Alterations in digestive enzyme activities during the development of diet-induced enteritis in Atlantic salmon, Salmo salar L.

• Introducing SBM to the salmon diet caused changes in digestive processes.
• The change in diet formulation and/or feed intake immediately affected intestines.
• Pancreatic protease activities generally increased over time.
• Pancreatic lipase and tissue enzyme activities remained unchanged or decreased.
• Distal intestinal tissue responses reflected progression of inflammatory changes.

This study followed the time-related changes in intestinal digestive physiology of post-smolt Atlantic salmon (Salmo salar L.; initial weight 500–600 g) during the development of soybean meal induced enteritis. Intestinal tissue and content (chyme) were sampled after 1, 2, 3, 5, 7, 10, 14, 17, and 21 days of dietary exposure to a 20% extracted soybean meal-containing (SBM) diet. Fish fed a fishmeal-based control diet sampled on day 0 (prior to SBM feeding) served as controls. From the different intestinal regions, histomorphological evaluation was conducted, activities of the pancreatic enzymes trypsin, chymotrypsin, elastase and lipase, as well as leucine aminopeptidase (LAP) were determined in chyme, and the brush border membrane (BBM) enzymes LAP and maltase were measured in homogenates of the intestinal wall. Epithelial cell functional differentiation was visually assessed using immunohistochemical expression of carbonic anhydrase 12 (CA-12). Histological signs of SBM-induced inflammation were observed in the distal intestine (DI) from day 5. At each given sampling time, pancreatic enzyme activities generally decreased aborally along the intestinal tract. Within each intestinal region, pancreatic protease activities tended to increase with exposure time to the SBM diet, with the greatest activity increases observed in the DI. The magnitude of the decrease in activity between the most proximal region and the most distal decreased with SBM exposure. DI tissue weight and BBM maltase and LAP activities were significantly reduced from day 1–2 and continued to decrease progressively with increasing SBM exposure time. Immunohistochemical staining of CA-12 also diminished over time starting at day 5. By day 7, very little CA-12 staining was observed. The data suggests an immediate and generalized physiological response to a change in diet and/or feed intake in all regions of the intestine, as well as a more localized functional loss in the DI that progressed with the development of SBM-induced inflammation from day 5. The diet change appeared to cause a rapid pancreatic response with increased secretion of all the enzymes investigated, while the longer term responses of the pancreatic proteases differed from that of lipase.