Responses of the Na+/H+ exchanger of european flounder red blood cells to hypertonic, β-adrenergic and acidotic stimuli

Acute CO2 tolerance in fishes is associated with air breathing but not the Root effect, red cell βNHE, or habitat

High CO₂ (hypercapnia) can impose significant physiological challenges associated with acid-base regulation in fishes, impairing whole animal performance and survival. Unlike other environmental conditions such as temperature and O₂, the acute CO₂ tolerance thresholds of fishes are not understood. While some fish species are highly tolerant, the extent of acute CO₂ tolerance and the associated physiological and ecological traits remain largely unknown. To investigate this, we used a recently developed ramping assay, termed the Carbon Dioxide maximum (CD_max), that increases CO₂ exposure until loss of equilibrium (LOE) is observed. We investigated if there was a relationship between CO₂ tolerance and the Root effect, β-adrenergic sodium proton exchanger (βNHE), air-breathing, and fish habitat in 17 species. We hypothesized that CO₂ tolerance would be higher in fishes that lack both a Root effect and βNHE, breathe air, and reside in tropical habitats. Our results showed that CD_max ranged from 2.7 to 26.7 kPa, while LOE was never reached in four species at the maximum PCO₂ we could measure (26.7 kPa); CO₂ tolerance was only associated with air-breathing, but not the presence of a Root effect or a red blood cell (RBC) βNHE, or fish habitat. This study demonstrates that the diverse group of fishes investigated here are incredibly tolerant of CO₂ and that although this tolerance is associated with air-breathing, further investigations are required to understand the basis for CO₂ tolerance.

Adrenergically induced translocation of red blood cell β-adrenergic sodium-proton exchangers has ecological relevance for hypoxic and hypercapnic white seabass

White seabass (Atractoscion nobilis) increasingly experience periods of low oxygen (O₂; hypoxia) and high carbon dioxide (CO₂, hypercapnia) due to climate change and eutrophication of the coastal waters of California. Hemoglobin (Hb) is the principal O₂ carrier in the blood and in many teleost fishes Hb-O₂ binding is compromised at low pH; however, the red blood cells (RBC) of some species regulate intracellular pH with adrenergically stimulated sodium-proton-exchangers (β-NHEs). We hypothesized that RBC β-NHEs in white seabass are an important mechanism that can protect the blood O₂-carrying capacity during hypoxia and hypercapnia. We determined the O₂-binding characteristics of white seabass blood, the cellular and subcellular response of RBCs to adrenergic stimulation, and quantified the protective effect of β-NHE activity on Hb-O₂ saturation. White seabass had typical teleost Hb characteristics, with a moderate O₂ affinity (Po₂ at half-saturation; P₅₀ 2.9 kPa) that was highly pH-sensitive (Bohr coefficient -0.92; Root effect 52%). Novel findings from super-resolution microscopy revealed β-NHE protein in vesicle-like structures and its translocation into the membrane after adrenergic stimulation. Microscopy data were corroborated by molecular and phylogenetic results and a functional characterization of β-NHE activity. The activation of RBC β-NHEs increased Hb-O₂ saturation by ∼8% in normoxic hypercapnia and by up to ∼20% in hypoxic normocapnia. Our results provide novel insight into the cellular mechanism of adrenergic RBC stimulation within an ecologically relevant context. β-NHE activity in white seabass has great potential to protect arterial O₂ transport during hypoxia and hypercapnia but is less effective during combinations of these stressors.

Sublethal effects of copper sulphate compared to copper nanoparticles in rainbow trout (Oncorhynchus mykiss) at low pH: physiology and metal accumulation

A few studies have investigated the interaction between copper toxicity and water pH in fishes, but little is known about the effects of acidic pH on the toxicity of copper nanoparticles (Cu-NPs). This study aimed to describe the sub-lethal toxic effects of Cu-NPs compared to CuSO4 at neutral and acidic water pH values in juvenile rainbow trout. Fish were exposed in triplicate (3 tanks/treatment) to control (no added Cu), or 20μgl(-1) of either Cu as CuSO4 or Cu-NPs, at pH 7 and 5 in a semi-static aqueous exposure regime for up to 7 days. Acidification of the water altered the mean primary particle size (at pH 7, 60±2nm and pH 5, 55±1nm) and dialysis experiments to measure dissolution showed an increased release of dissolved Cu from Cu-NPs at pH 5 compared to pH 7. Copper accumulation was observed in the gills of trout exposed to CuSO4 and Cu-NPs at pH 7 and 5, with a greater accumulation from the CuSO4 treatment than Cu-NPs at each pH. The liver also showed Cu accumulation with both Cu treatments at pH 7 only, whereas, the spleen and kidney did not show measurable accumulation of Cu at any of the water pH values. Exposure to acid water caused changes in the ionoregulatory physiology of control fish and also altered the observed effects of Cu exposure; at pH 5, branchial Na(+)/K(+)-ATPase activity was greater than at pH 7 and the inhibition of Na(+)/K(+)-ATPase activity caused by exposure to CuSO4 at pH 7 was also not observed. There were some changes in haematology and depletion of plasma Na(+) at pH 7 and 5 due to Cu exposure, but there were few material-type or pH effects. Overall, the data show that the accumulation of Cu is greater from CuSO4 than Cu-NPs; however, understanding of the effects of low pH on bioavailability of CuSO4 may not be directly transferred to Cu-NPs without further consideration of the physico-chemical behaviour of Cu-NPs in acid water.

A unique mode of tissue oxygenation and the adaptive radiation of teleost fishes

Teleost fishes constitute 95% of extant aquatic vertebrates, and we suggest that this is related in part to their unique mode of tissue oxygenation. We propose the following sequence of events in the evolution of their oxygen delivery system. First, loss of plasma-accessible carbonic anhydrase (CA) in the gill and venous circulations slowed the Jacobs-Stewart cycle and the transfer of acid between the plasma and the red blood cells (RBCs). This ameliorated the effects of a generalised acidosis (associated with an increased capacity for burst swimming) on haemoglobin (Hb)-O2 binding. Because RBC pH was uncoupled from plasma pH, the importance of Hb as a buffer was reduced. The decrease in buffering was mediated by a reduction in the number of histidine residues on the Hb molecule and resulted in enhanced coupling of O2 and CO2 transfer through the RBCs. In the absence of plasma CA, nearly all plasma bicarbonate ultimately dehydrated to CO2 occurred via the RBCs, and chloride/bicarbonate exchange was the rate-limiting step in CO2 excretion. This pattern of CO2 excretion across the gills resulted in disequilibrium states for CO2 hydration/dehydration reactions and thus elevated arterial and venous plasma bicarbonate levels. Plasma-accessible CA embedded in arterial endothelia was retained, which eliminated the localized bicarbonate disequilibrium forming CO2 that then moved into the RBCs. Consequently, RBC pH decreased which, in conjunction with pH-sensitive Bohr/Root Hbs, elevated arterial oxygen tensions and thus enhanced tissue oxygenation. Counter-current arrangement of capillaries (retia) at the eye and later the swim bladder evolved along with the gas gland at the swim bladder. Both arrangements enhanced and magnified CO2 and acid production and, therefore, oxygen secretion to those specialised tissues. The evolution of β-adrenergically stimulated RBC Na(+)/H(+) exchange protected gill O2 uptake during stress and further augmented plasma disequilibrium states for CO2 hydration/dehydration. Finally, RBC organophosphates (e.g. NTP) could be reduced during hypoxia to further increase Hb-O2 affinity without compromising tissue O2 delivery because high-affinity Hbs could still adequately deliver O2 to the tissues via Bohr/Root shifts. We suggest that the evolution of this unique mode of tissue O2 transfer evolved in the Triassic/Jurassic Period, when O2 levels were low, ultimately giving rise to the most extensive adaptive radiation of extant vertebrates, the teleost fishes.

Root effect hemoglobin may have evolved to enhance general tissue oxygen delivery

The Root effect is a pH-dependent reduction in hemoglobin-O2 carrying capacity. Specific to ray-finned fishes, the Root effect has been ascribed specialized roles in retinal oxygenation and swimbladder inflation. We report that when rainbow trout are exposed to elevated water carbon dioxide (CO2), red muscle partial pressure of oxygen (PO2) increases by 65%--evidence that Root hemoglobins enhance general tissue O2 delivery during acidotic stress. Inhibiting carbonic anhydrase (CA) in the plasma abolished this effect. We argue that CA activity in muscle capillaries short-circuits red blood cell (RBC) pH regulation. This acidifies RBCs, unloads O2 from hemoglobin, and elevates tissue PO2, which could double O2 delivery with no change in perfusion. This previously undescribed mechanism to enhance O2 delivery during stress may represent the incipient function of Root hemoglobins in fishes.

Plasma-accessible carbonic anhydrase at the tissue of a teleost fish may greatly enhance oxygen delivery: in vitro evidence in rainbow trout, Oncorhynchus mykiss

During a generalized acidosis in rainbow trout, catecholamines are released into the blood, activating red blood cell (RBC) Na+/H+ exchange (βNHE), thus protecting RBC intracellular pH (pHi) and subsequent O2 binding at the gill. Because of the presence of a Root effect (a reduction in oxygen carrying capacity of the blood with a reduction in pH), the latter could otherwise be impaired. However, plasma-accessible carbonic anhydrase (CA) at the tissues (and absence at the gills) may result in selective short-circuiting of RBC βNHE pH regulation. This would acidify the RBCs and greatly enhance O2 delivery by exploitation of the combined Bohr-Root effect, a mechanism not previously proposed. As proof-of-principle, an in vitro closed system was developed to continuously monitor extracellular pH (pHe) and O2 tension (PO2) of rainbow trout blood. In this closed system, adding CA to acidified, adrenergically stimulated RBCs short-circuited βNHE pH regulation, resulting in an increase in PO2 by >30 mmHg, depending on the starting Hb-O2 saturation and degree of initial acidification. Interestingly, in the absence of adrenergic stimulation, addition of CA still elevated PO2, albeit to a lesser extent, a response that was absent during general NHE inhibition. If plasma-accessible CA-mediated short-circuiting is operational in vivo, the combined Bohr-Root effect system unique to teleost fishes could markedly enhance tissue O2 delivery far in excess of that in vertebrates possessing a Bohr effect alone and may lead to insights about the early evolution of the Root effect.

Cell volume homeostatic mechanisms: effectors and signalling pathways

Cell volume homeostasis and its fine-tuning to the specific physiological context at any given moment are processes fundamental to normal cell function. The understanding of cell volume regulation owes much to August Krogh, yet has advanced greatly over the last decades. In this review, we outline the historical context of studies of cell volume regulation, focusing on the lineage started by Krogh, Bodil Schmidt-Nielsen, Hans-Henrik Ussing, and their students. The early work was focused on understanding the functional behaviour, kinetics and thermodynamics of the volume-regulatory ion transport mechanisms. Later work addressed the mechanisms through which cellular signalling pathways regulate the volume regulatory effectors or flux pathways. These studies were facilitated by the molecular identification of most of the relevant channels and transporters, and more recently also by the increased understanding of their structures. Finally, much current research in the field focuses on the most up- and downstream components of these paths: how cells sense changes in cell volume, and how cell volume changes in turn regulate cell function under physiological and pathophysiological conditions.

Use it or lose it? Sablefish, Anoplopoma fimbria, a species representing a fifth teleostean group where the βNHE associated with the red blood cell adrenergic stress response has been secondarily lost

Like most teleosts, sablefish (Anoplopoma fimbria Pallas 1814) blood exhibits a moderate Root effect (~35% maximal desaturation), where a reduction in blood pH dramatically reduces O2 carrying capacity, a mechanism important for oxygenating the eye and filling the swim bladder (SB) in teleosts. Although sablefish lack a SB, we observed a well-defined choroid rete at the eye. The adrenergically mediated cell swelling typically associated with a functional red blood cell (RBC) β-adrenergic Na+/H+ exchanger (βNHE), which would normally protect RBC pH, and thus O2 transport, during a generalized acidosis, was not observed in sablefish blood. Neither isoproterenol (a β-agonist) nor 8-bromo cAMP could elicit this response. Furthermore, RBC osmotic shrinkage, known to stimulate NHEs in general and βNHE in other teleosts such as trout and flounder, resulted in no significant regulatory volume increase (RVI), further supporting the absence of a functional RBC βNHE. The onset of the Root effect occurs at a much lower RBC pH (6.83–6.92) than in other teleosts, and thus RBC βNHE may not be required to protect O2 transport during a generalized acidosis in vivo. Phylogenetically, sablefish may represent a fifth group of teleosts exhibiting a secondary reduction or loss of βNHE activity. However, sablefish have not lost the choroid rete at the eye (unlike in the other four groups), which may still function with the Root effect to oxygenate the retina, but the low pH onset of the Root effect may ensure haemoglobin (Hb)-O2 binding is not compromised at the respiratory surface during a general acidosis in the absence of RBC βNHE. The sablefish may represent an anomaly within the framework of Root effect evolution, in that they possess a moderate Root effect and a choroid rete at the eye, but lack the RBC βNHE and the SB system.

31P nuclear magnetic resonance measurements of phosphate metabolites and intracellular pH in turbot Psetta maxima red blood cells using a novel flow method

A method for oxygenating and mixing suspensions of turbot Psetta maxima red blood cells (RBC) was tested in (31)P nuclear magnetic resonance (NMR) spectroscopy. In normoxia, the levels of inorganic phosphate (Pi) and nucleoside triphosphates (NTP) were stable up to 140 min and intracellular pH (pHi) was maintained and decreased oxygen partial pressure (P(O ( 2) )) from 30 to 15 and 600 Pa produced a significant fall in the intensity of NTP resonance, balanced by an increase in the Pi signal. Treatment of RBC with 0. 5 M isoproterenol during hypoxia exposure did not affect the pattern of changes in NTP or pHi induced by hypoxia and the effect was manifest only on Pi levels.

Oxygen-dependent ion transport in erythrocytes

The present contribution reviews current knowledge of apparently oxygen-dependent ion transport in erythrocytes and presents modern hypotheses on their regulatory mechanisms and physiological roles. In addition to molecular oxygen as such, reactive oxygen species, nitric oxide, carbon monoxide, regional variations of cellular ATP and hydrogen sulphide may play a role in the regulation of transport, provided that they are affected by oxygen tension. It appears that the transporter molecules themselves do not have direct oxygen sensors. Thus, the oxygen level must be sensed elsewhere, and the effect transduced to the transporter. The possible pathways involved in the regulation of transport, including haemoglobin as a sensor, and phosphorylation/dephosphorylation reactions both in the transporter and its upstream effectors, are discussed.

Minimal volume regulation after shrinkage of red blood cells from five species of reptiles

Red blood cells (RBCs) from most vertebrates restore volume upon hypertonic shrinkage and the mechanisms underlying this regulatory volume increase (RVI) have been studied extensively in these cells. Despite the phylogenetically interesting position of reptiles, very little is known about their red cell function. The present study demonstrates that oxygenated RBCs in all major groups of reptiles exhibit no or a very reduced RVI upon approximately 25% calculated hyperosmotic shrinkage. Thus, RBCs from the snakes Crotalus durissus and Python regius, the turtle Trachemys scripta and the alligator Alligator mississippiensis showed no statistically significant RVI within 120 min after shrinkage, while the lizard Tupinambis merianae showed 22% volume recovery after 120 min. Amiloride (10(-4) M) and bumetanide (10(-5) M) had no effect on the RVI in T. merianae, indicating no involvement of the Na(+)/H(+) exchanger (NHE) or the Na(+)/K(+)/2Cl(-) co-transporter (NKCC) or insentive transporters. Deoxygenation of RBCs from A. mississippiensis and T. merianae did not significantly affect RVI upon shrinkage. Deoxygenation per se of red blood cells from T. merianae elicited a slow volume increase, but the mechanism was not characterized. It seems, therefore, that the RVI response based on NHE activation was lost among the early sauropsids that gave rise to modern reptiles and birds, while it was retained in mammals. An RVI response has then reappeared in birds, but based on activation of the NKCC. Alternatively, the absence of the RVI response may represent the most ancient condition, and could have evolved several times within vertebrates.

Effect of osmotic shrinkage and hormones on the expression of Na+/H+ exchanger-1, Na+/K+/2Cl- cotransporter and Na+/K+ -ATPase in gill pavement cells of freshwater adapted Japanese eel, Anguilla japonica

It is well-known that gill epithelial cells are important in fish osmoregulation. However, studies on the effect of osmotic stress on the direct cellular responses of the gill epithelial cells are limited. In this paper, we aimed to determine the effects of osmotic hypertonicity, hormones and cellular signaling molecules on the expression of ion transporters in the cultured primary freshwater pavement cells (PVCs), prepared from freshwater-adapted eels (Anguilla japonica). Our data demonstrated that the hypertonic (500 mOsmol l(-1)) treatment of the isolated PVCs induced cell shrinkage, followed by regulatory volume increase (RVI). Application of blockers (i.e. ouabain, bumetanide and EIPA) demonstrated that Na+/K+ -ATPase, Na+/K+/2Cl- cotransporter (NKCC) and Na+/H+ exchanger-1 (NHE-1) were involved in RVI. Western blot analysis of the hypertonic-treated cells revealed a significant induction of NHE-1, NKCC and, alpha and beta subunits of Na+/K+ -ATPase. In nonshrunken cultured PVCs, we found that dexamethasone and dibutyryl cAMP treatments significantly stimulated the expression levels of the three ion transporters. Both prolactin and insulin-like growth factor-1, can only induce the expression of NKCC. The effect of thyroid hormone (T3) and dibutyryl cGMP was negligible. In this study, the induction of ion transporter expression was found to be post-transcriptionally regulated as no significant change in mRNA levels was detected. This observation implies that the regulation is rapid and is probably induced via nongenomic actions.

Oxygen-sensitive regulatory volume increase and Na transport in red blood cells from the cane toad,Bufo marinus

The red blood cells (RBCs) of cane toad, Bufo marinus, are only partially saturated with oxygen in most of the circulation due to cardiac shunts that cause desaturation of arterial blood. The present study examines the oxygen dependency of RBC ouabain-insensitive unidirectional Na transport,using 22Na, in control cells and in cells exposed to hyperosmotic shrinkage or the β-adrenergic agonist isoproterenol. Deoxygenation per se induced a slow, but significant Na influx, which was paralleled by a slow increase in RBC volume. Hyperosmotic shrinkage by a calculated 25% activated a robust Na influx that in the first 30 min had a strong PO2 dependency with maximal activation at low PO2 values and a P50of ∼5.5 kPa. This activation was completely abolished by the Na/H exchanger (NHE) inhibitor EIPA (10–4 mol l-1). Hyperosmotic shrinkage is particularly interesting in B. marinus as it withstands considerable elevation in extracellular osmolarity following dehydration. Parallel studies showed that deoxygenated B. marinusRBCs had a much faster regulatory volume increase (RVI) response than air-equilibrated RBCs, reflecting the difference in magnitude of Na influxes at the two PO2 values. The extent of RVI(∼60%) after 90 min, however, was similar under the two conditions,reflecting a more prolonged elevation of the shrinkage-induced Na influx in air-equilibrated RBCs. There were no significant differences in the ability to perform RVI between whole blood cells at a PCO2of 1 and 3 kPa or washed RBCs, and 10–4 mol l-1amiloride reduced the RVI under all conditions, whereas 10–5mol l-1 bumetanide had no effect. Isoproterenol(10–5 mol l-1) induced a significant and prolonged increase in an EIPA-sensitive and bumetanide-insensitive Na influx at low PO2 under iso-osmotic conditions, whilst there was no stimulation by isoproterenol for up to 45 min in air-equilibrated RBCs. The prolonged β-adrenergic activation of the Na influx at low PO2 is distinctly different from the rapid and transient stimulation in teleost RBCs, suggesting significant differences in the signal transduction pathways leading to transporter activation between vertebrate groups.

Calcium and cytoskeleton signaling during cell volume regulation in isolated nematocytes of Aiptasia mutabilis (Cnidaria: Anthozoa)

Cell volume regulation has not been completely clarified in Coelenterates. The present investigation focuses on cell volume regulation under anisosmotic conditions, both hyposmotic and hypertonic, and on the underlying signals in nematocytes isolated from the Coelenterate Aiptasia mutabilis living in sea water. Nematocytes, once isolated from acontia, that were submitted to either hyposmotic (35%) and hypertonic shock (45%) show RVD and RVI capabilities, respectively. In order to ascertain the role of Ca2+ in triggering such regulatory mechanisms and the possible involvement of cytoskeleton components, tests were performed by employing either Ca2+ free conditions, Gd3+ as Ca2+ channel blockers, TFP as calmodulin inhibitor, colchicine as microtubule inhibitor and cytochalasin B as microfilament polymerization inhibitor. Results show that isolated nematocytes of A. mutabilis can regulate their volume upon both hyposmotic and hypertonic challenge. Ca2+ both from external medium and from internal stores is needed to perform RVD mechanisms, whereas, intracellular Ca2+ seems to be mainly involved in RVI. Moreover cytoskeletal components may play an important role since a significant RVD and RVI inhibition was observed in treated cells. On the basis of our observations further studies are warranted to further verify the role of signals, including phosphatases and phosphorylases, in cell volume regulation of primitive eukaryotic cells.

Suppression of Huntington's disease pathology in Drosophila by human single-chain Fv antibodies

Misfolded neuronal proteins have been identified in a number of neurodegenerative disorders and have been implicated in the pathogenesis of diseases that include Alzheimer's disease, Parkinson's disease, prion-based dementia, Huntington's disease (HD), and other polyglutamine diseases. Although underlying mechanisms remain the subject of ongoing research, it is clear that aberrant processing, protein degradation, and aggregate formation or spurious protein association of the abnormal neuronal proteins may be critical factors in disease progression. Recent work in these diseases has demonstrated in vitro that specific engineered antibody species, peptides, or other general agents may suppress the formation of aggregates. We have modified an approach with intracellularly expressed single-chain Fv (sFv) antibodies (intrabodies) that bind with unique HD protein epitopes. In cell and tissue culture models of HD, anti-N-terminal huntingtin intrabodies (C4 sFv) reduce aggregation and cellular toxicity. Here, we present the crucial experiment of intrabody-mediated in vivo suppression of neuropathology, using a Drosophila model of HD. In the presence of the C4 sFv intrabody, the proportion of HD flies surviving to adulthood increases from 23% to 100%, and the mean and maximum lifespan of adult HD flies is significantly prolonged. Neurodegeneration and formation of visible huntingtin aggregates are slowed. We conclude from this investigation that engineered intrabodies are a potential new class of therapeutic agents for the treatment of neurodegenerative diseases. They may also serve as tools for drug discovery and validation of sites on mutant neuronal proteins that could be exploited for rational drug design.

Comparative biology of the ubiquitous Na+/H+ exchanger, NHE1: lessons from erythrocytes

By virtue of their electroneutral exchange of intracellular H+ for extracellular Na+, the Na+/H+ exchangers (NHE1-NHE8) play a pivotal role in many physiological processes. This review focuses on the ubiquitous plasma membrane isoform, NHE1. Particular attention is given to the roles and regulation of NHE1 in erythrocytes, in their own right and as model systems, but pertinent findings from non-erythroid cells are also discussed. NHE1 plays a key role in the regulation of cell volume and pH, and consequently in the control of such diverse processes as blood O2/CO2 transport, and cell proliferation, motility, and survival. Disturbances in NHE1 function are involved in important pathological states such as hypoxic cell damage and cancer development. NHE1 has a predicted topology of 12 transmembrane domains, and a hydrophilic C-terminus thought to be the major site for NHE1 regulation. NHE1 is highly conserved throughout the vertebrate phylum, particularly in the transmembrane region and the proximal part of the C-terminus. In non-erythroid, and probably also in erythroid cells, this part of the hydrophilic C-terminus interacts with multiple binding partners important for NHE1 function. Erythrocyte NHE1s from mammalian, amphibian, and teleost species are activated by cell shrinkage, decreased pH(i), inhibition of Ser/Thr protein phosphatases, and activation of Ser/Thr protein kinases, i.e., many of the stimuli activating NHE1 in non-erythroid cells. In erythrocytes of many lower vertebrates, NHE1 is activated during hypoxia and is an important modulator of hemoglobin oxygen affinity. Sensitivity of NHE1 to oxygenation status has recently been described also in non-erythroid mammalian cells.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Molecular cloning of NHE1 from winter flounder RBCs: activation by osmotic shrinkage, cAMP, and calyculin A

In this report, we describe the cloning, cellular localization, and functional characteristics of Na(+)/H(+) exchanger 1 (NHE1) from red blood cells of the winter flounder Pseudopleuronectes americanus (paNHE1). The paNHE1 protein localizes primarily to the marginal band and exhibits a 74% similarity to the trout beta-NHE, and 65% to the human NHE1 (hNHE1). Functionally, paNHE1 shares characteristics of both beta-NHE and hNHE1 in that it is activated both by manipulations that increase cAMP and by cell shrinkage, respectively. In accordance, the paNHE1 protein exhibits both protein kinase A consensus sites as in beta-NHE and a region of high homology to that required for shrinkage-dependent activation of hNHE1. After shrinkage-dependent activation of paNHE1 and resulting activation of a Cl(-)/HCO(3)(-) exchanger, their parallel operation results in net uptake of NaCl and osmotically obliged water. Activation of paNHE1 by cAMP is at least additive to that elicited by osmotic shrinkage, suggesting that these stimuli regulate paNHE1 by distinct mechanisms. Finally, exposure to the serine/threonine phosphatase inhibitor calyculin A potently activates paNHE1, and this activation is also additive to that induced by shrinkage or cAMP.

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.

Red blood cells from the South American rattlesnake (Crotalus durissus terrificus) regulate volume incompletely following osmotic shrinkage and swelling in vitro

The ability of rattlesnake (Crotalus durissus terrificus) red blood cells to volume regulate in vitro has been investigated. Blood was drawn through a catheter inserted in the dorsal aorta and equilibrated to gas mixtures of different composition. Cells shrunken osmotically by increasing the extracellular osmolarity from approximately 291 mosm l(-1) (n=3) to approximately 632 mosm l(-1) (calculated) only partially regulated their volume back towards the original volume either at pH 7.51+/-0.05 (mean+/-S.D., n=5) or pH 7. 20+/-0.06 (mean+/-S.D., n=3). There was no improvement of the regulatory volume increase at low haemoglobin oxygen saturation. The limited volume restoration was inhibited by separate additions of amiloride (10(-4) M) or DIDS (10(-4) M) suggesting involvement of the Na(+)/H(+) and Cl(-)/HCO(3)(-) exchangers. Cells that were swollen osmotically by an approximately 30% dilution of the extracellular medium also exhibited a limited ability to recover their volume. Therefore, these cells show little ability to volume regulate when exposed to in vitro conditions that shrink or swell the cell.

Oxygen-sensitive membrane transporters in vertebrate red cells

Oxygen is essential for all higher forms of animal life. It is required for oxidative phosphorylation, which forms the bulk of the energy supply of most animals. In many vertebrates, transport of O(2) from respiratory to other tissues, and of CO(2) in the opposite direction, involves red cells. These are highly specialised, adapted for their respiratory function. Intracellular haemoglobin, carbonic anhydrase and the membrane anion exchanger (AE1) increase the effective O(2)- and CO(2)-carrying capacity of red cells by approximately 100-fold. O(2) also has a pathological role. It is a very reactive species chemically, and oxidation, free radical generation and peroxide formation can be major hazards. Cells that come into contact with potentially damaging levels of O(2) have a variety of systems to protect them against oxidative damage. Those in red cells include catalase, superoxide dismutase and glutathione. In this review, we focus on a third role of O(2), as a regulator of membrane transport systems, a role with important consequences for the homeostasis of the red cell and also the organism as a whole. We show that regulation of red cell transporters by O(2) is widespread throughout the vertebrate kingdom. The effect of O(2) is selective but involves a wide range of transporters, including inorganic and organic systems, and both electroneutral and conductive pathways. Finally, we discuss what is known about the mechanism of the O(2) effect and comment on its physiological and pathological roles.