DIRECT AND INDIRECT EVIDENCE FOR POLYPEPTIDE ABSORPTION BY THE TELEOST GASTROINTESTINAL-TRACT

The ability of the gastrointestinal tract of chinook salmon, Oncorhynchus tshawytscha, to absorb polypeptides in vivo was investigated by reference to the appearance of orally administered adrenocorticotropic hormone (ACTH) within the blood of fish previously treated with dexamethasone (3μg g−l body weight) in order to suppress endogenous ACTH secretion. Further, since cortisol presence within plasma is dependent upon the availability of ACTH, dexamethasone blockade of endogenous ACTH secretion, in conjunction with subsequent measurements of plasma cortisol levels, provides a means by which biological patency of absorbed exogenous ACTH may be demonstrated. Levels of ACTH and cortisol in plasma of dexamethasone‐treated salmon were therefore measured for a period of 360 min immediately following oral intubation of ACTH (15μg g−1 body weight). Peak plasma presence of ACTH‐like immunoreactivity (676.6 ± 33.6 pg ml−1 plasma) and cortisol (227.1 ± 29.0 ng ml−1 plasma) were recorded 120 min after ACTH administration. Results from the experimental groups were compared to those of 15 control treatments. Since administration of ACTH to chinook salmon elicited a consistent and significant elevation in not only plasma ACTH but also cortisol presence, it is contended that the salmonid gut expresses an ability to absorb polypeptides of dietary origin. The significance of these findings with respect to the oral administration of biologically active peptides and proteins to fish of importance to aquaculture is discussed. Copyright © 1990, Wiley Blackwell. All rights reserved