Opposite effects of ACTH and glucocorticoids on adrenal DNA synthesis in vivo.
Administration of ACTH to rapidly growing weanling rats results in an increase of DNA synthesis in adrenal and a decrease in liver. Dexamethasone administration decreases both adrenal and liver DNA synthesis. When both hormones were administered to the same animals, the liver DNA synthesis was similar to that observed with dexamethasone alone, but the DNA synthesis in adrenal was lower than that obtained with ACTH alone, yet higher than that observed with dexamethasone. The plasma levels of corticosterone were similar in animals treated with ACTH or with ACTH plus dexamethasone. Aminoglutethimide stimulated adrenal DNA synthesis, but less than ACTH. This substance overcame partially the inhibitory effects of dexamethasone on liver DNA synthesis but did not in adrenal. When both ACTH and aminoglutethimide were given simultaneously, adrenal DNA synthesis was higher than that observed with each substance alone. In all experiments in which adrenal cytosol DNA polymerase was studied, the activity varied in the same direction as DNA synthesis. These results indicate opposing effects of ACTH and glucocorticoids on adrenal DNA synthesis. The finding of a glucocorticoid effect on the adrenal is supported by the demonstration of a glucocorticoid specific binding protein in adrenal cytosol. Cycloheximide blocks the stimulatory action of ACTH on both steroidogenesis and DNA synthesis. Actinomycin D, as well as dexamethasone, blocks only the DNA synthesis-promoting action of ACTH. This latter result suggests some differences in the metabolic pathways by which ACTH controls steroidogenesis and growth in the adrenal cell.