Characteristics of pancreatic function in fish

Great variation exists between fish species regarding the anatomy and physi ology of the digestive organs. Present knowledge on the fish pancreas is on ly fragmentary and further work is needed. In most species of fish the panc reas is not a distinct organ, but spread as a pancreas diffusum between or on the walls of internal organs and vessels. Endocrine pancreatic tissue is found in the vicinity of exocrine pancreatic tissue or localised in specia lised "Brockmann bodies". Pancreatic digestive enzymes in fish seem to have characteristics similar to those of the corresponding enzymes in other far m animals. One exception is the efficiency of nutrient hydrolysis by enzyme s in cold water species, which in many cases is an order of magnitude highe r than in mammals and birds. Carboxylester lipase is responsible for trigly ceride hydrolysis. Endocrine, paracrine and neural mechanisms are involved in the regulation of enzyme secretion from the fish pancreas. Cholecystokin in plays a major role. Products of nutrient hydrolysis are more potent stim ulators of enzyme secretion than the intact nutrients, at least in salmonid s. Enzyme secretion also appears to be regulated according to the feeding l evel and diet composition. The ability to adapt amylase secretion to dietar y carbohydrate level seems to be restricted to herbivorous and omnivorous s pecies. The major hormones produced in fish pancreatic islets are insulin, somatost atin-22-28 (SST-22-28), somatostatin-14 (SST-14), glucagon, glucagon-like-p eptide (GLP) and pancreatic polypeptide (PP). Insulin seems to have a posit ive effect on growth. Insulin and glucagon/GLP have opposing metabolic effe cts. Insulin stimulates anabolism and accumulation of macromolecules, and h as an inhibitory effect on regulatory enzymes catalysing lipolysis, glycoge nolysis, and gluconeogenesis. The same enzymes are stimulated by glucagon a nd GLP. Basic amino acids stimulate insulin, glucagon and GLP secretion. In fish, the basic amino acids stimulate insulin directly, while in mammals, the stimulatory effect on insulin is mediated by the glucagon family peptid es. Glucagon and GLP are strong insulin secretagogues in mammals, while in fish, glucagon and GLP have no effect on insulin secretion under most physi ological conditions. Basic amino acids are stronger insulin secretagogues i n fish than glucose. Somatostatins exert metabolic effects in fish indirect ly through a reduction of insulin and glucagon levels, and directly by enha ncing glycogenolysis and lipolysis. Pancreatic polypeptide from fish has be en found to stimulate appetite in mammals. Whether it also has this effect in fish remains to be learned.