Influence of exogenous and endogenous acth on adrenal cortex activity in calves during the early postnatal period.

A study was carried out on 66 black-and-white calves during the period from birth (day 0) to day 21 of life. Blood plasma adrenocorticotrophic hormone (ACTH) and cortisol concentrations were determined by radio-immunoassay and the influence of exogenous and endogenous ACTH on adrenocortical activity assessed. An ACTH-induced adrenal cortex response was observed as early as day 0, whereas a definite relationship between ACTH and cortisol concentrations emerged in the second week of life. It is concluded that the pituitary-adrenal axis is blocked during the first week of life and this effect seems to be related to high cortisol concentrations and an elevated reactivity threshold of the adrenal ACTH receptors.