The peripheral cytoplasm of adrenocortical cells: zone-specific responses to ACTH

Background: Differences in the cytoskeletal protein actin in cells from the zona glomerulosa and zona fasciculata would be of considerable interest because there is persuasive evidence that rat corticosteroids are secreted by mechanisms that are somewhat zone-specific. We have previously shown evidence that actin may be involved in steroid secretion, possibly in connection with changes in adrenocortical microvilli. However, the cells upon which the data were based were not separated according to zone of origin. Methods: Immunogold electron microscopy and morphometric procedures were used to determine whether ACTH-induced changes in the peripheral cytoplasm of isolated adrenocortical cells occur in both zona fasciculata and zona glomerulosa cells. Results: Actin immunoreactivity was more concentrated in the cytoplasm adjacent to the plasma membrane (including the cytoplasm within the microvilli) than it was in the internal cytoplasm in cells from both zones (4–6 times more concentrated in zona glomerulosa cells and 3–6 times more concentrated in zona fasciculata cells). However, the mean aggregate microvillar surface length (microvillar index) of untreated zona fasciculata cells previously reported (Loesser and Malamed, 1987) was 23% greater than that of untreated zona glomerulosa cells. Although ACTH (at a maximal steroidogenic concentration) had no effect on the peripheral cytoplasmic actin concentration of zona glomerulosa cells, there was a 24% increase in the aggregate microvillar length. In contrast, in zona fasciculata cells, ACTH treatment was accompanied by an increase in peripheral cytoplasmic actin concentration of 58–64% and an increase in aggregate microvillar surface length of 40% (previously reported (Loesser and Malamed, 1987)), almost twice that for zona glomerulosa cells. Conclusion: The results suggest that ACTH-induced hormone release from zona fasciculata cells is mediated by increases in peripheral cytoplasmic actin and aggregate microvillar length; in zona glomerulosa cells such changes are small or absent. © 1994 Wiley-Liss, Inc.