Abstract: To determine the relationship between circulating metabolic fuels and their local concentrations in peripheral tissues we measured glycerol, glucose, and amino acids by microdialysis in muscle and adipose interstitium of 10 fasted, nonobese human subjects during (a) baseline, (b) euglycemic hyperinsulinemia (3 mU/kg per min for 3 h) and, (c) local norepinephrine reuptake blockade (NOR). At baseline, interstitial glycerol was strikingly higher (P < 0.0001) in muscle (3710 microM) and adipose tissue (2760 microM) compared with plasma (87 microM), whereas interstitial glucose (muscle 3.3, fat 3.6 mM) was lower (P < 0.01) than plasma levels (4.8 mM). Taurine, glutamine, and alanine levels were higher in muscle than in adipose or plasma (P < 0.05). Euglycemic hyperinsulinemia did not affect interstitial glucose, but induced a fall in plasma glycerol and amino acids paralleled by similar changes in the interstitium of both tissues. Local NOR provoked a fivefold increase in glycerol (P < 0.001) and twofold increase in norepinephrine (P < 0.01) in both muscle and adipose tissues. To conclude, interstitial substrate levels in human skeletal muscle and adipose tissue differ substantially from those in the circulation and this disparity is most pronounced for glycerol which is raised in muscle as well as adipose tissue. In muscle, insulin suppressed and NOR increased interstitial glycerol concentrations. Our data suggest unexpectedly high rates of intramuscular lipolysis in humans that may play an important role in fuel metabolism.

Abstract: We examined the effect of two endogenous antioxidant agents, taurine and vitamin E, on renal function in experimental diabetes. Male Sprague-Dawley rats, rendered diabetic with streptozocin (STZ), ...

Abstract: Naturally occurring antimutagenic compounds are extensively analyzed for their capacity to protect cells from induced damage. We selected two agents, taurine and ellagic acid, treated in the literature as antioxidants, but whose activity is insufficiently known. This paper reports on the ability of these agents to act against damage induced by mitomycin-C and hydrogen peroxide in Chinese hamster ovary cells cultivated in vitro. Cytogenetic and cytofluorimetric analyses were performed. Ellagic acid proved to have more than one mechanism of action, probably as a scavenger of oxygen species produced by H2O2 treatment, and as a protector of the DNA double helix from alkylating agent injury. In our experimental conditions, taurine seems able to scavenge oxygen species.

Abstract: Most, but not all, cell types release intracellular organic solutes (e.g. taurine) in response to cell swelling to achieve cell volume regulation. Although this efflux is blocked by classical inhibitors of the electroneutral anion exchanger band 3 (AE1), it is thought to involve an anion channel. The role of band 3 in volume-dependent taurine transport was determined by expressing, in Xenopus oocytes, band 3 from erythrocytes which do (trout) or do not (mouse) release taurine when swollen. AE1 of both species elicited anion exchange activity, but only trout band 3 showed chloride channel activity and taurine transport. Chimeras constructed from trout and mouse band 3 allowed the identification of some protein domains critically associated with channel activity and taurine transport. The data provide evidence that swelling-induced taurine movements occur via an anion channel which is dependent on, or controlled by, band 3. They suggest the involvement of proteins of the band 3 (AE) family in cell volume regulation.

Abstract: beta-Amyloid protein (beta AP) has been frequently associated with the neuropathology of Alzheimer's disease (AD), although the mechanisms by which it can induce neurodegeneration are still unknown. Some studies in hippocampal cultured neurons suggest that beta AP, particularly its fragment 25-35, may induce neural growth or render neurons more vulnerable to excitotoxic insults by a mechanism involving intracellular Ca2+ dyshomeostasis. We have studied the effect of fragment 25-35 on the release of endogenous amino acids from hippocampal slices of young adult (3-3.5-month-old) and aged (23-25-month-old) rats, under basal, K(+)-depolarization, and post-depolarization conditions, in the presence and absence of Ca2+. In both young and aged tissue, the basal release of amino acids was not affected by the peptide. By contrast, 1-hr preincubation of slices from young animals with 10 microM 25-35 fragment resulted in a 140% increase of glutamate and aspartate release stimulated by K+ depolarization, compared with the control-stimulated release. These effects were strictly dependent on external Ca2+. Neither the K(+)-stimulated release of gamma-amino butyric acid (GABA) nor the release of glycine, glutamine, taurine, or alanine, which was not stimulated by high K+, were affected. Substance P and a scrambled sequence of the 25-35 fragment were without any effect per se, but substance P blocked the stimulatory effect of fragment 25-35 on glutamate and aspartate release. In slices from aged rats the basal release of glutamate was significantly higher (260%) than that in young tissue, and the K(+)-induced release of both aspartate and glutamate was also higher.(ABSTRACT TRUNCATED AT 250 WORDS)

Abstract: Although it is now well established that amino acids can improve preimplantation development of mouse embryos in vitro, the mechanisms by which they influence development have not been determined. To investigate these mechanisms, we compared the contents of seven abundant amino acids (alanine, aspartate, glutamate, glutamine, glycine, serine, and taurine) in 4-8-cell embryos and blastocysts developing in vivo with the contents in those developing from the 2-cell stage in vitro. We also studied the effects of five amino acids (alanine, glutamate, glutamine, glycine, and taurine) that are abundant in the oviductal lumen on the amino acid content of embryos developing in vitro. Blastocysts that developed in vitro contained about six times more alanine and about one-sixth as much taurine as blastocysts that developed in vivo, but they contained about the same amounts of glycine and serine. In the presence of glycine and four other abundant amino acids in oviductal secretions, however, blastocysts that developed in vitro had higher levels of both glycine and serine than those that developed in vivo. In contrast, glycine either alone or in combination with the other amino acids reduced the alanine content of blastocysts developing in vitro to nearer that of blastocysts developing in vivo. Similarly, taurine in the medium allowed blastocysts developed in vitro to increase their content of this amino acid to normal levels. The levels of taurine and, somewhat surprisingly, glutamine and glycine became abnormally low in embryos within 24 h of the onset of in vitro culture in medium that did not contain the amino acid. These results could, in part, account for the reduced viability of embryos that develop in vitro and for the beneficial effects of taurine, glutamine, and glycine in culture. We also detected a decrease in the glutamine, glutamate, and aspartate content of blastocysts as they approached implantation in vivo. These decreases may result from uterine suppression of the activities of the Na+-dependent systems that transport amino acids in blastocysts. Regulation of the levels of these amino acids in and around blastocysts may contribute to chemical signaling among uterine and embryonal tissues near the time of implantation.

Abstract: During volume regulation in hypotonic media, glial cells release a large portion of their amino acids These amino acid losses appear to be mediated by a diffusion type of transport and a swelling-activated chloride channel seems to be involved The objective of this project was to provide direct evidence that amino acids could diffuse through a Cl− channel Using a human glial cell line, Cl− currents activated in hypotonic media were measured in whole-cell patch clamp To measure the currents produced by amino acids, it was necessary to increase the pH of external solutions to basic values reaching 96 and 100 to raise the concentration of the anionic form of these amino acids Introducing external hypotonic media containing high concentrations of amino acids, like glycine, taurine, glutamine and glutamate, it was possible to measure their respective current-voltage curves with NMDG-Cl-filled pipettes From the reversal potentials, their permeability ratios with respect to chloride were determined It was found that the low molecular weight amino acids, like glycine, were most permeant, while the larger ones, like glutamine, had a lower permeability with respect to chloride The amino acids with two carboxyl groups, like glutamate, had a much lower permeability ratio The reversal potentials for some metabolites, like lactate and malate were also measured for comparison These results demonstrate that amino acids can diffuse through anion channels and that activation of these channels in pathological conditions could be at least partly responsible for the observed increase in external amino acids

Abstract: 1. The swelling of bovine articular chondrocytes isolated from, or in situ within, cartilage by hypotonic shock rapidly activated the efflux or influx of radiolabelled taurine, an amino acid involved in volume regulation in a range of other cell types. 2. When chondrocytes were isolated by the use of collagenase into media of 280 or 380 mosmol l-1, the activation of taurine efflux was at about the osmolarity of the isolating medium, but it was more marked for a given hypotonic shock in the cells isolated at the lower osmolarity. The volume of chondrocytes following isolation in these two osmolarities was the same, suggesting that the cells possess volume regulatory capacity. 3. In isolated chondrocytes, the induced pathway had some of the characteristics of a volume-activated channel, i.e. no transport saturation with increasing substrate concentration, and lack of trans acceleration. The pattern of inhibition of the volume-activated pathway by pharmacological blockers (e.g. pimozide, [(dihydro-indenyl)oxy]alkanoic acid (DIOA) and tamoxifen) differed from that described for a similar pathway in other cell types. 4. The transport of other potential osmolytes (uridine, sorbitol and inositol) was stimulated by cell swelling, independent of sodium and inhibited by pimozide with a selectivity ratio of taurine, 1.00; uridine, 0.84; sorbitol, 0.66; and inositol, 0.38. The selectivity of taurine: inositol was not altered at different cell volumes. 5. The intracellular taurine concentration of chondrocytes within cartilage was low (about 2 mmol (l cell water)-1) showing a negligible role for taurine as an osmolyte during recovery from cell swelling. The swelling-induced loss of taurine from chondrocytes in situ was largely inhibited by pimozide and other drugs, showing that despite the rigid nature of cartilage, the chondrocytes were osmotically sensitive within the extracellular matrix.

Abstract: Taurine and hypotaurine seem to be important compounds for sperm survival and capacitation, the fertilisation process and embryo development, and are present in both sperm and genital secretions. Hypotaurine has protective effects against peroxidative damage. We have established a simple method for hypotaurine quantification in sperm and genital secretions. The mean concentration of hypotaurine is significantly higher in bovine than in human spermatozoa and in seminal plasma. We observed that both molecules are secreted by cow, sow, goat and rabbit oviduct epithelial cell monolayers. In rabbit the release is ascorbic acid dependent. Goat oviduct epithelial cells are able to use the transsulfuration pathway to form hypotaurine and taurine from methionine. We were able to identify cysteine sulfinate decarboxylase (EC 4.1.1.29) activity in cow and goat tubal monolayers. Our results demonstrate that hypotaurine and taurine are secreted by oviduct epithelium, and synthesised by tubal cells via the cysteine sulfinic acid pathway. The data obtained emphasise the importance of hypotaurine and taurine for gamete maturation, fertilisation and early embryonic development.

Abstract: Plasma taurine concentrations were determined in 76 dogs with dilated cardiomyopathy (DCM), 28 dogs with acquired valvular disease (AVD), and 47 normal (control) dogs. The data were collected at 2 referral centers, The Animal Medical Center, New York, NY (AMC), and the University of California, Davis (UCD), and the studies were conducted independently. Different anticoagulants (sodium citrate at AMC and lithium heparin at UCD) were used to collect the plasma samples. Paired analysis of samples showed a significant difference in plasma taurine concentrations, depending on the anticoagulant used. Consequently, results from each clinic were analyzed separately. Plasma taurine concentrations were significantly higher in dogs with AVD (median, 133 nmol/mL; range, 25 to 229 nmol/mL) than in control dogs (median, 63 nmol/ml; range 44 to 224 nmol/mL) and dogs with DCM (median, 72 nmol/ mL; range, 1 to247 nmol/mL) at AMC (P< .001). Thenumber of dogs with AVD at UCD was too small to draw meaningful conclusions. At UCD, the median plasma taurine concentration was 98 nmol/mL (range, 28-1 69 nmol/mL) aurine (Zaminoethane sulfonic acid) is the most abunT dant free amino acid in the mammalian heart, composing more than 40% of the free amino acid pool in the normal dog's heart and more than 60% of the free amino acid pool in the heart of dogs with experimentally induced right ventricular hypertrophy and heart failure.' Its biochemical roles are not well defined and may vary in different tissues.2 Investigators have shown multiple cardiac effects for taurine, including positive and negative in~tropic,~ antiarrhythmic; arrhythmogenic' properties; enhanced sarcolemma1 calcium binding and transport6,'; carbohydrate metabolism alterations'; and osmotic regulation.' The high myocardial taurine concentration is maintained by active Increases in myocardial taurine concentration have been reported in humans with congestive heart fail~re,'~,'~ in rats with isoproterenol-induced cardiac hypertr~phy,'~ and in dogs and rabbits with experimentally produced congestive heart fail~re.','~,'~ Low plasma taurine concentration has been associated with reversible myocardial failure in cats."^'* Subsequent to this finding, taurine was added to many commercial cat foods, leading to a marked reduction in the prevalence of feline dilated cardiomyopathy (DCM). 19x20 Experimentally, taurine-depleted diets resulted in reduced left ventricular systolic function, with secondary eccentric hypertrophy in 30% of cats The possibility that taurine deficiency might be associated with heart disease in species other than the cat was suggested by a report correlating DCM and low plasma taurine concentrations in foxes.23 Because the above studies showed significant associations between taurine concentrations and cardiac function in cats and foxes with DCM, we initiated a study to evaluate plasma taurine concentrations in dogs with DCM or acquired (degenerative) valvular disease (AVD), and compared plasma taurine concentrations in these groups to those in a group of clinically normal dogs.

Abstract: Taurine mediates a plethora of membrane-linked effects in excitable tissues. To account for these multiple actions, four hypotheses have been proposed. One theory is based on the observation that taurine diminishes the inflammatory response of several cytotoxic oxidants. It is proposed that a reduction in the extent of membrane oxidative injury contributes to these cytoprotective actions. The second theory maintains that alterations in protein phosphorylation may underlie certain effects of taurine, particularly its effect on calcium transport. The third hypothesis assumes that the interaction of taurine with the neutral phospholipids leads to altered membrane calcium binding and function. The final theory ties the actions of taurine to inhibition of phospholipid N-methylation and the resulting changes in membrane composition and structure. While each of these hypotheses has merit, none of them can fully explain the membrane actions of taurine. Further studies are required to ascertain the importance of each theory.

Abstract: Taurolidine has bactericidal and antilipopolysaccharide properties. It is broken down into the amino acid taurine, which has been shown to modulate intracellular calcium activity, a critical component in the priming and activation of macrophages and polymorphonuclear leukocytes. We hypothesized that taurolidine may function to enhance immune activity in these cells. The aim of this study was to investigate the immunological effects of taurolidine and correlate findings with survival after a septic challenge in a murine model. Study 1: CD-1 mice underwent cecal ligation and puncture, were randomized to receive taurolidine (200 mg/kg body weight/i.p.) or saline control, and studied for end point survival. Study 2: CD-1 mice were randomized to receive taurolidine (200 mg/kg body weight/i.p.) or saline control. Peritoneal macrophages (PM luminal diameters) were assessed for O2-, NO, tumor necrosis factor-alpha (TNF-alpha), CD11b, phagocytosis, and PMN influx. O2-, TNF-alpha, CD11b expression, and phagocytosis were significantly increased in the taurolidine group. Study 3: PM luminal diameters were cultured in vitro +/- 0.5 mg/ml taurolidine and PM luminal diameter antimicrobial function assessed (O2-, NO, TNF-alpha, and phagocytosis). O2-, TNF-alpha, and phagocytosis were significantly increased, whereas NO was reduced. Study 4: PM luminal diameters were also cultured with taurine (0.5 mg/ml). Similar increase in O2-, TNF-alpha, and phagocytosis were identified. Intracellular PM luminal diameter [Ca2+] was also assessed and increases in free, unbound intracellular [Ca2+] occurred after taurine culture. Thus, in addition to its bactericidal and antilipopolysaccharide activity, taurolidine primes PM luminal diameters for enhanced antimicrobial activity and these effects appear mediated by the amino acid taurine.

Abstract: Ozone is a potent respiratory irritant known to induce lung injury in humans and experimental animals. The present studies determined if ozone-induced lung inflammation was modified by pretreatment of animals with taurine, a detoxifying antioxidant. Rats were pretreated for 10 days with 5% taurine in their drinking water (controls received water only) prior to exposure to 2 ppm ozone for 3 h. At 2, 6, 12, 24, 48, and 72 h after ozone exposure, rats were anesthetized and the lungs were perfusion-fixed for histopathologic evaluation. An additional group of rats was used to examine bronchoalveolar lavage cell counts and hydroxyproline levels. A count of bronchoalveolar lavage cells 48 h after ozone exposure showed significantly fewer total inflammatory cells and fewer polymorphonuclear leukocytes accompanied by a reduction in hydroxyproline in the lavage fluid of ozone-exposed rats pretreated with taurine compared to rats that did not receive taurine. Light microscopy revealed an inflammatory cell infiltrate in the lungs of rats exposed to ozone. This was followed by focal hyperplasia in the terminal and respiratory bronchioles. Rats pretreated with taurine and then exposed to ozone showed none of these alterations. In addition, although there was a significant reduction in cell proliferation as measured by DNA precursor incorporation in the lungs of rats pretreated with taurine prior to ozone exposure compared to unsupplemented rats, the distribution of labeled cells was the same in taurine supplemented and unsupplemented groups. Also, significantly higher levels of taurine were found in the plasma, whole blood, and lavage fluid of rats pretreated with dietary taurine compared to rats that received water only. The results suggest that supplemental taurine protects rat lung epithelium from acute ozone-induced lung inflammation and hyperplasia.

Abstract: Taurine (2-aminoethane sulfonic acid) is a physiologic amino acid involved in cellular osmoregulation in various species including man. This study was intended to compare the respective effects of cold storage and consecutive ischemic rewarming of the liver on postischemic hepatic flow and hepatocellular outcome upon reperfusion with or without the addition of taurine to the preservation medium. Livers from male Wistar rats were rinsed free of blood via the portal vein and stored ischemically at 4 °C in UW solution. Livers from group 1 were then rinsed again with 10 ml Ringer's solution and reperfused with Krebs-Henseleit buffer at a constant pressure of 10 mmHg for 45 min at 37 °C in a nonrecirculating manner. Livers from groups 2 and 3 were subjected to 30 min of warm ischemia subsequent to cold storage and prior to reperfusion with 10 mM taurine added to the UW solution in group 3. While there were only very few signs of hepatic injury in group 1, the additional period of warm ischemia (group 2) led to a significant reduction in early perfusate flow and enhanced enzyme leakage from the livers during postischemic rinse and reperfusion. Livers in group 3 exhibited an amelioration in hepatic circulation and significantly reduced enzyme release as compared to group 2. The results clearly demonstrate a remarkable impact of postischemic rewarming on graft viability. Furthermore, the addition of taurine to the preservation medium was shown to improve hepatic circulation and enhance viability of the liver upon reperfusion.

Abstract: The polyunsaturated fatty acids, arachidonic, linoleic, and linolenic acids, were potent blockers of regulatory volume decrease (RVD) and of the swelling-activated efflux of [3H]taurine, D-[3H]aspartate, [3H]inositol, and 125I (used as marker of Cl) from rat cerebellar astrocytes in culture. The monounsaturated oleic and ricinoleic acids and saturated fatty acids were ineffective. The amino acid and 125I fluxes were similarly inhibited by fatty acids, whereas inositol release was less sensitive. Polyunsaturated fatty acids appear to directly affect RVD in trypsinized astrocytes as the inhibition was immediate and fully reversible. Blockers of the arachidonic acid metabolic pathways, indomethacin (cyclooxygenase), esculetin (lipoxygenases), and metyrapone (P-450 monooxygenases), did not prevent the effect of arachidonic acid, suggesting that further metabolism is not required for displaying the effects of arachidonic acid on RVD and osmolyte fluxes. Some blockers of arachidonic acid metabolic pathways, such as nordihydroguaiaretic acid (lipoxygenases) and naphthoflavone (P-450 monooxygenases), also exhibited marked inhibitory effects on RVD and on osmolyte fluxes. The predominant arachidonic acid metabolite in astrocytes, 12-hydroxyeicosatetraenoic acid, did not affect RVD or osmolyte fluxes. These results suggest that arachidonic acid and other polyunsaturated fatty acids directly inhibit the permeability pathways correcting cell volume after swelling in cultured astrocytes.

Abstract: Taurine and/or hypotaurine are necessary compounds for sperm capacitation, fertilization and embryo development. Hypotaurine has a protective role against peroxidative damage. The object of this work was, on the one hand, to determine the precise amounts of hypotaurine and taurine in the sperm environment at the moment of fertilization, and on the other hand to evaluate the production of hypotaurine and taurine by oviduct epithelial cells. Hypotaurine and taurine were quantified in spermatozoa and seminal and tubal fluid of various species, and in secretions by oviduct epithelial cell layers in vitro. Significant amounts of taurine and hypotaurine were identified. Both compounds were quantified in pre-ovulatory follicular fluid, i.e. in one of the fluids present at the site of fertilization. We also observed that hypotaurine and taurine are synthesized and secreted in vitro by oviduct epithelial cells. We were able to demonstrate that hypotaurine is stable when added to an in-vitro fertilization (IVF) culture medium. The effects of this compound should be more carefully studied in human IVF.

Abstract: Nonfeeding larvae of the gastropod Haliotis rufescens maintained a constant amount of taurine during embryonic and larval development and, since no de novo synthesis of taurine was observed in these larvae, the maternal endowment of taurine to the egg was sufficient for larval development to metamorphosis. In contrast, feeding larvae of the bivalve Crassostrea gigas increased their taurine content by a factor of 43 during growth to metamorphosis (from 86 to 311 mm, valve length). Taurine was not present in algae used to feed the larvae, suggesting that de novo synthesis of taurine by the larvae met their requirements. In unfed larvae, cysteic acid, cysteine sulfinic acid and hypotaurine were labeled from a [35S]cysteine precursor, but taurine was not. Hyperosmotic treatment (from 33 to 44 salinity for up to 3 h) did not induce taurine synthesis in unfed larvae. However, larvae fed the alga Isochrysis galbana up-regulated their taurine synthesis from [35S]cysteine by a factor of 11 (fed, 11.7p2.2 fmol taurine larva-1 h-1; unfed controls, 1.08p0.33 fmol taurine larva-1 h-1; means p s.e.m.). Fed larvae also synthesized taurine from [35S]methionine (18.4 fmol larva-1 h-1). I. galbana contained 5 fmol cell-1 of cysteine and methionine (combined) and, based on known feeding rates, we calculated that there were sufficient taurine precursors in the algae to supply the taurine requirements of growing larvae. The lack of significant de novo taurine synthesis reported for adult bivalve molluscs has led to the conclusion that taurine is a dietary requirement. Our findings for larval forms differ in that there is significant de novo synthesis of taurine during development.

Abstract: Commercial dry and canned diets fed to cats cause ~two- and fourfold increase in the taurine requirement, respectively, as compared with that ob served for purified diets. In two experiments, the effect of source and level of protein and fiber in the diet on the concentration of taurine in plasma and whole blood of cats was studied. All diets contained 1 g taurine/kg dry matter. When a casein-based diet containing either 25% or 50% protein was given to cats for 6 wk, no difference in plasma taurine concentration was ob served; however, substituting soybean protein for ca sein resulted in a significant (P < 0.01) decrease in plasma taurine concentration of cats in the 50% soy bean protein group, but not in the 25% soybean protein group. In Experiment 2, the food intake of cats was limited (26 g dry matter/(kg body weight • d)|, and the protein was 30 or 60% of the diet. Cats fed 60% soybean protein or casein diets had significantly lower plasma taurine concentrations than cats fed a 30% casein diet, with the 60% soybean protein diet causing the greater decrease. There was no effect of either 2 or 4% soybean fiberon plasma taurine concentration as compared with the same diet without the added fiber. The taurine con centration in plasma was higher (P < 0.05) in male cats than in female cats. Protein source, amount in the diet and gender did not affect the whole blood taurine concentration. Cats given diets containing 60% casein or soybean protein diets excreted a greater amount of fecal total bile acid and total taurine than cats given a 30% casein diet. Cats with higher plasma concentra tions of taurine excreted a greater amount of free taurine in urine, and a lesser amount of taurine and bile acids in feces. These results show that although protein source (soybean protein) and the quantity in the diet have a significant effect on the excretion pattern of tau rine by cats, these effects are not sufficient to account for the marked increase in the taurine requirement found when canned heat-processed diets are fed. J. Nutr. 125: 2831-2837, 1995.

Abstract: To investigate the characteristics of extracellular amino acids released from the striatum, we performedin vivo microdialysis in non-anaesthetised, freely moving rats. Amino acids were determined after precolumn derivatisation witho-phthalaldehyde by high-performance liquid chromatography and fluorescence detection. The omission of Ca2+ in the perfusion medium partially decreased the basal concentration of aspartate and glutamate. This shows that a small fraction of basal concentration of aspartate and glutamate is of neuronal origin. The effect of high K+ and veratrine stimulation was evaluated in the presence or absence of Ca2+ or tetrodotoxin (1 μM). High K+ and veratrine caused a remarkable increase in the aspartate and glutamate efflux. The omission of Ca2+ only partially decreased K+-stimulated aspartate and glutamate efflux. Tetrodotoxin completely antagonised veratrine-stimulated aspartate and glutamate efflux. Although glycine and taurine releases were stimulated by high K+ and veratrine, their release was not always antagonised with Ca2+ omission or tetrodotoxin inclusion. Thus, the neuronal origin of stimulated release of glycine and taurine is unclear. Although tetrodotoxin sensitivity and Ca2+-dependency are regarded as a basic criterion for classical neurotransmitters in microdialysis experiments, they should not be adapted to the physiological characteristics of the release of amino acids.