A circadian rhythm in adrenal responsiveness to ACTH is not confirmed by in vitro studies

From in vitro studies a circadian variation in adrenal gland response to ACTH was observed. Regulation of this circadian rhythm in adrenal responsiveness was localized to the cellular level; involving variation in either receptor density or affinity. In the present study the circadian variation in adrenal gland responsiveness was studied using an in vitro approach. Levels of corticosterone secretion during the active and inactive phases were determined using incubated rat adrenal slices in the presence and absence of submaximal stimulation by the synthetic ACTH peptide, ACTH(1-24). Initially, a circadian difference in adrenal responsiveness appeared to exist in all groups except the betamethasone pretreated. However, data expressed in terms of corticosterone secretion/mg adrenal weight proved to be misleading due to the presence of an apparent circadian pattern in adrenal gland weight; the adrenal glands from animals in the dark being significantly less in weight than those from animals in the light phase. When the data were expressed in terms of actual gland output, no circadian variation in adrenal response to exogenous ACTH(1-24) stimulation was apparent. In conclusion, the present study does not support the concept of a circadian variation in the adrenal response to ACTH(1-24) stimulation resulting from a variation in the responsiveness of the adrenal cells themselves. However, the present study does not preclude the possibility of a circadian variation due to modulation of adrenal responsiveness by humoral factor(s) other than ACTH.