The Na+/K+/Cl- cotransport in C6 glioma cells. Properties and role in volume regulation

Reducing the late sodium current improves cardiac function during sodium pump inhibition by ouabain

Inhibition by cardiac glycosides of Na(+), K(+)-ATPase reduces sodium efflux from myocytes and may lead to Na(+) and Ca(2+) overload and detrimental effects on mechanical function, energy metabolism, and electrical activity. We hypothesized that inhibition of sodium persistent inward current (late I(Na)) would reduce ouabain's effect to cause cellular Na(+) loading and its detrimental metabolic (decrease of ATP) and functional (arrhythmias, contracture) effects. Therefore, we determined effects of ouabain on concentrations of intracellular sodium (Na(+)(i)) and high-energy phosphates using (23)Na and (31)P NMR, the amplitude of late I(Na) using the whole-cell patch-clamp technique, and contractility and electrical activity of guinea pig isolated hearts, papillary muscles, and ventricular myocytes in the absence and presence of inhibitors of late I(Na). Ouabain (1-1.3 μM) increased Na(+)(i) and late I(Na) of guinea pig isolated hearts and myocytes by 3.7- and 4.2-fold, respectively. The late I(Na) inhibitors ranolazine and tetrodotoxin significantly reduced ouabain-stimulated increases in Na(+)(i) and late I(Na). Reductions of ATP and phosphocreatine contents and increased diastolic tension in ouabain-treated hearts were also markedly attenuated by ranolazine. Furthermore, the ouabain-induced increase of late I(Na) was also attenuated by the Ca(2+)-calmodulin-dependent kinase I inhibitors KN-93 [N-[2-[[[3-(4-chlorophenyl)-2-propenyl]methylamino]methyl]phenyl]-N-(2-hydroxyethyl)-4-methoxybenzenesulphonamide] and autocamide-2 related inhibitory peptide, but not by KN-92 [2-[N-(4'-methoxybenzenesulfonyl)]amino-N-(4'-chlorophenyl)-2-propenyl-N-methylbenzylamine phosphate]. We conclude that ouabain-induced Na(+) and Ca(2+) overload is ameliorated by the inhibition of late I(Na).

Inhibition of the Sodium-Potassium-Chloride Cotransporter Isoform-1 reduces glioma invasion

Malignant gliomas metastasize throughout the brain by infiltrative cell migration into peritumoral areas. Invading cells undergo profound changes in cell shape and volume as they navigate extracellular spaces along blood vessels and white matter tracts. Volume changes are aided by the concerted release of osmotically active ions, most notably K(+) and Cl(-). Their efflux through ion channels along with obligated water causes rapid cell shrinkage. Suitable ionic gradients must be established and maintained through the activity of ion transport systems. Here, we show that the Sodium-Potassium-Chloride Cotransporter Isoform-1 (NKCC1) provides the major pathway for Cl(-) accumulation in glioma cells. NKCC1 localizes to the leading edge of invading processes, and pharmacologic inhibition using the loop diuretic bumetanide inhibits in vitro Transwell migration by 25% to 50%. Short hairpin RNA knockdowns of NKCC1 yielded a similar inhibition and a loss of bumetanide-sensitive cell volume regulation. A loss of NKCC1 function did not affect cell motility in two-dimensional assays lacking spatial constraints but manifested only when cells had to undergo volume changes during migration. Intracranial implantation of human gliomas into severe combined immunodeficient mice showed a marked reduction in cell invasion when NKCC1 function was disrupted genetically or by twice daily injection of the Food and Drug Administration-approved NKCC1 inhibitor Bumex. These data support the consideration of Bumex as adjuvant therapy for patients with high-grade gliomas.

Extracellular glutamine is a critical modulator for regulatory volume increase in human glioma cells

Mammalian cells regulate their volume to prevent unintentional changes in intracellular signaling, cell metabolism, and DNA integrity. Intentional cell volume changes occur as cells undergo proliferation, apoptosis, or cell migration. To regulate cell volume, cells use channels and transport systems to flux osmolytes across the plasma membrane followed by the obligatory movement of water. While essentially all cells are capable of regulatory volume decrease (RVD), regulatory volume increase (RVI) mechanisms have only been reported in some cell types. In this investigation, we used human glioma cells as a model system to determine conditions necessary for RVI. When exposed to hyperosmotic conditions through the addition of 30 mosM NaCl or sucrose, D54-MG and U251 glioma cell lines and glioma cells from acute patient biopsies shrunk transiently but were able to fully recover their original cell volume within 40-70 min. This ability was highly temperature sensitive and absolutely required the presence of low millimolar concentrations of l-glutamine in the extracellular solution. Other known substrates of glutamine transporters such as methyl-amino isobutyric acid (MeAIB), alanine, and threonine were unable to support RVI. The ability of cells to undergo RVI also required the presence of Na+, K+, and Cl- and was inhibited by the NKCC inhibitor, bumetanide, consistent with the involvement of a Na+/K+/2Cl- cotransporter (NKCC). Moreover, the expression of NKCC1 was demonstrated by Western blot. We concluded that regulatory volume increase in human glioma cells occurs through the uptake of Na+, K+, and Cl- by NKCC1 and is modulated by the presence of glutamine.

Physiology and pathophysiology of Na+/H+ exchange and Na+ -K+ -2Cl- cotransport in the heart, brain, and blood

Maintenance of a stable cell volume and intracellular pH is critical for normal cell function. Arguably, two of the most important ion transporters involved in these processes are the Na+/H+ exchanger isoform 1 (NHE1) and Na+ -K+ -2Cl- cotransporter isoform 1 (NKCC1). Both NHE1 and NKCC1 are stimulated by cell shrinkage and by numerous other stimuli, including a wide range of hormones and growth factors, and for NHE1, intracellular acidification. Both transporters can be important regulators of cell volume, yet their activity also, directly or indirectly, affects the intracellular concentrations of Na+, Ca2+, Cl-, K+, and H+. Conversely, when either transporter responds to a stimulus other than cell shrinkage and when the driving force is directed to promote Na+ entry, one consequence may be cell swelling. Thus stimulation of NHE1 and/or NKCC1 by a deviation from homeostasis of a given parameter may regulate that parameter at the expense of compromising others, a coupling that may contribute to irreversible cell damage in a number of pathophysiological conditions. This review addresses the roles of NHE1 and NKCC1 in the cellular responses to physiological and pathophysiological stress. The aim is to provide a comprehensive overview of the mechanisms and consequences of stress-induced stimulation of these transporters with focus on the heart, brain, and blood. The physiological stressors reviewed are metabolic/exercise stress, osmotic stress, and mechanical stress, conditions in which NHE1 and NKCC1 play important physiological roles. With respect to pathophysiology, the focus is on ischemia and severe hypoxia where the roles of NHE1 and NKCC1 have been widely studied yet remain controversial and incompletely elucidated.

Lactacidosis-induced glial cell swelling depends on extracellular Ca2+

Cerebral tissue acidosis following ischemia or traumatic brain injury contributes to cytotoxic brain edema formation. In vitro lactacidosis induces swelling of glial cells by intracellular Na+- and Cl--accumulation by the Na+/H+-antiporter, Cl-/HCO3--antiporters and the Na+-K+-2Cl--cotransport. The present study aimed to elucidate whether mechanisms of lactacidosis-induced glial swelling are dependent on intra- or extracellular Ca2+-ions. Therefore, C6 glioma cells were exposed to a lactacidosis of pH 6.2 in standard or calcium-free medium and following intracellular calcium chelation. Cell volume and intracellular pH were assessed by flow cytometry. Lactacidosis of pH 6.2 induced a prompt and sustained swelling of suspended C6 glioma cells reaching a maximum of 128% within 60 min. Omission of Ca2+ from the suspension medium strongly attenuated cell swelling while chelation of intracellular Ca2+ had no effects. Intracellular acidosis was not affected by either treatment. The present data show a strong dependency of lactacidosis-induced glial swelling upon extracellular Ca2+ while intracellular acidosis is not affected by omission of [Ca2+]e. Therefore, our data suggest that the Na+-K+-2Cl--cotransporter, the only so far known transporter involved in cell volume regulation but not in pHi regulation during lactacidosis, is activated in a [Ca2+]e-dependent manner.

Effects of the new ethacrynic acid derivative SA9000 on intraocular pressure in cats and monkeys

To evaluate the pharmacological characteristics of the new ethacrynic acid (ECA) derivative SA9000, we examined its ocular hypotensive effects in cats and cynomolgus monkeys, its corneal toxicity in rabbits, and its binding affinities for forty-three receptors, ion channels, and second messenger systems. A 20 microl injection into the anterior chamber of eye (intracameral injection) of 0.1 mM SA9000 significantly reduced intraocular pressure (IOP) 3.8 mmHg in cats. A 10 microl intracameral injection of 1 mM SA9000 significantly reduced IOP 7 mmHg in living monkeys without evidence of in vivo (or in vitro) toxicity. The ocular hypotensive effect of SA9000 in monkeys was greater than that of ECA. The morphology of corneal endothelial and epithelial cells in rabbit eyes after intracameral injection of SA9000 was observed using electron microphotography. SA9000 at 2 mM did not induce any abnormalities, indicating that it has no corneal toxicity at a concentration higher than the minimum needed for an ocular hypotensive effect (1 mM). SA9000 at 0.01 mM showed negligible binding affinity for, or inhibition of, forty-three different receptors, ion channel proteins, and second messenger systems. These findings indicate that SA9000 has the potential to be both effective and safe as an ocular hypotensive drug, although the mechanism of action remains unclear.

Regulation of Na(+)-K(+)-Cl(-) cotransporter in primary astrocytes by dibutyryl cAMP and high [K(+)](o)

In this study, we examined the Na(+)-K(+)-Cl(-) cotransporter activity and expression in rat cortical astrocyte differentiation. Astrocyte differentiation was induced by dibutyryl cAMP (DBcAMP, 0. 25 mM) for 7 days, and cells changed from a polygonal to process-bearing morphology. Basal activity of the cotransporter was significantly increased in DBcAMP-treated astrocytes (P < 0.05). Expression of an approximately 161-kDa cotransporter protein was increased by 91% in the DBcAMP-treated astrocytes. Moreover, the specific [(3)H]bumetanide binding was increased by 67% in the DBcAMP-treated astrocytes. Inhibition of protein synthesis by cyclohexamide (2-3 microgram/ml) significantly attenuated the DBcAMP-mediated upregulation of the cotransporter activity and expression. The Na(+)-K(+)-Cl(-) cotransporter in astrocytes has been suggested to play a role in K(+) uptake. In 75 mM extracellular K(+) concentration, the cotransporter-mediated K(+) influx was stimulated by 147% in nontreated cells and 79% in DBcAMP-treated cells (P < 0.05). To study whether this high K(+)-induced stimulation of the cotransporter is attributed to membrane depolarization and Ca(2+) influx, the role of the L-type voltage-dependent Ca(2+) channel was investigated. The high-K(+)-mediated stimulation of the cotransporter activity was abolished in the presence of either 0.5 or 1.0 microM of the L-type channel blocker nifedipine or Ca(2+)-free HEPES buffer. A rise in intracellular free Ca(2+) in astrocytes was observed in high K(+). These results provide the first evidence that the Na(+)-K(+)-Cl(-) cotransporter protein expression can be regulated selectively when intracellular cAMP is elevated. The study also demonstrates that the cotransporter in astrocytes is stimulated by high K(+) in a Ca(2+)-dependent manner.

Contribution of anion transporters to the acidosis-induced swelling and intracellular acidification of glial cells

This study examines the contribution of anion transporters to the swelling and intracellular acidification of glial cells from an extracellular lactacidosis, a condition well-known to accompany cerebral ischemia and traumatic brain injury. Suspended C6 glioma cells were exposed to lactacidosis in physiological or anion-depleted media, and different anion transport inhibitors were applied. Changes in cell volume and intracellular pH (pH(i)) were simultaneously quantified by flow cytometry. Extracellular lactacidosis (pH 6.2) led to an increase in cell volume to 125.1 +/- 2.5% of baseline within 60 min, whereas the pH(i) dropped from the physiological value of 7.13 +/- 0.05 to 6.32 +/- 0.03. Suspension in Cl(-)-free or HCO(3)(-)/CO(2)-free media or application of anion transport inhibitors [0.1 mM bumetanide or 0.5 mM 4, 4'-diisothio-cyanatostilbene-2,2'-disulfonic acid (DIDS)] did not affect cell volume during baseline conditions but significantly reduced cell swelling from lactacidosis. In addition, the Cl(-)-free or HCO(3)(-)/CO(2)-free media and DIDS attenuated intracellular acidosis on extracellular acidification. From these findings it is concluded that besides the known activation of the Na(+)/H(+) exchanger, activation of the Na(+)-independent Cl(-)/HCO(3)(-) exchanger and the Na(+)-K(+)-Cl(-) cotransporter contributes to acidosis-induced glial swelling and the intracellular acidification. Inhibition of these processes may be of interest for future strategies in the treatment of cytotoxic brain edema from cerebral ischemia or traumatic brain injury.

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.

Cl- transport in the lobster stretch receptor neurone

Experiments were performed to identify mechanisms underlying non-leakage and non-H+/HCO3--linked transmembrane Cl- transports in the slowly adapting stretch receptor neurone of the European lobster, using intracellular microelectrode and pharmacological techniques. In methodological tests, it was established that direct estimates of intracellular Cl- with ion-sensitive microelectrodes are statistically identical with indirect estimates by means of a GABA method, where 1-2 mM GABA is transforming the cell's membrane voltage into its Cl- equilibrium voltage from which the Cl- concentration is inferred by the Nernst equation. From experiments using sodium orthovanadate and ethacrynic acid, supposed to block primary Cl- pumps, and bumetanide, supposed to block Na-K-Cl co-transporters, it appeared that neither of the two Cl- transport systems exists in the stretch receptor neurone. It could be shown, however, that the cell is equipped with an electroneutral K-Cl co-transporter that (a) is blockable by furosemide in high (Km approximately 350 microM), by 4-acetamido-4'-isothiocyanato-stilbene-2,2-disulphonic acid (SITS) in medium-high (Km approximately 35 microM), and by 4, 4'-diisothiocyanostilbene-2,2'-disulphonic acid (DIDS) in low (Km approximately 15 microM) doses, (b) is (transiently) activatable by (1 mM) n-ethylmaleimide, (c) is not suppressed by extracellular Rb+ or NH4+, and (d) is not directly coupled to any transmembrane transports of Na+, H+ or HCO3-. From functional tests, with varying transmembrane K+ and Cl- gradients, evidence obtained that the K-Cl co-transporter is able to reverse its transport direction and to adjust its transport rate in a considerable range. As a whole, the results speak in favour of the K-Cl co-transporter being responsible (a) for normally keeping the intracellular Cl- concentration at low levels, for an optimization of the cell's inhibitory system, and (b) for achieving fast transmembrane shifts of K+ (and Cl-), as a means of stabilizing the cell's membrane excitability in conditions of varying extracellular K+ concentrations.

Activation and inactivation of taurine efflux in hyposmotic and isosmotic swelling in cortical astrocytes: role of ionic strength and cell volume decrease

A decrease in intracellular ionic strength appears involved in the activation of swelling-elicited 3H-taurine efflux in cortical cultured astrocytes. Hyposmotic (50%) or isosmotic urea-induced swelling leading to a decrease of intracellular ionic strength, activated 3H-taurine efflux from a rate constant of about 0.008 min(-1) to 0.33 min(-1) (hyposmotic) and 0.59 min(-1) (urea). This efflux rate was markedly lower (maximal 0.03 min(-1)) in isosmotic swelling caused by K+ accumulation, where there is no decrease in ionic strength, or in cold (10 degrees C) hyposmotic medium (maximal 0.18 min(-1)), where swelling is reduced and consequently intracellular ionic strength is less affected. Also, astrocytes pretreated with hyperosmotic medium, which recover cell volume by ion accumulation, did not release 3H-taurine when they swelled by switching to isosmotic medium, but when volume was recovered by accumulation of urea, taurine release was restored. These results point to a key role of ionic strength in the activation of osmosensitive 3H-taurine efflux. In contrast, its inactivation was independent of the change in ionic strength but appears related to the reduction in cell volume after swelling, since despite the extent or direction of the change in ionic strength, the 3H-taurine efflux did not inactivate in isosmotic KCl-elicited swelling when cell volume did not recover nor in hyposmotic swelling when RVD was impaired by replacing NaCl in the medium by permeant osmolytes.

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.

Sensitization to hyperthermia by intracellular acidification of C6 glioma cells

Hyperthermia has been introduced as a new modality of treatment for glioma. In these experiments, the cytotoxicity of hyperthermia in C6 glioma cells was enhanced by increasing the intracellular acidity with amiloride and/or 4,4'-diisothiocyanatostilbene-2,2' disulfonic acid (DIDS). Intracellular pH (pHi) is regulated mainly by Na+/H+ and HCO3-/Cl- antiports through the cell membrane, and amiloride acts on the former, DIDS on the latter to lower pHi. The cellular thermosensitivity to clinically achievable brain hyperthermia at 42 degrees C was enhanced by 0.5 mM amiloride (Na+/H+ antiport inhibitor). T0 values (T0 = the heating period required to reduce experimental survival rate by 1/e) at 42 degrees C without and with amiloride was 192 and 81 min, respectively. The addition of DIDS (HCO3-/Cl- antiport inhibitor) further enhanced. T0 value was 25 min. Fluorophotometric measurement of pHi was employed using the pH sensitive dye, bis(carboxyethyl)carboxyfluorescein, which is trapped in viable cells. The average pHi in control C6 glioma cells in pH 7.2 media was 7.21. In the untreated cells heated at 42 degrees C for 1 hour, the pHi was 7.12. The pHi of the cells heated in the presence of amiloride was decreased to 6.83. The pHi was further lowered to 6.67 by the treatment with amiloride in combination with DIDS for 2 hours. Hyperthermia with amiloride and DIDS may be a more effective treatment for malignant gliomas.

Potassium homeostasis and glial energy metabolism

Since capillaries appear not to contribute significantly to rapid removal of K+ from brain tissue, the K+ released into extracellular clefts by neurons at the onset of electrical activity is presumably removed either by redistribution in the clefts or by uptake into cells. What appear to be the three major processes require no energy from the glial cells. These are diffusion through the extracellular clefts, spatial buffering by glial cells, and net uptake of K+ into glial cells through glial K+ channels associated with uptake of Cl- through an independent Cl- conductance. There is a relatively slow uptake by the Na+/K+-ATPase, which directly consumes ATP. In addition, some glial cells take up K+ on the Na+/K+/2Cl- cotransporter, which leads indirectly to energy consumption when the Na+ is subsequently pumped out. Currently available data suggest that the glial energy metabolism devoted to K+ homeostasis is less than a tenth of the total tissue energy metabolism, even under conditions of pathologically high extracellular [K+]. Hence, in situ, it is possible that glial cells could function with much less ATP than neurons do. All the various routes of muffling of changes in extracellular [K+] can be modulated, directly or indirectly, by transmitters liberated by neurons. A consequence of this could be regulation of the entry of Na+ into glial cells such that the Na+/K+-ATPase is activated. The degree of activation might be adjusted so that the resulting activation of the glial glycolytic pathway is appropriate to the provision of the quantity of metabolic substrates required by the neurons.

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.

Electrolyte transport mechanisms involved in regulatory volume increase in C6 glioma cells

Volume regulation of C6 glioma cells was studied with an automatic system for monitoring cell thickness, while increasing bath osmolality from 300 to 440 mosmol/kgH2O. At 37 degrees C, tissues incubated in solutions containing active substances (inositol, D-biotin, hydrocortisone, prostaglandin E1, insulin, transferrin, sodium selenite, and 3,5,3'-triiodothyronine) responded to hyperosmotic challenge with a typical regulatory volume increase (RVI). Lowering temperature or removing the active substances inhibited osmoregulation. Bumetanide, amiloride, 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid, or ouabain significantly reduced RVI. Ion substitutions of Na+, Cl-, NaCl, or HCO3- also importantly affected the process. Extracellular acidification rate (EAR) was studied by microphysiometry. Hyperosmotic shock induced an increase in EAR with a time course that matched volume recovery. This increase in EAR was prevented by amiloride. The data show that under hyperosmotic conditions C6 cells are able to regulate their volume. Ion substitutions and application of blockers demonstrate that Na+/H+ and Cl-/HCO3- exchangers and Na(+)-K(-)-2Cl- cotransporter are involved in RVI. The rise in EAR is due to the enhanced activity of Na+/H+ antiporter, which seems to be volume dependent but not osmotic dependent.

Thyrotropin regulation of basolateral Cl- and I- effluxes in thyroid follicles in culture

This report describes chloride and iodide effluxes across the basolateral membrane of porcine thyroid follicles reconstituted in culture. Basolateral chloride efflux is activated by thyrotropin (TSH). TSH (10 mU/ml) induces a twofold increase in the initial rate of chloride efflux. Forskolin (FSK, 5 microM) which increases intracellular cAMP also stimulates the initial rate of chloride efflux 3.5-fold, whereas an increase in the free cytosolic Ca2+ with the ionophore A23187 or thapsigargin, fails to mimic the TSH effect. The chloride channel blocker 5-nitro-2(3-phenylpropylamino)benzoic acid (NPPB) dose dependently inhibits chloride efflux rates with the maximal and half maximal effects observed for 100 microM and 30 microM, respectively. Basolateral chloride efflux rates are also inhibited in the presence of the organic anion transporter blocker probenecid (5 mM) or the Cl-/HCO3- exchanger blocker 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS, 250 microM), respectively, by 60% and 40%, whereas it is not affected by ClO4 (100 microM). The initial rate of iodide efflux is weakly activated (1.4-fold) by TSH (10 mU/ml). TSH effect could be reproduced by agents known to activate Ca(2+)-dependent processes as A23187, ionomycin (1 microM), phorbol 12-myristate 13-acetate (TPA, 0.1 microM) and epidermal growth factor (EGF, 0.1 microM) which increase the initial rate of iodide efflux from 1.2- to 1.8-fold, whereas FSK is without effect. The chloride channel blocker NPPB (500 microM) is required to significantly inhibit the initial rate of iodide efflux by 30%. The initial rate of iodide efflux is also reduced by 30% in the presence of SITS (250 microM) or probenecid (5 mM) whereas it is activated by ClO4 (100 microM). We conclude that basolateral chloride and iodide effluxes are both regulated by TSH, using two different transduction pathways. Chloride efflux regulation may involve a cAMP transduction signal, whereas the regulation of iodide efflux may involve a Ca2+ signal. Furthermore, as the sensitivities of chloride and iodide effluxes for the anion transporter blockers (especially NPPB) are different, it seems likely that chloride and iodide use two different transport pathways.

Treatment of vasogenic brain edema with the novel Cl- transport inhibitor torasemide

The efficacy of the diuretic agent torasemide, which antagonizes the Na+/K+/Cl- cotransport and Cl- channels, was investigated to determine its inhibition of brain edema from a focal cerebral lesion. For this purpose, cold injury of the brain was induced in 50 Sprague-Dawley rats while monitoring arterial blood pressure. The brain was removed for gravimetric assessment of swelling of the traumatized hemisphere 24 h after trauma. The water content was also determined after drying the cerebral hemispheres for 24 h. Animals were divided into five groups. A control group with trauma received vehicle only; two other groups received 1.0 or 10.0 mg torasemide/kg body weight 30 minutes before and 6 h after trauma (n = 10-12). Administration of the drug after the insult was also investigated in animals with application of vehicle or 10.0 mg/kg of torasemide at 30 minutes and 6 h following the brain lesion (n = 8). Torasemide did not affect important physiologic variables, such as the arterial pO2, pCO2, pH, hemoglobin, hematocrit, or plasma osmolality, while increasing blood pressure (p < 0.01). The blood pressure response notwithstanding, treatment significantly attenuated hemispheric brain swelling from trauma. In control animals without treatment, cold injury led to hemispheric swelling of 8.89%. In animals with 1 mg torasemide/kg BW, brain swelling amounted to 8.51% and to 7.04% in animals receiving 10 mg/kg before and after the insult (p < 0.005). Treatment was also found to attenuate the increase in tissue water content from trauma, but without reaching statistical significance. Postinsult treatment with torasemide (10 mg/kg BW) at 30 minutes and 6 h after trauma was again associated with a significant reduction in hemispheric brain swelling, which in this group amounted to 7.46% compared with 9.76% in the untreated controls (p < 0.005). The increase in the cerebral water content from trauma was also significantly blunted in the latter experiments (p < 0.01). The present data indicate a remarkable therapeutic potential of the novel diuretic agent torasemide to reduce vasogenic brain edema from an acute cerebral lesion. It is surmised that the compound specifically interferes with Cl- transport mechanisms, which apparently are activated in edematous brain involving neuronal and glial cells, for example. This conclusion is supported by in vitro observations that torasemide inhibits the swelling of glial cells from acidosis. On the other hand, it is unlikely that gross dehydration of the brain secondary to the induction of diuresis by the agent played a role, because hematocrit and plasma osmolality were not found to be affected.

Regulatory volume decrease in cultured astrocytes. I. Potassium- and chloride-activated permeability

Regulatory volume decrease (RVD) in detached cerebellar astrocytes in culture after acute exposure to hyposmolarity was characterized in this and the accompanying paper [H. Pasantes-Morales, R. A. Murray, R. Sanches-Olea, and J. Moran. Am. J. Physiol. 266 (Cell Physiol. 35): C172-C178, 1994]. RVD was independent of extracellular calcium, was accelerated at pH 8-9 and retarded at pH 6, and was reduced at temperatures < 18 degrees C. The cationic pathway activated by hyposmolarity was specific for K+ and Rb+, since RVD was abolished and secondary swelling occurred when these ions replaced Na+. However, Li+, choline, tris(hydroxymethyl)aminomethane, and glucosamine, all as Cl- salts, did not affect RVD. The anion pathway was unselective, since RVD was inhibited when NaCl was replaced by anion K+ salts with a permeability rank of SCN- = I- > NO3- > Cl- > benzoate > acetate >> SO3- > gluconate. RVD was unaffected by bumetanide (50 microM) and weakly inhibited by furosemide (2 mM). Quinidine but not other K+ channel blockers inhibited RVD, and its effect was reversed by gramicidin. RVD was inhibited by 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid and dipyridamole but not by diphenylamine-2-carboxylate or anthracene-9-carboxylate. These results suggest that diffusion possibly via channels rather than cotransporters is involved in the swelling-activated K+ and Cl- fluxes. Gramicidin did not change astrocyte volume in isosmotic conditions, but greatly accelerated RVD, suggesting that low Cl- permeability in isosmotic conditions markedly increases by swelling, thus making K+ permeability the rate-limiting step for RVD.

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.

Na+, K+, Cl- cotransport and its regulation in Ehrlich ascites tumor cells. Ca2+/calmodulin and protein kinase C dependent pathways

Net Cl- uptake as well as unidirectional 36Cl influx during regulatory volume increase (RVI) require external K+. Half-maximal rate of bumetanide-sensitive 36Cl uptake is attained at about 3.3 mM external K+. The bumetanide-sensitive K+ influx found during RVI is strongly dependent on both Na+ and Cl-. The bumetanide-sensitive unidirectional Na+ influx during RVI is dependent on K+ as well as on Cl-. The cotransporter activated during RVI in Ehrlich cells, therefore, seems to transport Na+, K+ and Cl-. In the presence of ouabain and Ba+ the stoichiometry of the bumetanide-sensitive net fluxes can be measured at 1.0 Na+, 0.8 K+, 2.0 Cl- or approximately 1:Na, 1:K, 2:Cl. Under these circumstances the K+ and Cl- flux ratios (influx/efflux) for the bumetanide-sensitive component were estimated at 1.34 +/- 0.08 and 1.82 +/- 0.15 which should be compared to the gradient for the Na+, K+, 2Cl- cotransport system at 1.75 +/- 0.24. Addition of sucrose to hypertonicity causes the Ehrlich cells to shrink with no signs of RVI, whereas shrinkage with hypertonic standard medium (all extracellular ion concentrations increased) results in a RVI response towards the original cell volume. Under both conditions a bumetanide-sensitive unidirectional K+ influx is activated. During hypotonic conditions a small bumetanide-sensitive K+ influx is observed, indicating that the cotransport system is already activated. The cotransport is activated 10-15 fold by bradykinin, an agonist which stimulates phospholipase C resulting in release of internal Ca2+ and activation of protein kinase C. The anti-calmodulin drug pimozide inhibits most of the bumetanide-sensitive K+ influx during RVI. The cotransporter can be activated by the phorbol ester TPA. These results indicate that the stimulation of the Na+, K+, Cl- cotransport involves both Ca2+/calmodulin and protein kinase C.

Inability of Ehrlich ascites tumor cells to volume regulate following a hyperosmotic challenge

Ehrlich cells shrink when the osmolality of the suspending medium is increased and behave, at least initially, as osmometers. Subsequent behavior depends on the nature of the hyperosmotic solute but in no case did the cells exhibit regulatory volume increase. With hyperosmotic NaCl an osmometric response was found and the resultant volume maintained relatively constant. Continuous shrinkage was observed, however, with sucrose-induced hyperosmolality. In both cases increasing osmolality from 300 to 500 mOSM initiated significant changes in cellular electrolyte content, as well as intracellular pH. This was brought about by activation of the Na+/H+ exchanger, the Na/K pump, the Na+ + K+ + 2Cl cotransporter and by loss of K+ via a Ba-sensitive pathway. The cotransporter in response to elevated [Cl-]i (approximately 100 mM) and/or the increase in the outwardly directed gradient of chemical potential for Na+, K+ and Cl-, mediated net loss of ions which accounted for cell shrinkage in the sucrose-containing medium. In hyperosmotic NaCl, however, the net Cl- flux was almost zero suggesting minimal net cotransport activity. We conclude that volume stability following cell shrinkage depends on the transmembrane gradient of chemical potential for [Na+ + K+ + Cl-], as well as the ratio of intra- to extracellular [Cl-]. Both factors appear to influence the activity of the cotransport pathway.

Volume regulation in mouse pancreatic beta-cells is mediated by a furosemide-sensitive mechanism

A possible role for loop diuretic-sensitive Cl-/cation cotransport in volume regulation in the pancreatic beta-cells was investigated by measuring 86Rb+ efflux from beta-cell-rich pancreatic islets as well as the size of isolated beta-cells under different osmotic conditions. Lowering the osmolarity to 262 mosM (83% of control) resulted in a rapid cell swelling which was followed by regulatory volume decrease (RVD). RVD was completely inhibited by furosemide (1 mM), an inhibitor of Cl-/cation co-transport. The hypotonic medium (262 mosM) induced a rapid and strong increase in 86Rb+ efflux from beta-cell-rich mouse pancreatic islets and the furosemide-sensitive portion of the efflux was significantly increased. A slightly less hypotonic medium (285 mosM, 90% of control) induced only cell swelling and no RVD. With this medium only a marginal increase in 86Rb+ efflux was observed. Increasing the osmolarity by adding 50 mM NaCl (final osmolarity: 417 mosM, 132% of control) induced a rapid cell shrinkage but no regulatory volume increase (RVI). When the osmolarity was increased from a slightly hypotonic medium (262 mosM) to an isotonic medium (317 mosM) an initial cell shrinkage was followed by RVI. This RVI was inhibited by 1 mM furosemide. The data suggest that RVD as well as RVI in the beta-cells are mediated by loop diuretic-sensitive cotransport of chloride and cations and that these cells show a threshold for hypotonic stimulation of RVD.

Regulatory volume increase in Ehrlich ascites tumor cells

Ehrlich ascites tumor cells, shrunken as a result of KCl-depletion and Na+ loading, re-establish normal ionic concentrations by the combined activity of the Na+/K+ pump and the (2Cl- + K+ + Na+) cotransport system. Restoration of cell volume, however, correlates only with the increase in intracellular Cl-. This along with the finding that the equilibrium volume is linearly related to the steady state [Cl-] suggests that the extent to which cell volume increases is determined by Cl- transport. Net Cl- uptake, which is mediated almost exclusively by the cotransport system, is ultimately responsible for establishing the steady-state intracellular Cl- concentration. Transport mediated by this pathway ceases when the sum of the chemical potentials for Na+, K+ and Cl- approaches zero and corresponds with the establishment of a steady state for Cl-. These findings suggest that Cl- plays a key role in the regulation of net cotransport activity and thereby cell volume.

Volume regulation in mouse pancreatic islet cells as studied by a new technique of microperifusion

A new technique was designed to analyse whether pancreatic islet cells are able to regulate their volume in anisotonic media. The projected cell area of individual cells was continuously observed, and the corresponding volume calculated during microperifusion with media of different osmolarities. In isotonic medium (317 mosmol) the cell volume was stable during perifusion and decreased by 17 or 25% when the osmolarity was increased (sucrose) to 417 or 517 mosmol. Reducing the medium osmolarity to 285 mosmol resulted in a volume reduction of about 7%. No evidence for cell volume regulation was observed in these media. However, reducing the medium osmolarity to 262 mosmol induced an immediate and rapid cell swelling of approximately 14%, after which the initial cell volume was regained within 9 min. The data suggest that the pancreatic islet cells are equipped with mechanisms for regulatory volume decrease that appear to be activated when the cell volume is increased above a certain limit.

Sodium and potassium uptake in primary cultures of rat astroglial cells induced by long-term exposure to the basic astroglial growth factor (AGF2)

Astroglial cell cultures were derived from newborn rat forebrain and cultured for 5 days in serum containing-, and for an additional 4 days in a serum-free, defined medium. At the end of this 9-day-long period, basic astroglial growth factor (AGF2) was administered to the culture medium (10 ng per ml). Cells were subsequently cultured in AGF2 containing serum-free, defined medium for further two weeks. At definite intervals of culturing, unidirectional influx of both Na+ and K+ (INa and IK, respectively) was determined by applying 22Na and 42K. The AGF2-treated cultures showed highly increased, amiloride-sensitive INa at the early exposure period (2-8 hours), similar to that we have reported about cultured astroglia exposed to AGF2 for minutes. They also exhibited significant furosemide-sensitive-, while relatively poor ouabain-sensitive component of INa. However, at later periods of exposure to AGF2, INa was significantly reduced, particularly due to the decrease of its amiloride-sensitive component, while its furosemide-sensitive component further increased with the time of AGF2 treatment. In contrast to INa, the IK in the cultures exposed to AGF2 increased significantly in the course of the long-term exposure period, particularly the ouabain-, and furosemide-sensitive-components, while its amiloride-sensitive component, similarly to that of INa, decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

Lactic acid-induced swelling in C6 glial cells via Na+/H+ exchange

One of the primary consequences of ischemia is tissue acidification due to anaerobic production of lactic acid. Upon reperfusion and recovery of pH, cytotoxic edema often ensues. Na+/H+ exchange, a mechanism involved in the regulation of intracellular pH (pHi), is activated by low intracellular pH, is dependent on extracellular Na+, and is inhibited by low extracellular pH (pH less than 6) or by amiloride. In this study we explore the role of Na+/H+ exchange in cell swelling following cytoplasmic acidification of C6 glioma cells. Postischemic intracellular acidification was simulated in vitro by exposure of cells in suspension to: (1) 20 or 140 mM lactic acid; or (2) 10 microM oligomycin. pHi was monitored fluorimetrically using the intracellularly trapped pH-sensitive dye bis(carboxyethyl)carboxyfluorescein. Cell volume was measured electronically with a Coulter Counter/Channelyzer. Both simulations of ischemia caused intracellular acidification followed by recovery. pHi recovery was mediated by Na+/H+ exchange, since it was amiloride-sensitive and Na+-dependent. This pHi reversal following lactic acid-induced acidification was also inhibited at pHo less than 6. Volume measurements showed that cells suspended in 140 mM Na-lactate/lactic acid swelled by 19% over 15 min. This swelling was Na+-dependent, and inhibited by amiloride and pHo less than 6. These results suggest that Na+/H+ exchange may be involved in cell swelling following cytoplasmic acidification, and thus may be involved in postischemic cytotoxic brain edema.

Release of taurine from astrocytes during potassium-evoked swelling

Cultured astrocytes superfused with isosmotic solutions containing high concentrations of potassium, i.e., 25, 56, 75, and 100 mM, showed a proportional increase in cell volume corresponding to 25, 36, 57, and 75% greater than the cell volume in physiological solutions. This volume increase was abolished in low chloride or hypertonic solutions. The release of 3H-taurine previously accumulated by astrocytes was stimulated by potassium at all concentrations examined. During 4-minute exposure to 25, 56, 75, or 100 mM of potassium, cells released 13.5, 15.6, 20.2, or 36.2%, respectively, of the total labeled taurine accumulated during the preloading period. The potassium-stimulated release of 3H-taurine was calcium-independent and insensitive to BaCl2 and bumetanide. Substitution of chloride by gluconate to concentrations necessary to maintain the K+ X Cl- product constant abolished the potassium-stimulated release of 3H-taurine. Superfusion with solutions made hypertonic with sucrose also decreased the potassium-elicited efflux of 3H-taurine. In both conditions, the depolarizing effect of potassium measured with 3H-TPP+ was unchanged. High potassium concentrations and hyposmotic solutions released 3H-taurine by a nonadditive mechanism. These results indicate that in cultured astrocytes high concentrations of potassium produce both swelling and depolarization, but only swelling elicits the release of taurine. These observations suggest an involvement of taurine in cell-volume regulation in astrocytes.