Dietary regulation of ruminal bovine UT-B urea transporter expression and localization.

Facilitative UT-B urea transporters have been located in the gastrointestinal tract of numerous mammalian species. We have previously identified UT-B urea transporters within the epithelial layers of the bovine (b) rumen. The aim of this study was to test the hypothesis that ruminal bUT-B urea transporters are regulated by dietary intake. Six Limousine-cross steers (initial BW = 690 +/- 51 kg) were separated into 2 groups fed a basic silage-based diet (RS) or a concentrate-based diet (RC) for 37 d and compared for ruminal morphology, content, and bUT-B expression. Analysis by reverse transcription-PCR showed that ruminal bUT-B2 mRNA expression was greater in RC-fed than RS-fed animals. Utilizing an anti-bUT-B antibody, we also detected a significant increase in bUT-B2 protein expression in RC-fed rumen (P < 0.05, n = 3). In agreement with these findings, immunolocalization studies of RC-fed ruminal tissue showed strong bUT-B signals throughout all epithelial layers, in contrast to weaker staining in RS-fed rumen that was more localized to the stratum basale. This study therefore confirmed that ruminal bUT-B urea transporter expression and localization were indeed altered by changes in dietary intake. We conclude that UT-B transporters play a significant role in the dietary regulation of bovine nitrogen balance.