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Gastrointestinal recycling of urea involves entry through specific proteins such as UT-B and is thought to stimulate fermentation by supplying nitrogen for microbial protein synthesis. However, more typically, excessive quantities of NH3 are produced from dietary protein that have to be absorbed and converted to urea by the liver in an ATP-dependent process. The quantities of nitrogen recycled (e.g. 20 mol·d-1 in the rumen of cows) suggest additional functions of nitrogen recycling.
Urea transport rates across isolated pieces of ruminal epithelium were measured in Ussing chambers. The potential across the apical membrane and the tissue was investigated using microelectrode and Ussing chamber techniques in parallel.
Urea transport was stimulated by acidification of mucosal pH, reaching maximal values at pH 5.8. Luminal ammonia inhibited urea transport partially, with maximal effects at 5 mmol·l-1 ammonia. The effects of ammonia on urea transport could be reduced by clamping the tissue (serosa: + 25 mV), suggesting interaction between urea secretion and NH4+ absorption. Mucosal exposure of the tissue to NH4Cl at pH 6.4 resulted in a significant depolarization of both the apical membrane and PDt (basolateral vs. apical).
Entry of urea into the rumen is complexly regulated by ruminal pH and uptake of NH4+. At acidic pH, urea nitrogen primarily leaves the rumen in the form of NH4+ through apical and basolateral ion channels, thus contributing to the removal of ruminal protons. This suggests a dual role for urea recycling in the regulation of both ruminal fermentation and ruminal buffering.