The site and form of nitrogen (N) absorption across the gastrointestinal tract (GIT) in horses is unclear. While the role of the small intestine as a main site of amino acid (AA) absorption has been demonstrated using both in vivo and in vitro models of monogastric animals, contribution of the large intestine to N and AA homeostasis of the host is not known. Currently, there is very limited information on the anatomical site and mechanisms of AA uptake in the equine intestine. Understanding the functional and anatomical difference in AA absorption across the GIT of equids would provide critical information on the relative role of the large intestinal contribution to total GIT AA absorption and thus add to the overall understanding of protein utilization and AA requirements in equids. The main hypothesis of this dissertation was that the capacity to transport lysine across the epithelium of the equine large intestine mucosa is similar to that of the small intestine. The overall objective was to characterize the molecular entities and kinetic processes of L-lysine transport in the equine small and large intestine. To test the hypothesis, three specific aims were addressed: (1) to determine the mRNA abundance of AA transporter proteins b0,+AT (SLC7A9, of system b0,+), CAT-1 (SLC7A1, of system y+), and LAT-2 and LAT-3 (SLC7A8 and SLC43A1, respectively, of system L) in segments of the large intestine and the small intestine; (2) to characterize total (sum of Na+-dependent and Na+-independent) L-lysine transport kinetics across equine and porcine small and large intestinal brush border membrane (BBM); and (3) to determine the distribution and epithelial cell membrane localization (apical, i.e., BBM vs. serosal, i.e., basolateral membrane) of the CAT-1 and LAT-2 protein in the small and large intestine mucosa. Results show that 1) transporters b0,+AT and LAT-3 mRNA transcript abundance determined by qRT-PCR in the equine large intestine was similar to that of the small intestine; 2) L-lysine transport Vmax and KM across brush border membrane vesicles (BBMV) manufactured from equine intestinal epithelial cells did not differ between the large colon and the small intestine; 3) transporters CAT-1 and LAT-2 mRNA transcript abundance determined by qRT-PCR and protein abundance determined by immunohistochemistry and in the large intestine was lower than that in the small intestine; and 4) transporter LAT-2 appears to be localized to the equine intestinal basolateral epithelial membrane, while transporter CAT-1 is localized to the equine intestinal lamina propria and endothelial cells. In conclusion, the equine large intestine is capable of L-lysine transport, most of which would originate from microbial proteins, across the intestinal epithelium. Thus, in horses, the large intestine may contribute to overall lysine homeostasis similar to that of the small intestine. This implies that feeding forages of lower protein quality which closer meets, rather than exceeds, requirements for the horse may reduce excretion levels of urinary N, leading to increased environmental sustainability. Overall, knowledge of the large intestinal role in lysine absorption as demonstrated in vitro can shed light on the actual protein utilization and availability of less apparently digestible forages in horses.
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