Effect of hydrocortisone, adrenalin and actinomycin D on transition of cells to the DNA synthesis phase

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Injection of 5 μg/g of hydrocortisone or 0.1 μg/g of actinomycin D 9 h before the transition of cells to the S phase inhibited the initiation of DNA synthesis in squamous epithelial cells of the mouse forestomach. When these substances were injected 3 or 6 h before the transition of cells to the S phase there was no effect on initiation of DNA synthesis. Injection of 1 or 2.5 μg/g of adrenalin stimulated the transition of cells to the S phase, the stimulating effect appearing 9 h after injection of adrenalin. This shows that at some time ranging from 6 to 9 h before the transition to the S phase, the cell made a decision on initiation of DNA synthesis and that this decision was connected with the synthesis of RNA necessary for DNA synthesis. A part of the presynthetic period between decision on, and initiation of DNA synthesis was named as the R phase. Diurnal variations in index labelling were explained as a result of inhibition by glucocorticoids of the transition of cells to the R phase.