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

Abstract

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 (R = 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.

Localization of aquaporin-3 proteins in the bovine rumen

“…(Coyle et al, 2016). Overall, the abundant AQP3 transcript expression agrees with previous studies (Røjen et al, 2011;Saccà et al, 2018) and suggests that AQP3 may play an important role in bovine rumen function. To test for alternatively spliced AQP3 transcripts, we designed a forward primer against exon 1 (F1) and a reverse primer against exon 6 (R6).…”

“…Therefore, more detailed experiments are needed to comprehensively characterize the abundance and localization of AQGP. As stated, various studies (Røjen et al, 2011;Berends et al, 2014;Walpole et al, 2015;Saccà et al, 2018) have previously indicated AQP3 transcription is more responsive to diets. Therefore, in this present study we investigated the expression, abundance and localization of AQP3 transporters in the bovine rumen.…”

“…In line with this, linear increases of AQP3 and UT-B transcript expression were also observed, whereas no significant responses occurred for AQP7 and AQP10. Other studies have shown an increase in AQP3 and UT-B transcripts with the addition of solid feed to the diets of milk-fed calves (Berends et al, 2014), and a decrease in AQP3 expression in the rumen of fattening bulls when urea was introduced in the diet (Saccà et al, 2018). However, most of the work has been performed at a gene transcription level, while protein abundance has seldom been investigated.…”

Urea transport and hydrolysis in the rumen: A review

“…Their findings confirmed that UT-A or UT-B transporters were associated with urea transport in ruminal epithelia. Furthermore, when dietary treatments contain urea, AQP-3 gene expression is down-regulated, suggesting a portion of urea flux occurs via facilitated diffusion through AQP ( Saccà et al., 2018 ). The expression of UT-B and AQP-3 is up-regulated as an incremental supplementation of calves with solid feeds with minimal nitrogen contents, which is also an indication of facilitated urea recycling in ruminants ( Berends et al., 2014 ).…”

“…The activity of urea transporters are also affected by different dietary nitrogen contents. Saccà et al. (2018) and Røjen et al.…”