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After injection of 3H-taurine into eyeballs of frogs and maintenance for 3 h in darkness by a gentle shaking, an almost homogenous fraction of rod outer segments (ROS) was prepared. About a 22% decrease in tonicity caused by reducing NaCl in isotonic 225 mOsm normal solution caused a rapid increase in the rate coefficient of efflux of 3H-taurine from the ROS fraction. The peak level of increased efflux rate coefficient was 7-times higher than the basal isotonic level. This indicates that taurine could contribute essentially to the volume regulation, either via selective channels or a carrier transporter-mediated pathways. For further clarifying if taurine fluxes in the ROS are sensitive to the light, other experiments were performed. Neither light stimulation of dark-adapted ROSs fractions or dark stimulation of weakly illuminated ROSs revealed any detectable changes in the efflux rate coefficient of 3H-taurine. These results indicate that light-induced taurine efflux, if present in the ROS, must be small, compared with hypoosmotic induced efflux. Thus the question of light-induced release of taurine from ROS still remains to be clarified. In the second part of this study, using TLC (thin layer chromatography) in combination with 3H-taurine measurements we have tried to clarify whether frogs (Rana ridibunda) eye structures can synthesize tauret (retinylidenetaurine). In isolated retinal preparations almost no any noticeable radioactivity was detected compared with background level. The capability of the eye structures to synthesize tauret from 3H-taurine was revealed in the second whole eye injection experiment. About 0.3% of the total 3H-taurine pool taken up was converted into 3H-tauret in the dark-adapted frog retina. In the retina of frogs adapted to light compared with those which were dark adapted tauret quantities were remarkable lower--on average about half. These results are in agreement with our recent data obtained by HPLC, which indicate tauret levels several times higher in the dark-adapted frog retinae compared with those after long lasting light adaption. Taking into account these results one can conclude that the main structure able to synthesize 3H-tauret is probably pigment epithelium rather than retina.

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This investigation was improve the separation for tauret (retinylidene taurine) and to compare its content in the retina under dark and light adaptation. To prevent tauret hydrolysis, retinal samples were quickly frozen and lyophilized. Methanol extracts of dried retina and pigment epithelium from both dark- or light-adapted frogs, Rana ridibunda, were injected onto HPLC. Synthetic standard tauret appeared at 4.7 min after the solvent front. At the same time, an endogenous substance was eluted from the mixed retinal and pigment epithelial samples. The UV spectra of this endogenous compound matched with the spectra of synthetic tauret obtained under identical conditions, with lambda(max) = 446 nm at peak. We conclude that the HPLC system used permitted full separation of tauret from the methanol extracts of the retina and pigment epithelium. TLC and further HPLC analysis have shown that tauret quantities were several times higher in the retina and pigment epithelium of the frogs adapted to dark compared with those light-adapted (about 4 h under 1000 1x illumination). Tauret based vitamin A transport is probably involved in other systems as well, where along with its other known beneficial effects taurine probably is necessary to facilitate vitamin A transport.

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The potential role of taurine transport associated with volume regulation in renal tissue and isolated proximal renal tubules was studied in the teleost Carassius auratus (goldfish). The cellular taurine content in renal tissue fragments incubated in isosmotic solution (290 mOsm) (7.8 +/- 0.9 (SD) micromol g wet wt(-1)) decreased by 60% following exposure to hyposmotic medium (100 mOsm). The rate coefficient for [14C]taurine efflux in renal tissue and in isolated proximal renal tubules was strongly stimulated following hyposmotically or urea-activated cellular swelling. The stimulated basolateral taurine efflux pathway exhibited channel-like functional characteristics since (a) [14C]taurine influx was stimulated in parallel with the osmolality-dependent taurine efflux and (b) this efflux could not be stimulated by high medium taurine concentrations (40 mM) applied 10 min following the osmolality reduction (trans-stimulation test). Administration of the 5-lipoxygenase inhibitor ETH 615-139 (20 microM) during hyposmotic stimulation inhibited regulatory volume decreases but had no effect on taurine efflux. In addition, hyposmotically induced taurine efflux was slightly but significantly inhibited by the cyclooxygenase inhibitor indomethacin (10 microM). The taurine efflux was also dependent on both extra- and intracellular Ca2+. It is concluded that taurine is likely to coparticipate with KCl as an osmoeffector during RVD in Carassius proximal renal tubule cells. Cellular swelling seems to activate a basolateral taurine transport pathway with functional properties of a channel. This efflux mechanism appears to be partly regulated by Ca2+. Such a transport pathway could play a role in the cell volume regulatory mechanisms participating during transepithelial solute and water transport.

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Stimulation of flounder erythrocytes by noradrenaline under isosmotic conditions (330 mosmol kg-1) and physiological Na+ concentration (113 mmol l-1) caused swelling of the cells. The EC50 of this cell swelling was 0.65 µmol l-1 noradrenaline. The effect of the noradrenaline-induced cell swelling on the taurine channel under isosmotic conditions was negligible. However, when the cells were stimulated by noradrenaline (1.0 µmol l-1) before, simultaneously with or after reduction of osmolality (255 mosmol kg-1), the volume regulatory efflux of taurine mediated by the taurine channel was transiently accelerated. The rate coefficient for taurine efflux was more than four times higher than in osmolality-stimulated cells not exposed to noradrenaline. The present paper deals with the accelerating effect of noradrenaline on the taurine channel under hypo-osmotic conditions and the lack of effect of noradrenaline-induced cell swelling on the channel under iso-osmotic conditions. Noradrenaline initiated the cell swelling by interacting with ß-receptors which appeared to be more related to the mammalian ß1-receptors than to the ß2-receptors. The receptor interaction activated the adenylate cyclase system and, in the presence of 1.0 µmol l-1 noradrenaline, the cellular cyclic AMP concentration increased about 23 times. Noradrenaline also stimulated the Na+/H+ and Cl-/HCO3- antiporters and this affected the extracellular pH as well as the cell volume. Depending on the extracellular Na+ concentration, the incubation medium was acidified (113 mmol l-1 Na+) or alkalized (2.7 mmol l-1 Na+). Under these two conditions, the accelerating effects of noradrenaline on the taurine efflux were of similar magnitude. Similar effects on the cell volume, the extracellular pH and the volume regulatory taurine efflux were obtained in the presence of the cyclic AMP analogue 8-bromo-cyclic AMP. Under hypo-osmotic conditions in the absence of noradrenaline, the cellular level of cyclic AMP was not elevated. There was no significant positive correlation between the water content of the cells (cell volume) under different conditions in the presence or absence of noradrenaline and the state of activation of the osmolality-sensitive taurine channel. We conclude that the mechanism(s) which activate(s) the osmolality-sensitive taurine channel in flounder erythrocytes is transiently and strongly accelerated by noradrenaline, but not triggered by the noradrenaline-induced events. The acceleration does not appear to be due to increased activity of the antiporters, but to increased cellular levels of cyclic AMP.

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95% of the Na(+)-independent influx of taurine in flounder erythrocytes at normal osmolality (330 mosmol kg-1) and 0.30 mmol l-1 taurine was mediated by a saturable system (Vmax = 0.689 nmol g-1 dry mass min-1; Km = 0.47 mmol l-1). The influx was inhibited by taurine analogues, but was not significantly affected by reduced osmolality. This saturable influx of taurine was probably mediated by the so-called Na(+)-dependent influx system for taurine operating in the 0 Na+: 1 taurine mode. The remaining 5% of the Na(+)-independent influx was mediated by a diffusional pathway (Kd = 0.050 microliter g-1 dry mass min-1), since it did not show saturation kinetics, was not inhibited by taurine analogues and did not mediate counter-exchange. This non-saturable influx system for taurine was strongly, but transiently, stimulated by reduction of osmolality. The time course for this stimulatory effect was the same as that for the system that mediates the volume regulatory efflux of taurine. The relative inhibitory effect of bumetanide, furosemide, DIDS and quinine on the fluxes mediated by these two transport systems were also the same. We suggest that these unidirectional fluxes of taurine were mediated by only one transport system: a taurine channel. The effect of reduction of osmolality on the rate coefficient for efflux of beta-alanine was equal to the effect on the efflux of taurine, but greater than the effect on the efflux of choline. This difference probably reflects structural and/or electrical restrictions on the substrates to be transported by the taurine channel. The volume regulatory efflux of taurine was inhibited in the presence of the anti-calmodulin drug trifluoperazine and, in a Ca(2+)-free medium, added EGTA. The 5-lipoxygenase inhibitor nordihydroguaiaretic acid completely blocked the volume regulatory efflux of taurine. We suggest that both Ca2+/calmodulin and leukotrienes contribute to the control of the transport mediated by the taurine channel.

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The taurine transport of flounder erythrocytes is associated with a cell volume regulation in anisosmotic media. An osmolality reduction leads to a cell volume increase, which is followed by a volume readajustment towards the original level. A 75 mosM reduction is accompanied by a 33 mumol g dry wt.-1 reduction in the cellular taurine content. The reduction in osmolality activates the taurine release mechanism by transiently increasing the rate coefficient for taurine efflux. The rate coefficient for taurine influx is similarly stimulated. This influx is mediated by a Na+-independent transport system. The concomitant activation of influx and efflux suggests a coupling between these two systems. Higher taurine efflux and influx rate coefficients which decayed more slowly with time were measured in cells suspended in Na+-free (choline replacement) media than in the presence of Na+. This suggests that Na+ may play a role in the taurine release mechanism. Noradrenaline induced a cellular swelling at normal osmolality (330 mosM), but had only a minor effect on the taurine efflux and influx and the cellular taurine content. Urea-induced cellular swelling at normal osmolality initiated a volume regulatory process and activated the taurine release mechanism, similarly to an osmolality reduction. These results show that osmolality reduction and cellular swelling are no prerequisites for the activation of the taurine release mechanism and the cell volume readajustment. It is suggested that the dimension of an intracellular solute compartment determines the activation level of this mechanism.

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In brown trout surviving in acidified brook water, the plasma osmolality is reduced 15-25%. The decrease is much less than that expected from the measured reduction in the plasma concentration of Na+ and Cl-. This discrepancy cannot quantitatively be explained by the increase in plasma concentration of K+ (100%) and free amino compounds (mainly taurine; 280%) but appears mainly attributable to a reduction in plasma volume. The osmolality of heart ventricle cells is also reduced. Water content of the cells is unchanged, and there is a significant decrease in the intracellular concentration of K+, taurine and to a lesser extent glutamic acid, accounting for 25, 45 and 8%, respectively, of the osmolality reduction. These findings indicate the existence of a cell volume regulation mechanism in the fresh water brown trout which counteracts osmotic swelling of tissue cells during periods of salt loss in acidified water. The significance of this mechanism for survival under such conditions is discussed.

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