Age-related differences in corticosterone secretion in female rats.

The effects of age on steroidogenesis in rat zona fasciculata-reticularis (ZFR) cells were studied. Young, adult, and middle-aged rats were ovariectomized (Ovx) and received replacement therapy with oil or estradiol benzoate ([EB] 25 microg/mL/kg). Rat ZFR cells were incubated with corticotropin (ACTH), prolactin (PRL), or forskolin at 37 degrees C for 1 hour. The effects of age on the activity of steroidogenic enzymes of ZFR cells were measured by the amount of intermediate steroidal products separated by thin-layer chromatography. Plasma levels were higher for PRL (54% to 254%) and corticosterone (179% to 257%) in middle-aged versus young rats. In oil-treated Ovx rats, basal and ACTH-stimulated corticosterone release by ZFR cells were also greater in middle-aged compared with young rats. Replacement with EB in Ovx rats increased the ACTH-stimulated release of corticosterone. Administration of ovine PRL in vitro resulted in a dose-dependent increase of corticosterone production. In oil-treated middle-aged rats, ovine PRL-stimulated corticosterone release was higher than in young rats. Forskolin-induced production of cyclic adenosine 3',5'-monophosphate (cAMP) was greater in middle-aged versus young rats and correlated with the increase of corticosterone production. The activity of steroidogenic enzymes in rat ZFR cells was unchanged by age. These results suggest that the age-related increase of corticosterone production in female rats is associated with the stimulatory effect of PRL on ZFR cells and is due in part to an increase of cAMP generation.