Functional roles of aquaporins and urea transporters in urea flux across the ruminal epithelium

Abstract

In ruminants, urea that is recycled to the rumen is an important source of N for microbial growth. Urea transport (UT-B) proteins facilitate urea movement across the ruminal epithelium, although other mechanisms must be involved as inhibiting UT-B does not completely abolish urea transport (Stewart et al., 2005). Of the aquaporins (AQP), a family of membrane-spanning proteins predominantly involved in water transport, AQP-3, -7, and -10 are also permeable to urea (Litman et al., 2009); however, it is not clear if AQP contribute to urea transport across the ruminal epithelium. Røjen et al. (2011) observed that mRNA abundance for AQP-3, -7, and -10 in ruminal epithelium was altered by dietary N content, suggesting that AQP might be involved in trans-epithelial urea transport. Increasing ruminal carbohydrate digestion improves urea transfer to the rumen (Reynolds and Kristensen, 2008); however, the mechanisms responsible for this response remain obscure. Our objectives were to determine: (1) the relative functional roles of AQP and UT-B in ruminal urea transport; and (2) if functional adaptation of AQP and UT-B occurred in response to increased diet digestion.