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Background: Urea transferred from the blood to the digestive tract of ruminants and non-ruminants is a potentially important source of N for microbial growth in the gut (1), serving as a significant source of N when dietary intake is low. Recent studies have demonstrated urea transporter (UT) mRNA and protein in the rumen and colon epithelium of sheep (2) and cows (3). In this study, we investigated if urea flux across the rumen epithelium occurs paracellularly or transcellulary, and if there are indications for electrogenic cotransport.
Methods: Urea flux across sheep rumen epithelia was studied in Ussing-chambers. Isolated cells were monitored for effects of urea using the patch clamp technique.
Results: The Ussing chamber and patch clamp data did not show any impact of urea on transepithelial current, tissue conductance, transmembrane current or cell capacitance. Likewise, transepithelial potential had no impact on urea fluxes and no correlation with mannitol flux could be found. Conversely, reducing the mucosal pH from 7.4 to 6.4 resulted in an almost seven-fold increase in the serosal to mucosal urea flux rates, without affecting the mannitol flux in the same direction. Addition of amiloride to the mucosal side enhanced urea transport across the epithelium in a dose dependent manner, with a maximal rate of transport seen at a dose of 0.6 mmol/l.
Conclusion: The data suggest that transport of urea across the rumen epithelium involves an electrically silent mechanism. The response to variation of intracellular and extracellular pH suggests a mainly transcellular pathway, the exact nature of which remains to be determined.
1) Kennedy, PM; Milligan, LP (1980). Can. J. Anim. Sci. 60: 205-221.
2) Ritzhaupt, A; Wood, IS; Jackson, A.A.; Moran, JB; Shirazi-Beechey, PS (1998). Biochem. Soc. Trans. 26: S122.
3) Stewart GS; Graham C; Cattell S; Smith TPL; Simmons NL; Smith, CP (2005). Am. J. Physiol. 289: R605?R612.
Grants: Margarete-Markus-Charity; Alexander von Humboldt Foundation