Regulation of the epithelial Na(+) channel by extracellular acidification

Detrimental or beneficial: Role of endothelial ENaC in vascular function

In the past, it was believed that the expression of the epithelial sodium channel (ENaC) was restricted to epithelial tissues, such as the distal nephron, airway, sweat glands, and colon, where it is critical for sodium homeostasis. Over the past two decades, this paradigm has shifted due to the finding that ENaC is also expressed in various nonepithelial tissues, notably in vascular endothelial cells. In this review, the recent findings of the expression, regulation, and function of the endothelial ENaC (EnNaC) are discussed. The expression of EnNaC subunits is reported in a variety of endothelial cell lines and vasculatures, but this is controversial across different species and vessels and is not a universal finding in all vascular beds. The expression density of EnNaC is very faint compared to ENaC in the epithelium. To date, little is known about the regulatory mechanism of EnNaC. Through it can be regulated by aldosterone, the detailed downstream signaling remains elusive. EnNaC responds to increased extracellular sodium with the feedforward activation mechanism, which is quite different from the Na⁺ self-inhibition mechanism of ENaC. Functionally, EnNaC was shown to be a determinant of cellular mechanics and vascular tone as it can sense shear stress, and its activation or insertion into plasma membrane causes endothelial stiffness and reduced nitric oxide production. However, in some blood vessels, EnNaC is essential for maintaining the integrity of endothelial barrier function. In this context, we discuss the possible reasons for the distinct role of EnNaC in vasculatures.

An extracellular acidic cleft confers profound H+-sensitivity to epithelial sodium channels containing the δ-subunit in Xenopus laevis

The limited sodium availability of freshwater and terrestrial environments was a major physiological challenge during vertebrate evolution. The epithelial sodium channel (ENaC) is present in the apical membrane of sodium-absorbing vertebrate epithelia and evolved as part of a machinery for efficient sodium conservation. ENaC belongs to the degenerin/ENaC protein family and is the only member that opens without an external stimulus. We hypothesized that ENaC evolved from a proton-activated sodium channel present in ionocytes of freshwater vertebrates and therefore investigated whether such ancestral traits are present in ENaC isoforms of the aquatic pipid frog Xenopus laevis Using whole-cell and single-channel electrophysiology of Xenopus oocytes expressing ENaC isoforms assembled from αβγ- or δβγ-subunit combinations, we demonstrate that Xenopus δβγ-ENaC is profoundly activated by extracellular acidification within biologically relevant ranges (pH 8.0-6.0). This effect was not observed in Xenopus αβγ-ENaC or human ENaC orthologs. We show that protons interfere with allosteric ENaC inhibition by extracellular sodium ions, thereby increasing the probability of channel opening. Using homology modeling of ENaC structure and site-directed mutagenesis, we identified a cleft region within the extracellular loop of the δ-subunit that contains several acidic amino acid residues that confer proton-sensitivity and enable allosteric inhibition by extracellular sodium ions. We propose that Xenopus δβγ-ENaC can serve as a model for investigating ENaC transformation from a proton-activated toward a constitutively-active ion channel. Such transformation might have occurred during the evolution of tetrapod vertebrates to enable bulk sodium absorption during the water-to-land transition.

Acid and base secretion in freshly excised nasal tissue from cystic fibrosis patients with ΔF508 mutation

BACKGROUND

Cystic fibrosis (CF) is caused by a misfunctional CF transmembrane conductance regulator (CFTR) protein, which is believed to contributes to the regulation of the airway surface liquid (ASL) pH. This study investigated acid and base secretion in freshly excised human nasal tissues from CF patients homozygous for the ΔF508 mutation.

METHODS

Human nasal mucosa was collected during sinus surgery and investigated in Ussing chambers. Mucosal equilibrium pH values and rate of acid and base secretion were determined using the pH-stat technique.

RESULTS

The equilibrium pH of nasal epithelia from ΔF508 CF patients with chronic rhinosinusitis (CRS) was pH = 7.08 ± 0.09 and was significantly lower compared to nasal epithelia from CRS patients without CF (pH = 7.33 ± 0.06) and normal subjects (pH = 7.34 ± 0.08, n = 6). The rate of base secretion in CF nasal tissues was 11.8 ± 2.4 nmol · min(−1) · cm(−2), which was significantly lower than normal (57.2 ± 9.2 nmol · min(−1) · cm(−2)). The HCO3(−) secretory rate was further increased by forskolin by 16.1% in normal, but not in CF tissues.

CONCLUSION

Our data suggests that CF patients exhibited significantly lower base secretion by the nasal airway epithelium. It is possible that improper regulation of ASL pH in CF may negatively impact the innate host defense system.

Amiloride-sensitive and amiloride-insensitive responses to NaCl + acid mixtures in hamster chorda tympani nerve

Component signaling in taste mixtures containing both beneficial and dangerous chemicals depends on peripheral processing. Unidirectional mixture suppression of chorda tympani (CT) nerve responses to sucrose by quinine and acid is documented for golden hamsters (Mesocricetus auratus). To investigate mixtures of NaCl and acids, we recorded multifiber responses to 50 mM NaCl, 1 and 3 mM citric acid and acetic acid, 250 μM citric acid, 20 mM acetic acid, and all binary combinations of each acid with NaCl (with and without 30 μM amiloride added). By blocking epithelial Na(+) channels, amiloride treatment separated amiloride-sensitive NaCl-specific responses from amiloride-insensitive electrolyte-generalist responses, which encompass all of the CT response to the acids as well as responses to NaCl. Like CT sucrose responses, the amiloride-sensitive NaCl responses were suppressed by as much as 50% by citric acid (P = 0.001). The amiloride-insensitive electrolyte-generalist responses to NaCl + acid mixtures approximated the sum of NaCl and acid component responses. Thus, although NaCl-specific responses to NaCl were weakened in NaCl-acid mixtures, electrolyte-generalist responses to acid and NaCl, which tastes KCl-like, were transmitted undiminished in intensity to the central nervous system. The 2 distinct CT pathways are consistent with known rodent behavioral discriminations.

Insights into the molecular determinants of proton inhibition in an acid-inactivated degenerins and mammalian epithelial Na(+) channel

Mammalian ASIC channels of the DEG/ENaC superfamily are gated by extracellular protons and function to mediate touch and pain sensitivity, learning and memory, and fear conditioning. The recently solved crystal structure of chicken ASIC1a and preliminary functional studies suggested that a highly negatively charged pocket in the extracellular domain of the channel might be the primary proton binding domain. However, more recent extensive mutagenesis analysis paints a more complex mechanism of channel gating, involving binding of protons at sites immediately after the first transmembrane domain (TM1) and displacement of inhibitory Ca(2+) ions from the acidic pocket in the extracellular domain and from another Ca(2+) binding site at the mouth of the pore. We recently identified and functionally characterized Caenorhabditis elegans ACD-1, the first acid-inactivated DEG/ENaC channel. ACD-1 is expressed in C. elegans amphid glia and functions with neuronal DEG/ENaC channel DEG-1 to mediate acid avoidance and chemotaxis to the amino acid lysine. The post-TM1 residues that were proposed to bind protons in ASIC1a are not conserved in ACD-1, but some of the amino acids constituting the acidic pocket are. However, ACD-1 proton sensitivity is completely independent from extracellular Ca(2+), and protons appear to bind the channel in a less cooperative manner. We thus wondered if residues in the acidic pocket might contribute to ACD-1 acid sensitivity. We show here that while ACD-1 sensitivity to extracellular protons is influenced by mutations in the acidic pocket, other sites are likely to participate. We also report that one histidine at the base of the thumb and residues in the channel pore influence proton inhibition in a voltage-independent manner, suggesting that they affect the coupling of proton binding with the gating rather than proton binding itself. We conclude that ACD-1 inhibition by protons is likely mediated by binding of proton ions to multiple sites throughout the extracellular domain of the channel. Our data also support a model in which residues in the acidic pocket contribute to determining the channel state perhaps by changing the strength of the interaction between adjacent thumb and finger domains.

Effects of [Ca2+]i and pH on epithelial Na+ channel activity of cultured mouse cortical collecting ducts

[Ca2+]i and pH have been demonstrated to affect Na+ transport in epithelium mediated via the apical epithelial Na+ channel (ENaC). However, it still remains unclear whether the effects of [Ca2+]i and intracellular pH (pHi) on ENaC activity are direct. In this study, inside-out recording was employed to clarify the effects of pH(i) and [Ca2+]i on ENaC activity. We found that elevation of [Ca2+]i induced a significant inhibition of ENaC open probability without altering channel conductance. The inhibitory effect was due to a direct interaction between Ca2+ and ENaC, and is dependent on [Ca2+]i. pHi also directly regulated ENaC open probability. Lower pHi (<7.0) reduced the ENaC open probability as shown in shorter opening time, and higher pH(i) (>7.0) enhanced the ENaC open probability as shown in augmented opening time. pHi did not cause any alteration in channel conductance. The effects of pHi on ENaC open probability could be summarized as an S-shaped curve around pH 7.2.

Extracellular protons regulate human ENaC by modulating Na+ self-inhibition

The epithelial Na(+) channel, ENaC, is exposed to a wide range of proton concentrations in the kidney, lung, and sweat duct. We, therefore, tested whether pH alters ENaC activity. In Xenopus oocytes expressing human alpha-, beta-, and gammaENaC, amiloride-sensitive current was altered by protons in the physiologically relevant range (pH 8.5-6.0). Compared with pH 7.4, acidic pH increased ENaC current, whereas alkaline pH decreased current (pH(50) = 7.2). Acidic pH also increased ENaC current in H441 epithelia and in human primary airway epithelia. In contrast to human ENaC, pH did not alter rat ENaC current, indicating that there are species differences in ENaC regulation by protons. This resulted predominantly from species differences in gammaENaC. Maneuvers that lock ENaC in a high open-probability state ("DEG" mutation, proteolytic cleavage) abolished the effect of pH on human ENaC, indicating that protons alter ENaC current by modulating channel gating. Previous work showed that ENaC gating is regulated in part by extracellular Na(+) ("Na(+) self-inhibition"). Based on several observations, we conclude that protons regulate ENaC by altering Na(+) self-inhibition. First, protons reduced Na(+) self-inhibition in a dose-dependent manner. Second, ENaC regulation by pH was abolished by removing Na(+) from the extracellular bathing solution. Third, mutations that alter Na(+) self-inhibition produced corresponding changes in ENaC regulation by pH. Together, the data support a model in which protons modulate ENaC gating by relieving Na(+) self-inhibition. We speculate that this may be an important mechanism to facilitate epithelial Na(+) transport under conditions of acidosis.

Effect of cytosolic pH on epithelial Na+ channel in normal and cystic fibrosis sweat ducts

The activities of cystic fibrosis transmembrane conductance regulator (CFTR) Cl(-) channel and the amiloride-sensitive epithelial Na(+) channel (ENaC) are acutely coordinated in the sweat duct. However, the mechanisms responsible for cross-talk between these ion channels are unknown. Previous studies indicated that luminal pH of sweat ducts varies over 3 pH units and that the cytoplasmic pH affects both CFTR and ENaC. Therefore, using basolaterally alpha-toxin-permeabilized apical membrane preparations of sweat ducts as an experimental system, we tested the hypothesis that the cytosolic pH may mediate the cross-talk between CFTR and ENaC. We showed that while luminal pH had no effect, cytosolic pH acutely affected ENaC activity. That is, acidic pH inhibited, while basic pH activated, ENaC. pH regulation of ENaC appears to be independent of CFTR or endogenous kinase activities because basic pH independently stimulated ENaC (1) in normal ducts even when CFTR was deactivated, (2) in CF ducts that lack CFTR in the plasma membranes and (3) after blocking endogenous kinase activity with staurosporine. Considering the evidence of Na(+)/H(+) exchange (NHE) activity as shown by the expression of mRNA and function of NHE in the basolateral membrane of the sweat duct, we postulate that changes in cytosolic Na(+) ([Na(+)]( i )) may alter cytosolic pH (pH( i )) as salt loads into the cell during electrolyte absorption. These changes may play a role in coordinating the activities of ENaC and CFTR during transepithelial salt transport.

A glial DEG/ENaC channel functions with neuronal channel DEG-1 to mediate specific sensory functions in C. elegans

Mammalian neuronal DEG/ENaC channels known as ASICs (acid-sensing ion channels) mediate sensory perception and memory formation. ASICS are closed at rest and are gated by protons. Members of the DEG/ENaC family expressed in epithelial tissues are called ENaCs and mediate Na(+) transport across epithelia. ENaCs exhibit constitutive activity and strict Na(+) selectivity. We report here the analysis of the first DEG/ENaC in Caenorhabditis elegans with functional features of ENaCs that is involved in sensory perception. ACD-1 (acid-sensitive channel, degenerin-like) is constitutively open and impermeable to Ca(2+), yet it is required with neuronal DEG/ENaC channel DEG-1 for acid avoidance and chemotaxis to the amino acid lysine. Surprisingly, we document that ACD-1 is required in glia rather than neurons to orchestrate sensory perception. We also report that ACD-1 is inhibited by extracellular and intracellular acidification and, based on the analysis of an acid-hypersensitive ACD-1 mutant, we propose a mechanism of action of ACD-1 in sensory responses based on its sensitivity to protons. Our findings suggest that channels with ACD-1 features may be expressed in mammalian glia and have important functions in controlling neuronal function.

Intracellular calcium plays a role as the second messenger of hypotonic stress in gene regulation of SGK1 and ENaC in renal epithelial A6 cells

In A6 cells, a renal cell line derived from Xenopus laevis, hypotonic stress stimulates the amiloride-sensitive Na(+) transport. Hypotonic action on Na(+) transport consists of two phases, a nongenomic early phase and a genomic delayed phase. Although it has been reported that, during the genomic phase, hypotonic stress stimulates transcription of Na(+) transport-related genes, such as serum- and glucocorticoid-inducible kinase 1 (SGK1) and subunits of the epithelial Na(+) channel (ENaC), increasing Na(+) transport, the mechanism remains unknown. We focused the present study on the role of intracellular Ca(2+) in hypotonicity-induced SGK1 and ENaC subunit transcription. Since hypotonic stress raises intracellular Ca(2+) concentration in A6 cells, we hypothesized that Ca(2+)-dependent signals participate in the genomic action. Using real-time quantitative RT-PCR and Western blot techniques and measuring short-circuit currents, we observed that 1) BAPTA-AM and W7 blunted the hypotonicity-induced expression of SGK1 mRNA and protein, 2) ionomycin dose dependently stimulated expression of SGK1 mRNA and protein under an isotonic condition and the time course of the stimulatory effect of ionomycin on SGK1 mRNA was remarkably similar to that of hypotonic action on SGK1 mRNA, 3) hypotonic stress stimulated transcription of three ENaC subunits in an intracellular Ca(2+)-dependent manner, and 4) BAPTA-AM retarded the delayed phase of hypotonic stress-induced Na(+) transport but had no effect on the early phase. These observations indicate for the first time that intracellular Ca(2+) plays a role as the second messenger in hypotonic stress-induced Na(+) transport by stimulating transcription of SGK1 and ENaC subunits.

Amiloride-sensitive sodium absorption is different in vertebrates and invertebrates

Amiloride-sensitive Na+absorption is a well-described feature of numerous transporting epithelia in vertebrates. Yet, very little is known about this important physiological process regarding invertebrates. In the present paper, we compare vertebrate Na+absorption mediated by the amiloride-sensitive epithelial Na+channel (ENaC) and its invertebrate counterpart. We used the dorsal skin of the annelid Hirudo medicinalis as a model for the Na+absorption of invertebrate epithelia. In applying electrophysiological, molecular, and biochemical techniques we found striking functional and structural differences between vertebrate and invertebrate amiloride-sensitive Na+absorption. Using modified Ussing chambers, we analyzed the influence of different known blockers and effectors of vertebrate ENaC on leech epithelial Na+absorption. We demonstrate that the serine protease trypsin had no effect on the Na+transport across leech integument, while it strongly activates vertebrate ENaC. While protons, and the divalent cations Ni2+and Zn2+stimulate vertebrate ENaC, amiloride-sensitive Na+currents in leech integument were substantially reduced. For molecular studies, we constructed a cDNA library of Hirudo medicinalis and screened it with specific ENaC antibodies. We performed numerous PCR approaches using a vast number of different degenerated and specific ENaC primers to identify ENaC-like structures. Yet, both strategies did not reveal any ENaC-like sequence in leech integument. From these data we conclude that amiloride-sensitive Na+absorption in leech skin is not mediated by an ENaC-like Na+channel but by a still unknown invertebrate member of the ENaC/DEG family that we termed lENaTP (leech epithelial Na+transporting protein).

Effect of luminal acidity on the apical cation channel in rabbit esophageal epithelium

Esophageal epithelial cells contain an apical cation channel that actively absorbs sodium ions (Na(+)). Since these channels are exposed in vivo to acid reflux, we sought the impact of high acidity on Na(+) channel function in Ussing-chambered rabbit epithelium. Serosal nystatin abolished short-circuit current (I(sc)) and luminal pH titrated from pH 7.0 to pH > or = 2.0 had no effect on I(sc). Circuit analysis at pH 2.0 showed small, but significant, increases in apical and shunt resistances. At pH < 2.0, I(sc) increased whereas resistance (R(T)) decreased along with an increase in fluorescein flux. The change in I(sc), but not R(T), was reversible at pH 7.4. Reducing pH from 7.0 to 1.1 with H(2)SO(4) gave a similar pattern but higher I(sc) values, suggesting shunt permselectivity. A 10:1 Na(+) gradient after nystatin increased I(sc) by approximately 4 muAmps/cm(2) and this declined at pH < or = 3.5 until it reached approximately 0.0 at pH 2.0. Impedance analysis on acid-exposed (non-nystatin treated) tissues showed compensatory changes in apical (increase) and basolateral (decrease) resistance at modest luminal acidity that were poorly reversible at pH 2.0 and associated with declines in capacitance, a reflection of lower apical membrane area. In esophageal epithelium apical cation channels transport Na(+) at gradients as low as 10:1 but do not transport H(+) at gradients of 100,000:1 (luminal pH 2.0). Luminal acid also inhibits Na(+) transport via the channels and abolishes it at pH 2.0. These effects on the channel may serve as a protective function for esophageal epithelium exposed to acid reflux.

Mechanisms of acid and base secretion by the airway epithelium

One of the main functions of the airway epithelium is to inactivate and remove infectious particles from inhaled air and thereby prevent infection of the distal lung. This function is achieved by mucociliary and cough clearance and by antimicrobial factors present in the airway surface liquid (ASL). There are indications that airway defenses are affected by the pH of the ASL and historically, acidification of the airway surfaces has been suggested as a measure of airway disease. However, even in health, the ASL is slightly acidic, and this acidity might be part of normal airway defense. Only recently research has focused on the mechanisms responsible for acid and base secretion into the ASL. Advances resulted from research into the airway disease associated with cystic fibrosis (CF) after it was found that the CFTR Cl(-) channel conducts HCO (3) (-) and, therefore, may contribute to ASL pH. However, the acidity of the ASL indicated parallel mechanisms for H(+) secretion. Recent investigations identified several H(+) transporters in the apical membrane of the airway epithelium. These include H(+) channels and ATP-driven H(+) pumps, including a non-gastric isoform of the H(+)-K(+) ATPase and a vacuolar-type H(+) ATPase. Current knowledge of acid and base transporters and their potential roles in airway mucosal pH regulation is reviewed here.

Sperm channel diversity and functional multiplicity

Ion channels are extraordinarily efficient machines that move ions in diversely controlled manners, allowing cells to rapidly exchange information with the outside world and with other cells. Communication is the currency of fertilization, as it is of most fundamental cell signaling events. Ion channels are deeply involved in the dialogue between sperm, its surroundings, and the egg. How sperm swim, find the egg and fertilize it depend on ion permeability changes modulated by environmental cues and components of the egg outer layer. Different ion channels distinctly localized in these tiny, amazing cells perform specific decoding functions that shape the sophisticated behavior of sperm. It is not surprising that certain sperm ion channels are turning out to be unique. New strategies to characterize sperm ion transport have opened exciting possibilities to dissect sperm-egg signaling and unveil novel contraception targets.

Interregulation of proton-gated Na(+) channel 3 and cystic fibrosis transmembrane conductance regulator

Proton-gated Na(+) channels (ASIC) are new members of the epithelial sodium channel/degenerin gene family. ASIC3 mRNA has been detected in the homogenate of pulmonary tissues. However, whether ASIC3 is expressed in the apical membranes of lung epithelial cells and whether it regulates cystic fibrosis transmembrane conductance regulator (CFTR) function are not known at the present time. Using reverse transcription-PCR, we found that the ASIC3 mRNA was expressed in the human airway mucosal gland (Calu-3) and human airway epithelial (16HBE14o) cells. Indirect immunofluorescence microscopy revealed that ASIC3 was co-segregated with CFTR in the apical membranes of Calu-3 cells. Proton-gated, amiloride-sensitive short circuit Na(+) currents were recorded across Calu-3 monolayers mounted in an Ussing chamber. In whole-cell patch clamp studies, activation of CFTR channels with cAMP reduced proton-gated Na(+) current in Calu-3 cells from -154 +/- 28 to -33 +/- 16 pA (n = 5, p < 0.05) at -100 mV. On the other hand, cAMP-activated CFTR activity was significantly inhibited following constitutive activation of putative ASIC3 at pH 6.0. Immunoassays showed that both ASIC3 and CFTR proteins were expressed and co-immunoprecipitated mutually in Calu-3 cells. Similar results were obtained in human embryonic kidney 293T cells following transient co-transfection of ASIC3 and CFTR. Our results indicate that putative CFTR and ASIC3 channels functionally interact with each other, possibly via an intermolecular association. Because acidic luminal fluid in the cystic fibrosis airway and lung tends to stimulate ASIC3 channel expression and activity, the interaction of ASIC3 and CFTR may contribute to defective salt and fluid transepithelial transport in the cystic fibrotic pulmonary system.

Sodium and epithelial sodium channels participate in the regulation of the capacitation-associated hyperpolarization in mouse sperm

In a process called capacitation, mammalian sperm gain the ability to fertilize after residing in the female tract. During capacitation the mouse sperm plasma membrane potential (E(m)) hyperpolarizes. However, the mechanisms that regulate sperm E(m) are not well understood. Here we show that sperm hyperpolarize when external Na(+) is replaced by N-methyl-glucamine. Readdition of external Na(+) restores a more depolarized E(m) by a process that is inhibited by amiloride or by its more potent derivative 5-(N-ethyl-N-isopropyl)-amiloride hydrochloride. These findings indicate that under resting conditions an electrogenic Na(+) transporter, possibly involving an amiloride sensitive Na(+) channel, may contribute to the sperm resting E(m). Consistent with this proposal, patch clamp recordings from spermatogenic cells reveal an amiloride-sensitive inward Na(+) current whose characteristics match those of the epithelial Na(+) channel (ENaC) family of epithelial Na(+) channels. Indeed, ENaC-alpha and -delta mRNAs were detected by reverse transcription-PCR in extracts of isolated elongated spermatids, and ENaC-alpha and -delta proteins were found on immunoblots of sperm membrane preparations. Immunostaining indicated localization of ENaC-alpha to the flagellar midpiece and of ENaC-delta to the acrosome. Incubations known to produce capacitation in vitro or induction of capacitation by cell-permeant cAMP analogs decreased the depolarizing response to the addition of external Na(+). These results suggest that increases in cAMP content occurring during capacitation may inhibit ENaCs to produce a required hyperpolarization of the sperm membrane.

Innate host defense of the lung: effects of lung-lining fluid pH

Lung-lining fluid (LLF) is a primary constituent of the pulmonary host defense system. It is distributed continuously throughout the respiratory tract but is heterogeneous regarding its chemistry and physiology between the conducting airways and alveoli. The conducting airways are lined with airway surface liquid (ASL), a mucus gel-aqueous sol complex that interacts functionally with epithelial cilia as the mucociliary escalator. The alveoli are lined with alveolar subphase fluid (AVSF) and pulmonary surfactant. AVSF sterility is maintained in part by the phagocytic activity of resident alveolar macrophages. Normal ASL and AVSF are both more acidic than blood plasma. However, the details of acid-base regulation differ between the two media. Appreciable transepithelial acid-base flux is possible across the airway epithelium, whereas the alveolar epithelium is relatively impermeable to transepithelial acid-base flux. Moreover, one must consider the influence of resident macrophages on AVSF pH. Resident macrophages occupy a sizable fraction of AVSF by volume and are a substantial source of metabolic H+. The buffering capacities of ASL and AVSF probably are largely due to secreted peptides (e.g., ASL mucins and AVSF surfactant proteins). Acid-base exchange between the extracellular hydrophase and intracellular buffering systems of resident macrophages represents an additional buffer pool for AVSF. The pH of ASL and AVSF can be depressed by disease or inflammation. Low pH is predicted to suppress microbe clearance from the airways and alveoli, increase pathogen survival in both regions, and alter mediator release by resident macrophages and recruited leukocytes thereby increasing the propensity for bystander cell injury. Overall, ASL/AVSF pH is expected to be a major determinant of lung host defense responses.

Bisphosphonates stimulate an endogenous nonselective cation channel in Xenopus oocytes: potential mechanism of action

The mechanisms of action of bisphosphonates (BPs) have been poorly determined. Besides their actions on osteoclasts, these agents exhibit gastrointestinal complications. They have also recently been described as affecting various preparations that express an epithelial Na+ channel (ENaC). To understand the effects of BP on ion channels and the ENaC in particular, we used the Xenopus oocyte expression system. Alendronate, and similarly risedronate, two aminobisphosphonates, caused a large stimulation of an endogenous nonselective cation conductance (NSCC). This stimulation averaged 63 ± 12 μS ( n = 18) 60 min after the addition of 2 mM alendronate. The effects on the endogenous NSCC were blocked by extracellular acidification to pH 6.4. On the other hand, alendronate caused a small inhibition of ENaC conductance at pH 7.4 and 6.4, but the effects at pH 6.4 were more readily observed in the absence of changes of the endogenous conductance. The effects on membrane capacitance were also markedly different, with a clear decrease at pH 6.4 and no consistent changes at pH 7.4. The effects on the endogenous channel were further augmented by genistein and were inhibited by a tyrosine phosphatase inhibitor, indicating the involvement of the tyrosine kinase pathway. Stimulation of NSCC with BP is expected to cause membrane depolarization and may explain, in part, its mechanisms of action in inhibiting osteoclasts.

Opposite effects of Ni2+ on Xenopus and rat ENaCs expressed in Xenopus oocytes

The epithelial Na+ channel (ENaC) is modulated by various extracellular factors, including Na+, organic or inorganic cations, and serine proteases. To identify the effect of the divalent Ni2+ cation on ENaCs, we compared the Na+ permeability and amiloride kinetics of Xenopus ENaCs (xENaCs) and rat ENaCs (rENaCs) heterologously expressed in Xenopus oocytes. We found that the channel cloned from the kidney of the clawed toad Xenopus laevis [wild-type (WT) xENaC] was stimulated by external Ni2+, whereas the divalent cation inhibited the channel cloned from the rat colon (WT rENaC). The kinetics of amiloride binding were determined using noise analysis of blocker-induced fluctuation in current adapted for the transoocyte voltage-clamp method, and Na+ conductance was assessed using the dual electrode voltage-clamp (TEVC) technique. The inhibitory effect of Ni2+ on amiloride binding is not species dependent, because Ni2+ decreased the affinity (mainly reducing the association rate constant) of the blocker in both species in competition with Na+. Importantly, using the TEVC method, we found a prominent difference in channel conductance at hyperpolarizing voltage pulses. In WT xENaCs, the initial ohmic current response was stimulated by Ni2+, whereas the secondary voltage-activated current component remained unaffected. In WT rENaCs, only a voltage-dependent block by Ni2+ was obtained. To further study the origin of the xENaC stimulation by Ni2+, and based on the rationale of the well-known high affinity of Ni2+ for histidine residues, we designed alpha-subunit mutants of xENaCs by substituting histidines that were expressed in oocytes, together with WT beta- and gamma-subunits. Changing His215 to Asp in one putative amiloride-binding domain (WYRFHY) in the extracellular loop between Na+ channel membrane segments M1 and M2 had no influence on the stimulatory effect of Ni2+, and neither did complete deletion of this segment. Next, we mutated His416 flanked by His411 and Cys417, a unique site for possible heavy metal ion chelation, and, with this quality, most proximal (approximately 100 amino acids upstream of the second putative amiloride binding site at the pore entrance), was found localized at M2. Replacing His416 with arginine, aspartate, tyrosine, and alanine clearly affected amiloride binding in all cases, as well as Na+ conductance, as expressed in the xENaC current-voltage relationship, especially with regard to aspartate and tyrosine. However, similarly to those obtained with the WYRFHY stretch, none of these mutations could either abolish the stimulating effect of Ni2+ or reverse it to an inhibitory type.

Extracellular Ca2+regulates the stimulation of Na+transport in A6 renal epithelia

We investigated the involvement of intracellular and extracellular Ca2+in the stimulation of Na+transport during hyposmotic treatment of A6 renal epithelia. A sudden osmotic decrease elicits a biphasic stimulation of Na+transport, recorded as increase in amiloride-sensitive short-circuit current ( Isc) from 3.4 ± 0.4 to 24.0 ± 1.3 μA/cm2( n = 6). Changes in intracellular Ca2+concentration ([Ca2+]i) were prevented by blocking basolateral Ca2+entry with Mg2+and emptying the intracellular Ca2+stores before the hyposmotic challenge. This treatment did not noticeably affect the hypotonicity-induced stimulation of Isc. However, the absence of extracellular Ca2+severely attenuated Na+transport stimulation by the hyposmotic shock, and Iscmerely increased from 2.2 ± 0.3 to 4.8 ± 0.7 μA/cm2. Interestingly, several agonists of the Ca2+-sensing receptor, Mg2+(2 mM), Gd3+(0.1 mM), neomycin (0.1 mM), and spermine (1 mM) were able to substitute for extracellular Ca2+. When added to the basolateral solution, these agents restored the stimulatory effect of the hyposmotic solutions on Iscin the absence of extracellular Ca2+to levels that were comparable to control conditions. None of the above-mentioned agonists induced a change in [Ca2+]i. Quinacrine, an inhibitor of PLA2, overruled the effect of the agonists on Na+transport. In conclusion, we suggest that a Ca2+-sensing receptor in A6 epithelia mediates the stimulation of Na+transport without the interference of changes in [Ca2+]i.

Degenerin sites mediate proton activation of deltabetagamma-epithelial sodium channel

The delta-subunit of epithelial Na(+) channels (ENaC) is predominately expressed in brain, heart, and pancreas. The amiloride sensitivity, Na(+) conductance, and critical domains for gating are characterized as a cross between proton-activated Na(+) channels and alpha-ENaC. The hypothesis that external protons may activate human delta-ENaC was addressed by expressing deltabetagamma-hENaC in Xenopus oocytes and evaluating proton-activated current with the two-electrode voltage clamp technique. Our results showed that protons transiently evoked a Na(+) current with an EC(50) of pH 6 overlapped on the basal current of deltabetagamma-hENaC. Proton-activated current was not observed in uninjected oocytes. Studies on gating kinetics revealed that activation, desensitization, and recovery times of proton-activated Na(+) current were 3.8 +/- 0.5 s, 253 +/- 9.5 s, and 10 +/- 3.6 s, respectively (n = 4-12). Alkali metal cation selectivity of the proton-activated current was identical to that of the basal current of deltabetagamma-hENaC. The metabolic acids, lactate, pyruvate, and formate, modified the proton-activated current, as did hypo-osmotic stress. EDTA, hypo-osmolarity, and lactate enhanced proton activation synergistically. Our results suggest that delta-hENaC subunit is essential for proton-activated current and gamma-subunit may potentially regulate the response of delta-hENaC to protons. We have concluded that deltabetagamma-hENaC is a proton-activated cation channel whose closing gate can be regulated by a proton-induced conformational change. Proton-sensitivity of deltabetagamma-hENaC may be an important mechanism for integrating external ischemic signals in inflamed and hypoxic tissues.

Extracellular histidine residues crucial for Na+ self-inhibition of epithelial Na+ channels

Epithelial Na(+) channels (ENaC) participate in the regulation of extracellular fluid volume homeostasis and blood pressure. Channel activity is regulated by both extracellular and intracellular Na(+). The down-regulation of ENaC activity by external Na(+) is referred to as Na(+) self-inhibition. We investigated the structural determinants of Na(+) self-inhibition by expressing wild-type or mutant ENaCs in Xenopus oocytes and analyzing changes in whole-cell Na(+) currents following a rapid increase of bath Na(+) concentration. Our results indicated that wild-type mouse alphabetagammaENaC has intrinsic Na(+) self-inhibition similar to that reported for human, rat, and Xenopus ENaCs. Mutations at His(239) (gammaH239R, gammaH239D, and gammaH239C) in the extracellular loop of the gammaENaC subunit prevented Na(+) self-inhibition whereas mutations of the corresponding His(282) in alphaENaC (alphaH282D, alphaH282R, alphaH282W, and alphaH282C) significantly enhanced Na(+) self-inhibition. These results suggest that these two histidine residues within the extracellular loops are crucial structural determinants for Na(+) self-inhibition.

External nickel inhibits epithelial sodium channel by binding to histidine residues within the extracellular domains of alpha and gamma subunits and reducing channel open probability

Epithelial sodium channels (ENaC) are regulated by various intracellular and extracellular factors including divalent cations. We studied the inhibitory effect and mechanism of external Ni(2+) on cloned mouse alpha-beta-gamma ENaC expressed in Xenopus oocytes. Ni(2+) reduced amiloride-sensitive Na(+) currents of the wild type mouse ENaC in a dose-dependent manner. The Ni(2+) block was fast and partially reversible at low concentrations and irreversible at high concentrations. ENaC inhibition by Ni(2+) was accompanied by moderate inward rectification at concentrations higher than 0.1 mm. ENaC currents were also blocked by the histidine-reactive reagent diethyl pyrocarbonate. Pretreatment of the oocytes with the reagent reduced Ni(2+) inhibition of the remaining current. Mutations at alphaHis(282) and gammaHis(239) located within the extracellular loops significantly decreased Ni(2+) inhibition of ENaC currents. The mutation alphaH282D or double mutations alphaH282R/gammaH239R eliminated Ni(2+) block. All mutations at gammaHis(239) eliminated Ni(2+)-induced inward current rectification. Ni(2+) block was significantly enhanced by introduction of a histidine at alphaArg(280). Lowering extracellular pH to 5.5 and 4.4 decreased or eliminated Ni(2+) block. Although alphaH282C-beta-gamma channels were partially inhibited by the sulfhydryl-reactive reagent [2-(trimethylammonium)ethyl] methanethiosulfonate bromide (MTSET), alpha-beta-gamma H239C channels were insensitive to MTSET. From patch clamp studies, Ni(2+) did not affect unitary current but decreased open probability when perfused into the recording pipette. Our results suggest that external Ni(2+) reduces ENaC open probability by binding to a site consisting of alphaHis(282) and gammaHis(239) and that these histidine residues may participate in ENaC gating.

Modulation of rat chorda tympani NaCl responses and intracellular Na+ activity in polarized taste receptor cells by pH

Mixture interactions between sour and salt taste modalities were investigated in rats by direct measurement of intracellular pH (pH(i)) and Na(+) activity ([Na(+)](i)) in polarized fungiform taste receptor cells (TRCs) and by chorda tympani (CT) nerve recordings. Stimulating the lingual surface with NaCl solutions adjusted to pHs ranging between 2.0 and 10.3 increased the magnitude of NaCl CT responses linearly with increasing external pH (pH(o)). At pH 7.0, the epithelial sodium channel (ENaC) blocker, benzamil, decreased NaCl CT responses and inhibited further changes in CT responses induced by varying pH(o) to 2.0 or 10.3. At constant pH(o), buffering NaCl solutions with potassium acetate/acetic acid (KA/AA) or HCO(3)(-)/CO(2) inhibited NaCl CT responses relative to CT responses obtained with NaCl solutions buffered with HEPES. The carbonic anhydrase blockers, MK-507 and MK-417, attenuated the inhibition of NaCl CT responses in HCO(3)(-)/CO(2) buffer, suggesting a regulatory role for pH(i). In polarized TRCs step changes in apical pH(o) from 10.3 to 2.0 induced a linear decrease in pH(i) that remained within the physiological range (slope = 0.035; r(2) = 0.98). At constant pH(o), perfusing the apical membrane with Ringer's solutions buffered with KA/AA or HCO(3)(-)/CO(2) decreased resting TRC pH(i), and MK-507 or MK-417 attenuated the decrease in pH(i) in TRCs perfused with HCO(3)(-)/CO(2) buffer. In parallel experiments, TRC [Na(+)](i) decreased with (a) a decrease in apical pH, (b) exposing the apical membrane to amiloride or benzamil, (c) removal of apical Na(+), and (d) acid loading the cells with NH(4)Cl or sodium acetate at constant pH(o). Diethylpyrocarbonate and Zn(2+), modification reagents for histidine residues in proteins, attenuated the CO(2)-induced inhibition of NaCl CT responses and the pH(i)-induced inhibition of apical Na(+) influx in TRCs. We conclude that TRC pH(i) regulates Na(+)-influx through amiloride-sensitive apical ENaCs and hence modulates NaCl CT responses in acid/salt mixtures.

Role of PKCalpha in feedback regulation of Na(+) transport in an electrically tight epithelium

It has long been known that Na(+) channels in electrically tight epithelia are regulated by homeostatic mechanisms that maintain a steady state and allow new levels of transport to be sustained in hormonally challenged cells. Little is known about the potential pathways involved in these processes. In addition to short-term effect, recent evidence also indicates the involvement of PKC in the long-term regulation of the epithelial Na(+) channel (ENaC) at the protein level (40). To determine whether stimulation of ENaC involves feedback regulation of PKC levels, we utilized Western blot analysis to determine the distribution of PKC isoforms in polarized A6 epithelia. We found the presence of PKC isoforms in the conventional (alpha and gamma), novel (delta, eta, and epsilon), and atypical (iota, lambda, and zeta) groups. Steady-state stimulation of Na(+) transport with aldosterone was accompanied by a specific decrease of PKCalpha protein levels in both the cytoplasmic and membrane fractions. Similarly, overnight treatment with an uncharged amiloride analog (CDPC), a procedure that through feedback regulation causes a stimulation of Na(+) transport, also decreased PKCalpha levels. These effects were additive, indicating separate mechanisms that converge at the level of PKCalpha. These effects were not accompanied by changes of PKCalpha mRNA levels as determined by Northern blot analysis. We propose that this may represent a novel regulatory feedback mechanism necessary for sustaining an increase of Na(+) transport.

Up-regulation of acid-gated Na(+) channels (ASICs) by cystic fibrosis transmembrane conductance regulator co-expression in Xenopus oocytes

Cystic fibrosis transmembrane conductance regulator (CFTR) functions as both a chloride channel and an epithelial transport regulator, interacting with Na(+) (epithelial sodium channel), Cl(-), renal outer medullary potassium channel(+), and H(2)O channels and some exchangers (i.e. Na(+)/H(+)) and co-transporters (Na(+)-HCO(3)(minus sign), Na(+)-K(+)-2Cl(-)). Acid-sensitive ion channels (ASICs), members of the epithelial sodium channel/degenerin superfamily, were originally cloned from neuronal tissue, and recently localized in epithelia. Because CFTR has been immunocytochemically and functionally identified in rat, murine, and human brain, the regulation of ASICs by CFTR was tested in oocytes. Our observations show that the proton-gated Na(+) current formed by the heteromultimeric ASIC1a/2a channel was up-regulated by wild type but not by Delta F508-CFTR. In contrast, the acid-gated Na(+) current associated with either the homomultimeric ASIC1a or ASIC2a channel was not influenced by wild type CFTR. The apparent equilibrium dissociation constant for extracellular Na(+) for ASIC1a/2a was increased by CFTR, but CFTR had no effect on the gating behavior or acid sensitivity of ASIC1a/2a. CFTR had no effect on the pH activation of ASIC1a/2a. We conclude that wild type CFTR elevates the acid-gated Na(+) current of ASIC1a/2a in part by altering the kinetics of extracellular Na(+) interaction.

Ammonium interaction with the epithelial sodium channel

The purpose of this study was to investigate the direct effect of NH(3)/NH on mouse epithelial Na(+) channels (mENaC) expressed in Xenopus oocytes. Two-electrode voltage-clamp and ion-selective microelectrodes were used to measure the Na(+) current, intracellular pH (pH(i)), and ion activities in oocytes expressing mENaC. In oocytes expressing mENaC, removal of external Na(+) reversibly hyperpolarized membrane potential by 129 +/- 5.3 mV in the absence of 20 mM NH(4)Cl but only by 100 +/- 7.8 mV in its presence. Amiloride completely inhibited the changes in membrane potential. In oocytes expressing mENaC, butyrate (20 mM) caused a decrease in pH(i) (0.43 +/- 0.07) similar to the NH(4)Cl-induced pH(i) decrease (0.47 +/- 0.12). Removal of Na(+) in the presence of butyrate caused hyperpolarization that was not significantly different from that in the absence of butyrate at high pH(i) (in the absence of NH(4)Cl). Removal of external Na(+) resulted in an outward current of 3.7 +/- 0.8 microA (at -60 mV). The magnitude of this change in current was only 2.7 +/- 0.7 microA when Na(+) was removed in the presence of NH(4)Cl. In oocytes expressing mENaC, NH(4)Cl also caused a decrease in whole cell conductance at negative potential and an outward current at positive potential. In the presence of amiloride, steady-state current and the change in current caused by removal of Na(+) were not different from zero. These results indicate that NH(4)Cl inhibits Na(+) transport when mENaC is expressed in oocytes. The inhibition of voltage changes is not due to intracellular acidification caused by NH(4)Cl. Permeability and selectivity of ENaC to NH may play a role.