Effect of dietary nitrogen level and source on mRNA expression of urea transporters in the rumen epithelium of fattening bulls.

This paper aims to study the effect of the dietary treatments on mRNA expression of urea transporter B (UT-B) and some aquaporins (AQP) in rumen epithelium of Italian Simmental young bulls. Eighty animals allocated to 16 pens were fed from about 500 to 650 kg body weight with four experimental diets, which resulted from the combination of two crude protein levels (125 and 110 g/kg dry matter, diets M and L, respectively) and two nitrogen sources (soybean meal (SBM) or SBM partly replaced by an isonitrogenous mixture of corn and urea; diets -U and +U, respectively). At slaughtering samples of blood and rumen epithelium were collected from six bulls for each diet. Blood samples were analysed for haematological parameters and quantitative PCR was carried out on the mRNA extracted from the rumen epithelium samples. The bulls fed diets M had lower plasma concentrations of aspartate aminotransferase than those receiving diets L (78.9 vs. 88.3 U/l, p = 0.04). Plasma urea was higher (p = 0.03) for diets M and lower for diets +U (2.0 vs. 2.5 and 1.73 vs. 2.00 mmol/l, respectively, in M and L diets, p = 0.04). The effect of dietary treatments on rumen UT expression were limited to AQP3, which was down regulated (p = 0.01) in diets +U. Finally, a high positive correlation (R2  = 0.871) between the expressions of AQP7 and AQP10 was found. In conclusion, the AQP3 appears very responsive to dietary treatments and therefore it is a candidate to be further studied in rumen metabolism experiments. The close relationship between mRNA expression of AQP7 and AQP10 indicates a similar function of these two proteins.