Short-term effects of ACTH on protein synthesis in adrenal cortex cells of young rats.

Two units of ACTH were administered intraperitoneally to young 20 gm-rats which received an intravenous injection of L-leucine-3H thirteen min later. ACTH-injected rats, and control rats which received the isotope alone, were killed at 2-, 10-, 30- and 60-min intervals. Electron microscope autoradiographs in control animals showed strong amino-acid uptake at pulse time (2-min) in the cytoplasm of adrenal zona fasciculata cells. Label was shared between the endoplasmic reticulum (ER) and mitochondria, and a lower but still considerable uptake was seen in nucleoli. At first chase time interval (10-min) cytoplasmic labelling declined, while nuclear and nucleolar labelling increased, both changing little thereafter, and there was a 10-30 min Golgi peak. ACTH administration provoked an overall increase in amino-acid incorporation into cytoplasm, nucleus and nucleolus at pulse time, with no changes in the distribution of the reactions among organelles. Intensification of labelling was most evident over nucleoli, the grain density of which was four-times as high as in controls. The short-term increase in ER and mitochondrial protein synthesis observed after ACTH injections was considered to be consistent with the hypothesis that most newly-formed proteins in these cells may be involved in the regulation of steroidogenesis. The marked increase in nucleolar labelling suggested the presence of proteins involved in RNA synthesis.