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.

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.

Volume-activated trimethylamine oxide efflux in red blood cells of spiny dogfish (Squalus acanthias)

The aims of this study were to determine the pathway of swelling-activated trimethylamine oxide (TMAO) efflux and its regulation in spiny dogfish (Squalus acanthias) red blood cells and compare the characteristics of this efflux pathway with the volume-activated osmolyte (taurine) channel present in erythrocytes of fishes. The characteristics of the TMAO efflux pathway were similar to those of the taurine efflux pathway. The swelling-activated effluxes of both TMAO and taurine were significantly inhibited by known anion transport inhibitors (DIDS and niflumic acid) and by the general channel inhibitor quinine. Volume expansion by hypotonicity, ethylene glycol, and diethyl urea activated both TMAO and taurine effluxes similarly. Volume expansion by hypotonicity, ethylene glycol, and diethyl urea also stimulated the activity of tyrosine kinases p72syk and p56lyn, although the stimulations by the latter two treatments were less than by hypotonicity. The volume activations of both TMAO and taurine effluxes were inhibited by tyrosine kinase inhibitors, suggesting that activation of tyrosine kinases may play a role in activating the osmolyte effluxes. These results indicate that the volume-activated TMAO efflux occurs via the organic osmolyte (taurine) channel and may be regulated by the volume activation of tyrosine kinases.

Beta-adrenergic stimulation of volume-sensitive chloride transport in lamprey erythrocytes

We measured the effects of a beta-adrenergic agonist, isoproterenol, on chloride transport and volume regulation of lamprey (Lampetra fluviatilis) erythrocytes in isotonic (288 mosm L(-1)) and hypotonic (192 mosm L(-1)) medium. Isoproterenol at a high concentration (10(-5) M) did not influence chloride transport in isotonic medium but markedly increased chloride fluxes in hypotonic conditions: unidirectional flux increased from 100 mmol kg dcw(-1) h(-1) in the absence to 350 mmol kg dcw(-1) h(-1) (dcw=dry cell weight) in the presence of isoproterenol. Simultaneously, the half-time for volume recovery decreased from 27 to 9 min. Isoproterenol caused an increase in cellular cyclic AMP (cAMP) concentration. The stimulation of chloride transport in hypotonic conditions could be induced by application of the permeable cAMP analogue, 8-bromo-cyclic AMP, suggesting that the effect of beta-adrenergic stimulation on chloride transport occurs downstream of cAMP production. As isoproterenol did not affect unidirectional rubidium fluxes in hypotonic conditions, the transport pathway influenced by beta-adrenergic stimulation is most likely the swelling-activated chloride channel. Because the beta-adrenergic agonist only influenced the transport in hypotonic conditions despite the fact that cAMP concentration also increased in isotonic conditions, the activation may involve a volume-dependent conformational change in the chloride channel.

Mechanism of free radical-induced hemolysis of human erythrocytes: comparison of calculated rate constants for hemolysis with experimental rate constants

We previously developed a simple competitive reaction model between lipid peroxidation and protein oxidation in erythrocyte membranes that accounts for radical-induced hemolysis of human erythrocytes. In this study, we compared the rate constants calculated from the hemolysis curves of erythrocytes in the presence of radical initiators with those obtained from experiments using erythrocyte ghosts treated with radicals. 2,2'-Azobis(amidinopropane) dihydrochloride and 2,2'-azobis(2,4-dimethylvaleronitrile) were used as radical initiators. Plots of the logarithm of concentration of the radical initiator against the logarithm of the rate constant gave straight lines. The slope of the lines for the calculated lipid peroxidation was nearly equal with the experimental value. Similar results were obtained for oxidation of membrane proteins, except for band 3 oxidation. The values for the rate constants calculated from hemolysis curves seem to be accurate. The slope of the lines for the calculated rate constants for proteins was larger than the experimental value for band 3 oxidation, because band 3 oxidation is accompanied by aggregation or redistribution of band 3 proteins to form hemolytic holes. These results indicate that the competitive reaction model may be useful for analyzing radical-induced hemolysis.

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.

Fish red blood cells: characteristics and physiological role of the membrane ion transporters

Several membrane ion transporters playing a role in gas transport and exchanges, cell volume regulation and intracellular acid-base regulation have been identified in fish red blood cells (RBCs). This short review focuses on Na+/K+ATPase and its role in establishing the ionic gradients across the membrane, on the Cl-/HCO3- exchanger and its key role in respiration and possibly in inducing a chloride conductance, on the Na+/H+ exchanger and the recent advances on its molecular mechanisms of activation and regulation, on the different types of K-Cl cotransports, the different hypotheses and suggested models and their role in cell volume regulation. There is no evidence in the literature for ionic channels in fish RBCs. We present original data obtained with the patch-clamp technique that shows for the first time the existence of a DIDS-sensitive chloride anionic conductance measured in whole cell configuration and the presence of a stretch-activated nonselective cationic channel recorded in cell-attached and excised inside-out configuration. The part played by these ionic conductances is discussed in relation with their possible involvement in volume regulation.

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.

Organic osmolyte channels: transport characteristics and regulation

Erythrocytes of the skate (Raja erinacea) exposed to hypotonic stress swell and then undergo a volume regulatory decrease by releasing taurine and other osmolytes. Previous studies showed that taurine release occurs via a volume-activated, Na(+)-independent, bi-directional transporter that has the properties of a size-limited channel. We now report on the transport characteristics of this channel and its regulation. Kinetic, competition and inhibitor studies indicate that polyols (myo-inositol) and trimethylamines (betaine) are transported by the same channel as taurine. Although the identity of the channel is still unknown a variety of evidence suggests that band 3 is involved in either channel formation or regulation. Hypotonicity causes phosphorylation and structural changes in band 3. Under isotonic conditions band 3 is predominantly in the dimeric form. Hypotonicity causes a shift to tetrameric band 3. We hypothesize that the band 3 tetramer either forms or regulates an osmolyte channel. The finding that expression of band 3 protein increases osmolyte channel activity in Xenopus oocytes supports this hypothesis.

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.

Taurine efflux and the regulation of cell volume in incubated slices of rat cerebral cortex

The kinetics of efflux [3H]taurine have been examined in pre-loaded slices of rat cerebral cortex incubated in media of variable osmolality. Alterations in the rate of the slowest phase of efflux, considered to represent cellular loss, have been correlated with cell volumes, provisionally identified as the slice non-inulin space. Efflux was stimulated by reduction in medium osmolality, and impaired in hyperosmotic media; these variations were accompanied by moderate (non-osmometric) cell swelling and shrinkage, respectively. The rates of taurine efflux into media in which NaCl was partly replaced by sucrose, and measurement of the corresponding cell volumes, suggest that ionic strength, rather than osmolality or cell volume per se, may be a significant controlling factor. In both isosmolal and hyposmolal media efflux was significantly impaired by the anion transport inhibitor niflumic acid, with accompanying cell swelling, or by replacement of chloride by gluconate. In hyposmotic, but not isosmotic, media efflux was impaired, and cell volumes increased, in the presence of trifluoperazine or TMB-8, a reported blocker of intracellular calcium release, and the effects of niflumic acid and trifluoperazine on both variables were strongly additive. It is suggested that in both isosmotic and hyposmotic media taurine, efflux occurs by anionic transport, mainly through exchange with external chloride, whereas in hyposmotic media a second pathway is present, probably a volume-activated calmodulin-dependent channel.

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

Exposure of a human lung epithelial cancer cell-line to hypo-osmotic media led to a marked increase in the rate of efflux from the cells of taurine, a non-essential sulfonic amino acid. The osmotically-activated taurine efflux was inhibited by a range of known Cl- channel blockers, the most potent of which were NPPB and 1,9-dideoxyforskolin. These reagents were similarly effective at inhibiting the osmotically-activated efflux of I-, a known substrate of volume-activated Cl- channels. The results are consistent with the hypothesis that volume-regulatory taurine release from these cells is mediated by a volume-activated Cl- channel.

Species differences in cerebral taurine concentrations correlate with brain water content

The notion that taurine (Tau) has an osmoregulatory function in the mammalian brain has not been established, although it has been reported that the severity of hyponatremic edema is proportional to cerebral [Tau]. Tau pools are not easily altered in vivo, but the fact that there are large differences in cerebral taurine levels between mice, rats and guinea pigs offers an opportunity to determine whether endogenous Tau affects volume regulation of the brain in hyposmolal conditions. This issue was investigated by injecting saline or distilled water intraperitoneally at 150 ml/kg in anesthetized mice, rats and guinea pigs. The animals were decapitated 4 h later, and blood osmolality, cortical specific gravity, Na+, K+ and amino acid concentrations were determined. In controls, blood osmolality and specific gravity of the cortex were highest in the mouse (304 +/- 3 mmol/kg; 1.0488 +/- 0.0003 kg/l), followed by the rat (294 +/- 1; 1.0462 +/- 0.0002) and the guinea pig (285 +/- 2; 1.0445 +/- 0.0002). There was a correlation between these measures and cortical Tau levels which were 10.31 +/- 0.36 mmol/kg in mouse cortex, 6.31 +/- 0.18 in rat cortex and 1.37 +/- 0.06 in guinea pig cortex. Despite these differences, water-induced cerebrocortical swelling did not differ between the species studied. Interspecies variation in cortical osmolality did not relate to [Na+] and [K+], since the levels of these electrolytes were higher in the guinea pig cortex than in the rat and mouse cortex. After administration of water, the levels of Na+ and K+ were reduced in rat and guinea pig cortex, while only [Na+] was significantly decreased in mouse cortex.(ABSTRACT TRUNCATED AT 250 WORDS)