Myelopoiesis in Whole-Body-Irradiated Beagles: Influence of Dose-Rate on GM-CFU and CSA Kinetics

The influence of dose-rate (DR) (either 5.2 or 52 cGy/min.) on the regeneration of bone marrow (BM) myelopoietic progenitor cells was studied in beagles after exposure to whole-body-irradiation (235, 375 and 1500 cGy + autologous BM-transplantation). Myelopoietic progenitor cells were assayed as colonyforming units in agar cultures (GM-CFU), in correlation with the colony-stimulation activity (CSA) in serum. At 235 cGy, the influence of DR on the recovery of GM-CFU was insignificant. However, at 375 cGy, the recovery was critically dependent on the DR. Depletion of GM-CFU numbers elevated CSA levels above preirradiation values. The DR determines the regenerative ability when the dose itself is critical to survival of the least number of hematopoietic stem cells (HSC) necessary for restitution.