Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “TRANSPORTATION”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 559 records · Page 31Linked to original sources
PubMed1997

Membrane transport in hepatic clearance of drugs. II: Zonal distribution patterns of concentration-dependent transport and elimination processes.

PURPOSE: The objective of the present simulation study was to investigate the effects of hepatic zonal heterogeneity of membrane transporter proteins and intrinsic elimination activities on hepatic clearance (CL) and drug concentration gradient profiles in the sinusoidal blood and hepatocytes. METHODS: The model used in the simulations assumes an apparent unidirectional carrier-mediated transport and a bidirectional diffusion of substrates in the hepatic sinusoidal membrane as well as a nonlinear intrinsic elimination. Three different distribution patterns of the transporter and the metabolizing enzyme along the sinusoidal flow path were used for the simulations. The effects of changes in the Michaelis-Menten parameters for those nonlinear processes, and in the unbound fractions of the drug in blood and tissue components were investigated. RESULTS: Significant differences in CL occurred when the distribution patterns of the transporter and/or the metabolizing enzyme activities were altered under nonlinear conditions. The highest CL values were observed when the transporter and the metabolizing enzyme had similar distribution patterns within the liver acinus, while opposite distribution patterns produced the lowest CL values. Tissue concentration profiles were significantly affected by the distribution patterns of the transporter, but the changes in blood concentration profiles were relatively small. Altering protein binding in blood produced significant changes in CL, and blood and tissue concentration gradients, while altering protein binding in tissue affected only drug accumulation patterns within hepatocytes, regardless of the distribution patterns of the transporter or the metabolizing enzyme. CONCLUSIONS: The present simulations demonstrate that hepatic zonal heterogeneities in the transporter and the metabolizing enzyme activities can significantly influence hepatic clearance and/or drug concentration gradient profiles in the sinusoidal blood and hepatocytes.

Biological Transport↗
PubMed1986

Erythrocyte nucleoside and sugar transport. Endo-beta-galactosidase and endoglycosidase-F digestion of partially purified human and pig transporter proteins.

Nucleoside- and glucose-transport proteins isolated from human erythrocyte membranes were photoaffinity-labelled with [3H]nitrobenzylthioinosine and [3H]cytochalasin B, respectively, and subjected to endo-beta-galactosidase or endoglycosidase-F digestion. Without enzyme treatment the two radiolabelled transporters migrated on SDS/polyacrylamide gels with the same apparent Mr (average) of 55,000. Apparent Mr (average) values after endo-beta-galactosidase digestion were 47,000 and 48,000 for the nucleoside and glucose transporters respectively, and 44,000 and 45,000 respectively after endoglycosidase-F digestion. In contrast, endo-beta-galactosidase had no effect on the electrophoretic mobility of the nucleoside transporter isolated from pig erythrocytes. This transport system exhibited a higher Mr than the human protein, endoglycosidase-F treatment decreasing its apparent Mr (average) from 64,000 to 57,000. It is concluded that the human and pig erythrocyte nucleoside transporters are glycoproteins containing N-linked oligosaccharide. The data provide evidence of substantial carbohydrate and polypeptide differences between the human and pig erythrocyte nucleoside transporters, but evidence of molecular similarities between the human erythrocyte nucleoside and glucose transporters.

Affinity Labels↗
PubMed1997

Relative contributions of Na+-dependent phosphate co-transporters to phosphate transport in mouse kidney: RNase H-mediated hybrid depletion analysis.

Reabsorption of Pi in the proximal tubule of the kidney is an important determinant of Pi homoeostasis. At least three types (types I-III) of high-affinity Na+-dependent Pi co-transporters have been identified in mammalian kidneys. The relative roles of these three types of Na+/Pi co-transporters in Pi transport in mouse kidney cortex have now been investigated by RNase H-mediated hybrid depletion. Whereas isolated brush-border membrane vesicles showed the presence of two kinetically distinct Na+/Pi co-transport systems (high Km-low Vmax and low Km-high Vmax), Xenopus oocytes, microinjected with polyadenylated [poly(A)+] RNA from mouse kidney cortex, showed only the high-affinity Pi uptake system. Kidney poly(A)+ RNA was incubated in vitro with antisense oligonucleotides corresponding to Npt-1 (type I), NaPi -7 (type II) or Glvr-1 (type III) Na+/Pi co-transporter mRNAs, and then with RNase H. Injection of such treated RNA preparations into Xenopus oocytes revealed that an NaPi-7 antisense oligonucleotide that resulted in complete degradation of NaPi-7 mRNA (as revealed by Northern blot analysis), also induced complete inhibition of Pi uptake. Degradation of Npt-1 or Glvr-1 mRNAs induced by corresponding antisense oligonucleotides had no effect on Pi transport, which was subsequently measured in oocytes. These results indicate that the type II Na+/Pi co-transporter NaPi-7 mediated most Na+-dependent Pi transport in mouse kidney cortex.

Animals↗
PubMed2001

The carboxy-terminus of VirE2 from Agrobacterium tumefaciens is required for its transport to host cells by the virB-encoded type IV transport system.

Agrobacterium tumefaciens transfers DNA from the resident 'tumour-inducing' (Ti) plasmid into plant cells, where it can be stably integrated into the plant genome, ultimately resulting in crown gall tumour formation. The mobilized DNA molecule is a single-stranded intermediate with VirD2 covalently bound to its 5' end. Successful transport of the transferred DNA (T-DNA) and integration of the DNA into the genome requires that additional proteins be transported to the plant as well, including the single-stranded (ss)DNA-binding protein, VirE2. The transport of these two different substrates occurs as a result of the activities of a type IV secretion system encoded by the virB operon. Although the substrates have been identified, the mechanism of their transport remains unknown. In the experiments described here, a region in one of these substrates, VirE2, necessary for transport is identified. The addition of a C-terminal FLAG epitope tag to VirE2, or the deletion of its C-terminal 18 amino acids, renders it non-functional in A. tumefaciens. However, transgenic plants expressing either of these virE2 genes respond to virE2 mutants of A. tumefaciens by forming wild-type tumours. These results indicate that this region of VirE2 is necessary for the protein to be transported into the plant cells, but is not necessary for its function within the plant. Additionally, these studies demonstrate that mutant forms of VirE2 lacking this region do not disrupt the activities of the VirB transporter and support the hypothesis that VirE2 and the VirD2 T-strand are transported independently, even when they co-exist in the same cell.

Agrobacterium tumefaciens↗
PubMed2001

The spatial expression patterns of a phosphate transporter (MtPT1) from Medicago truncatula indicate a role in phosphate transport at the root/soil interface.

The movement of phosphate from the soil into plant root cells is the first of many crucial transport events required to supply phosphorous (P) to cells throughout the plant. In addition to the ability to acquire phosphate from the soil, the majority of the vascular plants are able to form arbuscular mycorrhizal associations in which phosphate may be delivered to the cortex via a fungus. Previously, we cloned two phosphate transporter genes, MtPT1 and MtPT2 from Medicago truncatula roots. Complementation of a yeast phosphate transport mutant revealed that MtPT1 is a functional phosphate transporter and Northern analyses revealed that MtPT1 is expressed exclusively in roots (Liu et al., 1998, Mol. Plant-Microbe Interact. 11, 14--22). Utilising an antibody specific for MtPT1, we have analysed the accumulation and spatial expression patterns of the MtPT1 transporter. MtPT1 transcript and protein levels show close correlation and increase dramatically in the roots in response to phosphate starvation. MtPT1 protein levels decrease in roots during development of a symbiosis with arbuscular mycorrhizal (AM) fungi, indicating that this transporter is not involved in symbiotic phosphate transport. Membrane fractionation and analysis of a MtPT1/GFP fusion protein revealed that MtPT1 is located in the plasma membrane, while in situ hybridisation and immunolocalisation demonstrate the presence of MtPT1 transcripts and protein in the epidermal cells and root hairs of M. truncatula roots. MtPT1 shows expression patterns consistent with a role specifically in the acquisition of phosphate from the soil and is distinct from the other phosphate transporter of this class described to date.

Antibody Specificity↗
PubMed1996

Cytokines and insulin induce cationic amino acid transporter (CAT) expression in cardiac myocytes. Regulation of L-arginine transport and no production by CAT-1, CAT-2A, and CAT-2B.

Cytokine-dependent production of nitric oxide (NO) by rat cardiac myocytes is a consequence of increased expression of the inducible isoform of nitric oxide synthase (iNOS or NOS2) and, in the presence of insulin, depresses the contractile function of these cells in vivo and in vitro. Experiments reported here show that L-lysine, a competitive antagonist of L-arginine uptake, suppressed NO production (detected as nitrite accumulation) by interleukin (IL)-1beta and interferon (IFN) gamma-pretreated cardiac myocytes by 70%, demonstrating that NO production is dependent on L-arginine uptake. Cardiac myocytes constitutively exhibit a high-affinity L-arginine transport system (Km = 125 microM; Vmax = 44 pmol/2 X 10(5) cells/min). Following a 24-h exposure to IL-1beta and IFNgamma, arginine uptake increases Vmax = 167 pmol/2 X 10(5) cells/min) and a second low-affinity L-arginine transporter activity appears (Km = 1.2 mM). To examine the molecular basis for these cytokine-induced changes in arginine transport, we examined expression of three related arginine transporters previously identified in other cell types. mRNA for the high-affinity cationic amino acid transporter-1 (CAT-1) is expressed in resting myocytes and steady-state levels increase by 10-fold following exposure to IL-1beta and IFNgamma. Only cytokine-pretreated myocytes expressed a second high-affinity L-arginine transporter, CAT-2B, as well as a low-affinity L-arginine transporter, CAT-2A. In addition, insulin, which potentiated cytokine-dependent NO production independent of any change in NOS activity, increased myocyte L-arginine uptake by 2-fold and steady-state levels of CAT-1, but not CAT-2A or CAT-2B mRNA. Thus, NO production by cardiac myocytes exposed to IL-1beta plus IFNgamma appears to be dependent on the coinduction of CAT-1, CAT-2A, and CAT-2B, while insulin independently augments L-arginine transport through CAT- 1.

Animals↗
PubMed2003

The human organic anion transport protein SLC21A6 is not sufficient for bilirubin transport.

A recent study (Cui, Y., Konig, J., Leier, I., Buchholz, U., and Keppler, D. (2001) J. Biol. Chem. 276, 9626-9630) suggests that human OATP2 (SLC21A6), also known as OATP-C and LST1, mediates hepatic bilirubin transport. Because of methodologic concerns, this study was designed to examine this issue using a bilirubin transport assay that was validated in overnight cultured rat hepatocytes. These studies showed that cultured rat hepatocytes transported bilirubin with kinetics virtually identical to the transport of sulfobromophthalein. This assay was then used to quantify bilirubin transport by HeLa cells that had been stably transfected with OATP2 under regulation of a metallothionein promoter. Immunoblot analysis revealed abundant expression of OATP2 after incubation of cells for 48 h in zinc, whereas uninduced cells had no expression of this protein. In OATP2-expressing (zinc-induced) HeLa cells at 37 degrees C, the uptake of [35S]sulfobromophthalein was substantial (51.6 +/- 16.5 pmol/15 min/mg protein, n = 5) with little cell-associated ligand in non-expressing (uninduced) cells (0.54 +/- 0.16 pmol/15 min/mg protein, n = 5, p < 0.002). In contrast, there was no difference (p > 0.2) in cell-associated [3H]bilirubin in induced (OATP2-expressing) as compared with uninduced cells (11.25 +/- 3.02 pmol/15 min/mg protein versus 9.15 +/- 1.68 pmol/min/mg protein, respectively, n = 5) We obtained similar results in OATP2-transfected HEK293 cells that were used in the original report. The existence of a bilirubin transporter has been an important field of investigation for many years. Although the current study indicates that a role for OATP2 in hepatocyte bilirubin transport is unlikely, it provides new and sensitive tools that can be adapted to examine the function of putative bilirubin transporters in the future.

Animals↗
PubMed1997

COPI-independent anterograde transport: cargo-selective ER to Golgi protein transport in yeast COPI mutants.

The coatomer (COPI) complex mediates Golgi to ER recycling of membrane proteins containing a dilysine retrieval motif. However, COPI was initially characterized as an anterograde-acting coat complex. To investigate the direct and primary role(s) of COPI in ER/Golgi transport and in the secretory pathway in general, we used PCR-based mutagenesis to generate new temperature-conditional mutant alleles of one COPI gene in Saccharomyces cerevisiae, SEC21 (gamma-COP). Unexpectedly, all of the new sec21 ts mutants exhibited striking, cargo-selective ER to Golgi transport defects. In these mutants, several proteins (i.e., CPY and alpha-factor) were completely blocked in the ER at nonpermissive temperature; however, other proteins (i.e., invertase and HSP150) in these and other COPI mutants were secreted normally. Nearly identical cargo-specific ER to Golgi transport defects were also induced by Brefeldin A. In contrast, all proteins tested required COPII (ER to Golgi coat complex), Sec18p (NSF), and Sec22p (v-SNARE) for ER to Golgi transport. Together, these data suggest that COPI plays a critical but indirect role in anterograde transport, perhaps by directing retrieval of transport factors required for packaging of certain cargo into ER to Golgi COPII vesicles. Interestingly, CPY-invertase hybrid proteins, like invertase but unlike CPY, escaped the sec21 ts mutant ER block, suggesting that packaging into COPII vesicles may be mediated by cis-acting sorting determinants in the cargo proteins themselves. These hybrid proteins were efficiently targeted to the vacuole, indicating that COPI is also not directly required for regulated Golgi to vacuole transport. Additionally, the sec21 mutants exhibited early Golgi-specific glycosylation defects and structural aberrations in early but not late Golgi compartments at nonpermissive temperature. Together, these studies demonstrate that although COPI plays an important and most likely direct role both in Golgi-ER retrieval and in maintenance/function of the cis-Golgi, COPI does not appear to be directly required for anterograde transport through the secretory pathway.

Alleles↗
PubMed1968

Intracellular transport of secretory proteins in the pancreatic exocrine cell. 3. Dissociation of intracellular transport from protein synthesis.

Experiments have been carried out to determine whether intracellular transport of pancreatic secretory proteins is obligatorily coupled to protein synthesis or whether it is a separable process which can be independently regulated. To this intent, guinea pig pancreatic slices were pulse labeled with leucine-(3)H for 3 min and incubated post-pulse for 37 min in chase medium containing cycloheximide up to concentrations sufficient to inhibit protein synthesis by 98%. In controls, newly synthesized secretory proteins are transported over this interval to condensing vacuoles of the Golgi complex. Since the latter are recovered in the zymogen granule fraction upon cell fractionation, intracellular transport was assayed by measuring the amount of protein radioactivity found in the zymogen granule fraction after a (3 + 37) min incubation. The results indicated that at maximum inhibition of protein synthesis (5 x 10(-4)M cycloheximide), transport proceeded with an efficiency approximately 80% of control. Parallel radioautographic studies on intact slices confirmed these data and further indicated that all the steps of intracellular transport, including discharge to the acinar lumen, were independent of protein synthesis. We conclude that: (1) transport and protein synthesis are separable processes; (2) intracellular transport is not the result of a continuous delivery of secretory proteins from attached polysomes to the cisternae of the rough endoplasmic reticulum; and (3) transport is not dependent on the synthesis of "specific" nonsecretory proteins within the time limits tested.

Animals↗
PubMed1974

Effects of serum on membrane transport. II. Serum and the stimulation of adenosine transport, a possible mechanism.

When monolayers of freshly obtained rabbit lung macrophages are exposed to the nucleoside analogue, showdomycin (sho), adenosine transport, measured over a 45 s interval, is irreversibly inhibited. Low doses of the drug or short periods of exposure, however, do not result in decreased transport, while higher concentrations or longer exposures result in exponential decline. The initial lag is not due to a long reaction time of sho with the transport carrier or to nonspecific sites absorbing the drug. Previously it was shown that preincubation of monolayers with normal rabbit serum (NRS) results in increased adenosine transport. When monolayers are first exposed to sho so as to inhibit transport to varying degrees and then incubated with NRS, transport is increased over the inhibited level. Several experiments make it unlikely that serum removes the drug from the cell surface in a nonspecific fashion. Moreover, serum given before, during, or after sho alters the dose response curve so that no shoulder is seen. One way to explain these results makes use of target theory: the adenosine transport system could be comprised mainly of "coupled" or "clustered" sites of which only one is active at any time as well as "hidden" sites which are inactive. When a site in a group is irreversibly inactivated by sho, another in the group becomes activated. Serum might activate or uncouple all sites and also cause the appearance of hidden sites, which previously neither transported nor bound sho.

Adenine Nucleotides↗
PubMed1993

Mapping of a sequence essential for the nuclear transport of the Xenopus ribosomal transcription factor xUBF using a simple coupled translation-transport and acid extraction approach.

The amino acid sequences directing the nuclear transport of the RNA polymerase I transcription factor xUBF have been studied by a novel combination of in oocyte-coupled translation-nuclear transport and selective HCl extraction. Synthetic mRNA was used to direct the translation of labeled xUBF and its mutants in microinjected oocytes. After manual dissection of nuclei and cytoplasm, labeled xUBF and mutants were isolated essentially pure by HCl extraction. Using deletion mutations, a sequence essential, but not necessarily sufficient, for nuclear transport was mapped to a 29-amino-acid segment lying between the most carboxy-terminal putative HMG-box DNA-binding domain, HMG-box 5, and the highly acidic carboxy-terminal domain. It was shown that deletion of only 5 amino acids from this segment eliminated xUBF transport, and it could be deduced that at least 11 of the 29 amino acids were essential for nuclear transport. The segment of xUBF necessary for nuclear transport contains a sequence conforming to the bipartite nuclear transport motif consensus, but this sequence in itself was insufficient for nuclear transport.

Amino Acid Sequence↗
PubMed2006

TCDB: the Transporter Classification Database for membrane transport protein analyses and information.

The Transporter Classification Database (TCDB) is a web accessible, curated, relational database containing sequence, classification, structural, functional and evolutionary information about transport systems from a variety of living organisms. TCDB is a curated repository for factual information compiled from >10,000 references, encompassing approximately 3000 representative transporters and putative transporters, classified into >400 families. The transporter classification (TC) system is an International Union of Biochemistry and Molecular Biology approved system of nomenclature for transport protein classification. TCDB is freely accessible at http://www.tcdb.org. The web interface provides several different methods for accessing the data, including step-by-step access to hierarchical classification, direct search by sequence or TC number and full-text searching. The functional ontology that underlies the database structure facilitates powerful query searches that yield valuable data in a quick and easy way. The TCDB website also offers several tools specifically designed for analyzing the unique characteristics of transport proteins. TCDB not only provides curated information and a tool for classifying newly identified membrane proteins, but also serves as a genome transporter-annotation tool.

Databases, Protein↗
PubMed2005

Functional characterization and expression analysis of a gene, OsENT2, encoding an equilibrative nucleoside transporter in rice suggest a function in cytokinin transport.

We identified four genes for potential equilibrative nucleoside transporters (ENTs) from rice (Oryza sativa; designated OsENT1 through OsENT4). Growth analysis of budding yeast (Saccharomyces cerevisiae) cells expressing OsENTs showed that OsENT2 transported adenosine and uridine with high affinity (adenosine, K(m) = 3.0 microm; uridine, K(m) = 0.7 microm). Purine or pyrimidine nucleosides and 2'-deoxynucleosides strongly inhibited adenosine transport via OsENT2, suggesting that OsENT2 possesses broad substrate specificity. OsENT2-mediated adenosine transport was resistant to the typical inhibitors of mammalian ENTs, nitrobenzylmercaptopurine ribonucleoside, dilazep, and dipyridamole. The transport activity was maximal at pH 5.0 and decreased slightly at lower as well as higher pH. In competition experiments with various cytokinins, adenosine transport by OsENT2 was inhibited by isopentenyladenine riboside (iPR). Direct measurements with radiolabeled cytokinins demonstrated that OsENT2 mediated uptake of iPR (K(m) = 32 microm) and trans-zeatin riboside (K(m) = 660 microm), suggesting that OsENT2 participates in iPR transport in planta. In mature plants, OsENT2 was predominantly expressed in roots. The OsENT2 promoter drove the expression of the beta-glucuronidase reporter gene in the scutellum during germination and in vascular tissues in germinated plants, suggesting a participation of OsENT2 in the retrieval of endosperm-derived nucleosides by the germinating embryo and in the long-distance transport of nucleosides in growing plants, respectively.

Adenosine↗
PubMed1981

Variable Ca2+ transport:phosphoprotein ratios in the early part of the GTP-driven calcium-transport reaction of the sarcoplasmic reticulum.

Initial Ca2+ transport and phosphoprotein formation of the sarcoplasmic reticulum membrane with GTP were investigated in a comparative study. While saturation of the high-affinity sites for Ca2+ binding and transporting as well as for GTP binding on the external surface of the membrane resulted in Ca2+ transport and phosphoprotein formation in a molar ratio of 2, the variation of the concentrations of the two reactants yielded ratios between 1.7 and 5.7. The ratios varied with a similar dependence on the concentrations of Ca2+ and GTP, except at 500 microM Ca2+, if the reaction was started by Ca2+ instead of GTP but the overall rates decreased. 1 mM DL-propranolol in the preincubation medium selectively inhibited Ca2+ transport but had no effect on initial phosphoprotein formation. These observations indicate that:L (a) phosphorylation of one enzyme molecule induces Ca2+ transport by a variable but limited number of neighbouring molecules, (b) not all Ca2+ bound is essential for phosphorylation but can be transported in parallel, (c) Ca2+ bound to low-affinity sites occupied at 500 microM Ca2+ in the reaction medium is also transported initially, (d) the accessibility of the high-affinity Ca2+ binding sites for DL-propranolol differs, (e) DL-propranolol interacts with Ca2+ binding and transporting sites only in that conformation of the enzyme that can be phosphorylated by the nucleotide.

Adenosine Triphosphate↗
PubMed2002

Lactate transport in red blood cells by monocarboxylate transporters.

The lactate transport activity of red blood cells (RBC) varies widely among different species; in equine RBC, the activity of the main lactate carrier, H+-monocarboxylate co-transporter (MCT), is distributed bimodally. The influence of lactate transport activity is measurable in vivo; after maximal exercise, the RBC lactate concentration in horses with high (HT) lactate transport activity is higher than in those with low (LT) activity. To study the expression of MCT in HT and LT horses, blood samples were taken from 10 horses at rest and after submaximal exercise. Blood and plasma lactate concentrations, lactate and pyruvate transport activities and the amounts of MCT1, MCT2 and MCT4 were measured. After exercise, RBC lactate concentration was higher in HT (n = 5) than in LT (n = 5) horses. At lactate concentrations of 0.25-30 mmol/l and at a pyruvate concentration of 1 mmol/l, transport activity was higher in HT horses. At a lactate concentration of 0.1 mmol/l, transport was similar. In Western blots, the signals for MCT1 and MCT2 were similar in both groups. The amount of CD147, a chaperone necessary for the activity of MCT1, was lower in LT horses. We suggest that MCT2 transports lactate at low concentrations, while MCT1 is needed at higher concentrations. MCT1 may be less active in LT horses and, therefore, during exercise their capacity to take up lactate is low. Further studies are needed to show whether the differences in lactate influx in RBC affect the function of erythrocytes or the performance capacity of horses.

Animals↗
PubMed1997

Intestinal brush-border membrane transport of monocarboxylic acids mediated by proton-coupled transport and anion antiport mechanisms.

Intestinal brush-border membrane transport of monocarboxylic acids was investigated by using rabbit intestinal brush-border membrane vesicles (BBMVs) and isolated intestinal tissues mounted on Ussing-type chambers. [3H]Mevalonic acid uptake by BBMVs showed an overshoot phenomenon in the presence of an inwardly directed proton gradient, but not in the presence of an inwardly directed sodium gradient or an outwardly directed HCO3- or chloride gradient. Initial uptake of mevalonic acid was saturable in the presence of a proton gradient. Uptake of [3H]mevalonic acid was inhibited by various monocarboxylic acids, including acetic acid, benzoic acid, lactic acid, nicotinic acid, pravastatin, salicylic acid and valproic acid, but not by dicarboxylic acid or amino acids. Acetic acid, which is transported by both anion antiport and proton-coupled transport systems, induced serosal bicarbonate-dependent alkalinization in the mucosal-side bathing solution of rabbit jejunal tissues, when examined in Ussing-type chambers. Pravastatin, which is a structural analogue of mevalonic acid and is absorbed via proton-coupled transport like mevalonic acid, did not. The result demonstrates that acetic acid is transported by the bicarbonate-dependent anion antiport system, whereas pravastatin is not. So, it is suggested that monocarboxylic acids are transported by at least two independent transporters, namely, a proton-coupled transporter for most monocarboxylic acids, including mevalonic acid, pravastatin and acetic acid, and an anion antiporter for acetic acid, but not for mevalonic acid or pravastatin. Activation of anion antiporter can induce HCO3- secretion in intact intestine.

Acetic Acid↗
PubMed1996

Noradrenaline transport and transporter mRNA of rat chromaffin cells are controlled by dexamethasone and nerve growth factor.

1. The biochemical basis for differences in noradrenaline (NA) transporter function between chromaffin cells in the adrenal medulla and those maintained in primary culture was investigated. 2. Intact adrenal medullae of neonatal rats accumulated small amounts of [3H]NA. In contrast, dissociated chromaffin cells placed in culture for 2-6 days accumulated 100-1000 times more [3H]NA. 3. Nerve growth factor (NGF) stimulated, whereas glucocorticoids dose dependently and reversibly inhibited, [3H]NA transport in chromaffin cells maintained in culture up to 6 days. During this period, no change in the morphological or biochemical characteristics of either NGF-treated or -untreated chromaffin cells was evident. 4. A rat NA transporter cDNA clone was isolated for use in the quantification of NA transporter mRNA. Intact adrenal medullae contained 40% less NA transporter mRNA than an equivalent number of chromaffin cells in culture. Furthermore, dexamethasone produced nearly 90% loss and NGF elicited approximately 60% increase in NA transporter mRNA levels in cultured cells. 5. In cultured cells, and possibly in vivo, glucocorticoids inhibit NA transporter function of chromaffin cells at least in part through a decrease in NA transporter mRNA.

Animals↗
PubMed2003

Aquaporin-1 and HCO3(-)-Cl- transporter-mediated transport of CO2 across the human erythrocyte membrane.

Recent studies have suggested that aquaporin-1 (AQP1) as well as the HCO3(-)-Cl- transporter may be involved in CO2 transport across biological membranes, but the physiological importance of this route of gas transport remained unknown. We studied CO2 transport in human red blood cell ghosts at physiological temperatures (37 degrees C). Replacement of inert with CO2-containing gas above a stirred cell suspension caused an outside-to-inside directed CO2 gradient and generated a rapid biphasic intracellular acidification. The gradient of the acidifying gas was kept small to favour high affinity entry of CO2 passing the membrane. All rates of acidification except that of the approach to physicochemical equilibrium of the uncatalysed reaction were restricted to the intracellular environment. Inhibition of carbonic anhydrase (CA) demonstrated that CO2-induced acidification required the catalytic activity of CA. Blockade of the function of either AQP1 (by HgCl2 at 65 microM) or the HCO3(-)-Cl- transporter (by DIDS at 15 microM) completely prevented fast acidification. These data indicate that, at low chemical gradients for CO2, nearly the entire CO2 transport across the red cell membrane is mediated by AQP1 and the HCO3--Cl- transporter. Therefore, these proteins may function as high affinity sites for CO2 transport across the erythrocyte membrane.

4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid↗