Circadian Variations and Extraadrenal Effect of ACTH on Insulinemia in Rabbit

Summary

The present experiment was designed to study the action of ACTH_1-24 on insulin secretion during the circadian cycle in normal rabbits and to provide evidence that ACTH_1-24 has an extra-adrenal effect on this secretion.

In normal rabbits intravenous administration of three doses of ACTH_1-24(1, 10, 100 μg/kg) at 10 a. m. increased plasma insulin levels. Hyperglycemia only occurred with doses of 10 and 100 μg/kg. A maximum insulin response was already obtained at 1 μg/kg. The same experiment performed at 12 p. m. also induced hyperinsulinemia which was only noted at 10 and 100 μg/kg; hyperglycemia was only observed after stimulation by the highest dose (100 μg/kg). ACTH was therefore more effective during the day; however, at 12 p. m. plasma insulin levels were the highest, but only with the maximum dose of ACTH (100 μg/kg).

The effect of ACTH_1-24 was evaluated throughout the day on normal and adrenalectomized rabbits. In normal animals injection of ACTH_1-24 increased plasma glucose and insulin levels both together. In the contrary, in rabbits deprived of adrenal glands, ACTH_1-24 induced high insulinemia along with hypoglycemia. We could, therefore, reasonably conclude that ACTH stimulates directly the pancreatic secretion of insulin.

Key words

Rabbit - Fasting - Adrenalectomy - ACTH_1-24 - Insulin - Glucose