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

Taurine Supplementation as a Neuroprotective Strategy upon Brain Dysfunction in Metabolic Syndrome and Diabetes

Obesity, type 2 diabetes, and their associated comorbidities impact brain metabolism and function and constitute risk factors for cognitive impairment. Alterations to taurine homeostasis can impact a number of biological processes, such as osmolarity control, calcium homeostasis, and inhibitory neurotransmission, and have been reported in both metabolic and neurodegenerative disorders. Models of neurodegenerative disorders show reduced brain taurine concentrations. On the other hand, models of insulin-dependent diabetes, insulin resistance, and diet-induced obesity display taurine accumulation in the hippocampus. Given the possible cytoprotective actions of taurine, such cerebral accumulation of taurine might constitute a compensatory mechanism that attempts to prevent neurodegeneration. The present article provides an overview of brain taurine homeostasis and reviews the mechanisms by which taurine can afford neuroprotection in individuals with obesity and diabetes. We conclude that further research is needed for understanding taurine homeostasis in metabolic disorders with an impact on brain function.

Role of hypertonic saline and mannitol in the management of raised intracranial pressure in children: A randomized comparative study

OBJECTIVE

To compare the efficacy and side effects of 3% hypertonic saline and mannitol in the management of raised intracranial pressure in children.

DESIGN

Prospective randomized study.

SETTING

Pediatric intensive care unit (PICU) in a tertiary care hospital.

SUBJECT

200 patients with raised intracranial pressure.

MATERIALS AND METHODS

Patients were randomized into two statistically comparable groups; Group A (n = 98) was treated with mannitol while Group B (n = 100) was treated with 3% hypertonic saline. Group C (n = 2) included those members of Group A in whom serum osmolality ≥320 mosmol/kg and were then treated with 3% hypertonic saline. Both Drugs were given at a loading dose of 5 ml/kg stat followed by 2 ml/kg in every 6 h(both have same osmolarity) for two days in their respective groups. Besides monitoring, blood pressure (NIBP), mean arterial pressure (pre and post 30 min of drug), serum sodium, chloride and osmolality were measured. Intracranial pressure was assessed indirectly by measuring mean arterial ressure "MAP". Student paired 't' test was applied.

RESULTS

Decrease in MAP was highly significant (P<0.001) at 0 h in males 0,6 h in females, and moderately significant at 12,36 h in females and significant(P<0.05) at 6,24,42 h in males of Group B. Decrease in coma hours was a highly significant finding (P<0.001) in Group B. In Group B, serum sodium and chloride increased significantly but remained within acceptable limits. There was no difference in osmolality and mortality (fisher Z).

CONCLUSION

Mannitol has several side effects, 3% hypertonic saline is a safe and effective alternative in managing cerebral edema.

The relationship between oxygen and adenosine in astrocytic cultures

Brain tissue oxygenation affects cerebral function and blood flow (CBF). Adenosine (Ado), a purine nucleoside, moderates neuronal activity, and arterial diameter. The cellular source of Ado in brain remains elusive; however, astrocytes are a logical site of production. Using astrocytic cultures, we tested the hypothesis that astrocytic derived Ado reflects cerebral oxygenation. We found that during alterations in pO(2), extracellular levels of Ado [Ado](e) changed rapidly. Graded reductions of oxygen tension revealed that[Ado](e) reached 10(-7) M to 10(-6) M with a pO(2) of 30-10mmHg, comparable with [Ado](e) and oxygen levels found in brain tissue during normoxemia. Higher O(2) levels were associated with a depression of [Ado](e). Under conditions of low pO(2) (pO(2) <or= 3 mmHg), inhibition of extracellular catabolism of adenosine monophosphate (AMP) prevented an increase of [Ado](e) and resulted in a rise in [AMP](e). The rise in [AMP](e) preceded the increase in [Ado](e). In the presence of nucleoside transporter inhibitors, accumulation of [Ado](e) persisted. On the basis of our studies in culture we conclude that astrocytes are a significant source of Ado and that during hypoxia, the changes in [Ado](e) are in a range to affect both neuronal activity as well as CBF.

Taurine down-regulates basal and osmolarity-induced gene expression of its transporter, but not the gene expression of its biosynthetic enzymes, in astrocyte primary cultures

Taurine content of astrocytes is primarily regulated by transport from the extracellular medium and endogenous biosynthesis from cysteine. We have investigated the gene expression of the taurine transporter (TauT) and the taurine biosynthetic enzymes, cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD), in astrocyte primary cultures in relationship to cell taurine content. TauT, CDO, and CSD mRNA levels were determined through quantitative RT-PCR. Cell taurine content was depleted by adapting the cells to a taurine-free chemically defined medium and increased by incubating the cells in the same medium containing exogenous taurine. With increased cell taurine content the level of TauT mRNA decreased, whereas the levels of CDO and CSD mRNA remained unchanged. In astrocytes exposed to a hyperosmotic medium the TauT mRNA level increased, whereas the CDO and CSD mRNA levels were not significantly altered. The osmolarity-induced up-regulation of TauT mRNA expression was fully prevented by increasing cell taurine content. Thus, the gene expression of the taurine transporter, but not that of the taurine biosynthetic enzymes, appears to be under the control of two antagonistic regulations, namely, a taurine-induced down-regulation and an osmolarity-induced up-regulation.

Gene expression of taurine transporter and taurine biosynthetic enzymes in brain of rats with acute or chronic hyperosmotic plasma. A comparative study with gene expression of myo-inositol transporter, betaine transporter and sorbitol biosynthetic enzyme

Cells exposed to hyperosmotic conditions maintain their volume by accumulating organic osmolytes. Taurine is considered as an osmolyte in brain cells. Accumulation of other osmolytes (sorbitol, myo-inositol and betaine), was shown in renal cells to result from an upregulation of the expression of the genes regulating osmolyte cell content. We have investigated the gene expression of the taurine transporter (TauT) and of the taurine biosynthetic enzymes, cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD) by measuring their mRNA levels in brain of salt-loaded rats. mRNA levels of genes previously identified as osmosensitive, namely aldose reductase (AR), myo-inositol transporter (SMIT) and betaine transporter (BGT1) were also determined. In whole brain, TauT-, SMIT- and BGT1-mRNA levels were significantly increased following acute salt-loading but SMIT-mRNA levels only remained elevated following chronic salt-loading while CDO-, CSD- and AR-mRNA levels remained unchanged in both conditions. Following acute salt-loading, mRNA levels of TauT, CDO, CSD, SMIT, BGT1 and AR were increased in cerebral cortex while SMIT- and BGT1-mRNA levels only were increased in striatum and habenula.TauT, CDO and CSD genes may be upregulated in brain of salt-loaded rats but the upregulation of the TauT gene appears more widespread. TauT, CDO and CSD are thus putative osmosensitive genes. However the actual pattern (amplitude, time course and regional occurrence) of the upregulation of each of the putative (TauT, CDO and CSD) and established (AR, SMIT and BGT1) osmosensitive genes differs markedly. This indicates that there exist other factors in brain cells which can selectively prevent the upregulation of these genes by hyperosmolarity.

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 efflux and intracellular pH during astrocyte volume regulation

Cytotoxic cerebral edema is characterized by enlarged astroglial cells. In tissue culture, osmotically swollen astrocytes return toward normal volume over a period of 15-30 min in a process termed regulatory volume decrease (RVD). RVD is due, in part, to net efflux of taurine and other amino acids. Our objective in these studies was to examine changes in astrocyte intracellular pH (pHi) which may be related to taurine loss during RVD. We hypothesized net efflux of anionic taurine abandons a proton inside the cell, thus lowering pHi. Primary cultures of cerebral astrocytes were prepared from neonatal rats pups and grown on glass coverslips. Confluent cells were loaded at 37 degrees C with the fluorescent pH indicator BCECF. Fluorescence intensity ratios for excitation wavelengths of 440 nm and 494 nm (530 nm emission) were computed every 2 sec. Intensity ratios were calibrated to pHi at the end of each experiment using 140 mM KCl plus 8.6 microM nigericin at pH 7.4. pHi was measured in isoosmotic Hepes-buffered saline (290 mOsm) and then in hypoosmotic Hepes-buffered saline (200 mOsm) in the presence of 0.5 mM amiloride. Some solutions also contained 150 microM niflumic acid (NA). Cellular taurine content was determined in parallel studies using HPLC. Changes in pHi were compared between groups using Student's t-test with Bonferroni correction. Significance was assumed if p < 0.05. In isoosmotic saline, mean +/- SEM pHi was 7.58 +/- 0.04 and decreased to 7.35 +/- 0.09 after adding amiloride. Hypoosmotic exposure caused a further drop in pHi of 0.29 +/- 0.03 within 15 min. Recovery of pHi in isoosmotic saline was amiloride-sensitive. Subsequent hypoosmotic exposure after recovery in isoosmotic saline produced a change in pHi which was 81 +/- 9% of the change measured during the initial hypoosmotic exposure. Taurine content decreased from 147 +/- 6 nmol/(mg protein) to 116 +/- 7 nmol(mg protein) during the 15 min hypoosmotic exposure in 0.5 mM amiloride. NA significantly reduced the hypoosmotically induced change in pHi to 0.17 +/- 0.02 while completely blocking taurine loss. Assuming an intracellular buffering power of 13 mM, the NA-sensitive intracellular acidification of cells during hypoosmotic exposure in the presence of 0.5 mM amiloride corresponds to 1.6 mequiv/l additional intracellular H+. This increase in intracellular H+ content is equivalent to approximately 32% of the NA-sensitive loss of taurine. The association of changes in pHi with taurine efflux is supported by these data; however, efflux of other weak acids may contribute to intracellular acidification during astrocyte RVD and a significant portion of taurine must leave the cell with a proton.

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.

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.

Hypoosmotic volume regulation and osmolyte transport in astrocytes is blocked by an anion transport inhibitor, L-644,711

Cell volume, potassium content, and potassium influx were measured in rat cerebral astrocytes grown in primary culture following exposure to hypoosmotic medium containing either 3.2 mM or 50 mM potassium. Some solutions also contained 1 mM L-644,711, an anion transport inhibitor. L-644,711 inhibited volume regulation and taurine efflux induced by hypoosmotic exposure in medium containing either potassium concentration. L-644,711 also inhibited potassium uptake associated and not associated with the sodium/potassium pump. The correlation of reduced taurine efflux and volume decrease produced by L-644,711 exposure indicates the important role for this amino acid in hypoosmotic astrocyte volume regulation. However, the effects of L-644,711 on potassium transport indicate that multiple actions of this drug may be important factors in its effect on astrocyte volume regulatory mechanisms.

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.

Taurine efflux and cell volume regulation in cerebral cortical slices during chronic hypernatraemia

Efflux of cellular taurine from pre-loaded cerebral cortical slices incubated in hypo- and hyperosmotic media has been studied in normal and chronically hypernatraemic rats. Significant differences in transport mechanisms between the two groups has been noted. Hyperosmotic media retard efflux in cells from normal animals, with associated cell shrinkage, but accelerate efflux in cells from hypernatraemic rats, in which cell volumes are well maintained at pre-hypernatraemic levels. In hypernatraemic rats an anionic component of taurine efflux, present in normal animals, is lacking. Conversely, a distinct, calmodulin-dependent component which in normal rats is stimulated only in hypo-osmotic media, is present in both hypo- and hyperosmotically incubated slices from hypernatraemic rats, and inhibition of calmodulin-activation leads to cell swelling. This altered pattern of efflux and cell volume-regulation persists for at least 5 h following recovery from hypernatraemia, but remits by 30 h, indicating slow down-regulation of the hypernatraemically activated calmodulin-dependent efflux pathway and re-expression of anionic taurine transport.

Osmoregulatory changes in myo-inositol content and Na+/myo-inositol cotransport in rat cortical astrocytes

Exposure of cortical astrocytes to 325, 350, or 390 mosM culture media for 48 h caused a 1.4-, 2.1-, and 3.5-fold increase, respectively, in cellular content of the compatible osmolyte myo-inositol. Elevated myo-inositol levels accounted for approximately 56-100% of the solute needed by the cells for complete volume regulation under hypertonic conditions. Myo-inositol accumulation was associated with 4-5-fold (peak rate) and 1.8-2-fold (steady-state rate) increases in the rate of Na(+)-dependent myo-inositol uptake when cells were acclimated to 390 mosM culture medium for 12 h or 24-96 h, respectively. When medium osmolality was elevated by 25 mosM, peak and steady-state increases in myo-inositol uptake of 1.7-fold and 1.3-fold, respectively, were observed. Exposure to 390 mosM medium for 12-48 h induced a 3-8-fold increase in cotransporter mRNA levels suggesting that the increase in myo-inositol uptake is brought about by increased cotransporter gene expression. Abrupt return of hypertonic cells to an isotonic medium induced a rapid increase in myo-inositol efflux and a return of cotransporter mRNA to control values in < 2 h. In contrast, the cotransporter remained fully activated at hypertonic levels for 16 h. Between 16-24 h after the transfer, the rate of myo-inositol uptake returned to control values. The remarkable sensitivity of the cotransporter to hypertonic stress indicates that upregulation of myo-inositol transport in glial cells is likely to occur in a variety of disease states that cause an elevation of plasma osmolality. Slow downregulation of the cotransporter may be responsible in part for the slow loss of myo-inositol and cerebral edema that occurs with too rapid correction of chronic plasma hypertonicity.

Simple high-performance liquid chromatographic analysis of free primary amino acid concentrations in rat plasma and cisternal cerebrospinal fluid

The quantitation of 16 acidic, basic, small and large neutral amino acids was performed using 10-microliters sample aliquots of cisternal cerebrospinal fluid (CSF) and blood plasma of rats. The analytical technique is based upon a two-buffer HPLC system with fluorimetric detection of pre-column derivatized primary amino acids with o-phthaldialdehyde (OPA). A modification of a well established method, the power of the present technique comes from an improved resolution and sensitivity by installing a column heater adjusted to 43 degrees C and strictly reducing any contamination by background amino acids. The analysis is simplified by separating the amino acid derivatives with a linear buffer gradient and less time consuming by the use of a short analytical column with a higher flow-rate. Analytical precision, linearity of response and reproducibility were highly acceptable at both CSF and plasma concentrations of amino acids without changing any of the separation or detection parameters.

Taurine efflux and the regulation of cell volume in incubated slices of rat cerebral cortex

The kinetics of efflux [3H]taurine have been examined in pre-loaded slices of rat cerebral cortex incubated in media of variable osmolality. Alterations in the rate of the slowest phase of efflux, considered to represent cellular loss, have been correlated with cell volumes, provisionally identified as the slice non-inulin space. Efflux was stimulated by reduction in medium osmolality, and impaired in hyperosmotic media; these variations were accompanied by moderate (non-osmometric) cell swelling and shrinkage, respectively. The rates of taurine efflux into media in which NaCl was partly replaced by sucrose, and measurement of the corresponding cell volumes, suggest that ionic strength, rather than osmolality or cell volume per se, may be a significant controlling factor. In both isosmolal and hyposmolal media efflux was significantly impaired by the anion transport inhibitor niflumic acid, with accompanying cell swelling, or by replacement of chloride by gluconate. In hyposmotic, but not isosmotic, media efflux was impaired, and cell volumes increased, in the presence of trifluoperazine or TMB-8, a reported blocker of intracellular calcium release, and the effects of niflumic acid and trifluoperazine on both variables were strongly additive. It is suggested that in both isosmotic and hyposmotic media taurine, efflux occurs by anionic transport, mainly through exchange with external chloride, whereas in hyposmotic media a second pathway is present, probably a volume-activated calmodulin-dependent channel.

Regulation of mammalian brain cell volume

Maintenance of brain cell volume is of crucial importance for normal central nervous system (CNS) function. This review considers volume regulation primarily in response to disturbances of body fluid osmolality. Brain cells counter the tendency to swell or shrink by appropriate adjustment of their internal osmotic potential. This is achieved by loss or uptake of inorganic ions and low molecular weight organic solutes (osmolytes). The latter comprise mainly amino acids, myoinositol, choline, and methylamines. Taurine may be of particular importance in volume control, especially in young animals. Brain cell volume regulation, however, is only one contributory factor to maintenance of constant brain volume (water content), and operates in parallel with important alterations in bulk fluid and electrolyte movement across the blood-brain barrier and between the interstitium and cerebrospinal fluid, which themselves moderate the requirement for transient alteration in cell volume during acute osmotic imbalance. Although altered cerebral content of inorganic ions and osmolytes are usually regarded as responses, respectively, to acute and chronic osmotic disturbances, osmolytes (especially taurine) may also participate in short-term cell volume regulation.

Brain osmolyte content and blood-brain barrier water permeability surface area product in osmotic edema

Brain edema was induced in adult rats by intraperitoneal injection of distilled water equivalent to 15% of the animal's body weight. Mean +/- SEM serum osmolality fell from 291 +/- 3 mOsm to 253 +/- 4 mOsm during the next hour while cerebral gray matter water content increased from 79.5 +/- 0.2% to 80.9 +/- 0.2%. Gray matter content of sodium, potassium, taurine, glycine, glutamine, and glutamate were unchanged. However, the blood-brain barrier permeability/surface area product for water decreased by 40%. This alteration in water permeability may represent a response to limit water influx during the first hour of hypoosmotic brain edema.

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.

Taurine release and swelling of cerebral cortex slices from adult and developing mice in media of different ionic compositions

The release of preloaded radiolabeled taurine was studied in superfused cerebral cortex slices obtained from adult and 3-day-old mice in media of varying ionic composition. Our aim was to establish whether the release of taurine from slices evokable by high concentrations of K+ could be attributed solely to cell volume changes or whether it results directly from depolarization of cell membranes. In both age groups hypoosmotic media enhanced the release of taurine. The enhancement was greater in 3-day-old than in adult mice. The K(+)-evoked release of taurine was likewise greater in slices from 3-day-old mice than in slices from adult mice. The K+ stimulation was totally preserved in adult mice and partially preserved in 3-day-old mice when the slices were superfused with Cl(-)-free media, with media in which the K+ x Cl- ionic product was kept constant and with hyperosmotic high-K+ media. The results were practically the same when the permeant anion acetate and the impermeant anion gluconate were used to replace the Cl- deficit. The unstimulated release of preloaded taurine was greatly enhanced in Cl(-)-free media in both age groups. There obtained no statistically significant correlation between the intracellular swelling of slices and the magnitude of taurine release under the present different experimental conditions in either age group. The results show that the K(+)-evoked release of taurine from superfused cerebral cortex slices cannot be solely attributed to depolarization-induced cell swelling. At least a part of the release results directly from membrane depolarization which besides exocytosis apparently also enhances the carrier-mediated release of taurine and inhibits the reuptake of taurine liberated from intracellular compartments.

Changes in taurine transport evoked by hyperosmolarity in cultured astrocytes

Cultured astrocytes grown chronically (1-3 days) in medium made hyperosmotic (450 mOsm) with NaCl or sucrose showed an increase in taurine concentration from 294 to 501 nmol/mg protein in NaCl and to 382 nmol/mg protein in sucrose. The effect of hyperosmolarity on taurine uptake and release was examined to investigate whether or not changes in these processes may account for the increase observed in cell taurine content. Hyperosmolarity significantly affected the two components of taurine uptake (i.e., the Na(+)-dependent and the diffusional component). The Vmax of the Na(+)-dependent, active transport increased 50%, whereas no change was observed in the Km. The diffusion coefficient was markedly decreased by hyperosmolarity, being 2.2 x 10(-4) and 6.6 x 10(-6) ml/min/mg protein in isosmotic and hyperosmotic conditions, respectively, indicating a blockade of the leak pathway. These changes in the active and passive components of taurine transport were opposite to those induced by hyperosmolarity. The effect of hyperosmolarity increasing cell taurine content was insensitive to cycloheximide and colchicine. The basal efflux of taurine from astrocytes also decreased in cells exposed to hyperosmotic medium, indicating that alterations in both influx and efflux of taurine are involved in the mechanism responsible for the increase in taurine levels induced by hyperosmolarity in astrocytes.

Astrocyte Heterogeneity: Endogenous Amino Acid Levels and Release Evoked by Non-N-Methyl-D-Aspartate Receptor Agonists and by Potassium-Induced Swelling in Type-1 and Type-2 Astrocytes

The aim of the present study was to determine whether endogenous amino acids are released from type-1 and type-2 astrocytes following non-N-methyl-D-aspartate (NMDA) receptor activation and whether such release is related to cell swelling. Amino acid levels and release were measured by HPLC in secondary cultures from neonatal rat cortex, highly enriched in type-1 or type-2 astrocytes. The following observations were made. (a) The endogenous level of several amino acids (glutamate, alanine, glutamine, asparagine, taurine, serine, and threonine) was substantially higher in type-1 than in type-2 astrocytes. (b) The spontaneous release of glutamine and taurine was higher in type-1 than in type-2 astrocytes; that of other amino acids was similar. (c) Exposure of type-2 astrocyte cultures to 50 microM kainate or quisqualate doubled the release of glutamate and caused a lower, but significant increase in that of aspartate, glycine, taurine, alanine, serine (only in the case of kainate), and glutamine (only in the case of quisqualate). These effects were reversed by the antagonist CNQX. (d) Exposure of type-1 astrocyte cultures to 50-200 microM kainate or 50 microM quisqualate did not affect endogenous amino acid release, even after treating the cultures with dibutyryl cyclic AMP. (e) Exposure of type-1 or type-2 astrocyte cultures to 50 mM KCl (replacing an equimolar concentration of NaCl) enhanced the release of taurine greater than glutamate greater than aspartate. The effect was somewhat more pronounced in type-2 than in type-1 astrocytes. Veratridine (50 microM) did not cause any increase in amino acid release. (f) The release of amino acids induced by high [K+] appeared to be related to cell swelling, in both type-1 and type-2 astrocytes. Swelling and K(+)-induced release were somewhat higher in type-2 than in type-1 astrocytes. In contrast, neither kainate nor quisqualate caused any appreciable increase in cell volume. It is concluded that non-NMDA receptor agonists stimulate the release of several endogenous amino acids (some of which are neuroactive) from type-2 but not from type-1 astrocytes. The effect does not seem to be related to cell swelling, which causes a different release profile in both type-1 and type-2 astrocytes. The absence of kainate- and quisqualate-evoked release in type-1 astrocytes suggests that the density of non-NMDA receptors in this cell type is very low.

Release of GABA and taurine from brain slices

In brain slices the mechanisms of release of GABA have been extensively studied, but those of taurine markedly less. The knowledge acquired from studies on GABA is, nevertheless, still fragmentary, not to speak of that obtained from the few studies on taurine, and firm conclusions are difficult, even impossible, to draw. This is mainly due to methodological matters, such as the diversity and pitfalls of the techniques applied. Brain slices are relatively easy to prepare and they represent a preparation that may most closely reflect relations prevailing in vivo, since the tissue structure and cellular integrity are largely preserved. In our opinion the most recommendable method at present is to superfuse freely floating agitated slices in continuously oxygenated medium. Taurine is metabolically rather inert in the brain, whereas the metabolism of GABA must be taken into account in all release studies. The use of inhibitors of GABA catabolism is discouraged, however, since a block in GABA metabolism may distort relations between different releasable pools of GABA in tissue. It is not known for sure how well, and homogeneously, incubation of slices with radioactive taurine labels the releasable pools but at least in the case of GABA there may prevail differences in the behavior of labeled and endogenous GABA. It is suggested therefore that the results obtained with radioactive GABA or taurine should be frequently checked and confirmed by analyzing the release of respective endogenous compounds. The spontaneous efflux of both GABA and taurine from brain slices is very slow. The magnitude of stimulation of GABA release by homoexchange is greater than that of taurine under the same experimental conditions. However, the release of both amino acids is generally enhanced by a great number of structural analogs, the most potent being those which are simultaneously the most potent inhibitors of uptake. This may result in part from inhibition of reuptake of amino acid molecules released from slices but the findings may also signify that the efflux of GABA and taurine is at least partially mediated by the membrane carriers operating in an outward direction. It is thus advisable not to interpret that stimulation of release in the presence of uptake inhibitors solely results from the block of reuptake of exocytotically released molecules, since changes in the carrier-mediated transport are also likely to occur upon stimulation. The electrical and K+ stimulation evoke the release of both GABA and taurine. The evoked release of GABA is several-fold greater than that of taurine in slices from the adult brain.(ABSTRACT TRUNCATED AT 400 WORDS)

HPLC-EC determination of free primary amino acid concentrations in cat cisternal cerebrospinal fluid

This paper describes an HPLC-EC method for measuring the concentrations of 9 free primary amino acids in cerebrospinal fluid (CSF) withdrawn from the cisterna magna of Nembutal-anesthetized adult cats. Amino acid derivatives were formed with o-phthalaldehyde and beta-mercaptoethanol; subsequently, excess thiol reagent was removed with iodoacetamide. During elution through a C18 5-micron column, the electrochemical detector's sensitivity was switched to accommodate the wide ranges of CSF amino acid concentrations. The analysis was acceptably precise and linear at and above the CSF levels and did not require CSF deproteinization. During the 23 min elution, the concentrations of 8 CSF amino acids were determined: alanine, asparagine, glutamate, glutamine, glycine, serine, taurine, and tyrosine; measurable concentrations were between 1 and 800 microM. The concentration of GABA was below its detection limit (0.5 microM). To assess the ability to detect small concentration increases which might occur due to experimental manipulations, the minimum detectable increments in CSF amino acid concentrations above endogenous levels were determined.