Short Chain Fatty Acids Accumulation Aggravates The Subacute Ruminal Acidosis Through Inhibiting SLC5A8 And MCT1 Expression

Subacute ruminal acidosis(SARA)is a well-recognized digestive disorder of high yielding dairy cows that has negative impact on both animal health and herd profitability.In the large-scale farms,high concentrate feeding was used to meet the energy needs of dairy cows,which produce large quantities of short-chain fatty acids(SCFA)through intraruminal microbial fermentation.When the production of SCFA exceeds the absorption and neutralization capacity of the rumen,SCFA accumulates in rumen.The accumulation of SCFA,main containing acetate,propionate and butyrate,may cause a depression of ruminal p H and increase the risk of SARA.SCFA is an important substrate of energy metabolism for ruminants,which provided a large amount of energy by metabolism in the body.Therefore,the study of the transport and absorption mechanism of SCFA in the rumen is of great concern for the ruminant energy supply.However,the influencing factors and mechanism of SCFA absorption are not fully understood.It has reported that sodium coupled monocarboxylate transporter 1(SLC5A8)and hydrogen coupled monocarboxylate transporter 1(MCT1)play an important role in the absorption and transport of SCFA in human and mouse intestinal epithelial cells,whereas there are rarely reported for SLC5A8 and MCT1 in ruminants.The expression,distribution and location of SLC5A8 and MCT1 in digestive tract of dairy cows were examined by combining in vivo and in vitro.(1)The expression and distribution of SLC5A8 and MCT1 in calves and adult bovine digestive tract were identified by Western blot and quantitative real-time PCR analysis.The results showed that the expression of SLC5A8 was detected in all regions of gastrointestinal tract in calf and adult cow,and was the highest in rumen.And the expression of SLC5A8 and MCT1 was significantly increased in the large intestine of adult cattle.(2)The difference of SLC5A8 and MCT1 expression was examined in rumen of normal and SARA dairy cows.The results showed that SARA inhibited the expression of SLC5A8 and MCT1.(3)The localization of SLC5A8 and MCT1 in rumen tissues was detected by immunohistochemistry.We found that SLC5A8 was localized in the rumen epithelium of the proximal chamber,and MCT1 was mainly located in the stratum basale of rumen epithelium.In order to determine the reason that the expression of SLC5A8 and MCT1 was inhibited in SARA cows,the rumen epithelial cells were treated with different concentration of SCFA(control group: 60 m M acetate,30 m M propionate and 20 m M butyrate;SARA group: 90 m M acetate,40 m M propionate and 30 m M butyrate).The expression level of SLC5A8 and MCT1 in control group was higher than that of SARA group,which is consistent with the results in vivo.To further determine which composition of SCFA effects the expression of SLC5A8,the rumen epithelial cells were treated with different concentrations of acetate(0,30,60 and 90 m M),propionate(0,20,30 and 40 m M)and butyrate(0,10,20 and 30 m M),respectively.The results showed that the expression of SLC5A8 was increased gradually with the increase of acetate dosages,whereas hight concentration of acetate inhibited the expression of MCT1.The expression of SLC5A8 and MCT1 was significantly lower in 40 m M propionate treatment group than 20 m M propionate treatment group.The expression of SLC5A8 was significantly lower in 30 m M butyrate treatment group than in 20 m M butyrate treatment group.Moreover,the results of immunocytofluorescence showed that SLC5A8 and MCT1 were mainly localized at the membranes of the ruminal epithelium cells.Collectively,these results demonstrate that high concentrations of SCFA inhibit the expression of SLC5A8 and MCT1 in rumen epithelium of SARA cows,which further decrease the absorption of SCFA,thereby aggravating the SARA.