Analysis of the preventive action of ACTH on dexamethasone-induced adrenocortical atrophy in the rat.

Adult female rats were treated for 4 or 6 days consecutively with dexamethasone (Dx; 100 micrograms kg-1 day-1) alone or with ACTH (100 micrograms kg-1 day-1) for 2, 4 or 6 days. After 2 days of ACTH administration, zona fasciculata cells increased in volume, and the blood concentration of corticosterone was raised. Basal corticosterone secretion by adrenal slices appeared to be enhanced by ACTH treatment only when calculating per adrenal pair (and not per unit wt of adrenals). A significant increase in the number of metaphase cells was observed after only 4 days of ACTH administration. It is suggested that (1) the first action of ACTH on Dx-suppressed rats is to induce the hypertrophy of adrenocortical cells; (2) the steroidogenic capacity of adrenocortical cells is directly proportional to their average volume; and (3) the stimulatory effect of ACTH on the proliferative activity of adrenocortical cells is subsequent to the stimulation of their specialized functions.