Oertzenweg 19 b
Tel.+49 30 838 62600 Fax.+49 30 838-62610
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.