© 2001 by The Society for Integrative and Comparative Biology
The Taurine Efflux Portal Used to Regulate Cell Volume in Response to Hypoosmotic Stress Seems to Be Similar in Many Cell Types: Lessons to Be Learned from Molluscan Red Blood Cells1
1 Department of Biology, University of South Florida, Tampa, Florida 33620
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 SYNOPSIS INTRODUCTIONGENERAL ASPECTS OF THE...THE TAURINE EFFLUX PATHWAYANION TRANSPORT INHIBITORS...CONTROL OF VOLUME SENSITIVE...ORGANIC OSMOLYTE TRANSPORT AND...TAURINE EFFLLUX CONTROL-A...NOETIA RBC CONCLUSIONReferences |
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The control of cell volume in all cell types is accomplished by the regulation of two general categories of osmolytes: inorganic ions, most commonly K+ and Cl–, and small molecular weight organic compounds, usually certain amino acids and certain quaternary ammonium compounds. The difference in who regulates what does not depend phylogeny, but instead upon the type of osmotic environment that a cell expects (in an evolutionary sense) to encounter. Cells that exist in extracellular osmotic concentrations up to 300–400 mosmol/kg (mosm) rely primarily on inorganic osmolytes for volume control, while cells that exist at greater osmotic concentrations rely more on organic osmolytes for volume control. Usually, strange or unique volume regulatory mechanisms are found in cells that exist in particularly demanding osmotic conditions. In order to provide further support the foregoing generalizations, the following paper will focus on comparisons between the hypoosmotically induced mechanism of taurine efflux regulation by red blood cells of the bivalve, Noetia ponderosa, probably the best understood "invertebrate" cell type in this regard, and taurine efflux from a variety of "vertebrate" cells.
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Symposia held to bring together investigators of osmoregulation, such as this one, often have the topic list arranged in some sort of phylogenetic order, at least in part. This is a convenient way to order the topics, but it can lead to the conclusion that osmoregulatory mechanisms are different among phyla while in fact, in animals, the cellular mechanisms are usually quite similar. Further, when the rare surprise mechanism is encountered, it is almost always in an organism that has unusual osmotic concentration-variation tolerance. For the rest, cellular osmoregulation is accomplished by controlling the intracellular amounts of the same few compounds, called osmolytes, by what seems to be very similar mechanisms. It is our intent here to illustrate this mechanistic similarity by reviewing the mechanism of the most widely utilized organic osmolyte, the sulfonyl amino acid, taurine.
In response to hypoosmotic conditions, cells counteract osmotic swelling through an increase in membrane permeability to specific intracellular osmolytes. The efflux of these osmolytes, along with osmotically obliged water, results in at least a partial recovery of cell volume. There are two general categories of osmolytes: inorganic ions, most commonly K+ and Cl–, and small molecular weight organic compounds, usually certain quaternary ammonium compounds and certain free amino acids (Pierce, 1982
). In general, cells that exist in extracellular osmotic concentrations up to 300–400 mosm (for example, cells from many types of freshwater animals and the majority of terrestrial vertebrate cells) rely primarily on inorganic osmolytes for cell volume control, while cells that exist at greater osmotic pressures (for example, cells from marine and brackish water organisms, dehydration-tolerant species and from certain regions of vertebrate kidneys) rely more on organic osmolytes to maintain cell volume. Some of the organic osmolytes (e.g., taurine, polyols, and glycine betaine), in addition to providing osmotic bulk, serve to stabilize proteins and protect cells from certain types of oxidative stress at high salt concentration (Yancey, 1994).
The initial studies, done many decades ago, on the cellular mechanisms of volume recovery after hypoosmotic stress were primarily on cells from terrestrial vertebrates (including human blood cells) and focused on the regulation of ionic osmolytes-the role of organic osmolytes was undiscovered. As a result, the regulation of membrane permeability to ions during volume recovery was studied extensively. As early as the 1950s, free amino acid regulation in response to osmotic stress was revealed in studies of volume recovery by salinity stressed marine organisms. By now it is well established that organic osmolytes occur in concentrations as high as hundreds of millimoles in the cytoplasm of many organisms ranging from bacteria to humans (reviewed in Chamberlain and Strange, 1989; Yancey, 1994) and a swelling-induced organic osmolyte efflux is a key component of cell volume regulation in bacteria (e.g., Berrier et al., 1992), algae (e.g., Kirst, 1977), higher plants (e.g., Handa et al., 1986) invertebrates (e.g., Amende and Pierce, 1980), lower vertebrates (e.g., Ballatori and Boyer, 1992), and mammals (e.g., Yancey, 1994). Furthermore, substantial evidence has accumulated showing that most cells have both ionic and organic osmolyte components that are regulated in a coordinated manner in the volume recovery response (Pierce, 1982).
The literature that has accumulated over 50 yr since the discovery of organic osmolyte regulation shows that, while several amino acids, including alanine, glycine, proline and glutamic acid (along with quaternary amines, such as glycine betaine) are often involved in some combination in the cellular response to osmotic stress, in the vast majority of cell types, taurine is often the predominant organic osmolyte. Nevertheless, in spite of both its intracellular abundance and broad phylogenetic occurrence, the regulation of membrane permeability to taurine during volume recovery from hypoosmotic stress has only been investigated in detail in a very few cell types. We have produced one of the more detailed studies of taurine regulation over the last 15 yr or so using the Noetia ponderosa (Mollusca, Bivalvia) red blood cell (RBC). On the pages that follow we will compare what is known about the mechanisms of taurine regulation in response to hypoosmotic stress in a variety of cell types with that in the N. ponderosa RBC and then present a hypothetical mechanism using that cell.
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GENERAL ASPECTS OF THE CONTROL OF TAURINE EFFLUX |
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The cells of marine invertebrates have a large intracellular pool of free amino acids which fluctuates with changes in salinity. Taurine is almost always a component of the osmolyte pool of these species and frequently it is the largest and only significant component (reviewed in Pierce, 1982
; Huxtable, 1990). In addition, loss of intracellular taurine plays an important role in cell volume recovery during hypoosmotic stress in a diverse group of vertebrate cell types (for example, erythrocytes from several species [Kirk et al., 1992], skate hepatocytes [Ballatori and Boyer, 1992], mammalian brain cells [Pasantes-Morales et al., 1993], Madin-Darby canine kidney [MDCK] cells [Robson and Hunter, 1994], Ehrlich ascites tumor cells [Hoffmann and Lambert, 1994], and C6 glioma cells [Jackson and Strange, 1993]). The pathway of taurine efflux from such cells during hypoosmotic stress seems to be quite similar, yet the control mechanisms regulating the pathway seem both complex and diverse, although the data are relatively few (see for example, Sukarev et al., 1994). |
THE TAURINE EFFLUX PATHWAY |
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Substantial kinetic, electrophysiological, and pharmacological evidence has been obtained in varying amounts from several cell types suggesting that an anion channel mediates taurine efflux during volume regulation. Volume-sensitive taurine efflux has channel-like characteristics: It is Na+-independent, relatively non-specific (among neutral and anionic species) and does not saturate over a broad concentration range. In addition, anion transport inhibitors block taurine efflux. In particular, stilbenedisulfonates, such as 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), are broadly effective, although other, less widely tested, anion transport inhibitors block taurine efflux as well (see below).
The first evidence that taurine (and other organic osmolytes) efflux during volume recovery following hypoosmotic stress was through an anion channel came from MDCK cells (Roy and Malo, 1992). Hypoosmotically-stimulated taurine efflux and the resulting volume recovery from these cells is Na+-independent and DIDS sensitive. In addition, incubation of MDCK cells in hypoosmotic media containing varying concentrations of taurine results in a [taurine]i that is linearly related to [taurine]o up to 60 mM (Roy and Malo, 1992), although the experiment was done in the wrong direction, physiologically. In addition, the three amino acids (glutamate, taurine, and glycine) that make up 80% of the organic osmolyte pool in these cells rapidly decrease in concentration during the 10 min following hypoosmotic stress. Single channel current measurements indicate that a swelling-activated anion channel has a relatively high permeability to these same amino acids, with taurine having the highest permeability ratio of Ptau/
= 0.75 (Banderali and Roy, 1992).
Similar to the MDCK cells, both the recovery of skate erythrocyte cell volume and taurine efflux resulting from hypoosmotic stress are blocked by DIDS (Goldstein and Musch, 1994). Furthermore, increased [taurine] in the hypoosmotic medium produced a linear increase in taurine influx over a 150-fold concentration range (up to 15 mM), well above the Michaelis transport Km of erythrocyte membrane amino acid carriers that have been characterized so far (Haynes and Goldstein, 1993). While the experiments with both the MDCK cells and skate erythrocytes measured taurine equilibration rather than efflux alone, the diffusional transport of taurine is characteristic of a channel. In addition, the pore size of the skate erythrocyte taurine channel has been estimated. Glycine, ß-alanine, taurine, proline, 
-aminobutyric acid and threonine exhibit volume-activated, DIDS inhibited uptake into the skate cells, whereas aspartic acid, leucine, methionine, and ornithine are not transported. On the basis of the size of these amino acids, molecules containing eight or fewer major atoms and having a molecular mass of <125–131 Da can pass through this channel but larger molecules can not. Indeed, several investigators have suggested that this channel is the main efflux portal for most of the organic osmolytes, not just taurine (see for example, Jackson and Strange, 1993; Goldstein and Davis, 1994). The channel is estimated to be 5.7–6.3 Å in diameter (Haynes and Goldstein, 1993).
The characteristics of taurine efflux from C6 rat glioma cells following hypoosmotic stress have led to the name, volume-sensitive, organic osmolyte-anion channel (VSOAC). This channel, which has been characterized by patch clamp techniques, is a swelling-activated, outwardly-rectifying, anion channel (Jackson et al., 1994) that carries structurally dissimilar organic osmolytes. Glutamate, taurine, inositol, and betaine all efflux through this anion channel following hypoosmotic stress. Furthermore, the characteristics of swelling-induced inositol and taurine efflux are virtually identical: the rate of efflux for both molecules is similar and transport inhibitors (including DIDS), fatty acids, PMA and forskolin, and ion substitution all have similar effects on the efflux of both molecules (Jackson and Strange, 1993). This channel has a relative cation permeability (Pcation/) of <0.03–0.05 and a relative taurine permeability (PTau/) of 
0.20 (Lytle and Forbush, 1992).
Similarly, hypoosmotically induced chloride and organic osmolyte conductances from skate hepatocytes are inhibited by a group of agents that affect efflux from C6 cells, including fatty acids, forskolin compounds, and DIDS. The permeability characteristics of skate hepatocyte channels are less selective; 
/ and 
/ are 0.2 and Pcholine/ is 0.55 (Ballatori et al., 1995).
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ANION TRANSPORT INHIBITORS EFFECT TAURINE EFFLUX |
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Pharmacological evidence from several cell types indicates that taurine efflux during hypoosmotic stress is accomplished via anion transporters or non-selective chloride channels. DIDS inhibits cell volume recovery (Fig. 1), taurine efflux, and Cl– efflux (Table 1) from N. ponderosa RBCs following hypoosmotic stress suggesting the presence of a VSOAC-like channel. In addition, niflumic acid, tamoxifen and 1,9-dideoxyforskolin (DDF), all anion transport inhibitors, have similar, although less potent, effects on volume regulation (Table 2) and efflux of taurine and Cl– from the N. ponderosa cells (Table 1). Although cell volume, taurine and Cl– effluxes and/or membrane conductances have not always been measured in the same experiment, a variety of cell types respond to these anion transport inhibitors in the same manner as the clam blood cells (Table 1).
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Although the pharmacological and physical behavior of the taurine efflux pathways in the cell types listed in Table 1 are similar, the identity of the channel is not established with certainty. Several volume sensitive anion channels have been identified, including CIC-2, a member of the CLC chloride channel family that is expressed almost ubiquitously in vertebrate cells (Thiemann et al., 1992
), human multidrug resistance phospho-glycoprotein(p-glycoprotein) (Valverde et al., 1992), and the swelling-induced chloride conductance regulatory protein, pICln (Krapivinsky et al., 1994; Musch et al., 1997). Any of these channels might play a role in cell volume recovery, and pICln shares many characteristics of VSOAC (reviewed in Strange et al., 1996), although some studies implicate pICln as a regulator of channel function instead of a channel itself (Paulmichl et al., 1992; Krapivinsky et al., 1994). pICln translocates from the cytosol to the plasma membrane in embryonic skate heart during hypoosmotic stress (Musch et al., 1997). However, when recombinant pICln was reconstituted into both artificial and biological membranes, the channel was at least seven times more selective for cations than for anions (Li et al., 1998). Thus the role that pICln plays in cell volume recovery is not as clear as first thought and, so far, neither pICln, ClC-2, nor p-glycoprotein have been proven as a pathway for organic osmolyte efflux from intact cells during volume regulation. So it is possible that the taurine channel is a different protein. |
CONTROL OF VOLUME SENSITIVE TAURINE EFFLUX |
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While the taurine efflux pathway seems similar among several cell types, the requirements of the mechanism that regulates the control of the pathway seem to vary among cell types. These requirements are described below:
Calcium and calmodulin
Both extracellular and intracellular [Ca2+] are too low in animal cells to be useful as an osmolyte. However, [Ca2+] often affects volume recovery of cells following a hypoosmotic stress. In many cases, removal of extracellular Ca2+ blocks volume recovery by somehow preventing osmolyte efflux. In other instances, the volume recovery process is not sensitive to [Ca2+]o, but changes in [Ca2+]i alter volume recovery following hypoosmotic stress, like [Ca2+]o, by altering the pattern of osmolyte efflux (reviews by McCarty and O'Neil, 1992; Foskett, 1994). An increase [Ca2+]i is the signal that initiates volume recovery after cell swelling in several cells types. This includes both inorganic osmolyte efflux, for instance K+-H+ exchange in Amphiuma RBCs (Cala et al., 1986), separate K+ and Cl– effluxes from Ehrlich ascites cells (Hoffmann et al., 1986), and taurine efflux from both N. ponderosa RBCs (Amende and Pierce, 1980; Pierce, 1994) and skate RBCs (Leite and Goldstein, 1987).
In other cases, changes in [Ca2+]i are not required for osmolyte efflux or volume recovery after hypoosmotic stress. For example, where it has been tested, activation of the VSOAC-like channels from vertebrate cells (described above) is not usually Ca2+ dependent, although in neuroblastoma cells, it is (reviewed in Strange et al., 1996).
Several calmodulin antagonists inhibit taurine efflux and volume regulation. Pimozide blocks cell volume recovery and taurine efflux from Ehrlich ascites tumor cells (Hoffmann and Simonsen, 1989). Trifluoperazine blocks recovery from cell swelling in astrocytes (Bender et al., 1992), but is ineffective in a similar pathway in flounder red cells (Fincham et al., 1987). In addition, trifluoperazine and chlorpromazine block volume recovery and taurine efflux from N. ponderosa RBCs (Pierce et al., 1988, 1989).
Protein phosphorylation and ATP
Protein phosphorylation is one of the most widely utilized mechanisms involved in the regulation of cellular function (reviewed in Palfrey, 1994, among many). Not surprisingly, phosphorylation seems to play a central role in the regulation of osmolyte concentrations in response to both hyper- and hypoosmotic stresses. In nearly every osmoregulatory system in which it has been tested, evidence for a role of volume-activated kinase and/or phosphatase activity has been found. Furthermore, in the few cases in which it has been studied, volume-activated phosphorylation of specific proteins occurs. Most research along these lines to date has involved the inorganic osmolytes. For example, according to the results of several pharmacological studies using kinase or phosphatase inhibitors, hypoosmotically-induced, inorganic osmolyte concentration regulation by both the Na+/K+/2Cl– transport protein and the KCl cotransporter is either directly or indirectly controlled by phosphorylations (Lauf, 1985; Jennings and Al-Rohil, 1990; Lytle and Forbush, 1992; Starke and Jennings, 1993; Leung et al., 1994; Larsen et al., 1994; Robsen and Hunter, 1997). These transporters are not involved in taurine regulation. However, less detailed evidence indicates that kinases are also involved in regulating VOSAC-like channels during volume regulation.
Protein kinase C
In frog proximal tubule cells, hypoosmotically induced Cl– conductance appears to be activated by a PKC-mediated phosphorylation (Robson and Hunter, 1994). In whole cell patches of these cells, the time-dependent activation of GCl requires ATP. In addition, inhibition of PKC by PKC pseudo-substrate abolishes this activation and PKC activation with 4b-phorbol 12-myristate, 13-acetate (PMA) potentiates it. However, in these cells G-protein activation with 10 µM GTPgS or 1 mM fluoride in the presence of ATP blocks the hypoosmotically induced GCl activation seen with ATP alone. In addition, the effect of G-protein stimulation is mirrored by application exogenous phosphatases, implying that G-proteins activate phosphatases which, in turn, down regulate GCl in these cells (Robson and Hunter, 1994).
MAP kinase
Phosphorylation pathways used for gene control may also be involved in direct channel activation during cell volume regulation after hypoosmotically induced swelling. The mitogen activated protein (MAP) kinases Erk-1 and Erk-2 can be activated by hypoosmotically induced swelling (Schliess et al., 1996). While these kinases are involved in the regulation of gene expression in hyperosmotic conditions (for instance MAP kinases are involved in the expression of osmoregulatory gene products, such as osmolyte transporters), recent evidence suggests that MAP kinases may be involved in the regulation of a VSOAC-like channel in rat astrocytes (Crepel et al., 1998). Western immunoblot analysis indicates that both tyrosine and MAP kinases are activated during swelling and that the tyrosine kinase inhibitors, genistein and tyrphostin A23, inhibit Cl– current. In addition, the MAP kinase kinase (MEK) inhibitor PD 98059 blocked both hypoosmotic activation of the Cl– current and Erk-1 and Erk-2 activation after hypoosmotic exposure. Swelling induced taurine efflux from these cells is pharamacologically similar to the Cl– current, although, the role that these kinases may play in taurine efflux has not been examined yet.
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ORGANIC OSMOLYTE TRANSPORT AND PROTEIN PHOSPHORYLATION |
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There is much less information regarding the role of protein phosphorylation in the regulation of organic osmolyte concentration. However, changes in both taurine efflux and membrane protein phosphorylation occur following a hypoosmotic stress in both skate and N. ponderosa RBCs. Three proteins (99, 67 and 34 kDa) are phosphorylated in the skate cell (Goldstein and Musch, 1994
). The 99 kDa protein immunoprecipitates with antibody to band 3, the membrane anion exchange protein, indicating that a hypoosmotically-induced phosphorylation of band 3 might be involved in regulating the volume recovery producing taurine efflux (Goldstein and Davis, 1994). Although band 3 may not be present in the N. ponderosa RBCs, the two phosphoproteins present in the molluscan cell have molecular weights (63 and 34 kDa) very close to the other skate phosphoproteins. In addition in the N. ponderosa cells, phosphorylation of these two proteins (Table 3), taurine efflux, and volume recovery (Fig. 2) are potentiated by okadaic acid indicating that a specific group of phosphatases is involved in the control of taurine efflux.
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ATP itself, and not specific phosphorylation, is required for taurine efflux in some cases. Activation of taurine efflux following hypoosmotic stress through VSOAC in the C6 glioma cell line requires non-hydrolytic binding of ATP. Depletion of intracellular ATP prevents taurine (and myo-inositol) efflux, but substitution of ATP with nonhydrolyzable ATP analogs restores efflux (Jackson et al., 1994
). In addition, some of the classic anion channel blockers affect taurine efflux by altering the [ATP]i. In skate hepatocytes, the anion channel blockers glibenclamide, 5-nitro-2-(3-phenylpropylamino) benzoic acid(NPPB), diphenylamine-2-carboylate, ketoconazole, gossypol, niflumic acid, and quinine affect taurine efflux by decreasing both [ATP]i and [ATP]i/[ADP]i, although anion transport inhibitors, DIDS and pyridoxal-5-phosphate, inhibit taurine efflux by blocking the anion channel itself (Ballatori et al., 1995). ATP depletion in N. ponderosa RBCs by 2,4-dinitro phenol or NaCN reduces both volume recovery and taurine efflux after hypoosmotic stress. Ouabain, however, has no effect on the volume recovery response (Amende and Pierce, 1980).
Band 3
While band 3 anion exchangers (also known as AE1) play a role in volume regulation, it is unclear whether band 3 is a channel regulator or the channel itself. In fact, recent evidence suggests that where it is involved in organic osmolyte efflux during cell volume regulation, band 3 regulates a separate channel. For example, Xenopus oocytes containing band 3 expressed from either trout or mouse erythrocytes, both display swelling-activated, DIDS-sensitive, taurine efflux even though mouse erythrocytes do not have a swelling activated organic osmolyte efflux. Chimeric constructs indicate that part of an intracellular loop sequence and the C-terminus end of band 3 are required for the taurine channel activity to be expressed, suggesting that band 3 is regulating an endogenous channel and is not the channel itself (Bize et al., 1998).
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TAURINE EFFLLUX CONTROL-A HYPOTHESIS |
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We have constructed a hypothetical mechanism of hypoosmotically-induced, taurine-mediated volume control, based on the foregoing information and the N. ponderosa RBC in particular (Fig. 3).
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The response involves both ionic and organic osmolyte components. At the onset of hypoosmotic stress, water enters the cell, cell volume increases, and several membrane permeability changes occur (1). The ionic component of volume regulation is an instantaneous, Ca2+-independent K+ and Cl– efflux (2) that prevents cell lysis but does not result in volume recovery (Pierce, 1982
; Pierce et al., 1988). Little is known about the regulation of this ionic osmolyte efflux in the N. ponderosa cells, except that K+ efflux is potentiated by a phorbol ester (TPA) known to increase PKC activity (Pierce et al., 1988).
The volume recovery is affected by a slower, Ca2+-dependent amino acid (mostly taurine) efflux (Amende and Pierce, 1980; Pierce et al., 1988). This component is initiated by influx of extracellular Ca2+ (3). Both volume recovery and taurine efflux are blocked or reduced in Ca2+-free medium (Amende and Pierce, 1980; Pierce et al., 1988) and 45Ca2+ influx occurs immediately following hypoosmotic stress (Smith and Pierce, 1987; Pierce et al., 1988), demonstrating that the source of the Ca2+ is extracellular. As [Ca2+]i rises, calmodulin is activated (4). Phenothiazines, trifluoperazine and chlorpromazine, which are both calmodulin inhibitors, block cell volume recovery and taurine efflux (Pierce et al., 1988, 1989).
In addition, two membrane proteins are phosphorylated during the response (34 kDa and 63 kDa) (5) (Politis and Pierce, 1991; Pierce, 1994). The 34 kDa protein is very insoluble and cell sections labeled with immuno-gold bound antibodies raised to this protein, and visualized with transmission EM, have localized the protein to the plasma membrane. The 63 kDa band often appears faintly on gels of the 34 kDa protein which has been purified by electro-elution from gel slices. Western blot analysis indicates that the antibody recognizes both the 34 kDa and 63 kDa bands in both the pure and ammonium sulfate fractions. Since our protein purification protocol excludes the 63 kDa band at the gel cutting step (Staining of the gel fragments remaining after removal of the 34 kDa protein segment clearly indicate that the 63 kDa protein is not included in the antigenic material injected into the rabbit), this result suggests that the 63 kDa band is likely a dimer of the 34 kDa protein. Furthermore, a variety of control experiments using pre-immune serum and protein extracts of rabbit red cells all indicate that the antibody is specific for the 34 and 63 kDa Noetia red cell phosphoproteins. The phosphorylation of both proteins is also blocked by phenothiazines (Pierce et al., 1988; Politis and Pierce, 1991) and Ca2+-free medium, and the time course of phosphorylation coincides with Ca2+ influx and taurine efflux (Politis and Pierce, 1991). These data suggest that increased [Ca2+]i activates calmodulin which, in turn, mediates phosphorylation of the 34 kDa protein through a kinase or phosphatase.
Recently, as described above, we have found that okadaic acid, a specific inhibitor of protein phosphatase 1 (PP1) and protein phosphatase 2A (PP2A), potentiates volume recovery, taurine efflux and phosphorylation of the 34 kDa membrane protein during hypoosmotic stress (6) (Warren and Pierce, 1995). These data provide more evidence that the levels of phosphorylation of the 34 kDa membrane protein are involved in the regulation of membrane permeability to taurine.
In addition, also as described above, the anion transport inhibitor DIDS, which blocks the VSOAC channel, blocks volume recovery, taurine efflux, and Cl– efflux from N. ponderosa RBCs following hypoosmotic stress (Warren and Pierce, 1998). This suggests that a VSOAC-like channel may be present in N. ponderosa RBCs (7).
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NOETIA RBC CONCLUSION |
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Several parts of the control mechanism of taurine efflux during hypoosmotic stress have been described in N. ponderosa RBCs. These include components of a signal transduction pathway that results in specific membrane protein phosphorylation that coincides with taurine efflux and volume recovery. In addition, our recent pharmacological evidence indicates that the VSOAC channel found in vertebrate cells is most likely present in N. ponderosa RBCs indicating that this cell type may be quite useful as a general model of the phenomenon. Since no one else has purified the taurine-anion channel from any other cell type as yet, in addition to identifying the 34 kDa protein, determining its similarity with other cell types is very important.
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1 From the Symposium Osmoregulation: An Integrated Approach presented at the Annual Meeting of the Society for Integrative and Comparative Biology, 4–8 January 2000, at Atlanta, Georgia.
2 E-mail: pierce{at}chuma1.cas.usf.edu
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Ballatori, N., A. T. Truong, P. S. Jackson, K. Strange, and J. L. Boyer. 1995. ATP depletion and inactivation of an ATP-sensitive taurine channel by classic ion channel blockers. Mol. Pharmacol, 48:472-476.[Abstract]
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Bender, A. S., J. T. Neary, and M. D. Norenberg. 1992. Involvement of second messengers and protein phosphorylation in astrocyte swelling. Canadian J. Physiol. Pharmacol, 70: s362-s366.
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Cala, H., L. Mandel, and E. Murphy. 1986. Volume regulation by Amphiuma red blood cells: Cytosolic free Ca2+ and alkalai metal-H+ exchange. Am. J. Physiol, 240:C423-C429.
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Crepel, V., W. Panenka, M. E. M. Kelly, and B. A. MacVicar. 1998. Mitogen-activated protein and tyrosine kinases in the activation of astrocyte volume-activated chloride current. J. Neurosci, 18:1196-1206.
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