Volume-regulatory taurine release from a human lung cancer cell line. Evidence for amino acid transport via a volume-activated chloride channel

RBC deformability and amino acid concentrations after hypo-osmotic challenge may reflect chronic cell hydration status in healthy young men

Biomarkers of chronic cell hydration status are needed to determine whether chronic hyperosmotic stress increases chronic disease risk in population-representative samples. In vitro, cells adapt to chronic hyperosmotic stress by upregulating protein breakdown to counter the osmotic gradient with higher intracellular amino acid concentrations. If cells are subsequently exposed to hypo-osmotic conditions, the adaptation results in excess cell swelling and/or efflux of free amino acids. This study explored whether increased red blood cell (RBC) swelling and/or plasma or urine amino acid concentrations after hypo-osmotic challenge might be informative about relative chronic hyperosmotic stress in free-living men. Five healthy men (20-25 years) with baseline total water intake below 2 L/day participated in an 8-week clinical study: four 2-week periods in a U-shaped A-B-C-A design. Intake of drinking water was increased by +0.8 ± 0.3 L/day in period 2, and +1.5 ± 0.3 L/day in period 3, and returned to baseline intake (0.4 ± 0.2 L/day) in period 4. Each week, fasting blood and urine were collected after a 750 mL bolus of drinking water, following overnight water restriction. The periods of higher water intake were associated with significant decreases in RBC deformability (index of cell swelling), plasma histidine, urine arginine, and urine glutamic acid. After 4 weeks of higher water intake, four out of five participants had ½ maximal RBC deformability below 400 mmol/kg; plasma histidine below 100 μmol/L; and/or undetectable urine arginine and urine glutamic acid concentrations. Work is warranted to pursue RBC deformability and amino acid concentrations after hypo-osmotic challenge as possible biomarkers of chronic cell hydration.

Swelling-induced taurine transport: relationship with chloride channels, anion-exchangers and other swelling-activated transport pathways

Cells have to regulate their volume in order to survive. Moreover, it is now evident that cell volume per se and the membrane transport processes which regulate it, comprise an important signalling unit. For example, macromolecular synthesis, apoptosis, cell growth and hormone secretion are all influenced by the cellular hydration state. Therefore, a thorough understanding of volume-activated transport processes could lead to new strategies being developed to control the function and growth of both normal and cancerous cells. Cell swelling stimulates the release of ions such as K(+) and Cl(-) together with organic osmolytes, especially the beta-amino acid taurine. Despite being the subject of intense research interest, the nature of the volume-activated taurine efflux pathway is still a matter of controversy. On the one hand it has been suggested that osmosensitive taurine efflux utilizes volume-sensitive anion channels whereas on the other it has been proposed that the band 3 anion-exchanger is a swelling-induced taurine efflux pathway. This article reviews the evidence for and against a role of anion channels and exchangers in osmosensitive taurine transport. Furthermore, the distinct possibility that neither pathway is involved in taurine transport is highlighted. The putative relationship between swelling-induced taurine transport and volume-activated anionic amino acid, alpha-neutral amino acid and K(+) transport is also examined.

Characterization of the Pharmacology of YM-198313 on Volume-Regulated Anion Channels

Activation of the volume-regulated anion channels (VRAC) is considered to be involved in arrhythmia, but it has not yet been fully elucidated because of the lack of its high affinitive and selective compounds. A newly synthesized compound, YM-198313 (sodium 4-({[2-(methylthio)benzyl]amino}-5-[(1-phenylethyl)thio]isothiazol-3-olate), strongly inhibited VRAC in HeLa cells with an IC50 of 3.03+/-0.05 microM. However, YM-198313 weakly affected both the Ca2+-activated Cl- channels in HTC cells and the cAMP-activated Cl- channels in T84 cells, demonstrating that this compound is selective for VRAC among Cl- channels. At 10 microM, YM-198313 almost completely (100+/-7.8%) inhibited the VRAC current in guinea pig atrial myocytes. However, at the same concentration, YM-198313 showed little inhibitory effect on the cardiac cation currents in ventricular myocytes. We believe that YM-198313 is a potent and selective VRAC inhibitor, therefore, it should be use to clarify the role VRAC plays in arrhythmia.

Differential effects of pyrethroids on volume-sensitive anion and organic osmolyte pathways

1. There are no effective ways of screening for potential modulators of volume-regulated anion channels in their native cell type. Generally, cell lines are used for this purpose. Using HeLa and C6 glioma cells, we identified the pyrethroids as a novel class of compounds that inhibit taurine efflux through volume-regulated anion transport pathways in these cells. Subsequently, we examined their effects on volume-regulated anion channels in guinea-pig ventricular myocytes to determine whether results obtained using cell lines could be extrapolated to other tissues. 2. Tetramethrin inhibited taurine efflux in both HeLa and C6 glioma cells with Ki values of approximately 26 and 16 micro mol/L, respectively. Bioallethrin and fenpropathrin inhibited volume-sensitive taurine efflux from C6 glioma cells, but not from HeLa cells. The Ki values for bioallethrin and fenpropathrin were 70 and 59 micro mol/L, respectively. 3. Volume-sensitive I- efflux was observed in HeLa cells but not in C6 glioma cells, suggesting that the taurine efflux pathway in C6 glioma cells may be different to that of the I- efflux pathway. Cyfluthrin, tetramethrin, fenpropathrin, tefluthrin and bioallethrin all significantly inhibited volume-sensitive I- efflux from HeLa cells at 100 micro mol/L. 4. Patch-clamp experiments have shown inhibition of ICl,vol in guinea-pig ventricular myocytes by fenpropathrin, but not tetramethrin or cypermethrin, at 100 micro mol/L. This revealed that further differences exist between ICl,vol in guinea-pig ventricular myocytes and the anion transport pathways in C6 glioma and HeLa cells. 5. In conclusion, we have shown that pyrethroids differentially inhibit volume-regulated anion and taurine efflux in a number of cell types. Because these compounds have different effects in different cells, it is likely that: (i) more than one pathway is involved in the volume-sensitive transport of anions and organic osmolytes; and (ii) the molecular identities of the channels underlying anion transport are different. Finally, for the reasons given above, care should be taken when extrapolating data from one cell type to another. However, in the absence of an existing high-throughput screen, taurine efflux still represents a viable route for the identification of potential modulators of volume-regulated ion channels.

Ca2+-mediated Potentiation of the Swelling-induced Taurine Efflux from HeLa Cells: On the Role of Calmodulin and Novel Protein Kinase C Isoforms

The present work sets out to investigate how Ca(2+) regulates the volume-sensitive taurine-release pathway in HeLa cells. Addition of Ca(2+)-mobilizing agonists at the time of exposure to hypotonic NaCl medium augments the swelling-induced taurine release and subsequently accelerates the inactivation of the release pathway. The accelerated inactivation is not observed in hypotonic Ca(2+)-free or high-K(+) media. Addition of Ca(2+)-mobilizing agonists also accelerates the regulatory volume decrease, which probably reflects activation of Ca(2+)-activated K(+) channels. The taurine release from control cells and cells exposed to Ca(2+) agonists is equally affected by changes in cell volume, application of DIDS and arachidonic acid, indicating that the volume-sensitive taurine leak pathway mediates the Ca(2+)-augmented taurine release. Exposure to Ca(2+)-mobilizing agonists prior to a hypotonic challenge also augments a subsequent swelling-induced taurine release even though the intracellular Ca(2+)-concentration has returned to the unstimulated level. The Ca(2+)-induced augmentation of the swelling-induced taurine release is abolished by inhibition of calmodulin, but unaffected by inhibition of calmodulin-dependent kinase II, myosin light chain kinase and calcineurin. The effect of Ca(2+)-mobilizing agonists is mimicked by protein kinase C (PKC) activation and abolished in the presence of the PKC inhibitor Gö6850 and following downregulation of phorbol ester-sensitive PKC isoforms. It is suggested that Ca(2+) regulates the volume-sensitive taurine-release pathway through activation of calmodulin and PKC isoforms belonging to the novel subclass (nPKC).

Osmotic swelling-provoked release of organic osmolytes in human intestinal epithelial cells

Human Intestine 407 cells respond to osmotic cell swelling by the activation of Cl−- and K+-selective ionic channels, as well as by stimulating an organic osmolyte release pathway readily permeable to taurine and phosphocholine. Unlike the activation of volume-regulated anion channels (VRAC), activation of the organic osmolyte release pathway shows a lag time of ∼30–60 s, and its activity persists for at least 8–12 min. In contrast to VRAC activation, stimulation of organic osmolyte release did not require protein tyrosine phosphorylation, active p21rho, or phosphatidylinositol 3-kinase activity and was insensitive to Cl−channel blockers. Treatment of the cells with putative organic anion transporter inhibitors reduced the release of taurine only partially or was found to be ineffective. The efflux was blocked by a subclass of organic cation transporter (OCT) inhibitors (cyanine-863 and decynium-22) but not by other OCT inhibitors (cimetidine, quinine, and verapamil). Brief treatment of the cells with phorbol esters potentiated the cell swelling-induced taurine efflux, whereas addition of the protein kinase C (PKC) inhibitor GF109203X largely inhibited the response, suggesting that PKC is involved. Increasing the level of intracellular Ca2+by using A-23187- or Ca2+-mobilizing hormones, however, did not affect the magnitude of the response. Taken together, the results indicate that the hypotonicity-induced efflux of organic osmolytes is independent of VRAC and involves a PKC-dependent step.

Creatine increases IGF-I and myogenic regulatory factor mRNA in C(2)C(12) cells

Addition of creatine to the differentiation medium of C(2)C(12) cells leads to hypertrophy of the myotubes. To investigate the implication of insulin-like growth factor I (IGF-I) and myogenic regulatory factors (MRFs) in this hypertrophy, their mRNA levels were assessed during the first 72 h of differentiation. Creatine significantly increased the IGF-I mRNA level over the whole investigated period of time, whereas the MRF mRNA levels were only augmented at precise moments, suggesting a general activation mechanism for IGF-I and a specifically regulated mechanism for MRF transcription. Our results suggest therefore that creatine-induced hypertrophy of C(2)C(12) cells is at least partially mediated by overexpression of IGF-I and MRFs.

Halide fluxes in epithelial cells measured with an automated cell plate reader

A method is introduced to measure chloride permeability in cultured epithelial cells using 6-methoxy-N-(3-sulfopropyl)quinolinium (SPQ) and 6-methoxy-N-ethylquinolinium iodide quinolinium (MEQ) as fluorescent chloride-sensitive probes. The method involves growing cells in multiwell plates, incubating cells with SPQ or MEQ, and then exchanging intracellular or extracellular halide ions with nitrate. The resulting time course of SPQ or MEQ fluorescence is followed by repetitive readings with a multiwell fluorescence plate reader. Exchange times are extracted by fitting the time course with a single exponential function of time. The method was validated by measuring the effect of chloride channel activators and blockers in A6 and MDCK cells. The baseline iodide/nitrate exchange time was 200-300 s. Isoproterenol (a modulator of cAMP-activated chloride channels) increased the exchange rate by a factor of 1.4+/-0.1; A23187 (a modulator of calcium-activated chloride channels) increased the rate by 3.4+/-0.4; bradykinin (also a modulator of calcium-activated chloride channels) increased the rate by 2.0+/-0.4; forskolin (a direct stimulator of adenylate cyclase) increased the rate by 2.7+/-0.3. Diphenylamine-2-carboxylate (a chloride channel blocker) decreased the rate by 0.12+/-0.03. These results indicate that our method is a valid indicator of halide-nitrate exchange in cultured epithelial cells.

Hypotonic cell swelling stimulates permeability to cAMP in a rat colonic cell line

This study characterized the membrane permeability to cAMP in a cell line derived from the rat colon (CC531(mdr+)) by comparison of fluxes of 3H-cAMP, 3H-8-bromo-cAMP, 3H-taurine, 3H-adenosine and 3H-5'AMP under various experimental conditions including cell membrane depolarization and hypotonic cell swelling. Cell volume was modified by changing the osmolality and composition of the extracellular medium. Incubation in iso- and hypotonic KCl media induced graded increases in cell volume and stable activation of volume-sensitive channels that was reflected in an increased efflux of 3H-taurine. Incubation in hypotonic KCl solution also enhanced the efflux of 3H-8-Br-cAMP (a non-hydrolysable analogue of cAMP). Both the efflux of 3H-taurine and of 3H-8-Br-cAMP were inhibited by 5-nitro-2-(3-phenylpropylamino)benzoate (NPPB, 100 microM) suggesting the involvement of volume-sensitive anion channels. To gain further insight into the route mediating cAMP permeability, the uptakes of 3H-cAMP, 3H-8-Br-cAMP and 3H-taurine were determined over short (5-min) periods. Uptakes of these substrates demonstrated close similarities: comparable increases were observed that correlated with the increases in cell volume in iso- and hypoosmotic KCl media; they were inhibited strongly by NPPB (100 microM) and metabolic inhibitors (deoxyglucose, 20 mM together with the mitochondrial uncoupler carbonylcyanide p-(trifluoromethoxy)phenylhydrazone, FCCP, 10 microM) while barely reduced by dipyridamole (100 microM) and they were not affected by adenosine (1 mM). In contrast, the uptakes of 3H-adenosine and 3H-5'AMP had strikingly different properties; they were insensitive to cell swelling; barely inhibited by NPPB (100 microM) and metabolic inhibitors (deoxyglucose and FCCP) while strongly reduced by dipyridamole (100 micro M). Unlike the uptakes of 3H-cAMP, 3H-8-Br-cAMP and 3H-taurine, the uptakes of 3H-adenosine and 3H-5'AMP were reduced in Na(+)-free media, suggesting the presence in this cell line of two different adenosine carriers, one sodium-dependent and one sodium-independent. Taken together the present data show that in this rat colonic cell line, cAMP permeability is increased by cell swelling in hypotonic KCl medium and inhibited by NPPB and metabolic inhibitors. The similarity of these characteristics to those of taurine permeability suggests the involvement of a volume-sensitive anion pathway.

Reactive oxygen species are important mediators of taurine release from skeletal muscle cells

The present study illustrates elements of the signal cascades involved in the activation of taurine efflux pathways in myotubes derived from skeletal muscle cells. Exposing primary skeletal muscle cells, loaded with (14)C-taurine, to 1) hypotonic media, 2) the phospholipase A(2) (PLA(2)) activator melittin, 3) anoxia, or 4) lysophosphatidyl choline (LPC) causes an increase in (14)C-taurine release and a concomitant production of reactive oxygen species (ROS). The antioxidants butulated hydroxy toluene and vitamin E inhibit the taurine efflux after cell swelling, anoxia, and addition of LPC. The muscle cells possess two separate taurine efflux pathways, i.e., a swelling- and melittin-induced pathway that requires 5-lipoxygenase activity for activation and a LPC-induced pathway. The two pathways are distinguished by their opposing sensitivity toward the anion channel blocker DIDS and cholesterol. These data provide evidence for PLA(2) products and ROS as key mediators of the signal cascade leading to taurine efflux in muscle.

Potentiation of glycine responses by dideoxyforskolin and tamoxifen in rat spinal neurons

Dideoxyforskolin, a forskolin analogue unable to stimulate adenylate cyclase, and tamoxifen, an antioestrogen widely used against breast cancer, are both known to block some Cl- channels. Their effects on Cl- responses to glycine or GABA have been tested here by using whole-cell recording from cultured spinal neurons. Dideoxyforskolin (4 or 16 microm) and tamoxifen (0.2-5 microm) both potentiate responses to low glycine concentrations. They also induce blocking effects, predominant at high glycine concentrations. At 5 microm, tamoxifen increased responses to 15 microm glycine by a factor >4.5, reaching 20 in some neurons. Potentiation by extracellular dideoxyforskolin or tamoxifen persisted after intracellular application of the modulator and was not due to Zn2+ contamination. Potentiation by tamoxifen also persisted in a Ca2+-free extracellular solution, after intracellular Ca2+ buffering and protein kinase C blockade. Thus, the critical sites of action are not intracellular. The EC50 for glycine was lowered 6.6-fold by 5 microm tamoxifen. The kinetics and voltage-dependence of the effects of tamoxifen on glycine responses support the idea that this hydrophobic drug may act from a site located within the membrane. Tamoxifen (5 micro m) also increased responses to 2 micro m GABA by a factor of 3.5, but barely affected peak responses to 20 microm GABA. The demonstration that tamoxifen affects some of the main inhibitory receptors should be useful for better evaluating its neurological effects. Furthermore, the results identify a new class of molecules that potentiate glycine receptor function.

Hypotonic-induced transport pathways in Xenopus laevis erythrocytes: taurine fluxes

Taurine fluxes in Xenopus laevis red cells were studied in vitro in media of different tonicities. Both influx and efflux increased 3-10 times reversibly when dilution of the medium exceeded 30%. The absolute values of uptake ranged between 5 and 30 micromol/l cells.h at extracellular taurine concentration of 1 mmol/l, but is poorly selective as almost the same uptake was measured for choline and sucrose. Q(10) of 2.77 and an activation energy of 71.90+/-7.37 kJ/mol were calculated for the uptake process. Taurine uptake was reduced 50% in the absence of Cl(-), whereas the alkali cations (Na(+), K(+), Li(+) and Rb(+)) supported it similarly. Taurine uptake was greatly increased in Ca(2+)-free solution, and was inhibited by alkaline pH. The inhibitor of anion exchange protein, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (IC(50)=25 microM) and the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid and [(dihydro-indenyl) oxy] alkanoic acid (IC(50)<20 microM) inhibited taurine uptake effectively. Isoproterenol did not affect taurine uptake in isotonic, nor in hypotonic solution. The uptake was reduced slowly to near the original, control level within 15-30 min in hypotonic solutions, indicating deactivation of the hypotonic-induced taurine pathway.

Taurine: evidence of physiological function in the retina

Taurine is a free amino acid found in high millimolar concentrations in mammalian tissue and is particularly abundant in the retina. Mammals synthesize taurine endogenously with varying abilities, with some species more dependent on dietary sources of taurine than others. Human children appear to be more dependent on dietary taurine than adults. Specifically, it has been established that visual dysfunction in both human and animal subjects results from taurine deficiency. Moreover, the deficiency is reversed with simple nutritional supplementation with taurine. The data suggest that taurine is an important neurochemical factor in the visual system. However, the exact function or functions of taurine in the retina are still unresolved despite continuing scientific study. Nevertheless, the importance of taurine in the retina is implied in the following experimental findings: (1) Taurine exhibits significant effects on biochemical systems in vitro. (2) The distribution of taurine is tightly regulated in the different retinal cell types through the development of the retina. (3) Taurine depletion results in significant retinal lesions. (4) Taurine release and uptake has been found to employ distinct regulatory mechanisms in the retina.

Molecular structure and physiological function of chloride channels

Cl- channels reside both in the plasma membrane and in intracellular organelles. Their functions range from ion homeostasis to cell volume regulation, transepithelial transport, and regulation of electrical excitability. Their physiological roles are impressively illustrated by various inherited diseases and knock-out mouse models. Thus the loss of distinct Cl- channels leads to an impairment of transepithelial transport in cystic fibrosis and Bartter's syndrome, to increased muscle excitability in myotonia congenita, to reduced endosomal acidification and impaired endocytosis in Dent's disease, and to impaired extracellular acidification by osteoclasts and osteopetrosis. The disruption of several Cl- channels in mice results in blindness. Several classes of Cl- channels have not yet been identified at the molecular level. Three molecularly distinct Cl- channel families (CLC, CFTR, and ligand-gated GABA and glycine receptors) are well established. Mutagenesis and functional studies have yielded considerable insights into their structure and function. Recently, the detailed structure of bacterial CLC proteins was determined by X-ray analysis of three-dimensional crystals. Nonetheless, they are less well understood than cation channels and show remarkably different biophysical and structural properties. Other gene families (CLIC or CLCA) were also reported to encode Cl- channels but are less well characterized. This review focuses on molecularly identified Cl- channels and their physiological roles.

Effects of calcium, calmodulin, protein kinase C and protein tyrosine kinases on volume-activated taurine efflux in human erythroleukemia cells

The effects of calcium, calmodulin, protein kinase C (PKC) and protein tyrosine kinase (PTK) modulators were examined on the volume-activated taurine efflux in the erythroleukemia cell line K562. Exposure to hypoosmotic solution significantly increased taurine efflux and intracellular calcium concentration ([Ca2+]i). The Ca2+ channel blockers La3+ (1 mM), verapamil (200 microM) and nifedipine (100 microM) inhibited the hypoosmotically-induced [Ca2+]i increase by more than 90%, while the volume-activated taurine efflux was inhibited by 61.3 +/- 9.5, 74.1 +/- 9.3 and 38.0 +/- 1.5%, respectively. Furthermore, the calmodulin inhibitors W7 (50 microM) and trifluoperazine (10 microM) and the Ca2+/calmodulin-dependent protein kinase II inhibitor KN-62 (2 microM) significantly blocked the volume-activated taurine efflux by 93.4 +/- 2.7, 77.9 +/- 3.5 and 61.3 +/- 15.8%, respectively. In contrast, the PKC inhibitor staurosporine (200 nM) or the PKC activator phorbol 12-myristate 13-acetate (100 nM) did not have significant effects on the volume-activated taurine efflux. However, pretreatment with PTK inhibitors genistein, tyrphostin A25, and tyrphostin A47 blocked the volume-activated taurine efflux. These results suggest that the volume-activated taurine efflux in K562 cells may not directly involve Ca2+, but may require the presence of calmodulin and/or PTK.

Cellular model for induction of drip loss in meat

Drip loss from porcine muscle (M. longissimus dorsi) contained high concentrations of K(+) ( approximately 135 mM) and organic osmolytes, for example, taurine ( approximately 15 mM), as well as significant amounts of protein ( approximately 125 mg.mL(-1)). Thus, the drip reflects release of intramuscular components. To simulate events taking place at the time of slaughter and leading to release of osmolytes and subsequent formation of drip loss, C2C12 myotubes were exposed to anoxia and reduction in pH (from 7.4 to 6.0). Anoxia and acidification increased the cellular Ca(2+) concentration ([Ca(2+)](i)) at a rate of 22-32 nM.min(-)(1). The anoxia-induced increase in [Ca(2+)](i) was mainly due to influx via sarcolemmal Na(+) channels. As mammalian cells swell and release lysophospholipids during anoxia, C2C12 cells and primary porcine muscle cells were exposed to either hypotonic shock or lysophosphatidylcholine (LPC) and the release of taurine was followed. The swelling-induced taurine efflux was blocked in the presence of the anion channel blocker (DIDS), the 5-lipooxygenase inhibitors (ETH 615-139 and NDGA) but unaffected by the presence of vitamin E. In contrast, the LPC-induced taurine release was unaffected by DIDS but abolished by antioxidants (butylated hydroxytoluene and vitamin E). Thus, stress-induced taurine release from muscles may precede by two different mechanisms, one being 5-lipooxygenase dependent and the other involving generation of reactive oxygen species. A model for the cellular events, preceding formation of drip in meat, is presented.

Cardiac chloride channels: physiology, pharmacology and approaches for identifying novel modulators of activity

Drugs that block cardiac cation channels have been marketed as the therapeutic answer to cardiac arrhythmia. However, such molecules have been only moderately successful at improving the survival of cardiac patients, and so new targets have been needed for future antiarrhythmic agents. This article outlines the properties and roles of Cl(-) channels, which are one of these new targets, and describes an approach for identifying novel CI(2) channel modulators.

Transport of milk constituents by the mammary gland

This review deals with the cellular mechanisms that transport milk constituents or the precursors of milk constituents into, out of, and across the mammary secretory cell. The various milk constituents are secreted by different intracellular routes, and these are outlined, including the paracellular pathway between interstitial fluid and milk that is present in some physiological states and in some species throughout lactation. Also considered are the in vivo and in vitro methods used to study mammary transport and secretory mechanisms. The main part of the review addresses the mechanisms responsible for uptake across the basolateral cell membrane and, in some cases, for transport into the Golgi apparatus and for movement across the apical membrane of sodium, potassium, chloride, water, phosphate, calcium, citrate, iodide, choline, carnitine, glucose, amino acids and peptides, and fatty acids. Recent work on the control of these processes, by volume-sensitive mechanisms for example, is emphasized. The review points out where future work is needed to gain an overall view of milk secretion, for example, in marsupials where milk composition changes markedly during development of the young, and particularly on the intracellular coordination of the transport processes that result in the production of milk of relatively constant composition at a particular stage of lactation in both placental and marsupial mammals.

Analysis of hyposmolarity-induced taurine efflux pathways in the bullfrog sympathetic ganglia

Hyposmolarity-induced taurine release was dependent on the decrease in medium osmolarity (5-50%) in the satellite glial cells of the bullfrog sympathetic ganglia. Release of GABA induced by hyposmolarity was much less than that of taurine. Omission of external Cl- replaced with gluconate totally suppressed taurine release, but only slightly suppressed GABA release. Bumetanide and furosemide, blockers of the Na+/K+/2Cl- cotransport system, inhibited taurine release by about 40%. Removal of external Na+ by replacement with choline, or omission of K+, suppressed taurine release by 40%. Antagonists of the Cl-/HCO3 exchange system, SITS, DIDS and niflumic acid, significantly reduced taurine release. The carbonic anhydrase inhibitor, acetazolamide, reduced the taurine release by 34%. Omission of external HCO3 by replacement with HEPES caused a 40% increase in the hyposmolarity-induced taurine release. Hyposmolarity-induced GABA release was not affected by bumetanide or SITS. Chloride channel blockers, 5-nitro-2-(3-phenylpropylamino) benzoic acid (NPPB) and N-phenylanthranilic acid (DPC), practically abolished taurine release. Blockers of K+ channels, clofilium and quinidine, had no effect on the taurine release. The hyposmolarity-induced taurine release was considerably enhanced by a simultaneous increase in external K+. GABA was not mediated by the same transport pathway as that of taurine. These results indicate that Cl- channels may be responsible for the hyposmolarity-induced taurine release, and that Na+/K+/2Cl- cotransporter and Cl-/HCO3 exchanger may contribute to maintain the intracellular Cl- levels higher than those predicted for a passive thermodynamic distribution in the hyposmolarity-induced taurine release.

The role of taurine in the regulation of brain cell volume in chronically hyponatraemic rats

Cells in slices prepared from the superficial cerebral cortex of normonatraemic rats underwent moderate swelling when exposed to low Na+ medium (122 mmol/l) accompanied by a large increase in the rate of efflux of preloaded taurine. In contrast, cells in slices from chronically (4 day) hyponatraemic rats did not increase in volume and the rate of taurine efflux was unchanged. The anion transport inhibitor 4,4'-diisothiocyanato-stilbene-2,2'-sulphonic acid (25 micromol/l) caused marked (-44%) reduction in taurine efflux in cells from normonatraemic rats; this response was strongly attenuated (-16%) by hyponatraemia. When slices from hyponatraemic rats were acutely exposed to medium containing 142 mmol/Na+ cells exhibited marked and paradoxical swelling. This response was completely abolished by the NaCl co-transport inhibitor bumetanide (50 micromol/l) and was not observed in slices that had not been pre-loaded with taurine. Forty eight hours after the start of the remission of hyponatraemia, cells from post-hyponatraemic rats displayed normal responses (i.e., moderate swelling and greatly accelerated taurine efflux) on exposure to 122 mmol/Na+. But at 24 h there was only partial restoration of the efflux response to 122 mmol/Na+, with an enhanced cell swelling response that was not significantly affected by bumetanide. It is concluded that (i) during chronic hyponatraemia, unlike acute hyposmotic stress, cortical cells preserve their volume and that this is not associated with any increase in the rate of taurine loss; there does however, appear to be a decrease in the anionic component of cellular taurine efflux; (ii) acute re-incubation of slices in medium containing 142 mmol/l Na+ is associated with cell swelling that may reflect up-regulation of Na/Cl/taurine co-transport; (iii) following restoration of normonatraemia the pattern of normal cellular response to acute hyposmotic stress is only gradually re-established.

Anion competition for a volume-regulated current

We have examined whether the anionic amino acids, glutamate and aspartate, permeate through the same volume-regulated conductance permeant to Cl- ions. Cell swelling was initiated in response to establishing a whole-cell configuration in the presence of a hyposmotic gradient. Volume-regulated anion currents carried by Cl-, glutamate, or aspartate developed with similar time courses and showed similar voltage-dependent inactivation. Permeability ratios (Paa/PCl) calculated from measured reversal potentials were dependent on the mole fraction ratio (MFR) of the permeant anions ([aa]/([aa] + [Cl-])). MFR was varied from 0.00 to 0.97. As the fraction of amino acid increased, Paa/PCl decreased. Current amplitude was similarly dependent on MFR. These results show that the permeation of anionic amino acids and that of Cl- ions are not independent of each other, indicating that the ion channel underlying the volume-regulated conductance can be occupied by more than one ion at a time. Application of Eyring rate theory indicated that the major barrier to Cl- ion permeation is at the intracellular side of the membrane, and that the major barrier to amino acid permeation is at the extracellular side of the membrane. The interactions between these permeant ions may have a physiological modulatory role in volume regulation through a volume-regulated anion conductance.

Volume-activated taurine efflux from the in situ perfused lactating rat mammary gland

The effect of cell swelling on the efflux of amino acids from the in situ perfused lactating rat mammary gland has been examined. Cell swelling, induced by a hyposmotic shock, increased the fractional release of [3H]taurine. In contrast, a hyposmotic shock did not stimulate the efflux of D-[3H]aspartate, suggesting that the effect of a hyposmotic challenge on taurine release cannot be attributed to cell lysis. Volume-activated taurine efflux was reversible, dependent upon the extent of the osmotic challenge and inactivated with a prolonged hyposmotic shock. The release of taurine was also reversibly increased following isosmotic cell swelling (using urea). The results confirm the presence of a volume-sensitive taurine efflux transport system in lactating rat mammary tissue and suggest that the volume-activated amino acid efflux pathway is located at the blood-facing aspect of the mammary epithelium.

Modulation of the swelling-activated amino acid channel of Leishmania major promastigotes by protein kinases

Leishmania promastigotes respond to hypotonic challenges by a mechanism of regulatory volume decrease (RVD), whereby anionic amino acid channels (HAAC) are hypotonically-activated and intracellular amino acids are released from the cells. Irrespective of the experimental conditions, restoration of isotonicity triggered an immediate blockage of the amino acid release. Both the speed and amplitude of the response depended on the hypotonic stimulus and on the operation of intracellular signaling mechanisms. The initial (5 s) hypotonic-induced release of amino acids (ri) and the steady state levels of amino acids attained (5 min) or amplitude (A), were markedly affected by modulators of protein kinase C: phorbol 12-myristate 13-acetate, 1-oleoyl-2-acetylglycerol and phorbol 12,13-diacetate whereas staurosporine and the related analog, bis-indolylmaleimide I (GF-109203.X) inhibited the RVD response. Agonists of cAMP-dependent protein kinase A such as forskolin or (8-(4-chlorophenylthio))-adenosine-3',5'cyclic-monophosphate enhanced the speed of the response but had little effect on its amplitude. Neither 4alpha-phorbol 12,13-didecanoate,1,9-dideoxyforskolin nor genistein, tamoxifen or thapsigargin had any apparent effect on either parameter tested. The most striking stimulation of hypotonic-induced amino acid release was exerted by arachidonic acid or by its non-metabolizable analog, 5,8,11,14-eicosatetraynoic acid (ETYA). These agents caused a major increase in the initial rate of amino acid release as well as a higher amplitude of the response, both of which were markedly inhibited by an anion channel blocker. The present studies indicate not only that hypotonicity is an obligatory and dominant component in HAAC activation, but implicate specific second messengers in the modulation of the RVD response. The modes of activation or attenuation of HAAC activity apparently differ for PKC and PKA modulators as well as for arachidonic acid. The involvement of Ca2+ in HAAC was studied in hypotonic challenged cells which were treated with intracellular Ca2+-chelators or Ca2+-free medium. These cells showed a lag in AA release and a modest inhibition of the amplitude. The inhibition of HAAC was markedly increased when cells were treated with the ionophore A23187 in Ca2+-free media. The HAAC activity was accompanied by a significant increase in internal Ca2+ when performed in Ca2+-containing medium (from 88+/-9 to 179+/-22 nM) but by no significant change when measured in Ca2+-free medium. These studies indicate that although Ca2+ might be involved in the early activation phase of HAAC, it is either not absolutely required or its action might be associated with localized events.

Volume-activated taurine transport is differentially activated in human cervical cancer HT-3 cells but not in human papillomavirus-immortalized Z183A and normal cervical epithelial cells

1. Previous studies demonstrate that volume-sensitive chloride currents are distinctly activated in cervical cancer cells, but not in human papillomavirus (HPV)-immortalized and normal cervical cells. In the present study, the Na(+)-independent volume-activated transport of taurine in three cervical cell types was investigated. 2. Osmotic swelling of cervical cancer HT-3 cells suspended in Na(+)-free hypotonic medium led to increased membrane uptake of taurine. This taurine uptake was effectively blocked by various Cl- channel blockers with a similar potency in blocking volume-sensitive Cl- channels: 1,9-dideoxyforskolin > 5-nitro-2-(3-phenyl-propylamino)-benzoic acid (NPPB) > 4-acetamido-4'-isothiocyanastilbene-2,2'-disulphonic acid (SITS) > 4,4'-diisothio-cyanatostilbene-2,2-disulphonic acid (DIDS) > furosemide. The taurine influx was also abolished by pertussis toxin. In contrast, Na(+)-independent volume-activated taurine transport was not significantly activated in HPV-immortalized Z183A cells and in normal cervical cells. 3. Exposure of HT-3 cells to hypotonic medium also resulted in a marked increase in taurine efflux. The volume-activated taurine efflux was osmolarity dependent and the pattern of pharmacological inhibition by Cl- channel blockers was indistinguishable from that for taurine uptake. 4. These results suggest that volume-sensitive Cl- channels in HT-3 cells can mediate the transport of amino acids. In addition, the pertussis toxin-sensitive G-protein is linked with the activation of this transport mechanism.

Thyroid hormone efflux from placental tissue is not stimulated during cell volume regulation

The effects of cell swelling induced by hyposmotic shock on efflux of hybrid hormones and selected amino acids from human placental tissue were examined. Decreasing the osmolarity of external medium from 290 to 140 mOsm/kg stimulated release of taurine, tryptophan and glutamine from placental tissue fragments. The efflux rate constant for taurine increased from 0.0069 +/- 0.0012/min to 0.0646 +/- 0.0217/min (n = 6) (P < 0.001), for tryptophan from 0.016 +/- 0.0010/min to 0.0295 +/- 0.0016/min (n = 6) (P < 0.001), and for glutamine from 0.0267 +/- 0.0027/min to 0.0659 +/- 0.0043/min (n = 4) (P < 0.001). In contrast, hyposmotic challenge did not affect release of triiodothyronine, thyroxine and leucine. These results indicate that transport processes involved in the regulation of cellular volume are unlikely to facilitate efflux of thyroid hormones from placental tissue, and therefore are unlikely to mediate transfer of thyroid hormones across the placenta. In addition, it is unlikely that the transport system facilitating the release of amino acids from placental tissue during regulatory volume decrease is one of the known amino acid carriers.

Anion exchanger AE1 as a candidate pathway for taurine transport in rat erythrocytes

Taurine has been shown to act as an osmolyte during the regulatory volume decrease process in a variety of cell types. The nature of the taurine efflux carrier is thought to consist of a diffusional pathway with pharmacological properties similar to a chloride channel or through an anion exchanger. We propose that taurine is a substrate of the anion exchanger AE1, also called band 3. Experiments were performed in rat-erythrocytes, which express large amounts of band 3. Taurine uptake and efflux transport experiments were determined in the presence of inhibitors of anion carriers and chloride channels. Both taurine uptake and efflux were inhibited by band 3 inhibitors 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 4,4'-dinitrostilbene-2,2'-disulfonic acid (DNDS), niflumic acid, or furosemide. Moreover, DIDS competes with taurine at a common binding site in the uptake process. Specific inhibitors of the electroneutral cotransport as well as inhibitors of the chloride channels were ineffective in blocking taurine transport. Thus we suggest that band 3 may be the protein responsible for taurine transport in rat erythrocytes.

Hypotonic stress induces translocation of the osmolyte channel protein pICln in embryonic skate (Raja eglanteria) heart

Volume expansion of cardiac cells from a wide variety of species stimulates the efflux of the beta-amino acid taurine through an osmolyte channel. Previous studies have suggested that the osmolyte channel in epithelial cells is a swelling-activated anion channel (pICln). In skate heart, a 37-kDa protein is present which is recognized by a specific antibody to a protein characterized in MDCK cells as pICln. This protein is present predominantly in the cytosol (only 10% in the membrane fraction) of heart incubated under isotonic conditions. After transfer to hypotonic medium (one-half osmolarity), the distribution of this protein is markedly altered and significant amounts of the protein are found in the membrane fraction. After hypotonic exposure, the amount of the protein in the membrane fraction rises to 38 +/- 11% (range 18-53, n = 3). The translocation to the membrane fraction suggests that this protein may play a role in the taurine efflux in this tissue stimulated by hypotonic stress.

Volume-sensitive taurine efflux from mammary tissue is not obliged to utilize volume-activated anion channels

Cell-swelling, induced by a hyposmotic shock, activates the release of taurine from lactating rat mammary tissue explants. The degree of stimulation of taurine efflux was dependent upon the extent of cell-swelling. Volume-sensitive taurine release was attenuated by the anion transport inhibitors NPPB, DIOA, DIDS, niflumate, flufenamate, mefenamate and diiodosalicylate but not by salicylate. Cell-swelling, following a hyposmotic challenge, did not increase the unidirectional efflux of radiolabelled I- or D-asparate from mammary tissue explants. The results suggest that although mammary tissue expresses a volume-sensitive amino acid transport system which is inhibited by anion transport blockers the pathway has no identity with volume-activated anion channels.

Volume regulation and the efflux of amino acids from cells in incubated slices of rat cerebral cortex. I. Characteristics of transport mechanisms

The characteristics of amino acid efflux from pre-loaded cells in incubated slices of rat cerebral cortex have been investigated under basal conditions (isosmotic media, 315 mosmol/kg) and following mild hyposmotic shock (265 mOsmol/kg). Rates of efflux have been correlated with the extent of cell swelling in hyposmotic media. Hyposmolality accelerated the slow phase of cellular efflux of L-aspartate (+ 29%), gamma-aminoisobutyric acid (GABA) (+ 38%), L-glutamate (+ 28%) and glycine (+ 26%). The anion transport inhibitor 4,4'-diisothiocyanatostilbene-2,2'-sulfonic acid (DIDS, 25 or 100 microM) as well as trifluoperazine (TFP, 25 microM), an inhibitor of calmodulin activation, both retarded efflux in hyposmotic media, with associated cell swelling (increase in slice non-inulin space). The effects of DIDS and TFP were not additive. N-Ethylmaleimide (NEM, 100 microM) significantly retarded the efflux of neutral amino acids, with cell swelling: these effects were less pronounced in cells loaded with acidic amino acids. It is concluded that the hyposmotically-activated efflux of carboxylic amino acids, and associated cell swelling limitation, requires calmodulin activation and the presence of free sulfydryl groups.

An amino acid channel activated by hypotonically induced swelling of Leishmania major promastigotes

Leishmania promastigotes accumulate amino acids (AAs) by an uphill transport mechanism that is dependent on membrane potential. The accumulated AAs provide the cell with an osmotic reservoir that can be utilized for osmoregulation. Exposure of Leishmania promastigotes to hypotonic media induced a rapid release of AAs that was proportional to the imposed osmotic gradients and independent of the ionic strength or the presence of Cl-, K+, Na+ or Ca2+ in the medium. The hypotonically activated AA release pathway was of relatively low chemical specificity. The solutes released included most of the zwitterionic and anionic AAs, predominantly alanine, hydroxyproline, glycine and glutamic acid, whereas cationic AAs were virtually excluded. AA release was markedly blocked by classical anion transport inhibitors such as the disulphonic stilbene 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS) and its dihydro derivative H2DIDS and others, by restoration of isotonicity or by lowering the temperature (4 degrees C). The temperature profile of AA release showed a low energy of activation (Ea 46 +/- 1.3 (S.E.M.) kJ/mol) in the range 15-30 degrees C and a very high Ea (147 +/- 8 kJ/mol) in the range 4-15 degrees C. Parasites exposed to hypotonic media containing AAs also showed a hypotonically stimulated AA uptake under favourable solute concentration gradients. This uptake was analogous for L- and D-isomers of threonine. After hypotonic exposure, cells underwent a depolarization that was largely prevented by anion transport blockers. On the basis of all these results we propose that after hypotonic stress Leishmania promastigotes restore their internal volume by a regulated release of AAs, which involves activation of channels that allow the passage of both neutral and anionic AAs and possibly other anionic substances.

Volume regulatory taurine release in human tracheal 9HTEo- and multidrug resistant 9HTEo-/Dx cells

The intracellular taurine release evoked by hypotonic shock is accomplished by volume-activated Cl- channels whose activity has been related to the expression of the multidrug resistance protein (MDR-1). We studied taurine transport in 9HTEo- cells and in the derived cell line 9HTEo-/Dx expressing MDR-1. [3H]taurine release from preloaded cells increased upon reduction of extracellular osmolality. This process was not inhibited by preincubation with phorbol 12-myristate 13-acetate but was reduced by inhibitors of volume-sensitive Cl- channels such as 1,9-dideoxiforskolin, La3+, and arachidonate. Verapamil, a substrate of MDR-1, increased the osmotically evoked taurine efflux. Replacement of extracellular Cl- with I- or gluconate or of extracellular Na+ with Li+ significantly reduced the taurine efflux, whereas substitution of N-methyl-D-glucamine for Na+ increased it. Application of ATP and 2-chloroadenosine stimulated the efflux in isotonic medium. No differences were seen between 9HTEo- and 9HTEo-/Dx cells with respect to hypotonically induced taurine efflux and the response to phorbol ester, channel blockers, ion replacement, and purinergic agents. Our results reveal novel properties of the osmotically induced taurine release and demonstrate its independence from MDR-1 gene expression.

Potent block of volume-activated chloride currents in endothelial cells by the uncharged form of quinine and quinidine

1. The effects of quinine and quinidine on the volume-activated chloride current (ICl(vol)) in cultured endothelial cells from bovine pulmonary artery were studied by use of the whole-cell patch-clamp technique. 2. At pH 7.4 both quinine and quinidine induced a fast and reversible block of ICl(vol)) with Ki values of 20 +/- 4 microM and 30 +/- 10 microM, respectively. 3. The blocking efficiency of both drugs increased dramatically with increasing extracellular pH, indicating that the blockade is mediated by the uncharged form of quinine and quinidine. 4. These results suggest a hydrophobic interaction with high affinity between volume-activated chloride channels and uncharged quinine and quinidine within the membrane bilayer of endothelial cells.

Volume-sensitive chloride currents in primary cultures of human fetal vas deferens epithelial cells

Using the patch-clamp technique, we have identified a large, outwardly rectifying, Cl--selective whole-cell current in primary cultures of human vas deferens epithelial cells. Whole-cell currents were time- and voltage-dependent and displayed inactivation following depolarising pulses >/= 60 mV. Currents were equally permeable to bromide (PBr/PCl = 1.05 +/- 0.04), iodide (PI/PCl = 1. 06 +/- 0.07) and Cl-, but significantly less permeable to gluconate (PGluc /PCl = 0.23 +/- 0.03). Currents spontaneously increased with time after establishing a whole-cell recording, but could be inhibited by exposure to a hypertonic bath solution which reduced inward currents by 68 +/- 4%. Subsequent exposure of the cells to a hypotonic bath solution led to a 418 +/- 110% increase in inward current, indicating that these currents are regulated by osmolarity. 4,4'-Diisothiocyanatostilbene-2,2'-disulphonic acid (100 microM) produced a rapid and reversible voltage-dependent block (60 +/- 5% and 10 +/- 7% inhibition of current, measured at +/- 60 mV, respectively). Dideoxyforskolin (50 microM) also reduced the volume-sensitive Cl- current, but with a much slower time course, by 41 +/- 13% and 32 +/- 16% (measured at +/- 60 mV, respectively). Tamoxifen (10 microM) had no effect on the whole-cell Cl- current. These results suggest that vas deferens epithelial cells possess a volume-sensitive Cl- conductance which has biophysical and pharmacological properties broadly similar to volume-sensitive Cl- currents previously described in a variety of cell types.

Volume-activated taurine permeability in cells of the human erythroleukemic cell line K562

The effects of hypotonic shock on cell volume, taurine influx and efflux were examined in the human erythroleukemic cell line K562. Cells exposed to hypotonic solutions exhibited a regulatory volume decrease (RVD) following rapid increases in cell volume. Cell swelling was associated with a increased taurine influx and efflux. The volume-activated taurine pathway was Na(+)-independent, and increased in parallel with increasing cell volume. The chloride channel blocker, 2,5-dichlorodiphenylamine-2-carboxylic acid (DCDPC), completely blocked the volume-activated taurine influx and efflux, while [dihydroindenyl)oxy]alkanoic acids (DIOA) and 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), an anion exchanger and anion channel blocker, respectively, also inhibited significantly. These results suggest that taurine transport is increased in response to hypotonic stress, which may be mediated via a volume-activated, DCDPC-sensitive anion channel.

Volume-sensitive chloride channels in the primary culture cells of human cervical carcinoma

Previous study shows volume-sensitive chloride currents are induced by hypotonicity in human cervical cancer cell lines, but not in normal cervical epithelium. To ascertain whether the preferential activation of these channels in cancer cell lines could be similarly and directly detected in cervical cancer tissues, we studied volume-sensitive chloride channels on the primary culture cells of invasive cervical carcinoma using the whole-cell patch-clamp technique. The process of regulatory volume decrease (RVD) was also studied using electronic cell sizing to measure cell volume. Results demonstrate that, in these cultured cells, RVD was mediated in part by chloride loss through the volume-sensitive Cl- channels. A small background current with a slope conductance of 0.32 +/- 0.07 nS/pF at +30 mV (n=60 cells from 10 different samples) was observed. Hypotonicity induced a fast activating and outward rectifying current which was reversed at about 0 mV, and the slope conductance at +30 mV was increased by 10-fold to 3.62 +/- 0.62 nS/pF. These effects were readily reversed by returning the cells to isotonic medium. Moreover, DIDS, NPPB, and 1,9-dideoxyforskolin, reversibly abolished the volume-sensitive Cl- currents. The EC50 required for the inhibitory effect of DIDS, NPPB and 1,9-dideoxyforskolin was 150, 120, and 50 microM, respectively. Volume-sensitive Cl- channels were ubiquitously expressed in cultured cells from 10 samples of different cancer stages, histopathologic types, and state of HPV DNA positivity. Interestingly, similar outward rectifying chloride currents were activated by intracellular 300 microM GTP gamma S. It is proposed that this Cl- conductance may play an important role leading to RVD in human cervical cancer.

Activation of a novel organic solute transporter in mammalian red blood cells

1. Suspending human red blood cells in isotonic sucrose (low ionic strength, LIS) medium induces a significant increase in membrane transport of glutamine, glutamate, lactate, histidine, taurine, glycine, serine, choline and carnitine but not sorbitol or sucrose. 2. Progressive lowering of ionic strength by sucrose or NaCl replacement gave a similar activation profile for taurine influx as found earlier for residual K+(86Rb+) flux. 3. The induced taurine transport could be measured as enhanced influx and efflux. Influx was linear with external concentration up to 10 mM, largely insensitive to alteration in cell volume, and did not vary with red blood cell age. 4. Unlike previous results for residual K+ transport, altering transmembrane potential with gluconate or glucuronate media led to an increase in taurine influx similar to that observed in LIS media. Varying medium pH confirmed the effect was not due to alteration in pH. 5. The LIS-induced flux was sensitive to a variety of 'classical' anion transport inhibitors in the order of potency DNDS, DIDS, NPPB, DIOA, niflumic acid, furosemide (frusemide), glibenclamide, nitrendipine and bumetanide. 6. The taurine flux showed a temperature dependence similar to that of the LIS-induced residual K+ flux. High hydrostatic pressure (40 MPa), however, inhibited taurine flux but stimulated residual K+ influx in LIS media. 7. A significant enhanced taurine flux could be demonstrated in red blood cells of other species, including horse, cattle, pig and high and low potassium type sheep. 8. It is concluded that lowering ionic strength activates a transport pathway for organic molecules sharing some similarities with background Cl- channels and LIS-induced residual K+ fluxes. In the latter context, however, there are certain significant differences (effect of transmembrane potential; volume; pressure sensitivity; species distribution) which may be important, and the unequivocal identity of the two transport processes remains to be confirmed.

Amino acid uptake and intracellular accumulation in Leishmania major promastigotes are largely determined by an H(+)-pump generated membrane potential

Leishmania major promastigotes maintain a relatively high pool of free amino acids (> 100 mM) under in vitro growth conditions. They also maintain a hyperpolarized plasma membrane which is primarily set by a dicyclohexylcarbodiimide (DCCD)-sensitive electrogenic H(+)-pump. We studied here the possible contribution of the membrane potential (Vm) and the transmembrane proton gradient (delta pH) to the mediated uptake of amino acids and their intracellular accumulation. Proline transport and accumulation were assessed by analysis of time-dependent changes in the internal pools of free amino acids and by uptake of radiolabelled proline. Proline uptake was markedly affected by changes in the Vm and considerably less by changes in delta pH. The most pronounced effects were obtained by treatment with either the H(+)-uncoupler carbonylcyanide chlorophenylhydrazone (CCCP), the cation ionophore gramicidin or by omitting Cl- from the medium (by exchange with gluconate or mannitol). Relatively smaller effects were obtained in the presence of the H(+)-ATPase inhibitor DCCD or with the anion transport blocker 4,4'-diisothiocyanodihydrostilbene-2,2'-disulfonic acid (H2DIDS). No significant effects were found with cells exposed to K+ in the presence of nigericin, to Na+ in the presence of monensin or to other cations substituting for Na+. These results suggest that neither extracellular Na+ or K+, per se, nor even intracellular pH, play a major role in proline uptake and accumulation. A significant stimulation in proline uptake induced by HCO3- could be associated with membrane hyperpolarization or intracellular alkalinization. The present observations indicate that uphill nutrient uptake by Leishmania promastigotes is largely determined by Vm. The relatively high intracellular pools of amino acids might be of physiological relevance to osmoregulation by parasites.

Activation of a Cl(-)-conductive pathway in primary cultures of rat inner medullary collecting duct (IMCD) cells under hypotonic stress

In intracellular recordings with conventional microelectrodes on rat IMCD cells, we find that hypotonic stress depolarizes membrane voltage and decreases cell input resistance. Ion substitution experiments reveal that these effects are largely due to the activation of a prominent Cl- conductance. After block of this conductance with dideoxyforskolin a smaller concomitant increase in K+ conductance becomes detectable.

Volume-activated amino acid efflux from term human placental tissue: stimulation of efflux via a pathway sensitive to anion transport inhibitors

The effect of a hyposmotic challenge and hence cell-swelling upon the efflux of a variety of solutes from isolated human placental tissue has been examined. A hyposmotic shock increased the fractional release of taurine, the most abundant free amino acid in placental tissue, via a pathway sensitive to niflumic acid, DIDS (4,4'-Diisothiocyanatostilbene-2',2'-disulphonic acid,) NPPB (5-Nitro-2(3-phenylpropylamino)benzoic acid) and DIOA (R(+)[2-n-butyl-6,7-dichloro-2-cyclopentyl-2,3-dihydro-1-oxo-1H-inden -5-y) oxy] acetic acid). In contrast, tamoxifen was without effect. The cell-swelling induced efflux of taurine was attenuated (40 per cent) by replacing external Cl- with NO3-. The efflux of glutamic acid was also markedly increased by a hyposmotic challenge. Niflumic acid inhibited both basal and volume-activated glutamic acid efflux. A hyposmotic shock also increased alpha-aminoisobutyric acid efflux but not that of 3-O-methylglucose and SO4(2)-. The results suggest that the human placenta can respond to cell-swelling by releasing organic osmolytes such as amino acids via a pathway which is sensitive to anion transport inhibitors. However, it appears that the volume-activated amino acid transport system is independent from the placental anion-exchange pathways. The efflux of these compounds may act with K+ and Cl- efflux to effect a regulatory volume decrease in placental tissue. In addition, volume-activated transport may play a role in transplacental amino acid transfer.

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.

Effects of extracellular bicarbonate ions and pH on volume-regulatory taurine efflux from rat cerebral cortical slices in vitro: evidence for separate neutral and anionic transport mechanisms

Net efflux of [3H]taurine from cells in pre-loaded slices of rat cerebral cortex has previously been shown to occur via an anionic pathway, believed to consist of exchange of anionic taurine for extracellular Cl-, and operating under both isomotic and hypoosmotic conditions, and a calmodulin-dependent mechanism, activated by hypoosmotic stress: the latter may comprise conductive channels (Law, R.O. (1994) Biochim. Biophys. Acta 1221, 21-28). Experiments have now been performed to examine two inter-related problems; firstly, how anion/anion exchange (assuming 1:1 stochiometry) could contribute to the regulation of brain cell volume, and secondly, whether the hypoosmotically-activated component of efflux represents a second anionic transport process or loss of neutral taurine. The former process has been shown to be strongly dependent upon extracellular pH and bicarbonate concentration, being accelerated by low pH (7.0) and high (60 mmol/l) bicarbonate, and retarded by alkalinization (pH 7.8) and low (2.5 mmol/l) bicarbonate. Taurine efflux is inhibited by acetazolamide, with accompanying cell swelling in both isoosmotic and hypoosmotic media. It is hypothesized that inwardly directed bicarbonate transport, in exchange for intracellular Cl-, operates in parallel with efflux of anionic taurine in exchange for extracellular Cl-, and it is the subsequent conversion of bicarbonate to CO2 and water, under the influence of carbonic anhydrase, that effects a volume-regulatory decrease in internal osmotic potential. The dependence of taurine efflux upon pH and bicarbonate persists in the presence of trifluoperazine (an inhibitor of calmodulin activation) but is abolished in hypoosmotic media by the anion transport inhibitor niflumic acid. Cell depolarization in high K+ has no effect on taurine efflux, which is envisaged as involving parallel electroneutral anion exchange processes (taurine/Cl- and Cl-/bicarbonate) augmented, in hypoosmotic media, by diffusive efflux of neutral taurine.