Proteases in the Atlantic Salmon, Salmo salar L.: Physiological and Biological Aspects

• Main Author: Einarsson, Sigfus
• Format: Thesis
• Language: English
• Published: ProQuest Dissertations & Theses 1993
• Subjects: Atlantic salmon; Salmo salar
• Online Access: http://theses.gla.ac.uk/74852/; http://theses.gla.ac.uk/74852/1/11007772.pdf

The digestive tract of the Atlantic salmon, Salmo salar L., was studied on a histological and an ultrastructural basis and was found to be fairly similar to the digestive tract of the rainbow trout, Oncorhynchus mykiss. Endocrine cells containing granules, hypothetically with a cholecys to kin inlike (CCK-like) hormone, were identified in the pyloric caecal region of the intestine. Supportive evidence is presented for the hypothesis that digestive enzymes from the pancreatic tissue are conveyed by many small ducts into the pyloric caeca and the pyloric caecal region of the intestinal wall. Pepsinogen was found to be stored in the oxynticopeptic cells of the stomach glands, in the cardiac and the transitional area of the stomach. The number of oxynticopeptic cells were gradually reduced in the caudal direction in the transitional area. The results support the hypothesis that pepsinogens have not undergone much change during vertebrate evolution. Trypsinogen and chymotrypsinogen were found to be stored in the secretory granules of the acinar cells, which were embedded in the fatty tissue surrounding the pyloric caeca. Optimal assay conditions, and optimal processing and storage of samples for pepsin, trypsin and chymotrypsin analysis were determined. These enzymes were found to follow the kinetics of single substrate reactions. Purified, natural, porcine CCK, sulphated, stimulated the secretion of trypsinogen and chymotrypsinogen both in vivo and in vitro. The effect of the CCK was found to be temperature dependent in vivo. It was also found to cause the discharge of the gallbladder in vivo. These findings support the hypothesis that the CCK-like peptides were established early in the evolution of vertebrates. Starvation for 2 days resulted in a slight increase of the stored pepsinogen, trypsinogen and chymotrypsinogen, but no reduction in these activities was found after at least 16 days. Starvation caused a drastic reduction of secreted trypsin and chymotrypsin activities to a minimum within 16 days, but not in secreted pepsin activity, which remained roughly the same. Ingested food stimulated immediate secretion and resynthesis of pepsinogen. Digesta from the stomach entering the pyloric caecal region of the intestine stimulated secretion of trypsinogen and chymotrypsinogen, caused the discharge of the gallbladder and probably peristalsis of the intestine, since trypsin and chymotrypsin activities appeared in all parts of the intestinal lumen at the same time. The relevance of these results for the identification of a CCK-like hormone is reviewed. The results may suggest that the pancreas secretion is under a feedback control. The intestinal lumen as well as the pancreas appear to be under a strict homeostatic control with regard to its content of trypsin and chymotrypsin. The arrest of growth of the lower modal group (LMG) fish during their first winter was not caused by a lack of digestive capacity, since quantities of pepsin, trypsin and chymotrypsin were no less than in the upper modal group (UMG) fish. A challenge with a pureified, natural, porcine CCK, sulphated, demonstrated that the pancreas of both groups was able to secrete both trypsinogen and chymotrypsinogen. The response to starvation and the response to CCK demonstrated that the stomach mucosa and the pancreas were secreting pepsinogen, trypsinogen and chymotrypsinogen more actively, in the UMG fish than in the LMG fish. This was also reflected in greater feeding activity of the UMG fish, which contained greater amounts of stomach digesta during the winter months. The suppression of appetite and the thresholds of response to plasma thyroxine in the LMG are reviewed.