Developmental regulation of expression of renal potassium secretory channels

The effect of high-dietary K+ (HK) on Kir4.1/Kir5.1 and ROMK in the distal convoluted tubule (DCT) is not affected by gender and Cl- content of the diet

Basolateral potassium channels in the distal convoluted tubule (DCT) are composed of inwardly-rectifying potassium channel 4.1 (Kir4.1) and Kir5.1. Kir4.1 interacts with Kir5.1 to form a 40 pS K⁺ channel which is the only type K⁺ channel expressed in the basolateral membrane of the DCT. Moreover, Kir4.1/Kir5.1 heterotetramer plays a key role in determining the expression and activity of thiazide-sensitive Na-Cl cotransport (NCC). In addition to Kir4.1/Kir5.1, Kir1.1 (ROMK) is expressed in the apical membrane of the late DCT (DCT2) and plays a key role in mediating epithelial Na⁺ channel (ENaC)-dependent K⁺ excretion. High dietary-K⁺-intake (HK) stimulates ROMK and inhibits Kir4.1/Kir5.1 in the DCT. Inhibition of Kir4.1/Kir5.1 is essential for HK-induced suppression of NCC whereas the stimulation of ROMK is important for increasing ENaC-dependent K⁺ excretion during HK. We have now used the patch-clamp-technique to examine whether gender and Cl^- content of K⁺-diet affect HK-induced inhibition of basolateral Kir4.1/Kir5.1 and HK-induced stimulation of ROMK. Single-channel-recording shows that basolateral 40 pS K⁺ channel (Kir4.1/Kir5.1) activity of the DCT defined by NP_o was 1.34 (1% KCl, normal K, NK), 0.95 (5% KCl) and 1.03 (5% K⁺-citrate) in male mice while it was 1.47, 1.02 and 1.05 in female mice. The whole-cell recording shows that Kir4.1/Kir5.1-mediated-K⁺ current of the early-DCT (DCT1) was 1,170 pA (NK), 725 pA (5% KCl) and 700 pA (5% K⁺-citrate) in male mice whereas it was 1,125 pA, 674 pA and 700 pA in female mice. Moreover, K⁺-currents (I_K) reversal potential of DCT (an index of membrane potential) was -63 mV (NK), -49 mV (5% KCl) and -49 mV (5% K-citrate) in the male mice whereas it was -63 mV, -50 mV and -50 mV in female mice. Finally, TPNQ-sensitive whole-cell ROMK-currents in the DCT2 /initial-connecting tubule (CNT) were 910 pA (NK), 1,520 pA (5% KCl) and 1,540 pA (5% K⁺-citrate) in male mice whereas the ROMK-mediated K⁺ currents were 1,005 pA, 1,590 pA and 1,570 pA in female mice. We conclude that the effect of HK intake on Kir4.1/Kir5.1 of the DCT and ROMK of DCT2/CNT is similar between male and female mice. Also, Cl^- content in HK diets has no effect on HK-induced inhibition of Kir4.1/Kir5.1 of the DCT and HK-induced stimulation of ROMK in DCT2/CNT.

Global knockout of ROMK potassium channel worsens cardiac ischemia-reperfusion injury but cardiomyocyte-specific knockout does not: Implications for the identity of mitoKATP

The renal-outer-medullary‑potassium (ROMK) channel, mutated in Bartter's syndrome, regulates ion exchange in kidney, but its extra-renal functions remain unknown. Additionally, ROMK was postulated to be the pore-forming subunit of the mitochondrial ATP-sensitive K+ channel (mitoKATP), a mediator of cardioprotection. Using global and cardiomyocyte-specific knockout mice (ROMK-GKO and ROMK-CKO respectively), we characterize the effects of ROMK knockout on mitochondrial ion handling, the response to pharmacological KATP channel modulators, and ischemia/reperfusion (I/R) injury. Mitochondria from ROMK-GKO hearts exhibited a lower threshold for Ca2+-triggered permeability transition pore (mPTP) opening but normal matrix volume changes during oxidative phosphorylation. Isolated perfused ROMK-GKO hearts exhibited impaired functional recovery and increased infarct size when I/R was preceded by an ischemic preconditioning (IPC) protocol. Because ROMK-GKO mice exhibited severe renal defects and cardiac remodeling, we further characterized ROMK-CKO hearts to avoid confounding systemic effects. Mitochondria from ROMK-CKO hearts had unchanged matrix volume responses during oxidative phosphorylation and still swelled upon addition of a mitoKATP opener, but exhibited a lower threshold for mPTP opening, similar to GKO mitochondria. Nevertheless, I/R induced damage was not exacerbated in ROMK-CKO hearts, either ex vivo or in vivo. Lastly, we examined the response of ROMK-CKO hearts to ex vivo I/R injury with or without IPC and found that IPC still protected these hearts, suggesting that cardiomyocyte ROMK does not participate significantly in the cardioprotective pathway elicited by IPC. Collectively, our findings from these novel strains of mice suggest that cardiomyocyte ROMK is not a central mediator of mitoKATP function, although it can affect mPTP activation threshold.

Potassium regulation in the neonate

Potassium, the major cation in intracelluar fluids, is essential for vital biological functions. Neonates maintain a net positive potassium balance, which is fundamental to ensure somatic growth but places these infants, especially those born prematurely, at risk for life-threatening disturbances in potassium concentration [K⁺] in the extracellular fluid compartment. Potassium conservation is achieved by maximizing gastrointestinal absorption and minimizing renal losses. A markedly low glomerular filtration rate, plus adaptations in tubular transport along the nephron, result in low potassium excretion in the urine of neonates. Careful evaluation of clinical data using reference values that are normal for the neonate's postmenstrual age is critical to avoid over-treating infants with laboratory results that represent physiologic values for their developmental stage. The treatment should be aimed at correcting the primary cause when possible. Alterations in the levels or sensitivity to aldosterone are common in neonates. In symptomatic patients, the disturbances in [K⁺] should be corrected promptly, with close electrocardiographic monitoring. Plasma [K⁺] should be monitored during the first 72 h of life in all premature infants born before 30 weeks of postmenstrual age as these infants are prone to develop non-oliguric hyperkalemia with potential serious complications.

Glomerular and tubular effects of nitric oxide (NO) are regulated by angiotensin II (Ang II) in an age-dependent manner through activation of both angiotensin receptors (AT1Rs and AT2Rs) in conscious lambs

Renin-angiotensin (RAS) and nitric oxide (NO) systems and the balance and interaction between them are considered of primary importance in maintaining fluid and electrolyte homeostasis. It has been suggested that the effects of NO may be modulated at least in part by the angiotensin (Ang) II, yet the roles of angiotensin receptor type 1 (AT1R) and type 2 (AT2R) are not well understood. Even though both Ang II and NO are elevated at birth and during the newborn period, their contribution to the adaptation of the newborn to life after birth as well as their physiological roles during development are poorly understood. The aim of this study was to determine if NO regulation of renal function during postnatal maturation is modulated by Ang II through activation of AT1R or AT2R or both receptors. Glomerular and tubular effects of either AT1R selective antagonist ZD 7155, AT2R selective antagonist PD 123319, and both antagonists ZD 7155 plus PD 123319, were measured in 1- (N = 9) and 6-week-old (N = 13) conscious, chronically instrumented lambs before and after removal of endogenous NO with L-arginine analogue, L-NAME. Two-way analysis of variance (ANOVA) procedures for repeated measures over time with factors age and treatment were used to compare the effects of the treatments on several glomerular and tubular variables in both groups. This study showed that L-NAME infusion after pre-treatment with ATR antagonists did not alter glomerular function in 1- or 6-week-old lambs. NO effects on electrolytes handling along the nephron during postnatal development were modulated by Ang II through AT1R and AT2R in an age-dependent manner. Selective inhibition of AT1R and AT2R increased excretion of Na⁺, K⁺, and Cl^- in 6- but not in 1-week-old lambs. In 6-week-old lambs, urinary flow rate increased by 200%, free water clearance increased by 50%, and urine osmolality decreased by 40% after L-NAME was added to the pre-treatment with ZD 7155 plus PD 123319. When L-NAME was added either to ZD 7155 or PD 123319, the same trend in the alterations of these variables was observed, albeit to a lower degree. In conclusion, in conscious animals, during postnatal maturation, Ang II modulates the effects of NO on glomerular function, fluid, and electrolyte homeostasis through AT1Rs and AT2Rs in an age-dependent manner. Under physiological conditions, AT2Rs may potentiate the effects of AT1R, providing evidence of a crosstalk between ATRs in modulating NO effects on fluid and electrolyte homeostasis during postnatal maturation. This study provides new insights on the regulation of renal function during early postnatal development showing that, compared with later in life, newborns have impaired capacity to regulate glomerular function, water, and electrolyte balance.

Potassium: friend or foe?

The kidney plays an essential role in maintaining homeostasis of ion concentrations in the blood. Because the concentration gradient of potassium across the cell membrane is a key determinant of the membrane potential of cells, even small deviations in serum potassium level from the normal setpoint can lead to severe muscle dysfunction, resulting in respiratory failure and cardiac arrest. Less severe hypo- and hyperkalemia are also associated with morbidity and mortality across various patient populations. In addition, deficiencies in potassium intake have been associated with hypertension and adverse cardiovascular and renal outcomes, likely due in part to the interrelated handling of sodium and potassium by the kidney. Here, data on the beneficial effects of potassium on blood pressure and cardiovascular and renal outcomes will be reviewed, along with the physiological basis for these effects. In some patient populations, however, potassium excess is deleterious. Risk factors for the development of hyperkalemia will be reviewed, as well as the risks and benefits of existing and emerging therapies for hyperkalemia.

Potassium Homeostasis: The Knowns, the Unknowns, and the Health Benefits

Potassium homeostasis has a very high priority because of its importance for membrane potential. Although extracellular K+ is only 2% of total body K+, our physiology was evolutionarily tuned for a high-K+, low-Na+ diet. We review how multiple systems interface to accomplish fine K+ balance and the consequences for health and disease.

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

Mice transgenic for genomic segments harboring PHAII (pseudohypoaldosteronism type II) mutant Wnk4 (with-No-Lysine kinase 4) (TgWnk4^PHAII) have hyperkalemia which is currently believed to be the result of high activity of Na-Cl cotransporter (NCC). This leads to decreasing Na⁺ delivery to the distal nephron segment including late distal convoluted tubule (DCT) and connecting tubule (CNT). Since epithelial Na⁺ channel (ENaC) and renal outer medullary K⁺ channel (ROMK or Kir4.1) are expressed in the late DCT and play an important role in mediating K⁺ secretion, the aim of the present study is to test whether ROMK and ENaC activity in the DCT/CNT are also compromised in the mice expressing PHAII mutant Wnk4. Western blot analysis shows that the expression of βENaC and γENaC subunits but not αENaC subunit was lower in TgWnk4^PHAII mice than that in wild-type (WT) and TgWnk4^WT mice. Patch-clamp experiments detected amiloride-sensitive Na⁺ currents and TPNQ-sensitive K⁺ currents in DCT2/CNT, suggesting the activity of ENaC and ROMK. However, both Na⁺ and ROMK currents in DCT2/CNT of TgWnk4^PHAII mice were significantly smaller than those in WT and TgWnk4^WT mice. In contrast, the basolateral K⁺ currents in the DCT were similar among three groups, despite higher NCC expression in TgWnk4^PHAII mice than those of WT and TgWnk4^WTmice. An increase in dietary K⁺ intake significantly increased both ENaC and ROMK currents in the DCT2/CNT of all three groups. However, high-K⁺ (HK) intake-induced stimulation of Na⁺ and K⁺ currents was smaller in TgWnk4^PHAII mice than those in WT and TgWnk4^WT mice. We conclude that ENaC and ROMK channel activity in DCT2/CNT are inhibited in TgWnk4^PHAII mice and that Wnk4^PHAII-induced inhibition of ENaC and ROMK may contribute to the suppression of K⁺ secretion in the DCT2/CNT in addition to increased NCC activity.

Basolateral Kir4.1 activity in the distal convoluted tubule regulates K secretion by determining NaCl cotransporter activity

PURPOSE OF REVIEW

Renal potassium (K) secretion plays a key role in maintaining K homeostasis. The classic mechanism of renal K secretion is focused on the connecting tubule and cortical collecting duct, in which K is uptaken by basolateral Na-K-ATPase and is secreted into the lumen by apical ROMK (Kir1.1) and Ca-activated big conductance K channel. Recently, genetic studies and animal models have indicated that inwardly rectifying K channel 4.1 (Kir4.1 or Kcnj10) in the distal convoluted tubule (DCT) may play a role in the regulation of K secretion in the aldosterone-sensitive distal nephron by targeting the NaCl cotransporter (NCC). This review summarizes recent progresses regarding the role of Kir4.1 in the regulation of NCC and K secretion.

RECENT FINDINGS

Kir4.1 is expressed in the basolateral membrane of the DCT, and plays a predominant role in contributing to the basolateral K conductance and in participating in the generation of negative membrane potential. Kir4.1 is also the substrate of src-family tyrosine kinase and the stimulation of src-family tyrosine kinase activates Kir4.1 activity in the DCT. The genetic deletion or functional inhibition of Kir4.1 depolarizes the membrane of the DCT, inhibits ste20-proline-alanine rich kinase, and suppresses NCC activity. Moreover, the downregulation of Kir4.1 increases epithelial Na channel expression in the collecting duct and urinary K excretion. Finally, mice with low Kir4.1 activity in the DCT are hypomagnesemia and hypokalemia.

SUMMARY

Recent progress in exploring the regulation and the function of Kir4.1 in the DCT strongly indicates that Kir4.1plays an important role in initiating the regulation of renal K secretion by targeting NCC and it may serves as a K sensor in the kidney.

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

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

WNK4 inhibits Ca(2+)-activated big-conductance potassium channels (BK) via mitogen-activated protein kinase-dependent pathway

We used the perforated whole-cell recording technique to examine the effect of with-no-lysine kinase 4 (WNK4) on the Ca(2+) activated big-conductance K channels (BK) in HEK293T cells transfected with BK-α subunit (BK-α). Expression of WNK4 inhibited BK channels and decreased the outward K currents. Coexpression of SGK1 abolished the inhibitory effect of WNK4 on BK channels and restored the outward K currents. Expression of WNK4(S1169D//1196D), in which both SGK1-phosphorylation sites (serine 1169 and 1196) were mutated to aspartate, had no effect on BK channels. Moreover, coexpression of SGK1 had no additional effect on K currents in the cells transfected with BKα+WNK4(S1169D//1196D), suggesting that SGK1 reversed WNK4-induced inhibition of BK channels by stimulating WNK4 phosphorylation. Expression of WNK4 but not WNK4(S1169D//1196D) increased the phosphorylation of ERK and p38 mitogen-activated protein kinase (MAPK); an effect was abolished by coexpression of SGK1. The role of ERK and p38 MAPK in mediating the effect of WNK4 on BK channels was further suggested by the finding that the inhibition of ERK and P38 MAPK completely abolished the inhibitory effect of WNK4 on BK channels. In contrast, inhibition of MAPK failed to abolish the inhibitory effect of WNK4 on ROMK channels in both HEK cells and Xenopus oocytes. Expression of dominant negative dynaminK44A (Dyn(K44A)) or treatment of the cells with dynasore, a dynamin inhibitor, not only increased K currents but also largely abolished the inhibitory effect of WNK4 on BK channels. However, inhibition of MAPK still increased the outward K currents in the cells transfected with BKα+WNK4 and treated with dynasore. Similar results were obtained in experiments performed in the native tissue in which inhibition of ERK and p38 MAPK increased BK channel activity in the cortical collecting duct (CCD) treated with dynasore. We concluded that WNK4 inhibited BK channels by stimulating ERK and p38 MAPK and that activation of MAPK by WNK4 may inhibit BK channels partially via a mechanism other than stimulating endocytosis.

Role of cytochrome P450 epoxygenase in regulating renal membrane transport and hypertension

PURPOSE OF REVIEW

Cytochrome P450 (CYP)-epoxygenase is highly expressed in the kidney and its metabolism of arachidonic acid plays important roles in regulating renal Na transport and in modulating vasoactivity in the kidney. In the past several years, progress has been made not only in characterizing the specific CYP-epoxygenases responsible for the regulation of membrane transport and vasoactivity in the kidney but also in exploring the mechanism by which they regulate renal Na transport and vasodilation of preglomerular arterioles. This review summarizes and updates recent progress in this area of research.

RECENT FINDINGS

CYP-epoxygenase metabolites of arachidonic acid inhibit epithelial Na channel (ENaC) in the cortical collecting duct (CCD), and 11,12-epoxyeicosatrienoic acid (11,12-EET) is mainly responsible for mediating the inhibitory effect on ENaC. Downregulation of CYP2C44 abolishes arachidonic acid mediated inhibition of ENaC and increases ENaC activity. In addition, 11,12-EET stimulates Ca-activated big conductance K channels in the CCD and afferent arterioles smooth muscles. Activation of big conductance K channels by 11,12-EET is responsible for EET-induced vasodilation in preglomerular arterioles. 11,12-EET-induced vasodilation is absent in preglomerular arterioles pretreated with okadaic acid.

SUMMARY

CYP-epoxygenase mediated suppression of renal Na transport is partially achieved by inhibition of ENaC activity in the CCD and CYP2C44-derived EETs are responsible for inhibition of ENaC. Stimulation of serine/threonine protein phosphatase 2A (PP2A) contributes to 11,12-EET-induced activation of big conductance K channels and vasodilation in preglomerular arterioles.

Regulation of large-conductance Ca2+-activated K+ channels by WNK4 kinase

Large-conductance, Ca(2+)-activated K(+) channels, commonly referred to as BK channels, have a major role in flow-induced K(+) secretion in the distal nephron. With-no-lysine kinase 4 (WNK4) is a serine-threonine kinase expressed in the distal nephron that inhibits ROMK activity and renal K(+) secretion. WNK4 mutations have been described in individuals with familial hyperkalemic hypertension (FHHt), a Mendelian disorder characterized by low-renin hypertension and hyperkalemia. As BK channels also have an important role in renal K(+) secretion, we examined whether they are regulated by WNK4 in a manner similar to ROMK. BK channel activity was inhibited in a rabbit intercalated cell line transfected with WNK4 or a WNK4 mutant found in individuals with FHHt. Coexpression of an epitope-tagged BK α-subunit with WNK4 or the WNK4 mutant in HEK293 cells reduced BK α-subunit plasma membrane and whole cell expression. A region within WNK4 encompassing the autoinhibitory domain and a coiled coil domain was required for WNK4 to inhibit BK α-subunit expression. The relative fraction of BK α-subunit that was ubiquitinated was significantly increased in cells expressing WNK4, compared with controls. Our results suggest that WNK4 inhibits BK channel activity, in part, by increasing channel degradation through an ubiquitin-dependent pathway. Based on these results, we propose that WNK4 provides a cellular mechanism for the coordinated regulation of two key secretory K(+) channels in the distal nephron, ROMK and BK.

Novel insights into TRPV4 function in the kidney

Kidneys are complex highly organized paired organs of nearly one million nephrons each. They rigorously process about 180 l of plasma daily to keep whole body homeostasis. To effectively perform such a titanic work, kidneys rely on mechanisms able to sense dynamic changes in composition and flow rates of protourine along the renal tubule. It is envisioned that Ca(2+)-permeable transient receptor potential (TRP) channels, and specifically mechanosensitive TRPV4, can serve to interpret these external mechanical cues in the form of elevated intracellular Ca(2+) concentration. This, in turn, initiates multiple cellular responses and adaptation mechanisms. The current review summarizes up-to-date knowledge about the sites of TRPV4 expression in renal tissue as well as discusses the functional role of the channel in cellular responses to hypotonicity and tubular flow. We will also provide insights as to how TRPV4 fits into classical polycystin mechanosensory complex in cilia and will speculate about previously underappreciated clinical implication of pharmacological TRPV4 targeting in treatment of polycystic kidney disease.

Inner mitochondrial maxi-K⁺ channels in neonatal renal tubular cells: novel therapeutic targets to control apoptosis

In developing kidneys, the total cell population is partly regulated by apoptosis. Despite our understanding of the molecular involvement in the regulatory pathway of apoptosis, we know little about the physiological involvement. Cardiomyocytes express large conductance voltage- and Ca(2+)-activated K(+) (maxi-K(+)) channels in their inner mitochondrial membranes. Triggering the mitochondrial K(+) influx necessary to inhibit apoptosis, the channels play cytoprotective roles during ischemic injury. Since proximal tubular cells in neonatal kidneys are physiologically under hypoxic stress, and since the channel activity is stimulated by hypoxia, those cells would share the same regulatory mechanism of apoptosis with ischemic cardiomyocytes. Therefore, we hypothesize here that the proximal tubular cells in neonatal kidneys would also express the maxi-K(+) channels in their inner mitochondrial membranes, and that the channels would play regulatory roles in apoptosis. Our hypothesis is unique because it sheds light for the first time on a physiological mechanism that involves the mitochondrial membranes in developing kidneys. It is also important because the idea could have novel therapeutic implications for kidney diseases that are associated with apoptosis.

Developmental changes in renal function

PURPOSE OF REVIEW

Although nephrogenesis in a term infant is complete, there are a number of functional changes that occur in the kidney as the infant matures. Understanding these changes will aid in the evaluation of neonates to delineate what is normal development versus a pathophysiologic problem. In addition, as many drugs are either cleared by the kidneys or can affect renal function, dosing regimens are dramatically different in the neonate as compared with the adult. These differences are greatly exaggerated in the preterm infant, making it more difficult to determine if there is a pathophysiologic problem.

RECENT FINDINGS

While investigators in recent years have made great strides in understanding the early embryology of the kidney and the molecular signals involved in the formation of the kidney, there remains a paucity of functional studies. The most recent studies have re-examined the changes in the serum creatinine in the newborn and how this impacts the excretion of drugs. Developmental changes in the renal tubule transport systems and their regulation have also been more extensively studied.

SUMMARY

The kidney undergoes many developmental physiologic changes as the neonate adapts to extra-uterine life. Understanding these changes will help in the medical management of these infants.

Renal outer medullary potassium channel knockout models reveal thick ascending limb function and dysfunction

The renal outer medullary potassium channel (ROMK) is an adenosine triphosphate-sensitive inward-rectifier potassium channel (Kir1.1 or KCNJ1) highly expressed in the cortical and medullary thick ascending limbs (TAL), connecting segment (CNT) and cortical collecting duct (CCD) in the mammalian kidney, where it serves to recycle potassium (K(+)) across the apical membrane in TAL and to secrete K(+) in the CNT and CCD. ROMK channel mutations cause type II Bartter's syndrome with salt wasting and dehydration, and ROMK knockout mice display a similar phenotype of Bartter's syndrome in humans. Studies from ROMK null mice indicate that ROMK is required to form both the small-conductance (30pS, SK) K channels and the 70pS (IK) K channels in the TAL. The availability of ROMK(-/-) mice has made it possible to study electrolyte transport along the nephron in order to understand the TAL function under physiological conditions and the compensatory mechanisms of salt and water transport under the conditions of TAL dysfunction. This review summarizes previous progress in the study of K(+) channel activity in the TAL and CCD, ion transporter expression and activities along the nephron, and renal functions under physiological and pathophysiological conditions using ROMK(-/-) mice.

Angiotensin II diminishes the effect of SGK1 on the WNK4-mediated inhibition of ROMK1 channels

ROMK1 channels are located in the apical membrane of the connecting tubule and cortical collecting duct and mediate the potassium secretion during normal dietary intake. We used a perforated whole-cell patch clamp to explore the effect of angiotensin II on these channels in HEK293 cells transfected with green fluorescent protein (GFP)-ROMK1. Angiotensin II inhibited ROMK1 channels in a dose-dependent manner, an effect abolished by losartan or by inhibition of protein kinase C. Furthermore, angiotensin II stimulated a protein kinase C-sensitive phosphorylation of tyrosine 416 within c-Src. Inhibition of protein tyrosine kinase attenuated the effect of angiotensin II. Western blot studies suggested that angiotensin II inhibited ROMK1 channels by enhancing its tyrosine phosphorylation, a notion supported by angiotensin II's failure to inhibit potassium channels in cells transfected with the ROMK1 tyrosine mutant (R1Y337A). However, angiotensin II restored the with-no-lysine kinase-4 (WNK4)-induced inhibition of R1Y337A in the presence of serum-glucocorticoids-induced kinase 1 (SGK1), which reversed the inhibitory effect of WNK4 on ROMK1. Moreover, protein tyrosine kinase inhibition abolished the angiotensin II-induced restoration of WNK4-mediated inhibition of ROMK1. Angiotensin II inhibited ROMK channels in the cortical collecting duct of rats on a low sodium diet, an effect blocked by protein tyrosine kinase inhibition. Thus, angiotensin II inhibits ROMK channels by two mechanisms: increasing tyrosine phosphorylation of the channel and synergizing the WNK4-induced inhibition. Hence, angiotensin II may have an important role in suppressing potassium secretion during volume depletion.

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

PURPOSE OF REVIEW

Studies on the mechanisms of distal K+ secretion have highlighted the importance of the renal outer-medullary K+ (ROMK) and maxi-K channels. This review considers several human disorders characterized by hypokalemia and hyperkalemia, as well as mouse models of these disorders, and the mechanisms by which ROMK and maxi-K may be dysregulated.

RECENT FINDINGS

Analysis of knockout mice lacking ROMK, a model for type II Bartter's syndrome, has shown a role for maxi-K in distal K+ secretion. Knockout mice lacking either the alpha or beta1 subunits of maxi-K also show deficits in flow-dependent K+ secretion. Analysis of transgenic and knock-in mouse models of pseudohypoaldosteronism type II, in which mutant forms of with-no-lysine kinase 4 are expressed, suggests ways in which ROMK and maxi-K may be dysregulated to result in hyperkalemia. Modeling studies also provide insights into the role of Na+ delivery vs. flow in K+ secretion.

SUMMARY

The importance of both ROMK and maxi-K to distal K+ secretion is now well established, but the relative role that each of these two channels plays in normal and diseased states has not been definitively established. Analysis of human and animal model data can generate hypotheses for future experiments.

Src family protein tyrosine kinase (PTK) modulates the effect of SGK1 and WNK4 on ROMK channels

WNK4 (with no lysine kinase 4) inhibits ROMK channel activity in the distal nephron by stimulating clathrin-dependent endocytosis, an effect attenuated by SGK1 (serum-glucocorticoids-induced kinase)-mediated phosphorylation. It has been suggested that increased ROMK activity because of SGK1-mediated inhibition of WNK4 plays a role in promoting renal K secretion in response to elevated serum K or high K (HK) intake. In contrast, intravascular volume depletion also increases SGK1 activity but fails to stimulate ROMK channels and K secretion. Because HK intake decreases Src family protein tyrosine kinase (PTK) activity an inhibitor of ROMK channels, it is possible that Src family PTK may modulate the effects of SGK1 on WNK4. Here, we show that c-Src prevents SGK1 from attenuating WNK4's inhibition of ROMK activity. This effect of c-Src was WNK4-dependent because c-Src had no effect on ROMK harboring mutation at the site of c-Src phosphorylation (R1Y337A) in the absence of WNK4. Moreover, expression c-Src diminished the SGK1-mediated increase in serine phosphorylation of WNK4, suggesting that c-Src enhances WNK4-mediated inhibition of ROMK channels by suppressing the SGK1-induced phosphorylation. This notion is also supported by the observation that c-Src was not able to modulate the interaction between SGK1 and WNK4 mutants (WNK4(S1169A) or WNK4(S1169D)) in which an SGK1-phosphorylation site (serine 1169) was mutated by alanine or aspartate. We conclude that c-Src inhibits SGK1-mediated phosphorylation hereby restoring the WNK4-mediated inhibition of ROMK channels thus suppressing K secretion.

Regulation of potassium (K) handling in the renal collecting duct

This review provides an overview of the molecular mechanisms of K transport in the mammalian connecting tubule (CNT) and cortical collecting duct (CCD), both nephron segments responsible for the regulation of renal K secretion. Aldosterone and dietary K intake are two of the most important factors regulating K secretion in the CNT and CCD. Recently, angiotensin II (AngII) has also been shown to play a role in the regulation of K secretion. In addition, genetic and molecular biological approaches have further identified new mechanisms by which aldosterone and dietary K intake regulate K transport. Thus, the interaction between serum-glucocorticoid-induced kinase 1 (SGK1) and with-no-lysine kinase 4 (WNK4) plays a significant role in mediating the effect of aldosterone on ROMK (Kir1.1), an important apical K channel modulating K secretion. Recent evidence suggests that WNK1, mitogen-activated protein kinases such as P38, ERK, and Src family protein tyrosine kinase are involved in mediating the effect of low K intake on apical K secretory channels.

Epoxyeicosatrienoic acid activates BK channels in the cortical collecting duct

The cortical collecting duct (CCD), which is involved in renal potassium (K) excretion, expresses cytochrome P450 (CYP)-epoxygenase. Here, we examined the effect of high dietary K on renal expression of CYP2C23 and CYP2J2 in the rat, as well as the role of CYP-epoxygenase-dependent metabolism of arachidonic acid in the regulation of Ca(2+)-activated big-conductance K (BK) channels. By Western blot analysis, high dietary K stimulated the expression of CYP2C23 but not CYP2J2 and increased 11,12-epoxyeicosatrienoic acid (11,12-EET) levels in isolated rat CCD tubules. Application of arachidonic acid increased BK channel activity, and this occurred to a greater extent in rats on a high-K diet compared with a normal-K diet. This effect was unlikely due to arachidonic acid-induced changes in membrane fluidity, because 11,14,17-eicosatrienoic acid did not alter BK channel activity. Inhibiting CYP-epoxygenase but not cyclooxygenase- or CYP-omega-hydroxylase-dependent pathways completely abolished the stimulatory effect of arachidonic acid on BK channel activity. In addition, application of 11,12-EET mimicked the effect of arachidonic acid on BK channel activity, even in the presence of CYP-epoxygenase inhibition. This effect seemed specific to 11,12-EET, because both 8,9- and 14,15-EET failed to stimulate BK channels. Finally, inhibition of CYP-epoxygenase abolished iberiotoxin-sensitive and flow-stimulated but not basal net K secretion in isolated microperfused CCD. In conclusion, high dietary K stimulates the renal CYP-epoxygenase pathway, which plays an important role in activating BK channels and flow-stimulated K secretion in the CCD.

K restriction inhibits protein phosphatase 2B (PP2B) and suppression of PP2B decreases ROMK channel activity in the CCD

We used Western blot analysis to examine the effect of dietary K intake on the expression of serine/threonine protein phosphatase in the kidney. K restriction significantly decreased the expression of catalytic subunit of protein phosphatase (PP)2B but increased the expression of PP2B regulatory subunit in both rat and mouse kidney. However, K depletion did not affect the expression of PP1 and PP2A. Treatment of M-1 cells, mouse cortical collecting duct (CCD) cells, or 293T cells with glucose oxidase (GO), which generates superoxide anions through glucose metabolism, mimicked the effect of K restriction on PP2B expression and significantly decreased expression of PP2B catalytic subunits. However, GO treatment increased expression of regulatory subunit of PP2B and had no effect on expression of PP1, PP2A, and protein tyrosine phosphatase 1D. Moreover, deletion of gp91-containing NADPH oxidase abolished the effect of K depletion on PP2B. Thus superoxide anions or related products may mediate the inhibitory effect of K restriction on the expression of PP2B catalytic subunit. We also used patch-clamp technique to study the effect of inhibiting PP2B on renal outer medullary K (ROMK) channels in the CCD. Application of cyclosporin A or FK506, inhibitors of PP2B, significantly decreased ROMK channels, and the effect of PP2B inhibitors was abolished by blocking p38 mitogen-activated protein kinase (MAPK) and ERK. Furthermore, Western blot demonstrated that inhibition of PP2B with cyclosporin A or small interfering RNA increased the phosphorylation of ERK and p38 MAPK. We conclude that K restriction suppresses the expression of PP2B catalytic subunits and that inhibition of PP2B decreases ROMK channel activity through stimulation of MAPK in the CCD.

Dual role of the TRPV4 channel as a sensor of flow and osmolality in renal epithelial cells

Gain/loss of function studies were utilized to assess the potential role of the endogenous vanilloid receptor TRPV4 as a sensor of flow and osmolality in M-1 collecting duct cells (CCD). TRPV4 mRNA and protein were detectable in M-1 cells and stably transfected HEK-293 cells, where the protein occurred as a glycosylated doublet on Western blots. Immunofluorescence imaging demonstrated expression of TRPV4 at the cell membranes of TRPV4-transfected HEK and M-1 cells and at the luminal membrane of mouse kidney CCD. By using intracellular calcium imaging techniques, calcium influx was monitored in cells grown on coverslips. Application of known activators of TRPV4, including 4α-PDD and hypotonic medium, induced strong calcium influx in M-1 cells and TRPV4-transfected HEK-293 cells but not in nontransfected cells. Applying increased flow/shear stress in a parallel plate chamber induced calcium influx in both M-1 and TRPV4-transfected HEK cells but not in nontransfected HEK cells. Furthermore, in loss-of-function studies employing small interference (si)RNA knockdown techniques, transfection of both M-1 and TRPV4-transfected HEK cells with siRNA specific for TRPV4, but not an inappropriate siRNA, led to a time-dependent decrease in TRPV4 expression that was accompanied by a loss of stimuli-induced calcium influx to flow and hypotonicity. It is concluded that TRPV4 displays a mechanosensitive nature with activation properties consistent with a molecular sensor of both fluid flow (or shear stress) and osmolality, or a component of a sensor complex, in flow-sensitive renal CCD.

A novel mutation in KCNJ1 in a Bartter syndrome case diagnosed as pseudohypoaldosteronism

Bartter syndrome (BS) is a genetic disorder with hypokalemic metabolic alkalosis and is classified into five types. One of these, type II BS (OMIM 241200), is classified as neonatal Bartter syndrome, which is caused by mutations in the KCNJ1 gene. Transient hyperkalemia and hyponatremia are usually noted in the early postnatal period, but as type II BS is a relatively rare disease, its exact clinical course and genetic background have not yet been thoroughly characterized. This report concerns a male type II BS patient with a novel mutation in the KCNJ1 gene. The unique clinical findings of this case are that hyperkalemia (8.9 mEq/l), hyponatremia, and metabolic acidosis detected in the early postnatal period led to a diagnosis of pseudohypoaldosteronism (PHA). As an adolescent, however, the patient currently shows normal potassium levels and normal renal function, although with hypercalciuria and nephrocalcinosis, without having received any treatment. In such cases, KCNJ1 mutations should be suspected. In our case, genetic analysis of the KCNJ1 gene identified a novel homozygous 1-bp deletion mutation (c.607 del. C in exon 5).

Role of gp91phox -containing NADPH oxidase in mediating the effect of K restriction on ROMK channels and renal K excretion

Previous study has demonstrated that superoxide and the related products are involved in mediating the effect of low K intake on renal K secretion and ROMK channel activity in the cortical collecting duct (CCD). This study investigated the role of gp91(phox)-containing NADPH oxidase (NOXII) in mediating the effect of low K intake on renal K excretion and ROMK channel activity in gp91(-/-) mice. K depletion increased superoxide levels, phosphorylation of c-Jun, expression of c-Src, and tyrosine phosphorylation of ROMK in renal cortex and outer medulla in wild-type (WT) mice. In contrast, tempol treatment in WT mice abolished whereas deletion of gp91 significantly attenuated the effect of low K intake on superoxide production, c-Jun phosphorylation, c-Src expression, and tyrosine phosphorylation of ROMK. Patch-clamp experiments demonstrated that low K intake decreased mean product of channel number (N) and open probability (P) (NP(o)) of ROMK channels from 1.1 to 0.4 in the CCD. However, the effect of low K intake on ROMK channel activity was significantly attenuated in the CCD from gp91(-/-) mice and completely abolished by tempol treatment. Immunocytochemical staining also was used to examine the ROMK distribution in WT, gp91(-/-), and WT mice with tempol treatment in response to K restriction. K restriction decreased apical staining of ROMK in WT mice. In contrast, a sharp apical ROMK staining was observed in the tempol-treated WT or gp91(-/-) mice. Metabolic cage study further showed that urinary K loss is significantly higher in gp91(-/-) mice than in WT mice. It is concluded that superoxide anions play a key role in suppressing K secretion during K restriction and that NOXII is involved in mediating the effect of low K intake on renal K secretion and ROMK channel activity.

The role of the BK channel in potassium homeostasis and flow-induced renal potassium excretion

The kidney is the major regulator of potassium homeostasis. In addition to the ROMK channels, large conductance Ca(2+)-activated K(+) (BK) channels are expressed in the apical membrane of the aldosterone sensitive distal nephron where they could contribute to renal K(+) secretion. We studied flow-induced K(+) secretion in BK channel alpha-subunit knockout (BK(-/-)) mice by acute pharmacologic blockade of vasopressin V(2) receptors, which caused similar diuresis in wild-type and knockout mice. However, wild-type mice, unlike the BK(-/-), had a concomitant increase in urinary K(+) excretion and a significant correlation between urinary flow rate and K(+) excretion. Both genotypes excreted similar urinary amounts of K(+) irrespective of K(+) diet. This was associated, however, with higher plasma aldosterone and stronger expression of ROMK in the apical membrane of the aldosterone-sensitive portions of the distal nephron in the knockout than in the wild-type under control diet and even more so with the high-K(+) diet. High-K(+) intake significantly increased the renal expression of the BK channel in the wild-type mouse. Finally, despite the higher plasma K(+) and aldosterone levels, BK(-/-) mice restrict urinary K(+) excretion when placed on a low-K(+) diet to the same extent as the wild-type. These studies suggest a role of the BK channel alpha-subunit in flow-induced K(+) secretion and in K(+) homeostasis. Higher aldosterone and an upregulation of ROMK may compensate for the absence of functional BK channels.

Flow-activated transport events along the nephron

PURPOSE OF REVIEW

It is well recognized that an increase in glomerular filtration rate leads to an increase in fluid, Na+ and HCO3- absorption in proximal tubules; however, underlying mechanisms of this modulation have not been delineated. This review provides an update of flow-activated transport events along the nephron. Transporters, flow-sensors and secondary messengers that may modulate flow are also discussed.

RECENT FINDINGS

We have demonstrated that both NHE3 and H-ATPase activities are modulated by axial flow in mouse proximal tubules in vitro. Experimental data and modeling calculations provide strong evidence that brush-border microvilli function as flow sensors in the proximal tubule. In addition, AngII receptor localization is regulated by flow in cultured proximal tubule cells, and flow induces eNOS translocation in TAL.

SUMMARY

Flow-modulated NHE3 activity is the regulatory mechanism for GTB. It is independent of neuron and systemic hormonal regulation, but requires the intact actin cytoskeleton to transmit the signal of altered axial flow sensed by brush-border microvilli. Unanswered questions include the identification of specific signaling transduction mechanisms and second messengers in response to flow. Whether the Na+/2Cl-/K+-cotransporter in TAL is flow-activated, and whether a divalent cation, Ca2+ and Mg2+ transport, can be regulated by flow is unknown.

Maxi-K channels contribute to urinary potassium excretion in the ROMK-deficient mouse model of Type II Bartter's syndrome and in adaptation to a high-K diet

Type II Bartter's syndrome is a hereditary hypokalemic renal salt-wasting disorder caused by mutations in the ROMK channel (Kir1.1; Kcnj1), mediating potassium recycling in the thick ascending limb of Henle's loop (TAL) and potassium secretion in the distal tubule and cortical collecting duct (CCT). Newborns with Type II Bartter are transiently hyperkalemic, consistent with loss of ROMK channel function in potassium secretion in distal convoluted tubule and CCT. Yet, these infants rapidly develop persistent hypokalemia owing to increased renal potassium excretion mediated by unknown mechanisms. Here, we used free-flow micropuncture and stationary microperfusion of the late distal tubule to explore the mechanism of renal potassium wasting in the Romk-deficient, Type II Bartter's mouse. We show that potassium absorption in the loop of Henle is reduced in Romk-deficient mice and can account for a significant fraction of renal potassium loss. In addition, we show that iberiotoxin (IBTX)-sensitive, flow-stimulated maxi-K channels account for sustained potassium secretion in the late distal tubule, despite loss of ROMK function. IBTX-sensitive potassium secretion is also increased in high-potassium-adapted wild-type mice. Thus, renal potassium wasting in Type II Bartter is due to both reduced reabsorption in the TAL and K secretion by max-K channels in the late distal tubule.

Extracellular zinc and ATP-gated P2X receptor calcium entry channels: New zinc receptors as physiological sensors and therapeutic targets

In this review, we focus on two attributes of P2X receptor channel function, one essential and one novel. First, we propose that P2X receptors are extracellular sensors as well as receptors and ion channels. In particular, the large extracellular domain (that comprises 70% of the molecular mass of the receptor channel protein) lends itself to be a cellular sensor. Moreover, its exquisite sensitivity to extracellular pH, ionic strength, and multiple ligands evokes the function of a sensor. Second, we propose that P2X receptors are extracellular zinc receptors as well as receptors for nucleotides. We provide novel data in multiple publications and illustrative data in this invited review to suggest that zinc triggers ATP-independent activation of P2X receptor channel function. In this light, P2X receptors are the cellular site of integration between autocrine and paracrine zinc signaling and autocrine and paracrine purinergic signaling. P2X receptors may sense changes in these ligands as well as in extracellular pH and ionic strength and transduce these sensations via calcium and/or sodium entry and changes in membrane potential.

Molecular diversity and function of voltage-gated (Kv) potassium channels in epithelial cells

Voltage-gated K+ channels belonging to Kv1-9 subfamilies are widely expressed in excitable cells where they play an essential role in membrane hyperpolarization during an action potential and in the propagation of action potentials along the plasma membrane. Early patch clamp studies on epithelial cells revealed the presence of K+ currents with biophysical and pharmacologic properties characteristic of Kv channels expressed in excitable cells. More recently, molecular approaches including PCR and the availability of more selective antibodies directed against Kv alpha and auxiliary subunits, have demonstrated that epithelial cells from various organ systems, express a remarkable diversity Kv channel subunits. Unlike neurons and myocytes however, epithelial cells do not typically generate action potentials or exhibit dynamic changes in membrane potential necessary for activation of Kv alpha subunits. Moreover, the fact that many Kv channels expressed in epithelial cells exhibit inactivation suggest that their activities are relatively transient, making it difficult to ascribe a functional role for these channels in transepithelial electrolyte or nutrient transport. Other proposed functions have included (i) cell migration and wound healing, (ii) cell proliferation and cancer, (iii) apoptosis and (iv) O2 sensing. Certain Kv channels, particularly Kv1 and Kv2 subfamily members, have been shown to be involved in the proliferation of prostate, colon, lung and breast carcinomas. In some instances, a significant increase in Kv channel expression has been correlated with tumorogenesis suggesting the possibility of using these proteins as markers for transformation and perhaps reducing the rate of tumor growth by selectively inhibiting their functional activity.

BK-β1 subunit: immunolocalization in the mammalian connecting tubule and its role in the kaliuretic response to volume expansion

Large, Ca2+-activated K+channels (BK), comprised of α- and β-subunits, mediate K+secretion during high flow rates in distal nephron segments. Because the BK-β1 subunit enhances Ca2+sensitivity of BK in a variety of cells, we determined its role in flow-induced K+secretion and its localization in the mammalian nephron. To determine the role of BK-β1 in the kaliuretic response to volume expansion, the rate of K+excretion (UKV) vs. varied urinary flow rates were determined in wild-type and BK-β1 knockout mice (BK-β1−/−). When flow rate was varied by volume expansion (2 ml·h−1·25 g body wt−1) for 30 to 60 min in wild-type mice, we found that the UKV increased significantly with increasing urine flow rates ( r2= 0.50, P < 0.00001, n = 31), as demonstrated previously in distal nephron of rats and rabbits. However, in BK-β1−/−mice, UKV did not vary with changing flow rates ( r2= 0.15, P = 0.08, n = 20). Using immunohistochemical techniques, we found that BK-β1 was strongly expressed in the apical membrane of the murine distal nephron and that 98% of BK-β1 protein detected by histochemistry colocalized with NCX, a marker of connecting tubules (CNT). Both BK-β1 and NCX colocalized with BK-α in separate experiments. Furthermore, we confirmed BK-β1 protein expression in the apical membrane of connecting tubules in rabbits. BK-β1 RNA from rabbit CNT was sequenced and was identical to previously published rabbit muscle sequences. These data show that the BK-β1 accessory subunit is present in the CNT segment of the mammalian distal nephron and has a significant role in the kaliuretic response to increased urinary flow induced by volume expansion.