Regulation of the mouse retinal taurine transporter (TAUT) by protein kinases in Xenopus oocytes

Signaling Pathway of Taurine-Induced Upregulation of TXNIP

Taurine, a sulfur-containing β-amino acid, is present at high concentrations in mammalian tissues and plays an important role in several essential biological processes. However, the genetic mechanisms involved in these physiological processes associated with taurine remain unclear. In this study, we investigated the regulatory mechanism underlying the taurine-induced transcriptional enhancement of the thioredoxin-interacting protein (TXNIP). The results showed that taurine significantly increased the luciferase activity of the human TXNIP promoter. Further, deletion analysis of the TXNIP promoter showed that taurine induced luciferase activity only in the TXNIP promoter region (+200 to +218). Furthermore, by employing a bioinformatic analysis using the TRANSFAC database, we focused on Tst-1 and Ets-1 as candidates involved in taurine-induced transcription and found that the mutation in the Ets-1 sequence did not enhance transcriptional activity by taurine. Additionally, chromatin immunoprecipitation assays indicated that the binding of Ets-1 to the TXNIP promoter region was enhanced by taurine. Taurine also increased the levels of phosphorylated Ets-1, indicating activation of Ets-1 pathway by taurine. Moreover, an ERK cascade inhibitor significantly suppressed the taurine-induced increase in TXNIP mRNA levels and transcriptional enhancement of TXNIP. These results suggest that taurine enhances TXNIP expression by activating transcription factor Ets-1 via the ERK cascade.

Taurine: A Maternally Derived Nutrient Linking Mother and Offspring

Mammals can obtain taurine from food and synthesize it from sulfur-containing amino acids. Mammalian fetuses and infants have little ability to synthesize taurine. Therefore, they are dependent on taurine given from mothers either via the placenta or via breast milk. Many lines of evidence demonstrate that maternally derived taurine is essential for offspring development, shaping various traits in adults. Various environmental factors, including maternal obesity, preeclampsia, and undernutrition, can affect the efficacy of taurine transfer via either the placenta or breast milk. Thus, maternally derived taurine during the perinatal period can influence the offspring’s development and even determine health and disease later in life. In this review, I will discuss the biological function of taurine during development and the regulatory mechanisms of taurine transport from mother to offspring. I also refer to the possible environmental factors affecting taurine functions in mother-offspring bonding during perinatal periods. The possible functions of taurine as a determinant of gut microbiota and in the context of the Developmental Origins of Health and Disease (DOHaD) hypothesis will also be discussed.

Role of 5-HT2 and 5-HT7 Serotonin Receptors, and Protein Kinases C and A on Taurine Transport in Lymphocytes of Rats Treated with Fluoxetine

Fluoxetine, an antidepressant and selective serotonin reuptake inhibitor, modulates immune cells in vitro. The present study investigates the influence of pharmacological agents which acts as agonist and antagonist of serotonin receptors ex vivo over taurine transport in lymphocytes of rats treated with fluoxetine by one week. The treatment with fluoxetine increase taurine transport and the incubation with the agonist of 5-HT2 receptor, 1-(2,5-dimethoxy-4-iodophenyl)-​2-aminopropane (DOI) counteract this effect, and ketanserin provoked no change in fluoxetine effect. While the agonist of 5-HT7 receptor, 4-[2-(methylthio)phenyl]-N-(1,2,3,4-tetrahydro-1-naphth alenyl)-1-piperazinehexanamide hydrochloride (LP44) had no significant effects, however the differences between Control and Fluoxetine groups were not observed, the antagonist (R)-3-[2-[2-(4-methylpiperidin-1-yl)ethyl]pyrrolidine-1-sulfonyl]phenol hydrochloride (SB269970) had no differences. Preincubation of cells with the diacylglycerol analogue, 1-oleoyl-2-acetyl-sn-glycerol (OAG) caused inhibition of fluoxetine treatment effect but this not occurred in presence of the PKC inhibitor, 1-O-hexadecyl-2-O-methyl-rac-glycerol (AMG-C16). Forskolin counteracted the effect of fluoxetine on taurine transport, since at the concentrations used, the rate of taurine transport in Fluoxetine group, returned to Control rate. No significant differences were observed with the PKA inhibitor. Although it is not possible to attribute a definitive role of 5-HT2 receptors in fluoxetine effect on taurine transport, its signaling might affect the function of it. Participation of PKC and PKA have an apparently relevant role in lymphocyte taurine transport.

Functional characterization of the human facilitative glucose transporter 12 (GLUT12) by electrophysiological methods

GLUT12 is a member of the facilitative family of glucose transporters. The goal of this study was to characterize the functional properties of GLUT12, expressed in Xenopus laevis oocytes, using radiotracer and electrophysiological methods. Our results showed that GLUT12 is a facilitative sugar transporter with substrate selectivity: d-glucose ≥ α-methyl-d-glucopyranoside (α-MG) > 2-deoxy-d-glucose(2-DOG) > d-fructose = d-galactose. α-MG is a characteristic substrate of the Na(+)/glucose (SGLT) family and has not been shown to be a substrate of any of the GLUTs. In the absence of sugar, (22)Na(+) was transported through GLUT12 at a higher rate (40%) than noninjected oocytes, indicating that there is a Na(+) leak through GLUT12. Genistein, an inhibitor of GLUT1, also inhibited sugar uptake by GLUT12. Glucose uptake was increased by the PKA activator 8-bromoadenosine 3',5'-cyclic monophosphate (8-Br-cAMP) but not by the PKC activator phorbol-12-myristate-13-acetate (PMA). In high K(+) concentrations, glucose uptake was blocked. Addition of glucose to the external solution induced an inward current with a reversal potential of approximately -15 mV and was blocked by Cl(-) channel blockers, indicating the current was carried by Cl(-) ions. The sugar-activated Cl(-) currents were unaffected by genistein. In high external K(+) concentrations, sugar-activated Cl(-) currents were also blocked, indicating that GLUT12 activity is voltage dependent. Furthermore, glucose-induced current was increased by the PKA activator 8-Br-cAMP but not by the PKC activator PMA. These new features of GLUT12 are very different from those described for other GLUTs, indicating that GLUT12 must have a specific physiological role within glucose homeostasis, still to be discovered.

Regulation of the human Na+-dependent glucose cotransporter hSGLT2

The human Na(+)-glucose cotransporter SGLT2 is expressed mainly in the kidney proximal convoluted tubule where it is considered to be responsible for the bulk of glucose reabsorption. Phosphorylation profiling has revealed that SGLT2 exists in a phosphorylated state in the rat renal proximal tubule cortex, so we decided to investigate the regulation of human SGLT2 (hSGLT2) by protein kinases. hSGLT2 was expressed in human embryonic kidney (HEK) 293T cells, and the activity of the protein was measured using radiotracer and whole cell patch-clamp electrophysiology assays before and after activation of protein kinases. 8-Bromo-adenosine cAMP (8-Br-cAMP) was used to activate protein kinase A, and sn-1,2-dioctanoylglycerol (DOG) was used to activate protein kinase C (PKC). 8-Br-cAMP stimulated D-[α-methyl-(14)C]glucopyranoside ([(14)C]α-MDG) uptake and Na(+)-glucose currents by 200% and DOG increased [(14)C]α-MDG uptake and Na(+)-glucose currents by 50%. In both cases the increase in SGLT2 activity was marked by an increase in the maximum rate of transport with no change in glucose affinity. These effects were completely negated by mutation of serine 624 to alanine. Insulin induced a 250% increase in Na(+)-glucose transport by wild-type but not S624A SGLT2. Parallel studies confirmed that the activity of hSGLT1 was regulated by PKA and PKC due to changes in the number of transporters in the cell membrane. hSGLT1 was relatively insensitive to insulin. We conclude that hSGLT1 and hSGLT2 are regulated by different mechanisms and suggest that insulin is an SGLT2 agonist in vivo.

Activation of Ca2+/calmodulin-dependent protein kinase II is involved in hyperosmotic induction of the human taurine transporter

We investigated the signaling pathways participating in hyperosmotic regulation of the human taurine transporter (TAUT) by using specific inhibitors of various intracellular signaling molecules. Among them, the specific inhibitor of calcium/calmodulin-dependent protein kinase II (Ca(2+)/CaM kinase II) completely repressed the hyperosmotic regulation of TAUT. The osmosensitive upregulation of TAUT was also significantly inhibited by calmodulin antagonists and calcium-chelators. The increased expression level of TAUT mRNA by hypertonicity was repressed by the specific Ca(2+)/CaM kinase II inhibitor. The activated form of Ca(2+)/CaM kinase II protein could only be detected in Caco-2 cells under hypertonic conditions.

Substrate-induced changes in the density of peptide transporter PEPT1 expressed in Xenopus oocytes

The adaptation of the capacity of the intestinal peptide transporter PEPT1 to varying substrate concentrations may be important with respect to its role in providing bulk quantities of amino acids for growth, development, and other nutritional needs. In the present study, we describe a novel phenomenon of the regulation of PEPT1 in the Xenopus oocyte system. Using electrophysiological and immunofluorescence methods, we demonstrate that a prolonged substrate exposure of rabbit PEPT1 (rPEPT1) caused a retrieval of transporters from the membrane. Capacitance as a measure of membrane surface area was increased in parallel with the increase in rPEPT1-mediated transport currents with a slope of approximately 5% of basal surface per 100 nA. Exposure of oocytes to the model peptide Gly-l-Gln for 2 h resulted in a decrease in maximal transport currents with no change of membrane capacitance. However, exposure to substrate for 5 h decreased transport currents but also, in parallel, surface area by endocytotic removal of transporter proteins from the surface. The reduction of the surface expression of rPEPT1 was confirmed by presteady-state current measurements and immunofluorescent labeling of rPEPT1. A similar simultaneous decrease of current and surface area was also observed when endocytosis was stimulated by the activation of PKC. Cytochalasin D inhibited all changes evoked by either dipeptide or PKC stimulation, whereas the PKC-selective inhibitor bisindolylmaleimide only affected PKC-stimulated endocytotic processes but not substrate-dependent retrieval of rPEPT1. Coexpression experiments with human Na(+)-glucose transporter 1 (hSGLT1) revealed that substrate exposure selectively affected PEPT1 but not the activity of hSGLT1.

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.

Taurine release in mouse brain stem slices under cell-damaging conditions

Taurine has been thought to be essential for the development and survival of neural cells and to protect them under cell-damaging conditions. In the brain stem taurine regulates many vital functions, including cardiovascular control and arterial blood pressure. We have recently characterized the release of taurine in the adult and developing brain stem under normal conditions. Now we studied the properties of preloaded [3H]taurine release under various cell-damaging conditions (hypoxia, hypoglycemia, ischemia, the presence of metabolic poisons and free radicals) in slices prepared from the mouse brain stem from developing (7-day-old) and young adult (3-month-old) mice, using a superfusion system. Taurine release was greatly enhanced under these cell-damaging conditions, the only exception being the presence of free radicals in both age groups. The ischemia-induced release was characterized to consist of both Ca2+-dependent and -independent components. Moreover, the release was mediated by Na+-, Cl--dependent transporters operating outwards, particularly in the immature brain stem. Cl- channel antagonists reduced the release at both ages, indicating that a part of the release occurs through ion channels, and protein kinase C appeared to be involved. The release was also modulated by cyclic GMP second messenger systems, since inhibitors of soluble guanylyl cyclase and phosphodiesterases suppressed ischemic taurine release. The inhibition of phospholipases also reduced taurine release at both ages. This ischemia-induced taurine release could constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

Regulation of taurine transport in rat hippocampal neurons by hypo-osmotic swelling

Taurine, an important mediator of cellular volume regulation in the central nervous system, is accumulated into neurons and glia by means of a highly specific sodium-dependent membrane transporter. During hyperosmotic cell shrinkage, net cellular taurine content increases as taurine transporter activity is enhanced via elevated gene expression of the transporter protein. In hypo-osmotic conditions, taurine is rapidly lost from cells by means of taurine-conducting membrane channels. We reasoned that changes in taurine transporter activity also might accompany cell swelling to minimize re-accumulation of taurine from the extracellular space. Thus, we determined the kinetic and pharmacological characteristics of neuronal taurine transport and the response to osmotic swelling. Accumulation of radioactive taurine is strongly temperature dependent and occurs via saturable and non-saturable pathways. At concentrations of taurine expected in extracellular fluid in vivo, 98% of taurine accumulation would occur via the saturable pathway. This pathway obeys Michaelis-Menten kinetics with a Km of 30.0 +/- 8.8 microm (mean +/- SE) and Jmax of 2.1 +/- 0.2 nmol/mg protein min. The saturable pathway is dependent on extracellular sodium with an effective binding constant of 80.0 +/- 3.1 mm and a Hill coefficient of 2.1 +/- 0.1. This pathway is inhibited by structural analogues of taurine and by the anion channel inhibitors, 4,4'-diisothiocyanostilbene-2, 2'-disulfonic acid (DIDS) and 5-nitro-2-(3 phenylpropylamino) benzoic acid (NPPB). NPPB, but not DIDS, also reduces the ATP content of the cell cultures. Osmotic swelling at constant extracellular sodium concentration reduces the Jmax of the saturable transport pathway by approximately 48%, increases Kdiff for the non-saturable pathway by 77%, but has no effect on cellular ATP content. These changes in taurine transport occurring in swollen neurons in vivo would contribute to net reduction of taurine content and resulting volume regulation.

Characteristics of taurine release in slices from adult and developing mouse brain stem

Taurine has been thought to function as a regulator of neuronal activity, neuromodulator and osmoregulator. Moreover, it is essential for the development and survival of neural cells and protects them under cell-damaging conditions. Taurine is also involved in many vital functions regulated by the brain stem, including cardiovascular control and arterial blood pressure. The release of taurine has been studied both in vivo and in vitro in higher brain areas, whereas the mechanisms of release have not been systematically characterized in the brain stem. The properties of release of preloaded [(3)H]taurine were now characterized in slices prepared from the mouse brain stem from developing (7-day-old) and young adult (3-month-old) mice, using a superfusion system. In general, taurine release was found to be similar to that in other brain areas, consisting of both Ca(2+)-dependent and Ca(2+)-independent components. Moreover, the release was mediated by Na(+)-, Cl(-)-dependent transporters operating outwards, as both Na(+)-free and Cl(-) -free conditions greatly enhanced it. Cl(-) channel antagonists and a Cl(-) transport inhibitor reduced the release at both ages, indicating that a part of the release occurs through ion channels. Protein kinases appeared not to be involved in taurine release in the brain stem, since substances affecting the activity of protein kinase C or tyrosine kinase had no significant effects. The release was modulated by cAMP second messenger systems and phospholipases at both ages. Furthermore, the metabotropic glutamate receptor agonists likewise suppressed the K(+)-stimulated release at both ages. In the immature brain stem, the ionotropic glutamate receptor agonists N-methyl-D-aspartate (NMDA) and 2-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) potentiated taurine release in a receptor-mediated manner. This could constitute an important mechanism against excitotoxicity, protecting the brain stem under cell-damaging conditions.

Characterization of a branched-chain amino-acid transporter SBAT1 (SLC6A15) that is expressed in human brain

The SLC6 gene family comprises membrane proteins that transport neurotransmitters, amino acids, or osmolytes. We report the first functional characterization of the human SLC6A15 gene, which codes for a sodium-coupled branched-chain amino-acid transporter 1 (SBAT1). SBAT1 expression is specific to the brain. When expressed in Xenopus oocytes, SBAT1 mediated Na+-coupled transport of hydrophobic, zwitterionic alpha-amino and imino acids. SBAT1 exhibited a strong preference for branched-chain amino acids (BCAA) and methionine (K0.5 80-160 microM). SBAT1 excluded aromatic or charged amino acids, beta-amino acids, glycine, and GABA. SBAT1-mediated transport of amino or imino acids was extremely temperature-dependent (Q10=9) and was inhibited at acidic pH. PKC activation reduced the plasma-membrane population of SBAT1 protein. SBAT1-mediated transport of BCAA, particularly leucine, may be an important source of amino nitrogen for neurotransmitter synthesis in glutamatergic and GABAergic neurons.

Taurine biosynthetic enzymes and taurine transporter: molecular identification and regulations

Many biological effects of taurine rely upon its cellular concentration, which is primarily controlled by taurine biosynthetic enzymes cysteine dioxygenase (CDO) and cysteine sulfinate decarboxylase (CSD) and taurine transporter (TauT). The cloning of CDO, CSD and TauT in various species provided first-hand information on these proteins, as well as molecular tools to investigate their regulations. CDO upregulation in hepatocytes in response to high sulfur amino acids appears clearly as the most spectacular among the regulations of the biosynthetic enzymes. Downregulation of TauT activity by activation of PKC appears particularly well documented. A unique serine residue could be identified as a phosphorylation site that leads to an inactive form of TauT. The previously revealed downregulation of TauT expression by taurine and hypertonicity-induced upregulation of TauT expression were shown to result from a modified transcription rate of TauT gene, but the precise molecular mechanisms are not yet formally established. Other regulations of taurine transporter expression were more recently reported, which involve glucose, tumor suppressor protein p53, tumor necrosis factor-alpha, and nitric oxide. This review reports the experimental models and data that support these various regulations but also points out the aspects that remain poorly understood or unknown concerning their molecular basis and physiological significance.

Regulation of the cellular content of the organic osmolyte taurine in mammalian cells

Change in the intracellular concentration of osmolytes or the extracellular tonicity results in a rapid transmembrane water flow in mammalian cells until intracellular and extracellular tonicities are equilibrated. Most cells respond to the osmotic cell swelling by activation of volume-sensitive flux pathways for ions and organic osmolytes to restore their original cell volume. Taurine is an important organic osmolyte in mammalian cells, and taurine release via a volume-sensitive taurine efflux pathway is increased and the active taurine uptake via the taurine specific taurine transporter TauT decreased following osmotic cell swelling. The cellular signaling cascades, the second messengers profile, the activation of specific transporters, and the subsequent time course for the readjustment of the cellular content of osmolytes and volume vary from cell type to cell type. Using Ehrlich ascites tumor cells, NIH3T3 mouse fibroblasts and HeLa cells as biological systems, it is revealed that phospholipase A2-mediated mobilization of arachidonic acid from phospholipids and subsequent oxidation of the fatty acid via lipoxygenase systems to potent eicosanoids are essential elements in the signaling cascade that is activated by cell swelling and leads to release of osmolytes. The cellular signaling cascade and the activity of the volume-sensitive taurine efflux pathway are modulated by elements of the cytoskeleton, protein tyrosine kinases/phosphatases, GTP-binding proteins, Ca2+/calmodulin, and reactive oxygen species and nucleotides. Serine/threonine phosphorylation of the active taurine uptake system TauT or a putative regulator, as well as change in the membrane potential, are important elements in the regulation of TauT activity. A model describing the cellular sequence, which is activated by cell swelling and leads to activation of the volume-sensitive efflux pathway, is presented at the end of the review.

Potentiation of effect of PKA stimulation of Xenopus CFTR by activation of PKC: role of NBD2

Activity of the human (h) cystic fibrosis transmembrane conductance regulator (CFTR) channel is predominantly regulated by PKA-mediated phosphorylation. In contrast, Xenopus (X)CFTR is more responsive to PKC than PKA stimulation. We investigated the interaction between the two kinases in XCFTR. We expressed XCFTR in Xenopus oocytes and maximally stimulated it with PKA agonists. The magnitude of activation after PKC stimulation was about eightfold that without pretreatment with PKC agonist. hCFTR, expressed in the same system, lacked this response. We name this phenomenon XCFTR-specific PKC potentiation effect. To ascertain its biophysical mechanism, we first tested for XCFTR channel insertion into the plasma membrane by a substituted-cysteine-accessibility method. No insertion was detected during kinase stimulation. Next, we studied single-channel properties and found that the single-channel open probability (Po) with PKA stimulation subsequent to PKC stimulation was 2.8-fold that observed in the absence of PKC preactivation and that single-channel conductance (gamma) was increased by approximately 22%. To ascertain which XCFTR regions are responsible for the potentiation, we constructed several XCFTR-hCFTR chimeras, expressed them in Xenopus oocytes, and tested them electrophysiologically. Two chimeras [hCFTR NH2-terminal region or regulatory (R) domain in XCFTR] showed a significant decrease in potentiation. In the chimera in which XCFTR nucleotide-binding domain (NBD)2 was replaced with the hCFTR sequence there was no potentiation whatsoever. The converse chimera (hCFTR with Xenopus NBD2) did not exhibit potentiation. These results indicate that potentiation by PKC involves a large increase in Po (with a small change in gamma) without CFTR channel insertion into the plasma membrane, that XCFTR NBD2 is necessary but not sufficient for the effect, and that the potentiation effect is likely to involve other CFTR domains.

Human placental taurine transporter in uncomplicated and IUGR pregnancies: cellular localization, protein expression, and regulation

Transplacental transfer is the fetus' primary source of taurine, an essential amino acid during fetal life. In intrauterine growth restriction (IUGR), placental transport capacity of taurine is reduced and fetal taurine levels are decreased. We characterized the protein expression of the taurine transporter (TAUT) in human placenta using immunocytochemistry and Western blotting, tested the hypothesis that placental protein expression of TAUT is reduced in IUGR, and investigated TAUT regulation by measuring the Na(+)-dependent taurine uptake in primary villous fragments after 1 h of incubation with different effectors. TAUT was primarily localized in the syncytiotrophoblast microvillous plasma membrane (MVM). TAUT was detected as a single 70-kDa band, and MVM TAUT expression was unaltered in IUGR. The PKC activator PMA and the nitric oxide (NO) donor 3-morpholinosydnonimine decreased TAUT activity (P < 0.05, n = 7-15). However, none of the tested hormones, e.g., leptin and growth hormone, altered TAUT activity significantly. PKC activity measured in MVM from control and IUGR placentas was not different. In conclusion, syncytiotrophoblast TAUT is strongly polarized to the maternal-facing plasma membrane. MVM TAUT expression is unaltered in IUGR, suggesting that the reduced MVM taurine transport in IUGR is due to changes in transporter activity. NO release downregulates placental TAUT activity, and it has previously been shown that IUGR is associated with increased fetoplacental NO levels. NO may therefore play an important role in downregulating MVM TAUT activity in IUGR.

Regulation of the human vitamin C transporters expressed in COS-1 cells by protein kinase C [corrected]

Protein kinase C (PKC) regulation of l-ascorbic acid transport mediated by the Na+/ascorbic acid transporters, hSVCT1 and hSVCT2, expressed in COS-1 cells was studied using recombinant carboxyl-terminal V5 epitope-tagged forms of the transporters. The PKC activator phorbol 12-myristate 13-acetate (PMA) caused a time-dependent and concentration-dependent decrease (40-60%) in ascorbic acid transport activity. Effects of PMA were not observed with the inactive phorbol ester 4 alpha-phorbol and were reversed by treatment of the cells with the PKC-specific inhibitor Ro-31-8220. Kinetically, the reduction in hSVCT1 and hSVCT2 activity arose from a decrease in maximal velocity with no change in the apparent affinity. Western blot and confocal microscopy analyses indicated that the total pool of hSVCT1 or hSVCT2 proteins expressed in the transfected COS-1 cells remained unaffected by PMA treatment. For hSVCT1 the decrease in L-ascorbic acid correlated with a redistribution of the transporter from the cell surface to intracellular membranes. However, for hSVCT2 there was no apparent change in transporter distribution, suggesting that the PKC-dependent modulation of L-ascorbic acid transport mediated by hSVCT2 was the result of reduced catalytic transport efficiency.

Regulation of taurine transport at the blood-brain barrier by tumor necrosis factor-alpha, taurine and hypertonicity

Taurine is the abundant sulfur-containing beta-amino acid in brain where it exerts a neuroprotective effect. Although it is known that the blood-brain barrier (BBB) mediates taurine transport, the regulation of taurine transport have not been clarified yet. A conditionally immortalized rat brain capillary endothelial cells (TR-BBB13), an in vitro model of the BBB, exhibited [3H]taurine uptake, which was dependent on both Na+ and Cl-, and inhibited by beta-alanine. Taurine transporter (TAUT) mRNA was detected in TR-BBB13 cells, and TAUT protein was also expressed at 70 kDa. TR-BBB13 cells exposed to 20 ng/mL TNF-alpha and under hypertonic conditions showed a 1.7-fold and 3.2-fold increase in [3H]taurine uptake, respectively. In contrast, lipopolysaccharide and diethyl maleate did not significantly affect taurine uptake. The taurine uptake was reduced by pre-treatment with excess taurine (50 mm). The mRNA level of the TAUT in TNF-alpha and following hypertonic treatment was greater than that in control cells, whereas that under excess taurine conditions was lower than in controls. Therefore, taurine transport activity at the BBB appears to be regulated at the transcriptional level by cell damage, osmolality and taurine in the brain.

Manganese causes differential regulation of glutamate transporter (GLAST) taurine transporter and metallothionein in cultured rat astrocytes

Neurotoxicity due to excessive brain manganese (Mn) can occur due to environmental (air pollution, soil, water) and/ or metabolic aberrations (decreased biliary excretion). Manganese is associated with oxidative stress, as well as alterations in neurotransmitter metabolism with concurrent neurobehavioral deficits. Based on the few existing studies that have examined brain regional [Mn], it is likely that in pathological conditions it can reach 100-500 microM. Amino acid (e.g. aspartate, glutamate, taurine), as well as divalent metal (e.g. zinc, manganese) concentrations are regulated by astrocytes in the brain. Recently, it has been reported that cultured rat primary astrocytes exposed to Mn displayed decreased glutamate uptake, thereby, increasing the excitotoxic potential of glutamate. Since the neurotoxic mechanism(s) Mn employs in terms of glutamate metabolism is unknown, a primary goal of this study was to link altered glutamate uptake in Mn exposed astrocytes to alterations in glutamate transporter message. Further, we wanted to examine the gene expression of metallothionein (MT) and taurine transporter (tau-T) as markers of Mn exposure. Glutamate uptake was decreased by nearly 40% in accordance with a 48% decrease in glutamate/aspartate transporter (GLAST) mRNA. Taurine uptake was unaffected by Mn exposure even though tau-T mRNA increased by 123%. MT mRNA decreased in these Mn exposed astrocytes possibly due to altered metal metabolism, although this was not examined. These data show that glutamate and taurine transport in Mn exposed astrocytes are temporally different.

Decreased taurine concentration in skeletal muscles after exercise for various durations

PURPOSE

To examine the changes of taurine concentrations in blood and skeletal muscles after transient exercise.

METHODS

Rats were placed on a treadmill set at 25 m.min-1. The animals were divided into four groups: control (no exercise) and exercise groups 1, 2, and 3. The exercise duration for groups 1, 2, and 3 was 30, 60, and 100 +/- 12.5 min (to exhaustion: mean +/- SD), respectively. We examined the plasma concentrations of taurine and lactate, the serum concentrations of sodium and chloride ions, as well as the skeletal muscle taurine content in the soleus (slow-twitch fiber dominant type), gastrocnemius (slow- and fast-twitch fiber mix type), and plantaris and extensor digitorum longus (fast-twitch fiber dominant type) muscles.

RESULTS

Although the plasma taurine concentration was not affected by the increased exercise duration, that in skeletal muscles was significantly decreased. The gastrocnemius and plantaris muscles from the exercise group 3 had a significantly lower concentration of taurine than those of the control group. The extensor digitorum longus taurine concentration from the different exercise groups was significantly decreased compared with that from the control group. However, there was no significant difference among the exercise groups.

CONCLUSION

Taurine concentration was decreased in all skeletal muscles after exercise, regardless of the duration. Moreover, this decrease was specific to fast-twitch dominant fibers. However, under these conditions, the plasma taurine concentration remained unchanged.

An improved method for real-time monitoring of membrane capacitance in Xenopus laevis oocytes

Measurements of membrane capacitance (C(m)) in Xenopus laevis oocytes offer unique experimental possibilities but are difficult to perform with current methods. To improve C(m) measurements in the two-electrode voltage clamp (TEVC) mode, we developed a paired-ramp protocol and tested its performance in a model circuit (with tunable C(m), membrane resistance R(m), and series resistance R(s)) and in Xenopus oocytes. In the cell model and with R(s) = 0 Omega, inaccuracy of C(m) estimates was <1% under widely varying conditions (R(m) ranging from 100 to 2000 kOmega, and C(m) from 50 to 1000 nF). With R(s) > 0 Omega, C(m) was underestimated by a relative error epsilon closely approximated as epsilon approximate 2 x R(s)/(R(s) + R(m)), in keeping with the theoretical prediction. Thus, epsilon may be neglected under standard conditions or, under extreme conditions, corrected for if R(s) is known. Relative imprecision of C(m) estimates was small, independent of R(s), and inversely related to C(m) (<1.5% at 50 nF, <0.4% at 200 nF). Averaging allowed reliable detection of C(m) deviations from 200 nF of 0.1 nF, i.e., 0.05%. In Xenopus oocytes, we could resolve C(m) changes that were small (e.g., DeltaC(m) approximate 2 nF upon 100 muM 8-Br-cAMP), fast (e.g., DeltaC(m)/Deltat approximate 20nF/30s upon 1 muM phorbol myristate acetate (PMA)) or extended and complex (e.g., fast increase, followed by prolonged C(m) decrease upon 1 muM PMA). Rapidly alternating between paired ramps and a second, step protocol allowed quasi-simultaneous monitoring of additional electrical parameters such as R(m), slope conductance g(m), and reversal potential E(rev). Taken together, our method is suited to monitor C(m) in Xenopus oocytes conveniently, with high temporal resolution, accuracy and precision, and in parallel with other electrical parameters. Thus, it may be useful for the study of endo- and exocytosis and of membrane protein regulation and for electrophysiological high-throughput screening.

Taurine release in the developing and adult mouse hippocampus: involvement of cyclic guanosine monophosphate

The inhibitory neuromodulator taurine is involved in osmoregulation and cell volume adjustments in the central nervous system. In addition, taurine protects neural cells from excitotoxicity and prevents harmful metabolic events evoked by cell-damaging conditions. The release of taurine in nervous cell preparations is greatly enhanced by glutamate receptor agonists and various cell-damaging conditions. NO-generating compounds also increase taurine release in the mouse hippocampus. The further involvement of the NO/cGMP pathway and protein kinases in preloaded [3H]taurine release from hippocampal slices from adult (3-month-old) and developing (7-day-old) mice in normoxia and in ischemia was now studied using a superfusion system. The release was enhanced by 8-Br-cGMP and the phosphodiesterase inhibitor 2-(2-propyloxyphenyl)-8-azapurin-6-one (zaprinast), particularly in the immature hippocampus, indicating that increased cGMP levels induce taurine release. The release was also increased by the inhibitor of soluble guanylyl cyclase, 1H-(1,2,4)oxadiazolo-(4,3a)quinoxalin-1-one (ODQ) and the protein kinase C activator 4beta-phorbol 12-myristate 13-acetate (PMA), but only in the adult hippocampus. The ischemia-induced release was also enhanced by increased cGMP levels in both adult and developing mice, whereas protein kinase inhibitors had no effects in any conditions. The results demonstrate that cGMP is able to modulate hippocampal taurine release in both adult and developing mice, the rise in cGMP levels evoking taurine release in normoxia and in ischemia. This could be part of the neuroprotective properties of taurine, being thus important particularly in cell-damaging conditions and in preventing excitotoxicity.

Inhibition of the glutamate transporter EAAC1 expressed in Xenopus oocytes by phorbol esters

Recent evidence indicates that second messengers and protein kinases regulate the activity and expression of glutamate transporters. The aim of the present study was to determine if direct activation of protein kinases C or A modulates the activity of the sodium-dependent glutamate transporter EAAC1. EAAC1 modulation was studied in cRNA-injected Xenopus oocytes by measuring [3H]L-glutamate uptake or glutamate-evoked uptake currents. We found that activation of PKA was ineffective, whereas treatment with the PKC agonist phorbol 12-myristate 13-acetate (PMA) caused a significant decrease in EAAC1 transport activity (IC(50)=44.7+/-12 nM). PMA-induced EAAC1 inhibition was PKC-mediated because the inhibition could be blocked by specific PKC inhibitors and incubation with the inactive 4alpha-phorbol-12,13-didecanoate (4alpha-PDD) did not affect EAAC1. Saturation studies of glutamate-evoked uptake currents showed that PMA-mediated inhibition was due to a decrease in I(max) with no change in K(m). PMA simultaneously decreased membrane capacitance (C(m)) and transport-associated current and increased cytosolic accumulation of EAAC1 protein, compared to control. These results suggest that PKC activation inhibits EAAC1 by promoting its retrieval from the plasma membrane. PMA also significantly decreased glutamate uptake in a Madin-Darby canine kidney (MDCK) cell line stably transfected with EAAC1 but enhanced EAAC1-mediated glutamate uptake in the rat C6 glioma cells, consistent with previous observations. Because activation of PKC by phorbol esters leads to opposite effects on EAAC1 activity in different culture models, we conclude that the PKC-mediated regulation of EAAC1 is cell-type specific.

Molecular characterization of taurine transport in bovine aortic endothelial cells

Cultured bovine aortic endothelial (BAE) cells expressed a Na(+)/Cl(-)-dependent taurine uptake activity that saturated with an apparent K(0.5) of approximately 4.9 microM for taurine and was inhibited by beta-alanine, guanidinoethane sulfonate, and homotaurine. We isolated a taurine transporter clone from a BAE cell cDNA library that revealed >91% sequence identity at the amino acid level to the previously cloned high-affinity mammalian taurine transporters. The biochemical and pharmacological properties of the bovine taurine transporter cDNA expressed in Xenopus oocyte was similar to those of the high-affinity taurine transporter. Surprisingly, F(-) blocked taurine uptake in BAE cells with an IC(50) of approximately 17.5 mM. The endogenous taurine uptake was also inhibited by the protein kinase C activator phorbol 12-myristate 13-acetate, but not by its inactive analog, 4 alpha-phorbol 12,13-didecanoate. The endogenous uptake was stimulated, however, by hypertonic stress and the increase was due to an increase in the V(max) of taurine uptake. Our results provide the first description of a molecular mechanism that may be responsible for maintaining the intracellular taurine content in the endothelial cells.

A hyperosmotic stress-induced mRNA of carp cell encodes Na(+)- and Cl(-)-dependent high affinity taurine transporter

A cDNA clone encoding a Na(+)- and Cl(-)-dependent high affinity taurine transporter was isolated from a common carp cell line, Epithelioma papulosum cyprini (EPC), as a hyperosmotic stress-inducible gene by RNA arbitrarily primed PCR. The clone contained a 2.5-kb cDNA fragment including an open reading frame of 1878 bp encoding a protein of 625 amino acids. The deduced amino acid sequence of carp taurine transporter shows 78-80% identity to those of cloned mammalian taurine transporters. The functional characteristics of the cloned transporter were analyzed by expression in COS-7 cells. Transfection with the cDNA induced Na(+)- and Cl(-)-dependent taurine transport activity with an apparent K(m) of 56 microM. The Na(+)/Cl(-)hepatopancreas. Taurine transporter mRNA level increased up to 7.5-fold on raising the ambient osmolality from 300 to 450 mosmol/kgH(2)O. These data suggest the significant role of taurine as an osmolyte in carp cells.

Taurine-induced synaptic potentiation: role of calcium and interaction with LTP

Taurine induces a long-lasting potentiation of excitatory synaptic potentials due to the enhancement of both synaptic efficacy and axon excitability in the CA1 area of rat hippocampal slices. In this study, we characterized the role of Ca2+ in the generation of these long-lasting taurine effects. Taurine perfusion in a free-Ca2+ medium did not induce changes in either field excitatory synaptic potentials (fEPSP) slope or fiber volley (FV) amplitude. Intracellular recordings with a micropipette filled with the Ca2+ chelator BAPTA, prevented the EPSP potentiation induced by taurine in the impaled cell, whereas a long-lasting potentiation of the simultaneously recorded fEPSP was obtained. The depletion of intracellular Ca2+ stores by thapsigargin (1 microM), an inhibitor of endosomal Ca2+-ATPase, transformed the taurine-induced potentiation into a transitory process that declined to basal values after taurine withdrawal. Taurine-induced potentiation was not significantly affected by kynurenate (glutamate receptor antagonist), or nifedipine (high-voltage-activated Ca2+ channel antagonist). But, the presence of nickel (50 microM), an antagonist of low-voltage-activated Ca2+ channel, inhibited the taurine-induced potentiation, indicating that Ca2+ influx through this type of Ca2+ channels could account for the Ca2+ requirement of the taurine-induced potentiation. Occlusion experiments between tetanus-induced long-term potentiation (LTP) and taurine-induced potentiation indicate that both processes share some common mechanisms during the maintenance period.

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.

Ser-322 is a critical site for PKC regulation of the MDCK cell taurine transporter (pNCT)

Previous studies have shown that the Madin-Darby canine kidney cell taurine transporter (pNCT) is downregulated by protein kinase C (PKC) activation. In this study, it is hypothesized that the highly conserved serine-322 (Ser-322) located in the fourth intracellular segment (S4) may play an important role in the function of taurine transporter, which is modulated by PKC phosphorylation. It is demonstrated that Ser-322 is the critical site of PKC phosphorylation, as determined by site-directed mutagenesis. When Ser-322 of pNCT was changed to alanine (S322A) and this mutant was evaluated in an oocyte expression system, taurine transport activity increased threefold compared with control (wild-type pNCT). Activation of PKC by the active phorbol ester 12-myristate 13-acetate did not influence taurine transport by mutant S322A. Kinetic analysis showed that the mutation of Ser-322 essentially changed the Vmax, rather than the Km, of the transporter. Mutation of all other PKC consensus sites did not affect transporter activity when expressed in the oocyte system. Western blot analysis showed that expression of taurine transporter protein was similar in oocytes injected with either wild-type or mutant pNCT cRNA, indicating that the enhanced taurine transport activity by mutant S322A was not caused by a greater amount of transporter expressed in the oocyte. Furthermore, this study demonstrated that the taurine transporter was phosphorylated after PKC activation, and this effect was not observed in mutant S322A. In conclusion, Ser-322 is critical in PKC regulation of taurine transporter activity. The steady-state taurine transporter activity is tightly controlled by endogenous PKC phosphorylation of Ser-322, which is located in the fourth intracellular segment of the taurine transporter.

Protein kinase C-mediated regulation of the renal Na(+)/dicarboxylate cotransporter, NaDC-1

The Na(+)/dicarboxylate cotransporter of the renal proximal tubule, NaDC-1, reabsorbs Krebs cycle intermediates, such as succinate and citrate, from the tubular filtrate. Although long-term regulation of this transporter by chronic metabolic acidosis and K(+) deficiency is well documented, there is no information on acute regulation of NaDC-1. In the present study, the transport of succinate in Xenopus oocytes expressing NaDC-1 was inhibited up to 95% by two activators of protein kinase C, phorbol 12-myristate, 13-acetate (PMA) and sn-1, 2-dioctanoylglycerol (DOG). Activation of protein kinase A had no effect on NaDC-1 activity. The inhibition of NaDC-1 transport by PMA was dose-dependent, and could be prevented by incubation of the oocytes with staurosporine. Mutations of the two consensus protein kinase C phosphorylation sites in NaDC-1 did not affect inhibition by PMA. The inhibitory effects of PMA were partially prevented by cytochalasin D, which disrupts microfilaments and endocytosis. PMA treatment was also associated with a decrease of approximately 30% in the amount of NaDC-1 protein found on the plasma membrane. We conclude that the inhibition of NaDC-1 transport activity by PMA occurs by a combination of endocytosis and inhibition of transport activity.

Protein kinase C activators induce membrane retrieval of type II Na+-phosphate cotransporters expressed in Xenopus oocytes

1. The rate of inorganic phosphate (Pi) reabsorption in the mammalian kidney is determined by the amount of type II sodium-coupled inorganic phosphate (Na+-Pi) cotransport protein present in the brush border membrane. Under physiological conditions, parathyroid hormone (PTH) leads to an inhibition of Na+-Pi cotransport activity, most probably mediated by the protein kinase A (PKA) and/or C (PKC) pathways. 2. In this study, PKC-induced inhibition of type II Na+-Pi cotransport activity was characterized in Xenopus laevis oocytes using electrophysiological and immunodetection techniques. Transport function was quantified in terms of Pi-activated current. 3. Oocytes expressing the type IIa rat renal, type IIb flounder renal or type IIb mouse intestinal Na+-Pi cotransporters lost > 50 % of Pi-activated transport function when exposed to the PKC activators DOG (1,2-dioctanoyl-sn-glycerol) or PMA (phorbol 12-myristate 13-acetate). DOG-induced inhibition was partially reduced with the PKC inhibitors staurosporine and bisindolylmaleimide I. Oocytes exposed to the inactive phorbol ester 4alpha-PDD (4alpha-phorbol 12,13-didecanoate) showed no significant loss of cotransporter function. 4. Oocytes expressing the rat renal Na+-SO42- cotransporter alone, or coexpressing this with the type IIa rat renal Na+-Pi cotransporter, showed no downregulation of SO42--activated cotransport activity by DOG. 5. Steady-state and presteady-state voltage-dependent kinetics of type II Na+-Pi cotransporter function were unaffected by DOG. 6. DOG induced a decrease in membrane capacitance which indicated a reduction in membrane area, thereby providing evidence for PKC-mediated endocytosis. 7. Immunocytochemical studies showed a redistribution of type II Na+-Pi cotransporters from the oolemma to the submembrane region after DOG treatment. Surface biotinylation confirmed a DOG-induced internalization of the transport protein. 8. These findings document a specific retrieval of exogenous type II Na+-Pi cotransporters induced by activation of a PKC pathway in the Xenopus oocyte.

Taurine-stimulated outgrowth from the retina is impaired by protein kinase C activators and phosphatase inhibitors

Taurine increases neurite elongation of post-crush goldfish retinal explants, as well as the number of outgrowing isolated cells from goldfish and rat retina in culture. The trophic effect of taurine is related to an elevation in calcium flux rather than increased cell proliferation. Since taurine regulates phosphorylation in rat retina, we investigated if this process could be involved in the mechanism of taurine action on outgrowth. Control and taurine-supplemented post-crush goldfish retinal explants were cultured in the presence of protein kinase C activators or phosphatase inhibitors, and the length of neurites was measured after five days in culture. In some cases, there was an inhibition of the stimulatory effect of taurine without a modification in basal outgrowth. In others, outgrowth of control explants was also reduced. A certain level of protein phosphorylation seems to be critical for the trophic effect of taurine in the retina.

Calyculin A modulates the kinetic constants for the Na+-coupled taurine transport in Ehrlich ascites tumour cells

The effect of the phosphatase inhibitor calyculin A (cal A) on the kinetic parameters of the Na+-coupled taurine uptake via the taurine transporter in the Ehrlich ascites tumour cells has been investigated. Preincubation with cal A (100 nM) reduces the initial taurine influx by about 20%, but has no effect on the diffusional component of the taurine influx or on the taurine release from cells suspended in isotonic or in hypotonic medium. Thus, cal A-sensitive phosphatases only affect taurine transport mediated by the Na+-dependent taurine transporter. Cal A increases the Michaelis-Menten constant for binding of taurine to the transporter from 31+/-6 to 45+/-4 microM and reduces the taurine transport capacity from 210+/-20 to 170+/-10 nmol x g dry wt(-1) x min(-1) [corrected]. The Michaelis-Menten constant for binding of Na+ to the taurine transporter is concomitantly increased from 96+/-11 to 129+/-8 mM and the Na+:taurine coupling ratio for activation of the transport cycle is reduced from 3.3+/-0.6 to 2.4+/-0.2. This suggests that cal A-sensitive phosphatases maintain a high affinity of the taurine transporter towards Na+ and taurine as well as a high taurine transport capacity in unpertubated Ehrlich cells.

Subcellular redistribution is involved in acute regulation of the brush border Na+/H+ exchanger isoform 3 in human colon adenocarcinoma cell line Caco-2. Protein kinase C-mediated inhibition of the exchanger

Na+/H+ exchanger isoform 3 (NHE3), an epithelial brush border isoform of the Na+/H+ exchanger gene family, plays an important role in reabsorption of Na+ in the small intestine, the colon, and the kidney. In several cell types, phorbol 12-myristate 13-acetate (PMA) acutely inhibits NHE3 activity by changes in Vmax, but the mechanism of this inhibition is unknown. We investigated the role of subcellular redistribution of NHE3 in the PMA-induced inhibition of endogenous brush border NHE3 in a model human colon adenocarcinoma cell line, Caco-2. Subcellular localization of NHE3 was examined by confocal morphometric analysis complemented with cell surface biotinylation and compared with NHE3 activity evaluated by fluorometric measurement of intracellular pH. PMA inhibited NHE3 activity by 28% (p < 0.01), which was associated with a decrease of the ratio of the brush border/subapical cytoplasmic compartment of NHE3 from approximately 4.3 to approximately 2.4. This translocation resulted in 10-15% of the total cell NHE3 being shifted from the brush border pool to the cytoplasmic pool. These effects were mediated by protein kinase C, since they were blocked by the protein kinase C inhibitor H7. We conclude that inhibition of NHE3 by protein kinase C in Caco-2 cells involves redistribution of the exchanger from brush border into a subapical cytoplasmic compartment, and that this mechanism contributes approximately 50% to the overall protein kinase C-induced inhibition of the exchanger.

Thyroid Na+/I- symporter. Mechanism, stoichiometry, and specificity

The rat thyroid Na+/I- symporter (NIS) was expressed in Xenopus laevis oocytes and characterized using electrophysiological, tracer uptake, and electron microscopic methods. NIS activity was found to be electrogenic and Na+-dependent (Na+ >> Li+ >> H+). The apparent affinity constants for Na+ and I- were 28 +/- 3 mM and 33 +/- 9 microM, respectively. Stoichiometry of Na+/anion cotransport was 2:1. NIS was capable of transporting a wide variety of anions (I-, ClO3-, SCN-, SeCN-, NO3-, Br-, BF4-, IO4-, BrO3-, but perchlorate (ClO4-) was not transported. In the absence of anion substrate, NIS exhibited a Na+-dependent leak current (approximately 35% of maximum substrate-induced current) with an apparent Na+ affinity of 74 +/- 14 mM and a Hill coefficient (n) of 1. In response to step voltage changes, NIS exhibited current transients that relaxed with a time constant of 8-14 ms. Presteady-state charge movements (integral of the current transients) versus voltage relations obey a Boltzmann relation. The voltage for half-maximal charge translocation (V0.5) was -15 +/- 3 mV, and the apparent valence of the movable charge was 1. Total charge was insensitive to [Na+]o, but V0.5 shifted to more negative potentials as [Na+]o was reduced. NIS charge movements are attributed to the conformational changes of the empty transporter within the membrane electric field. The turnover rate of NIS was >/=22 s-1 in the Na+ uniport mode and >/=36 s-1 in the Na+/I- cotransport mode. Transporter density in the plasma membrane was determined using freeze-fracture electron microscopy. Expression of NIS in oocytes led to a approximately 2. 5-fold increase in the density of plasma membrane protoplasmic face intramembrane particles. On the basis of the kinetic results, we propose an ordered simultaneous transport mechanism in which the binding of Na+ to NIS occurs first.