ABSTRACT
BACKGROUND: Experimental studies have shown decreased bile acid (BA) uptake and reduced excretion of cholephilic compounds in pregnant rodents. AIM: To assess the expression and function of the main BA importer, the Na(+)/taurocholate cotransporting polypeptide (Ntcp) in pregnant rats. METHODS: BA uptake and Ntcp expression were studied in control and timed-pregnant rats in late gestation. Ntcp protein, messenger RNA (mRNA) expression, and Ntcp tissue localization were determined by Northern blotting, Western analysis, and tissue immunofluorescence. The activity of three transactivators of the Ntcp promoter: hepatocyte nuclear factor 1-alpha (HNF1-alpha), nuclear receptor heterodimer retinoid X receptor:retinoid acid receptor (RXR:RAR) and signal transducer and activator of transcription 5 (Stat5) was assessed using gel electrophoretic mobility shift assays. RESULTS: A significantly reduced BA uptake and decreased Ntcp mRNA levels (-40%) and protein mass (-60%) was observed in pregnant rats. Nuclear extracts from pregnant rats showed a marked decrease of HNF1-alpha and RXR:RAR binding activities by -80 and -40% of basal activity, respectively. In contrast, binding activity of Stat-5 was increased by 50% in nuclear extracts from pregnant rats. CONCLUSIONS: Pregnancy is associated with reduced Ntcp expression and function in the rat. Our findings suggest that Ntcp down-regulation during pregnancy occurs primarily at the transcriptional level.
Subject(s)
Carrier Proteins/genetics , Carrier Proteins/metabolism , Membrane Transport Proteins , Mitochondrial Proteins , Pregnancy, Animal/metabolism , Saccharomyces cerevisiae Proteins , Taurocholic Acid/pharmacokinetics , Animals , Down-Regulation/physiology , Female , Fluorescent Antibody Technique , Gene Expression/physiology , Liver/metabolism , Organic Anion Transporters, Sodium-Dependent , Pregnancy , Promoter Regions, Genetic/physiology , RNA, Messenger/analysis , Rats , Rats, Sprague-Dawley , Ribosomal Proteins/genetics , Ribosomal Proteins/metabolism , Symporters , Transcription Factors/metabolism , TritiumABSTRACT
1. Micellar solubilization of lipolytic products is an important step in lipid absorption. However, micelles are not absorbed intact; dissociation of lipolytic products from bile salt micelles must occur. The dissociation of micelles has been postulated to occur in an acidic microclimate. 2. The effect of an acidic microclimate on the uptake of micellar fatty acid was examined in the rat intestine. We reported that the presence of a lower pH microclimate is associated with a higher fatty acid uptake, suggesting that a lower pH enhances fatty acid uptakes from the micelles. 3. Fatty acid uptake from solutions containing a constant amount of bile salt (10 mM) and varying amounts of fatty acid (3.3-26.4 mM) revealed a saturation phenomenon which reflects the fatty acid carrying capacity of a 10 mM-taurocholate solution. 4. There was a linear relationship between fatty acid uptake and fatty acid concentration when the micellar solutions contained a constant ratio of fatty acid and taurocholate (1.32). 5. Our results indicate that the fatty acid carrying capacity of the micelle and the number of micelles in the solution are both important determinants for the amount of fatty acids delivered to the microclimate. The amount of fatty acids derived from the dissociation of micelles within the microclimate determines fatty acid uptake by the intestine.
Subject(s)
Fatty Acids/pharmacokinetics , Intestinal Absorption/physiology , Jejunum/metabolism , Animals , Hydrogen-Ion Concentration , In Vitro Techniques , Kinetics , Male , Oleic Acids/pharmacokinetics , Rats , Rats, Inbred Strains , Taurocholic Acid/pharmacokineticsABSTRACT
The effect of streptozotocin (SZ) administration on sodium [14C]taurocholate (TC) transmural transfer was studied in the everted rat ileum. The excretion of fecal bile acids was also studied in living rats injected with that compound. The viability of the preparation used for the in vitro experiments was evaluated by light microscopy and by the rate of glucose uptake by tissue from the mucosal fluid. The results obtained showed that TC transfer to the serosal fluid was impaired after 24 h of SZ injection, as well as the active transport observed in control preparations. The amount of TC accumulated in the intestinal tissue was also diminished. In addition, total ATPase activity of tissue was decreased, and intracellular electrolyte concentration was altered. Therefore, a slower saturation of binding sites could be responsible for the effects of SZ on TC tissue accumulation, and a decreased ATPase activity for the impairment of the TC concentrative transport system. The results observed in vitro were supported by data in vivo because fecal bile acid excretion was significantly diminished in SZ-treated rats.