فرا‌تحلیل: تاثیر سطوح مختلف پروتئین‌خام و کربوهیدرات غیر الیافی خوراک بر بیان ژن ناقل اوره نوع ب

زمینه مطالعاتی: رابطه مستقیمی بین افزایش بیان ژن ناقل اوره نوع ب و جریان سروزی، مخاطی اوره در شکمبه وجود دارد. با این حال، هنوز نمی‌توان با اطمینان گفت بیان ژن ناقل اوره نوع ب تحت چه شرایطی افزایش می‌یابد. هدف: پژوهش حاضر با هدف بررسی تاثیر سطوح مختلف پروتئین‌خام و کربوهیدرات غیر الیافی خوراک بر بیان ژن ناقل اوره نوع ب به‌صورت فراتحلیل انجام شد. روش کار: پس از جستجو، 11 مقاله انتخاب شد و داده‌های مربوط به غلظت اسیدهای چرب فرار استیک، پروپیونیک و بوتیریک، نیتروژن آمونیاکی، ph و بیان ژن ناقل اوره نوع ب استخراج گردید. جهت انجام فراتحلیل از نرم‌افزار comprehensive metaanalysis (cma) نسخه 3 استفاده شد. با توجه به ماهیت پیوسته داده‌ها از تفاوت میانگین استاندارد شده و شاخص hedges’s g برای محاسبه اندازه اثر استفاده شد. از هر دو مدل اثرات ثابت و تصادفی برای ترکیب اندازه اثر استفاده شد. برای بررسی میزان سوگیری انتشار از نمودار قیفی و آزمون‌های آماری استفاده شد. نتایج: تاثیر پروتئین‌خام بر غلظت نیتروژن آمونیاکی (0001/0p˂) و بیان ژن ناقل اوره نوع ب (003/0p=) مثبت بود. استفاده از سطوح مختلف پروتئین‌خام تاثیر معنی‌داری بر ph و غلظت اسیدهای استیک، پروپونیک و بوتیریک نداشت. مقادیرi2 و q به‌ترتیب برای اندازه اثر غلظت نیتروژن آمونیاکی و بیان ژن ناقل اوره نوع ب نشان‌دهنده وجود ناهمگنی بالا متوسط بود. استفاده از کربوهیدرات غیر الیافی بر ph (001/0p˂) تاثیر منفی داشت. سطوح مختلف کربوهیدرات غیر الیافی تاثیر معنی‌دار مثبت بر غلظت اسید پروپیونیک (001/0p˂) و بیان ژن ناقل اوره نوع ب (001/0p˂) داشت. مقادیرi2 و q برای اندازه اثر ph و بیان ژن ناقل اوره نوع ب حاکی از وجود ناهمگنی بالا بود. نتیجه‌گیری نهایی: استفاده از سطوح مختلف پروتئین‌خام و کربوهیدرات ‌غیر الیافی در خوراک، افزایش بیان ژن ناقل اوره نوع ب را به‌دنبال دارد. با این حال به‌نظر می‌رسد، استفاده از مکمل‌های کربوهیدرات ‌غیر الیافی در خوراک تاثیر بیشتری بر افزایش بیان ژن ناقل اوره نوع ب داشته باشد.

The effect of dietary CP and NFC different levels on UTB expression: A metaanalysis

Introduction: 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 UTA and UTB genes. UTB 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 UTB 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.Materials and methods: 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 metaanalysis; and prepared comparisons for pH, VFAS, NNH3 and UTB expression. Metaanalyses were carried out using the Comprehensive MetaAnalysis 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).Results and discussion: The results of current metaanalysis showed that different levels of CP in diet, has a positive effects on ruminal NNH3 concentration and UTB 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 NNH3 concentration and UTB expression effect size indicated high and moderate heterogeneity, respectively. Marini and Van Amburgh (2003) noticed expression of ruminal UTB was changed by nitrogen intake levels. Marini and Van Amburgh (2003) observed greater expression of UTB (based upon visual evaluation) in ruminal papillae collected from the ventral sac of the rumen in dairy heifers fed highN diets (2.97 to 3.4% N) compared with lowN diets (1.45 to 1.89% N). Marini and Van Amburgh (2003) suggested that when a highN 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 highN diets are fed. Therefore, it is possible that the amount of crude protein increase UTB 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 UTB expression (<0.05). Dietary NFC different levels decrease ruminal pH (<0.05). Ruminal NNH3 concentration was not influenced by Dietary NFC different levels. The values of I2 and Q for pH and UTB expression effect size indicated high heterogeneity. Lu et al (2019) reported that the NFCrich diet caused a reduction of urinal ureaN 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 NFCrich diet, the concentration of ruminal NH3N was enhanced significantly, together with a significantly reduced urinal ureaN excretion. These data indicate that more ureaN 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 ureaN 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 nondegradable in anaerobic environments (Triolo et al., 2012). Dietary NFC, namely a fraction of easilyfermentable 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 metaanalysis, both dietary CP and NFC increase UTB expression. It seems however that the NFC have a greater effect on increasing UTB expression than CP.