The Na-K-2Cl cotransport system

Physiology and pathophysiology of SLC12A1/2 transporters

The electroneutral Na(+)-K(+)-Cl(-) cotransporters NKCC1 (encoded by the SLC12A2 gene) and NKCC2 (SLC12A1 gene) belong to the Na(+)-dependent subgroup of solute carrier 12 (SLC12) family of transporters. They mediate the electroneutral movement of Na(+) and K(+), tightly coupled to the movement of Cl(-) across cell membranes. As they use the energy of the ion gradients generated by the Na(+)/K(+)-ATPase to transport Na(+), K(+), and Cl(-) from the outside to the inside of a cell, they are considered secondary active transport mechanisms. NKCC-mediated transport occurs in a 1Na(+), 1K(+), and 2Cl(-) ratio, although NKCC1 has been shown to sometimes mediate partial reactions. Both transporters are blocked by bumetanide and furosemide, drugs which are commonly used in clinical medicine. NKCC2 is the molecular target of loop diuretics as it is expressed on the apical membrane of thick ascending limb of Henle epithelial cells, where it mediates NaCl reabsorption. NKCC1, in contrast, is found on the basolateral membrane of Cl(-) secretory epithelial cells, as well as in a variety of non-epithelial cells, where it mediates cell volume regulation and participates in Cl(-) homeostasis. Following their molecular identification two decades ago, much has been learned about their biophysical properties, their mode of operation, their regulation by kinases and phosphatases, and their physiological relevance. However, despite this tremendous amount of new information, there are still so many gaps in our knowledge. This review summarizes information that constitutes consensus in the field, but it also discusses current points of controversy and highlights many unanswered questions.

Na+,K+,2Cl- cotransport and intracellular chloride regulation in rat primary sensory neurons: thermodynamic and kinetic aspects

Adult primary afferent neurons are depolarized by GABA throughout their entire surface, including their somata located in dorsal root ganglia (DRG). Primary afferent depolarization (PAD) mediated by GABA released from spinal interneurons determines presynaptic inhibition, a key mechanism in somatosensory processing. The depolarization is due to Cl(-) efflux through GABA(A) channels; the outward Cl(-) gradient is generated by a Na+,K+,2Cl(-) cotransporter (NKCC) as first established in amphibians. Using fluorescence imaging microscopy we measured [Cl(-)]i and cell water volume (CWV) in dissociated rat DRG cells (P0-P21) loaded with N-(ethoxycarbonylmethyl)-6-methoxyquinolinium bromide and calcein, respectively. Basal [Cl(-)]i was 44.2 +/- 1.2 mM (mean +/- SE), Cl(-) equilibrium potential (E Cl) was -27.0 +/- 0.7 mV (n = 75). This [Cl(-)]i is about four times higher than electrochemical equilibrium. On isosmotic removal of external Cl(-), cells lost Cl(-) and shrank. On returning to control solution, cells reaccumulated Cl(-) and recovered CWV. Cl(-) reaccumulation had Na+-dependent (SDC) and Na+-independent (SIC) components. The SIC stabilized at [Cl(-)]i = 13.2 +/- 1.2 mM, suggesting that it was passive (E(Cl) = -60.5 +/- 3 mV). Bumetanide blocked CWV recovery and most (65%) of the SDC (IC50 = 5.7 microM), indicating that both were mediated by NKCC. Active Cl(-) uptake fell with increasing [Cl(-)]i and became negligible when [Cl(-)]i reached basal levels. The kinetics of active Cl(-) uptake suggests a negative feedback system in which intracellular Cl(-)regulates its own influx thereby keeping [Cl(-)]i constant, above electrochemical equilibrium but below the value that would attain if NKCC reached thermodynamic equilibrium.

The N-terminus of the norepinephrine transporter regulates the magnitude and selectivity of the transporter-associated leak current

The norepinephrine (NE) transporter (NET) mediates the removal of NE from synaptic spaces and is a major target for antidepressants, amphetamine and cocaine. Previously, we have shown that syntaxin 1A (SYN 1A) supports human NET (hNET) cell surface expression, that hNET/SYN 1A interactions are direct and mediated by the hNET N-terminus, and that the hNET/SYN 1A association limits substrate-induced hNET-associated currents [Sung, U., Apparsundaram, S., Galli, A., Kahlig, K.M., Savchenko, V., Schroeter, S., Quick, M.W., Blakely, R.D., 2003. A regulated interaction of syntaxin 1A with the antidepressant-sensitive norepinephrine transporter establishes catecholamine clearance capacity. J. Neurosci. 23, 1697-1709]. These data raise the possibility that the hNET N-terminus, and potentially its interaction with SYN 1A, might regulate other hNET conductance states, including the hNET-mediated leak current. Importantly for monoamine transporters, the leak conductance has been shown to play a critical role in regulating cell membrane potential and possibly neuronal excitability [Quick, M.W., 2003. Regulating the conducting states of a mammalian serotonin transporter. Neuron 40, 537-549]. Here we demonstrate that deletion of the binding domain for SYN 1A in the NET N-terminus robustly enhances the NET-mediated leak current as well as its selectivity for Cl- permeation under particular intracellular ionic compositions. In addition, we show that the NET N-terminus coordinates the ability of intracellular Na+ and Cl- to regulate the leak conductance. These data suggest that the NET N-terminus regulates and defines the ionic specificity of the NET-mediated leak current.

Cation-chloride co-transporters in neuronal communication, development and trauma

Electrical signaling in neurons is based on the operation of plasmalemmal ion pumps and carriers that establish transmembrane ion gradients, and on the operation of ion channels that generate current and voltage responses by dissipating these gradients. Although both voltage- and ligand-gated channels are being extensively studied, the central role of ion pumps and carriers is largely ignored in current neuroscience. Such an information gap is particularly evident with regard to neuronal Cl- regulation, despite its immense importance in the generation of inhibitory synaptic responses by GABA- and glycine-gated anion channels. The cation-chloride co-transporters (CCCs) have been identified as important regulators of neuronal Cl- concentration, and recent work indicates that CCCs play a key role in shaping GABA- and glycine-mediated signaling, influencing not only fast cell-to-cell communication but also various aspects of neuronal development, plasticity and trauma.

Cell volume homeostasis: ionic and nonionic mechanisms. The sodium pump in the emergence of animal cells

Plant cells and bacterial cells are surrounded by a massive polysaccharide wall, which constrains their high internal osmotic pressure (tens of atmospheres). Animal cells, in contrast, are in osmotic equilibrium with their environment, have no restraining surround, and can take on a variety of shapes and can change these from moment to moment. This osmotic balance is achieved, in the first place, by the action of the energy-consuming sodium pump, one of the P-type ATPase transport protein family, members of which are found also in bacteria. The pump's action brings about a transmembranal electrochemical gradient of sodium ions, harnessed in a range of transport systems which couple the dissipation of this gradient to establishing a gradient of the coupled substrate. These transport systems include many which are responsible for short-term regulation of the cell's volume in response to acute changes of their osmotic balance. Thus, the primary role of the sodium pump as a regulator of cell volume has been built upon to provide the basis for an enormous variety of physiological functions.

Vacuolation induced by VacA toxin of Helicobacter pylori requires the intracellular accumulation of membrane permeant bases, Cl(-) and water

The protein vacuolating toxin A (VacA) of Helicobacter pylori converts late endosomes into large vacuoles in the presence of permeant bases. Here it is shown that this phenomenon corresponds to an accumulation of permeant bases and Cl(-) in HeLa cells and requires the presence of extracellular Cl(-). The net influx of Cl(-) is due to electroneutral, Na(+), K(+), 2Cl(-) cotransporter-mediated transport. Cell vacuolation leads to cell volume increase, consistent with water flux into the cell, while hyper-osmotic media decreased vacuole formation. These data represent the first evidence that VacA-treated cells undergo an osmotic unbalance, reinforcing the hypothesis that the VacA chloride channel is responsible for cell vacuolation.

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.

Extracellular chloride and the maintenance of spontaneous epileptiform activity in rat hippocampal slices

Previous studies showed that furosemide blocks spontaneous epileptiform activity without diminishing synaptic transmission or reducing hyperexcited field responses to electrical stimuli. We now test the hypothesis that the antiepileptic effects of furosemide are mediated through its blockade of the Na+,K+,2Cl- cotransporter and thus should be mimicked by a reduction of extracellular chloride ([Cl-]o). In the first set of experiments, field recordings from the CA1 cell body layer of hippocampal slices showed that spontaneous bursting developed within 10-20 min in slices perfused with low-[Cl-]o (7 mM) medium but that this spontaneous epileptiform activity ceased after a further 10-20 min. Intracellular recordings from CA1 pyramidal cells showed that normal action potential discharge could be elicited by membrane depolarization, even after the tissue was perfused with low-[Cl-]o medium for >2 h. In a second set of experiments, spontaneous bursting activity was induced in slices by perfusion with high-[K+]o (10 mM), bicuculline (100 microM), or 4-aminopyridine (100 microM). In each case, recordings from the CA1 region showed that reduction of [Cl-]o to 21 mM reversibly blocked the bursting within 1 h. Similar to previous observations with furosemide treatment, low-[Cl-]o medium blocked spontaneous hypersynchronous discharges without reducing synaptic hyperexcitability (i.e., hyperexcitable field responses evoked by electrical stimulation). In a third set of experiments, prolonged exposure (>1 h after spontaneous bursting ceased) of slices to systematically varied [Cl-]o and [K+]o resulted in one of three types of events: 1) spontaneous, long-lasting, and repetitive negative field potential shifts (7 mM [Cl-]o; 3 mM [K+]o); 2) oscillations consisting of 5- to 10-mV negative shifts in the field potential, with a period of approximately 1 cycle/40 s (16 mM [Cl-]o; 12 mM [K+]o); and 3) shorter, infrequently occurring negative field shifts lasting 20-40 s (21 mM [Cl-]o; 3 mM [K+]o). Our observations indicate that the effects of low [Cl-]o on neuronal synchronization and spontaneous discharge are time dependent. Similar effects were seen with furosemide and low [Cl-]o, consistent with the hypothesis that the antiepileptic effect of furosemide is mediated by the drug's effect on chloride transporters. Finally, the results of altering extracellular potassium along with chloride suggest that blockade of the Na+, K+,2Cl- cotransporter, which normally transports chloride from the extracellular space into glial cells, is key to these antiepileptic effects.

Osmolarity, ionic flux, and changes in brain excitability

The majority of modern epilepsy research has focused on possible abnormalities in synaptic and intrinsic neuronal properties--as likely epileptogenic mechanisms as well as the targets for developing novel antiepileptic treatments. However, many other processes in the central nervous system contribute to neuronal excitability and synchronization. Regulation of ionic balance is one such set of critical processes, involving a complex array of molecules for moving ions into and out of brain cells--both neurons and glia. Alterations in extracellular-to-intracellular ion gradients can have both direct and indirect effects on neuronal discharge. We have found, for example, that when hippocampal slices are exposed to hypo-osmotic bathing medium, the cells not only swell, but there is also a significant increase in the amplitude of a delayed rectifier potassium current in inhibitory interneurons--an effect that may contribute to the increase in tissue excitability associated with hypo-osmolar treatments. In contrast, antagonists of the chloride co-transporter, furosemide or bumetanide, block epileptiform activity in both in vitro and in vivo preparations. This antiepileptic effect is presumably due to the drugs' ability to block chloride co-transport. Indeed, prolonged tissue exposure to low levels of extracellular chloride have a parallel action. These experiments indicate that manipulation of ionic balance may not only facilitate epileptiform activities, but may also provide insight into new therapeutic strategies to block seizures.

Characteristics of ionic transport processes in fish intestinal epithelial cells

A general mathematical version of the cell model of a leaky epithelium for the NaCl absorption is presented, analysed and integrated numerically. The model consists in the adequate differential equations that describe the rate of change of the intracellular ion concentrations and are expressed in strict accordance with the law of mass conservation. The model includes many state variables representing ion concentrations, the cell volume, and membrane potentials. Ion movements are described by the Michaelis-Menten kinetics or by the constant field flux equation (Goldman-Hodgkin-Katz). In this paper, we model the intracellular ion concentrations, change in the cell volume, the transmembrane flux and membrane potentials of intestinal epithelium of both fresh water and sea water fish, and generate several simulations (in both the steady state and the transient state analysis) that appear to accord with prior experimental data in this area. For the ion movements of the sea water fish intestine, there were included a Na+/K+ pump, a K(+)-Cl- symport system, the K+ and Cl- channels in the basolateral membrane, whereas a Na(+)-K(+)-2Cl- cotransporter for NaCl absorption and K+ channels are located in the apical membrane. In the fresh water fish intestinal cells, the NaCl absorption is performed by two coupled antiporters Na+/H+ and Cl-/HCO3- presumably responsible for the intracellular pH regulation. In this type of cells, Na+ and K+ channels are located within the apical membrane, whereas Cl- channels are located within the basolateral membrane. The osmotically induced water transport across the apical and basolateral membranes has been taken into account as well. The simulations plot the steady state values for membrane potential difference, short-circuit current and intracellular ionic concentrations using the magnitude of the transmembrane flux through the Na+/K+ pump and Na(+)-K(+)-2Cl- cotransporter, or the basolateral Cl- permeability as dependent variables. The model behaves appropriately with regard to several experimental studies regarding the hyperpolarization (sea water fish intestine) and depolarization (fresh water fish intestine) of the apical membrane potential and inhibition of the short-circuit flux with reduced NaCl absorption. The model is also used to make several analytical predictions regarding the response of the membrane potential and ionic concentrations to variations in the basolateral Cl- flux. Furthermore, maintaining conservation of both mass and electroneutrality and taking into account the osmolar forces is an important advantage, because it allows a rigorous analysis of the relationship between membrane potential difference, volume and flux. The model can be used in the analysis and planning of the experiments and is capable of predicting the instantaneous values of ionic fluxes and intracellular concentrations and of cell volume.

Transport mechanisms of seawater teleost chloride cells: an inclusive model of a multifunctional cell

This review assembles recent information on seawater-type chloride cells of marine teleost fish and evaluates the secretion of Na+, Cl-, K+, H+ and NH4+ and the absorption of Ca2+. The evidence for the distribution (apical vs basolateral) and the abundance of the various ion pumps, cotransporters, channels and exchangers is assessed and an inclusive model is constructed. Relationships among the transport systems are presented to suggest that many, if not all, of these systems may be operating simultaneously in individual, multifunctional chloride cells.

Changes in the volume of marginal cells induced by isotonic 'Cl- depletion/restoration': involvement of the Cl- channel and Na+-K+-Cl- cotransporter

Marginal cells constitute the endolymph-facing epithelium responsible for the secretion of endolymph by the stria vascularis in the inner ear. We have studied the possible involvement of Cl- conductance and Na+-K+-Cl- cotransport in the mechanism of changes in cell volume upon isotonic Cl- depletion/restoration. Changes in cell volume were estimated from video-microscopic images with the aid of an image processor. Marginal cells shrank to approximately 80% of their original volume in 30 s and to 65-70% in 90 s upon total replacement of [Cl]o (approximately 150 mM) by gluconate-, and the original volume of the shrunken cells was restored within 2 min after restoration of Cl-. The order of potency of anions to induce isotonic shrinkage was gluconate > I- > F- > Br-. The cell shrinkage caused by Cl- depletion was partially inhibited by 5-Nitro-2-(3-phenyl-propylamino)-benzoic acid (NPPB, 0.2 mM), but not by either 4-acetamido-4'-isothiocyanato-stilbene-2,2'-disulfonic acid (SITS, 0.5 mM), bumetanide (10 microM) or ouabain (1 mM). The cell shrinkage caused by a reduction of [Cl]o from approximately 150 mM to 7.5 mM was not affected by [K]o in the range of 3.6 mM to 72 mM. These results suggest that the main efflux pathway(s) responsible for the 'Cl removal'-induced shrinkage depends on volume-correlated Cl- conductance (Takeuchi and Irimajiri, J. Membrane Biol. 150, 47-62, 1996) and that this pathway(s) is essentially independent of the Na+-K+-Cl- cotransporter, the Na+,K+-ATPase, and the K+-Cl- cotransporter. With regard to volume recovery after isotonic shrinkage, its critical dependence on the simultaneous presence of Na+, K+ and Cl- in the bath and its substantial inhibition by bumetanide (10 microM) both indicate a major role for Na+-K+-Cl- cotransport. The strong influence on cell volume of solute fluxes working through the Cl- channel and the Na+-K+-Cl- cotransporter implies an essential role for these pathways in the ion transport mechanism(s) of the marginal cell.

Determination of Na+/Cl-, Na+/HCO3- and Na+/K+/2Cl- co-transporter activity in corneal endothelial cell plasma membrane vesicles

Corneal endothelial cell derived plasma membrane vesicles were used to investigate the presence of Na+/Cl-, Na+/HCO3- and Na+/K+/2Cl- co-transporter activity in the plasma membranes of these cells. Na+/H+ exchange was blocked by the presence of 1 mM amiloride in all determinations. The rate of accumulation of Na+ in the presence of chloride or bicarbonate was not significantly different from its accumulation in the presence of acetate, thiocyanate or gluconate. The addition of K+ to Na+ plus Cl- did not stimulate Na+ accumulation into the vesicles. The present work provides no evidence for Na+/K+/2Cl-, Na+/Cl- or Na+/HCO3- co-transport in corneal endothelial cell plasma membrane vesicles.

Stimulation of glutamate uptake and Na,K-ATPase activity in rat astrocytes exposed to ischemia-like insults

The postsynaptic actions of glutamate are rapidly terminated by high affinity glutamate uptake into glial cells. In this study we demonstrate the stimulation of both glutamate uptake and Na,K-ATPase activity in rat astrocyte cultures in response to sublethal ischemia-like insults. Primary cultures of neonatal rat cortical astrocytes were subjected to hypoxia, or to serum- and glucose-free medium, or to both conditions (ischemia). Cell death was assessed by propidium iodide staining of cell nuclei. To measure sodium pump activity and glutamate uptake, 3H-glutamate and 86Rb were both simultaneously added to the cell culture in the presence or absence of 2 mM ouabain. Na,K-ATPase activity was defined as ouabain-sensitive 86Rb uptake. Concomitant transient increases (2-3 times above control levels) of both Na,K-ATPase and glutamate transporter activities were observed in astrocytes after 4-24 h of hypoxia, 4 h of glucose deprivation, and 2-4 h of ischemia. A 24 h ischemia caused a profound loss of both activities in parallel with significant cell death. The addition of 5 mM glucose to the cells after 4 h ischemia prevented the loss of both sodium pump activity and glutamate uptake and rescued astrocytes from death observed at the end of 24 h ischemia. Reoxygenation after the 4 h ischemic event caused the selective inhibition of Na,K-ATPase activity. The observed increases in Na,K-ATPase activity and glutamate uptake in cultured astrocytes subjected to sublethal ischemia-like insults may model an important functional response of astrocytes in vivo by which they attempt to maintain ion and glutamate homeostasis under restricted energy and oxygen supply.

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.

Occlusion of K+ in the Na+/K+/2Cl- cotransporter of Ehrlich ascites tumor cells

Proteins of n-octyl glucoside solubilized membrane vesicles derived from Ehrlich ascites tumor cells can occlude 86Rb+.K+ displaces 86Rb+ and it is assumed that 86Rb+ can be used as a tracer to measure K+ occlusion. The following observations indicate that the Na+/K+/2Cl- cotransporter is responsible for this occlusion: (1) Na+ does not compete for the K+ binding site, but rather stimulates 86Rb+ occlusion. (2) K+ occlusion saturates with increasing [Na+] and [K+], the respective K0.5 values being 50 +/- 7 microM for Na+ and 371 +/- 63 microM for K+. (3) Preincubation with 1 mM ouabain does not inhibit 86Rb+ occlusion, arguing against the Na+/K+-ATPase as being responsible for the occlusion. This notion is supported by the K0.5 value for K+ being higher than reported for Na+/K+-ATPase and by the stimulatory effect of Na+. (4) The K+ occlusion is sensitive to [Cl-], and the occluded ion is protected by the presence of bumetanide during cation exchange chromatography. Our results suggest that occlusion measurements of substrate ions could be a profitable way to study the ion binding mechanism(s) of the Na+/K+/2Cl- cotransporter.

Apical localization of the Na-K-Cl cotransporter, rBSC1, on rat thick ascending limbs

A bumetanide-sensitive Na-K-Cl cotransporter (rBSC1) was recently cloned from a rat renal outer medulla (OM) cDNA library and shown to be expressed predominantly in the kidney. The purpose of the present study was to examine the nephron distribution of cotransporter transcripts and protein in rat kidney. In situ hybridization showed an intense signal only in the outer medulla and extending along cortical medullary rays consistent with expression of rBSC1 transcripts in medullary (MTAL) and cortical (CTAL) thick ascending limbs. Polyclonal antibodies raised in rabbits against a unique 67 amino acid segment from the carboxyl terminus of rBSC1 identified a broad major band of 130 to 160 (midpoint of 150) kDa and at least two minor bands of 50 to 70 kD on Western blotting of homogenates from cortex (C) and outer medulla (OM), but not inner medulla (IM), of rat kidney. Thus the Na-K-Cl cotransporter protein detected by the polyclonal rBSC1 antibody in rat kidney was similar in size to the major approximately 150 kD bumetanide binding protein detected by others in mouse and dog kidneys. Immunofluorescence studies using the anti-rBSC1 polyclonal antibody on rat kidney sections showed an intense signal limited to apical surfaces of MTAL and CTAL segments. Colocalization with anti-Tamm-Horsfall antibody which is present in all TABA cells except macula densa cells confirmed the absence of anti-rBSC1 fluorescence in the macula densa cells. These results are consistent with rBSC1 encoding the, or the major isoform of the, apical Na-K-Cl cotransporter in the thick ascending limb. The Na-K-Cl cotransporter functionally detected in macula densa cells may be encoded by a different BSC isoform.

Free radical-induced endothelial membrane dysfunction at the site of blood-brain barrier: relationship between lipid peroxidation, Na,K-ATPase activity, and 51Cr release

Na,K-ATPase activity, membrane lipid peroxidation (TBARM), and membrane 'leakiness' for small molecules were examined in rat cerebromicrovascular endothelial cells (RCEC) following exposure to hydrogen peroxide and xanthine/xanthine oxidase. Whereas short-term (15-30 min) exposure to either oxidant decreased ouabain-sensitive 86Rb uptake and increased TBARM in a concentration-dependent fashion, significant release of 51Cr (30-40%) from cells was observed only after one hour exposure to the oxidants. By comparison, much longer exposure times (i.e., 4 hours) were needed to induce significant lactate dehydrogenase release from oxidant-treated cells. The oxidant-evoked decrease in Na,K-ATPase activity and increases in TBARM and RCEC 'permeability' were abolished in the presence of the steroid antioxidants U-74500A and U-74389G (5-20 microM). Reduced glutathione (4 mM) partially attenuated oxidant-induced changes, whereas ascorbic acid (2 mM) and the disulfide bond-protecting agent, dithiothreitol (1 mM), were ineffective. These results suggest that the oxidant-induced loss of Na,K-ATPase activity in RCEC results primarily from changes in membrane lipids, and implicate both the inhibition of Na,K-ATPase and membrane lipid peroxidation in the mechanism responsible for the delayed free radical-induced increase in RCEC membrane 'permeability'.

Bicarbonate and chloride transport across rat ileal basolateral membrane

The mechanisms of HCO3- and Cl- transport across basolateral membranes from rat ileum were investigated in isolated vesicles by means of uptake experiments. Neither Cl-/HCO3- exchanger nor Na(+)-(HCO3-)n cotransport seem to be present in ileal basolateral membranes. Moreover Cl- uptake is unaffected by cis Na+ and/or K+ gradients, indicating the absence of Na(+)-Cl-, K(+)-Cl- and Na(+)-K(+)-2Cl- symport activity. An electrically conductive pathway seems to be responsible for both HCO3- and Cl- fluxes. Evidence is also given for the presence of a Na+/H+ exchanger at the basolateral pole of ileal enterocytes.

Improved hypothermic preservation of rat hearts by furosemide

The effect of furosemide, a blocker of the Na+/K+/Cl- cotransporter, on hypothermic preservation of rat hearts was studied with use of the Langendorff perfusion system and electron microscopy. Furosemide significantly improved the mechanical recovery and the coronary flow of the hearts preserved for 8 hours in St. Thomas' Hospital cardioplegic solution at a temperature of 4 degrees C. Furosemide at the concentration of 100 mumol/L was found to have an optimal effect, whereas at high concentrations (1000 mumol/L) it was found to have toxic effects. In addition, furosemide reduces the time elapsed between the end of the preservation time and the resumption of myocardial contractions. Ultrastructural evaluations were done in which the presence of swollen mitochondria was chosen as a criterion of hypothermic ischemic damage to the myocardium. Morphometric analysis indicated that the mitochondrial volume of hearts stored for 8 hours in St. Thomas' Hospital cardioplegic solution increased by 72% as compared with the mitochondrial volume of hearts that were not exposed to the hypothermic ischemic conditions (control group). The addition of 100 mumol/L furosemide to the cardioplegic solution resulted in a significant reduction of mitochondrial swelling during the period of 8 hours' storage, which amounted only to 28% as compared with the figure for the control group. The reduction of mitochondrial swelling by furosemide and the improved mechanical and coronary flow recoveries are thought to be related to the blocking of the sarcolemmal Na+/K+/Cl- cotransporter and consequently the reduction of the Na+ influx during hypothermic ischemic storage.

Inhibition by nitrendipine of 86Rb+ fluxes in subconfluent MDCK cells

Part of the natriuretic mechanism of dihydropyridine Ca2+ channel antagonists involves the inhibition of renal tubular sodium reabsorption. To identify the membrane ion transport system involved in this natriuretic action, we tested nitrendipine on unidirectional 86Rb+ fluxes in Madin-Darby canine kidney (MDCK) cells. To dissect between direct and indirect effects (via cytosolic Ca2+) of nitrendipine, the compound was re-examined on ion fluxes in human erythrocytes. In MDCK cells, external Ca2+ (3 mM), adrenalin (100 microM) and the Ca2+ ionophore A23187 (20 microM) strongly and transiently stimulated 86Rb+ efflux. All these stimulatory actions were fully inhibited by quinine (1 mM) suggesting that they reflect the opening of Ca(2+)-sensitive K+ channels. Nitrendipine was able to inhibit these Ca(2+)-sensitive K+ channels, bit this inhibitory action required concentrations of the compound (approximately 100 microM). Regarding 86Rb+ influx, the most significant result with nitrendipine was a partial inhibition of bumetanide-sensitive 86Rb+ influx. This effect represented a maximal flux inhibition of about 70% and required very low nitrendipine concentrations (IC50 approximately 1 nM). The Ca2+ ionophore A 23187 strongly stimulated bumetanide-sensitive 86Rb+ influx in MDCK cells. Conversely, a very important reduction (approximately 79%) of this influx component was found in Ca2+ depleted cells. In human red blood cells, Na+, K+, Cl- cotransport fluxes were resistant to nitrendipine, even at high concentrations of the compound (100-500 microM). Conversely, Ca(2+)-sensitive K+ channels were inhibited by nitrendipine with IC50 = 6 +/- 3 microM (mean +/- S.E.M., n = 3).(ABSTRACT TRUNCATED AT 250 WORDS)

Na-K-Cl cotransport regulates intracellular volume and monolayer permeability of trabecular meshwork cells

The trabecular meshwork (TM) of the eye plays a critical role in modulating intraocular pressure (IOP) through regulation of aqueous humor outflow, although the underlying mechanisms remain unknown. Ethacrynic acid, an agent known to inhibit Na-K-Cl cotransport of a number of cell types, recently has been reported to increase aqueous outflow and lower IOP through an unknown effect on the TM. In vascular endothelial cells and a variety of other cell types, the Na-K-Cl cotransporter functions to regulate intracellular volume. The present study was conducted to evaluate TM cells for the presence of Na-K-Cl cotransport activity and to test the hypothesis that modulation of cotransport activity alters intracellular volume and, consequently, permeability of the TM. We demonstrate here that bovine and human TM cells exhibit robust Na-K-Cl cotransport activity that is inhibited by bumetanide and by ethacrynic acid. Our studies also show that TM cell Na-K-Cl cotransport is modulated by a variety of hormones and neurotransmitters. Inhibition of the cotransporter either by bumetanide, ethacrynic acid, or inhibitory hormones reduces TM intracellular volume, whereas stimulatory hormones increase cell volume. In addition, shrinkage of the cells by hypertonic media stimulates cotransport activity and initiates a subsequent regulatory volume increase. Permeability of TM cell monolayers, assessed as transmonolayer flux of [14C]sucrose, is increased by hypertonicity-induced cell shrinkage and by bumetanide. These findings suggest that Na-K-Cl cotransport of TM cells is of central importance to regulation of intracellular volume and TM permeability. Defects of Na-K-Cl cotransport may underlie the pathophysiology of glaucoma.

Inward potassium transport systems in skeletal muscle derived cells are highly sensitive to oxidant exposure

Strenuous physical exercise causes a remarkable perturbation of K+ homeostasis in skeletal muscle tissue. Potassium efflux is crucial for a number of physiological control processes; however, exercise-induced perturbation of K+ homeostasis in skeletal muscle is suggested to be implicated in the generation of muscle fatigue. Physical exercise is also known to induce oxidative stress; a possible contribution of oxygen free radicals to the development of muscle fatigue has been hypothesized. To reveal the dose-dependent effect of oxidant exposure on inward and outward K+ (86RbCl) transporting systems, skeletal muscle derived L6 cells were treated with different concentrations of tert-butylhydroperoxide (TBOOH). We document the responses of (1) the ouabain-sensitive component of K+ influx (Na+,K+ pump), (2) bumetanide-sensitive ouabain-insensitive component of K+ influx (Na+,K+, 2Cl- cotransporter), (3) ouabain- and bumetanide-insensitive component of K+ influx (passive permeability of the cell membrane to inward K+), (4) ouabain-insensitive component of K+ efflux, and (5) passive leakage component of K+ efflux following exposure of L6 cells to oxidant treatment. Even very low doses of TBOOH (25 mumol) caused powerful activation of the Na+,K+ pump. Following TBOOH treatment, activity of the Na+,K+,2Cl- cotransporter was remarkably inhibited. Such a treatment also significantly decreased the permeability of the cell membrane to inward flux of K+ (passive influx). Thus, we observed that even very low doses of oxidant had remarkable specific effects on the different components of K+ influx in the skeletal muscle derived cells. However, K+ efflux mechanisms appeared to be rather insensitive to the extracellular oxidant challenge.

Increased cation transport in mdr1-gene-expressing K562 cells

Cation-transport properties were compared in a human leukemic cell line (K562) and its vincristine-selected, mdr1-gene-expressing sublines (K562/Vcr30 and K562/Vcr150) by the capacity of the cells to accumulate the potassium analogue thallium (201Tl). Determination of the time course of thallium accumulation in the absence and presence of ouabain, an inhibitor of sodium-potassium adenosine triphosphatase (ATPase), showed that the initial (at 20 min) rate of ouabain-resistant uptake was about 70% higher in the K562/Vcr30 cells than in the parental line. The maximal rate (Vmax) of ouabain-resistant uptake was 78 mmol/h for K562 cells and 115 mmol/h for K562/Vcr30 cells, and the Michaelis constant (Km) was 0.37 and 0.18 mmol, respectively. Bumetanide (50 microM), a specific inhibitor of ouabain-resistant Na-K-Cl cotransport, inhibited the elevated 201Tl uptake in K562/Vcr150 cells but had no effect on cellular vincristine accumulation. Incubation with different multidrug resistance (MDR)-reversing agents (verapamil as well as cyclosporin A and its analogue PSC833) had no significant effect on 201Tl uptake. Membrane depolarization by an elevation of the potassium concentration in the incubation medium did not affect vincristine accumulation in any cell line, which indicated that the changed drug-transport properties in mdr1-gene-expressing cells were not due to membrane hyperpolarization. It was concluded that P-glycoprotein-positive cells have a more efficient ouabain-resistant cation-transport mechanism than to cells without P-glycoprotein. A functional relationship between this phenomenon and MDR was not identified.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Potassium cotransport at the rat choroid plexus

The choroid plexuses are involved in cerebrospinal fluid (CSF) secretion and CSF K homeostasis. We examined K transport mechanisms present in the isolated rat choroid plexus that may be involved in these functions, predominantly using 86Rb as a marker for K. The study demonstrates that there are two primary uptake mechanisms. Ouabain-sensitive Na-K-adenosinetriphosphatase and bumetanide-sensitive cotransport, probably of the Na-K-2Cl form, account for 48 and 46% of uptake, respectively. Efflux studies demonstrate that the primary K efflux mechanism is also bumetanide-sensitive cotransport with the other major component probably being by K channels as it is inhibitable by barium or quinidine. Efflux via the cotransporter was not inhibited by R(+)-butylindazone, a KCl cotransport inhibitor, but it was enhanced in the presence of ouabain (P < 0.001) or increased extracellular Na concentration (P < 0.01). Furthermore, Na efflux was bumetanide sensitive (P < 0.05). In all, these data suggest that the efflux cotransporter is also of the Na-K-Cl form and that it is the same transporter as the influx mechanism operating in both directions. The evidence presented leads us to hypothesize that this cotransporter is on the apical membrane of the choroid plexus and that it may have a central role in CSF secretion and perhaps CSF K homeostasis.

Protein kinase C inhibits Na(+)-K(+)-2Cl- cotransporter activity in cultured rabbit nonpigmented ciliary epithelium

We examined the regulation of Na(+)-K(+)-2Cl- transporter activity by protein kinase C (PKC) in a cell line derived from rabbit nonpigmented ciliary epithelium. Na(+)-K(+)-2Cl- cotransporter activity was measured as the rate of bumetanide-sensitive potassium (86Rb) transport. Phorbol 12,13-dibutyrate (PBDu) was used to activate PKC. PBDu inhibited bumetanide-sensitive potassium (86Rb) uptake, with a half-maximal inhibitory concentration of approximately 0.1 microM. The inhibitory effect of PBDu on potassium uptake by the N(+)-K(+)-2Cl- cotransporter was abolished by PCK downregulation and diminished by 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine, a PKC inhibitor. PBDu inhibited Na(+)-K(+)-2Cl- cotransporter-mediated inward potassium (86Rb) transport by approximately 26% in control cells and by 40% in cells pretreated with ouabain. PKC activation also reduced the rate of bumetanide-sensitive potassium (86Rb) efflux in ouabain-treated cells but not in control (no oubain) cells. PBDu caused little change of intracellular sodium, potassium, or chloride, suggesting that an alteration of cytoplasmic ion composition is not responsible for the observed PBDu-induced changes in the rate of either inward or outward potassium movement mediated by the Na(+)-K(+)-2Cl- cotransporter.

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 cells in an electrically neutral manner, in most cases with a stoichiometry of 1Na:1K:2Cl. Na-K-Cl cotransporters are present in a wide variety of cells and tissues, including reabsorptive and secretory epithelia, nerve and muscle cells, endothelial cells, fibroblasts, and blood cells. Na-K-Cl cotransport plays a vital role in renal salt reabsorption and in salt secretion by intestinal, airway, salivary gland, and other secretory epithelia. Cotransport function also appears to be important in the maintenance and regulation of cell volume and of ion gradients by both epithelial and nonepithelial cells. Na-K-Cl cotransport activity is inhibited by "loop" diuretics, including the clinically efficacious agents bumetanide and furosemide. The regulation of Na-K-Cl cotransport is mediated, at least in some cases, through direct phosphorylation of the cotransport protein. Cotransporter regulation is highly tissue specific, perhaps in part related to the presence of different Na-K-Cl cotransporter isoforms. In epithelia, both absorptive (kidney-specific) and secretory isoforms have been identified by cDNA cloning and sequencing and Northern blot analysis; alternatively spliced variants of the kidney-specific isoform have also been identified. The absorptive and secretory isoforms exhibit approximately 60% identity at the amino acid sequence level; these sequences in turn show approximately 45% overall homology with those of thiazide-sensitive, bumetanide-insensitive, Na-Cl cotransport proteins of winter flounder urinary bladder and mammalian kidney. This review focuses on recent developments in the identification of Na-K-Cl cotransport proteins in epithelial and on the regulation of epithelial Na-K-Cl cotransporter function at cellular and molecular levels.

Characterization of the proteins of the intestinal Na(+)-K(+)-2Cl- cotransporter

Absorptive intestinal epithelia, such as that of the winter flounder, absorb salt via a bumetanide-sensitive Na(+)-K(+)-2Cl- cotransport mechanism on the brush-border membrane (BBM). The present study demonstrates the first molecular characterization of the intestinal Na(+)-K(+)-2Cl- cotransporter and its unique regulation. The photoaffinity bumetanide analogue, 4-[3H]benzoyl-5-sulfamoyl-3- (3-thenyloxy)benzoic acid, specifically labeled three groups of proteins in flounder intestinal microsomal membranes (MM): a approximately 180-kDa peptide, prominently labeled, and diffuse bands at approximately 110-70 and 50 kDa, less intensely labeled. Subcellular fractionation revealed a single prominently labeled protein of approximately 170 kDa in BBM but not in basolateral membranes (BLM) and little or no labeling of proteins of approximately 110-70 or 50 kDa. Polyclonal antiserum raised against the Ehrlich ascites cell cotransporter identified a 180-kDa peptide in MM and a 175-kDa peptide (pI approximately 5.4) in BBM but none in BLM or in the cytosol of flounder intestine. As predicted from the regulation of cotransport in this tissue, phosphorylation of this protein is increased by guanosine 3',5'-cyclic monophosphate (cGMP)-dependent but not by adenosine 3',5'-cyclic monophosphate-dependent protein kinase. In addition, phosphorylation of the protein is not increased by protein kinase C or Ca2+/calmodulin-dependent protein kinase but is increased by the phosphatase inhibitor calyculin A. Finally, calyculin A preserves the inhibitory effect of cGMP on ion transport, even in the absence of the nucleotide, suggesting that phosphorylation-dephosphorylation mechanisms are crucial in cotransporter regulation. Thus the flounder intestinal cotransporter is a approximately 175-kDa BBM protein that can be regulated by phosphorylation.

Diuretic drugs and the treatment of edema: from clinic to bench and back again

Despite wide variations in dietary NaCl intake, homeostatic mechanisms ensure that renal NaCl excretion matches intake at steady state. This does not imply, however, that extracellular fluid volume is maintained within narrow limits. In contrast with blood pressure, which appears to be tightly controlled, extracellular fluid volume varies significantly, even in normal individuals, when dietary NaCl intake changes. Cardiac, liver, or renal disease can perturb the relationship between NaCl intake and extracellular fluid volume and lead to symptomatic edema. All major classes of diuretic drugs in use today were developed between 1950 and 1970. These drugs were developed empirically, without knowledge of specific ion transport pathways, but experimental work during the past 15 years has shown that each major class of diuretic inhibits a specific ion transport protein in the kidney. These transport proteins have been characterized physiologically and the mechanisms by which each diuretic drug inhibits ion transport have been defined. Antibodies directed against these transport proteins have localized ion transport pathways to specific cell types along the nephron. Most recently, isoforms of each class of diuretic-sensitive Na transport pathway have been cloned. Ongoing experimental work is aimed at exploring relationships between families of transporters, determining the structural prerequisites for ion transport, and studying molecular mechanisms of transport regulation. Treatment of edema with diuretics is often straightforward, but can lead to adaptive changes in nephron structure and function. These adaptations can limit the effectiveness of diuretic drugs; maneuvers aimed at blocking these processes can be effective approaches to patients who are resistant to diuretic drugs.

Ion transport asymmetry and functional coupling in bovine pigmented and nonpigmented ciliary epithelial cells

The solute and water transport properties of the bovine ciliary epithelium were studied using isolated pigmented (PE) and nonpigmented (NPE) cells. It was shown that these cells were functionally coupled by demonstrating dye diffusion between paired PE and NPE cells after microinjection of lucifer yellow. Electronic cell sizing was used to measure cell volume changes of isolated PE and NPE cells in suspension after anisosmotic perturbations and after transport inhibition under isosmotic conditions. The PE cells showed the presence of a regulatory volume increase when subjected to osmotic shrinkage with NaCl, whereas the NPE cells did not demonstrate a regulatory volume increase under these conditions. In contrast, the NPE cells exhibited a regulatory volume decrease when subjected to osmotic swelling, whereas the PE cells did not recover from swelling. The regulatory volume decrease in NPE cells was inhibited by increased bath K or pretreatment with quinine (1 mM). The presence of a bumetanide-sensitive mechanism capable of moving measurable amounts of solute and water, probably Na-K-2Cl cotransport, was demonstrated in the PE cells but absent in the NPE cells. Bumetanide produced a dose-dependent shrinkage of PE cells at concentrations as low as 1 microM. Isosmotically reducing bath Cl, Na, or K concentration caused a rapid shrinkage of PE cells that was bumetanide inhibitable. The asymmetry of transport properties in PE and NPE cells supports a functional syncytium model of aqueous humor formation (39) across the two layers of the ciliary epithelium wherein ion uptake from the blood is carried out by the PE cells and ion extrusion by the NPE cells. Gap-junction coupling between the cells allows the ions taken up by the PE cells to move into the NPE cells. Extrusion of Na by the Na-K pump across the aqueous facing (basolateral) membranes of the NPE cells, most likely accompanied by Cl, determines the formation of the aqueous humor.

A method for analysis of cellular K-transport mechanisms through thallium (201Tl) uptake in human lymphocytes

Thallium (Tl) is the K-congener with the highest specific affinity to K-binding sites and the largest permeability in K-channels, and its usefulness for analysis of cellular K-transport mechanisms was investigated. The uptake of 201Tl was measured in cultured human lymphocytes (Jurkat cells and EBV-transformed B-cells) after various incubation conditions. In a complete culture medium the half-time of the Tl-uptake was 10 min at 37 degrees C and the steady state accumulation ratio (intracellular/extracellular) was c. 50. The steady state ouabain sensitive uptake was 81% in complete culture medium, it was reduced after substitution with Ringer solution and by a decrease in temperature (from 37 to 22 degrees C). The ouabain resistant uptake was higher at 22 degrees C than at 37 degrees C and it was almost completely inhibited in Na-free or Cl-free solution and by furosemide. The present results indicate that there are only two major routes for K-uptake in lymphocytes and that 201Tl may be a useful substitute for K in the analysis of cellular K-transport mechanisms.

Inhibition of Na(+)-K(+)-ATPase activates Na(+)-K(+)-2Cl- cotransporter activity in cultured ciliary epithelium

Inhibition of Na(+)-K(+)-ATPase activates Na(+)-K(+)-2Cl- cotransporter activity in cultured ciliary epithelium. Am. J. Physiol. 266 (Cell Physiol. 35): C198-C205, 1994.--86Rb uptake experiments were conducted to measure Na(+)-K(+)-ATPase activity and Na(+)-K(+)-2Cl- cotransporter activity in a cell line derived from rabbit nonpigmented ciliary epithelium. The presence of a Na(+)-K(+)-2Cl- cotransporter was supported by the observation of a bumetanide-sensitive 86Rb uptake component that was dependent on the extracellular concentration of both sodium and chloride. Potassium influx mediated by the Na(+)-K(+)-2Cl- cotransporter and Na(+)-K(+)-ATPase accounted for approximately 46 and 33% of total potassium uptake, respectively, whereas both ouabain- and bumetanide-resistant uptake accounted for 9%. Inhibition of the Na(+)-K(+)-ATPase had a stimulatory effect on Na(+)-K(+)-2Cl- cotransporter activity, which was dependent on the extent and duration of Na(+)-K(+)-ATPase inhibition. Ouabain treatment stimulated the potassium (86Rb) efflux rate and reduced intracellular potassium ([K]i). Potassium channel blockers suppressed the ouabain-activated potassium efflux and inhibited the ouabain-induced activation of the Na(+)-K(+)-2Cl- cotransporter. We conclude that Na(+)-K(+)-ATPase inhibition leads to the opening of potassium channels, which exacerbates the depletion of cellular potassium; Na(+)-K(+)-2Cl- cotransporter stimulation caused by the fall of [K]i overrides the tendency of increased cellular sodium to inhibit the cotransporter.

Identification of potassium flux pathways and their role in the cytotoxicity of estramustine in human malignant glioma, prostatic carcinoma and pulmonary carcinoma cell lines

Clinically-used drugs such as furosemide, bumetanide and cardiac glycosides, are modulators of transmembrane fluxes of cations. Recently, it has been suggested that the regulation of intracellular cation concentrations could be a primary target for anti-neoplastic drugs, and that the cytotoxic activity may be altered by inhibitors of cation fluxes at the level of the plasma membrane. Therefore, we investigated the mechanisms by which cations are translocated across the plasma membrane of malignant glioma (U251 MG), prostatic carcinoma (PC3) and pulmonary carcinoma (P31) cell lines. The interactions between cation flux inhibitors and the cytotoxicity of estramustine were also evaluated. Ouabain, the classical inhibitor of Na+, K+ATPase, markedly reduced 86Rb (K+) influx in all three lines, indicating that this ion transport system is present in the cells. Furosemide and especially bumetanide inhibited the 86Rb influx, indicating the presence of the Na+, K+, Cl- co-transport system. The potassium channel blocker, tetraethylammonium, but not apamin reduced the influx of 86Rb showing that high conductance K+ channels are present, but that channels of low conductance probably do not exist in these cell lines. The Na+, K+, Cl- co-transport inhibitors furosemide and bumetanide significantly reduced cytotoxicity of estramustine in P31 cells, whereas no interaction between other K+ flux inhibitors and the anti-neoplastic drugs were detected in any of the cell lines investigated. Thus, the data show that Na+, K+, ATPase and NA+, K+, Cl- co-transport systems and K+ channels of high conductance are present in malignant glioma (U251 MG), prostatic carcinoma (PC3) and pulmonary carcinoma (P31) cell lines, and that inhibition of the Na+, K+, Cl- co-transport system in P31 is associated with reduced cytotoxicity of estramustine. The results justify further studies evaluating the role of these cation flux pathways in terms of targets for anti-neoplastic therapy.

Identification, characterization and purification of a 160 kD bumetanide-binding glycoprotein from the rabbit parotid

We demonstrate the presence of a 160 kD protein in rabbit parotid basolateral membranes that can be labeled with the irreversible sulfhydryl reagent [14C]-N-ethylmaleimide in a bumetanide-protectable fashion. The specificity of this labeling, and our previous evidence for the existence of an essential sulfhydryl group closely associated with the bumetanide-binding site on the parotid Na(+)-K(+)-Cl-cotransporter (J. Membrane Biol. 112:51-58, 1989), provide strong evidence that this protein is a part or all of the parotid bumetanide-binding site. When this protein is treated with endoglycosidase F/N-glycosidase F to remove N-linked oligosaccharides, its apparent molecular weight decreases to 135 kD. The pI of this deglycosylated protein is approximately 6.4. The bumetanide-binding protein was purified using two preparative electrophoresis steps. First, a Triton X-100 extract enriched in this protein was run on preparative electrophoresis to obtain fractions containing proteins in the 160 kD range. These were then deglycosylated with endoglycosidase F/N-glycosidase F and selected fractions were pooled and rerun on preparative electrophoresis to obtain a final 135 kD fraction. The enrichment of the bumetanide-binding protein in this final 135 kD fraction estimated from [14C]-N-ethylmaleimide labeling was approximately 48 times relative to the starting membrane extract. Since the bumetanide-binding site represents approximately 2% of the total protein in this starting extract, this enrichment indicates a high degree of purity of this protein in the 135 kD fraction.

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

K efflux pathways responsible for regulatory volume decrease (RVD) were examined in bovine aortic endothelial cells. Hypotonic swelling produced a rapid and reversible threefold increase in bumetanide-insensitive 86Rb efflux. Swelling-activated 86Rb efflux was inhibited 43% when Cl was replaced with NO3, and this Cl-dependent efflux was inhibited by 1 mM furosemide. Neither Cl replacement nor furosemide inhibited the efflux stimulated by a Ca ionophore (A23187) in isotonic medium. Swelling-activated 86Rb efflux was also inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonate but not by dinitrostilbenedisulfonate. Cell swelling induced a volume-regulatory K loss that was incomplete in hypotonic medium but complete and more rapid when bumetanide was added or when cells were swollen isosmotically. K loss in the presence of bumetanide was partially blocked by furosemide. We conclude that two separate swelling-activated K fluxes mediate RVD in aortic endothelial cells: a Cl-dependent, furosemide-sensitive, but bumetanide-insensitive flux that is consistent with K-Cl cotransport, and a Cl-independent efflux that presumably is mediated by K channels.

Cl- and K+ transport in human biliary cell lines

BACKGROUND

The cellular mechanisms which contribute to billing secretion and absorption are not fully defined. The purpose of these studies was to evaluate the membrane ion transport properties of Mz-ChA-1 and Sk-ChA-1 cell lines derived from human biliary tumors.

METHODS

In cultured cells, 125I and 36Cl efflux rates were used to assess membrane anion permeability, and 86Rb efflux rates were used to assess K+ permeability.

RESULTS

Sections of tumors grown on BALB/Urd mice were used for morphological evaluation and for detection of cystic fibrosis transmembrane conductance regulator (CFTR), the protein product of the cystic fibrosis gene. There was organized development of ductular structures and cells stained for gamma-glutamyl transpeptidase and CK-19. Immunoperoxidase staining for CFTR, which is likely a Cl- channel, was also present. Increases in intracellular Ca2+ stimulated by exposure to ionomycin or thapsigargin increased efflux of 125I, 36Cl, and 86Rb. Efflux of 125I was greater than 36Cl, and anion efflux was inhibited by the Cl- channel blocker 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid. Increases in 5'-cyclic adenosine monophosphate increased efflux of 36Cl greater than 125I but had no effect on 86Rb efflux. Both cell lines possess bumetanide-sensitive 86Rb uptake consistent with possible Na+/K+/2Cl- cotransport.

CONCLUSIONS

These human cell lines retain certain phenotypic features of differentiated biliary cells and may be useful for further investigation of biliary fluid and electrolyte transport.

Mechanisms of sodium and chloride transport across isolated sheep reticulum

1. 22Na+ and 36Cl- fluxes across isolated reticular epithelium of sheep were measured by using the Ussing-chamber technique. 2. Net NaCl absorption driven by Na(+)-K(+)-ATPase was observed under short-circuit conditions. 3. Evaluation of fluxes measured under voltage-clamp conditions indicated that Na+ absorption is mainly electroneutral. 4. Mucosal application of bumetanide, hydrochlorothiazide, or low dose amiloride (10(-4) M) produced no changes in Na+ transport whereas addition of higher doses of amiloride (> or = 10(-3) M) led to a reduction in net Na+ transport. Short chain fatty acids (SCFA) enhanced the amiloride-sensitive Na+ transport. 5. Alterations of JmsNa induced by inhibitors or by SCFA were always accompanied by qualitatively similar changes of JsmNa. Amiloride-sensitive JsmNa was also decreased at low mucosal Na+ concentration. 6. DIDS, SITS, and nitrate reduced both JmsCl and JsmCl. SCFA did not influence chloride transport. 7. It is concluded that Na+ transport is mediated by Na(+)-H+ exchange and by transport processes operating as Na+ self-exchange. Mucosal-to-serosal chloride transport seems partly to depend on anion exchange systems.

Role of Na-K-Cl cotransport in vascular endothelial cell volume regulation

Vascular endothelial cells have been shown previously to possess a highly active Na-K-Cl cotransport system that mediates the major portion of total K influx and is regulated by a variety of vasoactive hormones and neurotransmitters. These observations suggest that the cotransporter may be an important component of endothelial cell function. The present study was conducted to investigate the role of Na-K-Cl cotransport in regulation of endothelial cell volume. Cultured bovine aortic endothelial cells were exposed to media of varying tonicities and Na-K-Cl cotransport activity assessed as bumetanide-sensitive K influx. Increasing the extracellular tonicity by increments as small as 10 mosM was found to cause significant stimulation of cotransport activity, and lowering tonicity reduced activity of the transporter. Exposure of endothelial cells to hypertonic medium was also found to increase bumetanide-sensitive net uptake of Na and K and total cellular Na and K content. Endothelial cell volume was evaluated by [14C]urea determination of intracellular water space in endothelial monolayers and by electronic cell sizing of suspended cells. Treatment of the cells with agents that stimulate Na-K-Cl cotransport activity was found to increase cell volume, whereas cotransport-inhibiting agents decreased cell volume. Exposure of the cells to hypertonic medium caused a rapid decrease in cell volume, followed by a regulatory volume increase that was greatly attenuated by bumetanide. The volume recovery was partially inhibited by the Na-H exchange inhibitor amiloride and was nearly abolished by bumetanide and amiloride in combination. Endothelial cells of pulmonary artery and cerebral microvessels were also found to exhibit increased Na-K-Cl cotransport activity on exposure to hypertonic media. These findings suggest that Na-K-Cl cotransport is of major importance in endothelial cell volume regulation.

Cl- transport in frog retinal pigment epithelium

Cl- transport across the retinal membrane of the frog retinal pigment epithelium was studied by means of double-barrelled Cl- selective microelectrodes. Three types of experiments were performed. In the first group of experiments, the ionic dependence of Cl- influx across the retinal membrane was studied. The intracellular Cl- activity was first decreased by perfusing the retinal side of the epithelium with low Cl- solutions (3.6 mM Cl-); then the perfusate was changed to high Cl- solutions (90.1 mM), and the resulting Cl- influx was studied. In these experiments, the combined presence of extracellular Na+ and K+ was a necessary condition for Cl- influx across the retinal membrane. This supports the hypothesis of Na+,K+,Cl- co-transport across this membrane. In a second group of experiments, the effect of furosemide was studied. Furosemide (100 microM) inhibited Cl- influx when the retinal extracellular Cl- concentration was increased from 3.6 to 90.1 mM. When administered to cells in steady state, furosemide in concentrations between 5 and 1000 microM decreased the intracellular Cl- activity. Michaelis-Menten analysis yielded a Ki for furosemide of 7 +/- 2 microM. The effect of furosemide on the intracellular Cl- activity required the combined presence of extracellular Na+ and K+. When the retinal extracellular K+ concentration was increased to between 0 and 10 mM, the furosemide-sensitive Cl- influx across the retinal membrane increased. Michaelis-Menten analysis yielded a half maximal stimulation at an extracellular K+ concentration of 0.5 mM. Stimulation of the epithelium with 1 mM cAMP and 0.5 mM IBMX reduced the effect of furosemide on the intracellular Cl- activity by 26%. In a third group of experiments, the effect of transepithelial currents on the intracellular Cl- activity was investigated. Currents that depolarized the choroidal membrane potential increased the intracellular Cl- activity; currents that hyperpolarized this membrane potential decreased the intracellular Cl- activity. These findings are compatible with conductive Cl- transport across the choroidal membrane. The apparent Cl- conductance of this membrane was estimated to be 0.59 mS cm-2. This represents 27% of the total conductance in the choroidal membrane. Administration of 1 mM cAMP and 0.5 mM IBMX caused a 21% increase in the apparent Cl- conductance of the choroidal membrane.