Taurine transport across hepatocyte plasma membranes: analysis in isolated rat liver sinusoidal plasma membrane vesicles

To elucidate the mechanism of taurine transport across the hepatic plasma membranes, rat liver sinusoidal plasma membrane vesicles were isolated and the transport process was analyzed. In the presence of a sodium gradient across the membranes (vesicle inside less than vesicle outside), an overshooting uptake of taurine occurred. In the presence of other ion gradients (K+, Li+, and choline+), taurine uptake was very small and no such overshoot was observed. Sodium-dependent uptake of taurine occurred into an osmotically active intravesicular space. Taurine uptake was stimulated by preloading vesicles with unlabeled taurine (transstimulation) in the presence of NaCl, but not in the presence of KCl. Sodium-dependent transport followed saturation kinetics with respect to taurine concentration; double-reciprocal plots of uptake versus taurine concentration gave a straight line from which an apparent Km value of 0.38 mM and Vmax of 0.27 nmol/20 s x mg of protein were obtained. Valinomycin-induced K+-diffusion potential failed to enhance the rate of taurine uptake, suggesting that taurine transport does not depend on membrane potential. Taurine transport was inhibited by structurally related omega-amino acids, such as beta-alanine and gamma-aminobutyric acid, but not by glycine, epsilon-aminocaproic acid, or other alpha-amino acids, such as L-alanine. These results suggest that Na+-dependent uptake of taurine might occur across the hepatic sinusoidal plasma membranes via a transport system that is specific for omega-amino acids having 2-3 carbon chain length.     

Na+-Dependent Transport of Taurine by Membrane Vesicles of Neuroblastoma ± Glioma Hybrid Cells

The transport of taurine into membrane vesicles prepared from neuroblastoma x glioma hybrid cells 108CC5 was studied. A great part of the taurine uptake by the membrane preparation is due to the transport into an osmotically sensitive space of membrane vesicles. Taurine uptake by membrane vesicles is an active transport driven by the concentration gradient of Na+ across the membrane (outside concentration greater than inside). The Km value of 36 microM for Na+-dependent taurine uptake indicates a high-affinity transport system. The rate of taurine transport by the membrane vesicles is enhanced by the K+ gradient (inside concentration greater than outside) and the K+ ionophore valinomycin. Taurine transport is inhibited by several structural analogs of taurine: hypotaurine, beta-alanine, and taurocyamine. All these results indicate that the taurine transport system of the membrane vesicles displays properties almost identical to those of intact neuroblastoma X glioma hybrid cells.     

Sodium-dependent high-affinity uptake of taurine by isolated rat brain capillaries

Transport of taurine has been demonstrated in capillary preparations from adult rat brains using [3H]taurine. Taurine transport is mediated by a saturable high-affinity system which is entirely dependent on sodium ions. The apparent maximal influx (Vmax) and half-saturation concentration (Km) corresponded to 1.06.10(-4) mumol/min per mg protein and 27.5 microM, respectively. Competition experiments in the presence of sodium ion showed that [3H]taurine uptake was strongly inhibited by 0.1 mM unlabeled structural analogues of taurine such as beta-alanine and hypotaurine as well as unlabeled taurine. gamma-Aminobutyric acid (GABA) (0.1 mM) inhibited the uptake of labeled taurine by 30%, whereas isethionic acid, L-methionine, L-2,4-diaminobutyric acid, glycine, L-cysteinesulfonic acid and cystamine did not exhibit any inhibitory effect. The results suggest that the Na+ gradient is the principal source of energy for taurine transport into isolated brain capillaries. This transport system may play an active role in the regulation of taurine concentration in the brain extracellular space.     

Insulin-stimulated taurine uptake in rat retina and retinal pigment epithelium.

Taurine is found at millimolar concentration in the retina and retinal pigment epithelium. High concentrations of taurine are essential for maintenance of retinal function. Taurine uptake by retina and retinal pigment epithelium was significantly enhanced by physiological concentrations of insulin as well as by high glucose concentrations. The results indicate that both, glucose and insulin enhanced taurine uptake occur through an increase in transport capacity which offset an additional, small decrease in affinity of the taurine carrier. Similar results were observed in retina and retinal pigment epithelium from streptozotocin-induced diabetic rats, suggesting that glucose and insulin regulate the taurine carrier through the same mechanism.     

Taurine transport by the flounder (Pseudopleuronectes americanus) intestine

Taurine transport by the flounder gut was characterized in isolated strips of intestine mounted between Ringer's solutions. Taurine was transported into the cell, against its concentration gradient, by a sodium-dependent system present in both the mucosal and serosal membranes. This system appears to be specific for beta-amino acids and is regulated by cyclic nucleotides (cGMP and cAMP). Kinetic analyses indicated that under physiological conditions the magnitudes of the bidirectional taurine uptake rates would favor net absorption of taurine from lumen to blood.     

Cellular uptake of taurine by lactating porcine mammary tissue

Milk taurine plays a critical role in neonatal development. Taurine uptake in lactating sow mammary tissue has not been characterized previously. The kinetic properties, ion dependence and substrate specificity of taurine uptake were characterized in mammary tissue collected from lactating sows at slaughter. Tissue explants were incubated in an isosmotic physiologic buffer with [³H]taurine tracer to measure taurine uptake. Taurine uptake was dependent upon the presence of extracellular sodium and chloride ions, which is consistent with the co-transport of sodium and chloride with taurine. Uptake was not dependent upon ion exchange mechanisms or upon furosemide-sensitive ion co-transport. Taurine uptake was saturable and exhibited an apparent K_m of 20 μM and a V_max of 386 μmol/kg cell water/30 min. Substrate specificity studies indicated a strong interaction of β-amino acids with the taurine transport system. Taurine transport in lactating sow mammary tissue is therefore a high affinity, sodium-dependent mechanism specific for β-amino acids, and is analogous to sodium-dependent taurine uptake in other tissues. The high affinity and high specificity of the taurine uptake system allows for concentration of taurine within the mammary cell and is ultimately responsible for provision of taurine required for neonatal development.     

The cytoprotective role of taurine in exercise-induced muscle injury

Summary.  Intense exercise is thought to increase oxidative stress and damage muscle tissue. Taurine is present in high concentration in skeletal muscle and may play a role in cellular defenses against free radical-mediated damage. The aim of this study was to determine if manipulating muscle levels of taurine would alter markers of free radical damage after exercise-induced injury. Adult male Sprague-Dawley rats were supplemented via the drinking water with either 3% (w/v) taurine (n = 10) or the competitive taurine transport inhibitor, β-alanine (n = 10), for one month. Controls (n = 20) drank tap water containing 0.02% taurine and all rats were placed on a taurine free diet. All the rats except one group of sedentary controls (n = 10) were subjected to 90 minutes of downhill treadmill running. Markers of cellular injury and free radical damage were determined along with tissue amino acid content. The 3% taurine treatment raised plasma levels about 2-fold and 3% β-alanine reduced plasma taurine levels about 50%. Taurine supplementation (TS) significantly increased plasma glutamate levels in exercised rats. Exercise reduced plasma methionine levels and taurine prevented its decline. Taurine supplementation increased muscle taurine content significantly in all muscles except the soleus. β-alanine decreased muscle taurine content about 50% in all the muscles examined. Lipid peroxidation (TBARS) was significantly increased by exercise in the extensor digitorium longus (EDL) and gastrocnemius (GAST) muscles. Both taurine and β-alanine completely blocked the increase in TBARs in the EDL, but had no effect in the GAST. Muscle content of the cytosolic enzyme, lactate dehydrogenase (LDH) was significantly decreased by exercise in the GAST muscle and this effect was attenuated by both taurine and β-alanine. Muscle myeloperoxidase (MPO) activity was significantly elevated in the gastrocnemius muscle, but diet had no effect. MPO activity was significantly increased by exercise in the liver and both taurine and β-alanine blocked this effect. There was no effect of either exercise or the diets on MPO activity in the lung or spleen. Running performance as assessed by a subjective rating scale was improved by taurine supplementation and there was a significant loss in body weight in the β-alanine-treated rats 24 hours after exercise. In summary, taurine supplementation or taurine depletion had measurable cytoprotective actions to attenuate exercise-induced injury. Received October 22, 2001 Accepted February 1, 2002     

Regulation of taurine transport in murine macrophages

Summary. We studied the regulation of taurine transport in ANA1 murine macrophage cell line. Taurine uptake was upregulated by hypertonicity and downregulated by bacterial lypopolysaccharide (LPS) and other stimuli leading to macrophage activation. However combined stimulation with LPS plus hypertonic shock evoked an increase of taurine uptake that was even higher than with hypertonic shock alone. Taurine transport was not modified by LPS in GG2EE macrophages derived from C3H/Hej mouse strain, which harbour a mutated Toll-like receptor 4 (TLR4) and thus are not activated by LPS. The extracellular signal-regulated kinase (ERK) inhibitor PD98059 abrogates the effect of both LPS and hyperosmotic shock on ANA1 taurine uptake, while the p38 inhibitor SB203580 reduces the taurine uptake in control conditions and impairs only the response to hypertonicity. These results suggest that the effect of LPS on taurine transport depends on ERK pathway and can be influenced by environmental conditions. Received September 1, 2000 Accepted December 6, 2000     

Potential benefits of taurine in the prevention of skeletal muscle impairment induced by disuse in the hindlimb-unloaded rat

Hindlimb unloading (HU) in rats induces severe atrophy and a slow-to-fast phenotype transition in postural slow-twitch muscles, as occurs in human disuse conditions, such as spaceflight or bed rest. In rats, a reduction of soleus muscle weight and a decrease of cross-sectional area (CSA) were observed as signs of atrophy. An increased expression of the fast-isoform of myosin heavy chain (MHC) showed the phenotype transition. In parallel the resting cytosolic calcium concentration (restCa) was decreased and the resting chloride conductance (gCl), which regulates muscle excitability, was increased toward the values of the fast-twitch muscles. Here, we investigated the possible role of taurine, which is known to modulate calcium homeostasis and gCl, in the restoration of muscle impairment due to 14-days-HU. We found elevated taurine content and higher expression of the taurine transporter TauT in the soleus muscle as compared to the fast-twitch extensor digitorum longus (EDL) muscle of control rats. Taurine level was reduced in the HU soleus muscle, although, TauT expression was not modified. Taurine oral supplementation (5?g/kg) fully prevented this loss, and preserved resting gCl and restCa together with the slow MHC phenotype. Taurine supplementation did not prevent the HU-induced drop of muscle weight or fiber CSA, but it restored the expression of MURF-1, an atrophy-related gene, suggesting a possible early protective effect of taurine. In conclusion, taurine prevented the HU-induced phenotypic transition of soleus muscle and might attenuate the atrophic process. These findings argue for the beneficial use of taurine in the treatment of disuse-induced muscle dysfunction.     

Taurine level in the blood and myocardium of rabbits with experimental heart failure

Experimental damage of the aortal heart valve in rabbits caused taurine accumulation in the heart. Taurine content in the heart increased for two months 2.7 times in the left ventricle and 1.8 times in the right one. Taurine concentration in the blood began rising 5-10 days after the operation, reached the maximal value of about 150% as compared with its initial level and then decreased to the level near the initial one. Animals with insignificant taurine accumulation in the heart died for the first two months after the operation. Results are discussed from consideration on the protective role of taurine for the myocardium under the heart failure.     

Transport, nutritional and metabolic studies of taurine in staphylococci

A specific, Na+-dependent, energy-requiring transport system for taurine has been reported recently in the Staphylococcus aureus M strain. Taurine was taken up vigorously by all S. aureus strains tested. The system was Na+-dependent, and Na+ decreased the Km but had no effect on the Vmax of the transport system. Among coagulase-negative staphylococci, the Staphylococcus epidermidis group (a taxonomically related group of species associated with humans or other primates) and the free-living, wide-ranging species Staphylococcus sciuri showed vigorous taurine uptake. Somewhat lower rates were found in the Staphylococcus saprophyticus group. Low or barely detectable uptake rates were noted in other staphylococcal species that were primarily of animal origin. No taurine uptake was detected in a variety of other bacterial species tested. Taurine uptake, which was not Na+-dependent, occurred in a Pseudomonas aeruginosa strain grown on taurine as sole energy, carbon, nitrogen, and sulphur source, but not when it was grown in a gluconate/salts medium. In nutritional studies we were unable to demonstrate a role for taurine as a sulphur source for S. aureus. [1,2-14C]- and [35S]taurine were taken up during overnight growth of cells, and radioactivity was distributed similarly among cellular fractions, indicating that the carbon and sulphur atoms of taurine were not cleaved and had the same fate. We were unable to demonstrate any catabolism of taurine in radiorespirometric experiments to detect evolution of 14CO2 by cells incubated with [1,2-14C]taurine. Thus, we found no evidence for a role of taurine in the energy, carbon and sulphur metabolism of S. aureus.     

Circadian rhythm and uptake of taurine by the rat pineal gland

Taurine is believed to be a modulator of membrane excitability in muscle and a neuroinhibitory transmitter in the central nervous system. The retina and pineal contain relatively large quantities of taurine. Taurine levels in the retina are reported to be responsive to variations in lighting conditions. We report here a carcadian rhythm for taurine in the mature male rat pineal gland. The maximum taurine concentration occurs at the midpoint of the light period, 24 ± 1.9 nmoles/gland, and the minimum at the beginning of the dark period, 13.9 ± 1.6 nmoles/gland. Sympathectomy by bilateral superior cervical ganglionectomy lowered pineal taurine levels. Constant light and blinding had no effect. Taurine was demonstrated to be taken up by the pineal gland $\text{in}$$\text{vitro}$ in organ culture. The uptake was saturable, K_m = 2.0 mM, and sodium dependent. The close structural analogs hypotaurine and β-alanine inhibited taurine uptake but α-alanine did not. We have demonstrated a circadian rhythm for taurine content in the rat pineal gland and the presence of a sodium-dependent transport system for taurine in the pineal $\text{in}$$\text{vitro}$ in organ culture.     

Lactate transport activity in rat skeletal muscle sarcolemmal vesicles after acute exhaustive exercise.

The effect of a single bout of exhaustive exercise on muscle lactate transport capacity was studied in rat skeletal muscle sarcolemmal (SL) vesicles. Rats were assigned to a control (C) group (n = 14) or an acutely exercised (E) group (n = 20). Exercise consisted of treadmill running (25 m/min, 10% grade) to exhaustion. SL vesicles purified from C and E rats were sealed because of sensitivity to osmotic forces. The time course of 1 mM lactate uptake in zero-trans conditions showed that the equilibrium level in the E group was significantly lower than in the C group (P < 0.05). The initial rate of 1 mM lactate uptake decreased significantly from 2.44 +/- 0.22 to 1.03 +/- 0.08 nmol. min(-1). mg protein(-1) (P < 0.05) after exercise, whereas that of 50 mM lactate uptake did not differ significantly between the two groups. For 100 mM external lactate concentration ([lactate]), exhaustive exercise increased initial rates of lactate uptake (219.6 +/- 36.3 to 465.4 +/- 80.2 nmol. min(-1). mg protein(-1), P < 0.05). Although saturation kinetics were observed in the C group with a maximal transport velocity of 233 nmol. min(-1). mg protein(-1) and a Michealis-Menten constant of 24.5 mM, saturation properties were not seen after exhaustive exercise in the E group, because initial rates of lactate uptake increased linearly with external [lactate]. We conclude that a single bout of exhaustive exercise significantly modified SL lactate transport activity, resulting in a decrease in 1 mM lactate uptake and was associated with alterations in the saturable properties at [lactate] above 50 mM. These results suggest that changes in sarcolemmal lactate transport activity may alter lactate and proton exchanges after exhaustive exercise.     

Modulation of taurine uptake in the goldfish retina and axonal transport to the tectum¶Effect of crushing the optic nerve or axotomy

Summary. Although there are a great number of studies concerning the uptake of taurine in several tissues, the regulation of taurine transport has not been studied in the retina after lesioning the optic nerve. In the present study, isolated retinal cells of the goldfish retina were used either immediatly after cell suspension or in culture. The high-affinity transport system of [³H]taurine in these cells was sodium-, temperature- and energy-dependent, and was inhibited by hypotaurine and β-alanine, but not by γ-aminobutyric acid. There was a decrease in the maximal velocity (V_max) without modifications in the substrate affinity (K_m) after optic axotomy. These changes were mantained for up to 15 days after the lesion. The results might be the summation of mechanisms for providing extracellular taurine to be taken up by other retinal cells or eye structures, or regulation by the substrate taurine, which increases after lesioning the optic nerve. The in vivo accumulation of [³H]taurine in the retina after intraocular injection of [³H]taurine was affected by crushing the optic nerve or by axotomy. A progressive retinal decrease in taurine transport was observed after crushing the optic nerve, starting at 7 hours after surgery on the nerve. The uptake of [³H]taurine by the tectum was compensated in the animals that were subjected to crushing of the optic nerve, since the concentration of [³H]taurine was only different from the control value 24 hours after the lesion, indicating an efficient transport by the remaining axons. On the contrary, the low levels of [³H]taurine in the tectum after axotomy might be an index of the non-axonal origin of taurine in the tectum. Axonal transport was illustrated by the differential presence of [³H]taurine in the intact or crushed optic nerve. The uptake of [³H]taurine into retinal cells in culture in the absence or in the presence of taurine might indicate the existence of an adaptive regulation of taurine transport in this tissue, however taurine transport probably differentially occurs in specific populations of retinal cells. The use of a purified preparation of cells might be useful for future studies on the modulation of taurine transport by taurine in the retina and its role during regeneration. Received June 11, 1999/Accepted August 31, 1999     

Characteristics of taurine transport system and its developmental pattern in mouse cerebral cortical neurons in primary culture

Developmental patterns and pharmacological and biochemical properties of taurine transport system were investigated using developing primary cultured neurons prepared from mouse cerebral cortex by trypsin treatment. [3H]Taurine was incorporated into neurons via a high-affinity transport system of which the Km value as well as the Vmax value increased during neuronal development in vitro. This transport system was also inhibited by sodium withdrawal from incubation medium and exposures for 15 h to several metabolic inhibitors such as 2,4-dinitrophenol and monoiodoacetate. In addition, [3H]taurine uptake in both neurons cultured for 3 and 14 days was competitively inhibited by beta-alanine, guanidinoethanesulfonate and hypotaurine. Cysteic acid and cysteine sulfinic acid, metabolic intermediates produced in the process of taurine biosynthesis in the brain from cysteine, induced significant reductions in [3H]taurine uptake in both types of cultured neurons, while cysteine, isethionic acid, cysteamine and cystamine exhibited no alterations in [3H]taurine transport. Moreover, non-competitive inhibition of [3H]taurine uptake by cysteic acid was observed in both neurons. These results clearly indicate that taurine uptake was mediated by the sodium- and energy-dependent transport system with high affinity in 14-day-old neurons as well as neurons cultured for 3 days and that both the Km and Vmax values of this transport system increase during neuronal development in vitro. The results described above suggest that the decrease in taurine content observed in developing brain is unlikely to be due to alteration in the capacity of the taurine transport system during neuronal development.     

The alpha-subunit of AMPK is essential for submaximal contraction-mediated glucose transport in skeletal muscle in vitro

AMP-activated protein kinase (AMPK) is a key signaling protein in the regulation of skeletal muscle glucose uptake, but its role in mediating contraction-induced glucose transport is still debated. The effect of contraction on glucose transport is impaired in EDL muscle of transgenic mice expressing a kinase-dead, dominant negative form of the AMPKalpha(2) subunit (KD-AMPKalpha(2) mice). However, maximal force production is reduced in this muscle, raising the possibility that the defect in glucose transport was due to a secondary decrease in force production and not impaired AMPKalpha(2) activity. Generation of force-frequency curves revealed that muscle force production is matched between wild-type (WT) and KD-AMPKalpha(2) mice at frequencies < or =50 Hz. Moreover, AMPK activation is already maximal at 50 Hz in muscles of WT mice. When EDL muscles from WT mice were stimulated at a frequency of 50 Hz for 2 min (200-ms train, 1/s, 30 volts), contraction caused an approximately 3.5-fold activation of AMPKalpha(2) activity and an approximately 2-fold stimulation of glucose uptake. Conversely, whereas force production was similar in EDL of KD-AMPKalpha(2) animals, no effect of contraction was observed on AMPKalpha(2) activity, and glucose uptake stimulation was reduced by 50% (P < 0.01) As expected, 5-aminoimidazole-4-carboxamide-1-beta-d-ribofuranosyl 5'-monophosphate (AICAR) caused a 2.3-fold stimulation of AMPKalpha(2) activity and a 1.7-fold increase in glucose uptake in EDL from WT mice, whereas no effect was detected in muscle from KD-AMPKalpha(2) mice. These data demonstrate that AMPK activation is essential for both AICAR and submaximal contraction-induced glucose transport in skeletal muscle but that AMPK-independent mechanisms are also involved.     

Taurine in the developing rabbit visual system: changes in concentration and axonal transport including a comparison with axonally transported proteins.

[35S]Taurine injected intravitreally into rabbits was transported axonally to the optic nerve terminals. Considerably more [35S]taurine was transported in young rabbits than in mature rabbits. The time course of taurine transport did not parallel that of proteins labeled with [3H]proline in the same system. The concentration of taurine in all components of the visual system, except retina, was greater in young animals than in mature animals, and was especially high in optic nerve. The possible functions of the high concentrations of taurine and the greater amount of axonally transported taurine in developing mammalian CNS are discussed.     

Taurine uptake by isolated alveolar macrophages and type II cells

Evidence suggests that taurine may protect cellular membranes against oxidants (Gordon et al., Am. J. Pathol. 125: 585-600, 1986). The present study was conducted to determine if alveolar macrophages and type II cells (which are relatively resistant to oxidant injury) possess a specialized transport system for the accumulation of taurine. The results indicate that both cell types contain more taurine than plasma or whole lung. Taurine influx exhibited both carrier-mediated and simple diffusion components. Carrier-mediated uptake displayed saturation kinetics (Km = 26.3 and 22.5 microM, while Vmax = 33.2 and 4.9 pmol.10(6) cells-1.min-1 for macrophages and type II cells, respectively). Taurine uptake was dependent on extracellular sodium and inhibited by metabolic inhibitors or ouabain. Total taurine uptake by type II cells was lower than that of alveolar macrophages. However, type II cells exhibited a higher intercellular concentration of taurine (14 vs. 4 mM) because of a higher ratio of carrier-mediated uptake to leakage than with alveolar macrophages. It is possible that this specialized transport system for taurine uptake may lend these cells resistant to oxidant injury.     

Effect of taurine on ATP-dependent calcium transport in guinea-pig cardiac muscle

The effect of taurine on ATP-dependent calcium transport was examined in guinea-pig cardiac ventricle homogenates and in microsomal preparations enriched in sarcoplasmic reticulum. Taurine (5?50 mM) did not affect ATP-dependent calcium binding or uptake in either of these preparations or alter the rate of decay of calcium uptake activity. Taurine (20 mM) also did not affect the oxalate-dependent calcium uptake stimulation noted in the presence of cyclic AMP-dependent protein kinase and cyclic AMP. The mechanism by which taurine alters cardiac function remains to be elucidated.