Angiotensin II inhibits the ROMK-like small conductance K channel in renal cortical collecting duct during dietary potassium restriction

The times they are K+-changin': bringing the potassium curriculum out of the 20th century

PURPOSE OF REVIEW

Although most of the current medical education literature has focused on teaching strategies, little attention has been devoted to selecting appropriate course content. Despite elegant descriptions of physiologic mechanisms in recent decades, medical school curricula and students continue to rely on outdated textbooks and certification examination study aids composed to fit an antiquated exam blueprint.

RECENT FINDINGS

Advances in our understanding of potassium physiology offer multiple examples of key concepts that deserve to be included in the modern-day renal physiology curriculum, including the relationship of potassium to blood pressure and the potassium 'switch', the aldosterone paradox, and novel pharmacologic agents that target dietary potassium absorption and potassium handling in the kidney.

SUMMARY

Key advances in our understanding and application of renal physiology to patient care have not been readily integrated into the nephrology curriculum of medical students. Difficult questions remain regarding when new concepts are sufficiently established to be introduced to medical students in the preclinical years.

Angiotensin II hypertension along the female rat tubule: predicted impact on coupled transport of Na+ and K. Am J Physiol Renal Physiol 2023;325:F733-F749. [PMID: 37823196 PMCID: PMC10878725 DOI: 10.1152/ajprenal.00232.2023] [Citation(s) in RCA: 0] [Impact Index Per Article: 0]

Chronic infusion of subpressor level of angiotensin II (ANG II) increases the abundance of Na+ transporters along the distal nephron, balanced by suppression of Na+ transporters along the proximal tubule and medullary thick ascending limb (defined as "proximal nephron"), which impacts K+ handling along the entire renal tubule. The objective of this study was to quantitatively assess the impact of chronic ANG II on the renal handling of Na+ and K+ in female rats, using a computational model of the female rat renal tubule. Our results indicate that the downregulation of proximal nephron Na+ reabsorption (TNa), which occurs in response to ANG II-triggered hypertension, involves changes in both transporter abundance and trafficking. Our model suggests that substantial (∼30%) downregulation of active NHE3 in proximal tubule (PT) microvilli is needed to reestablish the Na+ balance at 2 wk of ANG II infusion. The 35% decrease in SGLT2, a known NHE3 regulator, may contribute to this downregulation. Both depression of proximal nephron TNa and stimulation of distal ENaC raise urinary K+ excretion in ANG II-treated females, while K+ loss is slightly mitigated by cortical NKCC2 and NCC upregulation. Our model predicts that K+ excretion may be more significantly limited during ANG II infusion by ROMK inhibition in the distal nephron and/or KCC3 upregulation in the PT, which remain open questions for experimental validation. In summary, our analysis indicates that ANG II hypertension triggers a series of events from distal TNa stimulation followed by compensatory reduction in proximal nephron TNa and accompanying adjustments to limit excessive K+ secretion.NEW & NOTEWORTHY We used a computational model of the renal tubule to assess the impact of 2-wk angiotensin II (ANG II) infusion on the handling of Na+ and K+ in female rats. ANG II strongly stimulates distal Na+ reabsorption and K+ secretion. Simulations indicate that substantial downregulation of proximal tubule NHE3 is needed to reestablish Na+ balance at 2 wk. Proximal adaptations challenge K+ homeostasis, and regulation of distal NCC and specific K+ channels likely limit urinary K+ losses.

Dietary anions control potassium excretion: it is more than a poorly absorbable anion effect

The urinary potassium (K+) excretion machinery is upregulated with increasing dietary K+, but the role of accompanying dietary anions remains inadequately characterized. Poorly absorbable anions, including [Formula: see text], are thought to increase K+ secretion through a transepithelial voltage effect. Here, we tested if they also influence the K+ secretion machinery. Wild-type mice, aldosterone synthase (AS) knockout (KO) mice, or pendrin KO mice were randomized to control, high-KCl, or high-KHCO3 diets. The K+ secretory capacity was assessed in balance experiments. Protein abundance, modification, and localization of K+-secretory transporters were evaluated by Western blot analysis and confocal microscopy. Feeding the high-KHCO3 diet increased urinary K+ excretion and the transtubular K+ gradient significantly more than the high-KCl diet, coincident with more pronounced upregulation of epithelial Na+ channels (ENaC) and renal outer medullary K+ (ROMK) channels and apical localization in the distal nephron. Experiments in AS KO mice revealed that the enhanced effects of [Formula: see text] were aldosterone independent. The high-KHCO3 diet also uniquely increased the large-conductance Ca2+-activated K+ (BK) channel β4-subunit, stabilizing BKα on the apical membrane, the Cl-/[Formula: see text] exchanger, pendrin, and the apical KCl cotransporter (KCC3a), all of which are expressed specifically in pendrin-positive intercalated cells. Experiments in pendrin KO mice revealed that pendrin was required to increase K+ excretion with the high-KHCO3 diet. In summary, [Formula: see text] stimulates K+ excretion beyond a poorly absorbable anion effect, upregulating ENaC and ROMK in principal cells and BK, pendrin, and KCC3a in pendrin-positive intercalated cells. The adaptive mechanism prevents hyperkalemia and alkalosis with the consumption of alkaline ash-rich diets but may drive K+ wasting and hypokalemia in alkalosis.NEW & NOTEWORTHY Dietary anions profoundly impact K+ homeostasis. Here, we found that a K+-rich diet, containing [Formula: see text] as the counteranion, enhances the electrogenic K+ excretory machinery, epithelial Na+ channels, and renal outer medullary K+ channels, much more than a high-KCl diet. It also uniquely induces KCC3a and pendrin, in B-intercalated cells, providing an electroneutral KHCO3 secretion pathway. These findings reveal new K+ balance mechanisms that drive adaption to alkaline and K+-rich foods, which should guide new treatment strategies for K+ disorders.

Directing two-way traffic in the kidney: A tale of two ions

The kidneys regulate levels of Na+ and K+ in the body by varying urinary excretion of the electrolytes. Since transport of each of the two ions can affect the other, controlling both at the same time is a complex task. The kidneys meet this challenge in two ways. Some tubular segments change the coupling between Na+ and K+ transport. In addition, transport of Na+ can shift between segments where it is coupled to K+ reabsorption and segments where it is coupled to K+ secretion. This permits the kidney to maintain electrolyte balance with large variations in dietary intake.

Potassium Derangements: A Pathophysiological Review, Diagnostic Approach, and Clinical Management

Potassium is an essential cation critical in fluid and electrolyte balance, acid–base regulation, and neuromuscular functions. The normal serum potassium is kept within a narrow range of 3.5–5.2 meq/L while the intracellular concentration is approximately 140–150 meq/L. The total body potassium is about 45–55 mmol/kg; thus, a 70 kg male has an estimated ~136 g and 60 kg female has ~117 g of potassium. In total, 98% of the total body potassium is intracellular. Skeletal muscle contains ~80% of body potassium stores. The ratio of intracellular to extracellular potassium concentration (Ki/Ke) maintained by Na+/K+ ATPase determines the resting membrane potential. Disturbances of potassium homeostasis lead to hypo- and hyperkalemia, which if severe, can be life-threatening. Prompt diagnosis and management of these problems are important.

Paracrine and endocrine regulation of renal potassium secretion

The seminal studies conducted by Giebisch and colleagues in the 1960s paved the way for understanding the renal mechanisms involved in K+ homeostasis. It was demonstrated that differential handling of K+ in the distal segments of the nephron is crucial for proper K+ balance. Although aldosterone had been classically ascribed as the major ion transport regulator in the distal nephron, thereby contributing to K+ homeostasis, it became clear that aldosterone per se could not explain the kidney's ability to modulate kaliuresis in both acute and chronic settings. The existence of alternative kaliuretic and antikaliuretic mechanisms was suggested by physiological studies in the 1980s but only gained form and shape with the advent of molecular biology. It is now established that the kidneys recruit several endocrine and paracrine mechanisms for adequate kaliuretic response. These mechanisms include the direct effects of peritubular K+, a gut-kidney regulatory axis sensing dietary K+ levels, the kidney secretion of kallikrein during postprandial periods, the upregulation of angiotensin II receptors in the distal nephron during chronic changes in the K+ diet, and the local increase of prostaglandins by low K+ diet. This review discusses recent advances in the understanding of endocrine and paracrine mechanisms underlying the modulation of K+ secretion and how these mechanisms impact kaliuresis and K+ balance. We also highlight important unknowns about the regulation of renal K+ excretion under physiological circumstances.

The Angiotensin II Type 1 Receptor-Associated Protein Attenuates Angiotensin II-Mediated Inhibition of the Renal Outer Medullary Potassium Channel in Collecting Duct Cells

Adjustments in renal K⁺ excretion constitute a central mechanism for K⁺ homeostasis. The renal outer medullary potassium (ROMK) channel accounts for the major K⁺ secretory route in collecting ducts during basal conditions. Activation of the angiotensin II (Ang II) type 1 receptor (AT1R) by Ang II is known to inhibit ROMK activity under the setting of K⁺ dietary restriction, underscoring the role of the AT1R in K⁺ conservation. The present study aimed to investigate whether an AT1R binding partner, the AT1R-associated protein (ATRAP), impacts Ang II-mediated ROMK regulation in collecting duct cells and, if so, to gain insight into the potential underlying mechanisms. To this end, we overexpressed either ATRAP or β-galactosidase (LacZ; used as a control), in M-1 cells, a model line of cortical collecting duct cells. We then assessed ROMK channel activity by employing a novel fluorescence-based microplate assay. Experiments were performed in the presence of 10⁻¹⁰ M Ang II or vehicle for 40 min. We observed that Ang II-induced a significant inhibition of ROMK in LacZ, but not in ATRAP-overexpressed M-1 cells. Inhibition of ROMK-mediated K⁺ secretion by Ang II was accompanied by lower ROMK cell surface expression. Conversely, Ang II did not affect the ROMK-cell surface abundance in M-1 cells transfected with ATRAP. Additionally, diminished response to Ang II in M-1 cells overexpressing ATRAP was accompanied by decreased c-Src phosphorylation at the tyrosine 416. Unexpectedly, reduced phospho-c-Src levels were also found in M-1 cells, overexpressing ATRAP treated with vehicle, suggesting that ATRAP can also downregulate this kinase independently of Ang II-AT1R activation. Collectively, our data support that ATRAP attenuates inhibition of ROMK by Ang II in collecting duct cells, presumably by reducing c-Src activation and blocking ROMK internalization. The potential role of ATRAP in K⁺ homeostasis and/or disorders awaits further investigation.

Mechanisms coupling sodium and magnesium reabsorption in the distal convoluted tubule of the kidney

Hypomagnesaemia is a common feature of renal Na+ wasting disorders such as Gitelman and EAST/SeSAME syndrome. These genetic defects specifically affect Na+ reabsorption in the distal convoluted tubule, where Mg2+ reabsorption is tightly regulated. Apical uptake via TRPM6 Mg2+ channels and basolateral Mg2+ extrusion via a putative Na+ -Mg2+ exchanger determines Mg2+ reabsorption in the distal convoluted tubule. However, the mechanisms that explain the high incidence of hypomagnesaemia in patients with Na+ wasting disorders of the distal convoluted tubule are largely unknown. In this review, we describe three potential mechanisms by which Mg2+ reabsorption in the distal convoluted tubule is linked to Na+ reabsorption. First, decreased activity of the thiazide-sensitive Na+ /Cl- cotransporter (NCC) results in shortening of the segment, reducing the Mg2+ reabsorption capacity. Second, the activity of TRPM6 and NCC are determined by common regulatory pathways. Secondary effects of NCC dysregulation such as hormonal imbalance, therefore, might disturb TRPM6 expression. Third, the basolateral membrane potential, maintained by the K+ permeability and Na+ -K+ -ATPase activity, provides the driving force for Na+ and Mg2+ extrusion. Depolarisation of the basolateral membrane potential in Na+ wasting disorders of the distal convoluted tubule may therefore lead to reduced activity of the putative Na+ -Mg2+ exchanger SLC41A1. Elucidating the interconnections between Mg2+ and Na+ transport in the distal convoluted tubule is hampered by the currently available models. Our analysis indicates that the coupling of Na+ and Mg2+ reabsorption may be multifactorial and that advanced experimental models are required to study the molecular mechanisms.

Angiotensin II increases activity of the ClC-K2 Cl- channel in collecting duct intercalated cells by stimulating production of reactive oxygen species

The renal collecting duct plays a critical role in setting urinary volume and composition, with principal cells transporting Na+ and K+ and intercalated cells mediating Cl- reabsorption. Published evidence implies Angiotensin II (Ang II) is a potent regulator of the collecting duct apical transport systems in response to systemic volume depletion. However, virtually nothing is known about Ang II actions on the basolateral conductance of principal and intercalated cells. Here, we combined macroscopic and single channel patch clamp recordings from freshly isolated mouse collecting ducts with biochemical and fluorescence methods to demonstrate an acute stimulation of the basolateral Cl- conductance and specifically the ClC-K2 Cl- channel by nanomolar Ang II concentrations in intercalated cells. In contrast, Ang II did not exhibit measurable effects on the basolateral conductance and on Kir4.1/5.1 potassium channel activity in principal cells. Although both Ang II receptors AT1 and AT2 are expressed in collecting duct cells, we show that AT1 receptors were essential for stimulatory actions of Ang II on ClC-K2. Moreover, AT1R-/- mice had decreased renal ClC-K2 expression. We further demonstrated that activation of NADPH oxidases is the major signaling pathway downstream of Ang II-AT1R that leads to stimulation of ClC-K2. Treatment of freshly isolated collecting ducts with Ang II led to production of reactive oxygen species on the same timescale as single channel ClC-K2 activation. Overall, we propose that Ang II-dependent regulation of ClC-K2 in intercalated cells is instrumental for stimulation of Cl- reabsorption by the collecting duct, particularly during hypovolemic states.

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.

Sex difference in kidney electrolyte transport II: impact of K+ intake on thiazide-sensitive cation excretion in male and female mice

We studied sex differences in response to high K+ (HK) intake on thiazide-sensitive cation (Na+ and K+) excretion in wild-type (WT) and ANG II receptor subtype 1a (AT1aR) knockout (KO) mice. Renal clearance experiments were performed to examine Na+-Cl- cotransporter (NCC) activity on mice fed with control and HK (5% KCl, 7 days) diets. Hydrochlorothiazide (HCTZ)-induced changes in urine volume, glomerular filtration rate, absolute Na+ and K+ excretion, and fractional excretion were compared. HK-induced changes in NCC, Na+/H+ exchanger isoform 3 (NHE3), and ENaC expression were examined by Western blot analysis. In WT animals under the control diet, HCTZ-induced cation excretion was greater in female animals, reflecting larger increases in Na+ excretion, since there was little sex difference in HCTZ-induced K+ excretion. Under the HK diet, the sex difference in HCTZ-induced cation excretion was reduced because of larger increments in K+ excretion in male animals. The fraction of K+ excretion was 57 ± 5% in male WT animals and 36 ± 4% in female WT animals (P < 0.05), but this difference was absent in AT1aR KO mice. NCC abundance was higher in female animals than in male animals but decreased by similar fractions on HK diet. NHE3 abundance decreased, whereas cleaved forms of γ-ENaC increased, with HK in all groups; these changes were similar in male and female animals and were not significantly affected by AT1aR ablation. These results indicate that, with the HK diet, male animals display greater distal Na+ delivery and greater activation of K+ secretion mechanisms, all suggesting a more powerful male adaptation to HK intake.

Nephrotic syndrome is associated with increased plasma K+ concentration, intestinal K+ losses, and attenuated urinary K+ excretion: a study in rats and humans

The present study tested the hypotheses that nephrotic syndrome (NS) leads to renal K⁺ loss because of augmented epithelial Na⁺ channel (ENaC) activity followed by downregulation of renal K⁺ secretory pathways by suppressed aldosterone. The hypotheses were addressed by determining K⁺ balance and kidney abundance of K⁺ and Na⁺ transporter proteins in puromycin aminonucleoside (PAN)-induced rat nephrosis. The effects of amiloride and angiotensin II type 1 receptor and mineralocorticoid receptor (MR) antagonists were tested. Glucocorticoid-dependent MR activation was tested by suppression of endogenous glucocorticoid with dexamethasone. Urine and plasma samples were obtained from pediatric patients with NS in acute and remission phases. PAN-induced nephrotic rats had ENaC-dependent Na⁺ retention and displayed lower renal K⁺ excretion but elevated intestinal K⁺ secretion that resulted in less cumulated K⁺ in NS. Aldosterone was suppressed at day 8. The NS-associated changes in intestinal, but not renal, K⁺ handling responded to suppression of corticosterone, whereas angiotensin II type 1 receptor and MR blockers and amiloride had no effect on urine K⁺ excretion during NS. In PAN-induced nephrosis, kidney protein abundance of the renal outer medullary K⁺ channel and γ-ENaC were unchanged, whereas the Na⁺-Cl^- cotransporter was suppressed and Na⁺-K⁺-ATPase increased. Pediatric patients with acute NS displayed suppressed urine Na⁺-to-K⁺ ratios compared with remission and elevated plasma K⁺ concentration, whereas fractional K⁺ excretion did not differ. Acute NS is associated with less cumulated K⁺ in a rat model, whereas patients with acute NS have elevated plasma K⁺ and normal renal fractional K⁺ excretion. In NS rats, K⁺ balance is not coupled to ENaC activity but results from opposite changes in renal and fecal K⁺ excretion with a contribution from corticosteroid MR-driven colonic secretion.

Biomarkers of Oxidative Stress in Metabolic Syndrome and Associated Diseases

Metabolic syndrome (MS) represents worldwide public health issue characterized by a set of cardiovascular risk factors including obesity, diabetes, dyslipidemia, hypertension, and impaired glucose tolerance. The link between the MS and the associated diseases is represented by oxidative stress (OS) and by the intracellular redox imbalance, both caused by the persistence of chronic inflammatory conditions that characterize MS. The increase in oxidizing species formation in MS has been accepted as a major underlying mechanism for mitochondrial dysfunction, accumulation of protein and lipid oxidation products, and impairment of the antioxidant systems. These oxidative modifications are recognized as relevant OS biomarkers potentially able to (i) clarify the role of reactive oxygen and nitrogen species in the etiology of the MS, (ii) contribute to the diagnosis/evaluation of the disease's severity, and (iii) evaluate the utility of possible therapeutic strategies based on natural antioxidants. The antioxidant therapies indeed could be able to (i) counteract systemic as well as mitochondrial-derived OS, (ii) enhance the endogenous antioxidant defenses, (iii) alleviate MS symptoms, and (iv) prevent the complications linked to MS-derived cardiovascular diseases. The focus of this review is to summarize the current knowledge about the role of OS in the development of metabolic alterations characterizing MS, with particular regard to the occurrence of OS-correlated biomarkers, as well as to the use of therapeutic strategies based on natural antioxidants.

Angiotensin II Signal Transduction: An Update on Mechanisms of Physiology and Pathophysiology

The renin-angiotensin-aldosterone system plays crucial roles in cardiovascular physiology and pathophysiology. However, many of the signaling mechanisms have been unclear. The angiotensin II (ANG II) type 1 receptor (AT₁R) is believed to mediate most functions of ANG II in the system. AT₁R utilizes various signal transduction cascades causing hypertension, cardiovascular remodeling, and end organ damage. Moreover, functional cross-talk between AT₁R signaling pathways and other signaling pathways have been recognized. Accumulating evidence reveals the complexity of ANG II signal transduction in pathophysiology of the vasculature, heart, kidney, and brain, as well as several pathophysiological features, including inflammation, metabolic dysfunction, and aging. In this review, we provide a comprehensive update of the ANG II receptor signaling events and their functional significances for potential translation into therapeutic strategies. AT₁R remains central to the system in mediating physiological and pathophysiological functions of ANG II, and participation of specific signaling pathways becomes much clearer. There are still certain limitations and many controversies, and several noteworthy new concepts require further support. However, it is expected that rigorous translational research of the ANG II signaling pathways including those in large animals and humans will contribute to establishing effective new therapies against various diseases.

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.

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

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

Intracellular cAMP Sensor EPAC: Physiology, Pathophysiology, and Therapeutics Development

This review focuses on one family of the known cAMP receptors, the exchange proteins directly activated by cAMP (EPACs), also known as the cAMP-regulated guanine nucleotide exchange factors (cAMP-GEFs). Although EPAC proteins are fairly new additions to the growing list of cAMP effectors, and relatively "young" in the cAMP discovery timeline, the significance of an EPAC presence in different cell systems is extraordinary. The study of EPACs has considerably expanded the diversity and adaptive nature of cAMP signaling associated with numerous physiological and pathophysiological responses. This review comprehensively covers EPAC protein functions at the molecular, cellular, physiological, and pathophysiological levels; and in turn, the applications of employing EPAC-based biosensors as detection tools for dissecting cAMP signaling and the implications for targeting EPAC proteins for therapeutic development are also discussed.

Paracellular transport in the collecting duct

PURPOSE OF REVIEW

The paracellular pathway through the tight junction provides an important route for chloride reabsorption in the collecting duct of the kidney. This review describes recent findings of how defects in paracellular chloride permeation pathway may cause kidney diseases and how such a pathway may be regulated to maintain normal chloride homeostasis.

RECENT FINDINGS

The tight junction in the collecting duct expresses two important claudin genes - claudin-4 and claudin-8. Transgenic knockout of either claudin gene causes hypotension, hypochloremia, and metabolic alkalosis in experimental animals. The claudin-4 mediated chloride permeability can be regulated by a protease endogenously expressed by the collecting duct cell - channel-activating protease 1. Channel-activating protease 1 regulates the intercellular interaction of claudin-4 and its membrane stability. Kelch-like 3, previously identified as a causal gene for Gordon's syndrome, also known as pseudohypoaldosteronism II, directly interacts with claudin-8 and regulates its ubiquitination and degradation. The dominant pseudohypoaldosteronism-II mutation (R528H) in Kelch-like 3 abolishes claudin-8 binding, ubiquitination, and degradation.

SUMMARY

The paracellular chloride permeation pathway in the kidney is an important but understudied area in nephrology. It plays vital roles in renal salt handling and regulation of extracellular fluid volume and blood pressure. Two claudin proteins, claudin-4 and claudin-8, contribute to the function of this paracellular pathway. Deletion of either claudin protein from the collecting duct causes renal chloride reabsorption defects and low blood pressure. Claudins can be regulated on posttranslational levels by several mechanisms involving protease and ubiquitin ligase. Deregulation of claudins may cause human hypertension as exemplified in the Gordon's syndrome.

Effects of voluntary exercise on blood pressure, angiotensin II, aldosterone, and renal function in two-kidney, one-clip hypertensive rats

Spontaneous dynamic exercise promotes sympathoinhibition and decreases arterial pressure in two-kidney, one-clip (2K-1C) hypertensive rats. Renal sympathetic nerves stimulate renin secretion and increase renal tubular sodium reabsorption. We hypothesized that daily voluntary wheel running exercise by 2K-1C rats will decrease mean arterial pressure (MAP), plasma angiotensin II (Ang II), and aldosterone as well as normalize urinary sodium and potassium excretion independent of changes in glomerular filtration rate (GFR). Five-week-old male Sprague Dawley rats underwent sham clipping (Sham) or right renal artery clipping (2K-1C). Rats were randomized to standard caging (SED) or cages with running wheels (EX). After 12 weeks, rats were assigned to either collection of aortic blood for measurement of Ang II and aldosterone or assessment of inulin clearances and excretory function. Running distances were comparable in both EX groups. MAP was lower in 2K-1C EX vs 2K-1C SED rats (P<0.05). Plasma Ang II and aldosterone were significantly higher in 2K-1C SED rats and decreased in 2K-1C EX rats to levels similar to Sham SED or Sham EX rats. Clipped kidney weights were significantly lower in both 2K-1C groups, but GFR and urine flow rates were no different from right and left kidneys among the four groups. Total and fractional sodium excretion rates from the unclipped kidney of 2K-1C SED rats were higher vs either Sham group (P<0.05). Values in 2K-1C EX rats were similar to the Sham groups. Potassium excretion paralleled sodium excretion. These studies show that voluntary dynamic exercise in 2K-1C rats decreases plasma Ang II and aldosterone, which contribute to the lower arterial pressure without deleterious effects on GFR. The effects on sodium excretion underscore the impact of pressure natriuresis despite elevated plasma Ang II and aldosterone in sedentary 2K-1C rats. In contrast, potassium excretion is primarily regulated by circulating aldosterone and distal sodium delivery.

Effects of Reactive Oxygen Species on Tubular Transport along the Nephron

Reactive oxygen species (ROS) are oxygen-containing molecules naturally occurring in both inorganic and biological chemical systems. Due to their high reactivity and potentially damaging effects to biomolecules, cells express a battery of enzymes to rapidly metabolize them to innocuous intermediaries. Initially, ROS were considered by biologists as dangerous byproducts of respiration capable of causing oxidative stress, a condition in which overproduction of ROS leads to a reduction in protective molecules and enzymes and consequent damage to lipids, proteins, and DNA. In fact, ROS are used by immune systems to kill virus and bacteria, causing inflammation and local tissue damage. Today, we know that the functions of ROS are not so limited, and that they also act as signaling molecules mediating processes as diverse as gene expression, mechanosensation, and epithelial transport. In the kidney, ROS such as nitric oxide (NO), superoxide (O₂^-), and their derivative molecules hydrogen peroxide (H₂O₂) and peroxynitrite (ONO₂^-) regulate solute and water reabsorption, which is vital to maintain electrolyte homeostasis and extracellular fluid volume. This article reviews the effects of NO, O₂^-, ONO₂^-, and H₂O₂ on water and electrolyte reabsorption in proximal tubules, thick ascending limbs, and collecting ducts, and the effects of NO and O₂^- in the macula densa on tubuloglomerular feedback.

The adipose tissue and the involvement of the renin-angiotensin-aldosterone system in cardiometabolic syndrome

Cardiometabolic diseases are linked to a cluster of modifiable factors, including risk factors closely related to central adiposity. Chronic renin-angiotensin-aldosterone system (RAAS) activation has far-reaching effects on cardiometabolic risk and is a substantial contributor to this clinical condition. RAAS components are locally expressed in the vessels and adipose tissue. This review appoints RAAS, through the classical and the alternative view, as the main mediator of the cross-talk in cardiometabolic syndrome.

Potassium Supplementation Prevents Sodium Chloride Cotransporter Stimulation During Angiotensin II Hypertension

Angiotensin II (AngII) hypertension increases distal tubule Na-Cl cotransporter (NCC) abundance and phosphorylation (NCCp), as well as epithelial Na(+) channel abundance and activating cleavage. Acutely raising plasma [K(+)] by infusion or ingestion provokes a rapid decrease in NCCp that drives a compensatory kaliuresis. The first aim tested whether acutely raising plasma [K(+)] with a single 3-hour 2% potassium meal would lower NCCp in Sprague-Dawley rats after 14 days of AngII (400 ng/kg per minute). The potassium-rich meal neither decreased NCCp nor increased K(+) excretion. AngII-infused rats exhibited lower plasma [K(+)] versus controls (3.6±0.2 versus 4.5±0.1 mmol/L; P<0.05), suggesting that AngII-mediated epithelial Na(+) channel activation provokes K(+) depletion. The second aim tested whether doubling dietary potassium intake from 1% (A1K) to 2% (A2K) would prevent K(+) depletion during AngII infusion and, thus, prevent NCC accumulation. A2K-fed rats exhibited normal plasma [K(+)] and 2-fold higher K(+) excretion and plasma [aldosterone] versus A1K. In A1K rats, NCC, NCCpS71, and NCCpT53 abundance increased 1.5- to 3-fold versus controls (P<0.05). The rise in NCC and NCCp abundance was prevented in the A2K rats, yet blood pressure did not significantly decrease. Epithelial Na(+) channel subunit abundance and cleavage increased 1.5- to 3-fold in both A1K and A2K; ROMK (renal outer medulla K(+) channel abundance) abundance was unaffected by AngII or dietary K(+) In summary, the accumulation and phosphorylation of NCC seen during chronic AngII infusion hypertension is likely secondary to potassium deficiency driven by epithelial Na(+) channel stimulation.

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

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

Potassium homeostasis and dyskalemias: the respective roles of renal, extrarenal, and gut sensors in potassium handling

Integrated mechanisms controlling the maintenance of potassium homeostasis are well established and are defined by the classic "feedback control" of potassium balance. Recently, increasing investigative attention has focused on novel physiological paradigms that increase the complexity and precision of homeostasis. This review briefly considers the classic and well-established feedback control of potassium and then considers subsequent investigations that inform on an intriguing and not widely recognized complementary paradigm: the "feed-forward control of potassium balance." Feed-forward control refers to a pathway in a homeostatic system that responds to a signal in the environment in a predetermined manner, without responding to how the system subsequently reacts (i.e., without responding to feedback). Studies in several animal species, and recently in humans, have confirmed the presence of a feed-forward control mechanism that is capable of mediating potassium excretion independent of changes in serum potassium concentration and aldosterone. Knowledge imparted by this update of potassium homeostasis hopefully will facilitate the clinical management of hyperkalemia in patients with chronic and recurrent hyperkalemia. Awareness of this updated integrative control mechanism for potassium homeostasis is more relevant today when the medical community is increasingly focused on leveraging and expanding established renin-angiotensin-aldosterone system inhibitor treatment regimens and on successfully coping with the challenges of managing hyperkalemia provoked by renin-angiotensin-aldosterone system inhibitors. These new insights are relevant to the future design of clinical trials delineating renal potassium handling.

Gene Silencing and Haploinsufficiency of Csk Increase Blood Pressure

Objective

Recent genome-wide association studies have identified 33 human genetic loci that influence blood pressure. The 15q24 locus is one such locus that has been confirmed in Asians and Europeans. There are 21 genes in the locus within a 1-Mb boundary, but a functional link of these genes to blood pressure has not been reported. We aimed to identify a causative gene for blood pressure change in the 15q24 locus.

Methods and Results

CSK and ULK3 were selected as candidate genes based on eQTL analysis studies that showed the association between gene transcript levels and the lead SNP (rs1378942). Injection of siRNAs for mouse homologs Csk, Ulk3, and Cyp1a2 (negative control) showed reduced target gene mRNA levels in vivo. However, Csk siRNA only increased blood pressure while Ulk3 and Cyp1a2 siRNA did not change it. Further, blood pressure in Csk^+/- heterozygotes was higher than in wild-type, consistent with what we observed in Csk siRNA-injected mice. We confirmed that haploinsufficiency of Csk increased the active form of Src in Csk^+/- mice aorta. We also showed that inhibition of Src by PP2, a Src inhibitor decreased high blood pressure in Csk^+/- mice and the active Src in Csk^+/- mice aorta and in Csk knock-down vascular smooth muscle cells, suggesting blood pressure regulation by Csk through Src.

Conclusions

Our study demonstrates that Csk is a causative gene in the 15q24 locus and regulates blood pressure through Src, and these findings provide a novel therapeutic target for the treatment of hypertension.

Maintaining K+ balance on the low-Na+, high-K+ diet

A low-Na⁺, high-K⁺ diet (LNaHK) is considered a healthier alternative to the "Western" high-Na⁺ diet. Because the mechanism for K⁺ secretion involves Na⁺ reabsorptive exchange for secreted K⁺ in the distal nephron, it is not understood how K⁺ is eliminated with such low Na⁺ intake. Animals on a LNaHK diet produce an alkaline load, high urinary flows, and markedly elevated plasma ANG II and aldosterone levels to maintain their K⁺ balance. Recent studies have revealed a potential mechanism involving the actions of alkalosis, urinary flow, elevated ANG II, and aldosterone on two types of K⁺ channels, renal outer medullary K⁺ and large-conductance K⁺ channels, located in principal and intercalated cells. Here, we review these recent advances.

Roles and Regulation of Renal K Channels

More than two dozen types of potassium channels, with different biophysical and regulatory properties, are expressed in the kidney, influencing renal function in many important ways. Recently, a confluence of discoveries in areas from human genetics to physiology, cell biology, and biophysics has cast light on the special function of five different potassium channels in the distal nephron, encoded by the genes KCNJ1, KCNJ10, KCNJ16, KCNMA1, and KCNN3. Research aimed at understanding how these channels work in health and go awry in disease has transformed our understanding of potassium balance and provided new insights into mechanisms of renal sodium handling and the maintenance of blood pressure. This review focuses on recent advances in this rapidly evolving field.

Potassium and Its Discontents: New Insight, New Treatments

Hyperkalemia is common in patients with impaired kidney function or who take drugs that inhibit the renin-angiotensin-aldosterone axis. During the past decade, substantial advances in understanding how the body controls potassium excretion have been made, which may lead to improved standard of care for these patients. Renal potassium disposition is primarily handled by a short segment of the nephron, comprising part of the distal convoluted tubule and the connecting tubule, and regulation results from the interplay between aldosterone and plasma potassium. When dietary potassium intake and plasma potassium are low, the electroneutral sodium chloride cotransporter is activated, leading to salt retention. This effect limits sodium delivery to potassium secretory segments, limiting potassium losses. In contrast, when dietary potassium intake is high, aldosterone is stimulated. Simultaneously, potassium inhibits the sodium chloride cotransporter. Because more sodium is then delivered to potassium secretory segments, primed by aldosterone, kaliuresis results. When these processes are disrupted, hyperkalemia results. Recently, new agents capable of removing potassium from the body and treating hyperkalemia have been tested in clinical trials. This development suggests that more effective and safer approaches to the prevention and treatment of hyperkalemia may be on the horizon.

Changes in urinary excretion of water and sodium transporters during amiloride and bendroflumethiazide treatment

AIM: To quantify changes in urinary excretion of aquaporin2 water channels (u-AQP2), the sodium-potassium-chloride co-transporter (u-NKCC2) and the epithelial sodium channels (u-ENaC) during treatment with bendroflumethiazide (BFTZ), amiloride and placebo.

METHODS: In a randomized, double-blinded, placebo-controlled, 3-way crossover study we examined 23 healthy subjects on a standardized diet and fluid intake. The subjects were treated with amiloride 5 mg, BFTZ 1.25 mg or placebo twice a day for 4.5 d before each examination day. On the examination day, glomerular filtration rate was measured by the constant infusion clearance technique with ⁵¹Cr-EDTA as reference substance. To estimate the changes in water transport via AQP2 and sodium transport via NKCC2 and ENaC, u-NKCC2, the gamma fraction of ENaC (u-ENaCγ), and u-AQP2 were measured at baseline and after infusion with 3% hypertonic saline. U-NKCC2, u-ENaCγ, u-AQP2 and plasma concentrations of vasopressin (p-AVP), renin (PRC), angiotensin II (p-ANG II) and aldosterone (p-Aldo) were measured, by radioimmunoassay. Central blood pressure was estimated by applanation tonometry and body fluid volumes were estimated by bio-impedance spectroscopy. General linear model with repeated measures or related samples Friedman’s two-way analysis was used to compare differences. Post hoc Bonferroni correction was used for multiple comparisons of post infusion periods to baseline within each treatment group.

RESULTS: At baseline there were no differences in u-NKCC2, u-ENaCγ and u-AQP2. PRC, p-Ang II and p-Aldo were increased during active treatments (P < 0.001). After hypertonic saline, u-NKCC2 increased during amiloride (6% ± 34%; P = 0.081) and increased significantly during placebo (17% ± 24%; P = 0.010). U-AQP2 increased significantly during amiloride (31% ± 22%; P < 0.001) and placebo (34% ± 27%; P < 0.001), while u-NKCC2 and u-AQP2 did not change significantly during BFTZ (-7% ± 28%; P = 0.257 and 5% ± 16%; P = 0.261). U- ENaCγ increased in all three groups (P < 0.050). PRC, AngII and p-Aldo decreased to the same extent, while AVP increased, but to a smaller degree during BFTZ (P = 0.048). cDBP decreased significantly during BFTZ (P < 0.001), but not during amiloride or placebo. There were no significant differences in body fluid volumes.

CONCLUSION: After hypertonic saline, u-NKCC2 and u-AQP2 increased during amiloride, but not during BFTZ. Lower p-AVP during BFTZ potentially caused less stimulation of NKCC2 and AQP2 and subsequent lower reabsorption of water and sodium.

Angiotensin II signaling via protein kinase C phosphorylates Kelch-like 3, preventing WNK4 degradation

Hypertension contributes to the global burden of cardiovascular disease. Increased dietary K(+) reduces blood pressure; however, the mechanism has been obscure. Human genetic studies have suggested that the mechanism is an obligatory inverse relationship between renal salt reabsorption and K(+) secretion. Mutations in the kinases with-no-lysine 4 (WNK4) or WNK1, or in either Cullin 3 (CUL3) or Kelch-like 3 (KLHL3)--components of an E3 ubiquitin ligase complex that targets WNKs for degradation--cause constitutively increased renal salt reabsorption and impaired K(+) secretion, resulting in hypertension and hyperkalemia. The normal mechanisms that regulate the activity of this ubiquitin ligase and levels of WNKs have been unknown. We posited that missense mutations in KLHL3 that impair binding of WNK4 might represent a phenocopy of the normal physiologic response to volume depletion in which salt reabsorption is maximized. We show that KLHL3 is phosphorylated at serine 433 in the Kelch domain (a site frequently mutated in hypertension with hyperkalemia) by protein kinase C in cultured cells and that this phosphorylation prevents WNK4 binding and degradation. This phosphorylation can be induced by angiotensin II (AII) signaling. Consistent with these in vitro observations, AII administration to mice, even in the absence of volume depletion, induces renal KLHL3(S433) phosphorylation and increased levels of both WNK4 and the NaCl cotransporter. Thus, AII, which is selectively induced in volume depletion, provides the signal that prevents CUL3/KLHL3-mediated degradation of WNK4, directing the kidney to maximize renal salt reabsorption while inhibiting K(+) secretion in the setting of volume depletion.

Integration of the response to a dietary potassium load: a paleolithic perspective

Our purpose is to integrate new insights in potassium (K(+)) physiology to understand K(+) homeostasis and illustrate some of their clinical implications. Since control mechanisms that are essential for survival were likely developed in Paleolithic times, we think the physiology of K(+) homeostasis can be better revealed when viewed from what was required to avoid threats and achieve balance in Paleolithic times. Three issues will be highlighted. First, we shall consider the integrative physiology of the gastrointestinal tract and the role of lactic acid released from enterocytes following absorption of sugars (fruit and berries) to cause a shift of this K(+) load into the liver. Second, we shall discuss the integrative physiology of WNK kinases and modulation of delivery of bicarbonate to the distal nephron to switch the aldosterone response from sodium chloride retention to K(+) secretion when faced with a K(+) load. Third, we shall emphasize the role of intra-renal recycling of urea in achieving K(+) homeostasis when the diet contains protein and K(+).

Angiotensin II type 2 receptor regulates ROMK-like K⁺ channel activity in the renal cortical collecting duct during high dietary K⁺ adaptation

The kidney adjusts K⁺ excretion to match intake in part by regulation of the activity of apical K⁺ secretory channels, including renal outer medullary K⁺ (ROMK)-like K⁺ channels, in the cortical collecting duct (CCD). ANG II inhibits ROMK channels via the ANG II type 1 receptor (AT1R) during dietary K⁺ restriction. Because AT1Rs and ANG II type 2 receptors (AT2Rs) generally function in an antagonistic manner, we sought to characterize the regulation of ROMK channels by the AT2R. Patch-clamp experiments revealed that ANG II increased ROMK channel activity in CCDs isolated from high-K⁺ (HK)-fed but not normal K⁺ (NK)-fed rats. This response was blocked by PD-123319, an AT2R antagonist, but not by losartan, an AT1R antagonist, and was mimicked by the AT2R agonist CGP-42112. Nitric oxide (NO) synthase is present in CCD cells that express ROMK channels. Blockade of NO synthase with N-nitro-l-arginine methyl ester and free NO with 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide potassium salt completely abolished ANG II-stimulated ROMK channel activity. NO enhances the synthesis of cGMP, which inhibits phosphodiesterases (PDEs) that normally degrade cAMP; cAMP increases ROMK channel activity. Pretreatment of CCDs with IBMX, a broad-spectrum PDE inhibitor, or cilostamide, a PDE3 inhibitor, abolished the stimulatory effect of ANG II on ROMK channels. Furthermore, PKA inhibitor peptide, but not an activator of the exchange protein directly activated by cAMP (Epac), also prevented the stimulatory effect of ANG II. We conclude that ANG II acts at the AT2R to stimulate ROMK channel activity in CCDs from HK-fed rats, a response opposite to that mediated by the AT1R in dietary K⁺-restricted animals, via a NO/cGMP pathway linked to a cAMP-PKA pathway.

Regulation of Potassium Homeostasis

Potassium is the most abundant cation in the intracellular fluid, and maintaining the proper distribution of potassium across the cell membrane is critical for normal cell function. Long-term maintenance of potassium homeostasis is achieved by alterations in renal excretion of potassium in response to variations in intake. Understanding the mechanism and regulatory influences governing the internal distribution and renal clearance of potassium under normal circumstances can provide a framework for approaching disorders of potassium commonly encountered in clinical practice. This paper reviews key aspects of the normal regulation of potassium metabolism and is designed to serve as a readily accessible review for the well informed clinician as well as a resource for teaching trainees and medical students.

Regulation of renal potassium secretion: molecular mechanisms

A new understanding of renal potassium balance has emerged as the molecular underpinnings of potassium secretion have become illuminated, highlighting the key roles of apical potassium channels, renal outer medullary potassium channel (ROMK) and Big Potassium (BK), in the aldosterone-sensitive distal nephron and collecting duct. These channels act as the final-regulated components of the renal potassium secretory machinery. Their activity, number, and driving forces are precisely modulated to ensure potassium excretion matches dietary potassium intake. Recent identification of the underlying regulatory mechanisms at the molecular level provides a new appreciation of the physiology and reveals a molecular insight to explain the paradoxic actions of aldosterone on potassium secretion. Here, we review the current state of knowledge in the field.

The sodium chloride cotransporter SLC12A3: new roles in sodium, potassium, and blood pressure regulation

SLC12A3 encodes the thiazide-sensitive sodium chloride cotransporter (NCC), which is primarily expressed in the kidney, but also in intestine and bone. In the kidney, NCC is located in the apical plasma membrane of epithelial cells in the distal convoluted tubule. Although NCC reabsorbs only 5 to 10% of filtered sodium, it is important for the fine-tuning of renal sodium excretion in response to various hormonal and non-hormonal stimuli. Several new roles for NCC in the regulation of sodium, potassium, and blood pressure have been unraveled recently. For example, the recent discoveries that NCC is activated by angiotensin II but inhibited by dietary potassium shed light on how the kidney handles sodium during hypovolemia (high angiotensin II) and hyperkalemia. The additive effect of angiotensin II and aldosterone maximizes sodium reabsorption during hypovolemia, whereas the inhibitory effect of potassium on NCC increases delivery of sodium to the potassium-secreting portion of the nephron. In addition, great steps have been made in unraveling the molecular machinery that controls NCC. This complex network consists of kinases and ubiquitinases, including WNKs, SGK1, SPAK, Nedd4-2, Cullin-3, and Kelch-like 3. The pathophysiological significance of this network is illustrated by the fact that modification of each individual protein in the network changes NCC activity and results in salt-dependent hypotension or hypertension. This review aims to summarize these new insights in an integrated manner while identifying unanswered questions.

In vivo and ex vivo analysis of tubule function

Analysis of tubule function with in vivo and ex vivo approaches has been instrumental in revealing renal physiology. This work allows assignment of functional significance to known gene products expressed along the nephron, primary of which are proteins involved in electrolyte transport and regulation of these transporters. Not only we have learned much about the key roles played by these transport proteins and their proper regulation in normal physiology but also the combination of contemporary molecular biology and molecular genetics with in vivo and ex vivo analysis opened a new era of discovery informative about the root causes of many renal diseases. The power of in vivo and ex vivo analysis of tubule function is that it preserves the native setting and control of the tubule and proteins within tubule cells enabling them to be investigated in a "real-life" environment with a high degree of precision. In vivo and ex vivo analysis of tubule function continues to provide a powerful experimental outlet for testing, evaluating, and understanding physiology in the context of the novel information provided by sequencing of the human genome and contemporary genetic screening. These tools will continue to be a mainstay in renal laboratories as this discovery process continues and as we continue to identify new gene products functionally compromised in renal disease.

K+-induced natriuresis is preserved during Na+ depletion and accompanied by inhibition of the Na+-Cl- cotransporter

During hypovolemia and hyperkalemia, the kidneys defend homeostasis by Na(+) retention and K(+) secretion, respectively. Aldosterone mediates both effects, but it is unclear how the same hormone can evoke such different responses. To address this, we mimicked hypovolemia and hyperkalemia in four groups of rats with a control diet, low-Na(+) diet, high-K(+) diet, or combined diet. The low-Na(+) and combined diets increased plasma and kidney ANG II. The low-Na(+) and high-K(+) diets increased plasma aldosterone to a similar degree (3-fold), whereas the combined diet increased aldosterone to a greater extent (10-fold). Despite similar Na(+) intake and higher aldosterone, the high-K(+) and combined diets caused a greater natriuresis than the control and low-Na(+) diets, respectively (P < 0.001 for both). This K(+)-induced natriuresis was accompanied by a decreased abundance but not phosphorylation of the Na(+)-Cl(-) cotransporter (NCC). In contrast, the epithelial Na(+) channel (ENaC) increased in parallel with aldosterone, showing the highest expression with the combined diet. The high-K(+) and combined diets also increased WNK4 but decreased Nedd4-2 in the kidney. Total and phosphorylated Ste-20-related kinase were also increased but were retained in the cytoplasm of distal convoluted tubule cells. In summary, high dietary K(+) overrides the effects of ANG II and aldosterone on NCC to deliver sufficient Na(+) to ENaC for K(+) secretion. K(+) may inhibit NCC through WNK4 and help activate ENaC through Nedd4-2.

NADPH oxidases, reactive oxygen species, and the kidney: friend and foe

Reactive oxygen species (ROS) play an important role in normal cellular physiology. They regulate different biologic processes such as cell defense, hormone synthesis and signaling, activation of G protein-coupled receptors, and ion channels and kinases/phosphatases. ROS are also important regulators of transcription factors and gene expression. On the other hand, in pathologic conditions, a surplus of ROS in tissue results in oxidative stress with various injurious consequences such as inflammation and fibrosis. NADPH oxidases are one of the many sources of ROS in biologic systems, and there are seven isoforms (Nox1-5, Duox1, Duox2). Nox4 is the predominant form in the kidney, although Nox2 is also expressed. Nox4 has been implicated in the basal production of ROS in the kidney and in pathologic conditions such as diabetic nephropathy and CKD; upregulation of Nox4 may be important in renal oxidative stress and kidney injury. Although there is growing evidence indicating the involvement of NADPH oxidase in renal pathology, there is a paucity of information on the role of NADPH oxidase in the regulation of normal renal function. Here we provide an update on the role of NADPH oxidases and ROS in renal physiology and pathology.

Src family protein tyrosine kinase regulates the basolateral K channel in the distal convoluted tubule (DCT) by phosphorylation of KCNJ10 protein

The loss of function of the basolateral K channels in the distal nephron causes electrolyte imbalance. The aim of this study is to examine the role of Src family protein tyrosine kinase (SFK) in regulating K channels in the basolateral membrane of the mouse initial distal convoluted tubule (DCT1). Single-channel recordings confirmed that the 40-picosiemen (pS) K channel was the only type of K channel in the basolateral membrane of DCT1. The suppression of SFK reversibly inhibited the basolateral 40-pS K channel activity in cell-attached patches and decreased the Ba(2+)-sensitive whole-cell K currents in DCT1. Inhibition of SFK also shifted the K reversal potential from -65 to -43 mV, suggesting a role of SFK in determining the membrane potential in DCT1. Western blot analysis showed that KCNJ10 (Kir4.1), a key component of the basolateral 40-pS K channel in DCT1, was a tyrosine-phosphorylated protein. LC/MS analysis further confirmed that SFK phosphorylated KCNJ10 at Tyr(8) and Tyr(9). The single-channel recording detected the activity of a 19-pS K channel in KCNJ10-transfected HEK293T cells and a 40-pS K channel in the cells transfected with KCNJ10+KCNJ16 (Kir.5.1) that form a heterotetramer in the basolateral membrane of the DCT. Mutation of Tyr(9) did not alter the channel conductance of the homotetramer and heterotetramer. However, it decreased the whole-cell K currents, the probability of finding K channels, and surface expression of KCNJ10 in comparison to WT KCNJ10. We conclude that SFK stimulates the basolateral K channel activity in DCT1, at least partially, by phosphorylating Tyr(9) on KCNJ10. We speculate that the modulation of tyrosine phosphorylation of KCNJ10 should play a role in regulating membrane transport function in DCT1.

Direct activation of ENaC by angiotensin II: recent advances and new insights

Angiotensin II (Ang II) is the principal effector of the renin-angiotensin-aldosterone system (RAAS). It initiates myriad processes in multiple organs integrated to increase circulating volume and elevate systemic blood pressure. In the kidney, Ang II stimulates renal tubular water and salt reabsorption causing antinatriuresis and antidiuresis. Activation of the RAAS is known to enhance activity of the epithelial Na(+) channel (ENaC) in the aldosterone-sensitive distal nephron. In addition to its well described stimulatory actions on aldosterone secretion, Ang II is also capable of directly increasing ENaC activity. In this brief review, we discuss recent findings about non-classical Ang II actions on ENaC and speculate about its relevance for renal sodium handling.

Effects of angiotensin II on kinase-mediated sodium and potassium transport in the distal nephron

PURPOSE OF REVIEW

The aim is to review the recently reported effects of angiotensin II (Ang II) on sodium and potassium transport in the aldosterone-sensitive distal nephron, including the signaling pathways between receptor and transporter, and the (patho)physiological implications of these findings.

RECENT FINDINGS

Ang II can activate the sodium chloride cotransporter (NCC) through phosphorylation by Ste20-related, proline-alanine rich kinase (SPAK), an effect that is independent of aldosterone but dependent on with no lysine kinase 4 (WNK4). A low-sodium diet (high Ang II) activates NCC, whereas a high-potassium diet (low Ang II) inhibits NCC. NCC activation also contributes to Ang-II-mediated hypertension. Ang II also activates the epithelial sodium channel (ENaC) additively to aldosterone, and this effect appears to be mediated through protein kinase C and superoxide generation by nicotinamide adenine dinucleotide phosphate oxidase. While aldosterone activates the renal outer medullary potassium channel (ROMK), this channel is inhibited by Ang II. The key kinase responsible for this effect is c-Src, which phosphorylates ROMK and leaves WNK4 unphosphorylated to further inhibit ROMK.

SUMMARY

The effects of Ang II on NCC, ENaC, and ROMK help explain the renal response to hypovolemia which is to conserve both sodium and potassium. Pathophysiologically, Ang-II-induced activation of NCC appears to contribute to salt-sensitive hypertension.

Inhibition of ROMK channels by low extracellular K+ and oxidative stress

We tested the hypothesis that low luminal K⁺ inhibits the activity of ROMK channels in the rat cortical collecting duct. Whole-cell voltage-clamp measurements of the component of outward K⁺ current inhibited by the bee toxin Tertiapin-Q (ISK) showed that reducing the bath concentration ([K⁺]o) to 1 mM resulted in a decline of current over 2 min compared with that observed at 10 mM [K⁺]o. However, maintaining tubules in 1 mM [K⁺]o without establishing whole-cell clamp conditions did not affect ISK. The [K⁺]o-dependent decline was not prevented by increasing cytoplasmic-side pH or by inhibition of phosphatase activity. It was, however, abolished by the inclusion of 0.5 mM DTT in the pipette solution to prevent oxidation of the intracellular environment. Conversely, treatment of intact tubules with the oxidant H₂O₂ (100 μM) decreased ISK in a [K⁺]o-dependent manner. Treatment of the tubules with the phospholipase C inhibitor U73122 prevented the effect of low [K⁺]o, suggesting the involvement of this enzyme in the process. We examined these effects further using Xenopus oocytes expressing ROMK2 channels. A 50-min exposure to the permeant oxidizing agent tert-butyl hydroperoxide (t-BHP; 500 μM) did not affect outward K⁺ currents with [K⁺]o = 10 mM but reduced currents by 50% with [K⁺]o = 1 mM and by 75% with [K⁺]o = 0.1 mM. Pretreatment of the oocytes with U73122 prevented the effects of t-BHP. Under conditions of low dietary K intake, K⁺ secretion by distal nephron segments may be suppressed by a combination of low luminal [K⁺]o and oxidative stress.

Renal proteinase-activated receptor 2, a new actor in the control of blood pressure and plasma potassium level

Proteinase-activated receptor 2 (PAR2) is a G protein-coupled membrane receptor that is activated upon cleavage of its extracellular N-terminal domain by trypsin and related proteases. PAR2 is expressed in kidney collecting ducts, a main site of control of Na(+) and K(+) homeostasis, but its function remains unknown. We evaluated whether and how PAR2 might control electrolyte transport in collecting ducts, and thereby participate in the regulation of blood pressure and plasma K(+) concentration. PAR2 is expressed at the basolateral border of principal and intercalated cells of the collecting duct where it inhibits K(+) secretion and stimulates Na(+) reabsorption, respectively. Invalidation of PAR2 gene impairs the ability of the kidney to control Na(+) and K(+) balance and promotes hypotension and hypokalemia in response to Na(+) and K(+) depletion, respectively. This study not only reveals a new role of proteases in the control of blood pressure and plasma potassium level, but it also identifies a second membrane receptor, after angiotensin 2 receptor, that differentially controls sodium reabsorption and potassium secretion in the late distal tubule. Conversely to angiotensin 2 receptor, PAR2 is involved in the regulation of sodium and potassium balance in the context of either stimulation or nonstimulation of the renin/angiotensin/aldosterone system. Therefore PAR2 appears not only as a new actor of the aldosterone paradox, but also as an aldosterone-independent modulator of blood pressure and plasma potassium.

Key developments in renin-angiotensin-aldosterone system inhibition

The renin-angiotensin-aldosterone system (RAAS) was initially thought to be fairly simple. However, this idea has been challenged following the development of RAAS blockers, including renin inhibitors, angiotensin-converting-enzyme (ACE) inhibitors, type 1 angiotensin II (AT(1))-receptor blockers and mineralocorticoid-receptor antagonists. Consequently, new RAAS components and pathways that might contribute to the effectiveness of these drugs and/or their adverse effects have been identified. For example, an increase in renin levels during RAAS blockade might result in harmful effects via stimulation of the prorenin receptor (PRR), and prorenin-the inactive precursor of renin-might gain enzymatic activity on PRR binding. The increase in angiotensin II levels that occurs during AT(1)-receptor blockade might result in beneficial effects via stimulation of type 2 angiotensin II receptors. Moreover, angiotensin 1-7 levels increase during ACE inhibition and AT(1)-receptor blockade, resulting in Mas receptor activation and the induction of cardioprotective and renoprotective effects, including stimulation of tissue repair by stem cells. Finally, a role of angiotensin II in sodium and potassium handling in the distal nephron has been identified. This finding is likely to have important implications for understanding the effects of RAAS inhibition on whole body sodium and potassium balance.

Protein phosphatase 1 modulates the inhibitory effect of With-no-Lysine kinase 4 on ROMK channels

With-no-Lysine kinase 4 (WNK4) inhibited ROMK (Kir1.1) channels and the inhibitory effect of WNK4 was abolished by serum-glucocorticoid-induced kinase 1 (SGK1) but restored by c-Src. The aim of the present study is to explore the mechanism by which Src-family tyrosine kinase (SFK) modulates the effect of SGK1 on WNK4 and to test the role of SFK-WNK4-SGK1 interaction in regulating ROMK channels in the kidney. Immunoprecipitation demonstrated that protein phosphatase 1 (PP1) binds to WNK4 at amino acid (aa) residues 695-699 (PP1(#1)) and at aa 1211-1215 (PP1(#2)). WNK4(-PP1#1) and WNK4(-PP1#2), in which the PP1(#1) or PP1(#2) binding site was deleted or mutated, inhibited ROMK channels as potently as WNK4. However, c-Src restored the inhibitory effect of WNK4 but not WNK4(-PP1#1) on ROMK channels in the presence of SGK1. Moreover, expression of c-Src inhibited SGK1-induced phosphorylation of WNK4 but not WNK4(-PP1#1) at serine residue 1196 (Ser(1196)). In contrast, coexpression of c-Src restored the inhibitory effect of WNK4(-PP1#2) on ROMK in the presence of SGK1 and diminished SGK1-induced WNK4 phosphorylation at Ser(1196) in cells transfected with WNK4(-PP1#2). This suggests the possibility that c-Src regulates the interaction between WNK4 and SGK1 through activating PP1 binding to aa 695-9 thereby decreasing WNK4 phosphorylation and restoring the inhibitory effect of WNK4. This mechanism plays a role in suppressing ROMK channel activity during the volume depletion because inhibition of SFK or serine/threonine phosphatases increases ROMK channel activity in the cortical collecting duct of rats on a low-Na diet. We conclude that regulation of phosphatase activity by SFK plays a role in determining the effect of aldosterone on ROMK channels and on renal K secretion.

Regulation of transport in the connecting tubule and cortical collecting duct

The central goal of this overview article is to summarize recent findings in renal epithelial transport,focusing chiefly on the connecting tubule (CNT) and the cortical collecting duct (CCD).Mammalian CCD and CNT are involved in fine-tuning of electrolyte and fluid balance through reabsorption and secretion. Specific transporters and channels mediate vectorial movements of water and solutes in these segments. Although only a small percent of the glomerular filtrate reaches the CNT and CCD, these segments are critical for water and electrolyte homeostasis since several hormones, for example, aldosterone and arginine vasopressin, exert their main effects in these nephron sites. Importantly, hormones regulate the function of the entire nephron and kidney by affecting channels and transporters in the CNT and CCD. Knowledge about the physiological and pathophysiological regulation of transport in the CNT and CCD and particular roles of specific channels/transporters has increased tremendously over the last two decades.Recent studies shed new light on several key questions concerning the regulation of renal transport.Precise distribution patterns of transport proteins in the CCD and CNT will be reviewed, and their physiological roles and mechanisms mediating ion transport in these segments will also be covered. Special emphasis will be given to pathophysiological conditions appearing as a result of abnormalities in renal transport in the CNT and CCD.

Independent regulation of Na+ and K+ balance by the kidney

The understanding of the independent regulation of sodium and potassium by the kidney has remained elusive. Recent evidence now points to dissimilar regulatory mechanisms in ion handling, dependent on the presence of either aldosterone alone or angiotensin II with aldosterone among other factors. This review summarizes past and present information in an attempt to reconcile the current concepts of differential regulation of sodium and potassium balance through the with-no-lysine (K) kinase (WNK) system and the previous knowledge regarding ion transport mechanisms in the distal nephron.

Angiotensin II increases activity of the epithelial Na+ channel (ENaC) in distal nephron additively to aldosterone

Dietary salt intake controls epithelial Na+ channel (ENaC)-mediated Na+ reabsorption in the distal nephron by affecting status of the renin-angiotensin-aldosterone system (RAAS). Whereas regulation of ENaC by aldosterone is generally accepted, little is known about whether other components of RAAS, such as angiotensin II (Ang II), have nonredundant to aldosterone-stimulatory actions on ENaC. We combined patch clamp electrophysiology and immunohistochemistry in freshly isolated split-opened distal nephrons of mice to determine the mechanism and molecular signaling pathway of Ang II regulation of ENaC. We found that Ang II acutely increases ENaC Po, whereas prolonged exposure to Ang II also induces translocation of α-ENaC toward the apical membrane in situ. Ang II actions on ENaC Po persist in the presence of saturated mineralocorticoid status. Moreover, aldosterone fails to stimulate ENaC acutely, suggesting that Ang II and aldosterone have different time frames of ENaC activation. AT1 but not AT2 receptors mediate Ang II actions on ENaC. Unlike its effect in vasculature, Ang II did not increase [Ca2+]i in split-opened distal nephrons as demonstrated using ratiometric Fura-2-based microscopy. However, application of Ang II to mpkCCDc14 cells resulted in generation of reactive oxygen species, as probed with fluorescent methods. Consistently, inhibiting NADPH oxidase with apocynin abolished Ang II-mediated increases in ENaC Po in murine distal nephron. Therefore, we concluded that Ang II directly regulates ENaC activity in the distal nephron, and this effect complements regulation of ENaC by aldosterone. We propose that stimulation of AT1 receptors with subsequent activation of NADPH oxidase signaling pathway mediates Ang II actions on ENaC.