AGE-RELATED-CHANGES IN PLASMA-CORTICOSTERONE AND ALDOSTERONE RESPONSES TO EXOGENOUS ACTH IN THE RAT

Age related changes in the time courses of response of plasma corticos terone and aldosterone to exogenously applied ACTH were simultaneously studied in old (female and male) Long-Evans rats and compared to both young and adult pentobarbitone-anesthetized and dexamethasone-pretrea ted control rats. Acute intravenous injection of either 0.5 or 50.0 ng ACTH (1-24)/100 g body weight increased plasma concentrations of the two steroids with a similar time course in all groups of rats. However , we observed a significant age-related attenuation in the plasma cort icosteroid response. Thus, in old as compared to young rats there was a decrease of approximately 45, 40 and 30% in plasma corticosterone le vels respectively 8 min after the lower dose of ACTH in female and 45 min after the higher dose in female and male rats. Similarly, an atten uated (approximately -38%) response of plasma aldosterone levels, indu ced 45 min after the higher dose of ACTH, was observed both in old fem ale and male rats. These results suggest that the previously reported age related decreases of in vivo corticosterone and aldosterone secret ion are, at least in part, due to a reduced capacity of adrenocortical cells for steroid biosynthesis and release in response to stimulation by ACTH.