A radioimmunoassay using human ACTH as a standard was employed to follow 3 variables of pituitary ACTH secretion in intact rats. Plasma ACTH levels in nonstressed rats were higher between 6 PM and 3 AM (86–98 pg/ml) than those between 8 AM and 12 noon (39–46 pg/ml). This is the opposite of the diurnal variation pattern in humans. Plasma ACTH levels in unanesthetized, unexcited rats showed peaks 2 and 8 hr after a single intravenous injection of metyrapone. Plasma ACTH levels started to increase immediately after stress and reached almost maximum levels (932 pg/ml) 5 min after stress. This maximum response persisted for 60 min, followed by a gradual decrease throughout the remaining period of ether anesthesia and laparotomy. (Endocrinology88: 692, 1971)
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Ruhmann‐Wennhold et al. (1971) studied this question.