The effect of dietary CP and NFC different levels on UT-B expression: A meta-analysis

As is already known, urea transport across the ruminant wall is mediated via urea transporters in the epithelium membrane. Facilitative urea transporters are derived from the UT-A and UT-B genes. UT-B mRNA or protein expressions have been characterized in the rumen epithelium (Ludden et al 2009; Marini and Van Amburgh 2003; Marini et al 2004; Stewart et al 2005). Gene transcript abundance for UT-B was observed to be significantly correlated with the ruminal serosal to mucosal urea fluxes. However, the mechanism by which the increased gene expression occurred is unclear (Jin et al 2018). Transcriptome analysis has been used to analyze the rumen epithelium metabolic pathway changes under various conditions (Baldwin et al 2012; Dionissopoulos et al 2014; Naeem et al 2014), and this approach may provide a better means to understand the regulation of these urea transport mechanisms across the rumen wall.

A literature search was initially conducted using various data bases and investigated references in the papers. It was also based on the following key words: carbohydrate, dietary, protein and urea transporter. Then 11 studies were included in current meta-analysis; and prepared comparisons for pH, VFAS, N-NH3 and UT-B expression. Meta-analyses were carried out using the Comprehensive Meta-Analysis package, version 3. The effect sizes of across studies were calculated with fixed and random effect models. The presence of true heterogeneity among studies was identified with Cochran’s Q–tests and quantification of the degree of heterogeneity was done with the I2 index. Possible publication bias was evaluated with funnel plot and statistical tests (Vesterinen et al 2014).

The results of current meta-analysis showed that different levels of CP in diet, has a positive effects on ruminal N-NH3 concentration and UT-B expression (<0.05). However, dietary CP different levels had no significant effects on ruminal pH and VFAS concentration. The values of I2 and Q for ruminal N-NH3 concentration and UT-B expression effect size indicated high and moderate heterogeneity, respectively. Marini and Van Amburgh (2003) noticed expression of ruminal UT-B was changed by nitrogen intake levels. Marini and Van Amburgh (2003) observed greater expression of UT-B (based upon visual evaluation) in ruminal papillae collected from the ventral sac of the rumen in dairy heifers fed high-N diets (2.97 to 3.4% N) compared with low-N diets (1.45 to 1.89% N). Marini and Van Amburgh (2003) suggested that when a high-N diet is fed and ruminal ammonia is high, urea diffuses into the gastrointestinal tract via the paracellular space. Urease activity is known to be reduced by high ammonia concentrations (Bunting et al. 1989), a condition that arises when high-N diets are fed. Therefore, it is possible that the amount of crude protein increase UT-B abundance in the ventral rumen (Ludden et al. 2009). Also, Lu et al (2014) reported that by increasing nitrogen intake levels and increasing the blood urea nitrogen concentration to more than 5 mmol/l, the return of urea to the rumen and stimulate the expression of urea transporter B increase, linearly. Different levels of NFC in diet increase ruminal VFAS concentration and UT-B expression (<0.05). Dietary NFC different levels decrease ruminal pH (<0.05). Ruminal N-NH3 concentration was not influenced by Dietary NFC different levels. The values of I2 and Q for pH and UT-B expression effect size indicated high heterogeneity. Lu et al (2019) reported that the NFC-rich diet caused a reduction of urinal urea-N excretion by about 37%. Ma et al (2015) observed that, in lamb receiving a MNFC diet, the urinary N declined, but that fecal N did not change (Ma et al., 2015). Similarly, in growing steers, dietary carbohydrate facilitated urea being transferred from the blood directly to the rumen (Huntington, 1989; Younes et al., 2001). Lu et al (2019) observed that, in goats consuming an NFC-rich diet, the concentration of ruminal NH3-N was enhanced significantly, together with a significantly reduced urinal urea-N excretion. These data indicate that more urea-N was transferred into the rumen. This findings suggest that adequate dietary NFC feeding can improve nitrogen salvage in ruminant animals (Lu et al., 2019). The effect of dietary NFC on serum urea-N is associated with rumen microbes, which use plant carbohydrate as a fermentation substrate to obtain energy for microbial growth and as an indirect supply to host animals. Among the composition of plant carbohydrate, lignin is known to be non-degradable in anaerobic environments (Triolo et al., 2012). Dietary NFC, namely a fraction of easily-fermentable carbohydrate that includes starches, sugars, pectins, and glucans (Van Soest et al., 1991), leads to the rapid production of microbial metabolites, which are mainly SCFA (Hoover and Stokes, 1991). Previous studies have demonstrated that SCFA regulates blood urea entry into the rumen (Harmeyer and Martens, 1980; Remond et al., 2003). Remond et al. (2003) have reported that the highest rates of urea influx into the rumen and the highest NH3- N concentration in the rumen appears postprandially, when fermentation processes lead to a raise of SCFA in the rumen. Ruminal SCFA rapidly promotes blood urea across the rumen epithelium. A decrease of pH from 7.4 to 6.6 in the mucosal buffer exerted the same effect on flux Jsm urea as SCFA did (Lu et al., 2014). These data are in good agreement with another previous ex vivo study (Abdoun et al., 2010; Lu et al., 2014) providing supporting evidence that SCFA and pH stimulate blood urea entry into the rumen (Lu et al., 2019). According to the results of current meta-analysis, both dietary CP and NFC increase UT-B expression. It seems however that the NFC have a greater effect on increasing UT-B expression than CP.