The effect of length of adaptation to a high-grain diet and acidosis challenge and recovery on rumen papillae mRNA expression of genes relating to barrier function, inflammation and short-chain fatty acid transport in beef heifers

Abstract Text: Ruminal acidosis induced by high-grain diets can have serious negative effects on animal health and performance. The objective of this study was to investigate if the duration that ruminally cannulated beef heifers were fed a high-grain diet influences mRNA expression of genes relating to barrier function, immune response and short-chain fatty acid (SCFA) absorption in rumen papillae. Heifers were assigned to 1 of 4 blocks and randomly allocated to be long adapted (LA; 34 d; n = 8) or short adapted (SA; 8 d; n = 8) from a backgrounding diet to a high-grain (81% barley) finishing diet prior to inducing ruminal acidosis. Ruminal acidosis was induced by restricting DMI to 50% for 24 h followed by an intra-ruminal dose of barley grain (10% of DMI). Rumen papillae from the ventral sac were collected and stored in RNAlater during the baseline period (BASE; 6 d of prior to challenge), 24 h after challenge (Recovery1; REC1), and 8 d after challenge (REC2). Total mRNA was isolated from papillae using TriZol and quantitative real-time PCR was conducted to quantify relative expression of genes relating to gut barrier function (zonula occluden 1, claudin 1, and occuldin), immune response (TNF-α, Toll-like receptor 4;TLR4), and intracellular pH regulation and SCFA metabolism (Na⁺/H⁺ exchanger 1 and 3, monocarboxylate transporter 1 and 4; MCT1 and 4 respectively, and 3-hydroxy-3-methylglutaryl-CoA synthase 1 and 2; HMGS 1 and 2 respectively). Expression of mRNA were normalized for expression of GAPDH and β-actin and expressed as normalized relative fold-change. Data were analyzed using Proc Mixed in SAS and mean separation conducted using Tukey post-hoc separation test. Dietary treatment (P ≥ 0.19) and the treatment × period interaction (P ≥ 0.06) did not influence mRNA expression of genes of interest. Total mRNA expression was affected by period (P≤ 0.03) for TLR4, ZO1, claudin, occludin, MCT1 and MCT4. In general, REC1 showed least expression and expression during REC2 returned to values similar (TLR4, claudin 1, and occludin) or intermediate (ZO1, MCT1, and MCT4) to BASE. These results indicate that length of dietary adaptation used in our study was not sufficient to differentially influence the expression of mRNA for key genes influencing barrier function, immune response and SCFA transport following an acidosis challenge. However exposure to ruminal acidosis generally reduced mRNA expression with values returning to baseline conditions within 8 d.

Keywords: barrier function, cattle, immune response, rumen acidosis, short-chain fatty acids