INFLUENCE OF EXOGENOUS AND ENDOGENOUS ACTH ON ADRENAL-CORTEX ACTIVITYIN 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 adrenocorticotroph ic hormone (ACTH) and cortisol concentrations were determined by radio immunoassay and the influence of exogenous and endogenous ACTH on adre nocortical activity assessed. An ACTH-induced adrenal cortex response was observed as early as day 0, whereas a definite relationship betwee n 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 cortis ol concentrations and an elevated reactivity threshold of the adrenal ACTH receptors.