Pituitary-Adrenal Function in the Squirrel Monkey

Abstract

During the course of neuroendocrine investigations in squirrel monkeys “base line” values for plasma corticoids by radiostereoassay in excess of 400 μg/100 ml were observed. These values are higher than any reported in other mammalian species. Among possible explanations of this finding is that these animals are resistant to feedback inhibition of ACTH release, that there is an unusual circulating corticoid different from cortisol (the chief adrenal steroid found in other primates), or that there is an unusually high concentration of cortisol binding protein in this species. In order to answer these questions, studies have been made of the nature of the corticoids and the capacity of corticoid binding proteins of squirrel monkey plasma, the rate of disappearance from the circulation of injected 3H-cortisol, and the effects of capture, chair restraint, dexamethasone and ACTH on plasma cortisol. These studies indicate that the major steroid in squirrel monkey plasma is cortisol, that the values in resting, undisturbed caged animals are indeed uniquely high (405 μg/100 ml ±SE 43), and are capable of increasing in response to capture (672 μg/100 ml ±SE 72) and chair restraint (1024 μg/100 ml ±SE 52), that the bound fraction of plasma cortisol is not greater than in other primates, that the turnover rate of cortisol is extremely high (30 mg/day in chair restrained animals), that adrenal function in these animals is inhibited only by relatively large doses of dexamethasone, and that they are relatively unresponsive to exogenous ACTH. (Endocrinology86: 519, 1970)