Characteristics of lysophosphatidylcholine in its inhibition of taurine uptake by human intestinal Caco-2 cells

The characteristics of lysophosphatidylcholine (LPC) in its inhibition of the taurine uptake by human intestinal Caco-2 cells were investigated. By treating the cells with 200 microM of LPC, the taurine uptake was rapidly decreased by approximately 60%. This decrease was accompanied by an increase in the Km value for the uptake. A rapid uptake of LPC itself by the cells was also observed. The inhibitory activity of LPC was specific to the uptake of taurine and certain amino acids, while the uptake of glucose, glutamic acid and peptide (glycylglutamine) was not affected by LPC. The activity was dependent on the structure of a polar head and the bound fatty acid. The phosphorylcholine residue was likely to have played an important role, and surface active LPC with fatty acids of C14 or longer was highly inhibitory. These results suggest that the interaction of LPC with the taurine transporter in the intestinal cell membrane was the cause of the reduced taurine uptake.     

Characteristics of Lysophosphatidylcholine in Its Inhibition of Taurine Uptake by Human Intestinal Caco-2 Cells

The characteristics of lysophosphatidylcholine (LPC) in its inhibition of the taurine uptake by human intestinal Caco-2 cells were investigated. By treating the cells with 200 μM of LPC, the taurine uptake was rapidly decreased by approximately 60%. This decrease was accompanied by an increase in the K _m value for the uptake. A rapid uptake of LPC itself by the cells was also observed. The inhibitory activity of LPC was specific to the uptake of taurine and certain amino acids, while the uptake of glucose, glutamic acid and peptide (glycylglutamine) was not affected by LPC. The activity was dependent on the structure of a polar head and the bound fatty acid. The phosphorylcholine residue was likely to have played an important role, and surface active LPC with fatty acids of C14 or longer was highly inhibitory. These results suggest that the interaction of LPC with the taurine transporter in the intestinal cell membrane was the cause of the reduced taurine uptake.     

Taurine and cell volume maintenance in the shark rectal gland: cellular fluxes and kinetics

Tissue slices of shark rectal gland are studied to examine the kinetics of the cellular fluxes of taurine, a major intracellular osmolyte in this organ. Maintenance of high steady-state cell taurine (50 mM) is achieved by a ouabain-sensitive active Na+-dependent uptake process and a relatively slow efflux. Uptake kinetics are described by two saturable taurine transport components (high-affinity, Km 60 microM; and low-affinity, Km 9 mM). [14C]Taurine uptake is enhanced by external Cl-, inhibited by beta-alanine and unaffected by inhibitors of the Na+/K+/2Cl- co-transport system. Two cellular efflux components of taurine are documented. Incubation of slices in p-chloromercuribenzene sulfonate (1 mM) reduces taurine uptake, increases efflux of taurine and induces cell swelling. Studies of efflux in isotonic media with various cation and anion substitutions demonstrate that high-K+ markedly enhances taurine efflux irrespective of cell volume changes (i.e. membrane stretching is not involved). Moreover, iso-osmotic cell swelling induced in media containing propionate is not associated with enhanced efflux of taurine from the cells. It is suggested that external K+ exerts a specific effect on the cytoplasmic membrane to increase its permeability to taurine.     

Inhibition of myeloperoxidase by salicylhydroxamic acid.

Salicylhydroxamic acid inhibited the luminol-dependent chemiluminescence of human neutrophils stimulated by phorbol 12-myristate 13-acetate or the chemotactic peptide N-formylmethionyl-leucyl-phenylalanine (fMet-Leu-Phe). This compound had no inhibitory effect on the kinetics of O2.- generation or O2 uptake during the respiratory burst, but inhibited both the peroxidative activity of purified myeloperoxidase and the chemiluminescence generated by a cell-free myeloperoxidase/H2O2 system. The concentration of salicylhydroxamic acid necessary for complete inhibition of myeloperoxidase activity was 30-50 microM (I50 values of 3-5 microM) compared with the non-specific inhibitor NaN3, which exhibited maximal inhibition at 100-200 microM (I50 values of 30-50 microM). Whereas taurine inhibited the luminol chemiluminescence of an H2O2/HOC1 system by HOC1 scavenging, this compound had little effect on myeloperoxidase/H2O2-dependent luminol chemiluminescence; in contrast, 10 microM-salicylhydroxamic acid did not quench HOC1 significantly but greatly diminished myeloperoxidase/H2O2-dependent luminol chemiluminescence, indicating that its effects on myeloperoxidase chemiluminescence were largely due to peroxidase inhibition rather than non-specific HOC1 scavenging. Salicylhydroxamic acid prevented the formation of myeloperoxidase Compound II, but only at low H2O2 concentrations, suggesting that it may compete for the H2O2-binding site on the enzyme. These data suggest that salicylhydroxamic acid may be used as a potent inhibitor to delineate the function of myeloperoxidase in neutrophil-mediated inflammatory events.     

Polyene antibiotic amphotericin B in monomolecular layers: spectrophotometric and scanning force microscopic analysis

The effect of cytokines on the taurine uptake by human intestinal epithelial Caco-2 cells was investigated. Among the various cytokines tested, tumor necrosis factor alpha (TNF-alpha) markedly increased the taurine uptake by Caco-2 cells, resulting in an increase in the intracellular taurine level. TNF-alpha did not induce up-regulation of the taurine uptake in hepatic HepG2, renal human embryo kidney 293, and macrophage-like THP-1 cells. The uptake of glycine, L-leucine, and L-glutamic acid by Caco-2 cells was not affected by TNF-alpha. A kinetic analysis of the taurine uptake by TNF-alpha-treated Caco-2 cells suggests that this up-regulation was associated with both an increase in the amount of the taurine transporter (TAUT) and an increase in its affinity. TNF-alpha-treated cells showed a higher mRNA level of the TAUT than did the control cells.     

Inhibition of inositol 1,4,5-trisphosphate-mediated Ca2+ release by Ca2+ in cells from peripheral tissues

Permeabilized cells attached to culture plates were used to evaluate the inhibition of inositol 1,4,5-trisphosphate-mediated release (IPMCR) by Ca2+. In AR42J cells, a pancreatic acinar cell line, when permeabilization and Ca2+ uptake were carried out at low ionized Ca2+ (0.06 microM), Ca2+ had little effect on IPMCR. On the other hand, when permeabilization and Ca2+ uptake were performed at 5 microM Ca2+, IPMCR was inhibited by Ca2+ with an apparent affinity of 0.24 microM. This inhibition could be modified by exposing the cytosol of permeabilized cells to low Ca2+. Hence, permeabilizing the cells in the presence of 5 microM Ca2+ and then exposing them to Ca2+ concentrations between 0.01 and 5 microM before washing and Ca2+ uptake in the presence of 5 microM Ca2+ resulted in a Ca2(+)-dependent loss of inhibitory activity. The loss of inhibitory activity occurred with an apparent affinity for Ca2+ of 0.21 microM. A similar phenomenon with a comparable apparent dissociation constant for Ca2+ was found with three other cell types from peripheral tissues: the osteosarcoma cell line UMR-106-01, the kidney inner medullary cell line IMCD, and primary culture of urinary bladder smooth muscle cells. The properties of inhibition of IPMCR by Ca2+ in cells from peripheral tissues differ from those previously described in neuronal tissues and suggest that a different factor(s) mediates the inhibition of IPMCR by Ca2+ in cells from peripheral and neuronal tissues.     

Hypoxia-induced inhibition of whole cell membrane currents and ion transport of A549 cells

In excitable cells, hypoxia inhibits K channels, causes membrane depolarization, and initiates complex adaptive mechanisms. It is unclear whether K channels of alveolar epithelial cells reveal a similar response to hypoxia. A549 cells were exposed to hypoxia during whole cell patch-clamp measurements. Hypoxia reversibly inhibited a voltage-dependent outward current, consistent with a K current, because tetraethylamonium (TEA; 10 mM) abolished this effect; however, iberiotoxin (0.1 microM) does not. In normoxia, TEA and iberiotoxin inhibited whole cell current (-35%), whereas the K-channel inhibitors glibenclamide (1 microM), barium (1 mM), chromanol B293 (10 microM), and 4-aminopyridine (1 mM) were ineffective. (86)Rb uptake was measured to see whether K-channel modulation also affected transport activity. TEA, iberiotoxin, and 4-h hypoxia (1.5% O(2)) inhibited total (86)Rb uptake by 40, 20, and 35%, respectively. Increased extracellular K also inhibited (86)Rb uptake in a dose-dependent way. The K-channel opener 1-ethyl-2-benzimidazolinone (1 mM) increased (86)Rb uptake by 120% in normoxic and hypoxic cells by activation of Na-K pumps (+60%) and Na-K-2Cl cotransport (+170%). However, hypoxic transport inhibition was also seen in the presence of 1-ethyl-2-benzimidazolinone, TEA, and iberiotoxin. These results indicate that hypoxia, membrane depolarization, and K-channel inhibition decrease whole cell membrane currents and transport activity. It appears, therefore, that a hypoxia-induced change in membrane conductance and membrane potential might be a link between hypoxia and alveolar ion transport inhibition.     

Effects of calcium, calmodulin, protein kinase C and protein tyrosine kinases on volume-activated taurine efflux in human erythroleukemia cells.

The effects of calcium, calmodulin, protein kinase C (PKC) and protein tyrosine kinase (PTK) modulators were examined on the volume-activated taurine efflux in the erythroleukemia cell line K562. Exposure to hypoosmotic solution significantly increased taurine efflux and intracellular calcium concentration ([Ca2+]i). The Ca2+ channel blockers La3+ (1 mM), verapamil (200 microM) and nifedipine (100 microM) inhibited the hypoosmotically-induced [Ca2+]i increase by more than 90%, while the volume-activated taurine efflux was inhibited by 61.3 +/- 9.5, 74.1 +/- 9.3 and 38.0 +/- 1.5%, respectively. Furthermore, the calmodulin inhibitors W7 (50 microM) and trifluoperazine (10 microM) and the Ca2+/calmodulin-dependent protein kinase II inhibitor KN-62 (2 microM) significantly blocked the volume-activated taurine efflux by 93.4 +/- 2.7, 77.9 +/- 3.5 and 61.3 +/- 15.8%, respectively. In contrast, the PKC inhibitor staurosporine (200 nM) or the PKC activator phorbol 12-myristate 13-acetate (100 nM) did not have significant effects on the volume-activated taurine efflux. However, pretreatment with PTK inhibitors genistein, tyrphostin A25, and tyrphostin A47 blocked the volume-activated taurine efflux. These results suggest that the volume-activated taurine efflux in K562 cells may not directly involve Ca2+, but may require the presence of calmodulin and/or PTK.     

Expression of Madin-Darby canine kidney cell Na(+)-and Cl(-)-dependent taurine transporter in Xenopus laevis oocytes.

Expression of a Madin-Darby canine kidney (MDCK) cell taurine transporter was examined in Xenopus oocytes that had been injected with poly(A)+ RNA extracted from MDCK cells. Compared with water-injected oocytes, injection of total poly(A)+ RNA resulted in an increase in Na(+)-dependent taurine uptake which was directly related to the amount of RNA injected. The magnitude of expression in poly(A)+ RNA-injected oocytes was 5-10-fold higher than that of water-injected oocytes. Since the Vmax of taurine uptake in MDCK cells is increased by culture in hypertonic medium, we compared oocyte taurine uptake after injection with poly(A)+ RNA from MDCK cells cultured in hypertonic medium with uptake in oocytes injected with poly(A)+ RNA from hypertonic cells elicited twice the taurine uptake elicited by poly(A)+ RNA from isotonic cells. The transporter expressed in oocytes was like that in MDCK cells: it was completely dependent on external sodium and was also anion dependent (Cl- greater than or equal to Br- greater than SCN- much greater than gluconate-). Other beta-amino acids, beta-alanine and hypotaurine, inhibited taurine uptake, but L-alanine and 2-(methylamino) isobutyric acid did not. The apparent Km of the transporter was 7.0 microM. After size fractionation on a sucrose density gradient, poly(A)+ RNA encoding for the MDCK taurine transporter was found in the fraction whose average size was 4.4 kilobases.     

Exogenous ATP-stimulated calcium uptake in isolated rat intestinal epithelial cells

ATP in the extracellular medium is known to stimulate Ca uptake into avian intestinal epithelial cells. We have now demonstrated a similar effect of ATP in mammalian intestinal epithelial cells and have further characterized this effect. Exogenous ATP increased 45Ca uptake 2-6 fold in isolated rat small intestinal epithelial cells, with a maximal effect at 1 mM and an ED50 of 290 microM. A strict structural requirement for nucleotide-stimulated 45Ca uptake was observed. ADP was much less effective than ATP and gamma-thio-ATP, and 5'-AMP, cyclic AMP, adenosine, non-adenine nucleotides, non-hydrolyzable ATP analogs and ATP analogs with ring substitutions at the 8 position were inactive. Prenylamine (100 microM) completely inhibited ATP-stimulated 45Ca uptake, while verapamil (100 microM) had only a small effect. In the intact intestine, ATP increased short-circuit current (Isc) when added to the mucosal side of the tissue. This effect was reduced by 10 microM and abolished by 100 microM prenylamine. The effect of ATP on Isc was markedly reduced in Cl-free solutions and in reduced-Ca solutions. Serosal and mucosal addition of the nonhydrolyzable ATP analog, beta, gamma-methylene-ATP, and serosal addition of ATP had little or no effect on Isc. The similarities between the effects of ATP in isolated cells and in the intact intestine suggest that the effect of ATP on Isc may be at least partially mediated through stimulation of Ca uptake into the epithelial cells.     

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.     

Characterization of carnosine uptake and its physiological function in human intestinal epithelial Caco-2 cells

Carnosine (beta-Ala-L-His) is known to have the physiological functions of an antioxidant. Although dietary carnosine is thought to be absorbed across intestinal epithelial cells, the mechanism for this absorption is not yet well understood and its function in the intestinal tract is also obscure. The intestinal transport of carnosine was characterized in the present study by using human intestinal Caco-2 cells, and its physiological function in these cells was further examined. The carnosine uptake was proton-dependent, being activated by lowering the apical pH value. Its uptake was significantly inhibited by other dipeptides, whereas it was not inhibited by other amino acids. These characteristics of the carnosine uptake strongly suggest its transport into the cells via peptide transporter 1 (PepT1). Since carnosine has antioxidative activity, we studied its effect on the H2O2-induced secretion of inflammatory cytokines in Caco-2 cells. The H2O2 induced increase in IL-8 secretion was inhibited by a pretreatment with carnosine for 3 h, this inhibition being presented in a dose-dependent manner. These results suggest that carnosine had a protective effect against oxidative stress in intestinal epithelial cells.     

Characterization of the taurine transport pathway in A6 kidney cells

We investigated the role of taurine in cell homeostasis and characterized the taurine transport pathway in cultured kidney cells (A6). The taurine concentration in A6 cells varies with the osmolarity of the culture medium, suggesting that taurine participates in cell osmolarity. Under isosmotic conditions, 14C-taurine efflux through the apical membranes (aJtaur) was 6-7 times lower than that through the basolateral membranes (bJtaur). Under hyposmotic conditions, aJtaur remained almost unchanged. On the contrary, bJtaur increased 8 times in comparison with isosmotic conditions. In hyposmotic conditions, bJtaur was inhibited by 500 microM DIDS, 50 microM NPPB, 10 microM of the two oxonol derivatives DISBAC(2)3 and WW-791, and 100 microM ketoconazole. Conversely, 100 microM 1,9-dideoxyforskolin, 10 microM tamoxifen, 100 microM niflumic acid and 50 microM verapamil had no inhibitory effects. Cell volume regulation upon hyposmotic stress was also found to be inhibited by DISBAC(2)3 (K0.5 of 5+/-1 microM) and by ketoconazole. Nystatin was used to permeabilize the apical membranes with the aim to further characterize bJtaur. 14C-taurine transepithelial fluxes in nystatin-treated cells were found to be linear over taurine concentrations ranging from 3.5 microM to 35 mM. Clamping the transepithelial voltage at positive values (serosal side) slightly stimulated the 14C-taurine transport. Similar time courses of 14C-taurine, 36Cl and 86Rb transepithelial fluxes were found under osmotic stimulation followed by DIDS inhibition in nystatin-treated cells. In whole cell patch-clamp experiments, DISBAC(2)3 application resulted in a strong and reversible decrease of the global Cl- current which was stimulated by hyposmotic stress. Our study indicates that taurine participates in the control of A6 cell osmolarity and that the transporting taurine pathway (efflux) is on the basolateral membranes. In addition to usual chloride channel blockers, oxonol was found to be a potent blocker of the taurine transport and of the swelling-activated chloride current. Using a pharmacological approach, we could not distinguish between a common or different pathway for Cl- and taurine.     

Hydrogenase activity in Azospirillum brasilense is inhibited by nitrite, nitric oxide, carbon monoxide, and acetylene.

Nitrite, NO, CO, and C2H2 inhibited O2-dependent H2 uptake (H3H oxidation) in denitrifying Azospirillum brasilense Sp7 grown anaerobically on N2O or NO3-. The apparent Ki values for inhibition of O2-dependent H2 uptake were 20 microM for NO2-, 0.4 microM for NO, 28 microM for CO, and 88 microM for C2H2. These inhibitors also affected methylene blue-dependent H2 uptake, presumably by acting directly on the hydrogenase. Nitrite and NO inhibited H2 uptake irreversibly, whereas inhibition due to CO was easily reversed by repeatedly evacuating and backfilling with N2. The C2H2 inhibition was not readily reversed, partly due to difficulty in removing the last traces of this gas from solution. The NO2- inhibition of malate-dependent respiration was readily reversed by repeatedly washing the cells, in contrast to the effect of NO2- on H2-dependent respiration. These results suggest that the low hydrogenase activities observed in NO3(-)-grown cultures of A. brasilense may be due to the irreversible inhibition of hydrogenase by NO2- and NO produced by NO3- reduction.     

Hypotonic-induced transport pathways in Xenopus laevis erythrocytes: taurine fluxes

Taurine fluxes in Xenopus laevis red cells were studied in vitro in media of different tonicities. Both influx and efflux increased 3-10 times reversibly when dilution of the medium exceeded 30%. The absolute values of uptake ranged between 5 and 30 micromol/l cells.h at extracellular taurine concentration of 1 mmol/l, but is poorly selective as almost the same uptake was measured for choline and sucrose. Q(10) of 2.77 and an activation energy of 71.90+/-7.37 kJ/mol were calculated for the uptake process. Taurine uptake was reduced 50% in the absence of Cl(-), whereas the alkali cations (Na(+), K(+), Li(+) and Rb(+)) supported it similarly. Taurine uptake was greatly increased in Ca(2+)-free solution, and was inhibited by alkaline pH. The inhibitor of anion exchange protein, 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (IC(50)=25 microM) and the Cl(-) channel blockers 5-nitro-2-(3-phenylpropylamino) benzoic acid and [(dihydro-indenyl) oxy] alkanoic acid (IC(50)<20 microM) inhibited taurine uptake effectively. Isoproterenol did not affect taurine uptake in isotonic, nor in hypotonic solution. The uptake was reduced slowly to near the original, control level within 15-30 min in hypotonic solutions, indicating deactivation of the hypotonic-induced taurine pathway.     

Kinetics of the sodium-dependent glutamine transporter in human intestinal cell confluent monolayers.

The intestinal epithelium metabolism of glutamine plays a critical role in inter-organ nitrogen flow. Although it is known that glutamine is the primary oxidative energy source and nucleotide precursor in intestinal cells, the luminal uptake of glutamine by the apical surface of enterocytes is poorly understood. In this study we have uncovered the sodium-dependent transporter system responsible for L-glutamine uptake by the apical membrane of a human intestinal epithelial cell line. The sodium-dependent Michaelis constant (Km) = 247 +/- 45 microM glutamine, and Jmax = 4.44 +/- 0.65 x 10(-9) mole min-1(mg protein)-1 (37 degrees C). Glutamine shares the transporter with alanine, as demonstrated by unlabeled glutamine inhibition of [3H]alanine uptake kinetics with a purely competitive-type inhibition pattern, and glutamine inhibition Ki = 205 +/- 18 microM by Dixon analysis. The inhibition pattern for a series of amino acid analogs indicated that this intestinal apical membrane sodium-dependent transporter for glutamine is distinct from any other transport system found in membranes of non-intestinal cells.     

High-affinity maltose/trehalose transport system in the hyperthermophilic archaeon Thermococcus litoralis.

The hyperthermophilic marine archaeon Thermococcus litoralis exhibits high-affinity transport activity for maltose and trehalose at 85 degrees C. The K(m) for maltose transport was 22 nM, and that for trehalose was 17 nM. In cells that had been grown on peptone plus yeast extract, the Vmax for maltose uptake ranged from 3.2 to 7.5 nmol/min/mg of protein in different cell cultures. Cells grown in peptone without yeast extract did not show significant maltose or trehalose uptake. We found that the compound in yeast extract responsible for the induction of the maltose and trehalose transport system was trehalose. [14C]maltose uptake at 100 nM was not significantly inhibited by glucose, sucrose, or maltotriose at a 100 microM concentration but was completely inhibited by trehalose and maltose. The inhibitor constant, Ki, of trehalose for inhibiting maltose uptake was 21 nM. In contrast, the ability of maltose to inhibit the uptake of trehalose was not equally strong. With 20 nM [14C]trehalose as the substrate, a 10-fold excess of maltose was necessary to inhibit uptake to 50%. However, full inhibition was observed at 2 microM maltose. The detergent-solubilized membranes of trehalose-induced cells contained a high-affinity binding protein for maltose and trehalose, with an M(r) of 48,000, that exhibited the same substrate specificity as the transport system found in whole cells. We conclude that maltose and trehalose are transported by the same high-affinity membrane-associated system. This represents the first report on sugar transport in any hyperthermophilic archaeon.     

A(2b) receptor mediates adenosine inhibition of taurine efflux from pituicytes

BACKGROUND INFORMATION: Recent work suggests that part of the control of vasopressin output is mediated by taurine released from pituicytes, the astroglial cells of the neurohypophysis. Taurine release, in turn, is stimulated by hypotonic conditions and by vasopressin itself. As adenosine is generated from ATP co-released with vasopressin, it appeared important to study its effects on taurine efflux from pituicytes. RESULTS: We measured radioactive efflux from cultured pituicytes and whole neurohypophyses pre-loaded with [(3)H]taurine. Cultured pituicytes were also used to study adenosine-receptor mRNA expression. Taurine efflux elicited by hypotonic shocks is approximately 30-50% smaller in the presence of 10 microM adenosine or 1 microM NECA (5'-N-ethylcarboxamidoadenosine). Both compounds also inhibited basal efflux in a manner that was not immediately reversible. Agonists of the adenosine A1-, A2a- or A3-receptor subtypes have no relevant effect on basal taurine release, and the A1-receptor antagonist DPCPX (8-cyclopentyl-1,3-dipropylxanthine) has no effect on the inhibition of release by NECA. In turn, the A2b-receptor antagonists MRS 1706 {N-(4-acetylphenyl)-2-[4-(2,3,6,7-tetrahydro-2,6-dioxo-1,3-dipropyl-1H-purin-8-yl)phenoxy]acetamide} or alloxazine partially reverse the inhibition of basal or hypotonicity-evoked efflux by NECA. Both A1- and A2b-receptor mRNAs are expressed in pituicytes, which is consistent with an A1-receptor-mediated effect on cell morphology and an A2b-receptor-mediated effect on taurine release. Forskolin and dibutyryl cAMP mimic the inhibitory effects of purinergics on basal taurine efflux, and the adenylate cyclase inhibitor DDA (2',5'-dideoxyadenosine) partially reverses the inhibition of the hypotonic response by NECA.Conclusions. Our results suggest that purinergic inhibition of taurine efflux from pituicytes operates through A2b receptors coupled to intracellular cAMP increase. They point to a possible modulation of neurohypophysial hormone output by endogenous adenosine released in either physiological or pathological situations.     

Estrogen regulation and ion dependence of taurine uptake by MCF-7 human breast cancer cells

It has been reported that estrogen receptor-positive MCF-7 cells express TauT, a Na⁺-dependent taurine transporter. However, there is a paucity of information relating to the characteristics of taurine transport in this human breast cancer cell line. Therefore, we have examined the characteristics and regulation of taurine uptake by MCF-7 cells. Taurine uptake by MCF-7 cells showed an absolute dependence upon extracellular Na⁺. Although taurine uptake was reduced in Cl^- free medium a significant portion of taurine uptake persisted in the presence of NO₃ ^-. Taurine uptake by MCF-7 cells was inhibited by extracellular β-alanine but not by L-alanine or L-leucine. 17β-estadiol increased taurine uptake by MCF-7 cells: the V_max of influx was increased without affecting the K_m. The effect of 17β-estradiol on taurine uptake by MCF-7 cells was dependent upon the presence of extracellular Na⁺. In contrast, 17β-estradiol had no significant effect on the kinetic parameters of taurine uptake by estrogen receptor-negative MDA-MB-231 cells. It appears that estrogen regulates taurine uptake by MCF-7 cells via TauT. In addition, Na⁺-dependent taurine uptake may not be strictly dependent upon extracellular Cl^-.