A morphometric analysis of adrenocortical actin localized by immunoelectron microscopy: the effect of adrenocorticotropin.

The localization of actin and the effect of ACTH on its concentration was examined in freshly isolated rat adrenocortical cells. Lowicryl K4M-embedded cells were used for the immunoelectron localization of actin; gold was used as a label for immunoreactive sites. Actin was at least 4 times as concentrated at the cortical cytoplasm as in the lipid droplets and at least 5 times as concentrated in the microvilli as in the lipid droplets. ACTH stimulation approximately doubled the concentration of actin in the cortical cytoplasm and increased by 50% the concentration of actin in the microvilli. The microvillar contribution to the cell surface area was 40% higher in ACTH-stimulated cells than it was in unstimulated cells. These results provide quantitative evidence suggesting that actin and the microvilli participate in steroid secretion by the adrenocortical cell.