The Avian Endocrine Pancreas

The avian endocrine pancreas is comprised of A-islets containing A1- and A2- cell types, and B-islets containing A1- and B-cell types. The function of the A2- and B-cells is the secretion of glucagon and insulin, respectively, while that of the A2-cells is uncertain. The avian pancreas contains small amounts of insulin, has poor insulinogenic potential, and releases the hormone “sluggishly” in response to high glucose load. Fasting, hormones, and/or vagal stimulation do not alter insulin release. Avian insulin is not anti-lipolytic and is poorly lipogenic in in vitro avian systems. Both avian pancreas and plasma contain 5-10 times more glucagon than observed in mammals; however, no studies have been reported employing the avian hormone. Birds are extremely sensitive to mammalian glucagon, exhibiting a rapid and marked hyperglycemia, hepatic glycogenolysis, hyperglycerolemia, and hypertriglyceridemia. The lipolytic effects of glucagon are intensified in ‘vitro’ by insulin. A pancreatic polypeptide (APP) containing 36 amino acid residues has been isolated from the avian pancreas, but not from gut, liver, proventriculus, or gizzard. APP circulates normally, fluctuates with nutritional manipulation, and is found in all avian species investigated. At high levels APP induces hepatic glycogenolysis and hypoglycerolemia. At low levels APP is a powerful “gastric” secretogogue, encouraging rapid proventricular volume, acid, pepsin, and protein release.