Inhibition of DNA synthesis in Ehrlich ascites cells by actinomycin D. II. The presynthetic block in the cell cycle.

Per cent of labeled mitoses Interval between H3-Tdr and act.D mean (range)No act.D 61 (50-80) 30 Min 150, 270, * 20 lsc of H3-Tdr per mouse. Acinomycin D, 0.016 iug/gm body weight.See text for details. Labeled Cells per 1,000 After one injectionAfter two injections Controls 550 (520-580) 708 (685-720)Act.D 545 (490-600) 588 (505-650) * Tumor-bearing mice were injected with 10 pc of HL-Tdr, and an aliquot of tumor was aspirated from the peritoneal cavity after 30 min.After another 30 min, half of the animals were injected with actinomycin D (0.016 gg/gm body weight); injections were repeated every 2 hr for a total of four injections.One hour after the last injection of act.D, all mice were reinjected with 10 Ac of H'-Tdr, and killed after another 30 min.Smears for auto- radiography were prepared from each mouse after the first injection of radioisotope and again after killing.