The structural and functional adaptations of the rumen epithelium during grain-induced ruminal acidosis

The experiments in this thesis were designed to enhance the understanding of rumen epithelial adaptation to high grain diets. All experiments presented were conducted in vivo in our target animal model, the dairy cow. The first experiment, summarized as a case report, describes the technique for inducing ruminal acidosis and harvesting rumen papillae to study the adaptation of the rumen epithelium. In this study, we found that the structure and function of the rumen epithelium changed rapidly and the structural integrity of the rumen epithelium was compromised.;The second experiment was used to generate hypotheses to explain the adaptation of the rumen epithelium Four rumen-cannulated non-lactating dairy cattle were transitioned from a high forage diet to a high grain diet, and the structural adaptations. were characterized using light scanning and transmission electron microscopy. In addition, global gene expression was assessed using a microarray and quantitative real-time PCR. In Chapter 4, we found that during the grain challenge the structural integrity of the rumen epithelium was compromised as evidenced by increased tissue damage, and reduced epithelial thickness and tight junctions between cells. The expression of all genes in specified gene families were screened using a microarray and the differential gene expression of IGF-binding proteins (IGFBP; IGFBP3, IGFBP5 and IGFBP6) were validated using qRT-PCR. Further, the expression of desmoglein 1 (DSG1), an important component of desmosomes, was the most responsive gene to shifts in dietary rapidly fermentable carbohydrates. To develop more hypotheses, the microarray data was analyzed using Ingenuity Pathway Analysis to uncover other responsive pathways. The pathway results indicated that genes involved in cholesterol biosynthesis and LXR/RXR activation are responsive during a grain challenge. From these findings, a model describing the regulation of genes involved in cholesterol homeostasis in the rumen epithelium was proposed.;The third experiment was conducted to assess if the molecular adaptation of the rumen epithelium developed in the previous experiment also occurs in lactating dairy cattle. A severe state of ruminal acidosis was induced, causing the differential expression of IGFBP3 and IGFBP5. Despite increases in circulating beta-hydroxybutyrate, there were no differences in ketogenic gene expression; however, there was a down-regulation of a cholesterol biosynthesis gene. Collectively, these experiments offer a unique insight into genes associated with a decline in rumen epithelial barrier function and altered SCFA metabolism during grain-induced ruminal acidosis.