Protection of cycling CFUs against hydroxyurea by low doses of actinomycin D

Low dose (80 micrograms/kg) Actinomycin D (AD) produced a significant but transient inhibition of proliferation of the haemopoietic stem cells (CFUs) in chimaeras or in mice regenerating after sublethal irradiation. The same dose of AD had no effect on the resting CFUs population. During the period of proliferation inhibition, CFUs proved to be insensitive to the killing effect of [3H]thymidine in vitro and hydroxyurea (HU) in vivo. In Ehrlich ascites tumour (EAT) bearing mice enhanced CFUs turnover rate was found. Eighty micrograms/kg AD produced a selective effect in these mice: it protected the proliferating CFUs population without diminishing the effect of hydroxyurea on the tumour cells.