The Effect of Insulin on the Nitrogen Metabolism of Teleost Fishes

Abstract

The work presented here has investigated certain aspects of the action of insulin in controlling the overall metabolism of nitrogen in the goldfish (Carassius auratus). Morphological investigations of the Islets of Langerhans have shown them to be closely associated with the ducts of the diffuse exocrine pancreas and thus are present throughout the pleuroperitoneal cavity: The islets are discrete rounded richly vascularized bodies occurring in greater numbers around the cystic duct common bile duct and the gall bladder. Two distinct cell types, alpha and beta cells, are morphologically identifiable in all goldfish islets. Ammonia production by goldfish was increased markedly after the administration of insulin and tolbutamide to the ambient water at 23°C. In addition, the amino-acid nitrogen content of muscle treated with insulin and tolbutamide was elevated, while alloxan lowered the content. The total nitrogen concentration of goldfish muscle was increased after injection of insulin and tolbutamide, but alloxan treatment reduced the total nitrogen content of goldfish muscle, Leucine- 14C-  incorporation used as an index of protein metabolism was significantly increased in muscle after insulin and tolbutamide treatment. The effect of insulin, tolbutamide and alloxan on the total RNA and the total DNA content of goldfish muscle has also been studied. Insulin and tolbutamide markedly increased the total RNA concentration of goldfish muscle, while alloxan significantly lowered the total RNA content of the same. An attempt was made to study the secretion of insulin in response to high and low protein ingestion by Rainbow trout . The ingestion of high protein diet resulted in a significant increase in the plasma insulin level and an increase in the me@ body weight of the trout. From this it is concluded that insulin plays a definite role in controlling the overall metabolism of nitrogen in goldfish and Rainbow trout