AGE-RELATED DECREASES IN PLASMA ADRENOCORTICOTROPIC HORMONE, CORTICOSTERONE, AND ALDOSTERONE RESPONSES TO EXOGENOUS CORTICOTROPIN-RELEASINGHORMONE IN THE RAT

Age-related changes in plasma adrenocorticotropic hormone (ACTH), cort icosterone, and aldosterone responses to exogenous corticotropin-relea sing hormone (CRH) were studied concomitantly in both old and adult Lo ng-Evans female rats. All animals were pretreated with dexamethasone a nd pentobarbital anesthetized. An acute intravenous injection of 1 mu g rat CRH/100 g body weight markedly increased the plasma concentratio ns of ACTH, corticosterone, and aldosterone with similar temporal kine tics in the two groups of rats. However, the incremental CRH responses were significantly lower in old (approximately -70, -45-60, and -30-5 0%, respectively, for ACTH, corticosterone, and aldosterone) as compar ed with adult rats. Together with our recent findings, the present res ults suggest that the previously reported reduced secretion of cortico steroids in aged rats is due to both an impaired steroidogenetic capac ity of adrenocortical cells to respond to ACTH and to a decreased abil ity of corticotropes to produce ACTH in response to CRH.