| The aim of the study is to investigate the morphology, histology and ultrastructure of digestive tract of Glyptosternum maculatum, and the distribution and characteristics of main digestive enzymes, and the changes of digestive enzymes vary from age, also the in-vitro digestibility on seven feed ingredients of crude enzymes. Such information of digestive physiology of G. maculatum of notable economic importance and with high potential for controlled rearing is considered valuable, not only for adding to current knowledge on the biology of Sisoridae fishes, but also for understanding formulation of any artificial diets. The main results are below:1. The features of digestive tract of G. maculatum are summarized below:(1) Inferior mouth with thin papillate lips, blunt snout and broad mouth width. Premaxillary teeth are short and conical, arranged in irregular rows on as a patch. Tongue degenerate. The number of outer gill raker in the fist gill arch is 5-9. The mucosa of both upper and lower wall of buccopharynx is stratified squamous epithelium with goblet cells and taste bud.(2) The oesophagus is stubby with thick muscularis. The mucosa fold is longitudinal, lined with many goblet cells, and few taste buds are found in the forepart of oesophagus.(3) The U shape stomach was divided into three portions obviously:cardiac, fundic and pyloric parts. In the cardiac and fundic stomach, there were a great amount of gastric glands, whereas there is a gradual loss of glands towards the pyloric stomach, and no gastric glands are found in the posterior part of pyloric stomach. The mucosa fold of the stomach is longitudinal with zigzagging bend, and second fold was transverse or wavy. At the ultrastuctural level, three type cells (mucous, glandular and endocrine cell) were found in the stomach, and glandular cell with a great amount of pepsinogen granules.(4) The intestine is short with 1-2 times convolution, and the relative intestine length was 0.9. The wall of intestine tube is thick, and the height of mucosa fold and thickness of muscularis decrease from the anterior intestine to posterior intestine. The intestinal epithelium is composed of two main cell types:enterocytes and goblet cells. (5) Liver and pancreas is separate, and the pancreas mainly distribute close to stomach, anterior intestine and gall bladder, as well as the lipid tissue of mesenterium. Liver has right and left lobe, above the oesophagus and stomach, and gall bladder is attached to the left lobe of liver. The color of liver and gall bladder is puce and bottle green, respectively. The subdominant liver is under the skin of pectoral fin, with joint belt connected to dominant liver in the abdominal cavity.2. The descending order of protease activity in different digestive sections is: stomach, anterior intestine, dominant liver, subdominant liver, posterior intestine and middle intestine, and the activity in the stomach is significantly higher than that in the anterior intestine and two parts of liver (p<0.05), while there is no significant difference between dominant and subdominant liver, as well as between posterior intestine and middle intestine (p>0.05). The amylase activity is highest in anterior intestine, following in the dominant and subdominant liver, and the amylase in the in anterior intestine is significantly higher than that in two parts of liver (p<0.05). The amylase is lowest in the stomach, which had no significant differences compared to that in posterior intestine and middle intestine (p>0.05). Lipase activity is highest in anterior intestine, following in the dominant liver and posterior intestine, the lipase activity of three above digestive sections with significant differences (p<0.05). Lipase activity is lowest in the subdominant liver, and lipase activity has significant difference among the middle intestine, stomach and subdominant liver (p<0.05), as well as lipase activity between dominant and subdominant liver with significant difference (p<0.05). The highest trypsin activity is found in the anterior intestine, and highest chymotrypsin activity is in the two parts of liver, both digestive enzymes with lowest value in the stomach. Alkaline phosphatase activity is highest in the anterior intestine, following in the middle intestine and posterior intestine, and the activity in anterior intestine is significantly higher than that in the middle and posterior intestine (p<0.05), however with nosignificant difference between middle and posterior intestine (p>0.05). Alkaline phosphatase activity is lower in the stomach, dominant and subdominant liver and above three digestive sections with no significant difference. Leucine aminopeptidase activity is similar to alkaline phosphatase activity, highest activity in the anterior intestine, following in the middle intestine and posterior intestine, lower in the stomach, dominant and subdominant liver, and the activity in anterior intestine is significantly higher than that in the middle and posterior intestine (p<0.05). Leucine aminopeptidase activity of the stomach, dominant and subdominant liver has no significant difference (p>0.05).3. The optimal reaction temperature for pepsin is 30℃, and for protease in the anterior, middle and posterior intestine is 50℃. The optimal reaction temperature in four digestive sections is 30℃. The optimal temperature of lipase is 30℃in stomach,50℃in anterior intestine,40℃in middle and posterior intestine. The optimal pH for pepsin is 2.0, and pH 9.0-10.0 is the optimal pH for intestine protease. The optimal pH for amylase is 6.0 in the stomach, and 7.0 for three parts of intestine. The optimal pH for lipase is 6.0, and 8.0 for three parts of intestine. The thermal stability for gastrointestinal protease is ranged from 20 to 50℃. Pepsin is stable in the pH range 1.0-4.5, and intestine protease is stable in the pH range 7.5-11.0. The best concentration of substrate for pepsin and intestine protease is 2.5% and 2.0% casein, respectively. And the best concentration of substrate for stomach and intestine amylase is 2.5% and 3.0% soluble starch, respectively. Na+ and Hg2+promote pepsin activity, while Cu2+ and Fe3+ inhibit the pepsin activity. Ca2+, Fe2+ and Mg2+promote protease activity of intestine, while Cu2+, Zn2+and Fe3+ inhibit intestine protease activity. The protease activity in the stomach and intestine is inhibited by PMSF, TPCK, TLCK, Pepstatin A andβ-mercaptoethanol.4. Digestive enzymes vary from age. The highest protease and amylase is found the 9 age group, lowest value in 7 and 15 age groups, and the enzymes activity increase first then decrease with age increasing from 3 to 15. Alkaline phosphatase activity vary according to age change is similar to the trend of protease and amylase. The lipase activities are higher in the 3 and 5 age-groups than other age-groups, and the activity decrease with age increasing from 3 to 15. The trend of change of trypsin activity, chymotrypsin activity and leucine aminopeptidase activity also decrease with age increasing from 3 to 15. 5. The descending order of in-vitro protein and dry matter digestibility of crude enzymes of different digestive sections on seven feed ingredients was:stomach, anterior intestine, middle intestine and posterior intestine, and the in-vitro digestibility of crude enzymes on animal feed ingredients was higher than that of plant feed ingredients. The descending order of producing amino acid rate to the enzymolysis reaction by crude enzymes of different digestive sections was anterior intestine middle intestine, stomach and posterior intestine.
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