Effects Of Dietary Concentrate Level On Expression Of Several Transporters Involved In SCFA Absorption In Rumen Epithelium And Its Underlying Mechanism

Past research has demonstrated that diets could modulate SCFA absorption capacity of rumen via the changed expression of transporters, which are involved in SCFA absorption. Despite, the underlying mechanisms are not completely clear. The current experiments therefore studied the effects of dietary concentrate level on expression of transporters related to SCFA absorption in rumen epithelium. Furthermore, the underlying mechanisms were also discussed in this paper.1. Effect of dietary concentrate level on mRNA expression of transporters related to SCFA absorption in ruminal epithelium of goats and its underlying mechanism.Goats (n= 12) were randomly allocated to 2 groups and fed either a low concentrate diet (LC,10% concentrate, n=6) or a medium concentrate diet (MC,35% concentrate, n= 6) in 2 equal portions daily. Goats were fed separately with their respective diet for 3 wk. The goats were slaughtered 6 h after the morning feeding on d 22. In vivo, goats in the MC treatment exhibited a greater ruminal SCFA concentration compared with those in the LC, and pH decreased from 7.03 to 6.63. Correspondingly, mRNA expression of candidates for SCFA-/HCO3-exchangers, namely downregulated in adenoma (DRA), putative anion transporter 1 (PAT1) and anion exchanger 2 (AE2) were increased in MC group. Further, upregulation in monocarboxylate transporter 1 (MCT1) and monocarboxylate transporter 4 (MCT4) mRNA abundances was observed in MC group. The expression of genes which helps the rumen epithelial cells to maintain intracellular pH (pHi), including Na+/H+ exchanger (NHE) 1, NHE2, NHE3, vacuolar H+ATPase subunit B (vH+ATPase) and Na+/K+ATPase pump subunit al (Na+/K+ATPase) were also enhanced in the MC group relative to the LC. During in vitro studies with isolated rumen epithelial cells from goats, exposure to pH of 6.8 increased MCT1 mRNA expression after 24 h of culture, while the mRNA expression of AE2 was downregulated. The presence of SCFA (20 mM) in the medium increased DRA, PAT1, AE2, MCT1 and Na+/K+ATPase mRNA expression. The expression of vH+ATPase was decreased by increased SCFA concentration. The mRNA expression of MCT4 did not vary in vitro with pH (6.8) or SCFA (20 mM). The expression of DRA was increased by synergistic effects of higher SCFA concentration and lower pH, so did MCT1. Thus, diet-dependent rumen epithelial mRNA expression changes of genes involved in SCFA absorption are probably related to ruminal SCFA concentration and pH.2. Effects of dietary concentrate level and fasting on expression of the NHEl and NHE3 in the rumen epithelium of goats and its underlying mechanism.The goats (n=26) were randomly allocated to two experiments (in Exp.1, n=16 and in Exp.2, n=10) and received a diet of either peanut straw ad libitum (PNS) or peanut straw ad libitum plus concentrates (PNSC). Concentrate (400g/d) was provided in four equal portions at 0800 h,1100 h,1400 h, and 1700 h daily to PNSC goat. On d 43 the goats in Exp.1 were fed, followed the normal continuous feeding protocol, and slaughtered at 1900 h. In Exp.2 the goats of both groups were fasted for 16 h before slaughter. In Exp.1, goats in the PNSC treatment exhibited a greater ruminal SCFA concentration and lowwer ruminal pH compared with those in the PNS treatment. Correspondingly, the mRNA and protein expression of NHE1 and NHE3 in the rumen epithelium was greater for goats in the PNSC treatment than for those in the PNS treatment. However, in Exp.2,16 h of fasting abolished differences in ruminal SCFA concentration, pH, and NHE expression between goats in the PNSC and PNS treatments. Further, the mRNA and protein expression of NHEl and NHE3 were decreased in the PNSC goats in Exp.2, compared with those in the PNS treatment in Exp.1. The expression of genes which related to protein degradation, including Nedd4-1-. β-Arrestin-1、proteasome subunit C8 and C9 were enhanced in the goats fasted for 16 h. During in vitro studies with isolated rumen epithelial cells from goats, both low pH (6.8) and SCFA (20 mM) treatment could enhance the protein and mRNA expression of NHE1 and NHE3. Besides, they could also influence the mRNA expression of genes related to protein degradation. These results suggested that the dietary concentrate level and fasting could influence the abundance of NHE1 and NHE3 in rumen epithelium by regulating their expression and degradation. Furthermore, this process is probably related to ruminal SCFA concentration and ruminal pH.3. Effects of dietary concentrate level on expression of NHE1 and NHE3 in rumen epithelium of cows and its underlying mechanism.Four non-lactating, farrow Holstein cows, fitted with ruminal cannulas, were used in this experiment. The cows were fed in equal allotments at 0800 and 1700. The experiment was conducted over 1 month, during which time the animals were fed either a high concentrate diet (HC,60% concentrate) or a low concentrate diet (LC,20% concentrate) in two phases. Each experimental phase consisted of 15 days, and samples were collected on d 15 of each experimental stage. In vivo, NHE1 and NHE3 expression increased in rumen epithelium of cows ingesting a diet rich in concentrate, accompanied by increased SCFA concentrations but decreased pH in rumen, and increased insulin concentration in plasma compared to cows fed by a diet low in concentrate. Furthermore, the phosphorylation of ERK1/2, AKT, and p70S6K1 were higher in the rumen epithelium of the HC group than those in the LC group. During in vitro studies with isolated rumen epithelial cells from cows, insulin increased phosphorylation of ERK 1/2, AKT and p70S6K1 in rumen epithelial cells. And this promotion was diminished by Insulin receptor inhibitor. Insulin also stimulated NHE1 and NHE3 expression. But this increase was suppressed by Insulin receptor inhibitor, ERK inhibitor and AKT inhibitor, respectively. Collectively, these results indicate that diet-dependent change of NHE1 and NHE3 abundance were mediated, at least in part, by plasma insulin through the ERK and AKT pathway.In summary, dietary modulation of the expression of transporters involved in short chain fatty acid absorption in rumen epithelium is associated with the ruminal SCFA, ruminal pH and plasma insulin.