Insulin and the glucose-glucagon feedback mechanism in the duck

The avian pancreas in health and disease

The avian endocrine pancreas shares some similarities with mammals but also some clinically relevant differences in anatomy and physiology. Diabetes mellitus, an uncommon disease of pet birds, is a challenging condition because of lack of knowledge of the exact pathophysiology and responses to insulin therapy. This article reviews the anatomy and physiology of the avian pancreas and describes the etiology, diagnosis, and treatment of diabetes mellitus in pet birds.

Growth hormone and somatostatin in the plasma of transiently diabetic ducks: basal variation and response to glucose

In the duck, subtotal pancreatectomy induces a transient diabetes, with decreased insulin and glucagon basal levels as well as responses to glucose. At the same time, a transient increase in basal peripheral somatostatin occurs, followed by an increase in growth hormone in the postdiabetic state. Intravenous glucose induces a slight decrease in somatostatin secretion in normal, but not in diabetic animals, and no significant variation in growth hormone secretion at any state. An obvious role of growth hormone or somatostatin in the development of this transient diabetes in the duck could not be detected in this study.

Growth hormone secretion and pancreatic function following somatostatin infusion in ducks (Anas platyrhynchos)

The intravenous infusion of somatostatin (800 ng/kg min) reduced the concentration of growth hormone (GH) in the plasma of 4 to 5, 6 to 7 and 8 to 9 week-old ducklings, but not in adult ducks. The inhibition of GH secretion was not due to accompanying changes in pancreatic function, since the infusion of a lower dose of somatostatin (200 ng/kg min) increased glucagon release and decreased plasma free fatty acids (FFA), as observed with the higher dose, but had no effect on GH concentrations. The withdrawal of somatostatin inhibition resulted in rebound GH secretion in immature birds, the magnitude of which was directly related to the pre-treatment level. Following somatostatin infusion (800 ng/kg min) no modification in GH concentration was observed in adult ducks. These results demonstrate that basal GH release in young birds is not autonomous and is suppressible by somatostatin. The data provide further evidence for age-related changes in the control of avian GH and insulin release and for the independence of the effects of somatostatin on the pituitary and pancreas glands.

Effect of insulin on glucagon secretion mediated via glucose metabolism of pancreatic A cells in ducks

A possible action of insulin via glucose metabolism on the pancreatic A cell response to glucose, was studied in ducks. 2-Deoxyglucose, a nonmetabolizable analogue of glucose was used. In normal ducks, the hyperglycaemia induced by 2-deoxyglucose (IV: 0.5 g/kg) resulted in hyperglucagonaemia, while the same degree of hyperglycaemia, induced by glucose infusion (IV injection 25 mg/kg, and infusion 5 mg/kg/min) immediately suppressed glucagon secretion. In diabetic ducks, two days after subtotal pancreatectomy, glucose responsiveness of the A cell was abolished, but could be restored by insulin treatment before (IM 0.2 U/kg insulin + 8 micrograms/kg glucagon every 6 h) and during (IV 3.6 mU/kg + infusion 0.9 mU/kg/min) the glucose test (IV: 0.5 g/kg). The normal response of the A cell to glucose was not observed in diabetic insulin-treated ducks after the administration of 2-deoxyglucose (IV: 0.5 g/kg). These data suggest an inhibitory effect of the metabolism of glucose on the release of glucagon. In addition, the action of insulin on the A cell may be mediated by its effect on glucose metabolism within the A cell.

Effect of free fatty acids and amino acids on glucagon and insulin secretions in normal and diabetic ducks

The relationship between two metabolites free fatty acids (FFA) and amino acids (AA), and the two main pancreatic hormones, insulin and glucagon, was studied by infusing small amounts of these metabolites into normal and diabetic Peking ducks, i.e. two days after subtotal pancreatectomy. Infusion of oleic acid (0.365 g/kg/30 min as an emulsion in plasma) indicated a suppressive effect of free fatty acids on glucagon secretion, but was without effect on insulin secretion, in normal as well as in diabetic ducks, indicating that insulin might not be directly involved in the FFA-glucagon feedback in the duck. Infusions of arginine for one hour (1 g/kg/h) into normal ducks, hyperglycaemic normal birds (as a result of glucose infusion: 1 g/kg/h) and diabetic ducks, suggested the persistence of amino acid effect on glucagon secretion, and a slight reduction of the effect on insulin secretion in diabetes. This suggests that insulin may not be involved in amino acid-induced glucagon secretion in the duck.

Roles of insulin and glucose in the regulation of plasma FFA in the duck. I. In the absence of pancreatic glucagon

The present study was undertaken to determine how free fatty acid (FFA) release by adipose tissue can occur in the absence of pancreatic glucagon, the most potent lipolytic hormone in the duck. Two possible explanations have successively been investigated. Replacement therapy experiments showed that glucose uptake was implicated in the phenomenon; indeed, when pre-operative insulin and glucose levels were restored after total pancreatectomy, plasma FFA progressively fell to a level significantly lower than the pre-operative value. The same effect was observed when the ducks were kept in a continuous state of hyperglycaemia, whereas "normal" levels of insulin or glucose alone were shown to be ineffective. On the other hand a compensatory mechanism for the lack of glucagon, by other lipolytic agents [with the intestine or the anterior pituitary as possible sources] has been excluded by associating the removal of these two organs with pancreatectomy. These experiments suggest that the lack of pancreatic glucagon induced by total pancreatectomy is essentially masked by a failure in glucose uptake due to the operation.