Volume-sensitive taurine transport in fish erythrocytes

Molecular basis of the unique osmoregulatory strategy in the inshore hagfish, Eptatretus burgeri

Hagfishes are characterized by omo- and iono-conforming nature similar to marine invertebrates. Conventionally, hagfishes had been recognized as the most primitive living vertebrate that retains plesiomorphic features. However, some of the "ancestral" features of hagfishes, such as rudimentary eyes and the lack of vertebrae, have been proven to be deceptive. Similarly, by the principle of maximum parsimony, the unique body fluid regulatory strategy of hagfishes seems to be apomorphic, since the lamprey, another cyclostome, adopts osmo- and iono-regulatory mechanisms as in jawed vertebrates. Although hagfishes are unequivocally important in discussing the origin and evolution of the vertebrate osmoregulatory system, the molecular basis for the body fluid homeostasis in hagfishes has been poorly understood. In the present study, we explored this matter in the inshore hagfish, Eptatretus burgeri, by analyzing the transcriptomes obtained from the gill, kidney, and muscle of the animals acclimated to distinct environmental salinities. Together with the measurement of parameters in the muscular fluid compartment, our data indicate that the hagfish possesses an ability to conduct free amino acid (FAA)-based osmoregulation at a cellular level, which is in coordination with the renal and branchial FAA absorption. We also revealed that the hagfish does possess the orthologs of the known osmoregulatory genes and that the transepithelial movement of inorganic ions in the hagfish gill and kidney is more complex than previously thought. These observations pose a challenge to the conventional view that the physiological features of hagfishes have been inherited from the last common ancestor of the extant vertebrates.

Babesia divergens-infected red blood cells take up glutamate via an EAAT3 independent mechanism

Human red blood cells infected with the malaria parasite Plasmodium falciparum show an increased permeability to a number of solutes. We have previously demonstrated that such infected cells take up glutamate via a member of the excitatory amino acid transporter protein family (EAAT), namely EAAT3. Babesia divergens is a parasite that also infects human erythrocytes, and also induces increased solute permeability, including for glutamate. Here we have investigated whether glutamate uptake in B. divergens infected human red blood cells is also dependent on EAAT3 activity. We find that, although B. divergens infected cells do take up glutamate, this uptake is independent on EAAT3. Thus, though infecting the same host cell, two related parasites have developed distinct pathways to obtain access to nutrients from the extracellular milieu.

Radiation protection following nuclear power accidents: a survey of putative mechanisms involved in the radioprotective actions of taurine during and after radiation exposure

There are several animal experiments showing that high doses of ionizing radiation lead to strongly enhanced leakage of taurine from damaged cells into the extracellular fluid, followed by enhanced urinary excretion. This radiation-induced taurine depletion can itself have various harmful effects (as will also be the case when taurine depletion is due to other causes, such as alcohol abuse or cancer therapy with cytotoxic drugs), but taurine supplementation has been shown to have radioprotective effects apparently going beyond what might be expected just as a consequence of correcting the harmful consequences of taurine deficiency per se. The mechanisms accounting for the radioprotective effects of taurine are, however, very incompletely understood. In this article an attempt is made to survey various mechanisms that potentially might be involved as parts of the explanation for the overall beneficial effect of high levels of taurine that has been found in experiments with animals or isolated cells exposed to high doses of ionizing radiation. It is proposed that taurine may have radioprotective effects by a combination of several mechanisms: (1) during the exposure to ionizing radiation by functioning as an antioxidant, but perhaps more because it counteracts the prooxidant catalytic effect of iron rather than functioning as an important scavenger of harmful molecules itself, (2) after the ionizing radiation exposure by helping to reduce the intensity of the post-traumatic inflammatory response, and thus reducing the extent of tissue damage that develops because of severe inflammation rather than as a direct effect of the ionizing radiation per se, (3) by functioning as a growth factor helping to enhance the growth rate of leukocytes and leukocyte progenitor cells and perhaps also of other rapidly proliferating cell types, such as enterocyte progenitor cells, which may be important for immunological recovery and perhaps also for rapid repair of various damaged tissues, especially in the intestines, and (4) by functioning as an antifibrogenic agent. A detailed discussion is given of possible mechanisms involved both in the antioxidant effects of taurine, in its anti-inflammatory effects and in its role as a growth factor for leukocytes and nerve cells, which might be closely related to its role as an osmolyte important for cellular volume regulation because of the close connection between cell volume regulation and the regulation of protein synthesis as well as cellular protein degradation. While taurine supplementation alone would be expected to exert a therapeutic effect far better than negligible in patients that have been exposed to high doses of ionizing radiation, it may on theoretical grounds be expected that much better results may be obtained by using taurine as part of a multifactorial treatment strategy, where it may interact synergistically with several other nutrients, hormones or other drugs for optimizing antioxidant protection and minimizing harmful posttraumatic inflammatory reactions, while using other nutrients to optimize DNA and tissue repair processes, and using a combination of good diet, immunostimulatory hormones and perhaps other nontoxic immunostimulants (such as beta-glucans) for optimizing the recovery of antiviral and antibacterial immune functions. Similar multifactorial treatment strategies may presumably be helpful in several other disease situations (including severe infectious diseases and severe asthma) as well as for treatment of acute intoxications or acute injuries (both mechanical ones and severe burns) where severely enhanced oxidative and/or nitrative stress and/or too much secretion of vasodilatory neuropeptides from C-fibres are important parts of the pathogenetic mechanisms that may lead to the death of the patient. Some case histories (with discussion of some of those mechanisms that may have been responsible for the observed therapeutic outcome) are given for illustration of the likely validity of these concepts and their relevance both for treatment of severe infections and non-infectious inflammatory diseases such as asthma and rheumatoid arthritis.

Non-electrolyte permeability of deoxygenated sickle cells compared

The passive permeability pathways of red cells are poorly defined, with the exception of the Gardos channel. Several cation and anion pathways can be induced by a variety of manoeuvres, however, including treatment with oxidants, low ionic strength (LIS), shrinkage, swelling and also infection with the intra-erythrocytic malaria parasite. Several of these stimuli (malaria, swelling, LIS), in addition, also activate a non-electrolyte this permeability. Sickle cells uniquely show a deoxygenation-induced pathway, which is termed P(sickle) and is usually considered to be a conductive cationic pathway. In this report, we explore further the extent to which this permeability pathway of deoxygenated sickle cells is available for non-electrolyte transport. We show that a number of solutes are permeable, with greater permeability to sugars (notably lactose and maltose) and smaller molecules, and less to charged or zwitterionic species. Red cells from heterozygous HbSC patients also showed deoxygenation-induced haemolysis in isosmotic sucrose solution, though to a slightly lesser extent than for red cells from homozygous sickle cell patients. In contrast to sickle cells, red cells from beta-thalassaemic patients did not show haemolysis in isosmotic sucrose solutions, regardless of the O(2) tension. Of the secondary cellular changes resulting from incubation in non-electrolyte solutions (which include imposition of a highly positive membrane potential, marked intracellular alkalinisation and cell shrinkage), none appear to correlate with activation of the non-electrolyte permeability. Rather, findings indicate that it is low ionic strength per se that is responsible. Normal red cells also show changes in ionic and non-electrolyte permeability in low ionic strength media, and these permeabilities are compared to those found in deoxygenated sickle cells. The extent to which these different permeabilities in normal and sickle red cells can be ascribed to one or more common pathways remains to be determined.

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.

The effect of mefloquine and volume-regulated anion channel inhibitors on induced transport in Plasmodium falciparum-infected human red blood cells

The malaria parasite Plasmodium falciparum activates new permeation pathways (NPP) in the host cell membrane of infected human red blood cells (RBCs), which are permeable to anions, cations and a range of organic solutes. It has been suggested from inhibitor and substrate selectivity studies that the NPP may be identical to the volume-activated anion channel (VRAC) present in many mammalian cell types. Here we have tested several known inhibitors of VRAC on the transport of choline and lactate in malaria-infected human RBCs and on parasite growth. Mefloquine, tamoxifen and clomiphene were all without effect on malaria-induced transport at concentrations up to 10 microM and only mefloquine (IC(50) = 24 nM) and, to a lesser degree, clomiphene (IC(50) = 6.2 microM) inhibited parasite growth below this level. It is concluded that the antimalarial effect of mefloquine does not involve the inhibition of malaria-induced transport via the NPP and there is no evidence at present for VRAC and the NPP being identical.

Potentized Mercuric chloride and Nux vomica Facilitate Water Permeability in Erythrocytes of a Fresh-Water CatfishClarius batrachusUnder Acute Ethanol Intoxication

OBJECTIVES

The primary biomolecular target of a homeopathic potency is unknown. If it is a plasma membrane protein such as water-channel protein, the drug would alter water permeation in cells. Therefore, the objective is to see if potentized homeopathic drugs like Mercuric chloride 30c and Nux vomica 30c could alter permeation of water through the erythrocytes of a fresh water fish under acute ethanol intoxication.

LOCATION

The work was carried out in the Zoology Laboratory of Visva Bharati University, Santiniketan, West Bengal, India.

SUBJECT

Live freshwater catfish.

DESIGN

Erythrocytes collected from fish with and without ethanol intoxication were incubated in distilled water at 30 degrees C for 30 minutes with Ethanol 30c (control), Merc cor 30c (test 1), and Nux vomica 30c (test 2). Merc cor 30c and Nux vom 30c were prepared by successive dilution of the respective mother tinctures with 90% ethanol (1:100) followed by sonication at 20 kHz for 30 seconds in 30 steps. Ethanol 30c was prepared in the same way from 90% ethanol diluted with 90% ethanol. In another experiment, fish were pretreated with Ethanol 30c and Nux vom 30c followed by ethanol injection at 2 g/kg of body weight. Then their erythrocytes were tested in vitro with the same potencies. After centrifugation of blood samples, fluid part was removed, erythrocyte pellets dried in a BioChemical Oxygen Demand (BOD; Atlas Surgical, New Delhi, India) incubator at 90 degrees C for 12 hours and intracellular water content measured.

RESULTS

Red blood cells (RBCs) from ethanol-injected fish permeated more water than those from normal fish. Water permeation was enhanced with Merc cor 30c and Nux vom 30c. RBCs from fish pretreated with Nux vom 30c imbibed more water in in vitro treatments than those from fish pretreated with Ethanol 30c.

CONCLUSION

Because water channel proteins or aquaporins are mainly responsible for water transport through the plasma membrane of RBCs, it is thought that potentized drugs interact with these proteins, thereby facilitating water influx in the cells.

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.

Osmoregulation and cell volume regulation in the preimplantation embryo

The early preimplantation mammalian embryo possesses mechanisms that regulate intracellular osmolarity and cell volume. While transport of osmotically active inorganic ions might play a role in this process in embryos, the major mechanisms that have been identified and studied are those that employ organic osmolytes. Organic osmolytes provide a substantial portion of intracellular osmotic support in embryos and are required for their development under in vivo conditions. The main osmolytes that have been identified in cleavage stage embryos are accumulated via two transport systems of the neurotransmitter transporter family active in early preimplantation embryos--the glycine transport system (GLY) and the beta-amino acid transport system (system beta). While system beta has been established to have a similar role in many other cells, this is a novel function for the GLY transport system. The intracellular concentration of organic osmolytes such as glycine in early preimplantation embryos is regulated by tonicity, allowing the embryo to regulate its volume against shrinkage and to control its internal osmolarity. In addition, the cells of the embryo can regulate against an increase in volume via controlled release of osmolytes from the cytoplasm. This is mediated by a swelling-activated anion channel that is also highly permeable to a range of organic osmolytes, and which closely resembles similar channels found in many other cell types (VSOAC channels). Together, these mechanisms appear to regulate cell volume in the egg through the early cleavage stages of embryogenesis, after which there are indications that the mechanisms of osmoregulation change.

Copper ion redox state is critical for its effects on ion transport pathways and methaemoglobin formation in trout erythrocytes

We have studied the mechanism of copper uptake by the cells, its oxidative action and effects on ion transport systems using rainbow trout erythrocytes. Cupric ions enter trout erythrocytes as negatively charged complexes with chloride and hydroxyl anions via the band 3-mediated Cl-/HCO3- exchanger. Replacement of Cl- by gluconate, and complexation of cupric ions with histidine abolish rapid Cu2+ uptake. Within the cell cupric ions interact with haemoglobin, causing methaemoglobin formation by direct electron transfer from heme Fe2+ to Cu2+, and consecutive proton release. Ascorbate-mediated reduction of cupric ions to cuprous decreases copper-induced metHb formation and proton release. Moreover, cuprous ions stimulate Na+H+ exchange and residual Na+ transport causing net Na+ accumulation in the cells. The effect requires copper binding to an externally facing thiol group. Copper-induced Na+ accumulation is accompanied by K+ loss occurring mainly via K+-Cl- cotransporter. Taurine efflux is also stimulated by copper exposure. However, net loss of osmolytes is not as pronounced as Na+ uptake and modest swelling of the cells occurs after 5 min of copper exposure. Taken together the results indicate that copper toxicity, including copper transport into the cells and its interactions with ion transport processes, depend on the valency and complex formation of copper ions.

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.

Membrane transport in the malaria-infected erythrocyte

The malaria parasite is a unicellular eukaryotic organism which, during the course of its complex life cycle, invades the red blood cells of its vertebrate host. As it grows and multiplies within its host blood cell, the parasite modifies the membrane permeability and cytosolic composition of the host cell. The intracellular parasite is enclosed within a so-called parasitophorous vacuolar membrane, tubular extensions of which radiate out into the host cell compartment. Like all eukaryote cells, the parasite has at its surface a plasma membrane, as well as having a variety of internal membrane-bound organelles that perform a range of functions. This review focuses on the transport properties of the different membranes of the malaria-infected erythrocyte, as well as on the role played by the various membrane transport systems in the uptake of solutes from the extracellular medium, the disposal of metabolic wastes, and the origin and maintenance of electrochemical ion gradients. Such systems are of considerable interest from the point of view of antimalarial chemotherapy, both as drug targets in their own right and as routes for targeting cytotoxic agents into the intracellular parasite.

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.

The role of anion and cation channels in volume regulatory responses in trout red blood cells

(1) An outwardly rectifying chloride channel (ORCC) of large conductance has been detected under isotonic conditions (320 mosM 1(-1)) in the plasma membrane of trout red blood cells (RBCs) using the excised inside-out configuration. The channel, with a permeability ratio P(Cl)/Pcation of 12, was inhibited by the Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)-benzoate (NPPB) (50 microM), and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS) (100 microM) in the bathing solution. (2) In hypotonic conditions (215 mosM 1(-1)), 44% of cell-attached patches showed spontaneous single channel activity identified as nonselective cationic (NSC) channels. A second group, corresponding to 7% of cell-attached patches, showed spontaneous activity corresponding to a channel type presenting outward rectification and anionic selectivity. Finally, 49% of patches displayed a complex spontaneous signal corresponding to the superimposition of inward and outward currents probably due to activation of different channel types. (3) Giga-seals obtained without suction in intact cells under isotonic conditions possessed NSC channels that were quiescent but which could be activated either by mechanical deformation of cell membrane or by hypotonic cell swelling. (4) Hypotonically swollen RBCs exhibited regulatory volume decrease (RVD) over 3 h, which was linked to a fivefold to sixfold increase in unidirectional fluxes of K+, a net loss of intracellular K+ and net gain of extracellular Na+. RVD and the hypotonically activated, unidirectional K+ influx continued after replacement of Cl- by methylsulfonate (MeSF) albeit more slowly. (5) The NSC channel inhibitor, barium, and the Cl- channel inhibitor, NPPB, both inhibited the RVD response by approximately 50% in Cl- containing saline. When Cl- was replaced by MeSF, the inhibition was > 90% suggesting that NSC channels and ORCC play key roles in the chloride-independent component of RVD.

Na+-dependent transport of pyruvate in erythrocytes of the Pacific hagfish (Eptatretus stouti)

We investigated the mechanisms of pyruvate transport in the erythrocytes of an ancient marine agnathan, the Pacific hagfish (Eptatretus stouti), and a sedentary euryhaline teleost, the starry flounder (Platichthys stellatus). Uptake of [14C]pyruvate (50 µM, 10°C) by flounder erythrocytes was slow (t1/2 (half-life) ~ 30 min), nonconcentrative, and mediated by the band 3 Cl-/HCO-3 exchanger in combination with a process similar to the H+/monocarboxylate symporter present in freshwater teleosts and mammalian erythrocytes. In contrast, pyruvate uptake by hagfish erythrocytes (50 µM, 10°C) was rapid (t1/2 ~ 1.5 min) and, in 10 min, reached an intracellular concentration more than 20-fold higher than that present in the extracellular medium. Pyruvate accounted for almost 90% of the accumulated intracellular radioactivity, the remaining label being incorporated into tricarboxylic acid cycle intermediates and glutamate. Influx of pyruvate was saturable (apparent Km = 12 mM) and inhibited by p-chloromercuriphenylsulphonate (PCMBS) (Ki = 71 µM) and 4,4'-diisothiocyanatostilbene-2,2'-disulphonate (DIDS) (Ki = 0.49 mM). Transport was inhibited poorly by α-cyano-4-hydroxycinnamate (CIN) (Ki > 4 mM) and was not coupled to the movement of protons. Instead, the influx of pyruvate was Na+ dependent. A sigmoidal relationship between pyruvate transport and extracellular Na+ concentration was observed, suggesting a Na+:pyruvate coupling ratio greater than 1:1. In contrast with previously described Na+-dependent monocarboxylate transport activities in mammalian renal and intestinal epithelia, the hagfish erythrocyte system did not transport lactate.

A hyperosmotic stress-induced mRNA of carp cell encodes Na(+)- and Cl(-)-dependent high affinity taurine transporter

A cDNA clone encoding a Na(+)- and Cl(-)-dependent high affinity taurine transporter was isolated from a common carp cell line, Epithelioma papulosum cyprini (EPC), as a hyperosmotic stress-inducible gene by RNA arbitrarily primed PCR. The clone contained a 2.5-kb cDNA fragment including an open reading frame of 1878 bp encoding a protein of 625 amino acids. The deduced amino acid sequence of carp taurine transporter shows 78-80% identity to those of cloned mammalian taurine transporters. The functional characteristics of the cloned transporter were analyzed by expression in COS-7 cells. Transfection with the cDNA induced Na(+)- and Cl(-)-dependent taurine transport activity with an apparent K(m) of 56 microM. The Na(+)/Cl(-)hepatopancreas. Taurine transporter mRNA level increased up to 7.5-fold on raising the ambient osmolality from 300 to 450 mosmol/kgH(2)O. These data suggest the significant role of taurine as an osmolyte in carp cells.

Swelling activation of transport pathways in erythrocytes: effects of Cl-, ionic strength, and volume changes

If swelling of a cell is induced by a decrease in external medium tonicity, the regulatory response is more complex than if swelling of similar magnitude is due to salt uptake. The present results provide an explanation. In fish erythrocytes, two distinct transport pathways were swelling activated: a channel of broad specificity and a K+-Cl- cotransporter. Each was activated by a specific signal: the channel by a decrease in intracellular ionic strength and the K+-Cl- cotransporter by cell enlargement. A decrease in ionic strength also affected K+-Cl- cotransport activity, but by acting as a negative modulator of the cotransport. Thus cells swollen by salt accumulation respond by activating exclusively the K+-Cl- cotransport, leading to a Cl--dependent K+ loss. By contrast, cells swollen by electrolyte dilution respond by activating both pathways, leading to a reduced loss of electrolytes and a large loss of taurine. Thus two swelling-sensitive pathways, differently regulated, would allow control of the ionic composition of a cell exposed to different volume perturbations.

Transport of uncharged organic solutes in Xenopus oocytes expressing red cell anion exchangers (AE1s)

When expressed in Xenopus oocytes, the trout red cell anion exchanger tAE1, but not the mouse exchanger mAE1, elicited a transport of electroneutral solutes (sorbitol, urea) in addition to the expected anion exchange activity. Chimeras constructed from mAE1 and tAE1 allowed us to identify the tAE1 domains involved in the induction of these transports. Expression of tAE1 (but not mAE1) is known to generate an anion conductance associated with a taurine transport. The present data provide evidence that (i) the capacity of tAE1 and tAE1 chimeras to generate urea and sorbitol permeability also was associated with an anion conductance; (ii) the same inhibitors affected both the permeability of solutes and anion conductance; and (iii) no measurable water transport was associated with the tAE1-dependent conductance. These results support the view that fish red blood cells, to achieve cell volume regulation in response to hypotonic swelling, activate a tAE1-associated anion channel that can mediate the passive transport of taurine and electroneutral solutes.

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.

Volume-activated DIDS-sensitive whole-cell chloride currents in trout red blood cells

1. The nystatin-perforated whole-cell recording mode of the patch-clamp technique was used to investigate the membrane conductance of trout (Oncorhynchus mykiss) red blood cells in the steady state, 5 min after exposure to hyposmotic medium and 10 min after return to normal isosmotic medium. 2. Whole-cell I-V relations showed outward rectification when red blood cells were bathed in isosmotic (320 mosmol l-1) saline solution and the patch pipette was filled with 117 mM KCl. The membrane conductance was 2.58 +/- 0.59 nS (number of experiments, n = 18) between 0 and 100 mV and 1.32 +/- 0.19 nS (n = 18) between 0 and -100 mV. Removal of Cl- from the extracellular side or incubation with the Cl- channel blocker DIDS caused a reduction in whole-cell membrane conductance by more than 50%, indicating that the membrane current was generated by Cl- ions. The remaining conductance was voltage independent and probably due to non-selective cation conductance. 3. The membrane conductance increased approximately 2-fold after cell swelling induced by exposure to hyposmotic saline solution (215 mosmol l-1). This effect was abolished in Cl(-)-free hyposmotic medium or in the presence of DIDS. 4. The return to isosmotic solution produced a fall in membrane conductance to, or below, control values. 5. We conclude that trout red blood cells possess a significant Cl- conductance in the steady state which is reversibly activated during cell swelling and contributes to volume recovery.

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.

Glycine transport by the red cells of channel catfish

In humans, glycine enters the red cell via four distinct plasma membrane carrier systems. The purpose of the present experiments was to measure the mode of transport of glycine by channel catfish (Ictalurus punctatus) red cells. About 54% of the glycine was transported by system L, while 16.1% of the glycine was transported by system Gly. A further 15.6% of transport was via system ASC and system asc together. An unidentified Na+-independent system was responsible for the transport of 7.2% of the glycine. No solute appeared to be carried into the cell by band 3. The remainder of the glycine entered the cell by diffusion. The Na+-independent system exhibited a Ktvalue of 57 ± 12 (mean ± standard deviation) μM and Vmaxof 142 ± 27 nmol∙g hemoglobin−1∙min−1(this compares with system L, which exhibited a Ktvalue of 65 ± 21 μM and Vmaxof 516 ± 117 nmol∙g hemoglobin−1∙min−1). These results demonstrate that channel catfish red cells are capable of transporting glycine by three of the four transporters involved in human red cells, although the relative contributions differ markedly, and by an additional unidentified transport system not requiring Na+. The differences in glycine transport between human and catfish red cell membranes can be attributed to evolutionary influences.

Volume-associated osmolyte fluxes in cell lines with or without the anion exchanger

To investigate the involvement of a red cell-type anion exchanger in the volume-sensitive amino acid release, the hyposmolarity-evoked release of D-[3H]aspartate and [3H]taurine was examined in three cell lines: 1) wild-type Chinese hamster ovary (CHO-K1) cells, expressing an anion exchanger activity (Cl-/SO4(2-)) functionally similar to the erythroid band 3; 2) a mutant CHO cell type (CHO 605) lacking this anion exchanger activity; and 3) 293 cells in which the Cl-/HCO3(-) anion exchanger is absent. All cell types accumulated D-[3H]aspartate and [3H]taurine under isosmotic conditions, and, similarly, in the three cell lines, cell swelling evoked by hyposmolarity induced a rapid and transient increase in the amino acid efflux. Blockers of the anion exchanger and/or Cl- channels [niflumic acid, dipyridamole, diphenylamine-2-carboxylate,5-nitro-2-(3-phenylpropylamino)-benzoi c acid, and 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid] were potent inhibitors of amino acid efflux in the three cell lines. 125I- efflux, used as a marker for Cl- fluxes, was also markedly increased in response to cell swelling in all cell lines, and this efflux was inhibited by the anion exchanger/Cl- channel blockers. These results do not support a role for an anion exchanger in the hyposmolarity-induced amino acid efflux and suggest that amino acids and Cl- may be transported by the same or a similar mechanism, presumably an anion channel-like structure.

Taurine efflux is a cell volume regulatory process in proximal renal tubules from the teleost Carassius auratus

The potential role of taurine transport associated with volume regulation in renal tissue and isolated proximal renal tubules was studied in the teleost Carassius auratus (goldfish). The cellular taurine content in renal tissue fragments incubated in isosmotic solution (290 mOsm) (7.8 +/- 0.9 (SD) micromol g wet wt(-1)) decreased by 60% following exposure to hyposmotic medium (100 mOsm). The rate coefficient for [14C]taurine efflux in renal tissue and in isolated proximal renal tubules was strongly stimulated following hyposmotically or urea-activated cellular swelling. The stimulated basolateral taurine efflux pathway exhibited channel-like functional characteristics since (a) [14C]taurine influx was stimulated in parallel with the osmolality-dependent taurine efflux and (b) this efflux could not be stimulated by high medium taurine concentrations (40 mM) applied 10 min following the osmolality reduction (trans-stimulation test). Administration of the 5-lipoxygenase inhibitor ETH 615-139 (20 microM) during hyposmotic stimulation inhibited regulatory volume decreases but had no effect on taurine efflux. In addition, hyposmotically induced taurine efflux was slightly but significantly inhibited by the cyclooxygenase inhibitor indomethacin (10 microM). The taurine efflux was also dependent on both extra- and intracellular Ca2+. It is concluded that taurine is likely to coparticipate with KCl as an osmoeffector during RVD in Carassius proximal renal tubule cells. Cellular swelling seems to activate a basolateral taurine transport pathway with functional properties of a channel. This efflux mechanism appears to be partly regulated by Ca2+. Such a transport pathway could play a role in the cell volume regulatory mechanisms participating during transepithelial solute and water transport.

Volume-sensitive taurine transport in bovine articular chondrocytes

1. The swelling of bovine articular chondrocytes isolated from, or in situ within, cartilage by hypotonic shock rapidly activated the efflux or influx of radiolabelled taurine, an amino acid involved in volume regulation in a range of other cell types. 2. When chondrocytes were isolated by the use of collagenase into media of 280 or 380 mosmol l-1, the activation of taurine efflux was at about the osmolarity of the isolating medium, but it was more marked for a given hypotonic shock in the cells isolated at the lower osmolarity. The volume of chondrocytes following isolation in these two osmolarities was the same, suggesting that the cells possess volume regulatory capacity. 3. In isolated chondrocytes, the induced pathway had some of the characteristics of a volume-activated channel, i.e. no transport saturation with increasing substrate concentration, and lack of trans acceleration. The pattern of inhibition of the volume-activated pathway by pharmacological blockers (e.g. pimozide, [(dihydro-indenyl)oxy]alkanoic acid (DIOA) and tamoxifen) differed from that described for a similar pathway in other cell types. 4. The transport of other potential osmolytes (uridine, sorbitol and inositol) was stimulated by cell swelling, independent of sodium and inhibited by pimozide with a selectivity ratio of taurine, 1.00; uridine, 0.84; sorbitol, 0.66; and inositol, 0.38. The selectivity of taurine: inositol was not altered at different cell volumes. 5. The intracellular taurine concentration of chondrocytes within cartilage was low (about 2 mmol (l cell water)-1) showing a negligible role for taurine as an osmolyte during recovery from cell swelling. The swelling-induced loss of taurine from chondrocytes in situ was largely inhibited by pimozide and other drugs, showing that despite the rigid nature of cartilage, the chondrocytes were osmotically sensitive within the extracellular matrix.

Substrate transport and utilization in fish erythrocytes

In contrast to mammalian erythrocytes which fulfil their energy requirements via anaerobic glycolysis, fish erythrocytes obtain most of their energy aerobically via the Krebs cycle. This enables them to use various substrates, including monocarboxylic acids, glucose and amino acids. This review discusses the membrane permeability of these substrates, and their relative importance in the energy production of fish erythrocytes. Agnathans are characterized by a high permeability to all of the potential substrates, glucose, monocarboxylic and amino acids. In contrast, teleost erythrocytes are often characterized by low glucose permeability. It appears that monocarboxylic acids and certain amino acids such as glutamine may be more important in energy production of teleost erythrocytes than glucose.

Osmotic regulation of taurine transport via system beta and novel processes in mouse preimplantation conceptuses

Taurine was shown recently to increase the frequency at which 2-cell mouse conceptuses develop into blastocysts in vitro. For this reason and because taurine helps cells adapt to external stresses, we studied transport of this and related amino acids by preimplantation mouse conceptuses. The most conspicuous component of taurine transport in conceptuses at the 1-cell through blastocyst stages of development was both Na(+)- and Cl(-)-dependent. This Na(+)- and Cl(-)-dependent transport system interacted relatively strongly with beta- but not alpha-amino acids. By these criteria, transport system beta is responsible for Na(+)-dependent taurine transport in preimplantation mouse conceptuses. Moreover, detection of mRNA encoding the taurine transport protein (TAUT) in early conceptuses supports the theory that TAUT is a major component of system beta. Transport of taurine by system beta in 1-cell conceptuses was slower in hypotonic than in hypertonic media, whereas the reverse was true for system beta in blastocysts. In contrast, hypotonically stimulated Na(+)-independent taurine transport was, of course, more rapid in hypotonic than in hypertonic media in both 1-cell conceptuses and blastocysts. Transport via this hypotonically stimulated process also showed no sign of saturation by up to 10 mM taurine. Hypotonically stimulated taurine transport appeared transiently in 1-cell conceptuses under hypotonic conditions until they had recovered their initial volumes. Hence, we suggest that a decrease in taurine uptake via system beta and an increase in taurine exodus via the Na(+)-independent, nonsaturable transport process could contribute to the regulatory volume decrease in 1-cell conceptuses in hypotonic medium. Since taurine uptake by system beta in blastocysts is, however, higher in hypotonic than in hypertonic media, taurine uptake by system beta in blastocysts might intensify a tendency to increase cell volume in hypotonic medium. Such an increase in taurine uptake could further favor anabolic changes associated with cell swelling. In addition to contributing to regulation of cellular volume and perhaps metabolism, the hypotonically stimulated Na(+)-independent transport processes in early embryos have novel characteristics. Hypotonically stimulated Na(+)-independent taurine transport was inhibited by niflumate, N-ethylmaleimide and NaN3 but not by furosemide, iodoacetate, KCN, ouabain or alpha- or beta-amino acids. Furthermore, 4,4'-diisothiocyanostilbene-2,2'-disulfonate inhibited this transport in 1-cell conceptuses but not in blastocysts. Hence, different hypotonically stimulated Na(+)-independent taurine transport processes appear to be present in 1-cell conceptuses vs. blastocysts. The functions of these and other instances of developmental regulation of expression of transport processes in preimplantation conceptuses remain largely to be elucidated.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

The protective action of betaine on the deleterious effects of NaCl on preimplantation mouse embryos in vitro

The development of outbred mouse (CF1) zygotes in vitro has been studied using medium SOM in which the concentrations of NaCl (85, 105, 125 mM), glutamine (0, 1, 2 mM), and betaine (0, 1, 2 mM) were varied. The effects of the compounds were studied using a 3(3) factorial experimental arrangement. The inhibitory effect of relatively high concentrations of NaCl and the protective effect of glutamine were confirmed. Betaine, an organic osmolyte, can also protect against the deleterious effects of relatively high concentrations of NaCl. The intracellular contents of potassium and sodium have also been measured in single zygotes using X-ray electron probe spectrometry. When medium SOM contains 85 mM or 125 mM NaCl, the intracellular content of Na rises and the content of K decreases. These changes are partially reduced in the presence of 125 mM NaCl if betaine is also in the medium. Betaine has no effect on the intracellular content of K and Na if the concentration of NaCl is 85 mM. These results suggest that organic osmolytes may be required in embryo culture media to prevent excessive changes in the intracellular ionic concentration.

Cold activation of Na influx through the Na-H exchange pathway in guinea pig red cells

Previous work showed that amiloride partially inhibits the net gain of Na in cold-stored red cells of guinea pig and that the proportion of unidirectional Na influx sensitive to amiloride increases dramatically with cooling. This study shows that at 37 degrees C amiloride-sensitive (AS) Na influx in guinea pig red blood cells is activated by cytoplasmic H+, hypertonic incubation, phorbol ester in the presence of extracellular Ca2+ and is correlated with cation-dependent H+ loss from acidified cells. Cytoplasmic acidification increases AS Na efflux into Na-free medium. These properties are consistent with the presence of a Na-H exchanger with a H+ regulatory site. Elevation of cytoplasmic free Mg2+ above 3 mM greatly increases AS Na influx: this correlates with a Na-dependent loss of Mg2+, indicating the presence of a Na-Mg exchanger. At 20 degrees C activators of Na-H exchange have little or no further stimulatory effect on the already elevated AS Na influx. AS Na influx is much larger than either Na-dependent H+ loss or AS Na efflux at 20 degrees C. The affinity of the AS Na influx for cytoplasmic H+ is greater at 20 degrees C than at 37 degrees C. Depletion of cytoplasmic Mg2+ does not abolish the high AS Na influx at 20 degrees C. Thus, elevation of AS Na influx with cooling appears to be due to increased activity of a Na-H exchanger (operating in a "slippage" mode) caused by greater sensitivity to H+ at a regulatory site.

Oxygenation-activated K fluxes in trout red blood cells

The effect of oxygenation on the dissipative fluxes of K in trout red blood cells has been determined. Unidirectional influx under low oxygen tension (PO2 = 1 kPa) was 0.56 +/- 0.07 mmol.l-1 packed cells.h-1. Within a few minutes of equilibration with high oxygen tension (PO2 = 120 kPa), influx was increased 14-fold, and this was associated with a progressive loss of KCl and a cell shrinkage. K influx progressively declined over the following 3 h to levels close to those characteristic of cells at low oxygen tension. Replacement of medium Cl by NO3- or methane sulfonate inhibited the stimulation due to high oxygen as did furosemide and low extracellular pH. The oxygenation-stimulated influx was highly volume sensitive, being increased by up to 100% by osmotic swelling and decreased by osmotic shrinkage. By contrast, the small influx under low oxygen tension was unaffected by either Cl replacement or by shrinkage and increased only with extreme swelling. Thus high oxygen tension activated a Cl-dependent and furosemide-sensitive K flux. Once activated, the mechanism was rapidly deactivated on transfer back to low oxygen tension but slowly deactivated when maintained at high PO2. The oxygenation-stimulated flux mechanism promotes a rapid and more complete volume regulatory decrease than in cells at low oxygen tension.

Regulation of Na+/H+ exchange and pH in erythrocytes of fish

1. The function of trout RBC Na+/H+ antiport is unrelated to cell volume or cell pH regulation. Its role is to improve oxygen transport capacity when the supply of oxygen becomes limited. 2. Antiport activation, mediated by cAMP, promotes complex changes in blood pH which have been analyzed in vivo and in vitro. 3. The regulation of antiport (activation, desensitization, control by molecular oxygen and by a newly discovered cytosolic protein, arrestin) is presented. 4. Molecular cloning of the antiport shows that two typical site motifs of phosphorylation by cAMP-dependent protein kinase are localized on the cytoplasmic region.

Cation and harmaline interactions with Na(+)-independent dibasic amino acid transport system y+ in human erythrocytes and in erythrocytes from a primitive vertebrate the pacific hagfish (Eptatretus stouti)

Transport systems y+, asc and ASC exhibit dual interactions with dibasic and neutral amino acids. For conventional Na(+)-dependent neutral amino acid system ASC, side chain amino and guanido groups bind to the Na+ site on the transporter. The topographically equivalent recognition site on related system asc binds harmaline (a Na(+)-site inhibitor) with the same affinity as asc (apparent Ki range 1-4 mM), but exhibits no detectable affinity for Ha. Although also classified as Na(+)-independent, dibasic amino acid transport system y+ accepts neutral amino acids when Na+ or another acceptable cation is also present. This latter observation implies that the y+ translocation site binds Na+ and suggests possible functional and structural similarities with ASC/asc. In the present series of experiments with human erythrocytes, system y(+)-mediated lysine uptake (5 microM, 20 degrees C) was found to be 3-fold higher in isotonic sucrose medium than in normal 150 mM NaCl medium. This difference was not a secondary consequence of changes in membrane potential, but resulted from Na+ functioning as a competitive inhibitor of transport. Apparent Km and Vmax values for lysine transport at 20 degrees C were 15.2 microM and 183 mumol/l cells per h, respectively, in sucrose medium and 59.4 microM and 228 mumol/l cells per h in Na+ medium. Similar results were obtained with y+ in erythrocytes of a primitive vertebrate, the Pacific hagfish (Eptatretus stouti), indicating that Na(+)-inhibition is a general property of this class of amino acid transporter. At a permeant concentration of 5 microM, the IC50 value for Na(+)-inhibition of lysine uptake by human erythrocytes was 27 mM. Other inorganic and organic cations, including K+ and guanidinium+, also inhibited transport. In parallel with its actions on ASC/asc harmaline competitively inhibited lysine uptake by human cells in sucrose medium. As predicted from mutually competitive binding to the y+ translocation site, the presence of 150 mM Na+ increased the harmaline inhibition constant (Ki) from 0.23 mM in sucrose medium to 0.75 mM in NaCl medium. We interpret these observations as further evidence that y+, asc and ASC represent a family of closely related transporters with a common evolutionary origin.