Studies on cytogenesis in adult rat adrenal cortex: Circadian and zonal variations and their modulation by adrenocorticotropic hormone

Circadian rhythms and zonal variations in the cell proliferation of adult r at adrenal cortex were studied by following the cells in the DNA-synthesizi ng stage (S-phase) as assessed by 5-bromo-2'-deoxyuridine incorporation int o the cell-nuclei and/or by visualizing proliferating cell nuclear antigen. The S-phase cells were observed throughout the day in two regions of the a drenal cortex: (i) a region from the inner half of the zona glomerulosa to near the outer margin of the tons fasciculata, and (ii) the outer one-fourt h portion of the tons fasciculata, Very little change in number was observe d in the former region between day and night, while a burst of cell prolife ration occurred in early morning at 3-4 a.m. in the latter region. A promin ent rise in the plasma adrenocorticotropic hormone (ACTH) concentration pre ceded the burst of cell proliferation by about 4 h, Upon raising the plasma ACTH concentration by administration of ACTH or metyrapone, prominent cell proliferation also occurred in the same portion of the tons fasciculata 4- 6 h after the provoked ACTH surge. Thus at least two sites in rat adrenal c ortex are responsible for cytogenesis in this endocrine organ, and respond differentially to day/night cycles and circulating ACTH levels.