Characterization of urea transport mechanisms in the intestinal tract of growing pigs. 
Pigs are capable of nitrogen salvage via urea recycling, which involves the movement of urea in the gastrointestinal tract.  Aquaporins (AQP) and urea transporter B (UT-B) are involved in urea recycling in ruminants; however, their contribution to urea flux in the intestinal tract of the pig is not known.  The objective of this study was to characterize the presence and relative contribution of known urea transporters to urea flux in the growing pig.  Intestinal tissue samples (duodenum, jejunum, ileum, cecum, and colon) were obtained from nine barrows (50.8 ± 0.9 kg) and analyzed for mRNA abundance of UT-B and AQP-3, -7, and -10.  Immediately after tissue collection, samples from the jejunum and cecum were placed in Ussing chambers for analysis of the serosal-to-mucosal urea flux (Jsm-urea) with no inhibition or when incubated in the presence of phloretin to inhibit UT-B-mediated transport, NiCl2 to inhibit AQP-mediated transport, or both inhibitors.  UT-B expression was greatest (P < 0.05) in the cecum, whereas AQP-3, -7, and -10 expression was greatest (P < 0.05) in the jejunum.  The Jsm-urea was greater in the cecum than the jejunum (67.8 .  42.7 ± 5.01 µmol·cm-2·h-1; P < 0.05), confirming the capacity for urea recycling in the gut in pigs; however, flux rate was not influenced (P > 0.05) by urea transporter inhibitors.  The results of this study suggest that, although known urea transporters are expressed in the gastrointestinal tract of pigs, they may not play a significant functional role in transepithelial urea transport. NEW & NOTEWORTHY We characterized the location and contribution of known urea transporters to urea flux in the pig.  Aquaporins are located throughout the intestinal tract, and urea transporter B is expressed only in the cecum.  Urea flux occurred in both the jejunum and cecum.  Transporter inhibitors had no affect on urea flux, suggesting that their contribution to urea transport in the intestinal tract is limited.  Further work is required to determine which factors contribute to urea flux in swine.