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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

UT-B is expressed in bovine rumen: potential role in ruminal urea transport

¹Faculty of Life Sciences, Stopford Building, The University of Manchester, Manchester; ²School of Cellular and Molecular Biosciences, Division of Physiological Sciences, University Medical School, University of Newcastle-upon-Tyne, Newcastle-upon-Tyne, United Kingdom; and ³United States Meat Animal Research Center, United States Department of Agriculture/Animal Research Service, Clay Center, Nebraska

Submitted 23 February 2005 ; accepted in final form 12 April 2005

The UT-A (SLC14a2) and UT-B (SLC14a1) genes encode a family of specialized urea transporter proteins that regulate urea movement across plasma membranes. In this report, we describe the structure of the bovine UT-B (bUT-B) gene and characterize UT-B expression in bovine rumen. Northern analysis using a full-length bUT-B probe detected a 3.7-kb UT-B signal in rumen. RT-PCR of bovine mRNA revealed the presence of two UT-B splice variants, bUT-B1 and bUT-B2, with bUT-B2 the predominant variant in rumen. Immunoblotting studies of bovine rumen tissue, using an antibody targeted to the NH2-terminus of mouse UT-B, confirmed the presence of 43- to 54-kDa UT-B proteins. Immunolocalization studies showed that UT-B was mainly located on cell plasma membranes in epithelial layers of the bovine rumen. Ussing chamber measurements of ruminal transepithelial transport of ¹⁴C-labeled urea indicated that urea flux was characteristically inhibited by phloretin. We conclude that bUT-B is expressed in the bovine rumen and may function to transport urea into the rumen as part of the ruminant urea nitrogen salvaging process.

facilitative transporter; phloretin; thionicotinamide; transepithelial flux; nitrogen balance

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