Inhibitory effect of ammonia on urea flux across rumen epithelium depends on level of serosal urea

Abstract

Ruminants perpetually depend on their ability to transfer urea from blood to the rumen in order to maintain a positive N balance. Estimates indicate that 40 to 80% of endogenously-produced urea can be recycled to the rumen where it provides ruminally-available N for microbial growth (Lapierre and Lobley, 2001), so it is important to understand the regulatory mechanisms that control trans-epithelial urea transfer into the rumen. Increasing ruminal ammonia (NH3-N) concentration decreases trans-epithelial urea transfer from blood into the rumen (Abdoun et al., 2009); however, the mechanisms that are responsible for this response are uncertain. Facilitative urea transporter proteins (UT-B) are expressed in ruminal epithelium and the addition of phloretin, an inhibitor of UT-B function, reduces trans-epithelial flux of urea in isolated ruminal epithelium (Stewart et al., 2005), thus providing proof that these UT-B might have a functional role in ruminal urea transfer. However, the functional role of UT-B in mediating the effects of ruminal NH3-N on trans-epithelial flux of urea remains obscure. We evaluated the effects of mucosal NH3 and serosal urea concentrations on total and UT-B-dependent (i.e. phloretin-sensitive) urea flux across the isolated bovine ruminal epithelium.