Hormonal regulation of adrenal microvillar channel formation.

This study examined the in vivo relationship between expression of the HDL receptor scavenger receptor class B (SR-BI) and corresponding structural changes in the rat adrenocortical cell microvillar compartment. Using hormonal stimulation and withdrawal protocols, we were able to manipulate adrenal SR-BI levels and carry out qualitative and quantitative measurements correlating SR-BI expression with microvillar mass and microvillar channel formation. Young male rats were used as controls or treated with adrenocorticotropin hormone (ACTH) (24 h), 17alpha-ethinyl estradiol (17alpha-E2) (5 days), or dexamethasone (DEX) (24 h). Quantitative Western blot analysis and immunocytochemistry indicated that ACTH and 17alpha-E2 treatment greatly increased SR-BI expression in the adrenal (especially in the microvillar compartment of adrenocortical cells), whereas DEX treatment led to a decrease of SR-BI by all measurements. At the same time, striking ultrastructural changes occurred in the adrenocortical cell microvillar compartment: e.g., microvillar area and microvillar channel formation and complexity dramatically increased (compared with control values) after ACTH or 17alpha-E2 treatment, whereas the same values declined after DEX treatment. These measurements illustrate the exceptional flexibility and responsiveness of the microvillar compartment to hormonal stimuli, and suggest that regulation of SR-BI expression and structural configuration of the surface of steroidogenic cells goes hand in hand.