet. In contrast, inhibition of the cytochrome P-450 metabolism of arachidonic acid increased NPo from 0.64 to 0.81 in the tubules from animals on a high-K diet. Furthermore, the sensitivity of the 70-pS K channel to 20-HETE was the same between rats on a high-K diet and on a K-deficient diet. Finally, the pretreatment of the tubules with DDMS increased NPo of the 70-pS K channels in the mTAL from rats on a K-deficient diet to 0.76. We conclude that an increase in 20-HETE production is involved in reducing the activity of the apical 70-pS K channels in the mTAL from rats on a K-deficient diet.

Renal potassium transport: mechanisms and regulation

The regulation of potassium metabolism involves mechanisms for the appropriate distribution between the intra- and extracellular fluid compartments and for the excretion by the kidney. Clearance and single nephron studies show that renal excretion is determined by regulated potassium secretion and potassium reabsorption, respectively, in principal and intercalated cells of the distal nephron. Measurement of the electrochemical driving forces acting on potassium transport across individual cell membranes and characterization of several ATPases and potassium channels provide insights into the transport and regulation of renal potassium excretion.

Regulation of renal Na+,K(+)-ATPase in rat thick ascending limb during K+ depletion: evidence for modulation of Na+ affinity

1. NaCl reabsorption along the loop of Henle is reduced in K(+)-depleted rats. Because Na+,K(+)-ATPase energizes this transport and because K+ depletion is known to induce an upregulation of Na+,K(+)-ATPase in most tissues, the regulation of this enzyme was investigated at the level of single thick ascending limbs of the loop of Henle freshly microdissected from rats fed either a normal (control rats) or a low-K+ diet (LK rats). 2. Within 2 weeks of K+ depletion, Na+,K(+)-ATPase activity and [3H]ouabain binding were increased by 30-50% in the medullary portion of the thick ascending limb (MTAL). 3. Despite this increase in the number of Na+,K(+)-ATPase units, the transport capacity of the Na+,K+ pump, determined by ouabain-sensitive Rb+ uptake in the presence of an extracellular concentration of Rb+ mimicking the kalaemia determined in control (4.0 mM Rb+) and LK rats (2.3 mM Rb+), was reduced in MTAL from LK rats. 4. Inhibition of the Na+,K+ pump was not accounted for by changes in either extracellular K+ or intracellular Na+ concentrations, but by a decrease in the pump affinity for Na+. 5. Because this change in the apparent affinity of the Na+,K+ pump for Na+ was detectable in intact but not in permeabilized MTAL cells, it is probably induced by a rapidly reversible cytosolic factor.