Effects of salinity on digestive enzyme activities of juvenile Acipenser schrenckii

Influence of salinity on digestive enzyme activity of juvenile Acipenser schrenckii was investigated in this study. We measured and compared activities of three kinds of digestive enzymes (protease,amylase and lipase) in five digestive organs (pyloric caecum,ileum,duodenum,stomach and liver) of juvenile Acipenser schrenckii cultured in salinities 0,10 and 25 for 10 days,respectively. During the experimental period, major environmental factors,such as water temperature,pH and DO,were measured once daily,and those environmental factors were controlled at the same levels in each experimental tank with temperature(20±1)℃, pH 7.5±0.5 and DO6.0 mg/L. Prior to sampling,fish were starved for 24 h and were then randomly killed for enzyme activity assay. The alimentary canal and liver were extracted on ice and cleaned with chilly saline water (0.65%). The alimentary canal was divided into pyloric caecum,stomach,duodenum and ileum portions. The digestive organs were minced and homogenized respectively. Each homogenized sample was then centrifuged at 1 700 g for 20 min and the supernatant was collected and prepared for enzyme assay at 4 ℃ . The results showed that:1) The same digestive enzyme activity was different among digestive organs measured under the same salinity. The pyloric caecum had the greatest protease activity (in freshwater and salinity 25) and amylase (in freshwater),followed by ileum,duodenum,stomach and liver,while the descending order of protease activity (in salinity 10) and amylase (in salinities 10 and 25 ) was ileum,pyloric caecum,duodenum,stomach and liver. For lipase activity,the rising order was pyloric caecum,liver,stomach,duodenum and ileum at freshwater and salinity 10. At salinity 25,the order remained except that,liver tissue had greater lipase activity than stomach. 2) The activity of the same digestive enzymes in the same digestive organs measured was different in different salinity. Pyloric caecum,duodenum,stomach and liver tissues,except for ileum tissue,had higher protease, amylase,lipase activities in freshwater than 25 salinity. It could be inferred that salinity can inhibit the activities of protease,amylase,lipase in pyloric caecum,duodenum,stomach and liver tissues. However,the highest activities of protease,amylase,lipase in ileum appeared at salinity 10(P0.05),which indicated that salinity could increase the enzyme activity within some salinity range and could inhibit the activity beyond the salinity range.