1,721,080 research outputs found
Water channel proteins in the gastrointestinal tract.
Full text linkWater transport through the human digestive system is physiologically crucial for maintaining body water homeostasis and ensure digestive and absorptive functions. Within the gastrointestinal tract, water recirculates, being secreted with the digestive juices and then almost entirely absorbed by the small and large intestine. The importance of aquaporins (AQPs), transmembrane water channel proteins, in the rapid passage of water across plasma membranes in the gastrointestinal tract appears immediately evident. Several AQP isoforms are found in gastrointestinal epithelia, with AQP1, 3, 7, 10 and 11 being the most abundantly expressed in the whole gut. On the other hand, AQP4 and 8 are located selectively in the stomach and colon, respectively. Here we review AQP expression and localization at the tissue, cellular and subcellular level in gastrointestinal epithelia, and their modification in various gut diseases
Thiamine intestinal transport and related issues: recent aspects.
Full text linkIn the intestinal lumen thiamine is in free form and very low concentrations. Absorption takes place primarily in the proximal part of the small intestine by means of a dual mechanism, which is saturable at low (physiological) concentrations and diffusive at higher. Thiamine undergoes intracellular phosphorylation mainly to thiamine pyrophosphate, while at the serosal side only free thiamine is present. Thiamine uptake is enhanced by thiamine deficiency, and reduced by thyroid hormone and diabetes. The entry of thiamine into the enterocyte, as evaluated in brush border membrane vesicles of rat small intestine in the absence of H+ gradient, is Na+- and biotransformation-independent, completely inhibited by thiamine analogs and reduced by ethanol administration and aging. The transport involves a saturable mechanism at low concentrations of vitamin and simple diffusion at higher. Outwardly oriented H+ gradients enhance thiamine transport, whose saturable component is a Na+-independent electroneutral uphill process utilizing energy supplied by the H+ gradient, and involving a thiamine/ H+ 1:1 stoichiometric exchange. The exit of thiamine from the enterocyte, as evaluated in basolateral membrane vesicles, is Na+-dependent, directly coupled to ATP hydrolysis by Na+-K+-ATPase, and inhibited by thiamine analogs. Transport of thiamine by renal brush border membrane vesicles is similar to the intestinal as far as both H+ gradient influence and specificity are concerned. In the erythrocyte thiamine transport is a Na+-independent, electroneutral process yet with two components: saturable, prevailing at low thiamine concentrations, and diffusive at higher. The saturable (specific) component is missing in patients of the rare disease known as thiamine-responsive megaloblastic anaemia (TRMA), producing a general disturbance of thiamine transport up to thiamine deficiency. The TRMA gene is located in chromosome 1q23.3. Recently, the thiamine transporter has been cloned: it is a protein of 497 amino acid residues with high homology with the reduced-folate transporter
Thiamine mono- and pyrophosphatase and nucleoside phosphatases in neuronal and glial cell enriched fractions from human brains
No full textA thiamine/H+ antiport mechanism for thiamine entry into brush border membrane vesicles from rat small intestine.
Full text linkOutwardly oriented H+ gradients greatly enhanced thiamine transport rate in brush border membrane vesicles from duodenal and jejunal mucosa of adult Wistar rats. At a gradient pHin5:pHout7.5, thiamine uptake showed an overshoot, which at 15 sec was three times as large as the uptake observed in the absence of the gradient. Under the same conditions, the binding component of uptake accounted for only 10-13% of intravesicular transport. At the same gradient, the Km and Jmax values of the saturable component of the thiamine uptake curve after a 6 sec incubation time were 6.2 +/- 1.4 microM and 14.9 +/- 3 pmol.mg-1 protein.6 sec-1 respectively. These values were about 3 and 5 times higher, respectively, than those recorded in the absence of H+ gradient. The saturable component of the thiamine antiport had a stoichiometric thiamine: H+ ratio of 1:1 and was inhibited by thiamine analogues, guanidine, guanidine derivatives, inhibitors of the guanidine/H+ antiport, and imipramine. Conversely, the guanidine/H+ antiport was inhibited by unlabeled thiamine and thiamine analogues; omeprazole caused an approximately fourfold increase in thiamine transport rate. In the absence of H+ gradient, changes in transmembrane electrical potential did not affect thiamine uptake. At equilibrium, the percentage membrane-bound thiamine taken up was positively correlated with the pH of the incubation medium, and increased from about 10% at pH 5 to 99% at pH 9
Distribution of thiamine, thiamine phosphates, and thiamine metabolizing enzymes in neuronal and glial cell enriched fractions of rat brain.
Full text linkThe distribution of thiamine, thiamine phosphoesters, and the thiamine pyrophosphate synthetizing [thiamine-pyrophosphokinase (TPKase)] as well as hydrolyzing [thiamine pyrophosphatase (TPPase) and thiamine monophosphatase (TMPase)] enzymes was determined in neuronal and glial enriched fractions prepared from rat brain. Nucleoside diphosphatases [inosine diphosphatase (IDPase) and uridine diphosphatase (UDPase)] and nucleoside monophosphatases [uridine monophosphatase (UMPase) and inosine monophosphatase (IMPase)] were also determined. Thiamine and thiamine mono- and pyrophosphate were present in neuronal enriched fractions at concentrations 2.8, 3.6, and 4.6 times higher than in glial fractions. TMPase was found only in glial enriched fractions, whereas the levels of TPKase, UMPase, IMPase, IDPase, UDPase, and TPPase were 2.0-, 2.2-, 1.3-, 2.8-, 3.7-, and 20.8-fold higher in neuronal than in glial fractions
A monocarboxylate transporter MCT1 is located at the basolateral pole of rat jejunum.
Full text linkWe have functionally expressed and identified a monocarboxylate transporter (MCT1) from rat jejunal enterocyte and we provide evidence for its basolateral localization. Poly(A)+ RNA isolated from rat jejunum was injected into Xenopus laevis oocytes and expression of a proton-lactate symporter was investigated by means of L-[14C]lactate uptake. The existence of an endogenous capacity for L-lactate transport was demonstrated; when, however, oocytes were injected with jejunal mRNA, an expressed L-lactate uptake was seen which differed from the endogenous transporter since it was significantly pH dependent. After sucrose density gradient fractionation, the highest expression of the pH-dependent lactate uptake was detected with the mRNA size fraction of about 2-3 kb in length. The substrate specificity, stereoselectivity and sensitivity to pCMBS (an organomercurial thiol reagent that modifies cysteine residues) of the expressed transport were in good agreement with results previously obtained using isolated jejunal basolateral membranes. Using the reverse transcriptase-polymerase chain reaction, the presence of mRNA coding for the MCT1 isoform was demonstrated in jejunal enterocytes. These data, together with previous results, suggest that MCT1 is a major route for lactate efflux across the basolateral membrane of rat jejunum; this is in contrast to current opinion which restricts the presence of MCT1 to the apical membrane of the whole small intestine
PREVALENZA DEI PROCESSI METABOLICI NEL TRASPORTO DELLA RIBOFLAVINA IN ENTEROCITI ISOLATI DI RATTO
No full textIn isolated rat enterocytes, both normal (normoenergized) and rotenone-deenergized, riboflavin intracellular metabolic processes, operating in association with membrane specific transport mechanism, were investigated. The time course contents of unlabeled (endogenous) and labeled (exogenous) riboflavin compounds (riboflavin, RF; flavin mononucleotide, FMN; and flavin-adenine dinucleotide, FAD) were determined by a HPLC analytical procedure before and after incubation with tritium labeled riboflavin. In normoenergized enterocytes total labeled riboflavin content, which is the sum of riboflavin membrane transported and intracellular metabolized, steadily increased to a plateau up to 20 min incubation; FMN content reached a plateau after 3-20 min, while FAD and free RF increased constantly. The phosphorylated forms prevailed over the free RF form, being about 60% of total flavins. In deenergized enterocytes total riboflavin content was significantly lower than in normoenergized enterocytes and reached a plateau after only 3 min incubation; FMN and FAD contents were significantly lower than in normoenergized enterocytes and free RF represented the prevailing form (about 70% of total RF content). In both normoenergized and deenergized enterocytes the contents of unlabeled total RF, FMN and FAD significantly decreased after 20 min incubation, whereas free RF significantly increased only in normoenergized enterocytes
Thiamin pyrophosphate-dependent enzymes in neuronal, glial and synaptosomal enriched fractions from rat brain
No full textExpression and localization of ryanodine receptors in the frog semicircular canal.
Full text linkSeveral experiments suggest an important role for store-released Ca2+ in hair cell organs: drugs targeting IP3 and ryanodine (RyRs) receptors affect release from hair cells, and stores are thought to be involved in vesicle recycling at ribbon synapses. In this work we investigated the semicircular canal distribution of RyRs by immunofluorescence, using slice preparations of the sensory epithelium (to distinguish cell types) and flat mounts of the simpler nonsensory regions. RyRs were present in hair cells, mostly in supranuclear spots, but not in supporting cells; as regards nonsensory regions, they were also localized in dark cells and cells from the ductus. No labeling was found in nerve terminals, although nerve branches could be observed in proximity to hair cell RyR spots. The differential expression of RyR isoforms was studied by RT-PCR and immunoblotting, showing the presence of RyRα in both ampulla and canal arm and RyRβ in the ampulla only
- …
