Swelling-activated K fluxes in vascular endothelial cells: volume regulation via K-Cl cotransport and K channels

A Novel Cell Volume Sensor for Real-Time Analysis of Ca2+-Activated K+ Channel

Potassium channels play a vital role in cell volume regulation. A cell volume sensor was constructed by integrating regulatory volume decrease (RVD) with quartz-crystal microbalance (QCM) for studying potassium channels and their expression. The sensor successfully monitored the K+ channel's activities during RVD by sensitive and noninvasive means. It showed that Ca2+ activated the K+ channel (KCa) and enhanced the RVD level. The inhibition of blockers on K+ channels exhibited an obvious difference in RVD level between normal and cancerous nasopharyngeal cells, suggesting that the KCa channel contributes a dominant role to the RVD function and provides an approach to identify the activation of various K+ channels.

Early Endothelial Signaling Transduction in Developing Lung Edema

The lung promptly responds to edemagenic conditions through functional adaptations that contrast the increase in microvascular filtration. This review presents evidence for early signaling transduction by endothelial lung cells in two experimental animal models of edema, hypoxia exposure, and fluid overload (hydraulic edema). The potential role of specialized sites of the plasma membranes considered mobile signaling platforms, referred to as membrane rafts, that include caveolae and lipid rafts, is presented. The hypothesis is put forward that early changes in the lipid composition of the bilayer of the plasma membrane might trigger the signal transduction process when facing changes in the pericellular microenvironment caused by edema. Evidence is provided that for an increase in the extravascular lung water volume not exceeding 10%, changes in the composition of the plasma membrane of endothelial cells are evoked in response to mechanical stimuli from the interstitial compartment as well as chemical stimuli relating with changes in the concentration of the disassembled portions of structural macromolecules. In hypoxia, thinning of endothelial cells, a decrease in caveolae and AQP-1, and an increase in lipid rafts are observed. The interpretation of this response is that it favors oxygen diffusion and hinder trans-cellular water fluxes. In hydraulic edema, which generates greater capillary water leakages, an increase in cell volume and opposite changes in membrane rafts were observed; further, the remarkable increase in caveolae suggests a potential abluminal-luminal vesicular-dependent fluid reabsorption.

Role of the endothelial reverse mode sodium-calcium exchanger in the dilation of the rat middle cerebral artery during hypoosmotic hyponatremia

A decrease in serum sodium ion concentration below 135 mmol L^-1 is usually accompanied by a decrease in plasma osmolality (hypoosmotic hyponatremia) and leads to the disorder of intracranial homeostasis mainly due to cellular swelling. Recently, using an in vitro model of hypoosmotic hyponatremia, we have found that a decrease in sodium ion concentration in the perfusate to 121 mmol L^-1 relaxes the isolated rat middle cerebral artery (MCA). The aim of the present study was to explore the mechanism responsible for this relaxation. Isolated, pressurized, and perfused MCAs placed in a vessel chamber were subjected to a decrease in sodium ion concentration to 121 mmol L^-1. Changes in the diameter of the vessels were monitored with a video camera. The removal of the endothelium and inhibition of nitric oxide-dependent signaling or the reverse mode sodium-calcium exchanger (NCX) were used to study the mechanism of the dilation of the vessel during hyponatremia. The dilation of the MCA (19 ± 5%, p < 0.005) in a low-sodium buffer was absent after removal of the endothelium or administration of the inhibitor of the reverse mode of sodium-calcium exchange and was reversed to constriction after the inhibition of nitric oxide (NO)/cGMP signaling. The dilation of the middle cerebral artery of the rat in a 121 mmol L^-1 Na⁺ buffer depends on NO signaling and reverse mode of sodium-calcium exchange. These results suggest that constriction of large cerebral arteries with impaired NO-dependent signaling may be observed in response to hypoosmotic hyponatremia.

Aldosterone up-regulates voltage-gated potassium currents and NKCC1 protein membrane fractions

Na+-K+-2Cl- Cotransporter (NKCC1) is a protein that aids in the active transport of sodium, potassium, and chloride ions across cell membranes. It has been shown that long-term systemic treatment with aldosterone (ALD) can enhance NKCC1 protein expression and activity in the aging cochlea resulting in improved hearing. In the present work, we used a cell line with confirmed NKCC1 expression to demonstrate that in vitro application of ALD increased outward voltage-gated potassium currents significantly, and simultaneously upregulated whole lysate and membrane portion NKCC1 protein expression. These ALD-induced changes were blocked by applying the mineralocorticoid receptor antagonist eplerenone. However, application of the NKCC1 inhibitor bumetanide or the potassium channel antagonist Tetraethyl ammonium had no effect. In addition, NKKC1 mRNA levels remained stable, indicating that ALD modulates NKCC1 protein expression via the activation of mineralocorticoid receptors and post-transcriptional modifications. Further, in vitro electrophysiology experiments, with ALD in the presence of NKCC1, K+ channel and mineralocorticoid receptor inhibitors, revealed interactions between NKCC1 and outward K+ channels, mediated by a mineralocorticoid receptor-ALD complex. These results provide evidence of the therapeutic potential of ALD for the prevention/treatment of inner ear disorders such as age-related hearing loss.

K+-Cl- cotransporter 1 (KCC1): a housekeeping membrane protein that plays key supplemental roles in hematopoietic and cancer cells

During the 1970s, a Na+-independent, ouabain-insensitive, N-ethylmaleimide-stimulated K+-Cl- cotransport mechanism was identified in red blood cells for the first time and in a variety of cell types afterward. During and just after the mid-1990s, three closely related isoforms were shown to account for this mechanism. They were termed K+-Cl- cotransporter 1 (KCC1), KCC3, and KCC4 according to the nomenclature of Gillen et al. (1996) who had been the first research group to uncover the molecular identity of a KCC, that is, of KCC1 in rabbit kidney. Since then, KCC1 has been found to be the most widely distributed KCC isoform and considered to act as a housekeeping membrane protein. It has perhaps received less attention than the other isoforms for this reason, but as will be discussed in the following review, there is probably more to KCC1 than meets the eye. In particular, the so-called housekeeping gene also appears to play crucial and specific roles in normal as well as pathological hematopoietic and in cancer cells.

Arachidonic Acid Activates K-Cl-cotransport in HepG2 Human Hepatoblastoma Cells

K(+)-Cl(-)-cotransport (KCC) has been reported to have various cellular functions, including proliferation and apoptosis of human cancer cells. However, the signal transduction pathways that control the activity of KCC are currently not well understood. In this study we investigated the possible role of phospholipase A(2) (PLA(2))-arachidonic acid (AA) signal in the regulatory mechanism of KCC activity. Exogenous application of AA significantly induced K(+) efflux in a dose-dependent manner, which was completely blocked by R-(+)-[2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden-5-yl]oxy]acetic acid (DIOA), a specific KCC inhibitor. N-Ethylmaleimide (NEM), a KCC activator-induced K(+) efflux was significantly suppressed by bromoenol lactone (BEL), an inhibitor of the calcium-independent PLA(2) (iPLA(2)), whereas it was not significantly altered by arachidonyl trifluoromethylketone (AACOCF(3)) and p-bromophenacyl bromide (BPB), inhibitors of the calcium-dependent cytosolic PLA(2) (cPLA(2)) and the secretory PLA(2) (sPLA(2)), respectively. NEM increased AA liberation in a dose- and time-dependent manner, which was markedly prevented only by BEL. In addition, the NEM-induced ROS generation was significantly reduced by DPI and BEL, whereas AACOCF(3) and BPB did not have an influence. The NEM-induced KCC activation and ROS production was not significantly affected by treatment with indomethacin (Indo) and nordihydroguaiaretic acid (NDGA), selective inhibitors of cyclooxygenase (COX) and lipoxygenase (LOX), respectively. Treatment with 5,8,11,14-eicosatetraynoic acid (ETYA), a non-metabolizable analogue of AA, markedly produced ROS and activated the KCC. Collectively, these results suggest that iPLA(2)-AA signal may be essentially involved in the mechanism of ROS-mediated KCC activation in HepG2 cells.

K-Cl cotransport function and its potential contribution to cardiovascular disease

K-Cl cotransport is the coupled electroneutral movement of K and Cl ions carried out by at least four protein isoforms, KCC1-4. These transporters belong to the SLC12A family of coupled cotransporters and, due to their multiple functions, play an important role in the maintenance of cellular homeostasis. Significant information exists on the overall function of these transporters, but less is known about the role of the specific isoforms. Most functional studies were done on K-Cl cotransport fluxes without knowing the molecular details, and only recently attention has been paid to the isoforms and their individual contribution to the fluxes. This review summarizes briefly and updates the information on the overall functions of this transporter, and offers some ideas on its potential contribution to the pathophysiological basis of cardiovascular disease. By virtue of its properties and the cellular ionic distribution, K-Cl cotransport participates in volume regulation of the nucleated and some enucleated cells studied thus far. One of the hallmarks in cardiovascular disease is the inability of the organism to maintain water and electrolyte balance in effectors and/or target tissues. Oxidative stress is another compounding factor in cardiovascular disease and of great significance in our modern life styles. Several functions of the transporter are modulated by oxidative stress, which in turn may cause the transporter to operate in either "overdrive" with the purpose to counteract homeostatic changes, or not to respond at all, again setting the stage for pathological changes leading to cardiovascular disease. Intracellular Mg, a second messenger, acts as an inhibitor of K-Cl cotransport and plays a crucial role in regulating the activity of protein kinases and phosphatases, which, in turn, regulate a myriad of cellular functions. Although the role of Mg in cardiovascular disease has been dealt with for several decades, this chapter is evolving nowadays at a faster pace and the relationships between Mg, K-Cl cotransport, and cardiovascular disease is an area that awaits further experimentation. We envision that further studies on the role of K-Cl cotransport, and ideally on its specific isoforms, in mammalian cells will add missing links and help to understand the cellular mechanisms involved in the pathophysiology of cardiovascular disease.

Reduced hyperpolarization in endothelial cells of rabbit aortic valve following chronic nitroglycerine administration

This study was undertaken to determine whether long-term in vivo administration of nitroglycerine (NTG) downregulates the hyperpolarization induced by acetylcholine (ACh) in aortic valve endothelial cells (AVECs) of the rabbit and, if so, whether antioxidant agents can normalize this downregulated hyperpolarization. ACh (0.03-3 microM) induced a hyperpolarization through activations of both apamin- and charybdotoxin-sensitive Ca2+-activated K+ channels (K(Ca)) in rabbit AVECs. The intermediate-conductance K(Ca) channel (IK(Ca)) activator 1-ethyl-2-benzimidazolinone (1-EBIO, 0.3 mM) induced a hyperpolarization of the same magnitude as ACh (3 microM). The ACh-induced hyperpolarization was significantly weaker, although the ACh-induced [Ca2+]i increase was unchanged, in NTG-treated rabbits (versus NTG-untreated control rabbits). The hyperpolarization induced by 1-EBIO was also weaker in NTG-treated rabbits. The reduced ACh-induced hyperpolarization seen in NTG-treated rabbits was not modified by in vitro application of the superoxide scavengers Mn-TBAP, tiron or ascorbate, but it was normalized when ascorbate was coadministered with NTG in vivo. Superoxide production within the endothelial cell (estimated by ethidium fluorescence) was increased in NTG-treated rabbits and this increased production was normalized by in vivo coadministration of ascorbate with the NTG. It is suggested that long-term in vivo administration of NTG downregulates the ACh-induced hyperpolarization in rabbit AVECs, possibly through chronic actions mediated by superoxide.

Comparison of K-Cl cotransport expression in high and low K dog erythrocytes

K-Cl cotransport plays a crucial role in regulatory volume decrease of erythrocytes. K-Cl cotransport activities in dog erythrocytes with an inherited high Na-K pump activity (HK) and normal erythrocytes (LK) were compared. Nitrite (NO(2)) stimulated K-Cl cotransport activity in HK cells around 14-fold at 2.4 mM, and it also increased the Km value of this cotransporter. Real-time PCR and western blot analysis revealed that K-Cl cotransporter 1 was dominant, and that the quantity of K-Cl cotransporter 1 protein was comparable between HK and LK erythrocytes. These results suggest that the difference in cotransport activity was not caused by the amount of K-Cl cotransport protein but by a difference in the regulation system, which is susceptible to oxidant.

NH2-terminal heterogeneity in the KCC3 K+-Cl− cotransporter

The SLC12A6 gene encoding the K+-Cl− cotransporter KCC3 is expressed in multiple tissues, including kidney. Here, we report the molecular characterization of several NH2-terminal isoforms of human and mouse KCC3, along with intrarenal localization and functional characterization in Xenopus laevis oocytes. Two major isoforms, KCC3a and KCC3b, are generated by transcriptional initiation 5′ of two distinct first coding exons. Northern blot analysis of mouse tissues indicates that KCC3b expression is particularly robust in the kidney, which also expresses KCC3a. Western blotting of mouse tissue using an exon 3-specific antibody reveals that the kidney is also unique in expressing immunoreactive protein of a lower mass, suggestive evidence that the shorter KCC3b protein predominates in kidney. Immunofluorescence reveals basolateral expression of KCC3 protein along the entire length of the proximal tubule, in both the mouse and rat. Removal of the 15-residue exon 2 by alternative splicing generates the KCC3a-x2M and KCC3b-x2M isoforms; other splicing events at an alternative acceptor site within exon 1a generate the KCC3a-S isoform, which is 60 residues shorter than KCC3a. This variation in sequence of NH2-terminal cytoplasmic domains occurs proximal to a stretch of highly conserved residues and affects the content of putative phosphorylation sites. Kinetic characterization of KCC3a in X. laevis oocytes reveals apparent Kms for Rb+ and Cl− of 10.7 ± 2.5 and 7.3 ± 1.2 mM, respectively, with an anion selectivity of Br− > Cl− > PO4 = I− = SCN− = gluconate. All five NH2-terminal isoforms are activated by cell swelling (hypotonic conditions), with no activity under isotonic conditions. Although the isoforms do not differ in the osmotic set point of swelling activation, this activation is more rapid for the KCC3a-x2M and KCC3a-S proteins. In summary, there is significant NH2-terminal heterogeneity of KCC3, with particularly robust expression of KCC3b in the kidney. Basolateral swelling-activated K+-Cl− cotransport mediated by KCC3 likely functions in cell volume regulation during the transepithelial transport of both salt and solutes by the proximal tubule.

Regulation of K-Cl cotransport: from function to genes

This review intends to summarize the vast literature on K-Cl cotransport (COT) regulation from a functional and genetic viewpoint. Special attention has been given to the signaling pathways involved in the transporter's regulation found in several tissues and cell types, and more specifically, in vascular smooth muscle cells (VSMCs). The number of publications on K-Cl COT has been steadily increasing since its discovery at the beginning of the 1980s, with red blood cells (RBCs) from different species (human, sheep, dog, rabbit, guinea pig, turkey, duck, frog, rat, mouse, fish, and lamprey) being the most studied model. Other tissues/cell types under study are brain, kidney, epithelia, muscle/smooth muscle, tumor cells, heart, liver, insect cells, endothelial cells, bone, platelets, thymocytes and Leishmania donovani. One of the salient properties of K-Cl-COT is its activation by cell swelling and its participation in the recovery of cell volume, a process known as regulatory volume decrease (RVD). Activation by thiol modification with N-ethylmaleimide (NEM) has spawned investigations on the redox dependence of K-Cl COT, and is used as a positive control for the operation of the system in many tissues and cells. The most accepted model of K-Cl COT regulation proposes protein kinases and phosphatases linked in a chain of phosphorylation/dephosphorylation events. More recent studies include regulatory pathways involving the phosphatidyl inositol/protein kinase C (PKC)-mediated pathway for regulation by lithium (Li) in low-K sheep red blood cells (LK SRBCs), and the nitric oxide (NO)/cGMP/protein kinase G (PKG) pathway as well as the platelet-derived growth factor (PDGF)-mediated mechanism in VSMCs. Studies on VSM transfected cells containing the PKG catalytic domain demonstrated the participation of this enzyme in K-Cl COT regulation. Commonly used vasodilators activate K-Cl COT in a dose-dependent manner through the NO/cGMP/PKG pathway. Interaction between the cotransporter and the cytoskeleton appears to depend on the cellular origin and experimental conditions. Pathophysiologically, K-Cl COT is altered in sickle cell anemia and neuropathies, and it has also been proposed to play a role in blood pressure control. Four closely related human genes code for KCCs (KCC1-4). Although considerable information is accumulating on tissue distribution, function and pathologies associated with the different isoforms, little is known about the genetic regulation of the KCC genes in terms of transcriptional and post-transcriptional regulation. A few reports indicate that the NO/cGMP/PKG signaling pathway regulates KCC1 and KCC3 mRNA expression in VSMCs at the post-transcriptional level. However, the detailed mechanisms of post-transcriptional regulation of KCC genes and of regulation of KCC2 and KCC4 mRNA expression are unknown. The K-Cl COT field is expected to expand further over the next decades, as new isoforms and/or regulatory pathways are discovered and its implication in health and disease is revealed.

Molecular Physiology and Pathophysiology of Electroneutral Cation-Chloride Cotransporters

Electroneutral cation-Cl−cotransporters compose a family of solute carriers in which cation (Na+or K+) movement through the plasma membrane is always accompanied by Cl−in a 1:1 stoichiometry. Seven well-characterized members include one gene encoding the thiazide-sensitive Na+−Cl−cotransporter, two genes encoding loop diuretic-sensitive Na+−K+−2Cl−cotransporters, and four genes encoding K+−Cl−cotransporters. These membrane proteins are involved in several physiological activities including transepithelial ion absorption and secretion, cell volume regulation, and setting intracellular Cl−concentration below or above its electrochemical potential equilibrium. In addition, members of this family play an important role in cardiovascular and neuronal pharmacology and pathophysiology. Some of these cotransporters serve as targets for loop diuretics and thiazide-type diuretics, which are among the most commonly prescribed drugs in the world, and inactivating mutations of three members of the family cause inherited diseases such as Bartter's, Gitelman's, and Anderman's diseases. Major advances have been made in the past decade as consequences of molecular identification of all members in this family. This work is a comprehensive review of the knowledge that has evolved in this area and includes molecular biology of each gene, functional properties of identified cotransporters, structure-function relationships, and physiological and pathophysiological roles of each cotransporter.

Role of reactive oxygen species in apoptosis induced by N-ethylmaleimide in HepG2 human hepatoblastoma cells

We have previously reported that N-ethylmaleimide induces apoptosis through activation of K(+), Cl(-)-cotransport in HepG2 human hepatoblastoma cells. In this study, we investigated the role for reactive oxygen species as a mediator of the apoptosis induced by N-ethylmaleimide. N-ethylmaleimide induced a significant elevation of intracellular level of reactive oxygen species. Treatment with antioxidants (N-acetyl cysteine, N,N'-diphenyl-p-phenylenediamine) which markedly suppressed generation of reactive oxygen species, significantly inhibited the N-ethylmaleimide-induced activation of K(+), Cl(-)-cotransport and apoptosis. Inhibitors of NADPH oxidase (diphenylene iodonium, apocynin, D-(+)-neopterine) also significantly blunted the generation of reactive oxygen species, activation of K(+), Cl(-)-cotransport and apoptosis induced by N-ethylmaleimide. These results suggest that reactive oxygen species generated through activation of NADPH oxidase may play a role in the N-ethylmaleimide-induced stimulation of K(+), Cl(-)-cotransport and apoptosis in HepG2 cells.

Mechanisms of cell volume regulation and possible nature of the cell volume sensor

In animal organisms, cell volume undergoes dynamic changes in many physiological and pathological processes. To protect themselves against lysis and apoptosis and to maintain an optimal concentration of intracellular enzymes and metabolites, most animal cells actively regulate their volume. In the present review, we shortly summarize the data on ion transport mechanisms involved in regulatory volume decrease (RVD) and regulatory volume increase (RVI) with an emphasis on unresolved aspects of this problem such as: (i) how cells sense their volume changes; (ii) what signals are generated upon cell volume alterations; and (iii) how these signals are transferred to the ion transport systems executing cell volume regulation.

Evidence supporting a role for KCl cotransporter in the thick ascending limb of Henle's loop

BACKGROUND

A basolateral Ba(2+)-sensitive KCl cotransporter has previously been proposed as participating in basolateral K+ recycling and transepithelial NaCl reabsorption in the thick ascending limb of Henle's loop (TAL). The aim of the present study was to answer the question as to whether this cotransporter plays a role in transepithelial K+ reabsorption and whether dietary Mg(2+) deficiency, known to regulate the KCl cotransporter in erythrocytes, also regulates KCl transport in the TAL.

METHODS

The effects of a low-Mg(2+) diet were investigated on urinary and plasma K+ concentration in control mice and Mg(2+)-deficient mice. Transepithelial Na+, Cl- and K+ net fluxes (J(Na), J(Cl), J(K)), determined in isolated perfused TALs with electron probe analysis or cation-exchange high-performance liquid chromatography (HPLC) and electrophysiological parameters (V(te), R(te)), were measured in both animal groups. Expression of transcripts for the KCl cotransporter and its possible regulation by low-Mg(2+) were studied by RT-PCR in microdissected mouse cortical TAL (CTAL) and medullary TAL (MTAL) segments.

RESULTS

In isolated perfused CTALs, basolateral Ba(2+) and amiloride induced a large K+ net secretion towards the tubular lumen, paralleled by a 50% decrease in transepithelial NaCl reabsorption. KCC1 transcripts were found in the mouse CTAL and MTAL. A low-Mg(2+) diet led to diminished urinary K+ excretion, lowered plasma K+ concentration and up-regulation of KCC1 transcripts in the TAL. For low-Mg(2+) diet, this upregulation was associated with increased transepithelial K+ reabsorption in the in vitro-perfused CTAL.

CONCLUSIONS

Our study provides evidence that the KCl cotransporter, which is functionally expressed in the TAL, plays an important role in transepithelial K+ reabsorption. Direct inhibition of this transporter by Ba(2+) and its indirect inhibition by amiloride lead to a strong transepithelial K+ secretion and diminished NaCl reabsorption in the TAL. Up-regulation of KCC1 mRNA by dietary Mg(2+) restriction is associated with an increased K+ reabsorption in the in vitro perfused CTAL.

Renal potassium-chloride cotransporters

The electroneutral cotransport of potassium and chloride is mediated by potassium-chloride transporters, which are encoded by members of the gene family of cation-chloride cotransporters. A significant body of evidence argues for swelling-activated, basolateral potassium-chloride transport in the proximal tubule and thick ascending limb, with a potential role in transepithelial salt transport. However, the lack of specific inhibitors has impeded progress in this area. The cloning of the four potassium-chloride cotransporter genes has sparked new interest in this transport pathway, and promises to yield novel insights into their roles in cellular and renal physiology.

Role of reactive oxygen species generated by NADPH oxidase in the mechanism of activation of K(+)-Cl(-)-cotransport by N-ethylmaleimide in HepG2 human hepatoma cells

K(+)-Cl(-)-cotransport (KCC) is ubiquitously present in all cells, and plays an essential role in ion and volume regulation. In this study we investigated the role of reactive oxygen species (ROS) in regulation of KCC in HepG2 human hepatoblastoma cells. N-ethylmaleimide (NEM), a KCC activator, induced Cl(-)-dependent K+ efflux, which was markedly prevented by KCC inhibitors (calyculin-A, genistein and BaCl2), indicating that KCC is activated by NEM in the HepG2 cells. Treatment with NEM also induced a sustained increase in the level of intracellular ROS assessed by 2',7'-dichlorofluorescein fluorescence. Antioxidants, N-acetyl cysteine or N,N'-diphenyl-p-phenylenediamine significantly inhibited both ROS generation and KCC activation induced by NEM. The NEM-induced ROS production was significantly suppressed by inhibitors of NADPH oxidase (diphenylene iodonium, apocynin and neopterine). These inhibitors also significantly inhibited the NEM-induced KCC activation. Taken together, these results suggest that ROS generated by NADPH oxidase may mediate the NEM-induced activation of KCC in human hepatoma cells.

Volume-activated K(+)(Rb(+)) efflux in lactating rat mammary tissue

The effect of cell swelling, induced by a hyposmotic shock, on K(+)(Rb(+)) efflux from lactating rat mammary tissue explants has been studied. A hyposmotic challenge increased the fractional release of K(+)(Rb(+)) from mammary tissue in the absence and presence of the loop-diuretic bumetanide (100 microM). However, the volume-sensitive moiety of K(+)(Rb(+)) efflux was proportionately larger when bumetanide was present in the incubation medium. On the other hand, a hyposmotic shock appeared to reduce the bumetanide-sensitive component of K(+)(Rb(+)) efflux. The increase in K(+)(Rb(+)) efflux, induced by cell swelling, was dependent upon the extent of the hyposmotic challenge. In the presence of bumetanide, substituting Cl(-) with NO(3)(-) reduced the initial increase in volume-sensitive K(+)(Rb(+)) efflux. However, volume-sensitive K(+)(Rb(+)) release was prolonged in the presence of NO(3)(-). Volume-activated K(+)(Rb(+)) efflux from rat mammary tissue explants was inhibited by quinine. Cell swelling increased the intracellular concentration of Ca(2+) in a fashion which depended on the presence of extracellular Ca(2+). However, removing extracellular Ca(2+) did not inhibit volume-activated K(+)(Rb(+)) efflux from rat mammary tissue explants. The results are consistent with the presence of volume-activated K(+) channels in lactating rat mammary tissue. Volume-activated K(+) efflux may play a central role in mammary cell volume regulation.

Basolateral K-Cl cotransporter regulates colonic potassium absorption in potassium depletion

Active potassium absorption in the rat distal colon is electroneutral, Na(+)-independent, partially chloride-dependent, and energized by an apical membrane H,K-ATPase. Both dietary sodium and dietary potassium depletion substantially increase active potassium absorption. We have recently reported that sodium depletion up-regulates H,K-ATPase alpha-subunit mRNA and protein expression, whereas potassium depletion up-regulates H,K-ATPase beta-subunit mRNA and protein expression. Because overall potassium absorption is non-conductive, K-Cl cotransport (KCC) at the basolateral membrane may also be involved in potassium absorption. Although KCC1 has not been cloned from the colon, we established, in Northern blot analysis with mRNA from the rat distal colon using rabbit kidney KCC1 cDNA as a probe, the presence of an expected size mRNA in the rat colon. This KCC1 mRNA is substantially increased by potassium depletion but only minimally by sodium depletion. KCC1-specific antibody identified a 155-kDa protein in rat colonic basolateral membrane. Potassium depletion but not sodium depletion resulted in an increase in KCC1 protein expression in basolateral membrane. The increase of colonic KCC1 mRNA abundance and KCC1 protein expression in potassium depletion of the rat colonic basolateral membrane suggests that K-Cl cotransporter: 1) is involved in transepithelial potassium absorption and 2) regulates the increase in potassium absorption induced by dietary potassium depletion. We conclude that active potassium absorption in the rat distal colon involves the coordinated regulation of both apical membrane H,K-ATPase and basolateral membrane KCC1 protein.

Functional comparison of the K+-Cl- cotransporters KCC1 and KCC4

The K(+)-Cl(-) cotransporters (KCCs) are members of the cation-chloride cotransporter gene family and fall into two phylogenetic subgroups: KCC2 paired with KCC4 and KCC1 paired with KCC3. We report a functional comparison in Xenopus oocytes of KCC1 and KCC4, widely expressed representatives of these two subgroups. KCC1 and KCC4 exhibit differential sensitivity to transport inhibitors, such that KCC4 is much less sensitive to bumetanide and furosemide. The efficacy of these anion inhibitors is critically dependent on the concentration of extracellular K(+), with much higher inhibition in 50 mm K(+) versus 2 mm K(+). KCC4 is also uniquely sensitive to 10 mm barium and to 2 mm trichlormethiazide. Kinetic characterization reveals divergent affinities for K(+) (K(m) values of approximately 25.5 and 17.5 mm for KCC1 and KCC4, respectively), probably due to variation within the second transmembrane segment. Although the two isoforms have equivalent affinities for Cl(-), they differ in the anion selectivity of K(+) transport (Cl(-) > SCN(-) = Br(-) > PO(4)(-3) > I(-) for KCC1 and Cl(-) > Br(-) > PO(4)(-3) = I(-) > SCN(-) for KCC4). Both KCCs express minimal K(+)-Cl(-) cotransport under isotonic conditions, with significant activation by cell swelling under hypotonic conditions. The cysteine-alkylating agent N-ethylmaleimide activates K(+)-Cl(-) cotransport in isotonic conditions but abrogates hypotonic activation, an unexpected dissociation of N-ethylmaleimide sensitivity and volume sensitivity. Although KCC4 is consistently more volume-sensitive, the hypotonic activation of both isoforms is critically dependent on protein phosphatase 1. Overall, the functional comparison of these cloned K(+)-Cl(-) cotransporters reveals important functional, pharmacological, and kinetic differences with both physiological and mechanistic implications.

Molecular water pumps

There is good evidence that cotransporters of the symport type behave as molecular water pumps, in which a water flux is coupled to the substrate fluxes. The free energy stored in the substrate gradients is utilized, by a mechanism within the protein, for the transport of water. Accordingly, the water flux is secondary active and can proceed uphill against the water chemical potential difference. The effect has been recognized in all symports studied so far (Table 1). It has been studied in details for the K+/Cl- cotransporter in the choroid plexus epithelium, the H+/lactate cotransporter in the retinal pigment epithelium, the intestinal Na+/glucose cotransporter (SGLT1) and the renal Na+/dicarboxylate cotransporter both expressed in Xenopus oocytes. The generality of the phenomenon among symports with widely different primary structures suggests that the property of molecular water pumps derives from a pattern of conformational changes common for this type of membrane proteins. Most of the data on molecular water pumps are derived from fluxes initiated by rapid changes in the composition of the external solution. There was no experimental evidence for unstirred layers in such experiments, in accordance with theoretical evaluations. Even the experimental introduction of unstirred layers did not lead to any measurable water fluxes. The majority of the experimental data supports a molecular model where water is cotransported: A well defined number of water molecules act as a substrate on equal footing with the non-aqueous substrates. The ratio of any two of the fluxes is constant, given by the properties of the protein, and is independent of the driving forces or other external parameters. The detailed mechanism behind the molecular water pumps is as yet unknown. It is, however, possible to combine well established phenomena for enzymes into a working model. For example, uptake and release of water is associated with conformational changes during enzymatic action; a specific sequence of allosteric conformations in a membrane bound enzyme would give rise to vectorial transport of water across the membrane. In addition to their recognized functions, cotransporters have the additional property of water channels. Compared to aquaporins, the unitary water permeability is about two orders of magnitude lower. It is suggested that the water permeability is determined from chemical associations between the water molecule and sites within the pore, probably in the form of hydrogen-bonds. The existence of a passive water permeability suggests an alternative model for the molecular water pump: The water flux couples to the flux of non-aqueous substrates in a hyperosmolar compartment within the protein. Molecular water pumps allow cellular water homeostasis to be viewed as a balance between pumps and leaks. This enables cells to maintain their intracellular osmolarity despite external variations. Molecular water pumps could be relevant for a wide range of physiological functions, from volume regulation in contractile vacuoles in amoeba to phloem transport in plants (Zeuthen 1992, 1996). They could be important building blocks in a general model for vectorial water transport across epithelia. A simplified model of a leaky epithelium incorporating K+/Cl-/H2O and Na+/glucose/H2O cotransport in combination with channels and primary active transport gives good quantitative predictions of several properties. In particular of how epithelial cell layers can transport water uphill.

Vasoconstrictors and nitrovasodilators reciprocally regulate the Na+-K+-2Cl- cotransporter in rat aorta

Little is known about the function and regulation of the Na+-K+-2Cl- cotransporter NKCC1 in vascular smooth muscle. The activity of NKCC1 was measured as the bumetanide-sensitive efflux of 86Rb+ from intact smooth muscle of the rat aorta. Hypertonic shrinkage (440 mosmol/kgH2O) rapidly doubled cotransporter activity, consistent with its volume-regulatory function. NKCC1 was also acutely activated by the vasoconstrictors ANG II (52%), phenylephrine (50%), endothelin (53%), and 30 mM KCl (54%). Both nitric oxide and nitroprusside inhibited basal NKCC1 activity (39 and 34%, respectively), and nitroprusside completely reversed the stimulation by phenylephrine. The phosphorylation of NKCC1 was increased by hypertonic shrinkage, phenylephrine, and KCl and was reduced by nitroprusside. The inhibition of NKCC1 significantly reduced the contraction of rat aorta induced by phenylephrine (63% at 10 nM, 26% at 30 nM) but not by KCl. We conclude that the Na+-K+-2Cl- cotransporter in vascular smooth muscle is reciprocally regulated by vasoconstrictors and nitrovasodilators and contributes to smooth muscle contraction, indicating that alterations in NKCC1 could influence vascular smooth muscle tone in vivo.

Physiological significance of volume-regulatory transporters

Research over the past 25 years has identified specific ion transporters and channels that are activated by acute changes in cell volume and that serve to restore steady-state volume. The mechanism by which cells sense changes in cell volume and activate the appropriate transporters remains a mystery, but recent studies are providing important clues. A curious aspect of volume regulation in mammalian cells is that it is often absent or incomplete in anisosmotic media, whereas complete volume regulation is observed with isosmotic shrinkage and swelling. The basis for this may lie in an important role of intracellular Cl- in controlling volume-regulatory transporters. This is physiologically relevant, since the principal threat to cell volume in vivo is not changes in extracellular osmolarity but rather changes in the cellular content of osmotically active molecules. Volume-regulatory transporters are also closely linked to cell growth and metabolism, producing requisite changes in cell volume that may also signal subsequent growth and metabolic events. Thus, despite the relatively constant osmolarity in mammals, volume-regulatory transporters have important roles in mammalian physiology.

Acetylcholine-induced membrane potential changes in endothelial cells of rabbit aortic valve

1. Using a microelectrode technique, acetylcholine (ACh)-induced membrane potential changes were characterized using various types of inhibitors of K+ and Cl- channels in rabbit aortic valve endothelial cells (RAVEC). 2. ACh produced transient then sustained membrane hyperpolarizations. Withdrawal of ACh evoked a transient depolarization. 3. High K+ blocked and low K+ potentiated the two ACh-induced hyperpolarizations. Charybdotoxin (ChTX) attenuated the ACh-induced transient and sustained hyperpolarizations; apamin inhibited only the sustained hyperpolarization. In the combined presence of ChTX and apamin, ACh produced a depolarization. 4. In Ca2+-free solution or in the presence of Co2+ or Ni2+, ACh produced a transient hyperpolarization followed by a depolarization. In BAPTA-AM-treated cells, ACh produced only a depolarization. 5. A low concentration of A23187 attenuated the ACh-induced transient, but not the sustained, hyperpolarization. In the presence of cyclopiazonic acid, the hyperpolarization induced by ACh was maintained after ACh removal; this maintained hyperpolarization was blocked by Co2+. 6. Both NPPB and hypertonic solution inhibited the membrane depolarization seen after ACh washout. Bumetanide also attenuated this depolarization. 7. It is concluded that in RAVEC, ACh produces a two-component hyperpolarization followed by a depolarization. It is suggested that ACh-induced Ca2+ release from the storage sites causes a transient hyperpolarization due to activation of ChTX-sensitive K+ channels and that ACh-activated Ca2+ influx causes a sustained hyperpolarization by activating both ChTX- and apamin-sensitive K+ channels. Both volume-sensitive Cl- channels and the Na+-K+-Cl- cotransporter probably contribute to the ACh-induced depolarization.

K-Cl cotransporter expression in the human kidney

The K-Cl cotransporter protein KCC1 is a membrane transport protein that mediates the coupled, electroneutral transport of K and Cl across plasma membranes. The precise cell type(s) in the kidney that express the K-Cl cotransporter have remained unknown. The aim of the present investigation was to define the distribution of KCC1 mRNA in the human kidney. We used in situ hybridization with a nonradioactive digoxigenin-labeled riboprobe. We identified abundant KCC1 mRNA expression in the epithelial cells throughout the distal and proximal renal tubular epithelium. The transporter was also expressed in glomerular mesangial cells and endothelial cells of the renal vessels. These findings suggest that the K-Cl cotransporter may have an important role in transepithelial K and Cl reabsorption.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Fluid and ion transport in corneal endothelium: insensitivity to modulators of Na(+)-K(+)-2Cl- cotransport

The role of Na(+)-K(+)-2Cl- cotransport in ion and fluid transport of the corneal endothelium was examined by measuring changes in corneal hydration and uptake of 86Rb by the endothelial cell layer. Isolated, intact rabbit corneas maintain normal hydration when they are superfused at the endothelial surface with bicarbonate (HCO3-)-Ringer solutions as a result of equilibrium between active ion and fluid transport out of the stromal tissue and leak of fluid into stromal tissue from the aqueous humor. Furosemide and bumetanide did not alter this equilibrium when they were added to the superfusion medium. Uptake of 86Rb by the endothelium of the incubated cornea was increased 25% by bumetanide, but uptake in the presence of ouabain (70% less than that of controls) was not changed by bumetanide. In Na(+)-free medium, uptake of 86Rb was reduced by 58%, but it was unchanged in Cl(-)-free medium. Calyculin A, a protein phosphatase inhibitor and activator of Na(+)-K(+)-Cl- cotransport, was without effect on 86Rb uptake. Hypertonicity (345 mosmol/kg) increased uptake slightly, whereas hypotonicity (226 mosmol/kg) caused a 33% decrease. Neither of these changes was significantly different when bumetanide was present in the media. It is concluded that Na(+)-K(+)-2Cl- cotransporter activity is not exhibited by the in situ corneal endothelium and does not play a role in the ion and fluid transport of this cell layer. Its presence in cultured endothelial cells may reflect the reported importance of this protein in growth, proliferation, and differentiation.

A role for chloride in the suppressive effect of acetylcholine on afferent vestibular activity

Afferents of the frog semicircular canal (SCC) respond to acetylcholine (ACh) application (0.3-1.0 mM) with a facilitation of their activity while frog saccular afferents respond with suppression (Guth et al., 1994). All recordings are of resting (i.e., non-stimulated) multiunit activity as previously reported (Guth et al., 1994). Substitution of 80% of external chloride (Cl-) by large, poorly permeant anions of different structures (isethionate, methanesulfonate, methylsulfate, and gluconate) reduced the suppressive effect of ACh in the frog saccular afferents. This substitution did not affect the facilitatory response of SCC afferents to ACh. Chloride channel blockers were also used to test further whether Cl- is involved in the ACh suppressive effect. These included: niflumic and flufenamic acids, picrotoxin, 5-nitro-2-(-3-phenylpropylamino)benzoic acid (NPPB), and 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS). As with the Cl- substitutions, all of these agents reduced the suppressive response to ACh in the saccule, but not the facilitatory response seen in the SCC. The suppressive effect of ACh on saccular afferents is considered to be due to activation of a nicotinic-like receptor (Guth et al., 1994; Guth and Norris, 1996). Taking into account the effects of both Cl- substitutions and Cl- channel blockers, we conclude that changes in Cl- availability influence the suppressive effect of ACh and that therefore Cl- may be involved in this effect.

Ion channels in vascular endothelium

The functional impact of ion channels in vascular endothelial cells (ECs) is still a matter of controversy. This review describes different types of ion channels in ECs and their role in electrogenesis, Ca2+ signaling, vessel permeability, cell-cell communication, mechano-sensor functions, and pH and volume regulation. One major function of ion channels in ECs is the control of Ca2+ influx either by a direct modulation of the Ca2+ influx pathway or by indirect modulation of K+ and Cl- channels, thereby clamping the membrane at a sufficiently negative potential to provide the necessary driving force for a sustained Ca2+ influx. We discuss various mechanisms of Ca2+ influx stimulation: those that activate nonselective, Ca(2+)-permeable cation channels or those that activate Ca(2+)-selective channels, exclusively or partially operated by the filling state of intracellular Ca2+ stores. We also describe the role of various Ca(2+)- and shear stress-activated K+ channels and different types of Cl- channels for the regulation of the membrane potential.

Swelling-induced activation of Na+,K+,2Cl- cotransport in C6 glioma cells: kinetic properties and intracellular signalling mechanisms

Swelling of C6 glioma cells in hypotonic medium (180 mOsm) results in two- to three-fold activation of K+ (86Rb+) influx suppressed by 10 microM bumetanide. Bumetanide-sensitive transport of 86Rb+ is dependent on extracellular K+, Na+ and Cl- both in iso-osmotic conditions and under hypo-osmotic shock, supporting the notion that it is mediated by Na+,K+,2Cl- cotransport. Inhibitors of protein kinase C (10 microM polymyxin B and l microM staurosporine) had no significant effect on basal cotransport but reduced its hypotonic stimulation by 70-80%. Similar results were obtained with calmodulin antagonist R24571 (10 microM), indicating Ca2+/calmodulin-dependence of the process. Influence of polymyxin B and R24571 was not additive. Swelling-activated Na+,K+,2Cl- cotransport was also suppressed by protein kinase C activator PMA (l microM). By contrast, preincubation of cells with inhibitors of protein phosphatases (100 microM vanadate, 5 mM fluoride and 0.5 microM okadaic acid) activated greatly the bumetanide-sensitive 86Rb+ uptake in isotonic conditions, while a subsequent hypotonic swelling led to smaller or no increment. These results indicate the involvement of Ca2+/calmodulin-dependent staurosporine/polymyxin B-sensitive protein kinase other than protein kinase C in swelling-induced activation of Na+,K+,2Cl- cotransport in glial cells.

The influence of bumetanide on the membrane potential of mouse skeletal muscle cells in isotonic and hypertonic media

1. Increasing the medium osmolality, with a non-ionic osmoticant, from control (289 mOsm) to 319 mOsm or 344 mOsm in the lumbrical muscle cell of the mouse, resulted in a depolarization of the membrane potential (Vm) of 5.9 mV and 10.9 mV, respectively. 2. In control medium, the blockers of chloride related cotransport bumetanide and furosemide, induced a hyperpolarization of -3.6 and -3.0 mV and prevented the depolarization due to hypertonicity. When bumetanide was added in hypertonic media Vm fully repolarized to control values. 3. In a medium of 266 mOsm, the hyperpolarization by bumetanide was absent. 4. At 344 mOsm the half-maximal effective concentration (IC50) was 0.5 microM for bumetanide and 21 microM for furosemide. 5. In solutions containing 1.25 mM sodium the depolarization by hypertonicity was reduced to 2.3 mV. 6. Reducing chloride permeability, by anthracene 9 carboxylic acid (9-AC) in 289 mOsm, induced a small but significant hyperpolarization of -2.6 mV. Increasing medium osmolality to 344 mOsm enlarged this hyperpolarization significantly to -7.6 mV. 7. In a solution of 344 mOsm containing 100 microM ouabain, the bumetanide-induced hyperpolarization of Vm was absent. 8. The results indicate that a Na-K-2Cl cotransporter is present in mouse lumbrical muscle fibre and that its contribution to Vm is dependent on medium osmolality.

Molecular cloning and functional expression of the K-Cl cotransporter from rabbit, rat, and human. A new member of the cation-chloride cotransporter family

We report the cloning, sequence analysis, tissue distribution, and functional expression of the K-Cl cotransport protein, KCC1. KCC1 was identified by searching the human expressed sequence tag data base, based on the expectation that it would be distantly related to the Na-K-Cl cotransporter. Rabbit KCC1 (rbKCC1) and rat KCC1 (rtKCC1) were cloned by screening rabbit kidney and rat brain cDNA libraries using homologous cDNA probes. Human KCC1 (hKCC1) was obtained from I.M.A.G.E. clones and in part by reverse transcription-polymerase chain reaction; it exhibits 97% identity with rbKCC1. KCC1 encodes a 1085-residue polypeptide with substantial sequence homology (24-25% identity) to the bumetanide-sensitive Na-K-Cl cotransporter (NKCC or BSC) and the thiazide-sensitive Na-Cl cotransporter (NCC or TSC). Hydropathy analysis of KCC1 indicates structural homology to NKCC, including 12 transmembrane domains, a large extracellular loop with potential N-linked glycosylation sites, and cytoplasmic N- and C-terminal regions. Northern blot analysis revealed a ubiquitously expressed 3. 8-kilobase transcript. Much of the genomic sequence of hKCC1 is in the data base, and the gene has been previously localized to 16q22.1 (Larsen, F., Solhein, J., Kristensen, T., Kolsto, A. B., and Prydz, H.(1993) Hum. Mol. Genet. 2, 1589-1595). Epitope-tagged rbKCC1 was stably expressed in human embryonic kidney (HEK 293) cells, resulting in production of a approximately150-kDa glycoprotein. The initial rate of 86Rb efflux from cells expressing rbKCC1 was more than 7 times greater than efflux from control cells and was inhibited by 2 mM furosemide; 86Rb efflux was stimulated by cell swelling. Uptake of 86Rb into rbKCC1 cells after a 15-min pretreatment with 1 mM N-ethylmaleimide was dependent on external chloride but not on external sodium, and was inhibited by furosemide with a Ki of approximately 40 microM and by bumetanide with a Ki of approximately 60 microM. These data demonstrate that the KCC1 cDNAs encode a widely expressed K-Cl cotransporter with the characteristics of the K-Cl transporter that has been characterized in red cells.

Red blood cell cation transports in uraemic anaemia: evidence for an increased K/Cl co-transport activity. Effects of dialysis and erythropoietin treatment

This study examines the role of uraemia and the effect of different dialysis treatments on red cell cation transport. We evaluated the main cation transport systems in erythrocytes of non-dialysed end-stage renal disease (ESRD) subjects, of patients undergoing haemodialysis (HD) and continuous ambulatory peritoneal dialysis (CAPD), as well as the changes induced by human recombinant erythropoietin (r-HuEPO) administration. In uraemic undialysed and dialysed patients, we observed an increase in K/Cl co-transport activity and in shrinkage-induced amiloride-sensitive (HMA-sensitive) Na efflux (Na/H exchange) and a decrease in Na/K pump and Na/K/Cl co-transport activity, while Na/Li exchange was increased only in dialysed patients. In uraemic erythrocytes, we showed for the first time an increased K/Cl co-transport activity, which was cell age independent. Generally, the different method of dialysis (CAPD or HD) did not modify the cation transport abnormalities observed. During the treatment with r-HuEPO, all the systems, with the exception of the Na/K pump and Na/K/Cl co-transport, increased their activities following the increase of circulating young red cells. The changes produced under r-HuEPO administration were transient and cation transports returned to the baseline values within 100 days of treatment, indicating a primary and prominent pathogenetic role of uraemia in modulating the red cell membrane cation transport activities.

Outward chloride/potassium co-transport in insect neurosecretory cells (DUM neurones)

The mechanism underlying outward chloride transport in the cell body and in the neuritic field of cockroach Dorsal Unpaired Median (DUM) neurones was assessed using the intracellular microelectrode technique. The chloride equilibrium potential was indirectly estimated from the reversal potentials of responses to gamma-aminobutyric acid (GABA) pressure ejections and of inhibitory postsynaptic potential (IPSP) evoked by electrical stimulation of the anterior connectives. Changes in intracellular chloride concentration [Cl-]i following various treatments were estimated from the amplitude changes of soma GABA responses and IPSP. Decreasing external Cl- concentration reduced the amplitude of GABA-mediated inhibitory events without affecting the membrane potential. Cl-/K+ co-transport was assessed by increasing external K+ concentration. The rate of outward Cl- movement was reduced furosemide but not by SITS or DIDS. All these results suggest that Cl- is not passively distributed in DUM neurones and that an active outwardly directed Cl-/K+ co-transport is implicated in the regulation of [Cl-]i.