Anisosmotic cell volume regulation: a comparative view

Metabolomic Profile of the Fungus Cryomyces antarcticus Under Simulated Martian and Space Conditions as Support for Life-Detection Missions on Mars

The identification of traces of life beyond Earth (e.g., Mars, icy moons) is a challenging task because terrestrial chemical-based molecules may be destroyed by the harsh conditions experienced on extraterrestrial planetary surfaces. For this reason, studying the effects on biomolecules of extremophilic microorganisms through astrobiological ground-based space simulation experiments is significant to support the interpretation of the data that will be gained and collected during the ongoing and future space exploration missions. Here, the stability of the biomolecules of the cryptoendolithic black fungus Cryomyces antarcticus, grown on two Martian regolith analogues and on Antarctic sandstone, were analysed through a metabolomic approach, after its exposure to Science Verification Tests (SVTs) performed in the frame of the European Space Agency (ESA) Biology and Mars Experiment (BIOMEX) project. These tests are building a set of ground-based experiments performed before the space exposure aboard the International Space Station (ISS). The analysis aimed to investigate the effects of different mineral mixtures on fungal colonies and the stability of the biomolecules synthetised by the fungus under simulated Martian and space conditions. The identification of a specific group of molecules showing good stability after the treatments allow the creation of a molecular database that should support the analysis of future data sets that will be collected in the ongoing and next space exploration missions.

Poroelastic osmoregulation of living cell volume

Cells maintain their volume through fine intracellular osmolarity regulation. Osmotic challenges drive fluid into or out of cells causing swelling or shrinkage, respectively. The dynamics of cell volume changes depending on the rheology of the cellular constituents and on how fast the fluid permeates through the membrane and cytoplasm. We investigated whether and how poroelasticity can describe volume dynamics in response to osmotic shocks. We exposed cells to osmotic perturbations and used defocusing epifluorescence microscopy on membrane-attached fluorescent nanospheres to track volume dynamics with high spatiotemporal resolution. We found that a poroelastic model that considers both geometrical and pressurization rates captures fluid-cytoskeleton interactions, which are rate-limiting factors in controlling volume changes at short timescales. Linking cellular responses to osmotic shocks and cell mechanics through poroelasticity can predict the cell state in health, disease, or in response to novel therapeutics.

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Cell height changes can be finely captured by defocusing microscopy

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Water permeation and cellular deformability regulate dynamics of cell volume changes

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Poroelasticity describes the dynamics of cell volume changes

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The response of cell to hypo or hyperosmotic shocks are modeled by poroelasticity

Liver cell hydration and integrin signaling

Liver cell hydration (cell volume) is dynamic and can change within minutes under the influence of hormones, nutrients, and oxidative stress. Such volume changes were identified as a novel and important modulator of cell function. It provides an early example for the interaction between a physical parameter (cell volume) on the one hand and metabolism, transport, and gene expression on the other. Such events involve mechanotransduction (osmosensing) which triggers signaling cascades towards liver function (osmosignaling). This article reviews our own work on this topic with emphasis on the role of β1 integrins as (osmo-)mechanosensors in the liver, but also on their role in bile acid signaling.

Potassium (K+) channels in the pulmonary vasculature: Implications in pulmonary hypertension Physiological, pathophysiological and pharmacological regulation

The large K⁺ channel functional diversity in the pulmonary vasculature results from the multitude of genes expressed encoding K⁺ channels, alternative RNA splicing, the post-transcriptional modifications, the presence of homomeric or heteromeric assemblies of the pore-forming α-subunits and the existence of accessory β-subunits modulating the functional properties of the channel. K⁺ channels can also be regulated at multiple levels by different factors controlling channel activity, trafficking, recycling and degradation. The activity of these channels is the primary determinant of membrane potential (Em) in pulmonary artery smooth muscle cells (PASMC), providing an essential regulatory mechanism to dilate or contract pulmonary arteries (PA). K⁺ channels are also expressed in pulmonary artery endothelial cells (PAEC) where they control resting Em, Ca²⁺ entry and the production of different vasoactive factors. The activity of K⁺ channels is also important in regulating the population and phenotype of PASMC in the pulmonary vasculature, since they are involved in cell apoptosis, survival and proliferation. Notably, K⁺ channels play a major role in the development of pulmonary hypertension (PH). Impaired K⁺ channel activity in PH results from: 1) loss of function mutations, 2) downregulation of its expression, which involves transcription factors and microRNAs, or 3) decreased channel current as a result of increased vasoactive factors (e.g., hypoxia, 5-HT, endothelin-1 or thromboxane), exposure to drugs with channel-blocking properties, or by a reduction in factors that positively regulate K⁺ channel activity (e.g., NO and prostacyclin). Restoring K⁺ channel expression, its intracellular trafficking and the channel activity is an attractive therapeutic strategy in PH.

Contrasting strategies of osmotic and ionic regulation in freshwater crabs and shrimps: gene expression of gill ion transporters

Owing to their extraordinary niche diversity, the Crustacea are ideal for comprehending the evolution of osmoregulation. The processes that effect systemic hydro-electrolytic homeostasis maintain hemolymph ionic composition via membrane transporters located in highly specialized gill ionocytes. We evaluated physiological and molecular hyper- and hypo-osmoregulatory mechanisms in two phylogenetically distant, freshwater crustaceans, the crab Dilocarcinus pagei and the shrimp Macrobrachium jelskii, when osmotically challenged for up to 10 days. When in distilled water, D. pagei survived without mortality, hemolymph osmolality and [Cl-] increased briefly, stabilizing at initial values, while [Na+] decreased continually. Expression of gill V-type H+-ATPase (V-ATPase), Na+/K+-ATPase and Na+/K+/2Cl- symporter genes was unchanged. In M. jelskii, hemolymph osmolality, [Cl-] and [Na+] decreased continually for 12 h, the shrimps surviving only around 15-24 h exposure. Gill transporter gene expression increased 2- to 5-fold. After 10 days exposure to brackish water (25‰S), D. pagei was isosmotic, iso-chloremic and iso-natriuremic. Gill V-ATPase expression decreased while Na+/K+-ATPase and Na+/K+/2Cl- symporter expression was unchanged. In M. jelskii (20‰S), hemolymph was hypo-regulated, particularly [Cl-]. Transporter expression initially increased 3- to 12-fold, declining to control values. Gill V-ATPase expression underlies the ability of D. pagei to survive in fresh water while V-ATPase, Na+/K+-ATPase and Na+/K+/2Cl- symporter expression enables M. jelskii to confront hyper/hypo-osmotic challenges. These findings reveal divergent responses in two unrelated crustaceans inhabiting a similar osmotic niche. While D. pagei does not secrete salt, tolerating elevated cellular isosmoticity, M. jelskii exhibits clear hypo-osmoregulatory ability. Each species has evolved distinct strategies at the transcriptional and systemic levels during its adaptation to fresh water.

The Biology of the Mesothelium during Peritoneal Dialysis

Substantial derangements of mesothelial biology are observed during experimental simulations of dialysis conditions, inferred from the content of human dialysis effluent and visualized by microscopy of human mesothelial biopsies. Canosmotically active solutions be made biocompatible with the osmoregulatory system of the mesothelium? Can the contributions of the mesothelium to host defenses against inflammation and/or infection be supported during CAPD? Do underlying metabolic derangements present in various kidney diseases and end-stage renal disease, regardless of cause, require customized CAPD protocols and solutions? Use of dialysis solutions less directly toxic to the mesothelium is a necessary step toward some day manipulating peritoneal biology by pharmacological and therapeutic modalities.

Cell Volume Measurements by Optical Transmission Microscopy

Cell volume is an important parameter in studying cell adaptation to anisosmotic stress, activation of monovalent ion channels, and cell death. This article describes a method for measurement of the volumes of adherent cells using a standard light microscope. A coverslip with attached cells is placed in a shallow chamber in a medium containing a strongly absorbing and cell-impermeant dye, Acid Blue 9. When such a sample is imaged in transmitted light at a wavelength of maximum dye absorption (630 nm), the resulting contrast quantitatively reflects cell thickness; once the thickness is known at every point, the volume can be computed as well. Technical details, interpretation of data, and possible artifacts are discussed. © 2019 by John Wiley & Sons, Inc.

Osmoionic homeostasis in bivalve mollusks from different osmotic niches: Physiological patterns and evolutionary perspectives

Physiological knowledge gained from questions focused on the challenges faced and strategies recruited by organisms in their habitats assumes fundamental importance about understanding the ability to survive when subjected to unfavorable situations. In the aquatic environment, salinity is particularly recognized as one of the main abiotic factors that affects the physiology of organisms. Although the physiological patterns and challenges imposed by each occupied environment are distinct, they tend to converge to osmotic oscillations. From a comparative perspective, we aimed to characterize the osmoregulatory patterns of the bivalve mollusks Corbicula largillierti (purple Asian cockle), Erodona mactroides (lagoon cockle), and Amarilladesma mactroides (white clam) - inhabitants of different osmotic niches - when submitted to hypo- and/or hyperosmotic salinity variations. We determined the hemolymph osmotic and ionic concentrations, tissue hydration, and the intracellular isosmotic regulation (IIR) from the use of osmolytes (organic and inorganic) after exposure to species-specific salinity intervals. Additionally, we incorporated phylogenetic perspectives to infer and even broaden the understanding about the patterns that comprise the osmoionic physiology of Bivalvia representatives. According to the variables analyzed in the hemolymph, the three species presented a pattern of osmoconformation. Furthermore, both ionic regulation and conformation patterns were observed in freshwater, estuarine, and marine species. The patterns verified experimentally show greater use of inorganic osmolytes compared to the participation of organic molecules, which varied according to the osmotic niche occupied in the IIR for the mantle, adductor muscle, and gills. This finding widens the classic vision about the preferential use of certain osmolytes by animals from distinct niches. Our phylogenetic perspective also indicates that environmental salinity drives physiological trait variations, including hemolymph osmolality and the ion composition of the extracellular fluid (sodium, chloride, magnesium, and calcium). We also highlight the important role played by the shared ancestry, which influences the interspecific variability of the hemolymph K⁺ in selected representatives of Bivalvia.

Detection of Glycemia and Osmolarity Changes Using Eye Examinations

Background: Glycated hemoglobin (HbA1c) is an index of the average blood glucose level over the preceding 2-3 months. In experimental studies, the lens responded to changes in osmolarity by forming vacuoles. By observing the vacuoles of the lens during eye examination, can we detect changes in osmolarity and glycemia over the last 6 months through HbA1c levels? Methods: In total, 400 patients (mean age, 67.7 ± 9.8 years), including those with diabetes mellitus, hypertension, and heart failure, were included in the study. The control group contained 70 patients matched in terms of age and sex and who had no prior disease (mean age, 67.8 ± 9.4 years). Monthly Na, glucose, and blood urea nitrogen values were used to calculate changes in osmolarity over 6 months. HbA1c values were also recorded. Biomicroscopy was used to evaluate lens vacuolation; all vacuoles were digitally photographed and converted to ImageJ format. Results: The sensitivity and specificity of using large vacuoles to detect HbA1c ≥10% were 88.0% (95% confidence interval [CI]: 68.8-97.4) and 82.6% (95% CI: 74.1-89.2), respectively. The sensitivity and specificity of detecting a 10 mOsm/kg change in osmolarity were 61% (95% CI: 48.9-72.4) and 94.5% (95% CI: 91.5-96.7), respectively. Conclusions: Lens vacuoles, which can be observed with a simple and quick examination, can be used to detect HbA1c levels and osmolarity changes over the last 6 months. Because of their relationship to the severity of retinopathy, vacuoles can also be used as a weak control indicator.

Cell Volume Control in Healthy Brain and Neuropathologies

Regulation of cellular volume is a critical homeostatic process that is intimately linked to ionic and osmotic balance in the brain tissue. Because the brain is encased in the rigid skull and has a very complex cellular architecture, even minute changes in the volume of extracellular and intracellular compartments have a very strong impact on tissue excitability and function. The failure of cell volume control is a major feature of several neuropathologies, such as hyponatremia, stroke, epilepsy, hyperammonemia, and others. There is strong evidence that such dysregulation, especially uncontrolled cell swelling, plays a major role in adverse pathological outcomes. To protect themselves, brain cells utilize a variety of mechanisms to maintain their optimal volume, primarily by releasing or taking in ions and small organic molecules through diverse volume-sensitive ion channels and transporters. In principle, the mechanisms of cell volume regulation are not unique to the brain and share many commonalities with other tissues. However, because ions and some organic osmolytes (e.g., major amino acid neurotransmitters) have a strong impact on neuronal excitability, cell volume regulation in the brain is a surprisingly treacherous process, which may cause more harm than good. This topical review covers the established and emerging information in this rapidly developing area of physiology.

Salt water and skin interactions: new lines of evidence

In Health Resort Medicine, both balneotherapy and thalassotherapy, salt waters and their peloids, or mud products are mainly used to treat rheumatic and skin disorders. These therapeutic agents act jointly via numerous mechanical, thermal, and chemical mechanisms. In this review, we examine a new mechanism of action specific to saline waters. When topically administered, this water rich in sodium and chloride penetrates the skin where it is able to modify cellular osmotic pressure and stimulate nerve receptors in the skin via cell membrane ion channels known as "Piezo" proteins. We describe several models of cutaneous adsorption/desorption and penetration of dissolved ions in mineral waters through the skin (osmosis and cell volume mechanisms in keratinocytes) and examine the role of these resources in stimulating cutaneous nerve receptors. The actions of salt mineral waters are mediated by a mechanism conditioned by the concentration and quality of their salts involving cellular osmosis-mediated activation/inhibition of cell apoptotic or necrotic processes. In turn, this osmotic mechanism modulates the recently described mechanosensitive piezoelectric channels.

Intracellular Pressure: A Driver of Cell Morphology and Movement

Intracellular pressure, generated by actomyosin contractility and the directional flow of water across the plasma membrane, can rapidly reprogram cell shape and behavior. Recent work demonstrates that cells can generate intracellular pressure with a range spanning at least two orders of magnitude; significantly, pressure is implicated as an important regulator of cell dynamics, such as cell division and migration. Changes to intracellular pressure can dictate the mechanisms by which single human cells move through three-dimensional environments. In this review, we chronicle the classic as well as recent evidence demonstrating how intracellular pressure is generated and maintained in metazoan cells. Furthermore, we highlight how this potentially ubiquitous physical characteristic is emerging as an important driver of cell morphology and behavior.

Upregulations of Clcn3 and P-Gp Provoked by Lens Osmotic Expansion in Rat Galactosemic Cataract

OBJECTIVE

Lens osmotic expansion, provoked by overactivated aldose reductase (AR), is the most essential event of sugar cataract. Chloride channel 3 (Clcn3) is a volume-sensitive channel, mainly participating in the regulation of cell fundamental volume, and P-glycoprotein (P-gp) acts as its modulator. We aim to study whether P-gp and Clcn3 are involved in lens osmotic expansion of galactosemic cataract.

METHODS AND RESULTS

In vitro, lens epithelial cells (LECs) were primarily cultured in gradient galactose medium (10-60 mM), more and more vacuoles appeared in LEC cytoplasm, and mRNA and protein levels of AR, P-gp, and Clcn3 were synchronously upregulated along with the increase of galactose concentration. In vivo, we focused on the early stage of rat galactosemic cataract, amount of vacuoles arose from equatorial area and scattered to the whole anterior capsule of lenses from the 3rd day to the 9th day, and mRNA and protein levels of P-gp and Clcn3 reached the peak around the 9th or 12th day.

CONCLUSION

Galactosemia caused the osmotic stress in lenses; it also markedly leads to the upregulations of AR, P-gp, and Clcn3 in LECs, together resulting in obvious osmotic expansion in vitro and in vivo.

Pitfall in the Diagnosis of Diabetes Insipidus and Pregnancy

Diabetes insipidus (DI) during pregnancy and the perinatal period is an uncommon medical problem characterized by polyuria and excessive thirst. Diagnosis of DI may be overlooked in the setting of pregnancy, a time when increased water intake and urine output are commonly reported. We report two cases: one of transient DI in a young woman during her third trimester of twin pregnancy in association with acute fatty liver and hypertension and one of postpartum DI secondary to Sheehan syndrome from rupture of a splenic artery aneurysm. These cases illustrate the spectrum with which DI related to pregnancy and delivery can present and highlight the difficulty in making the diagnosis since the symptoms are often initially overlooked.

Research and progress on ClC‑2 (Review)

Chloride channel 2 (ClC-2) is one of the nine mammalian members of the ClC family. The present review discusses the molecular properties of ClC‑2, including CLCN2, ClC‑2 promoter and the structural properties of ClC‑2 protein; physiological properties; functional properties, including the regulation of cell volume. The effects of ClC‑2 on the digestive, respiratory, circulatory, nervous and optical systems are also discussed, in addition to the mechanisms involved in the regulation of ClC‑2. The review then discusses the diseases associated with ClC‑2, including degeneration of the retina, Sjögren's syndrome, age‑related cataracts, degeneration of the testes, azoospermia, lung cancer, constipation, repair of impaired intestinal mucosa barrier, leukemia, cystic fibrosis, leukoencephalopathy, epilepsy and diabetes mellitus. It was concluded that future investigations of ClC‑2 are likely to be focused on developing specific drugs, activators and inhibitors regulating the expression of ClC‑2 to treat diseases associated with ClC‑2. The determination of CLCN2 is required to prevent and treat several diseases associated with ClC‑2.

Turning down the volume: Astrocyte volume change in the generation and termination of epileptic seizures

Approximately 1% of the global population suffers from epilepsy, a class of disorders characterized by recurrent and unpredictable seizures. Of these cases roughly one-third are refractory to current antiepileptic drugs, which typically target neuronal excitability directly. The events leading to seizure generation and epileptogenesis remain largely unknown, hindering development of new treatments. Some recent experimental models of epilepsy have provided compelling evidence that glial cells, especially astrocytes, could be central to seizure development. One of the proposed mechanisms for astrocyte involvement in seizures is astrocyte swelling, which may promote pathological neuronal firing and synchrony through reduction of the extracellular space and elevated glutamate concentrations. In this review, we discuss the common conditions under which astrocytes swell, the resultant effects on neural excitability, and how seizure development may ultimately be influenced by these effects.

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.

Postoperative fluid retention after heart surgery is accompanied by a strongly positive sodium balance and a negative potassium balance

The conventional model on the distribution of electrolyte infusions states that water will distribute proportionally over both the intracellular (ICV) and extracellular (ECV) volumes, while potassium homes to the ICV and sodium to the ECV. Therefore, total body potassium is the most accurate measure of ICV and thus potassium balances can be used to quantify changes in ICV. In cardiothoracic patients admitted to the ICU we performed complementary balance studies to measure changes in ICV and ECV. In 39 patients, fluid, sodium, potassium, and electrolyte‐free water (EFW) balances were determined to detect changes in ICV and ECV. Cumulatively over 4 days, these patients received a mean ± SE infusion of 14.0 ± 0.6 L containing 1465 ± 79 mmol sodium, 196 ± 11 mmol potassium and 2.1 ± 0.1 L EFW. This resulted in strongly positive fluid (4.0 ± 0.6 L) and sodium (814 ± 75 mmol) balances but in negative potassium (−101 ± 14 mmol) and EFW (−1.1 ± 0.2 L) balances. We subsequently compared potassium balances (528 patients) and fluid balances (117 patients) between patients who were assigned to either a 4.0 or 4.5 mmol/L blood potassium target. Although fluid balances were similar in both groups, the additionally administered potassium (76 ± 23 mmol) in the higher target group was fully excreted by the kidneys (70 ± 23 mmol). These findings indicate that even in the context of rapid and profound volume expansion neither water nor potassium moves into the ICV.

Volume regulatory hormones and plasma volume in pregnant women with sickle cell disorder

Background:

Sickle cell disease (haemoglobin SS (HbSS)) mainly affects those of West African origin and is associated with hypervolaemia. Plasma volume rises by up to 50% in normal pregnancy but was previously found to be paradoxically contracted in late sickle cell pregnancy. The renin–angiotensin–aldosterone system is activated very early in human pregnancy to support the plasma volume expansion. We hypothesised that activation of the renin–angiotensin–aldosterone system would be blunted in pregnant women with sickle cell disease.

Materials and methods:

We measured plasma volume and concentrations of plasma renin, angiotensinogen, aldosterone and other volume-related hormones in a cross-sectional study of pregnant and non-pregnant Nigerian women with HbSS or HbAA.

Results:

Plasma volume was higher in non-pregnant HbSS than HbAA women, but had not risen by 16 weeks, unlike plasma volume in HbAA women. The concentration of plasma renin also rose significantly less by 16 weeks in HbSS; angiotensinogen and aldosterone concentrations increased.

Conclusions:

The lower plasma renin concentration at 16 weeks with HbSS could be either primary or secondary to vasoconstriction related to inadequate vasodilator activity. The contracted plasma volume might then stimulate aldosterone synthesis by non-angiotensin II dependent stimulation. Studies of vasodilators such as nitric oxide, vasodilator eicosanoids or the PlGF/VEGF/sFlT-1 axis in pregnant HbSS and HbAA women will test this hypothesis.

Mechanisms of Renal Vasodilation and Hyperfiltration During Pregnancy

Glomerular filtration rate (GFR) and renal plasma flow (RPF) increase by 40-65% and 50-85%, respectively, during normal pregnancy in women. Studies using the gravid rat as a model have greatly enhanced our understanding of mechanisms underlying these remarkable changes in the renal circulation during gestation. Hyperfiltration appears to be almost completely due to the increase in RPF, the latter attributable to profound reductions in both the renal afferent and efferent arteriolar resistances. The major pregnancy hormone involved is relaxin. The mediators downstream from relaxin include endothelin (ET) and nitric oxide (NO). New evidence indicates that relaxin increases vascular gelatinase activity during pregnancy, thereby converting big ET to ET(1-32), which leads to renal vasodilation, hyperfiltration, and reduced myogenic reactivity of small renal arteries via the endothelial ET(B) receptor and NO. Whether the chronic volume expansion characteristic of pregnancy contributes to the maintenance of gestational renal changes requires further investigation. Additional studies are also needed to further delineate the molecular basis of these mechanisms and, importantly, to investigate whether they apply to women.

Hypotonicity stimulates potassium flux through the WNK-SPAK/OSR1 kinase cascade and the Ncc69 sodium-potassium-2-chloride cotransporter in the Drosophila renal tubule

The ability to osmoregulate is fundamental to life. Adult Drosophila melanogaster maintain hemolymph osmolarity within a narrow range. Osmolarity modulates transepithelial ion and water flux in the Malpighian (renal) tubules of the fly, which are in direct contact with hemolymph in vivo, but the mechanisms causing increased transepithelial flux in response to hypotonicity are unknown. Fly renal tubules secrete a KCl-rich fluid. We have previously demonstrated a requirement for Ncc69, the fly sodium-potassium-2-chloride cotransporter (NKCC), in tubule K(+) secretion. Mammalian NKCCs are regulated by a kinase cascade consisting of the with-no-lysine (WNK) and Ste20-related proline/alanine-rich (SPAK)/oxidative stress response (OSR1) kinases. Here, we show that decreasing Drosophila WNK activity causes a reduction in K(+) flux. Similarly, knocking down the SPAK/OSR1 homolog fray also decreases K(+) flux. We demonstrate that a hierarchical WNK-Fray signaling cascade regulates K(+) flux through Ncc69, because (i) a constitutively active Fray mutant rescues the wnk knockdown phenotype, (ii) Fray directly phosphorylates Ncc69 in vitro, and (iii) the effect of wnk and fray knockdown is abolished in Ncc69 mutants. The stimulatory effect of hypotonicity on K(+) flux is absent in wnk, fray, or Ncc69 mutant tubules, suggesting that the Drosophila WNK-SPAK/OSR1-NKCC cascade is an essential molecular pathway for osmoregulation, through its effect on transepithelial ion flux and fluid generation by the renal tubule.

Osmotic homeostasis

Alterations in water homeostasis can disturb cell size and function. Although most cells can internally regulate cell volume in response to osmolar stress, neurons are particularly at risk given a combination of complex cell function and space restriction within the calvarium. Thus, regulating water balance is fundamental to survival. Through specialized neuronal "osmoreceptors" that sense changes in plasma osmolality, vasopressin release and thirst are titrated in order to achieve water balance. Fine-tuning of water absorption occurs along the collecting duct, and depends on unique structural modifications of renal tubular epithelium that confer a wide range of water permeability. In this article, we review the mechanisms that ensure water homeostasis as well as the fundamentals of disorders of water balance.

Immunomodulating effects of environmentally realistic copper concentrations in Mytilus edulis adapted to naturally low salinities

The monitoring of organisms' health conditions by the assessment of their immunocompetence may serve as an important criterion for the achievement of the Good Environmental Status (GES) as defined in the Marine Strategy Framework Directive (EU). In this context, the complex role of natural environmental stressors, e.g. salinity, and interfering or superimposing effects of anthropogenic chemicals, should be carefully considered, especially in scenarios of low to moderate contamination. Organisms from the Baltic Sea have adapted to the ambient salinity regime, however energetically costly osmoregulating processes may have an impact on the capability to respond to additional stress such as contamination. The assessment of multiple stressors, encompassing natural and anthropogenic factors, influencing an organisms' health was the main aim of the present study. Immune responses of Mytilus edulis, collected and kept at natural salinities of 12‰ (LS) and 20‰ (MS), respectively, were compared after short-term exposure (1, 7 and 13 days) to low copper concentrations (5, 9 and 16 μg/L Cu). A significant interaction of salinity and copper exposure was observed in copper accumulation. LS mussels accumulated markedly more copper than MS mussels. No combined effects were detected in cellular responses. Bacterial clearance was mostly achieved by phagocytosis, as revealed by a strong positive correlation between bacterial counts and phagocytic activity, which was particularly pronounced in LS mussels. MS mussels, on the other hand, seemingly accomplished bacterial clearance by employing additional humoral factors (16 μg/L Cu). The greatest separating factor in the PCA biplot between LS and MS mussels was the proportion of granulocytes and hyalinocytes while functional parameters (phagocytic activity and bacterial clearance) were hardly affected by salinity, but rather by copper exposure. In conclusion, immune responses of the blue mussel may be suitable and sensitive biomarkers for the assessment of ecosystem health in brackish waters (10-20‰S).

Reset osmostat in pregnancy: a case report

The reset osmostat syndrome, a form of inappropriate antidiuretic hormone secretion (SIADH), occurs when the threshold for antidiuretic hormone secretion is moved downward. There is evidence to suggest a "reset osmostat phenomenon" in normal pregnancies, whereby the average plasma-osmolality is decreased by 5-10 mOsm/kg. We present a case of a non-physiologic reset osmostat in a pregnant patient, thought to be caused by large intracranial arteriovenous malformations and intraventricular hemorrhage. The presence of a reset osmostat should be suspected in any patient with apparent SIADH who has mild hyponatremia that is stable over many days despite variations in sodium and water intake. Therapeutic efforts to raise the serum sodium concentration appear to be unnecessary.

Regulatory volume decrease in COS-7 cells at 22 °C and its influence on the Boyle van't Hoff relation and the determination of the osmotically inactive volume

Cryobiological analyses assume that the direction and rate of water movements across cell membranes and equilibrium cell volumes are determined solely by differences in the chemical potentials of intra- and extra-cellular water. A consequence of this assumption is that cells obey the Boyle van't Hoff (BvH) law which states that cell volumes are a linear function of reciprocal osmolality. Extrapolation of the BvH plot to infinite osmolality yields a quantity b, the fractional volume of the cell occupied by solids. In many cells, however, a cell volume excursion above the isotonic volume initiates an energy-requiring response that causes the swollen cells to shrink back to or towards isotonic volume. It is referred to as regulatory volume decrease (RVD). We have observed a strong RVD in COS-7 cells. If not eliminated by keeping exposure times short, this RVD produces a b that is 60% too high (0.48 vs. 0.30). These results indicate the importance of examining cells for volume regulatory mechanisms before performing measurements to determine their osmotic parameters.

Importance of Residual Water Permeability on the Excretion of Water during Water Diuresis in Rats

When the concentration of sodium (Na(+)) in arterial plasma (P(Na)) declines sufficiently to inhibit the release of vasopressin, water will be excreted promptly when the vast majority of aquaporin 2 water channels (AQP2) have been removed from luminal membranes of late distal nephron segments. In this setting, the volume of filtrate delivered distally sets the upper limit on the magnitude of the water diuresis. Since there is an unknown volume of water reabsorbed in the late distal nephron, our objective was to provide a quantitative assessment of this parameter. Accordingly, rats were given a large oral water load, while minimizing non-osmotic stimuli for the release of vasopressin. The composition of plasma and urine were measured. The renal papilla was excised during the water diuresis to assess the osmotic driving force for water reabsorption in the inner medullary collecting duct. During water diuresis, the concentration of creatinine in the urine was 13-fold higher than in plasma, which implies that ~8% of filtered water was excreted. The papillary interstitial osmolality was 600 mOsm/L > the urine osmolality. Since 17% of filtered water is delivered to the earliest distal convoluted tubule micropuncture site, we conclude that half of the water delivered to the late distal nephron is reabsorbed downstream during water diuresis. The enormous osmotic driving force for the reabsorption of water in the inner medullary collecting duct may play a role in this reabsorption of water. Possible clinical implications are illustrated in the discussion of a case example.

Hyponatremia associated coma due to pituitary apoplexy in early pregnancy: a case report

Pituitary apoplexy in pregnancy is rare. Its clinical features may range from unspecific complaints to panhypopituitarism resulting even in coma and death. Therefore, alertness to signs and symptoms of acute loss of pituitary function in pregnancy is mandatory. We report a woman in her 7th week of her first gestation presenting with sudden coma due to severe hyponatremia. Secondary adrenal insufficiency could be identified as the underlying cause. Panhypopituitarism including central diabetes insipidus and spontaneous abortion developed during the follow-up. Magnetic resonance imaging showed pituitary apoplexy without a pre-existing pituitary mass. The clinical course was notable for severe complications, including neurological deficits through cerebral ischemia, but eventual recovery could be achieved. We discuss the diagnostic difficulties in the evaluation of pituitary disease in pregnancy.

Cloning and regulation of expression of the Na+–Cl––taurine transporter in gill cells of freshwater Japanese eels

Our previous studies have demonstrated the hypertonic-induced expression of osmotic stress transcription factor and the regulatory volume increase (RVI)response in gill cells isolated from freshwater eels. In this study, we aimed to clone one of the organic osmolyte transporters, the Na+–Cl––taurine transporter (TauT),and to characterize its expression in anisosmotic conditions, using both in vivo and in vitro approaches. A cDNA clone encoding TauT was isolated from gill tissues of Japanese eels, Anguilla japonica. The deduced amino acid sequence shows 88–90% identity to other reported piscine TauT sequences. Our data indicated that TauT mRNA was detectable in both freshwater and seawater fish gills. The expression level of TauT mRNA increased in gills of seawater-acclimating fish. A high abundance of TauT protein was found to be localized in seawater gill chloride cells. Using primary gill cell culture, expression of the gene was induced when the ambient osmolarity was raised from 320 to 500 mosmol l–1. Hypertonic treatment of the culture caused an increase of F-actin distribution in the cell periphery. Treatment of the cells with colchicine or cytochalasin D significantly reduced TauT transcript level following hypertonic exposure. The inhibition of myosin light chain (MLC) kinase by ML-7 had a significant additive effect on hypertonic-induced TauT expression. Collectively, the data of this study reveal, for the first time, the regulation of TauT expression in gill cells of euryhaline fish. We have demonstrated the involvement of ionic strength, the cytoskeleton and MLC kinase in the regulation of TauT expression. The results shed light on the osmosensing and hyperosmotic adaption in fish gills.

The cloning of eel osmotic stress transcription factor and the regulation of its expression in primary gill cell culture

In the present study, we aimed to clone an osmotic stress transcriptional factor (Ostf) from gill cells of Japanese eels. In addition, we measured its expression in Percoll-gradient-isolated gill chloride (CC) and pavement (PVC) cells and determined the regulation of its expression in primary gill cell culture. Using degenerative primers and RACE techniques, we cloned a cDNA of 615bp, encompassing the coding sequence of Ostf (204 amino acids). The cloned Ostf1 DNA sequence shared 84% DNA homology with the Ostf1 of tilapia. In general, the basal Ostf expression level was found to be significantly higher in CCs than in PVCs. In the direct transfer of fish from freshwater to seawater, a significant but transient induction of Ostf mRNA in CCs and PVCs was measured after 6h of acclimation. Compared with gill CCs, the level of induction measured at PVCs was lower. In the seawater-to-freshwater transfer, no significant change in Ostf transcript levels was detected in either CCs or PVCs. To decipher the regulatory mechanism of Ostf expression, we conducted experiments using primary gill cell culture to specifically address the involvement of two putative osmosensors (i.e. intracellular ion strength/macromolecular crowding and cytoskeleton) in the regulation of Ostf expression. Hypertonic treatment using impermeable solutes (i.e. NaCl, 500 mOsmol l(-1)) induced Ostf mRNA expression in 6h, but no noticeable effect was measured using permeable solute (i.e. urea, 500 mOsmol l(-1)). The induction was transcriptionally regulated and was abolished by the addition of organic osmolytes (i.e. betaine, inositol or taurine) into the culture media. Addition of colchicine (an inhibitor of microtubule polymerization) to hypertonic (with added NaCl, 500 mOsmol l(-1)) cells reduced Ostf mRNA expression, suggesting that an increase in intracellular ionic strength and the integrity of the cytoskeleton are involved in the activation of Ostf mRNA expression in the cells. Collectively, the results of this study reveal, for the first time, the differential expression of Ostf in isolated CCs and PVCs. The resulting knowledge can shed light on how Ostf participates in hyperosmotic adaptation in fish gills.