Effects of Heat Stress on the Ruminal Epithelial Barrier of Dairy Cows revealed by Micromorphological Observation and Transcriptomic Analysis

Heat stress (HS) alters the rumen fermentation of dairy cows thereby affecting the metabolism of rumen papillae and thus the epithelial barrier function. The aim of the present study was to investigate if HS damage the barrier function of ruminal epithelia. Eight multiparous Holstein dairy cows with rumen cannula were randomly equally allocated to two replicates (n=4), with each replicate being subjected to heat stress (HS) or thermal neutrality and pair-feeding (PFTN) in four environmental chambers. Compared with PFTN, HS raised respiration rate (by 2.65-fold), rectal temperature (by 1.33°C), and skin temperature (by 4.59°C) but decreased milk yield (by 34.22%) and content of milk protein (by 10.44%). Additionally, HS increased the concentrations of total volatile fatty acids (by 31.60%), acetate (by 39.51%), propionate (by 47.29%), and valerate (by 61.54%). Micromorphological observation showed that HS aggravated the shedding of the corneum and destroyed the physical barrier of the ruminal epithelium to certain extent. Transcriptomics analysis of the rumen papillae using the KEEG database revealed upregulation of 238 genes downregulation of 263 (<i>P</i>_adj ≤ 0.05) by HS, including the pathways associated with DNA replication and repair and amino acid metabolism. Transcriptomic analysis using the Database for Annotation, Visualization and Integrated Discovery (DAVID) showed that the biological processes related to sister chromatid segregation, double-strand break repair, meiotic nuclear division, etc. were also up-regulated by HS, while the MAPK and NF-kB cell signaling pathways were downregulated. However, no heat stress-specific change in the expression of tight junction protein or TLR4 signaling was found in the rumen papillae. These data suggest that HS negatively affected the physical barrier of the ruminal epithelium to some extent but did not break the ruminal epithelium. Heat stress invoked mechanisms to maintain the integrity of the rumen epithelial barrier by upregulating the expression of heat shock protein and repairments in rumen papillae. The increase in amino acid metabolism in rumen papillae might affect the nutrient utilization of the whole body in dairy cows. The findings of this study may inform future research to better understand how heat stress affects the physiology and productivity of lactating cows and the development of mitigation strategies.