Respiration in rat cerebral astrocytes from primary culture

High frequency stimulation induces sonic hedgehog release from hippocampal neurons

Sonic hedgehog (SHH) as a secreted protein is important for neuronal development in the central nervous system (CNS). However, the mechanism about SHH release remains largely unknown. Here, we showed that SHH was expressed mainly in the synaptic vesicles of hippocampus in both young postnatal and adult rats. High, but not low, frequency stimulation, induces SHH release from the neurons. Moreover, removal of extracellular Ca²⁺, application of tetrodotoxin (TTX), an inhibitor of voltage-dependent sodium channels, or downregulation of soluble n-ethylmaleimide-sensitive fusion protein attachment protein receptors (SNAREs) proteins, all blocked SHH release from the neurons in response to HFS. Our findings suggest a novel mechanism to control SHH release from the hippocampal neurons.

Astrocyte Cultures Mimicking Brain Astrocytes in Gene Expression, Signaling, Metabolism and K+ Uptake and Showing Astrocytic Gene Expression Overlooked by Immunohistochemistry and In Situ Hybridization

Based on differences in gene expression between cultured astrocytes and freshly isolated brain astrocytes it has been claimed that cultured astrocytes poorly reflect the characteristics of their in vivo counterparts. This paper shows that this is not the case with the cultures of mouse astrocytes we have used since 1978. The culture is prepared following guidelines provided by Drs. Monique Sensenbrenner and John Booher, with the difference that dibutyryl cyclic AMP is added to the culture medium from the beginning of the third week. This addition has only minor effects on glucose and glutamate metabolism, but it is crucial for effects by elevated K⁺ concentrations and for Ca²⁺ homeostasis, important aspects of astrocyte function. Work by Liang Peng and her colleagues has shown identity between not only gene expression but also drug-induced gene upregulations and editings in astrocytes cultured by this method and astrocytes freshly isolated from brains of drug-treated animals. Dr. Norenberg's laboratory has demonstrated identical upregulation of the cotransporter NKCC1 in ammonia-exposed astrocytes and rats with liver failure. Similarity between cultured and freshly isolated astrocytes has also been shown in metabolism, K⁺ uptake and several aspects of signaling. However, others have shown that the gene for the glutamate transporter GLT1 is not expressed, and rat cultures show some abnormalities in K⁺ effects. Nevertheless, the overall reliability of the cultured cells is important because immunohistochemistry and in situ hybridization poorly demonstrate many astrocytic genes, e.g., those of nucleoside transporters, and even microarray analysis of isolated cells can be misleading.

Regulation of taurine transport in rat hippocampal neurons by hypo-osmotic swelling

Taurine, an important mediator of cellular volume regulation in the central nervous system, is accumulated into neurons and glia by means of a highly specific sodium-dependent membrane transporter. During hyperosmotic cell shrinkage, net cellular taurine content increases as taurine transporter activity is enhanced via elevated gene expression of the transporter protein. In hypo-osmotic conditions, taurine is rapidly lost from cells by means of taurine-conducting membrane channels. We reasoned that changes in taurine transporter activity also might accompany cell swelling to minimize re-accumulation of taurine from the extracellular space. Thus, we determined the kinetic and pharmacological characteristics of neuronal taurine transport and the response to osmotic swelling. Accumulation of radioactive taurine is strongly temperature dependent and occurs via saturable and non-saturable pathways. At concentrations of taurine expected in extracellular fluid in vivo, 98% of taurine accumulation would occur via the saturable pathway. This pathway obeys Michaelis-Menten kinetics with a Km of 30.0 +/- 8.8 microm (mean +/- SE) and Jmax of 2.1 +/- 0.2 nmol/mg protein min. The saturable pathway is dependent on extracellular sodium with an effective binding constant of 80.0 +/- 3.1 mm and a Hill coefficient of 2.1 +/- 0.1. This pathway is inhibited by structural analogues of taurine and by the anion channel inhibitors, 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS) and 5-nitro-2-(3 phenylpropylamino) benzoic acid (NPPB). NPPB, but not DIDS, also reduces the ATP content of the cell cultures. Osmotic swelling at constant extracellular sodium concentration reduces the Jmax of the saturable transport pathway by approximately 48%, increases Kdiff for the non-saturable pathway by 77%, but has no effect on cellular ATP content. These changes in taurine transport occurring in swollen neurons in vivo would contribute to net reduction of taurine content and resulting volume regulation.

Simultaneous measurement of water volume and pH in single cells using BCECF and fluorescence imaging microscopy

Regulation and maintenance of cell water volume and intracellular pH (pHi) are vital functions that are interdependent; cell volume regulation affects, and is in turn affected by, changes in pHi. Disruption of either function underlies various pathologies. To study the interaction and kinetics of these two mechanisms, we developed and validated a quantitative fluorescence imaging microscopy method to measure simultaneous changes in pHi and volume in single cells loaded with the fluorescent probe BCECF. CWV is measured at the excitation isosbestic wavelength, whereas pHi is determined ratiometrically. The method has a time resolution of <1 s and sensitivity to osmotic changes of approximately 1%. It can be applied in real time to virtually any cell type attached to a coverslip, independently of cellular shape and geometry. Calibration procedures and algorithms developed to transform fluorescence signals into changes in cell water volume (CWV) and examples of applications are presented.

Astrocytic contributions to bioenergetics of cerebral ischemia

Astrocytes are multifunctional cells that interact with neurons and other astrocytes in signaling and metabolic functions, and their resistance to pathophysiological conditions can help restrict loss of tissue after an ischemic event provided adequate nutrients are supplied to support their requirements. Astrocytes have substantial oxidative capacity and mechanisms to upregulate glycolytic capability when respiration is impaired. An astrocytic enzyme that synthesizes a powerful activator of glycolysis is not present in neurons, endowing astrocytes with the ability to sustain ATP production under restrictive conditions. The monocarboxylic acid transporter (MCT) isoforms predominating in astrocytes are optimized to facilitate very large increases in lactate flux as lactate concentration increases within (1-3 mM) and above (>3 mM) the normal range. In sharp contrast, the major neuronal MCT serves as a barrier to increased transmembrane transport as lactate rises above 1 mM, restricting both entry and efflux. Lactate can serve as fuel during recovery from ischemia but direct evidence that lactate is oxidized by neurons (vs. astrocytes) to maintain synaptic function is lacking. Astrocytes have critical roles in regulation of ionic homeostasis and control of extracellular glutamate levels, and spreading depression associated with ischemia places high demands on energy supplies in astrocytes and contributes to metabolic exhaustion and demise. Disruption of Ca2+ homeostasis, generation of oxygen free radicals and nitric oxide, and mitochondrial depolarization contribute to astrocyte death during and after a metabolic insult. Novel pharmaceutical agents targeted to astrocytes and hyperoxic therapy that restores penumbral oxygen level during energy failure might improve postischemic outcome.

Volume-regulated anion conductance in cultured rat cerebral astrocytes requires calmodulin activity

We examined the calmodulin dependence of anion channel activation during hypo-osmotic swelling in rat cerebral astrocytes. Control cells bathed in iso-osmotic (290 mOsm) phosphate-buffered saline (PBS) and recorded using a patch electrode containing 140 mM KCl increased membrane conductance threefold over basal levels after 12 min in hypo-osmotic (200 mOsm) PBS. Cells injected with monoclonal anticalmodulin antibody demonstrated no increase in membrane conductance during a subsequent exposure to hypo-osmotic PBS. In contrast, cells iontophoretically injected with monoclonal antiglial fibrillary acidic protein antibody or with anticalmodulin antibody absorbed with an excess of free calmodulin demonstrated an increase in conductance during hypo-osmotic exposure similar to that of control cells. Conductance in iso-osmotic conditions was unchanged by antibody injection. Similar results were obtained when using patch electrode and bath solutions containing chloride as the only cell permeant ion, indicating a calmodulin-dependent anion current is activated with this degree of hypo-osmotic treatment. Western blots confirmed the specificity of the anticalmodulin and antiglial fibrillary acidic protein antibodies used in this study for proteins of 17 and 51 kD, respectively. In addition, in vitro studies demonstrated inhibition of the calmodulin-dependent activation of phosphodiesterase by the anticalmodulin antibody. Thus, binding of this antibody to calmodulin causes functional inhibition of calmodulin activity. No change in the intensity or cellular distribution of calmodulin immunostaining was observed during 30 min of hypo-osmotic exposure. However, increased immunostaining for activated calmodulin kinase IIalpha was observed after 10 min of hypo-osmotic exposure, suggesting initiation of calmodulin-dependent processes by cell swelling. The data indicate calmodulin activity is critical for activation of volume-regulated anion channels in rat cerebral astrocytes.

Calcium/calmodulin-modulated chloride and taurine conductances in cultured rat astrocytes

Osmotically swollen rat cerebral astrocytes develop an increased anion conductance which can mediate chloride and taurine release. We used whole cell patch clamp to study mechanisms that modulate this conductance. Astrocyte chloride conductance increased within 4 min of exposure to 200 mOsm medium and was 670+/-123% of its initial value after 15 min (mean+/-S.E.M.). This conductance was substantially reduced in 0.1 mM extracellular calcium with 20 mM BAPTA added to the electrode solution and was completely inhibited with calcium-free perfusion solution containing 1 mM EDTA (n=4). The conductance increase in 200 mOsm medium also was inhibited in a dose-dependent manner by nimodipine with a calculated K(i) of 0.31+/-0.4 microM and mean+/-S.E.M. inhibition of 84.4+/-4% at 100 microM nimodipine. In the presence of 100 microM W-7, a calmodulin antagonist, the mean+/-S.E.M. conductance increase after 15 min was 223+/-40% of the initial value while 300 microM W-7 or 100 microM trifluoperazine inhibited the conductance increase completely (n=6). With taurine as the major anion in electrode and perfusion solutions, a significant conductance increase was observed in 200 mOsm medium. This conductance increase was inhibited by 300 microM W-7 or 100 microM nimodipine. We conclude extracellular calcium influx via L-type calcium channels leads to increased astrocyte anion conductance in 200 mOsm conditions via calmodulin-dependent activation of anion channels. Efflux of anionic taurine from swollen astrocytes also may be affected by calcium influx through a similar calcium/calmodulin-dependent process.

Asymmetrical response of p38 kinase activation to volume changes in primary rat astrocytes

Activation of p38 kinase by osmotic stress has been documented in many cells; however, no report has distinguished the effects of cell volume on p38 activity from the effects of the altered osmotic condition per se. Here we report asymmetrical activation of astrocyte p38 mitogen-activated protein (MAP) kinase in response to volume increases and volume decreases. We separate effects of cell volume changes from the effects of osmotic exposure on p38 activation. Exposure to 400, 500, or 600 mOsm phosphate-buffered saline (PBS) caused cell shrinkage and an osmolality-dependent increase in p38 activity to 175%, 409%, or 518%, respectively, compared with cells maintained in control conditions (290 mOsm). Likewise, hyposmotic conditions ranging from 250 to 57 mOsm PBS caused the same activation of p38 (approximately 300% of the control value within 10 min). The activity in hyposmotic conditions did not diminish over 30 min despite cell volume recovery, indicating a dependence of extracellular osmolality or ionic strength rather than cell volume. Cells that were returned to isosmotic conditions following 30 min in 250, 150, or 57 mOsm PBS shrunk to 73%, 39%, or 26% of the control cell volume, respectively. In these cells, the activity of p38 increased further from approximately 300% of the control values in each hyposmotic condition to as much as 500% of the control activity as a function of the degree of cell shrinkage. Thus, p38 may be activated by cell shrinkage in hyperosmotic or in isoosmotic conditions, indicating reduced cell volume is a more important determinant of this enzyme activity than extracellular osmolality. Our results indicate distinct mechanisms of p38 activation in astrocytes exposed to hyperosmotic or hyposmotic PBS.

Osmolyte contents of cultured astrocytes grown in hypoosmotic medium

Primary rat cerebral astrocyte cultures were grown for 2 weeks in isoosmotic medium (305 mosmol) and then placed in similar medium with a reduced NaCl concentration. During the first hour of growth in this moderately hypoosmotic medium (240 mosmol), the cells lose 88% of their taurine contents, 62% of their alanine contents, and 54% of their aspartate contents while regaining normal volume. Loss of these amino acids accounts for 43% of observed volume regulation. Contents of these amino acids remain decreased during 24 h of growth in hypoosmotic medium. In contrast, potassium, glutamate, glutamine, and asparagine contents are not changed, relative to cells in isoosmotic medium, at time points between 1 h and 24 h of hypoosmotic exposure. The data suggest astrocytes contribute to net loss of amino acids, but not potassium, from brains exposed to hypoosmotic conditions in situ.

Taurine synthesis and cysteine metabolism in cultured rat astrocytes: effects of hyperosmotic exposure

We investigated mechanisms controlling taurine synthesis in cultured rat cerebral astrocytes. The mean +/- SE rate of taurine synthesis from extracellular cysteine was 21.2 +/- 2.0 pmol.mg protein-1.min-1, whereas taurine degradation was < 1.3% of this rate. Eliminating cellular glutathione and inhibiting glutathione biosynthesis increased taurine synthesis from extracellular cysteine by 39%. In cell homogenates, cysteine dioxygenase (CDO) and cysteine-sulfinate decarboxylase activities were 2.4 +/- 0.2 and 8.3 +/- 2.8 nmol.mg protein-1.min-1, respectively. CDO activity was strongly dependent on cysteine concentration over physiological and pathophysiological ranges of intracellular cysteine concentration. Growth in hyperosmotic medium caused a greater increase in culture medium taurine content than that measured from cells in isosmotic growth medium. Hyperosmotic treatment transiently increased the rate of cysteine accumulation and cellular cysteine and glutathione contents but had no effect on the synthesis rate of taurine from extracellular cysteine. Thus cysteine is accumulated and then metabolized to taurine through CDO, whose activity depends on the intracellular cysteine concentration and appears to be rate limiting for taurine synthesis. Hyperosmotic exposure increases net taurine production yet has no effect on taurine synthesis from exogenously applied cysteine. Availability of substrate from intracellular pools must contribute to maintenance of high intracellular taurine during hyperosmotic exposure.

Increased potassium, chloride, and taurine conductances in astrocytes during hypoosmotic swelling

Membrane conductances during hypoosmotic swelling were characterized in rat astrocytes in primary tissue culture. Using whole cell patch clamp techniques, mean +/- SEM cell conductance in isoosmotic phosphate-buffered saline (PBS) was 55.6 +/- 5.8 pS/pF. Cell conductance (mean +/- SEM) increased from this initial value to 187 +/- 46%, 561 +/- 188%, and 1216 +/- 376% within 9 min of exposure to 220 mOsm, 190 mOsm, and 145 mOsm PBS, respectively. With each of these hypoosmotic exposures, no change occurred in membrane capacitance. When CsCl replaced KCl in the microelectrode solution, a similar conductance increase was obtained at each osmolality. However, when gluconate salts were used in place of chloride salts in the electrode solution, no significant conductance increase was observed with 190 mOsm PBS. With a KCl microelectrode solution, all conductance increase which occurred in 190 mOsm PBS was inhibited by 200 microM niflumic acid, but not by 5 mM BaCl(2). Both niflumic acid and BaCl(2) inhibited 60-80% of the conductance increase of cells in 145 mOsm PBS. Using a microelectrode solution containing taurine as the major anion, membrane conductance increased 5-fold when cells were placed in 250 mOsm medium. This conductance increase was completely inhibited by 200 microM niflumic acid. Thus, independent chloride and potassium conductances are activated by hypoosmotic swelling of cultured astrocytes while plasma membrane area is unaltered. The chloride conductance pathway is activated at a significantly lower degree of hypoosmotic exposure than that which activates the potassium pathway and may be permeable to anionic taurine. These conductance pathways may mediate diffusive loss of potassium, chloride, and taurine from these cells during volume regulation following hypoosmotic swelling.

Shrinkage-induced activation of Na+/H+ exchange: role of cell density and myosin light chain phosphorylation

Previously, we suggested that myosin light chain kinase (MLCK) is involved in shrinkage-induced activation of the Na+/H+ exchanger in rat astrocytes. Here we have studied the effects of hyperosmotic exposure in C6 glioma cells, a common model for astrocytes. Shrinkage-induced activation of the Na+/H+ exchanger in C6 cells is directly proportional to the degree of shrinkage, results in an alkaline shift in the pK' of the exchanger, is dependent on ATP, and is inhibited by ML-7 (an MLCK inhibitor) and by various calmodulin inhibitors. Cell shrinkage also results in increased phosphorylation of myosin light chain (MLC). Interestingly, shrinkage-induced activation of the exchanger does not occur in subconfluent C6 cells. However, phosphorylation of MLC still occurs in subconfluent cultures of C6 cells on shrinkage, suggesting that the lack of activation in these cells occurs at a point between MLC phosphorylation and Na+/H+ exchange activation. The lack of activation of Na+/H+ exchange in subconfluent C6 cells can be utilized to further elucidate the shrinkage-induced activation pathway.

Hyperosmotic exposure alters total taurine quantity and cellular transport in rat astrocyte cultures

Taurine content and cellular taurine transport were characterized in astrocytes from rat cerebral cortex after growth in isoosmotic or hyperosmotic culture conditions to investigate mechanisms of taurine accumulation during conditions of increased osmolality. Total taurine content of the culture dishes was significantly (P < 0.05) elevated after 8, 24, and 48 h of hyperosmotic exposure compared to cultures grown for the same period in isoosmotic (300 mOsm, control) conditions. Hyperosmotic medium elevated intracellular taurine (nmol/mg protein) levels by 29-108% over control cultures. Significant (P < 0.02) increases in carrier-mediated taurine uptake rates were observed in astrocytes exposed to 350, 400, and 450 mOsm culture medium for 24 h compared to control cultures at the same time point. The increase in uptake rate decreased to control values by 48 h in 450 mOsm treated cultures. The carrier-mediated transport binding constant for taurine uptake, Km, was not altered at any time after hyperosmotic treatment. Maximal velocity of uptake, V(max), increased by 70% and 36% after 24 h growth in 400 and 450 mOsm culture medium, respectively, compared to control cells at the same time. After 48 h of hyperosmotic exposure, V(max) returned to control values. The diffusional transport rate for taurine efflux, Kdiff, was not affected by hyperosmotic exposure at any time point. Taurine release rates were increased by over two-fold during the first 8 h of exposure to 450 mOsm medium compared with cells grown in control conditions. After 24 and 48 h hyperosmotic exposure, release rates decreased to 44-72% of the release from control cultures. These data indicate at least three mechanisms contribute to taurine accumulation in cultured cerebral astrocytes exposed to hyperosmotic conditions. These mechanisms are (i) an increased rate of taurine uptake from the extracellular space within 24 h, (ii) a decrease in net taurine efflux by 48 h, and (iii) an enhanced rate of taurine synthesis.

Acidosis causes failure of astrocyte glutamate uptake during hypoxia

Failure of glutamate uptake during ischemia can lead to neurotoxic accumulations of glutamate in brain extracellular space. Hypoxia and acidosis are metabolic consequences of ischemia that may individually or in combination impair glutamate uptake. We used primary rat astrocyte cultures to study the effects of acidosis, chemical hypoxia, and the combination of acidosis plus chemical hypoxia on glutamate uptake. Chemical hypoxia alone reduced uptake by 35-45%. Reduction in pH from 7.4 to 5.8 also caused a significant but incomplete inhibition of glutamate uptake, and this effect was more pronounced in medium buffered with CO2/bicarbonate. However, the combination of chemical hypoxia plus acidosis reduced glutamate uptake to below 10% of controls. Astrocyte ATP levels, like glutamate uptake, were significantly reduced by chemical hypoxia and further reduced by the combination of hypoxia plus acidosis. Acidosis under normoxic conditions had no significant effect on astrocyte ATP levels. These results suggest two mechanisms by which acidosis may contribute to failure of astrocyte glutamate uptake during ischemia: Acidosis may act in concert with hypoxia to cause ATP depletion, and acidosis may also have direct effects on glutamate transporters unrelated to effects on cellular ATP levels. pH effects on glutamate uptake may be an important factor affecting neuronal survival during incomplete ischemia.

Hypoosmotic volume regulation of astrocytes in elevated extracellular potassium

Cellular volume and potassium contents were determined in rat astrocytes from primary culture following suspension in isoosmotic (269 mOsm) and hypoosmotic (136 mOsm) phosphate-buffered saline (PBS) containing various potassium concentrations. Within 1 min of suspension in hypoosmotic PBS, cells swelled to 135% of their volume in isoosmotic PBS. This initial swelling was not altered by varying the potassium concentration of the hypoosmotic PBS. After suspension in hypoosmotic PBS containing 3.2 mM potassium, a regulatory volume decrease (RVD) was observed. Higher concentrations of potassium in hypoosmotic PBS inhibited RVD following osmotic swelling. Cells swollen in hypoosmotic PBS containing 50 mM potassium continued to swell for 7 min, reaching a volume of 141% of their initial isoosmotic volume. After 7 min, these cells demonstrated a subsequent decrease in volume. The swelling observed between 1-7 min after suspension in hypoosmotic PBS containing 50 mM potassium was not affected by 10 microM gadolinium, 1 mM quinine, 1 mM DIDS (4,4'-diisothiocyanato-2,2'-stilbenedisulfonic acid), 1 mM SITS (4-acetamido-4'-isothiocyanato-2,2'-stilbenedisulfonic acid), 1 mM furosemide, or 100 microM bumetanide. Normal RVD was obtained in hypoosmotic PBS containing 50 mM potassium, if chloride was replaced with gluconate (but not nitrate) to reduce the extracellular K.Cl product to that of hypoosmotic PBS containing 3.2 mM potassium. The volume decrease seen between 7-30 min after exposure to hypoosmotic PBS containing 50 mM potassium was blocked by 1 mM DIDS, 1 mM SITS, or 1 mM furosemide. Cellular potassium content was elevated by approximately 60% after 7 min exposure to isoosmotic or hypoosmotic PBS containing 50 mM potassium. In hypoosmotic PBS, this increase in cellular potassium was reduced with replacement of chloride by gluconate, but not by nitrate. The results indicate that astrocytes swollen in PBS containing elevated potassium concentrations continue to swell, in part, by accumulation of potassium plus chloride mediated by an approach to Donnan equilibrium. Cotransport carriers or stretch-activated channels do not play a role in the enhanced swelling observed in hypoosmotic PBS containing 50 mM potassium. We suggest that a voltage-sensitive chloride channel mediates this continuation of cell swelling. This mechanism may be important in the persistent swelling of astrocytes observed in pathologic conditions such as trauma and seizures where extracellular potassium is elevated, or when other factors are present which may cause astroglial depolarization.

Excitatory amino acid release from astrocytes during energy failure by reversal of sodium-dependent uptake

Non-synaptic release may be the major route of excitatory amino acid (EAA) efflux during cerebral ischemia. Possible routes of non-synaptic release include non-specific anion channels, reversal of Na(+)-, Cl(-)-, or Ca(2+)-dependent uptake, and cell lysis. In the present study we employ a novel approach to show reversal of Na(+)-dependent uptake as a major route of EAA efflux from astrocyte cultures under conditions of energy failure. Primary rat astrocyte cultures were subjected to combined blockade of glycolytic and oxidative metabolism after incubation with [3H]-D-aspartate (D-ASP). Energy failure produced an efflux of D-ASP that was maximal by 90 minutes. The efflux over this period was reduced by more than 50% in cells that had been pre-loaded with PDC (L-transpyrrolidine-2,4-dicarboxylic acid) or TBHA (threo-beta-hydroxyaspartic acid), compounds that are competitive inhibitors of Na(+)-dependent glutamate uptake. The effect of pre-loading with the inhibitors was concentration dependent. No effect was seen if the inhibitors were added after induction of energy failure, suggesting that the attenuation of D-ASP efflux resulted from binding of the inhibitors to an intracellular site. These results provide strong evidence that EAA efflux from astrocytes under conditions of energy failure occurs largely through reversal of Na(+)-dependent uptake.

Intracellular pH regulation in primary rat astrocytes and C6 glioma cells

We used the pH-sensitive fluorescent dye BCECF to study intracellular pH (pHi) regulation in primary cultures of rat astrocytes and C6 glioma cells. Both cell types contain three pH-regulating transporters: 1) alkalinizing Na+/H+ exchange; 2) alkalinizing Na+ + HCO3-/Cl- exchange; and 3) acidifying Cl-/HCO3- exchange. Na+/H+ exchange was most evident in the absence of CO2; recovery from acidification was Na+ dependent and amiloride sensitive. Exposure to CO2 caused a cell alkalinization that was inhibited by DIDS, dependent on external Na+, and inhibited 75% in the absence of Cl- (thus mediated by Na+ + HCO3-/Cl- exchange). When pHi was increased above the normal steady-state pHi, a DIDS-inhibitable and Na(+)-independent acidifying recovery was evident, indicating the presence of Cl-/HCO3- exchange. Astrocytes, but not C6 cells, contain a fourth pH-regulating transporter, Na(+)-HCO3- cotransport; in the presence of CO2, depolarization caused an alkalinization of 0.12 +/- 0.01 (n = 8) and increased the rate of CO2-induced alkalinization from 0.23 +/- 0.02 to 0.42 +/- 0.03 pH unit/min. Since C6 cells lack the Na(+)-HCO3- cotransporter, they are an inferior model of pHi regulation in glia. Our results differ from previous observations in glia in that: 1) Na+/H+ exchange was entirely inhibited by amiloride; 2) Na+ + HCO3-/Cl- exchange was present and largely responsible for CO2-induced alkalinization; 3) Cl-/HCO3- exchange was only active at pHi values above steady state; and 4) depolarization-induced alkalinization of astrocytes was seen only in the presence of CO2.

Taurine transport in rat astrocytes adapted to hyperosmotic conditions

[3H]Taurine uptake and release was characterized in astrocytes from rat cerebral cortex grown in normal and hyperosmotic culture conditions to investigate mechanisms of cell volume regulation and adaptation to states of altered osmolality. In high concentrations of taurine (1 mM), uptake was linear in both osmotic conditions for at least 30 min. The uptake rate in 1 mM taurine was not affected by exposure to hyperosmotic conditions. The mean +/- S.E.M. apparent binding constant for carrier-mediated taurine transport, Km, was not altered by hyperosmotic conditions (22.8 +/- 5.1 microM in iso-osmotic media, 21.3 +/- 11.9 microM in hyperosmotic media). However, maximal velocity of uptake, Vmax (mean +/- S.E.M.), of taurine was significantly lower in hyperosmotically treated astrocytes (0.175 +/- 0.035 nmol/mg protein.min) compared with the Vmax of iso-osmotically treated astrocytes (0.299 +/- 0.026 nmol/mg protein.min). The diffusional transport rate, Kdiff, was not affected by growth in hyperosmotic conditions (0.221 +/- 0.033 microliter/mg protein.min in iso-osmotic media, 0.295 +/- 0.043 microliter/mg protein.min in hyperosmotic media). Taurine release rate, expressed as a percent of the total cell content, was not affected by hyperosmotic exposure. However, astrocytes grown in hyperosmotic conditions contain nearly 60% more taurine than control cells. Thus, the absolute rate of taurine release (mean +/- S.E.M.) was significantly larger (P < 0.05) in hyperosmotic cells (0.1592 +/- 0.0082 nmol/mg protein.min) compared with control cells (0.0943 +/- 0.0096 nmol/mg protein.min). Quantitative analysis of these data indicate that maintenance of elevated taurine contents by cultured cerebral astrocytes exposed to hyperosmotic conditions is not due to alterations in rates of transport.

Astrocyte volume regulation and ATP and phosphocreatine concentrations after exposure to salicylate, ammonium, and fatty acids

Cellular volume regulation following swelling in hypo-osmotic phosphate-buffered saline (PBS) and ATP and phosphocreatine concentrations of cells incubated in iso-osmotic or hypo-osmotic PBS were measured in primary cultured rat cerebral astrocytes exposed for 30 min to NH4Cl, salicylate, hexanoate, octanoate, and/or dodecanoate. These compounds have been implicated in the pathogenesis of cerebral edema in Reye's Syndrome. NH4Cl (0.10 - 10 mM) had no effect on astrocyte volume regulation or ATP concentration. Salicylate significantly reduced ATP concentrations at 3.0 mM and 10 mM but had no effect on volume regulation. Hexanoate (10 mM and 30 mM) decreased astrocyte ATP content by over 80% while octanoate (10 mM) reduced ATP content by more than 50%. Concentrations of these fatty acids at or below 3.0 mM had no effect on ATP content. Volume regulation was inhibited by 3.0 mM hexanoate and 3.0 mM octanoate but not lower concentrations. Dodecanoate (0.1-3.0 mM) decreased cellular ATP content by 33-51% in iso-osmotic PBS solutions. Phosphocreatine content was reduced by exposure to salicylate or octanoate at concentrations which had no effect on ATP content. These results indicate that astrocyte energy metabolism and volume regulation may be compromised by agents associated with cerebral edema in Reye's Syndrome. Analysis of the dose-dependence of these effects suggests that inhibition of astrocyte energy metabolism is not sufficient to affect volume regulation.

Astrocyte glutamate uptake during chemical hypoxia in vitro

Glutamate uptake was measured in primary rat cortical astrocyte cultures exposed to sodium azide, 2,4-dinitrophenol, or antimycin A to assess the ability of astrocytes to function under hypoxic conditions. Uptake was maintained at 54-63% of control values despite maximal inhibition of oxidative ATP production. In contrast, the glycolytic inhibitor sodium fluoride (20 mM) reduced glutamate uptake by more than 95% when glucose was the only available substrate. These data suggest that glutamate uptake is largely maintained during hypoxia provided glucose remains available. Astrocyte glutamate uptake may aid neuronal survival during conditions such as incomplete ischemia where oxygen but not glucose is depleted.

Amino acid content of rat cerebral astrocytes adapted to hyperosmotic medium in vitro

Rat cerebral astrocytes from confluent primary cultures were grown for two weeks in medium made hyperosmotic with additional NaCl. At the time the cells were harvested (four weeks in culture), the medium osmolality of experimental cultures was approximately 600 mOsm. Amino acid, protein, and potassium contents and the cell volume were measured. Compared to cells maintained in control medium (approximately 300 mOsm), cells grown in hyperosmotic conditions had over two times the content of taurine and five times the content of glutamine. Alanine, aspartate, glutamate, glycine, and tyrosine contents also were elevated in these hyperosmotic-treated cells, while asparagine contents were unchanged relative to control cells. Cell volume and potassium content were decreased to approximately 50% of control levels by the hyperosmotic treatment while total protein content per cell was unchanged relative to cells from control cultures. Seven min after hyperosmotic-exposed cells were rapidly diluted into PBS with osmolality equal to about 330 mOsm, cell contents of alanine, asparagine, glutamine, glutamate, glycine, taurine, and tyrosine fell toward control levels. The data indicate that significant alterations in intracellular osmolytes occur in astrocytes adapted to hyperosmotic conditions. We suggest that a loss of intracellular potassium is at least partially compensated by accumulation of taurine, glutamine, and perhaps other amino acids acting as intracellular osmolytes.

Effect of ferric nitrilotriacetate on predominantly cortical glial cell cultures

Cultured glial cells were exposed to ferric nitrilotriacetate (Fe-NTA) at varying concentrations. Studies of the exposed glial cells were performed at days 29 and 36 post-conceptional age (culture days 8 and 15). In addition to morphologic studies, biochemical assays including [3H]-flunitrazepam (FLU) specific binding, Ro5-4864-displaceable 3H-FLU binding, and protein determinations were performed. At day 29 post-conceptional age, significant decreases in 3H-FLU specific binding, Ro5-4864-displaceable 3H-FLU binding, and protein determinations were discernible only in the presence of 100 microM Fe-NTA. At day 36 post-conceptional age 3H-FLU specific binding was significantly decreased at 20, 60, and 100 microM Fe-NTA concentrations, while Ro5-4864-displaceable 3H-FLU binding and protein determinations were significantly reduced at 60 and 100 microM Fe-NTA concentrations. The effects of Fe-NTA exposure appear to be both concentration and duration-of-exposure related. When compared to previously reported neuronal cell culture studies utilizing 3H-FLU specific binding, Ro5-4864-displaceable 3H-FLU binding, and protein determinations, glial cells appear to be significantly more resistant to chelated iron exposure.

Control of astrocyte volume by intracellular and extracellular Ca2+

Astrocytes from primary culture were exposed to conditions that affect intracellular and extracellular Ca2+ concentrations. Astrocyte cell volume was increased approximately 16% after a 30 min exposure to isoosmotic phosphate-buffered saline (PBS) containing the Ca2+ buffer EDTA. Cell volume returned to control values within 30 min of resuspension in normal PBS. Cellular calcium content was not affected by these treatments; however, the recovery of normal cell volume following EDTA exposure was inhibited by 0.1-1.0 mM quinine HCl in a dose-dependent fashion suggesting that a potassium channel controlled by the intracellular Ca2+ concentration is important in this volume response. Intracellular accumulation of an exogenous Ca2+ buffer, BAPTA, also produced cell swelling that persisted following resuspension in normal PBS. Lowering the extracellular Ca2+ concentration with EDTA enhanced the swelling of BAPTA-loaded cells. These data suggest that conditions leading to a decrease in free intracellular Ca2+ concentration may influence astrocyte volume by a mechanism similar to that described in other cell types.

Octanoic acid inhibits astrocyte volume control: implications for cerebral edema in Reye's syndrome

Octanoic acid has been implicated in the pathogenesis of cytotoxic cerebral edema in Reye's syndrome. Using astrocytes from primary culture, we studied the dose-dependent effects of octanoate on cellular volume regulation and metabolism. Astrocyte volume recovery following hypoosmotic swelling was stimulated by 1.0 mM octanoate and inhibited by 3.0 mM octanoate. Parallel effects were obtained at these concentrations on the activity of the Na+,K+-dependent ATPase. Cellular ATP concentrations also were reduced 36% with the higher octanoate concentration. These effects of octanoate may contribute to the severe astrocyte swelling observed in the brains of Reye's syndrome patients.

Reduction of dye coupling in glial cultures by microinjection of antibodies against the liver gap junction polypeptide

Intracellular injection of antibodies to the 27-kDa liver gap junction polypeptide have been shown previously to uncouple pairs of cultured mammalian hepatocytes, cardiac myocytes, and ganglionic neurons (Proc. Natl. Acad. Sci. U.S.A., 82 (1985) 2412-2416). In confluent primary cultures of astrocytes, similar injections significantly reduced dye coupling for cells closer than 80 micron to the injected glial cell. Western blots identified a 27-kDa protein in extracts of the astrocyte cultures that cross-react with the gap junction-specific antibodies. These results suggest that homologous gap junction polypeptides exist in liver and glial cells.

Cell cycle-specific requirement for mevalonate, but not for cholesterol, for DNA synthesis in glial primary cultures

The requirement for the sterol biosynthetic pathway for the occurrence of DNA synthesis in glial cells and, in particular, the relative roles of cholesterol and of mevalonate have been studied. Primary cultures of developing glial cells were synchronized by reducing the content of fetal calf serum (FCS) in the culture medium from 10% to 0.1% (vol/vol) for 48 h between days 4 and 6 in culture. Reversal of the resulting quiescent state by the return of the cultures to 10% serum caused after 24 h a marked increase in DNA synthesis, and this increase was prevented by the simultaneous addition of mevinolin, a specific inhibitor of the sterol biosynthetic pathway at the 3-hydroxy-3-methylglutaryl coenzyme A reductase step, at the time of serum repletion. A dose-dependent reversal of the mevinolin inhibition of DNA synthesis occurred with simultaneous addition of mevalonate to the culture medium. The induction of DNA synthesis by serum repletion, its inhibition by mevinolin, and the reversal of the inhibition by mevalonate were unaffected by a 95% reduction in exogenous cholesterol produced by utilization of lipoprotein-poor serum (LPPS) rather than FCS. Similarly, return of quiescent cultures to 10% LPPS containing mevinolin and sufficient low-density lipoprotein (LDL) to raise the cholesterol concentration 80-fold failed to restore DNA synthesis. In addition, reversal of the mevinolin inhibition of DNA synthesis by mevalonate occurred despite the continuous presence of mevinolin if mevalonate was added as late as 12 h after serum repletion, but not if added after 16 h or more.(ABSTRACT TRUNCATED AT 250 WORDS)

Lead toxicity in primary cultured cerebral astrocytes and cerebellar granular neurons

Neurons are more sensitive than astrocytes to lead toxicity in vivo. In order to understand the bases for the differences in brain cell responses to lead, the effects of lead acetate on cell morphology and on aerobic energy metabolism were studied in rat primary cultured neurons and astrocytes. By transmission electron microscopy, neuronal cell damage was seen with exposure to lead concentrations which were much lower than those required for similar changes in the astrocyte. As previously described in our studies of in vivo lead exposure, astrocytes in primary culture concentrated lead in nuclear, cytoplasmic, and lysosomal inclusions while neurons showed lead densities only in lysosomes. With acute lead exposures, inhibition of maximal respiratory capacity was greater and occurred at lower lead concentrations in neurons than in astrocytes. Similarly, respiratory rates were inhibited at lower lead concentrations in cerebral cortical slices from 8-day-old rat pups compared to those from adults. We conclude that primary cultured brain cells are appropriate in vitro systems for studying the in vivo cellular responses to lead. As in vivo, neurons are more sensitive than astrocytes to lead toxicity. In both cells, inhibition of aerobic energy metabolism appears to be closely associated with cell damage. The capacity of the astrocyte to sequester lead in nonmitochondrial intracellular sites may be critical in resistance to lead toxicity in vitro and in the mature brain.

Transferrin binding by mammalian cortical cells

Predominantly neuronal (neuronal) or non-neuronal (glial) cerebral cortical cell cultures were employed to study the kinetics and changes with maturation of 125I-diferric-transferrin uptake. The diferric-transferrin association curve of neuronal cultures at 37 degrees C was nonphasic and indicated equilibrium at 90 minutes. Dissociation was completed by 70 minutes. Diferric-transferrin specific uptake (80% of total) in neuronal cells (evaluated at days 6, 9, 13, 16, and 23 in culture) increased with maturation. Scatchard transformation of the data revealed increasing Bmax from day 6 to day 16 in culture (1626 to 2740 fmoles/mg protein). However, the K uptake was statistically unchanged over time and equaled 48.7 +/- 13.9 nM (mean +/- SD). In contrast, association studies of glial cultures documented equilibrium by 45 minutes and dissociation by 40 minutes. The concentration curves for diferric-transferrin uptake in glial cells, evaluated at days 11, 15, and 18 in culture, revealed virtually identical uptake at the three ages studied, but the percent specific uptake (58%) was less than for neurons (88%). Scatchard transformation of the data revealed no statistical alteration of Bmax or K uptake from days 11 to 18 in culture. Bmax ranged from 595 to 751 fmol/mg protein; overall K uptake was 48.3 +/- 13.2 nM (mean +/- SD).

Energy-dependent volume regulation in primary cultured cerebral astrocytes

Cell volume regulation and energy metabolism were studied in primary cultured cerebral astrocytes during exposure to media of altered osmolarity. Cells suspended in medium containing 1/2 the normal concentration of NaCl (hypoosmotic) swell immediately to a volume 40-50% larger than cells suspended in isoosmotic medium. The cell volume in hypoosmotic medium then decreases over 30 min to a volume approximately 25% larger than cells in isoosmotic medium. In hyperosmotic medium (containing twice the normal concentration of NaCl), astrocytes shrink by 29%. Little volume change occurs following this initial shrinkage. Cells resuspended in isoosmotic medium after a 30 min incubation in hypoosmotic medium shrink immediately to a volume 10% less than the volume of cells incubated continuously in isoosmotic medium. Thus, the regulatory volume decrease (RVD) in hypoosmotic medium involves a net reduction of intracellular osmoles. The RVD is partially blocked by inhibitors of mitochondrial electron transport but is unaffected by an inhibitor of glycolysis or by an uncoupler of oxidative phosphorylation. Inhibition of RVD by these metabolic agents is correlated with decreased cellular ATP levels. Ouabain, added immediately after hypoosmotic induced swelling, completely inhibits RVD, but does not alter cell volume if added after RVD has taken place. Ouabain also inhibits cell respiration 27% more in hypoosmotic medium than in isoosmotic medium indicating that the (Na,K)-ATPase-coupled ion pump is more active in the hypoosmotic medium. These data suggest that the cell volume response of astrocytes in hypoosmotic medium involves the net movement of osmoles by a mechanism dependent on cellular energy and tightly coupled to the (Na,K)-ATPase ion pump. This process may be important in the energy-dependent osmoregulation in the brain, a critical role attributed to the astrocyte in vivo.

Effects of barbiturates on energy metabolism by cultured astrocytes and neurons in the presence of normal and elevated concentrations of potassium

Rates of consumption of oxygen and of the formation of CO2 from [U-14C]glucose were studied in primary cultures of either astrocytes or neurons of the cerebral cortex. An increase of the extracellular concentration of potassium from 5 to 55 mM caused an increase in uptake of oxygen and in the production of CO2 in the astrocytes but not in the neurons. Pentobarbital (0.25-1.0 mM) or phenobarbital (1 mM) abolished the stimulation of oxygen uptake and/or the production of CO2 produced by potassium in the astrocytes with less (CO2 production) or no (oxygen uptake) effect at a normal concentration of potassium. In the neurons, pentobarbital (0.1-1.0 mM) caused, in contrast, a moderate inhibition of the production of CO2 and uptake of oxygen which was at least as pronounced at the small concentration of potassium. These results suggest that the pronounced inhibition of the stimulation of uptake of oxygen induced by potassium in brain slices is exerted on astrocytes, whereas the more modest decrease in uptake of oxygen at a small concentration of potassium is a neuronal phenomenon.

Obligatory relationship between the sterol biosynthetic pathway and DNA synthesis and cellular proliferation in glial primary cultures

Primary cultures of newborn rat brain, which are composed predominantly of astroglia, were used to examine the relationship between the sterol biosynthetic pathway and DNA synthesis and cellular proliferation. Reduction of the fetal calf serum content of the culture medium from 10 to 0.1% (vol/vol) for an interval of 48 h between days 4 and 6 in culture resulted in a quiescent state characterized by inhibition of DNA synthesis and cellular proliferation. When 10% fetal calf serum was returned to the medium for these quiescent cells, within 24 h DNA synthesis increased markedly. Preceding the rise in DNA synthesis was an increase in sterol synthesis, which occurred within 12 h of the return of the quiescent cells to the 10% fetal calf serum. Exposure of the quiescent cells to mevinolin, a specific inhibitor of sterol synthesis at the 3-hydroxy-3-methylglutaryl-CoA reductase step, completely inhibited the increase in DNA synthesis that followed serum repletion. The increase in total protein synthesis that followed serum repletion was not similarly inhibited by mevinolin. When mevinolin was removed after causing the 24-h inhibition of DNA synthesis, the cultured cells underwent active DNA synthesis and proliferation. Thus, inhibition of the sterol biosynthetic pathway resulted in a specific and reversible inhibition of DNA synthesis and glial proliferation in developing glial cells. These findings establish a valuable system for the examination of glial proliferation, i.e., primary glial cultures subjected to serum depletion and subsequent repletion. Moreover, the data establish an obligatory relationship between the sterol biosynthetic pathway and DNA synthesis and cellular proliferation in developing glia.

Chloroquine reduces neuronal and glial iron uptake

The effect of chloroquine, a lysosomotropic agent, on iron uptake into neuronal and glial cell cultures is reported. Chloroquine significantly inhibited iron uptake in both neuronal and glial cells. These findings suggest that iron transport into both neuronal and glial cells is mediated by the transferrin-iron complex.

Oxidative metabolism in cultured astroglial cells from rat brain

Growth, morphology, glutamine synthetase activity, cytochrome C oxidase activity and respiratory activity of rat brain cultures enriched in astrocytes were studied during four weeks in culture. Two different polarographic methods were used for measurement of respiratory activity: one newly developed perfusion method leaving the cellular monolayer morphologically intact and still attached to the culture dish, and one traditional stirring method involving the removal of cells from the culture vessel. Regardless of the method used, a stable respiratory activity was registrated throughout the four weeks of culturing. Also the cytochrome C oxidase activity remained unchanged. In perfusion all absolute values for respiration were found to be higher than those obtained with the stirring method. The use of the stirring technique resulted in a doubling of oxygen consumption upon succinate addition. No such effect was seen in perfusion. It can thus be concluded that the removal of cultured astrocytic cells from their substratum alters their respiratory activity and their response to added substrates.

Iron uptake by glial cells

Dynamic studies of iron metabolism in brain are generally unavailable despite the fact that a number of neurologic conditions are associated with excessive accumulation of iron in central nervous tissue. Cortical non-neuronal (glial) cultures were prepared from fetal mouse brain. After 13 days the cultures were exposed to radiolabeled iron. Brisk and linear total iron uptake and ferritin iron uptake occurred over 4 hours. When methylamine or ammonium chloride was added, (both known inhibitors of transferrin iron release because of their lysosomotropic properties), total iron uptake was diminished. Further studies indicated that methylamine inhibits glial cell ferritin iron incorporation. Glial cell iron transport is similar to previously reported neuronal cell iron transport (1) but glial cell iron uptake proceeds at a faster rate and is more susceptible to the inhibition of certain lysosomotropic agents. The data reinforces the likelihood that iron uptake by nervous tissues is transferrin-mediated.

Use of a perfusion technique for measurements of respiratory activity in cultured cells

A method for measuring respiratory activity in anchorage-dependent cultured cells has been developed. This method is based on a technique that permits the perfusion of standard plastic culture dishes with attached cells. Basal respiratory activities were studied in two continuous cell lines of neural origin, neuroblastoma C1300 clone 41A3 and glioma 138MG. As compared to traditional measurements on detached cells, a fourfold increase in value was obtained. Investigations on membrane permeability suggested that the observed difference could be attributed to alterations in cell membrane integrity. Pretreatment with dibutyryl cyclic AMP, known to induce a morphological and biochemical differentiation in C1300 and 138MG cells, caused in both cell lines an enhanced respiration.

Effect of "flow anoxia" and "non flow anoxia" on the NAD/NADH redox state of the intact brain cortex of the cat

In the present study, we compared the nicotinamide adenine dinucleotide (NAD) reducing potencies of "flow anoxia" and "non flow anoxia" in the cat brain cortex. In animals anaesthetized with alpha D-glucochloralose "flow anoxia" and "non flow anoxia" were produced by ventilating for 2 and 25 min, respectively, with nitrogen gas. Following "non flow anoxia" the brain cortices of dead animals were superfused with oxygen saturated artificial cerebrospinal fluid (mock CSF), and subsequently with CSF containing various concentrations (10(-3 -10 -1) M) of potassium cyanide. NADH (reduced NAD) fluorescence of the brain cortex was measured through a cranial window with a microscope fluororeflectometer. Ventilating the animals for 2 and 25 min with nitrogen gas increased cortical NADH fluorescence (NAD reduction) by 43.5 +/- 2.8% and 135.3 +/- 6.1%, respectively. Oxygen saturated CSF superfusion of the ischemic brain cortex restored the cortical NAD/NADH redox state to the preanoxic level (oxidation of NADH). 10(-1) M cyanide, applied after superfusion of the brain cortex with oxygen saturated CSF resulted in comparable NAD reduction to that produced by "non flow anoxia". On the basis of these findings it is suggested that "non flow anoxia" leads to much greater cortical NAD reduction than "flow anoxia", because oxygen tension in the cortex may not fall to zero mm Hg during nitrogen anoxia lasting for 2 min. Besides this, a more pronounced substrate mobilization and acidosis may also contribute to the greater NAD reducing potency of "now flow anoxia".(ABSTRACT TRUNCATED AT 250 WORDS)

Dibutyryl cyclic AMP causes intermediate filament accumulation and actin reorganization in astrocytes

We have examined the effects of dibutyryl-cyclic AMP (dBcAMP) on the organization and expression of filamentous proteins in astroglia. The drug produced several effects on astrocytes grown in primary cultures. Cultures ceased to grow, and cells changed shape to a contracted form, displaying thin cytoplasmic processes. Cellular levels of the intermediate filament (IF) proteins, vimentin and glial fibrillary acidic protein (GFAP), and actin, insoluble in Triton X-100, were examined by polyacrylamide gel electrophoretic analysis. The cellular content of both of the IF proteins increased concurrently, approximately doubling during a 2-week course of treatment. The content of actin associated with the Triton residue decreased, however, a biochemical alteration which correlated with a loss of stress fibers in treated cells. Treatment with sodium butyrate did not change either cell shape or cytoskeletal protein content. Filament protein expression in astrocytes can, therefore, be modulated via cAMP-dependent mechanisms. The effects do not, however, appear specific for the GFAP-type of intermediate filament.

Respiration in primary cultured cerebellar granule neurons and cerebral cortical neurons

Respiration was measured polarographically in primary cultures enriched with cerebellar granule neurons or cerebral cortical neurons. The basal respiratory rate, measured on the sixth day after culturing, was 12.00 natom equiv. O/mg protein/min for the cortical neurons and 12.70 natom equiv. O/mg protein/min for the granule neurons. Maximal stimulation by 2,4-dinitrophenol produced a 20-40% increase over the basal rate for both neuronal types. Oligomycin inhibited neuronal basal respiration by 45%. These respiratory rates in neurons from primary culture are markedly lower than those measured in astrocytes grown under similar conditions.

Synthesis and turnover of cytoskeletal proteins in cultured astrocytes

We previously reported that the cytoskeleton of rat astrocytes in primary culture contains vimentin, glial fibrillary acidic protein (GFAP), and actin. These proteins were found in a fraction insoluble in Triton X-100 and thought to be assembled in filamentous structures. We now used primary astrocyte cultures to study the kinetics of synthesis and turnover of these cytoskeletal proteins. The intermediate filament proteins were among the most actively synthesized by astrocytes. High levels of synthesis were detectable by the third day of culture in the early log phase of growth, and the pattern of labeling at day 3 was similar to that at 14 days when the cultures had reached confluency. In short-term incorporation experiments vimentin, GFAP, and actin in the Triton-insoluble fraction were labeled within 5 min after exposure of the cultures to radioactive leucine. We did not detect any saturation of labeling for up to 6 h of incubation. The turnover of filament proteins studied by following the decay of radioactivity from prelabeled vimentin, GFAP, and cytoskeletal actin displayed biphasic decay kinetics for all three proteins. In the initial phase a fast-decaying pool with a half-life of 12-18 h contributed about 40% of the total activity in each protein. A major portion, about 60%, of each protein, however, decayed much more slowly, exhibiting a half-life of about 8 days.

Respiration and cell volume of primary cultured cerebral astrocytes in media of various osmolarities

Respiration and cell volume of cerebral astrocytes from primary culture were measured in media of various osmolarities. Respiration was measured in 3T3 fibroblasts under similar conditions. Uncoupled respiration and respiration independent of oxidative phosphorylation were obtained by adding dinitrophenol and oligomycin, respectively. In NaCl media, dinitrophenol-stimulated respiration was inhibited at low and high osmolarities. With increasing osmolarity from 363 to 1185 mOsm, oligomycin-insensitive respiration increased and became the predominant respiratory component. The same respiratory changes in response to altered osmolarity were observed in 3T3 fibroblasts. In astrocytes, the oligomycin-insensitive respiration also increased in hyperosmolar sodium acetate media but was unchanged with increasing osmolarity in choline chloride or sucrose media. The increase in oligomycin-insensitive respiration in hyperosmolar NaCl media was blocked by amiloride, an inhibitor of passive Na+ movement. In contrast to amiloride, ouabain, an inhibitor of Na+, K+-ATPase, inhibited a constant amount of respiration with increasing NaCl concentration. The relationship of astrocyte volume to osmolarity was the same in hyper-osmolar media containing NaCl or sucrose. Cell volumes were greater in hypo-osmolar NaCl than in sucrose media. Our results suggest the presence of a Na+-dependent respiratory component in primary cultured cerebral astrocytes in media of increased osmolarity. This respiratory component is not coupled to oxidative phosphorylation or a Na+-K+-ATPase. It may be important in the proposed physiologic role of the astrocyte in maintaining brain extracellular water content and electrolyte concentrations.

In vitro cellular respiration at elevated temperatures in developing rat cerebral cortex

Cellular respiration in vitro was studied in cerebral cortical tissue from rats 2-60 days of age. Respiration was measured polarographically over the temperature range 34-44 degrees C in tissue slices in a basal condition; maximally stimulated by an uncoupler of oxidative phosphorylation, dinitrophenol; and inhibited by a blocker of mitochondrial oxidative phosphorylation, oligomycin. Basal respiration at 34 degrees C increased about 80% between 7 and 30 days of age. Oligomycin-insensitive respiration did not change with age. Dinitrophenol-stimulated respiration was unchanged from 2 to 10 days and then increased over 100% between 10 and 15 days of age. The Q10 for dinitrophenol-stimulated respiration increased from a value of 1 in tissue from rats 2-10 days of age to about 2 in tissue from rats 15 days and older. Our results confirm the previously reported maturational increases in basal respiration and in respiratory capacity in rat cerebral cortical tissue. The maturational increase in maximal respiratory capacity occurs in a short age interval coincident with a marked increase in the Q10 for the hyperthermic temperature range. Both these properties may be important in the increasing resistance to hyperthermia-induced seizures and their functional sequelae in the rat pup.

Factors influencing the growth and respiration of rat cerebral astrocytes in primary culture

Cell densities and respiratory rates of astrocytes from neonatal rat brain grown in primary culture were determined after 20-30 days in vitro. Cells grown in flasks reached lower densities (microgram DNA/cm2) and higher protein:DNA ratios than cells grown in petri dishes. Respiratory rates were lower for cells grown in flasks compared to cells grown in dishes. The pH of the medium in flasks fell below 6.9 between feedings while the pH of the medium in dishes remained at about 7.2. Cells grown in dishes with the medium pH adjusted to 6.8 also showed lower final cell densities, higher protein:DNA ratios, and lower respiratory rates, compared to cells grown under similar conditions at pH 7.5. Intermediate values of each parameter were found in cells grown at pH 7.5 for one week and then at 6.8 for 20 days. We conclude that the effects of ambient pH account for the differences in growth characteristics and respiratory rates of astrocytes grown in dishes versus those grown in flasks.