Na-K-Cl cotransport in nystatin-treated tracheal cells: regulation by isoproterenol, apical UTP, and [Cl]i

WNK4 kinase: role of chloride sensing in the distal convoluted tubule

PURPOSE OF REVIEW

This review focuses on recent efforts in identifying with-no-lysine kinase 4 (WNK4) as a physiological intracellular chloride sensor and exploring regulators of intracellular chloride concentration ([Cl-]i) in the distal convoluted tubule (DCT).

RECENT FINDINGS

The discovery of WNK1's chloride-binding site provides the mechanistic details of the chloride-sensing regulation of WNK kinases. The subsequent in-vitro studies reveal that the chloride sensitivities of WNK kinases were variable. Because of its highest chloride sensitivity and dominant expression, WNK4 emerges as the leading candidate of the chloride sensor in DCT. The presentation of hypertension and increased sodium-chloride cotransporter (NCC) activity in chloride-insensitive WNK4 mice proved that WNK4 is inhibitable by physiological [Cl-]i in DCT. The chloride-mediated WNK4 regulation is responsible for hypokalemia-induced NCC activation but unnecessary for hyperkalemia-induced NCC deactivation. This chloride-sensing mechanism requires basolateral potassium and chloride channels or cotransporters, including Kir4.1/5.1, ClC-Kb, and possibly KCCs, to modulate [Cl-]i in response to the changes of plasma potassium.

SUMMARY

WNK4 is both a master NCC stimulator and an in-vivo chloride sensor in DCT. The understanding of chloride-mediated regulation of WNK4 explains the inverse relationship between dietary potassium intake and NCC activity.

β2 adrenoceptor signaling regulates ion transport in 16HBE14o- human airway epithelial cells

We investigated the regulation of Cl^- secretion by adrenoceptors in polarized 16HBE14o- human bronchial epithelial cells. Treatment with the nonselective β adrenoceptor agonist isoprenaline stimulated an increase in short-circuit current (I_SC ), which was inhibited by the β adrenoceptor blocker propranolol. Treatment with procaterol, an agonist specific for the β₂ adrenoceptor subtype, stimulated a similar increase in I_SC , which was inhibited by the β₂ adrenoceptor antagonist ICI 118551. Inhibitors of cystic fibrosis transmembrane conductance regulator (CFTR) and calcium-activated Cl^- channel (CaCC), but not K⁺ channel blockers, were able to inhibit the increase in I_SC . "Trimultaneous" recording of I_SC and intracellular cyclic adenosine monophosphate (cAMP) and Ca²⁺ levels in 16HBE14o- epithelia confirmed that the I_SC induced by isoprenaline or procaterol involved both cAMP and Ca²⁺ signaling. Our results demonstrate that β₂ adrenoceptors regulate Cl^- secretion in the human airway epithelium by activating apical CFTRs and CaCCs via cAMP-dependent and intracellular Ca²⁺ -dependent mechanisms, respectively.

CFTR mediated chloride secretion in the avian renal proximal tubule

In primary cell cultures of the avian (Gallus gallus) renal proximal tubule parathyroid hormone and cAMP activation generate a Cl(-)-dependent short circuit current (I(SC)) response, consistent with net transepithelial Cl(-) secretion. In this study we investigated the expression and physiological function of the Na-K-2Cl (NKCC) transporter and CFTR chloride channel, both associated with Cl(-) secretion in a variety of tissues, in these proximal tubule cells. Using both RT-PCR and immunoblotting approaches, we showed that NKCC and CFTR are expressed, both in proximal tubule primary cultures and in a proximal tubule fraction of non-cultured (native tissue) fragments. We also used electrophysiological methods to assess the functional contribution of NKCC and CFTR to forskolin-activated I(SC) responses in filter grown cultured monolayers. Bumetanide (10 μM), a specific blocker of NKCC, inhibited forskolin activated I(SC) by about 40%, suggesting that basolateral uptake of Cl(-) is partially mediated by NKCC transport. In monolayers permeabilized on the basolateral side with nystatin, forskolin activated an apical Cl(-) conductance, manifested as bidirectional diffusion currents in the presence of oppositely directed Cl(-) gradients. Under these conditions the apical conductance appeared to show some bias towards apical-to-basolateral Cl(-) current. Two selective CFTR blockers, CFTR Inhibitor 172 and GlyH-101 (both at 20 μM) inhibited the forskolin activated diffusion currents by 38-68%, with GlyH-101 having a greater effect. These data support the conclusion that avian renal proximal tubules utilize an apical CFTR Cl(-) channel to mediate cAMP-activated Cl(-) secretion.

Basolateral Cl- uptake mechanisms in Xenopus laevis lung epithelium

A thin liquid layer covers the lungs of air-breathing vertebrates. Active ion transport processes via the pulmonary epithelial cells regulate the maintenance of this layer. This study focuses on basolateral Cl(-) uptake mechanisms in native lungs of Xenopus laevis and the involvement of the Na(+)/K(+)/2 Cl(-) cotransporter (NKCC) and HCO(3)(-)/Cl(-) anion exchanger (AE), in particular. Western blot analysis and immunofluorescence staining revealed the expression of the NKCC protein in the Xenopus lung. Ussing chamber experiments demonstrated that the NKCC inhibitors (bumetanide and furosemide) were ineffective at blocking the cotransporter under basal conditions, as well as under pharmacologically stimulated Cl(-)-secreting conditions (forskolin and chlorzoxazone application). However, functional evidence for the NKCC was detected by generating a transepithelial Cl(-) gradient. Further, we were interested in the involvement of the HCO(3)(-)/Cl(-) anion exchanger to transepithelial ion transport processes. Basolateral application of DIDS, an inhibitor of the AE, resulted in a significantly decreased the short-circuit current (I(SC)). The effect of DIDS was diminished by acetazolamide and reduced by increased external HCO(3)(-) concentrations. Cl(-) secretion induced by forskolin was decreased by DIDS, but this effect was abolished in the presence of HCO(3)(-). These experiments indicate that the AE at least partially contributes to Cl(-) secretion. Taken together, our data show that in Xenopus lung epithelia, the AE, rather than the NKCC, is involved in basolateral Cl(-) uptake, which contrasts with the common model for Cl(-) secretion in pulmonary epithelia.

Modelling dysregulated Na+ absorption in airway epithelial cells with mucosal nystatin treatment

In cystic fibrosis (CF), the absence of functional CFTR leads to dysregulated Na(+) absorption across airway epithelia. We established an in vitro model of dysregulated Na(+) absorption by treating polarized normal human bronchial epithelial cells (HBEs) with nystatin (Nys), a polyene antibiotic that enables monovalent cations to permeate biological membranes. Acute mucosal Nys produced a rapid increase in short circuit current (I(sc)) that reflected increased transepithelial Na(+) absorption and required Na(+)/K(+)ATPase activity. The acute increase in I(sc) was associated with increased mucosal liquid absorption. Prolonged mucosal Nys treatment resulted in sustained Na(+) hyperabsorption, associated with increased mucosal liquid absorption in comparison with naïve (nontreated, kept under air-liquid interface conditions) or vehicle-treated cultures. Nys treatment was not toxic. Increased lactate accumulation in Nys-treated culture media suggested a higher metabolic rate associated with the higher energy demand for Na(+) transport. After chronic Nys treatment, the increased I(sc) was rapidly lost when the cultures were mounted in Ussing chambers, indicating that Nys could be rapidly removed from the apical membrane. Importantly, chronic Nys treatment promoted sustained mucosal liquid depletion and caused mucus dehydration, compaction, and adhesion to the apical surface of Nys-treated cultures. We conclude that mucosal Nys treatment of HBEs provides a simple in vitro model to recapitulate the Na(+) and volume hyperabsorptive features of CF airway epithelia.

Hypoxia effects on cell volume and ion uptake of cerebral microvascular endothelial cells

Increased transport of Na across an intact blood-brain barrier (BBB) contributes to cerebral edema formation in ischemic stroke. Our previous studies have shown that ischemic factors stimulate activity of a luminal BBB Na-K-Cl cotransporter, and we have hypothesized that during ischemia, the cotransporter together with the abluminal Na/K pump mediates increased transport of Na from blood into the brain. However, it is possible that elevated Na-K-Cl cotransporter activity could also cause cell swelling if it outpaces ion efflux pathways. The present study was conducted to evaluate the effects of hypoxia on intracellular volume of BBB cells. Cerebral microvascular endothelial cell (CMEC) monolayers were exposed to varying levels of hypoxia for 1 to 5 h in an O(2)-controlled glove box, and cell volume was assessed using 3-O-methyl-D-[(3)H]glucose and [(14)C]sucrose as markers of total and extracellular water space, respectively. Cells exposed to either 7.5%, 3%, or 1% O(2) showed gradual increases in volume (compared with 19% O(2) normoxic controls) that became significant after 3 or more hours. By ion chromatography methods, we also found that a 30-min exposure to 7.5% O(2) caused an increase in bumetanide-sensitive net Na uptake by the cells without increasing cell Na content. CMEC Na content was significantly increased, however, following 3 or more hours of exposure to 7.5% O(2). These findings are consistent with the hypothesis that during cerebral ischemia, the BBB Na-K-Cl cotransporter is stimulated to mediate transendothelial uptake of Na into the brain and that increased cotransporter activity also contributes to gradual swelling of the cells.

Physiological significance of hypotonicity-induced regulatory volume decrease: reduction in intracellular Cl- concentration acting as an intracellular signaling

Regulatory volume decrease (RVD) occurs after hypotonicity-caused cell swelling. RVD is caused by activation of ion channels and transporters, which cause effluxes of K(+), Cl(-), and H(2)O, leading to cell shrinkage. Recently, we showed that hypotonicity stimulated transepithelial Na(+) reabsorption via elevation of epithelial Na(+) channel (alpha-ENaC) expression in renal epithelia A6 cells in an RVD-dependent manner and that reduction of intracellular Cl(-) concentration ([Cl(-)](i)) stimulated the Na(+) reabsorption. These suggest that RVD would reveal its stimulatory action on the Na(+) reabsorption by reducing [Cl(-)](i). However, the reduction of [Cl(-)](i) during RVD has not been definitely analyzed due to technical difficulties involved in halide-sensitive fluorescent dyes. In the present study, we developed a new method for the measurement of [Cl(-)](i) change during RVD by using a high-resolution flow cytometer with a halide-specific fluorescent dye, N-(6-methoxyquinolyl) acetoethyl ester. The [Cl(-)](i) in A6 cells in an isotonic medium was 43.6 +/- 3.1 mM. After hypotonic shock (268 to 134 mosmol/kgH(2)O), a rapid increase of cell volume followed by RVD occurred. The RVD caused drastic diminution of [Cl(-)](i) from 43.6 to 10.8 mM. Under an RVD-blocked condition with NPPB (Cl(-) channel blocker) or quinine (K(+) channel blocker), we did not detect the reduction of [Cl(-)](i). Based on these observations, we conclude that one of the physiological significances of RVD is the reduction of [Cl(-)](i) and that RVD shows its action via reduction of [Cl(-)](i) acting as an intracellular signal regulating cellular physiological functions.

Regulation of ion transport via apical purinergic receptors in intact rabbit airway epithelium

We investigated purinergic receptors involved in ion transport regulation in the intact rabbit nasal airway epithelium. Stimulation of apical membrane P2Y receptors with ATP or UTP (200 microM) induced transient increases in short-circuit current (Isc) of 13 and 6% followed by sustained inhibitions to 8 and 17% below control level, respectively. Serosal application of nucleotides had no effect. The ATP-induced response appeared to involve additional activation of apical adenosine (P1) and P2X receptors. The inhibitory effect of ATP and UTP on Isc was eliminated by pretreatment with amiloride (100 microM), while the stimulatory effect was potentiated, indicating that ATP and UTP inhibit Na+ and stimulate Cl- current. Ionomycin (1 microM) induced responses similar to UTP and ATP and desensitized the epithelium to the nucleotides, indicating involvement of intracellular Ca2+ (Ca2+ i. Furthermore, ATP, UTP and ionomycin induced 21, 24, and 21% decreases, respectively, in transepithelial conductance. Measurements of unidirectional isotope fluxes showed a 39% decrease in the dominant net Na+ absorption in response to ATP, while the smaller net Cl- secretion increased only insignificantly and unidirectional Cl- fluxes decreased significantly. The results suggest that nucleotides released to the airway surface liquid exert an autocrine regulation of epithelial NaCl absorption mainly by inhibiting the amiloride-sensitive epithelial Na+ channel (ENaC) and paracellular anion conductance via a P2Y receptor-dependent increase in Ca2+ i, while stimulation of Cl- secretion is of minor importance.

Involvement of cytosolic Cl- in osmoregulation of alpha-ENaC gene expression

Hypotonicity stimulates transepithelial Na(+) reabsorption in renal A6 cells, but the mechanism for this stimulation is not fully understood. In the present study, we found that hypotonicity stimulated Na(+) reabsorption through increases in mRNA expression of the alpha-subunit of the epithelial Na(+) channel (alpha-ENaC). Hypotonicity decreases cytosolic Cl(-) concentration; therefore, we hypothesized that hypotonicity-induced decreases in cytosolic Cl(-) concentration could act as a signal to regulate Na(+) reabsorption through changes in alpha-ENaC mRNA expression. Treatment with the flavone apigenin, which activates the Na(+)-K(+)-2Cl(-) cotransporter and increases cytosolic Cl(-) concentration, markedly suppressed the hypotonicity-induced increase in alpha-ENaC mRNA expression. On the other hand, blockade of the Na(+)-K(+)-2Cl(-) cotransporter decreases cytosolic Cl(-) concentration and increased alpha-ENaC mRNA expression and Na(+) reabsorption. Blocking Cl(-) channels with 5-nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) inhibited the hypotonicity-induced decrease in cytosolic Cl(-) concentration and suppressed the hypotonicity-induced increase in alpha-ENaC mRNA expression. Coapplication of NPPB and apigenin synergistically suppressed alpha-ENaC mRNA expression. Thus, in every case, changes in cytosolic Cl(-) concentration were associated with changes in alpha-ENaC mRNA expression and changes in Na(+) reabsorption: decreases in cytosolic Cl(-) concentration increased alpha-ENaC mRNA and increased Na(+) reabsorption, whereas increases in cytosolic Cl(-) concentration decreased alpha-ENaC mRNA and decreased Na(+) reabsorption. These findings support the hypothesis that changes in cytosolic Cl(-) concentration are an important and novel signal in hypotonicity-induced regulation of alpha-ENaC expression and Na(+) reabsorption.

Beta 2-adrenergic regulation of ciliary beat frequency in rat bronchiolar epithelium: potentiation by isosmotic cell shrinkage

Single bronchiolar ciliary cells were isolated from rat lungs. The beta(2)-adrenergic regulation of ciliary beat frequency (CBF) was studied using video-optical microscopy. Terbutaline (a beta(2)-adrenergic agonist) increased CBF in a dose-dependent manner, and it also decreased the volume of the ciliary cells. These terbutaline actions were inhibited by a PKA inhibitor (H-89) and mimicked by forskolin, IBMX and DBcAMP. Ion transport inhibitors were used to isosmotically manipulate the volume of the terbutaline-stimulated bronchiolar ciliary cells. Amiloride (1 microM) and bumetanide (20 microM) potentiated cell shrinkage and the CBF increase, and they shifted the terbutaline dose-response curve to the lower-concentration side. Quinidine (500 microM), in contrast, increased cell volume and suppressed the CBF increase. Moreover, a KCl solution containing amiloride (1 microM) and strophanthidin (100 microM) increased cell volume and suppressed the CBF increase, and then the subsequent removal of either amiloride or strophanthidin decreased cell volume and further increased CBF. NPPB (10 microM) or glybenclamide (200 microM) had no effect on the action of terbutaline. Thus, in terbutaline-stimulated ciliary cells, cell shrinkage enhances the CBF increase; in contrast, cell swelling suppresses it. However, the results of direct manupulation of cell volume by applying osmotic stresses (hyperosmotic shrinkage or hyposmotic swelling) were the opposite of the findings of the isosmotic experiments: hyposmotic cell swelling enhanced the CBF increase, while isosmotic swelling suppressed it. These results suggest that isosmotic and non-isosmotic volume changes in terbutaline-stimulated bronchiolar ciliary cells may trigger different signalling pathways. In conclusion, terbutaline increases CBF and decreases the volume of rat bronchiolar ciliary cells via cAMP accumulation under isosmotic conditions, and the isosmotic cell shrinkage enhances the CBF increase by increasing cAMP sensitivity.

Bicarbonate-dependent chloride secretion in Calu-3 epithelia in response to 7,8-benzoquinoline

Stimulation of Calu-3 epithelia with 7,8-benzoquinoline, under short circuit current conditions, produced a current increase that was completely accounted for by the net flux of chloride, measured simultaneously with 36Cl-. Nevertheless the current stimulated by 7,8-benzoquinoline was sensitive to acetazolamide, which caused up to 50 % inhibition of the stimulated current, the remainder being sensitive to the Na+-K+-2Cl- cotransport inhibitor bumetanide. The effects of acetazolamide could be mimicked by either amiloride or by the di-sodium salt of 4,4'-dinitrostilbene-2,2'-disulphonic acid (DNDS) added to the basolateral side of the epithelium, but their actions were not additive. Amiloride was needed in sufficient concentration to inhibit the sodium-proton exchanger NHE1. DNDS blocks both the chloride-bicarbonate exchanger AE2 and the sodium-bicarbonate transporter NBC1. However, since 7,8-benzoquinoline activates basolateral K+ channels, causing hyperpolarisation, it is unlikely NBC1 is active after addition of 7,8-benzoquinoline. The effect of DNDS is, therefore, mainly on AE2. It is concluded that chloride enters the basolateral aspect of the cells using the Na+-K+-2Cl- cotransporter and a parallel arrangement of NHE1 with AE2, these latter two being sensitive to acetazolamide because of their association with the cytoplasmic form of carbonic anhydrase CAII. The effects of acetazolamide could be mimicked by removal of HCO3-/CO2 from the bathing medium, and furthermore showed that the NHE1-AE2 mechanism is particularly important when the transport rate is high. Thus part of the current stimulated by 7,8-benzoquinoline and inhibited by acetazolamide or HCO3-/CO2 removal can be said to represent bicarbonate-dependent chloride secretion.

Coordinate modulation of Na-K-2Cl cotransport and K-Cl cotransport by cell volume and chloride

Na-K-2Cl cotransporter (NKCC) and K-Cl cotransporter (KCC) play key roles in cell volume regulation and epithelial Cl(-) transport. Reductions in either cell volume or cytosolic Cl(-) concentration ([Cl(-)](i)) stimulate a corrective uptake of KCl and water via NKCC, whereas cell swelling triggers KCl loss via KCC. The dependence of these transporters on volume and [Cl(-)](i) was evaluated in model duck red blood cells. Replacement of [Cl(-)](i) with methanesulfonate elevated the volume set point at which NKCC activates and KCC inactivates. The set point was insensitive to cytosolic ionic strength. Reducing [Cl(-)](i) at a constant driving force for inward NKCC and outward KCC caused the cells to adopt the new set point volume. Phosphopeptide maps of NKCC indicated that activation by cell shrinkage or low [Cl(-)](i) is associated with phosphorylation of a similar constellation of Ser/Thr sites. Like shrinkage, reduction of [Cl(-)](i) accelerated NKCC phosphorylation after abrupt inhibition of the deactivating phosphatase with calyculin A in vivo, whereas [Cl(-)] had no specific effect on dephosphorylation in vitro. Our results indicate that NKCC and KCC are reciprocally regulated by a negative feedback system dually modulated by cell volume and [Cl(-)]. The major effect of Cl(-) on NKCC is exerted through the volume-sensitive kinase that phosphorylates the transport protein.

General models for water transport across leaky epithelia

The group of leaky epithelia, such as proximal tubule and small intestine, have several common properties in regard to salt and water transport. The fluid transport is isotonic, the transport rate increases in dilute solutions, and water can be transported uphill. Yet, it is difficult to find common features that could form the basis for a general transport model. The direction of transepithelial water transport does not correlate with the direction of the primary active Na+ transport, or with the ultrastucture as defined by the location of apical and basolateral membranes, of the junctional complex and the lateral intercellular spaces. The presence of specific water channels, aquaporins, increases the water permeability of the epithelial cell membranes, i.e., the kidney proximal tubule. Yet other leaky epithelia, for example, the retinal pigment epithelium, have no known aquaporins. There is, however, a general correlation between the direction of transepithelial transport and the direction of transport via cotransporters of the symport type. A simple epithelial model based on water permeabilities, a hyperosmolar compartment and restricted salt diffusion, is unable to explain epithelial transport phenomena, in particular the ability for uphill water transport. The inclusion of cotransporters as molecular water pumps in these models alleviates this problem.

Modulation of Na-K-2Cl cotransport by intracellular Cl(-) and protein kinase C-delta in Calu-3 cells

In this study, we tested the hypothesis that intracellular Cl(-) (Cl) regulates the activity of protein kinase C (PKC)-delta and thus the activation of Na-K-Cl cotransport (NKCC1) in a Calu-3 cell line. The alpha(1)-adrenergic agonist methoxamine (MOX) and hypertonic sucrose increased Cl and increased or decreased intracellular volume, respectively, without changing Cl concentration ([Cl(-)](i)). Titration of [Cl(-)](i) from 20-140 mM in nystatin-permeabilized cell monolayers did not affect the baseline activity of PKC-delta, PKC-zeta, or rottlerin-sensitive NKCC1. At 200 mM Cl(-), rottlerin-sensitive NKCC1 was activated, and PKC isotypes were localized predominantly to a particulate fraction. MOX induced a biphasic increase in NKCC1 activity and PKC-delta in activity and particulate localization of PKC-delta and -zeta. Activity of NKCC1 and PKC-delta decreased with increasing Cl from 20 to 80 mM Cl then increased at 140-200 mM Cl apparently as an additive effect to high [Cl(-)](i) levels. Rottlerin inhibited the effects of MOX, which indicates that PKC-delta was required for activation of NKCC1. The results indicate that, in airway epithelial cells, a Cl electrochemical gradient alone is not sufficient to stimulate NKCC1 activity; rather, elevated activity of PKC-delta is necessary. Further, high Cl levels induce a subcellular redistribution of PKC-delta, which results in increased enzyme activity.

Phosphorylation of the salivary Na(+)-K(+)-2Cl(-) cotransporter

We studied the phosphorylation of the secretory Na(+)-K(+)-2Cl(-) cotransporter (NKCC1) in rat parotid acinar cells. We have previously shown that NKCC1 activity in these cells is dramatically upregulated in response to beta-adrenergic stimulation and that this upregulation correlates with NKCC1 phosphorylation, possibly due to protein kinase A (PKA). We show here that when ATP is added to purified acinar basolateral membranes (BLM), NKCC1 is phosphorylated as a result of membrane-associated protein kinase activity. Additional NKCC1 phosphorylation is seen when PKA is added to BLMs, but our data indicate that this is due to an effect of PKA on endogenous membrane kinase or phosphatase activities, rather than its direct phosphorylation of NKCC1. Also, phosphopeptide mapping demonstrates that these phosphorylations do not take place at the site associated with the upregulation of NKCC1 by beta-adrenergic stimulation. However, this upregulatory phosphorylation can be mimicked by the addition of cAMP to permeabilized acini, and this effect can be blocked by a specific PKA inhibitor. These latter results provide good evidence that PKA is indeed involved in the upregulatory phosphorylation of NKCC1 and suggest that an additional factor present in the acinar cell but absent from isolated membranes is required to bring about the phosphorylation.

Insulin-independent, MAPK-dependent stimulation of NKCC activity in skeletal muscle

Na(+)-K(+)-Cl(-) cotransporter (NKCC) activity in quiescent skeletal muscle is modest. However, ex vivo stimulation of muscle for as little as 18 contractions (1 min, 0.3 Hz) dramatically increased the activity of the cotransporter, measured as the bumetanide-sensitive (86)Rb influx, in both soleus and plantaris muscles. This activation of cotransporter activity remained relatively constant for up to 10-Hz stimulation for 1 min, falling off at higher frequencies (30-Hz stimulation for 1 min). Similarly, stimulation of skeletal muscle with adrenergic receptor agonists phenylephrine, isoproterenol, or epinephrine produced a dramatic stimulation of NKCC activity. It did not appear that stimulation of NKCC activity was a reflection of increased Na(+)-K(+)-ATPase activity because insulin treatment did not stimulate NKCC activity, despite insulin's well-known stimulation of Na(+)-K(+)-ATPase activity. Stimulation of NKCC activity could be blocked by pretreatment with inhibitors of mitogen-activated protein kinase (MAPK) kinase 1/2 (MEK1/2) activity, indicating that activation of the extracellular signal-regulated kinase 1/2 (ERK1/2) MAPKs may be required. These data indicate a regulated NKCC activity in skeletal muscle that may provide a significant pathway for potassium transport into skeletal muscle fibers.

Cl(-)-dependent secretory mechanisms in isolated rat bile duct epithelial units

Cholangiocytes absorb and secrete fluid, modifying primary canalicular bile. In several Cl(-)-secreting epithelia, Na(+)-K(+)-2Cl(-) cotransport is a basolateral Cl(-) uptake pathway facilitating apical Cl(-) secretion. To determine if cholangiocytes possess similar mechanisms independent of CO2/HCO, we assessed Cl(-)-dependent secretion in rat liver isolated polarized bile duct units (IBDUs) by using videomicroscopy. Without CO2/HCO, forskolin (FSK) stimulated secretion entirely dependent on Na(+) and Cl(-) and inhibited by Na(+)-K(+)-2Cl(-) inhibitor bumetanide. Carbonic anhydrase inhibitor ethoxyzolamide had no effect on FSK-stimulated secretion, indicating negligible endogenous CO2/HCO transport. In contrast, FSK-stimulated secretion was inhibited approximately 85% by K(+) channel inhibitor Ba(2+) and blocked completely by bumetanide plus Ba(2+). IBDU Na(+)-K(+)-2Cl(-) cotransport activity was assessed by recording intracellular pH during NH4Cl exposure. Bumetanide inhibited initial acidification rates due to NH entry in the presence and absence of CO2/HCO. In contrast, when stimulated by FSK, a 35% increase in Na(+)-K(+)-2Cl(-) cotransport activity occurred without CO2/HCO. These data suggest a cellular model of HCO-independent secretion in which Na(+)-K(+)-2Cl(-) cotransport maintains high intracellular Cl(-) concentration. Intracellular cAMP concentration increases activate basolateral K(+) conductance, raises apical Cl(-) permeability, and causes transcellular Cl(-) movement into the lumen. Polarized IBDU cholangiocytes are capable of vectorial Cl(-)-dependent fluid secretion independent of HCO. Bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransport, Cl(-)/HCO exchange, and Ba(2+)-sensitive K(+) channels are important components of stimulated fluid secretion in intrahepatic bile duct epithelium.

Characterization of Rb uptake into Sf9 cells using cation chromatography: evidence for a K-Cl cotransporter

To assess cation-chloride cotransporter activity in Sf9 cells, cation chromatography was used to measure initial uptake rates of Rb. Rb exchanged with cellular K, with 30% of cellular K replaced after a 40 min exposure to Rb. Rb uptake into Sf9 cells was not inhibited by 50 µmol l(-1) ouabain. Rb uptake was approximately 65% inhibited by 250 µmol l(-1) bumetanide added to the assay solution, and was more than 95% inhibited when cells were pre-incubated for 20 min with bumetanide (100 and 1000 µmol l(-1)). Uptake of Rb and Cl followed simple Michaelis-Menten kinetics, with a K(m) for Rb of 17.1+/-2.2 mmol l(-1) and a K(m) for Cl of 93.7+/-5.6 mmol l(-1). Rb uptake was not dependent upon extracellular Na. Two min exposures to solutions with reduced [Na] or [Cl] produced small but significant changes in cellular Na content. We conclude that the primary Rb uptake pathway in Sf9 cells is a K-Cl cotransporter and that cation chromatography can be used to effectively study kinetic parameters of cotransporter function in tissue culture cells. Characterization of baseline cation-chloride cotransporter activity in Sf9 cells strengthens their utility as a tool for expression and characterization of exogenous proteins.

Ion transport and regulation of respiratory tract fluid output in dogs

To investigate the regulation of respiratory tract fluid output (RTFO), we collected the RTFO in an anesthetized canine model after a series of pharmacological interventions (inhibition of Na(+)-K(+)-ATPase or Na(+)-K(+)-2Cl(-) cotransporter, 250 microl) and physiological challenges (ionic and/or osmotic perturbation in airway lumen, 250 microl). Whereas 250 microl of aerosolized 0.9% saline caused a transient increase in RTFO, a 250-microl bumetanide-induced increase in RTFO was evident for 18 min and a 250-microl acetylstrophanthidin-induced increase in RTFO persisted for at least 30 min. Dry air ventilation decreased the responses of RTFO to the saline (sham) and acetylstrophanthidin intervention but not the bumetanide intervention. Delivery of 250 mosmol/kgH(2)O ion-free mannitol (250 microl) caused marked increases in RTFO that were little affected by the administration of acetylstrophanthidin or bumetanide 30 min before these challenges. A 250-microl 550 mosmol/kgH(2)O ion-free mannitol challenge caused a more marked and prolonged increase in RTFO. Thus aerosol delivery of a low dose of a cardiac glycoside or a near-isosmotic, ion-free, impermeant osmolyte solution may be therapeutically useful by increasing the clearance of secretions from the tracheobronchial airways.

Catecholaminergic regulation of Na-K-Cl cotransport in pigmented ciliary epithelium: differences between PE and NPE

Pigmented (PE) and nonpigmented (NPE) ciliary epithelial cells comprise the ciliary epithelium, the site of aqueous humor formation in the eye. In man, catecholamines increase the rate of aqueous humor formation, but the mechanism underlying these effects is not understood. Recent evidence suggests that Na-K-Cl cotransport plays a central role in blood-to-aqueous chloride transport across ciliary epithelium in cow and rabbit. We therefore investigated whether catecholamines stimulate Na-K-Cl cotransport in human PE cells. Na-K-Cl cotransporter protein was detected as a 170 kDa protein band on immunoblots. Immunofluorescence microscopy detected cotransporter on the basolateral membranes of the PE layer of ciliary epithelium from a human donor. Cotransporter immunofluorescence was also detected in cultured PE cells. Na-K-Cl cotransport activity measured as ouabain-insensitive bumetanide-sensitive(86)Rb uptake was stimulated by isoproterenol 1.6-fold, with an EC(50) = 28 n M and maximal stimulation at 1 microM. Other transport mechanisms involved in(86)Rb uptake were not affected. Stimulation by 1 microM isoproterenol was blocked by 10 n M ICI 118,551, a beta(2)-specific receptor antagonist, whereas the receptor subtype-specific antagonists yohimbine (alpha(2)), prazosin (alpha(1)) and atenolol (beta(1)) were ineffective. Norepinephrine stimulation (EC(50) = 280 n M) was also blocked by ICI 118,551. Dopamine stimulated Na-K-Cl cotransport 1.6-fold with an EC(50) = 14 microM. The dopamine effect could not be blocked by 10 microM SCH 23390, a D1-antagonist, but was abolished by ICI 118,551. Forskolin and CPT-cAMP stimulated Na-K-Cl cotransport 1.79- and 1.71-fold, respectively, whereas the inactive forskolin analogue 1,9-dideoxyforskolin had no effect. However, high concentrations of the PKA inhibitors PKI amide 14-22 and KT 5720 were needed to inhibit both PKA activity in cell lysates and isoproterenol stimulation of cotransport. This finding may indicate the presence of a novel PKA isoform in PE cells. Inhibitors of other protein kinases, including myosin light chain kinase, protein kinase G, calmodulin-dependent kinase and tyrosine kinase, were without effect on stimulated Na-K-Cl cotransport. When EC(50)s for catecholaminergic stimulations of Na-K-Cl cotransport in PE were compared to those in NPE, values within five-fold of one another were seen for isoproterenol and norepinephrine. In contrast, dopamine was 28-fold more potent in NPE than in PE. The data suggest that both PE and NPE possess beta(2)adrenergic receptors, but only NPE cells possess dopamine D1 receptors linked to Na-K-Cl cotransport.

The Na-K-Cl cotransporter of secretory epithelia

The Na-K-Cl cotransporters are a class of ion transport proteins that transport Na, K, and Cl ions into and out of cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. To date, two Na-K-Cl cotransporter isoforms have been identified: NKCC1, which is present in a wide variety of secretory epithelia and non-epithelial cells; and NKCC2, which is present exclusively in the kidney, in the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Both NKCC isoforms represent part of a diverse family of cation-chloride cotransport proteins that share a common predicted membrane topology; this family also includes Na-Cl cotransporters and multiple K-Cl cotransporter isoforms. In secretory epithelia, the regulation of NKCC1, which is typically present on the basolateral membrane, is tightly coordinated with that of other transporters, including apical Cl channels, to maintain cell volume and integrity during active salt and fluid secretion. Changes in intracellular [Cl] ([Cl]i) appear to be involved in this regulation of NKCC1, which is directly phosphorylated by an unknown protein kinase in response to various secretagogues as well as reductions in [Cl]i and cell volume. This review focuses on structure-function relationships within NKCC1 and on recent developments pertaining to NKCC1 regulation at cellular and molecular levels.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Characterization of a phosphorylation event resulting in upregulation of the salivary Na(+)-K(+)-2Cl(-) cotransporter

Previous studies from our laboratory have shown a close correlation between increased Na(+)-K(+)-2Cl(-) cotransporter activity and increased cotransporter phosphorylation after beta-adrenergic stimulation of rat parotid acinar cells. We demonstrate here that these effects are paralleled by an increase in the number of high-affinity binding sites for the cotransporter inhibitor bumetanide in membranes prepared from stimulated acini. We also show that the sensitivity of cotransporter fluxes to inhibition by bumetanide is the same in both resting and isoproterenol-stimulated cells, consistent with the hypothesis that beta-adrenergic stimulation and the accompanying phosphorylation result in the activation of previously quiescent transporters rather than in a change in the properties of already active proteins. In addition, we demonstrate that the increased phosphorylation on the cotransporter resulting from beta-adrenergic stimulation is localized to a 30-kDa phosphopeptide obtained by cyanogen bromide digestion. Immunoprecipitation and Western blotting experiments demonstrate that this peptide is derived from the NH(2)-terminal cytosolic tail of the cotransporter, which surprisingly does not contain the sole protein kinase A consensus site on the molecule.

Intracellular Cl regulates Na-K-Cl cotransport activity in human trabecular meshwork cells

The trabecular meshwork (TM) of the eye plays a central role in modulating intraocular pressure by regulating aqueous humor outflow, although the mechanisms are largely unknown. We and others have shown previously that aqueous humor outflow facility is modulated by conditions that alter TM cell volume. We have also shown that the Na-K-Cl cotransport system is a primary regulator of TM cell volume and that its activity appears to be coordinated with net efflux pathways to maintain steady-state volume. However, the cellular mechanisms that regulate cotransport activity and cell volume in TM cells have yet to be elucidated. The present study was conducted to investigate the hypothesis that intracellular Cl concentration ([Cl]i) acts to regulate TM cell Na-K-Cl cotransport activity, as has been shown previously for some other cell types. We demonstrate here that the human TM cell Na-K-Cl cotransporter is highly sensitive to changes in [Cl]i. Our findings reveal a marked stimulation of Na-K-Cl cotransport activity, assessed as ouabain-insensitive, bumetanide-sensitive K influx, in TM cells following preincubation of cells with Cl-free medium as a means of reducing [Cl]i. In contrast, preincubation of cells with media containing elevated K concentrations as a means of increasing [Cl]i results in inhibition of Na-K-Cl cotransport activity. The effects of reducing [Cl]i, as well as elevating [Cl]i, on Na-K-Cl cotransport activity are concentration dependent. Furthermore, the stimulatory effect of reduced [Cl]i is additive with cell-shrinkage-induced stimulation of the cotransporter. Our studies also show that TM cell Na-K-Cl cotransport activity is altered by a variety of Cl channel modulators, presumably through changes in [Cl]i. These findings support the hypothesis that regulation of Na-K-Cl cotransport activity, and thus cell volume, by [Cl]i may participate in modulating outflow facility across the TM.

Effects of UTP on Na+, Cl- and K+ transport in primary cultures from human sweat gland coils

Extracellular ATP and UTP can increase membrane permeability in the sweat gland, but the intracellular signalling regulating the response to these agonists is poorly understood. Stimulation of Cl- transport by nucleotides has been suggested as a pharmacological therapy to improve Cl- secretion in patients with cystic fibrosis. In the present study, regulation of Na+, Cl- and K+ transport in primary cultures of cells from the secretory coil of human sweat glands was investigated by electron probe X-ray microanalysis. Stimulation with 200 microM UTP for 2 min at room temperature caused a significant increase in intracellular Na but did not affect Cl and K. After 5 min, the Na concentration was still increased, but now also a significant decrease in Cl and K was observed, indicating an increase in Cl- and K+ permeability. The effect of UTP on Cl- secretion was enhanced in Mg2+-deficient buffer, indicating that the response is elicited by the extracellular fully ionized form of UTP (UTP4+), but not by MgUTP2+. The effects of UTP were abolished in Ca2+-deficient buffer supplemented with EGTA. Alloxan, an adenylate cyclase inhibitor, did not inhibit the response to UTP. These results indicate that the membrane Cl- and K+ permeability elicited by UTP in primary coil cell cultures is Ca2+-dependent. The response to UTP did not attenuate at 8 degrees C, suggesting that it could be activated, in part, via ligand-gated ion channels. The effect of UTP was not decreased in the presence of ouabain. Pre-treatment of the cells with pertussis toxin (24 h) had minor effects on Cl- secretion activated by UTP, indicating a role for G proteins in the UTP activation of Cl- secretion.

Functional interaction of the K-Cl cotransporter (KCC1) with the Na-K-Cl cotransporter in HEK-293 cells

We have studied the regulation of the K-Cl cotransporter KCC1 and its functional interaction with the Na-K-Cl cotransporter. K-Cl cotransporter activity was substantially activated in HEK-293 cells overexpressing KCC1 (KCC1-HEK) by hypotonic cell swelling, 50 mM external K, and pretreatment with N-ethylmaleimide (NEM). Bumetanide inhibited 86Rb efflux in KCC1-HEK cells after cell swelling [inhibition constant (Ki) approximately 190 microM] and pretreatment with NEM (Ki approximately 60 microM). Thus regulation of KCC1 is consistent with properties of the red cell K-Cl cotransporter. To investigate functional interactions between K-Cl and Na-K-Cl cotransporters, we studied the relationship between Na-K-Cl cotransporter activation and intracellular Cl concentration ([Cl]i). Without stimulation, KCC1-HEK cells had greater Na-K-Cl cotransporter activity than controls. Endogenous Na-K-Cl cotransporter of KCC1-HEK cells was activated <2-fold by low-Cl hypotonic prestimulation, compared with 10-fold activation in HEK-293 cells and >20-fold activation in cells overexpressing the Na-K-Cl cotransporter (NKCC1-HEK). KCC1-HEK cells had lower resting [Cl]i than HEK-293 cells; cell volume was not different among cell lines. We found a steep relationship between [Cl]i and Na-K-Cl cotransport activity within the physiological range, supporting a primary role for [Cl]i in activation of Na-K-Cl cotransport and in apical-basolateral cross talk in ion-transporting epithelia.

Na-K-2Cl cotransporter gene expression and function during enterocyte differentiation. Modulation of Cl- secretory capacity by butyrate

The basolateral Na-K-2Cl cotransporter (NKCC1) is a key component of the intestinal crypt cell secretory apparatus. Its fate during the transition to absorptive enterocyte and the potential impact of its altered expression on secretory output have not been addressed. In this report, NKCC1 mRNA was found to be expressed in rat jejunal crypt but not villus cells. Butyrate treatment of intestinal epithelial HT29 cells induced a differentiation pattern that recapitulated the rat intestinal crypt-villus axis, with NKCC1 mRNA levels decreasing in a time- and dose-dependent fashion in parallel with upregulation of apical brush-border markers. Butyrate but not acetate or proprionate decreased basal and cAMP-stimulated bumetanide-sensitive K+ (86Rb) uptake in both HT29 cells and the Cl--secreting T84 line. Butyrate markedly decreased transepithelial Cl- secretion in confluent T84 monolayers without effect on cAMP-regulated apical Cl- efflux. We conclude that NKCC1 regulation during enterocyte differentiation occurs at the level of gene expression, and that selective downregulation of NKCC1 gene expression and function by butyrate leads to a profound decrease in transepithelial Cl- secretion. These data emphasize the importance of NKCC1 in determining epithelial secretory capacity and suggest the possibility of modulation of the enterocytic transport phenotype as therapy for diarrheal disorders.

Comparison of Na-K-Cl cotransporters. NKCC1, NKCC2, and the HEK cell Na-L-Cl cotransporter

The Na-K-Cl cotransporter (NKCC) mediates the coupled movement of ions into most animal cells, playing important roles in maintenance of cell volume and in epithelial Cl transport. Two forms of NKCC have been described: NKCC1, the "housekeeping" isoform that is also responsible for Cl accumulation in secretory epithelial cells, and NKCC2, which mediates apical Na+K+Cl entry into renal epithelial cells. Here we examine the kinetic properties of NKCC1, NKCC2, and the endogenous HEK-293 cell cotransporter. Stable expression of rabbit NKCC2A was obtained in HEK-293 cells utilizing a chimera (h1r2A0.7) in which the 5'-untranslated region and cDNA encoding 104 amino acids of the N terminus are replaced by the corresponding sequence of NKCC1. h1r2A0.7 exhibits Na and Cl affinities near those of NKCC1, but it has a 4-fold lower Rb affinity, and a 3-fold higher affinity for the inhibitor bumetanide. The activity of h1r2A0.7 is increased on incubation in low [Cl] media as is NKCC1, but the resting level of activity is higher in h1r2A0.7 and activation is more rapid. h1r2A0.7 exhibits an appropriate volume response, unlike NKCC1 for which concomitant changes in [Cl]i appear to be the overriding factor. These results support a model in which apical NKCC2 activity is matched to basolateral Cl exit through changes in [Cl]i. Reverse transcriptase-polymerase chain reaction of HEK-293 cell mRNA is positive with NKCC1 primers and negative with NKCC2 primers. Surprisingly, we found that the behavior of the endogenous HEK cell Na-K-Cl cotransporter is unlike either of the two forms which have been described: compared with NKCC1, HEK cell cotransporter has a 2.5-fold lower Na affinity, an 8-fold lower Rb affinity, and a 4-fold higher bumetanide affinity. These results suggest the presence of a novel isoform of NKCC in HEK-293 cells.

The Na-K-Cl cotransporters

The Na-K-Cl cotransporters are a class of membrane proteins that transport Na, K, and Cl ions into and out of a wide variety of epithelial and nonepithelial cells. The transport process mediated by Na-K-Cl cotransporters is characterized by electroneutrality (almost always with stoichiometry of 1Na:1K:2Cl) and inhibition by the "loop" diuretics bumetanide, benzmetanide, and furosemide. Presently, two distinct Na-K-Cl cotransporter isoforms have been identified by cDNA cloning and expression; genes encoding these two isoforms are located on different chromosomes and their gene products share approximately 60% amino acid sequence identity. The NKCC1 (CCC1, BSC2) isoform is present in a wide variety of tissues; most epithelia containing NKCC1 are secretory epithelia with the Na-K-Cl cotransporter localized to the basolateral membrane. By contrast, NKCC2 (CCC2, BSC1) is found only in the kidney, localized to the apical membrane of the epithelial cells of the thick ascending limb of Henle's loop and of the macula densa. Mutations in the NKCC2 gene result in Bartter's syndrome, an inherited disease characterized by hypokalemic metabolic alkalosis, hypercalciuria, salt wasting, and volume depletion. The two Na-K-Cl cotransporter isoforms are also part of a superfamily of cation-chloride cotransporters, which includes electroneutral K-Cl and Na-Cl cotransporters. Na-K-Cl cotransporter activity is affected by a large variety of hormonal stimuli as well as by changes in cell volume; in many tissues this regulation (particularly of the NKCCI isoform) occurs through direct phosphorylation/dephosphorylation of the cotransport protein itself though the specific protein kinases involved remain unknown. An important regulator of cotransporter activity in secretory epithelia and other cells as well is intracellular [Cl] ([Cl]i), with a reduction in [Cl]i being the apparent means by which basolateral Na-K-Cl cotransport activity is increased and thus coordinated with that of stimulated apical Cl channels in actively secreting epithelia.

Osmotic regulation of intestinal epithelial Na(+)-K(+)-Cl- cotransport: role of Cl- and F-actin

Previous data indicate that adenosine 3',5'-cyclic monophosphate activates the epithelial basolateral Na(+)-K(+)-Cl- cotransporter in microfilament-dependent fashion in part by direct action but also in response to apical Cl- loss (due to cell shrinkage or decreased intracellular Cl-). To further address the actin dependence of Na(+)-K(+)-Cl- cotransport, human epithelial T84 monolayers were exposed to anisotonicity, and isotopic flux analysis was performed. Na(+)-K(+)-Cl- cotransport was activated by hypertonicity induced by added mannitol but not added NaCl. Cotransport was also markedly activated by hypotonic stress, a response that appeared to be due in part to reduction of extracellular Cl- concentration and also to activation of K+ and Cl- efflux pathways. Stabilization of actin with phalloidin blunted cotransporter activation by hypotonicity and abolished hypotonic activation of K+ and Cl- efflux. However, phalloidin did not prevent activation of cotransport by hypertonicity or isosmotic reduction of extracellular Cl-. Conversely, hypertonic but not hypotonic activation was attenuated by the microfilament disassembler cytochalasin D. The results emphasize the complex interrelationship among intracellular Cl- activity, cell volume, and the actin cytoskeleton in the regulation of epithelial Cl- transport.

Use of artificial ion channels for quasi-intracellular recording of cerebral cortex neuron activity

Action potentials and synaptic potentials were recorded in vivo from cortical neurons in baby rats aged 20-25 days using a new method based on the ionophore nystatin. Nystatin solution was used to fill a standard glass extracellular microelectrode, and became inserted into membranes. Spikes which were initially recorded as extracellular spikes showed increases in amplitude and were transformed into unipolar quasi-intracellular spikes at 0.5-5.0 min after formation of the high-conductance contact. This method allows stable recording of neuronal activity from cells for at least 1 h, and provides a good signal-to-noise ratio. The electrode does not puncture the neuron membrane, with the result that experiments do not require any isolation from vibration. Thus, the results obtained demonstrate that the method is highly efficient for recording the in vivo activity of small nerve cells.

Antisense oligodeoxynucleotide to PKC-delta blocks alpha 1-adrenergic activation of Na-K-2Cl cotransport

A role for protein kinase C (PKC)-delta and -zeta isotypes in alpha 1-adrenergic regulation of human tracheal epithelial Na-K-2Cl cotransport was studied with the use of isotype-specific PKC inhibitors and antisense oligodeoxy-nucleotides to PKC-delta or -zeta mRNA. Rottlerin, a PKC-delta inhibitor, blocked 72% of basolateral-to-apical, bumetanide-sensitive 36Cl flux in nystatin-permeabilized cell monolayers stimulated with methoxamine, an alpha 1-adrenergic agonist, with a 50% inhibitory concentration of 2.3 microM. Methoxamine increased PKC activity in cytosol and a particulate fraction; the response was insensitive to PKC-alpha and -beta II isotype-specific inhibitors, but was blocked by general PKC inhibitors and rottlerin. Rottlerin also inhibited methoxamine-induced PKC activity in immune complexes of PKC-delta, but not PKC-zeta. At the subcellular level, methoxamine selectively elevated cytosolic PKC-delta activity and particulate PKC-zeta activity. Pretreatment of cell monolayers with antisense oligodeoxynucleotide to PKC-delta for 48 h reduced the amount of whole cell and cytosolic PKC-delta, diminished whole cell and cytosolic PKC-delta activity, and blocked methoxamine-stimulated Na-K-2Cl cotransport. Sense oligodeoxynucleotide to PKC-delta and antisense oligodeoxynucleotide to PKC-zeta did not alter methoxamine-induced cotransport activity. These results demonstrate the selective activation of Na-K-2Cl cotransport by cytosolic PKC-delta.

Characteristics of membrane transport processes of macula densa cells

1. Macula densa (MD) cells are located within the thick ascending limb (TAL) and have their apical surface in contact with tubular fluid and their basilar region in contact with the glomerulus. These cells sense changes in luminal fluid sodium chloride concentration ([NaCl]) and transmit signals resulting in changes in vascular resistance (tubuloglomerular feedback) and renin release. 2. Current efforts have focused on understanding the cellular transport mechanisms of MD cells. Progress in this area has benefited from the use of the isolated perfused TAL-glomerular preparation, which permits direct access to MD cells. 3. Using microelectrodes to measure basolateral membrane potential (VBL) of MD cells, it was found that VBL was very sensitive to changes in luminal fluid [NaCl]. As [NaCl] was elevated from 20 to 150 mmol/L, VBL was found to depolarize by over 30 mV. 4. Basolateral membrane potential measurements were also used to identify an apical Na+:2Cl-:K+ cotransport pathway in MD cells that is the major pathway for NaCl entry into these cells. 5. Other work identified a basolateral chloride channel that is presumed to be responsible for changes in VBL during alterations in luminal [NaCl]. This channel, which is the predominant conductance across the basolateral membrane, may be regulated by intracellular Ca2+ and cAMP. 6. An apical Na+:H+ exchanger in MD cells was detected by measuring changes in intracellular pH using the fluorescent probe 2',7'-bis-(2-carboxyethyl)-5(and-6) carboxyfluorescein. 7. Using patch-clamp techniques, a high density of pH- and Ca(2+)-sensitive K+ channels was observed at the apical membrane of MD cells. 8. Other studies found that, at the normal physiological conditions prevailing at the end of the TAL (luminal [NaCl] of 20-60 mmol/L), reabsorption mediated by MD cells is very sensitive to changes in luminal [NaCl].

Upregulation of Na(+)-K(+)-2Cl- cotransporter activity in rat parotid acinar cells by muscarinic stimulation

1. The effects of fluid secretory stimuli on the Na(+)-K(+)-2Cl- cotransporter in rat parotid acini were investigated. Cotransporter activity was measured using NH4+ as a K+ surrogate and following cotransporter-mediated NH4+ fluxes by monitoring intracellular pH. 2. A dramatic upregulation (15- to 20-fold) of acinar Na(+)-K(+)-2Cl- cotransporter activity was induced by muscarinic, alpha 1-adrenergic and peptidergic stimuli. A half-maximal effect of the muscarinic agonist carbachol was observed at approximately 0.5 microM. 3. Our results indicate that the rise in intracellular calcium concentration ([Ca2+]i) which accompanies these stimuli is both a necessary and a sufficient condition for this effect; but it is not a consequence of the KCl loss and concomitant isotonic shrinkage caused by increased [Ca2+]i as it persists when these effects are prevented. 4. The effect of muscarinic stimulation on the cotransporter can, however, be blocked by inhibitors of phospholipase A2 (4-bromophenacylbromide and manoalide), by a general inhibitor of arachidonic acid metabolism (5,8,11,14-eicosatetraynoic acid) and by specific inhibitors of the cytochrome P450 pathway (methoxsalen and ketoconazole). 5. These latter results argue strongly for the involvement of a product of the cytochrome P450 pathway of arachidonic acid metabolism in upregulation of the salivary Na(+)-K(+)-2Cl- cotransporter. 6. Owing to the complexity of the arachidonic acid cascade a wide variety of agents could potentially interfere with this upregulation of the cotransporter, and thereby result in decreased salivary fluid production. We suggest that such an effect could underlie the dry mouth (xerostomia) that occurs as an unexplained side-effect of many commonly prescribed medications.

Chloride accumulation in freshly isolated Ehrlich ascites tumor cells: the role of the Na/K/2Cl cotransporter

When Ehrlich ascites tumor cells are removed from the peritoneal cavity and incubated in a saline solution, cells lose water, sodium, lactate and hydrogen ions and gain chloride. The gain of intracellular chloride exceeds that predicted from passive distribution. As chloride has been purported to play a role in volume regulation, it was of interest to identify factors responsible for controlling or maintaining intracellular chloride out of electrochemical equilibrium in Ehrlich cells. The results demonstrate that chloride accumulation in freshly isolated Ehrlich cells is sensitive to bumetanide, low extracellular K+ and low extracellular Na+, and is insensitive to DIDS. We conclude that chloride accumulation occurs due to the activity of the Na/K/2Cl cotransporter.

Elevated [Cl-]i, and [Na+]i inhibit Na+, K+, Cl- cotransport by different mechanisms in squid giant axons

Bumetanide-sensitive (BS) unidirectional fluxes of (36)Cl- or (22)Na+ were measured in internally dialyzed squid giant axons while varying the intra- or extracellular concentrations of Na+ and/or Cl-. Raising either [Cl-]i or [Na+]i resulted in a concentration-dependent reduction of the BS influx of both (36)Cl- and (22)Na+. Raising [Cl-]i above 200 mM completely blocked BS influxes. However, raising [Na+]i to 290 mM resulted in saturable but incomplete inhibition of both BS Na+ influx and BS Cl- influx. The consequences of varying intracellular Cl- on cotransporter effluxes were complex. At lower [Cl-]i values (below 100 mM) intracellular Cl- activated cotransporter effluxes. Surprisingly, however, raising [Cl-]i levels > 125 mM resulted in a [Cl-]i-dependent inhibition of BS effluxes of both Na+ and Cl-. On the other hand, raising [Na+]i resulted only in the activation of the BS Na+ efflux; intracellular Na+ did not inhibit BS efflux even at 290 mM. The inhibitory effects of intracellular Na+ on cotransporter-mediated influxes, and lack of inhibitory effects on BS effluxes, are consistent with the trans-side inhibition expected for an ordered binding/release model of cotransporter operation. However, the inhibitory effects of intracellular Cl- on both influxes and effluxes are not explained by such a model. These data suggest that Cl may interact with an intracellular site (or sites), which does not mediate Cl transport, but does modulate the transport activity of the Na+, K+, Cl- cotransporter.

[Cl-]i-dependent phosphorylation of the Na-K-Cl cotransport protein of dog tracheal epithelial cells

Basolateral Na-K-Cl cotransport activity in primary cultures of dog tracheal epithelial cells is stimulated by beta-adrenergic agents, such as isoproterenol, and by apical UTP, which acts through an apical P2-purinergic receptor. While at least part of the stimulatory effect of isoproterenol appears to involve direct activation of the cotransporter via cAMP-dependent protein kinase, cotransport stimulation by apical UTP is entirely secondary to apical Cl- efflux and a resultant decrease in intracellular [Cl-] ([Cl-]i) and/or cell shrinkage (Haas, M., and McBrayer, D. G. (1994) Am. J. Physiol. 266, C1440-C1452). In the secretory epithelia of the shark rectal gland and avian salt gland, Na-K-Cl cotransport activation by both cAMP-dependent and cAMP-independent secretagogues has been shown to be accompanied by phosphorylation of the cotransport protein itself (Lytle, C., and Forbush, B., III (1992) J. Biol. Chem. 267, 25438-25443; Torchia, J., Lytle, C., Pon, D. J., Forbush, B., III, and Sen, A. K. (1992) J. Biol. Chem. 267, 25444-25450). In the present study, we immunoprecipitate the approximately 170-kDa Na-K-Cl cotransport protein of dog tracheal epithelial cells with a monoclonal antibody against the cotransporter of the intestinal cell line T84. Incubation of confluent primary cultures of tracheal cells with isoproterenol and apical UTP increases basolateral-to-apical 36Cl- flux 3.4- and 2.6-fold, respectively, and produces similar increases (3.2- and 2.8-fold, respectively) in 32P incorporation into the approximately 170-kDa cotransport protein. Decreasing [Cl-]i (without concomitant cell shrinkage) by incubating cultures with apical nystatin and reduced apical [Cl-] ([Cl-]alpha) likewise increases both cotransport activity and cotransport protein phosphorylation. These effects become more pronounced with greater reductions in [Cl-]alpha; after 20 min of incubation with nystatin and 32 mM [Cl-]alpha, cotransport activity and 32P incorporation into the cotransport protein are increased 2.8- and 2.7-fold, respectively, similar to increases seen with apical UTP. 2-3-fold increases in cotransporter activity and phosphorylation are also seen in nystatin-treated cells under hypertonic conditions (50 mM sucrose added apically and basolaterally). These findings suggest a close correlation between Na-K-Cl cotransport activity and phosphorylation of the approximately 170-kDa cotransport protein. The latter is phosphorylated in response to both reduced [Cl-]i and cell shrinkage, either or both of which are likely to be involved in secondary cotransport activation in response to apical UTP.

Molecular characterization of the epithelial Na-K-Cl cotransporter isoforms

Recent advances in the molecular characterization of specific isoforms of the Na-K-Cl cotransporter have allowed rapid progress in the study of the structure, function, and regulation of these members of a family of Cl-dependent cation cotransporters. Two distinct isoforms have been identified, one from Cl(-)-secretory epithelia and another found specifically in the diluting segment of the vertebrate kidney, a Cl(-)-absorptive epithelium. The discovery of three alternatively spliced variants of the absorptive isoform, which differ only by 31 amino acids and which appear to be differentially distributed within the mammalian thick ascending limb of the loop of Henle, highlight this spliced region as an important functional component of the protein.

Primary structure, functional expression, and chromosomal localization of the bumetanide-sensitive Na-K-Cl cotransporter in human colon

By moving chloride into epithelial cells, the Na-K-Cl cotransporter aids transcellular movement of chloride across both secretory and absorptive epithelia. Using cDNA probes from the recently identified elasmobranch secretory Na-K-Cl cotransporter (sNKCC1) (Xu, J. C., Lytle, C. Zhu, T. T., Payne, J. A., Benz, E., and Forbush, B., III (1994) Proc. Natl. Acad. Sci. 91, 2201-2205), we have identified the human homologue. By screening cDNA libraries of a human colonic carcinoma line, T84 cell, we identified a sequence of 4115 bases from overlapping clones. The deduced protein is 1212 amino acids in length, and analysis of the primary structure indicates 12 transmembrane segments. The primary structure is 74% identical to sNKCC1, 91% identical to a mouse Na-K-Cl cotransporter (mNKCC1), 58% identical to rabbit and rat renal Na-K-Cl cotransporters (NKCC2), and 43% identical to the thiazide-sensitive Na-Cl cotransporters from flounder urinary bladder and rat kidney. Similar to sNKCC1 and mNKCC1, the 5'-end of the human colonic cotransporter is rich in G + C content. Interestingly, a triple repeat (GCG)7 occurs within the 5'-coding region and contributes to a large alanine repeat (Ala15). Two sites for N-linked glycosylation are predicted on an extracellular loop between putative transmembrane segments 7 and 8. A single potential site for phosphorylation by protein kinase A is present in the predicted cytoplasmic C-terminal domain. Northern blot analysis revealed a 7.4-7.5-kilobase transcript in T84 cells and shark rectal gland and a approximately 7.2-kilobase transcript in mammalian colon, kidney, lung, and stomach. Metaphase spreads from lymphocytes were probed with biotin-labeled cDNA and avidin fluorescein (the cotransporter gene was localized to human chromosome 5 at position 5q23.3). Human embryonic kidney cells stably transfected with the full-length cDNA expressed a approximately 170-kDa protein recognized by anti-cotransporter antibodies. Following treatment with N-glycosidase F, the molecular mass of the expressed protein was similar to that predicted for the core protein from the cDNA sequence (132-kDa) and identical to that of deglycosylated T84 cotransporter (approximately 135-kDa). The stably transfected cells exhibited a approximately 15-fold greater bumetanide-sensitive 86Rb influx than control cells, and this flux required external sodium and chloride. Flux kinetics were consistent with an electroneutral cotransport of 1Na:1K:2Cl. Preincubation in chloride-free media was necessary to activate fully the expressed cotransporter, suggesting a [Cl]-dependent regulatory mechanism.

Molecular mechanisms for passive and active transport of water

Water crosses cell membranes by passive transport and by secondary active cotransport along with ions. While the first concept is well established, the second is new. The two modes of transport allow cellular H2O homeostasis to be viewed as a balance between H2O leaks and H2O pumps. Consequently, cells can be hyperosmolar relative to their surroundings during steady states. Under physiological conditions, cells from leaky epithelia may be hyperosmolar by roughly 5 mosm liter-1, under dilute conditions, hyperosmolarities up to 40 mosm liter-1 have been recorded. Most intracellular H2O is free to serve as solvent for small inorganic ions. The mechanism of transport across the membrane depends on how H2O interacts with the proteinaceous or lipoid pathways. Osmotic transport of H2O through specific H2O channels such as CHIP 28 is hydraulic if the pore is impermeable to the solute and diffusive if the pore is permeable. Cotransport of ions and H2O can be a result of conformational changes in proteins, which in addition to ion transport also translocate H2O bound to or occlude in the protein. A cellular model of a leaky epithelium based on H2O leaks and H2O pumps quantitatively predicts a number of so-far unexplained observations of H2O transport.