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    Modulation of urea transport across rumen sheep epithelium in vitro by SCFA and CO2 (2010)

    Art
    Zeitschriftenartikel / wissenschaftlicher Beitrag
    Autoren
    Abdoun, K.
    Stumpff, F.
    Rabbani, I.
    Martens, H.
    Quelle
    American journal of physiology : Gastrointestinal and liver physiology; 298(2) — S. G190–G202
    ISSN: 0193-1857
    Sprache
    Englisch
    Verweise
    Pubmed: 19926818
    Kontakt
    Institut für Veterinär-Physiologie

    Oertzenweg 19 b
    14163 Berlin
    +49 30 838 62600
    physiologie@vetmed.fu-berlin.de

    Abstract / Zusammenfassung

    Urea transport across the gastrointestinal tract involves transporters of the UT-B group, the regulation of which is poorly understood. The classical stimulatory effect of CO2 and the effect of short chain fatty acids (SCFA) on the ruminal recycling of urea was investigated by using Ussing chamber and microelectrode techniques with isolated ruminal epithelium of sheep. The flux of urea was found to be phloretin-sensitive and passive. At a luminal pH of 6.4, but not at 7.4, the addition of SCFA (40 mmol?l-1) or CO2/HCO3- (10%/25 mmol?l-1) led to a four-fold increase in urea flux. The stepwise reduction of luminal pH in the presence of SCFA from 7.4 to 5.4 led to a bell-shape modification of urea transport, with a maximum at pH 6.2. Lowering the pH in the absence of SCFA or CO2 had no effect. Inhibition of Na+/H+ exchange increased urea flux at pH 7.4, with a decrease being seen at pH 6.4. In experiments with double-barrelled, pH-sensitive microelectrodes, we confirmed the presence of an apical pH microclimate and demonstrated the acidifying effects of SCFA on the underlying epithelium. We confirm that the permeability of the ruminal epithelium to urea involves a phloretin-sensitive pathway. We present clear evidence for the regulation of urea transport by strategies that alter cytosolic pH (pHi), with permeability being highest after a moderate decrease. The well-known postprandial stimulation of urea transport to the rumen in vivo may involve acute pH dependent effects of intraruminal SCFA and CO2.on the function of existing urea transporting proteins.