Cellular volume regulation in salinity stressed molluscs: The response ofNoetia ponderosa (Arcidae) red blood cells to osmotic variation

Expression of Na+/K+-ATPase Was Affected by Salinity Change in Pacific abalone Haliotis discus hannai

Na⁺/K⁺-ATPase (NKA) belongs to the P-type ATPase family, whose members are located in the cell membrane and are distributed in diverse tissues and cells. The main function of the NKA is to regulate osmotic pressure. To better understand the role of NKA in osmoregulation, we first cloned and characterized the full-length cDNAs of NKA α subunit and β subunit from Pacific abalone Haliotis discus hannai in the current study. The predicted protein sequence of the NKA α subunit, as the catalytic subunit, was well conserved. In contrast, the protein sequence of the β subunit had low similarity with those of other species. Phylogenetic analysis revealed that both the α and β subunits of the NKA protein of Pacific abalone were clustered with those of the Gastropoda. Then, the relationship between salinity changes and the NKA was investigated. Sudden salinity changes (with low-salinity seawater (LSW) or high-salinity seawater (HSW)) led to clear changes in ion concentration (Na⁺ and K⁺) in hemolymph; however, the relative stability of ion concentrations in tissue revealed that Pacific abalone has a strong osmotic pressure regulation ability when faced with these salinity changes. Meanwhile, the expression and activity of the NKA was significantly decreased (in LSW group) or increased (in HSW group) during the ion concentration re-establishing stages, which was consistent with the coordinated regulation of ion concentration in hemolymph. Moreover, a positive correlation between cyclic adenosine monophosphate (cAMP) concentrations and NKA mRNA expression (NKA activity) was observed in mantle and gill. Therefore, the sudden salinity changes may affect NKA transcription activation, translation and enzyme activity via a cAMP-mediated pathway.

Oyster's cells regulatory volume decrease: A new tool for evaluating the toxicity of low concentration hydrocarbons in marine waters

Human activities require fossil fuels for transport and energy, a substantial part of which can accidentally or voluntarily (oil spillage) flow to the marine environment and cause adverse effects in human and ecosystems' health. This experiment was designed to estimate the suitability of an original cellular biomarker to early quantify the biological risk associated to hydrocarbons pollutants in seawater. Oocytes and hepatopancreas cells, isolated from oyster (Crassostrea gigas), were tested for their capacity to regulate their volume following a hypo-osmotic challenge. Cell volumes were estimated from cell images recorded at regular time intervals during a 90min-period. When exposed to diluted seawater (osmolalities from 895 to 712mosmkg(-1)), both cell types first swell and then undergo a shrinkage known as Regulatory Volume Decrease (RVD). This process is inversely proportional to the magnitude of the osmotic shock and is best fitted using a first-order exponential decay model. The Recovered Volume Factor (RVF) calculated from this model appears to be an accurate tool to compare cells responses. As shown by an about 50% decrease in RVF, the RVD process was significantly inhibited in cells sampled from oysters previously exposed to a low concentration of diesel oil (8.4mgL(-1) during 24h). This toxic effect was interpreted as a decreased permeability of the cell membranes resulting from an alteration of their lipidic structure by diesel oil compounds. In contrast, the previous contact of oysters with diesel did not induce any rise in the gills glutathione S-transferase specific activity. Therefore, this work demonstrates that the study of the RVD process of cells selected from sentinel animal species could be an alternative bioassay for the monitoring of hydrocarbons and probably, of various chemicals in the environment liable to alter the cellular regulations. Especially, given the high sensitivity of this biomarker compared with a proven one, it could become a relevant and accurate tool to estimate the biological hazards of micropollutants in the water.

Anisosmotic cell volume regulation: a comparative view

A variety of organisms and cell types spanning the five taxonomic kingdoms are exposed, either naturally or through experimental means, to osmotic stresses. A common physiological response to these challenges is maintenance of cell volume through changes in the concentration of intracellular inorganic and organic solutes, collectively termed osmolytes. Research on the mechanisms by which the concentration of these solutes is regulated has proceeded along several experimental lines. Extensive studies on osmotically activated ion transport pathways have been carried out in vertebrate cells and tissues. Much of our knowledge on organic osmolytes has come from investigations on invertebrates, bacteria, and protists. The relative simplicity of bacterial genetics has provided a powerful and elegant tool to explore the modifications of gene expression during volume regulation. An implication of this diverse experimental approach is that phylogenetically divergent organisms employ uniquely adapted mechanisms of cell volume regulation. Given the probability that changes in extracellular osmolality were physiological stresses faced by the earliest organisms, it is more likely that cell volume regulation proceeds by highly conserved physiological processes. We review volume regulation from a comparative perspective, drawing examples from all five taxonomic kingdoms. Specifically, we discuss the role of inorganic and organic solutes in volume maintenance and the mechanisms by which the concentrations of these osmolytes are regulated. In addition, the processes that may transduce volume perturbations into regulatory responses, such as stretch activation of ion channels, intracellular signaling, and genomic regulation, are discussed. Throughout this review we emphasize areas we feel are important for future research.

Does taurine act as an osmoregulatory substance in the rat brain?

The effects of hypotonic media on extracellular free amino acid levels were studied 'in vivo' in the rat dentate gyrus by means of the brain dialysis technique. Extracellular taurine levels increased specifically during perfusions with Krebs-Ringer bicarbonate in which the NaCl concentration was reduced by 25 or 50 mmol/l (hypotonic solutions). These taurine increases were markedly reduced in the presence of furosemide. With further NaCl reductions the enhanced taurine levels remained stable, whereas other amino acids such as glutamate started in increase in a dose-dependent manner. Isoosmolar replacement of NaCl by sucrose did not affect extracellular amino acid levels. These results indicate the possible involvement of taurine in osmoregulatory processes in the brain.

A Ca2+ influx in response to hypo-osmotic stress may alter osmolyte permeability by a phenothiazine-sensitive mechanism

The phenomenon of cell volume recovery following a hypo-osmotic stress mediated by intracellular osmolyte regulation is well known. In many, perhaps all, cell types, the osmolytes involved are usually inorganic ions and amino acids. The details of the regulatory mechanisms for the organic-type osmolytes are not well known. We have found that an immediate influx of external Ca2+ occurs coincident with the application of a hypo-osmotic stress into red cells of two invertebrate species. In both, the influx is initiated by the osmotic stress, not the concomitant ionic decrease. Volume recovery in clam red blood cells is blocked by phenothiazines. In addition, the effect of the phenothiazines is to reduce the amino acid efflux; the ionic portion of the volume response is unaffected. In contrast, the phenothiazines potentiate the volume recovery in worm red coelomocytes. A23187 also potentiates the volume recovery of the worm red cells. The results suggest that the Ca2+ influx is involved in the mechanism that alters cell membrane permeability permitting the amino acid efflux by a mechanism that may involve calmodulin.

Volume regulation and metabolism of suspended C6 glioma cells: an in vitro model to study cytotoxic brain edema

The in vitro model presented provides an approach to study the nature of cell volume control as well as of swelling mechanisms under pathophysiological conditions. Pertinent parameters of cell volume control can be analyzed in isolation due to a virtually infinite extracellular environment precluding secondary effects of the suspended cells. Exposure of C6 glial cells to hypotonic medium was investigated as a model to study fundamental aspects of cell volume control. In confirmation of studies on other cell types glial cells suspended in hypotonic medium recover cell volume after transient swelling. Normalization of cell volume is associated with stimulation of respiration. Moreover, normalization of cell volume in hypotonic medium can be pharmacologically influenced. Addition of naftidrofuryl which enhances cellular O2-consumption led to acceleration of cell volume recovery. On the other hand, inhibition of Na+-K+-ATPase by ouabain did not prevent regulatory volume decrease ruling out a major role of the Na+-transport enzyme in this process. Contrary to hypotonic suspension, hypertonic exposure did not result in volume regulation during an observation period of 3 h. However, this may not necessarily exclude a capability of cell volume to normalize in hypertonic conditions as observed in vivo. Volume control of glial cells in abnormal osmotic medium may--on a cellular basis--reflect fundamental adaptive processes of central nervous tissue. Knowledge of the physiological and biochemical basis of cell volume control is not only of scientific interest but also of therapeutical significance in patients suffering from cytotoxic brain edema.

Cell volume regulation in fish heart ventricles with special reference to taurine

A review of the cell volume regulation mechanism in heart ventricles of teleosts reveal that the mechanism is not only restricted to euryhaline species in a changing salinity regime but also is manifested in fresh-water fish. Taurine is the dominating amino acid and the main cellular osmo-effector in teleost hearts (accounting for 40-50% of the osmolality change). During hypo-osmotic regulation, cellular taurine is reduced by an efflux from the cells, whereas intracellular synthesis of taurine most probably accompanies hyper-osmotic regulation. Vertebrate hearts seem to have a high concentration of taurine and it may in general in vertebrate hearts also play a pivotal role in cellular osmoregulatory function.

Protective effect of taurine on the light-induced disruption of isolated frog rod outer segments

Isolated frog rod outer segments (ROS) incubated in a Krebs-bicarbonate medium, and illuminated for 2 h, show a profound alteration in their structure. This is characterized by distention of discs, vesiculation, and a marked swelling. The light-induced ROS disruption requires the presence of bicarbonate and sodium chloride. Replacement of bicarbonate by TRIS or HEPES protects ROS structure. Also, substitution of sodium chloride by sucrose or choline chloride maintains unaltered the ROS structure. Deletion of calcium, magnesium, or phosphate does not modify the effect produced by illumination. An increased accumulation of labeled bicarbonate and tritiated water is observed in illuminated ROS, as compared with controls in the dark. The presence of taurine, GABA, or glycine, at concentrations of 5-25 mM, effectively counteracts the light-induced ROS disruption. Taurine (25 mM) reduces labeled bicarbonate and tritiated water levels to those observed in the dark incubated ROS.