FEEDING LIVE BAIT AND FEED ON THE PEPTIDASE ACTIVITY, TRANSPORT AND ABSORPTION OF SMALL PEPTIDES IN JUVENILE MANDARIN FISH

In this study, we examined Siniperca chuatsi with an initial weight of (6.77±0.64) g to explore the difference of digestive and absorption ability between live bait and feed from the aspects of digestive tract peptidase activity and small peptide transport and absorption. Overall, fish were divided into three groups by random: live bait group (the following group was fed with live bait only for 22d), feeding group (the following group was fed with feed only for 22d) and re-feeding group (the following group was fed with 5d of domestication, 15d of feed and 2d of live bait). At 22d post feeding, the isolated intestinal and pyloric caecum sampled at 0.5h, 1h, 2h, 3h, 6h, 12h and 16h after eating were used to measure activities of aminopeptidase and carboxypeptidase and gene expression of mall peptide transport vector genes: Pept1a, Pept1b and Pept2. Besides, the content of residual small peptides in intestinal contents and feces 12h after eating were measured. The results showed that there was no significant change in the contents of carboxypeptidase and aminopeptidase in each group, and the enzyme activity increased first and then decreased; The peak value of peptidase activity in the live bait group was significantly higher than that in the re-feeding group (P<0.01); In foregut, midgut and pyloric caecum, the mRNA expression of Pept1a and Pept1b was significantly higher while that of Pept2 was comparatively lower than those in hindgut (P<0.01); After feeding live bait, the gene expression level of small peptide transport vector in the re-feeding group increased rapidly compared with that in the feed group (P<0.01); At 12h post feeding, the content of small peptides in intestinal contents in the re-feeding group was significantly higher than that in the live bait group and feed group, and the absorption efficiency of small peptides increased with the extension of digestion time. However, the content of small peptides in feces of feed group was significantly higher than that of live bait group and re-feeding group, and the content of essential amino acids in small peptides not transported was higher. In conclusion, feeding S. chuatsi compound feed has adverse effects on the activity of intestinal peptidase, small peptide transporter gene expression and small peptide absorption rate. The decomposition of protein and the efficiency of small peptide transport and absorption of mandarin fish which intake artificial feed are significantly lower than those that intake live bait.