Colonic bacteria express a urease enzyme that enables them to use host-derived urea as a nitrogen source (Fuller & Reeds, 1998). We have previously reported the possible role in this host-microbial relationship of facilitative UT-A urea transporters located in the mouse colon, and have identified phloretin-inhibitable urea transport in this tissue (Stewart et al. 2002). The aim of this study was to further characterise the urea transport in mouse colon in order to investigate the precise role of UT-A transporters.

Tissue was obtained from humanely killed male adult MF1 mice. Refractive light flux experiments using colonic plasma membrane vesicles confirmed the presence of a significant urea flux (n = 10, P < 0.05, ANOVA) that was inhibited by 0.1 mM phloretin (n = 4, P < 0.01, ANOVA). Although a similar flux was observed with glutamate, this was not inhibited by phloretin (n = 3, P > 0.05, ANOVA). Since UT-B transporters are inhibited by mercury compounds while UT-A proteins are not (Martial et al. 1996), the fact that 0.5 mM mercury chloride had no effect on the urea flux (n = 3, P > 0.05, ANOVA) suggests UT-B proteins are not involved. Immunoblotting experiments confirmed that UT-B proteins were not present in the colonic vesicles, but that 34- and 100-kDa UT-A proteins were present. Immunoblots of serially centrifuged samples of mouse colon also indicated UT-A proteins were found in plasma membrane-containing fractions, while UT-B proteins were restricted to fractions containing intracellular proteins. Our results therefore suggest that the UT-A transporters located in the mouse colon are responsible for the observed phloretin-inhibitable urea flux.

This work was funded by the BBSRC and The Royal Society.