The Avian Endocrine Pancreas

The avian endocrine pancreas is comprised of A-islets containingA₁- and A₂- cell types, and B-islets containing A₁- and B-celltypes. The function of the A₂- and B-cells is the secretionof glucagon and insulin, respectively, while that of the A₂-cellsis uncertain. The avian pancreas contains small amounts of insulin, has poorinsulinogenic potential, and releases the hormone "sluggishly"in response to high glucose load. Fasting, hormones, and/orvagal stimulation do not alter insulin release. Avian insulinis not anti-lipolytic and is poorly lipogenic in in vitro aviansystems. Both avian pancreas and plasma contain 5-10 times more glucagonthan observed in mammals; however, no studies have been reportedemploying the avian hormone. Birds are extremely sensitive tomammalian 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 residueshas 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 allavian species investigated. At high levels APP induces hepaticglycogenolysis and hypoglycerolemia. At low levels APP is apowerful "gastric" secretogogue, encouraging rapid proventricularvolume, acid, pepsin, and protein release.