Differential suppression of the outer and inner zones of the adrenal cortex of the guinea pig.

To examine the regulation of the zona fasciculata and the zona reticularis of the guinea pig adrenal cortex, animals were placed on a chronic regimen of dexamethasone. Changes in adrenal zonal weight, cholesterol side-chain cleavage activity, and tissue and serum steroid concentrations were measured after 1 month of dexamethasone administration. With dexamethasone treatment, the weight of the outer zone (glomerulosa/fasciculata) decreased significantly, while the weight of the inner zone (reticularis) did not change. Cholesterol side-chain cleavage activity in mitochondria isolated from the outer zone also declined significantly, while a similar activity in the inner zone did not change. The serum cortisol concentration in response to dexamethasone administration decreased by 85%, as did the concentrations of cortisol and progesterone in the outer zone; the concentration of cortisol in the inner zone decreased by only one third, while the concentration of progesterone did not change. These results in association with previous reports indicate that the zona reticularis of the guinea pig adrenal cortex (in contrast to the zona fasciculata) is not regulated by ACTH in terms of either steroidogenesis or maintenance of cell growth.