Swelling and Volume Control in Brain Astroglial Cells

Bursting at the Seams: Molecular Mechanisms Mediating Astrocyte Swelling

Brain swelling is one of the most robust predictors of outcome following brain injury, including ischemic, traumatic, hemorrhagic, metabolic or other injury. Depending on the specific type of insult, brain swelling can arise from the combined space-occupying effects of extravasated blood, extracellular edema fluid, cellular swelling, vascular engorgement and hydrocephalus. Of these, arguably the least well appreciated is cellular swelling. Here, we explore current knowledge regarding swelling of astrocytes, the most abundant cell type in the brain, and the one most likely to contribute to pathological brain swelling. We review the major molecular mechanisms identified to date that contribute to or mitigate astrocyte swelling via ion transport, and we touch upon the implications of astrocyte swelling in health and disease.

■ Review : Cell Volume in the CNS: Regulation and Implications for Nervous System Function and Pathology

Swelling of cells in the nervous system is frequently associated with pathological states such as cerebral ischemia. The major cell type that swells in gray matter appears to be the astrocyte, although swelling of neuronal dendrites also occurs. Such swelling probably affects function by reducing the volume of the extracellular space. In addition the properties of the swollen cells themselves are altered, such as the swelling-induced release of excitatory amino acids, which are likely to be deleterious. Recent work has shown that these effects, linked to astrocytic swelling, may be involved in pathological states such as cerebral ischemia and trauma. Increased understanding of such swelling in the CNS will thus be of great importance in understanding mechanisms of brain damage and may provide specific sites for therapeutic intervention. NEUROSCIENTIST 6:14-25, 2000

Effects of Hyponatremia on the Brain

Hyponatremia is a very common electrolyte disorder, especially in the elderly, and is associated with significant morbidity, mortality and disability. In particular, the consequences of acute hyponatremia on the brain may be severe, including permanent disability and death. Also chronic hyponatremia can affect the health status, causing attention deficit, gait instability, increased risk of falls and fractures, and osteoporosis. Furthermore, an overly rapid correction of hyponatremia can be associated with irreversible brain damage, which may be the result of the osmotic demyelination syndrome. This review analyzes the detrimental consequences of acute and chronic hyponatremia and its inappropriate correction on the brain and the underlying physiopathological mechanisms, with a particular attention to the less known in vivo and in vitro effects of chronic hyponatremia.

Comparative evaluation of brain neurometabolites and DTI indices following whole body and cranial irradiation: a magnetic resonance imaging and spectroscopy study

Understanding early differential response of brain during whole body radiation or cranial radiation exposure is of significant importance for better injury management during accidental or intentional exposure to ionizing radiation. We investigated the early microstructural and metabolic profiles using in vivo diffusion tensor imaging (DTI) and proton magnetic resonance spectroscopy ((1)H MRS) following whole body and cranial radiation exposure of 8 Gy in mice using a 7.0 T animal MRI system and compared profiles with sham controls at days 1, 3, 5 and 10 post irradiation. A significant decrease in fractional anisotropy (FA) values was found in hippocampus, thalamic and hypothalamic regions (p < 0.05) in both whole body and cranial irradiated groups compared with controls, suggesting radiation induced reactive astrogliosis or neuroinflammatory response. In animals exposed to whole body radiation, FA was significantly decreased in some additional brain regions such as sensory motor cortex and corpus callosum in comparison with cranial irradiation groups and controls. Changes in FA were observed till day 10 post irradiation in both the groups. However, MRS study from hippocampus revealed changes only in the whole body radiation dose group. Significant reduction in the ratios of the metabolites myoinositol (mI, p = 0.02) and taurine (tau, p = 0.03) to total creatine were observed, and these metabolic alterations persisted till day 10 post irradiation. To the best of our knowledge this study has for the first time documented a comparative account of microstructural and metabolic aspects of whole body and cranial radiation induced early brain injury using in vivo MRI. Overall our findings suggest differential response at microstructure and metabolite levels following cranial or whole body radiation exposure.

Altered brain metabolism after whole body irradiation in mice: a preliminary in vivo 1H MRS study

Abstract Purpose: In the classical description of acute radiation syndrome, the role of central nervous system (CNS) is underestimated. It is now well recognised that ionising radiation-induced oxidative stress may bring about functional changes in the brain. In this study, we prospectively evaluated metabolic changes in the brain after whole body irradiation in mice using in vivo proton ((1)H) nuclear magnetic resonance spectroscopy (MRS).

MATERIAL AND METHODS

Young adult mice were exposed to whole body irradiation of 8 Gy and controls were sham irradiated. In vivo (1)H MRS from cortex-hippocampus and hypothalamic-thalamic region of brain at different time points, i.e., as early as 6 hours, day 1, 2, 3, 5 and 10 post irradiation was carried out at 7 Tesla animal magnetic resonance imaging system. Brain metabolites were measured and quantitative analysis of detectable metabolites was performed by linear combination of model (LCModel).

RESULTS

Significant reduction in myoinositol (p = 0.03) and taurine (p = 0.02) ratios were observed in cortex-hippocampus region as early as day 2 post irradiation compared to controls. These metabolic alterations remained sustained over day 10 post irradiation.

CONCLUSIONS

The results of this preliminary study suggest that the alteration/reduction in the mI and Tau concentration may be associated with physiological perturbations in astrocytes or radiation induced neuro-inflammatory response triggered in microglial cell.

Targeted overexpression of endothelin-1 in astrocytes leads to more severe cytotoxic brain edema and higher mortality

Transgenic mice overexpressing endothelin-1 (ET-1) in astrocytes (GET-1) displayed more severe brain edema and neurologic dysfunction after experimental ischemic stroke. However, it was not clear whether astrocytic ET-1 contributed to cytotoxic or vasogenic edema associated with stroke. In this study, the role of astrocytic ET-1 in cytotoxic edema and brain injury was investigated. Upon acute water intoxication, the GET-1 mice had a lower survival rate and more severe neurologic deficits. Such an exacerbated condition in the GET-1 mice may be a result of a significant increase in cerebral water content and increased expression of the water channel protein, aquaporin 4 (AQP-4). The GET-1 mice treated with OPC-31260, a nonpeptide arginine vasopressin V(2) receptor antagonist, were alleviated from the cerebral water accumulation and neurologic deficit during the early time period after water intoxication. In addition, a significant reduction of AQP-4 expression was observed in astrocytic end-feet AQP-4 in the hippocampus of the GET-1 mice treated with OPC-31260. Therefore, ET-1-induced AQP-4 expression and cerebral water accumulation are the key factors in brain edema associated with acute water intoxication. The V(2) receptor antagonist, OPC-31260, may be one of the effective drugs for the early treatment of ET-1-induced cytotoxic edema and brain injury.

Protective role of reactive astrocytes in brain ischemia

Reactive astrocytes are thought to protect the penumbra during brain ischemia, but direct evidence has been lacking due to the absence of suitable experimental models. Previously, we generated mice deficient in two intermediate filament (IF) proteins, glial fibrillary acidic protein (GFAP) and vimentin, whose upregulation is the hallmark of reactive astrocytes. GFAP(-/-)Vim(-/-) mice exhibit attenuated posttraumatic reactive gliosis, improved integration of neural grafts, and posttraumatic regeneration. Seven days after middle cerebral artery (MCA) transection, infarct volume was 210 to 350% higher in GFAP(-/-)Vim(-/-) than in wild-type (WT) mice; GFAP(-/-), Vim(-/-) and WT mice had the same infarct volume. Endothelin B receptor (ET(B)R) immunoreactivity was strong on cultured astrocytes and reactive astrocytes around infarct in WT mice but undetectable in GFAP(-/-)Vim(-/-) astrocytes. In WT astrocytes, ET(B)R colocalized extensively with bundles of IFs. GFAP(-/-)Vim(-/-) astrocytes showed attenuated endothelin-3-induced blockage of gap junctions. Total and glutamate transporter-1 (GLT-1)-mediated glutamate transport was lower in GFAP(-/-)Vim(-/-) than in WT mice. DNA array analysis and quantitative real-time PCR showed downregulation of plasminogen activator inhibitor-1 (PAI-1), an inhibitor of tissue plasminogen activator. Thus, reactive astrocytes have a protective role in brain ischemia, and the absence of astrocyte IFs is linked to changes in glutamate transport, ET(B)R-mediated control of gap junctions, and PAI-1 expression.

Astrocyte intermediate filaments in CNS pathologies and regeneration

Astroglial cells are the most abundant cells in the mammalian central nervous system (CNS), yet our knowledge about their function in health and disease has been limited. This review focuses on the recent work addressing the function of intermediate filaments in astroglial cells under severe mechanical or osmotic stress, in hypoxia, and in brain and spinal cord injury. Recent data show that when astrocyte intermediate filaments are genetically ablated in mice, reactive gliosis is attenuated and the course of several CNS pathologies is altered, while the signs of CNS regeneration become more prominent. GFAP is the principal astrocyte intermediate filament protein and dominant mutations in the GFAP gene have been shown to lead to Alexander disease, a fatal neurodegenerative condition in humans.

Glial diffusion barriers during aging and pathological states

In conclusion, glial cells control not only ECS ionic composition, but also ECS size and geometry. Since ECS ionic and volume changes have been shown to play an important role in modulating the complex synaptic and extrasynaptic signal transmission in the CNS, glial cells may thus affect neuronal interaction, synchronization and neuron-glia communication. As shown in Fig. 2, a link between ionic and volume changes and signal transmission has been proposed as a model for the non-specific feedback mechanism suppressing neuronal activity (Syková, 1997; Ransom, 2000). First, neuronal activity results in the accumulation of [K+]e, which in turn depolarizes glial cells, and this depolarization induces an alkaline shift in glial pHi. Second, the glial cells extrude acid and the resulting acid shift causes a decrease in the neuronal excitability. Because ionic transmembrane shifts are always accompanied by water, this feedback mechanism is amplified by activity-related glial swelling compensated for by ECS volume shrinkage and by increased tortuosity, presumably by the crowding of molecules of the ECS matrix and/or by the swelling of fine glial processes. This, in turn, results in a larger accumulation of ions and other neuroactive substances in the brain due to increased diffusion hinderance in the ECS. Astrocyte hypertrophy, proliferation and swelling influence the size of the ECS volume and tortuosity around neurons, slowing diffusion in the ECS. Their organization may also affect diffusion anisotropy, which could be an underlying mechanism for the specificity of extrasynaptic transmission, including 'cross-talk' between distinct synapses (Barbour and Hausser, 1997; Kullmann and Asztely, 1998). An increased concentration of transmitter released into a synapse (e.g. repetitive adequate stimuli or during high frequency electrical stimulation which induces LTP) results in a significant activation of high-affinity receptors at neighboring synapses. The efficacy of such synaptic cross-talk would be dependent on the extracellular space surrounding the synapses, i.e. on intersynaptic geometry and diffusion parameters. Other recent studies have also suggested an important role for proteoglycans, known to participate in multiple cellular processes, such as axonal outgrowth, axonal branching and synaptogenesis (Hardington and Fosang, 1992; Margolis and Margolis, 1993) that are important for the formation of memory traces. Recent observation of a decrease of fibronectin and chondroitin sulfate proteoglycan staining in the hippocampus of behaviorally impaired aged rats (Syková et al., 1998a,b) supports this hypothesis. It is reasonable to assume that besides neuronal and glial processes, macromolecules of the extracellular matrix contribute to diffusion barriers in the ECS. It is therefore apparent that glial cells play an important role in the local architecture of the CNS and they may also be involved in the modulation of signal transmission, in plastic changes, LTP, LTD and in changes of behavior and memory formation.

Glial cells and volume transmission in the CNS

Although synaptic transmission is an important means of communication between neurons, neurons themselves and neurons and glia also communicate by extrasynaptic "volume" transmission, which is mediated by diffusion in the extracellular space (ECS). The ECS of the central nervous system (CNS) is the microenvironment of neurons and glial cells. The composition and size of ECS change dynamically during neuronal activity as well as during pathological states. Following their release, a number of neuroactive substances, including ions, mediators, metabolites and neurotransmitters, diffuse via the ECS to targets distant from their release sites. Glial cells affect the composition and volume of the ECS and therefore also extracellular diffusion, particularly during development, aging and pathological states such as ischemia, injury, X-irradiation, gliosis, demyelination and often in grafted tissue. Recent studies also indicate that diffusion in the ECS is affected by ECS volume inhomogeneities, which are the result of a more compacted space in certain regions, e.g. in the vicinity of oligodendrocytes. Besides glial cells, the extracellular matrix also changes ECS geometry and forms diffusion barriers, which may also result in diffusion anisotropy. Glial cells therefore play an important role in extrasynaptic transmission, for example in functions such as vigilance, sleep, depression, chronic pain, LTP, LTD, memory formation and other plastic changes in the CNS. In turn, ECS diffusion parameters affect neuron-glia communication, ionic homeostasis and movement and/or accumulation of neuroactive substances in the brain.

Glial cells in neurotoxicity development

Neuroglial cells of the central nervous system include the astrocytes, oligodendrocytes, and microglia. Their counterparts in the peripheral nervous system are the Schwann cells. The term neuroglia comes from an erroneous concept originally coined by Virchow (1850), in which he envisioned the neurons to be embedded in a layer of connective tissue. The term, or its shortened form--glia, has persisted as the preferred generic term for these cells. A reciprocal relationship exists between neurons and glia, and this association is vital for mutual differentiation, development, and functioning of these cell types. Therefore, perturbations in glial cell function, as well as glial metabolism of chemicals to active intermediates, can lead to neuronal dysfunction. The purpose of this review is to explore neuroglial sites of neurotoxicant actions, discuss potential mechanisms of glial-induced or glial-mediated central nervous system and peripheral nervous system damage, and review the role of glial cells in neurotoxicity development.

Glial swelling and astrogliosis produce diffusion barriers in the rat spinal cord

Cell swelling and astrogliosis (manifested as an increase in GFAP) were evoked in isolated rat spinal cords of 4-21-day-old rats by incubation in either 50 mM K+ or hypotonic solution (235 mosmol kg(-1)). Application of K+ and hypotonic solution resulted at first in a decrease of extracellular space (ECS) volume fraction alpha (ECS volume/total tissue volume) and an increase in tortuosity lambda (lambda2 = free/apparent diffusion coefficient) in spinal gray (GM) and white matter (WM). These changes resulted from cell swelling, since the total water content (TW) in spinal cord was unchanged and the changes were blocked in Cl- -free solution and slowed down by furosemide and bumetanide. Diffusion in WM was anisotropic, i.e., more facilitated along fibers (x-axis) than across them (y- or z-axis). The increase of lambda(y,z) was greater than that of lambda(x), reaching unusually high values above 2.4. In GM only, during continuous 45 min application, alpha and lambda started to return towards control values, apparently due to cell shrinkage of previously swollen cells since TW remained unchanged. This return was blocked by fluoroacetate, suggesting that most of the changes were due to the swelling of glia. A 45 min application of 50 mM K+ and, to a lesser degree, of hypotonic solution evoked astrogliosis, which persisted after washing out these solutions with physiological saline. During astrogliosis lambda increased again to values as high as 2.0, while alpha either returned to or increased above control values. This persistent increase in lambda after washout was also found in WM, and, in addition, the typical diffusion anisotropy was diminished. Our data show that glial swelling and astrogliosis are associated with a persistent increase in ECS diffusion barriers. This could lead to the impairment of the diffusion of neuroactive substances, extrasynaptic transmission, "crosstalk" between synapses and neuron-glia communication.

Altered taurine release following hypotonic stress in astrocytes from mice deficient for GFAP and vimentin

Astrocytes maintain their volume in response to changes in osmotic pressure in their environment by an afflux/influx of ions and organic osmoequivalents. The initial swelling of an astrocyte transferred to a hypoosmotic medium is thus reversed within minutes. The mechanisms which trigger this process as well as the sensors for cell volume are largely unknown, however, the cytoskeleton appears to be involved. We have addressed the role of one component of the cytoskeleton, the intermediate filaments, in the maintenance of astrocytic cell volume. Astrocytes from wild type mice were compared with cells from mice deficient for either glial fibrillary acidic protein (GFAP-/-) or vimentin (vimentin-/-) and with astrocytes from mice deficient for both proteins (GFAP-/-vim-/-). Whereas GFAP-/- and vimentin-/- cultured or reactive astrocytes retain intermediate filaments, the GFAP-/-vim-/- astrocytes are completely devoid of these structures. The rate of efflux of the preloaded osmoequivalent 3H-taurine from primary and passaged cultures of astrocytes was monitored. A reduction of NaCl (25 mM) in the perfusion medium led to a 400-900% increase of 3H-taurine afflux in astrocytes from wild type mice. The stimulated efflux was not significantly affected in astrocytes from GFAP-/- or vimentin-/- mice. However, the efflux from astrocytes from GFAP-/-vim-/- mice was 25-46% lower than the wild type levels. The results strengthen the role of the cytoskeleton in astrocyte volume regulation and suggest an involvement of intermediate filaments in the process.

Glutamate induced astroglial swelling--methods and mechanisms

Glutamate (Glu) plays an important role in the early development of brain injuries caused by ischemia, i.e. stroke, or brain trauma. Glu induces a rapid astroglial swelling which, in turn, deranges the composition of neuroactive substances in the extracellular space. We report that Glu can induce astroglial cell swelling by interaction with metabotropic Glu receptors (mGluRs). Furthermore, the Na(+)-K(+)-2Cl- cotransporter, a Na(+)-K(+) ATPase, and the Na(+)-dependent electrogenic Glu carrier seem to be involved in this Glu-induced astroglial cell swelling. Two methods for studying cell swelling arc described. One is based on variations in the signal emitted by the fluorescent probe fura-2/AM when excited at its isosbestic point. These variations were shown to be directly proportional to variations in intracellular volume. Relative changes in cell volume and intracellular calcium concentration could be detected simultaneously in single astroglial cells. The other method used permits the cell volume to be calculated in relative terms with the aid of image processing techniques.

Volume expansion-sensing outward-rectifier Cl- channel: fresh start to the molecular identity and volume sensor

The maintenance of a constant volume in the face of extracellular and intracellular osmotic perturbation is essential for the normal function and survival of animal cells. Osmotically swollen cells restore their volume, exhibiting a regulatory volume decrease by releasing intracellular K+, Cl-, organic solutes, and obligated water. In many cell types, the volume regulatory effluxes of Cl- and some organic osmolytes are known to be induced by swelling-induced activation of anion channels that are characterized by their moderate outward rectification, cytosolic ATP dependency, and intermediate unitary conductance (10-100 pS). Recently, simultaneous measurements of cell size by light microscopy and whole cell Cl- current have shown that the Cl- current density is proportionally increased with an increase in the outer surface area, which is mainly achieved through unfolding of membrane invaginations by volume expansion. Thus this anion channel can somehow sense volume expansion and can be called the volume expansion-sensing outwardly rectifying (VSOR) anion channel. Its molecular identity and activation mechanism are yet to be elucidated. Three cloned proteins, ClC-2, P-glycoprotein, and pIcln, have been proposed as candidates for the VSOR anion channel. The unitary conductance, voltage dependency, anion selectivity, pH dependency, and pharmacology of the VSOR anion channel are distinct from the ClC-2 Cl- channel, which is also known to be sensitive to volume changes. Recent patch-clamp studies in combination with molecular biological techniques have shown that P-glycoprotein is not itself the channel protein but is a regulator of its volume sensitivity. Although there is still debate about another candidate protein, pIcln, the most recent study has suggested that this is likely to be a regulator of some other distinct Cl- channel. Identification of the VSOR anion channel protein per se, its volume-sensing mechanism, and its accessory/regulatory proteins at the molecular level is currently a subject of utmost physiological importance.

Evidence against volume regulation by cortical brain cells during acute osmotic stress

The cell bodies of neurons and glia examined in culture respond to severe osmotic stress (100 to 200 mOsm) by passive volume change that is followed within several minutes by volume regulation, even in the face of maintained osmotic change. However, in clinical situations, the brain does not experience such precipitous and severe changes in brain hydration. In this study we examined if there is evidence from the hippocampal slice preparation supporting the type of volume regulation observed in cultured brain cells. Within the CA1 region we imaged changes in light transmittance (LT), recorded the evoked field potential, and monitored tissue resistance (all measures of cell volume change) during the first hour of osmotic stress to search for evidence of volume regulation. During superfusion of hypo-osmotic aCSF (-40 mOsm), LT increased 24 to 28% in the dendritic regions of CA1 neurons. The LT reached a plateau which was maintained throughout a 45-min application interval, more than enough time to reveal a regulatory volume decrease. Upon return to control saline, LT immediately returned to baseline and settled there. Hypo-osmolality reversibly increased the relative tissue resistance (RREL) measured across the CA1 region with a time course identical to the increase in LT. Conversely, hyperosmotic aCSF (mannitol, +40 mOsm) decreased both RREL by 8% and LT by 15.5% with no indication of a regulatory volume increase. The CA1 cell body layer showed only slight hypo-osmotic swelling whereas exposure to the glutamate agonist quinolinic acid caused pronounced swelling in this region. Even when osmolality was decreased by 120 mOsm for 20 min, dendritic regions responded passively with no regulatory volume decrease. However, when aCSF Cl- was substituted, the CA1 dendritic regions displayed immediate swelling followed by a dramatic volume reduction under normosmotic conditions, indicating that such behavior can be evoked by extreme aCSF dilution. We conclude that in the brain slice preparation, the cortical cells do not exhibit classic volume regulation in response to sudden physiological changes in osmolality. Moreover it is the dendritic region, not the cell body region, that displays dynamic volume change during osmotic challenge.

Metallothionein induction protects swollen rat primary astrocyte cultures from methylmercury-induced inhibition of regulatory volume decrease

Metallothionein (MT) proteins have been postulated to play a role in the detoxification of heavy metals. Since methylmercury (MeHg) preferentially accumulates in astrocytes, and MT-1 and MT-2 are astrocyte-specific MT isoforms, we investigated the ability of MTs to attenuate MeHg-induced cytotoxicity. The toxic effects of MeHg on astrocytes were investigated in a model of regulatory volume decrease (RVD) in which the cells are swollen by exposure to a hypotonic buffer. Preexposure to CdCl2 (1 microM) for 72, 96 or 120 h, prior to acute exposure to hypotonic buffer and MeHg (10 microM) led to a time-dependent increase in the intracellular levels of astrocyte MT proteins. The acute MeHg-induced inhibition of RVD was significantly, and almost fully reversed by preexposure to CdCl2. This reversal was time-dependent, 120-h preexposure to CdCl2 producing the greatest reversibility. Furthermore, the ability of astrocytes to efficiently volume regulate in the presence of MeHg-containing hypotonic buffer was highly correlated (r = 0.99) with the intracellular levels of MT proteins. The release of [3H]taurine, an osmolyte involved in the RVD process was also measured. The inhibitory effect of MeHg on [3H]taurine in swollen cells was significantly, and fully reversed by CdCl2 preexposure. The study suggests that astrocytes induced to express high levels of MT proteins are resistant to the acute inhibitory effect of MeHg on RVD.

Inhibition of regulatory volume decrease in swollen rat primary astrocyte cultures by methylmercury is due to increased amiloride-sensitive Na+ uptake

Primary astrocyte cultures from neonatal rats were swollen by exposure to hypotonic buffer with and without 10 microM methylmercury (MeHg). We investigated the effects of MeHg on K+ (using 86Rb), taurine, D-aspartate (a non metabolizable analogue of glutamate) and Na+ fluxes during regulatory volume decrease (RVD), with an electrical impedance method for determination of cell volume, coupled with on-line measurements of efflux of radioactive ions and amino acids. Addition of 10 microM MeHg completely inhibited RVD in swollen astrocytes, increased the uptake of 22Na+, increased 86Rb release, and decreased 3H-taurine release. There was no effect on the rate of release of 3H-D-aspartate from swollen astrocytes. 0.5 mM amiloride completely inhibited MeHg-induced increased Na+ influx during RVD, while 1 mM furosemide had no effect. When Na+ in the hypotonic buffer was replaced with N-methyl-D-glucamine (NMDG), RVD in the presence of MeHg was indistinguishable from controls. These results indicate that MeHg increases cellular permeability to ions such as Na+ and K+, and that an increase in Na+ permeability via Na+/H+ exchange, offsetting K+ loss, is the primary mechanism in its inhibition of RVD in swollen astrocytes.

Changes in the extracellular glutamate concentrations in the rat cortex following localized by hyperthermia

The mechanisms involved in cellular injury caused by hyperthermia in the central nervous system (CNS) remain unknown. In order to test the hypothesis that glutamate excitotoxicity may play a role in hyperthermia-induced CNS injury, we measured the extracellular glutamate concentrations, using intracerebral microdialysis, in the rat brain following localized hyperthermia The glutamate concentration in the dialysate was not increased by mild hyperthermia (41 degrees C), but it reached 250% of the control level 40 min after a 20-min period of moderate hyperthermia (43 degrees C) and then decreased rapidly When serve hyperthermia (45 degrees C) was induced, the glutamate concentration reached approximately 300% of the control level and was maintained at that level for 100 min after hyperthermia cessation. The elevated extracellular glutamate concentrations by local hyperthermia reached neurotoxic levels. The results presented here suggest that glutamate-mediated, 'excitotoxic' process plays an important role in hyperthermia-induced cellular injury in the CNS.

Changes in organic solutes, volume, energy state, and metabolism associated with osmotic stress in a glial cell line: a multinuclear NMR study

Diffusion-weighted in vivo 1H-NMR spectroscopy of F98 glioma cells embedded in basement membrane gel threads showed that the initial cell swelling to about 180% of the original volume induced under hypotonic stress was followed by a regulatory volume decrease to nearly 100% of the control volume in Dulbecco's modified Eagle's medium (DMEM) but only to 130% in Krebs-Henseleit buffer (KHB, containing only glucose as a substrate) after 7 h. The initial cell shrinkage to approx. 70% induced by the hypertonic stress was compensated by a regulatory volume increase which after 7 h reached almost 100% of the control value in KHB and 75% in DMEM. 1H-, 13C- and 31P-NMR spectroscopy of perchloric acid extracts showed that these volume regulatory processes were accompanied by pronounced changes in the content of organic osmolytes. Adaptation of intra- to extracellular osmolarity was preferentially mediated by a decrease in the cytosolic taurine level under hypotonic stress and by an intracellular accumulation of amino acids under hypertonic stress. If these solutes were not available in sufficient quantities (as in KHB), the osmolarity of the cytosol was increasingly modified by biosynthesis of products and intermediates of essential metabolic pathways, such as alanine, glutamate and glycerophosphocholine in addition to ethanolamine. The cellular nucleoside triphosphate level measured by in vivo 31P-NMR spectroscopy indicated that the energy state of the cells was more easily sustained under hypotonic than hypertonic conditions.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Single-channel characteristics of the large-conductance anion channel in rat cortical astrocytes in primary culture

Cultured rat cortical astrocytes, in addition to a variety of voltage-sensitive potassium channels, also express anion channels. However, the behavior and regulation of these anion channels have been far less studied. This paper describes a patch-clamp study on a voltage-sensitive 200-300 pS high-conductance single-channel anion current, which seems to possess at least five different open sublevels or, alternatively, be formed from five or more small-conductance ion channels linked together. This channel is voltage dependent, showing a bell-shaped open probability curve with highest open probability close to the reversal potential (zero-current). Although potassium channels are commonly detected in astrocytes in cell-attached and excised patches with both normal osmolarity and hypoosmotic solutions, the occurrence of the anion channel is clearly increased in isolated patches when hypoosmotic bath solutions are used. Also, cell aging in culture and the preparation of secondary cell cultures by trypsinization seem to increase the rate of occurrence of the anion channel. Though this channel is more routinely seen when a membrane patch is excised from the cell, occasionally cell-attached configurations with instant channel activity can be formed. While the modulation of this anion channel was being studied, it was found to be blocked by an anion transport inhibitor, L-644,711, reported to affect cell volume regulation in astrocytes.