Distribution and Relative Abundance of Nutrient Transporter mRNA in the Gastrointestinal Tract of Black Bears
Gilbert, Elizabeth Ruth
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Black bears are omnivorous, and tend to be opportunistic feeders, in that they will eat what is readily abundant or available. The end-products of intestinal digestion are absorbed by the body through the action of transporter proteins expressed on the brushborder membrane of small intestinal epithelial cells. The goal of this study was to increase the understanding of the physiological processes associated with nutrient assimilation by black bears. Distribution and relative abundance of mRNA of a peptide transporter (PepT1), a glucose transporter (SGLT1), two AA transporters (NBAT, bo,+AT), and a digestive enzyme, aminopeptidase N (APN), in the intestinal tract of black bears were investigated. Ten bears were used for this study. For tissue collection, the intestine was removed from the animal and divided into five sections. Each collected section was opened longitudinally, rinsed in ice-cold PBS, and the mucosal scrapings were stored at -80C. Total RNA was extracted and quantified by spectrophotometry. Abundance of PepT1, SGLT1, NBAT, bo,+AT, and APN mRNA was determined by performing Northern blots, using bear cDNA probes. Northern blot data were quantified by densitometric analysis, with the abundance of each gene expressed relative to GAPDH. Abundance of PepT1 (P < 0.05), APN (P < 0.05), and SGLT1 (P < 0.0001) changed quadratically from the proximal to the distal intestine with abundance being greatest in the midregion. Abundance of bo,+AT mRNA increased linearly (P < 0.05) from the proximal to distal intestine. Abundance of NBAT mRNA did not change among intestinal segments.The absolute number of molecules of mRNA/ng of total RNA for each gene was determined using Real-Time PCR. Similar to the Northern results, abundance of PepT1 (P < 0.0003), SGLT1 (P < 0.0003), and APN (P < 0.02) changed quadratically from the proximal to distal intestine with abundance being greatest in the mid-region, and bo,+AT mRNA increased linearly (P < 0.0001) from the proximal to distal intestine. NBAT mRNA abundance also increased linearly (P < 0.0001) from proximal to distal intestine. PepT1 mRNA was present at tenfold or greater levels than AA transporter mRNA in all segments of the intestine, suggesting that di- and tripeptides constitute the major form in which AAs are absorbed. NBAT and bo,+AT mRNA abundance was greater towards the distal portion of the intestine, suggesting their importance in salvaging remaining unabsorbed AAs.These results indicate that the mRNA of nutrient transporters examined and APN are differentially expressed throughout the gastrointestinal tract of black bears, suggesting their involvement in nutrient assimilation.
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