Recuperation from Lethal Injury by Whole-Body Irradiation: II. Kinetic Aspects in Radiosensitive BALB/c Mice, and Cyclic Fine Structure during the Four Days after Conditioning Irradiation
Mature male or female BALB/c mice were whole-body irradiated, either with single 250-kv doses of short duration or with graded series of 2nd doses at different times after 3 conditioning doses (Dc)[long dash]309 R, 154 R, or 50 R. Second doses were administered every 3 hrs. during the first 12 hrs. and every 6 to 12 hrs. thereafter. The LD50''s were determined from the mortalities for the 30 days after irradiation, and these data were used for the calculation of residual injury[long dash], that fraction of the Dc still effective at time t. As in the case of C57BL mice, which were tested previously in a comparable series of experiments, all the Dc''s induced a state of refractoriness relative to the radiosensitivity of unirradiated controls. This relatively refractory state occurred at approximately 6 to 9 hrs. postconditioning and was succeeded, at approximately 9 to 15 hrs., by an increase in radiosensitivity (a maximum in the curve of residual injury decay). The data were suggestive of a continuation of this initial radiosensitivity fluctuation during the 12- to 48-hr. post-conditioning period, but were inadequate to establish this with certainty in this series of experiments. Additional experiments were undertaken to examine more closely this kind of possible cyclic fine structure during recuperation. After exposure of both C57BL and BALB/c mice to conditioning doses of 150 R and 300 R, groups of 16 mice were irradiated with a constant 2nd dose at usually 1 1/2-hr. increments, thereafter. The resultant 30-day mortalities of groups thus treated were then subjected to close inspection according to rigorous but arbitrary criteria in order to detect meaningful minima and maxima. In addition, the same data were subjected to time series analysis with the aid of a computer. Periodic fluctuations were detected by both techniques of data evaluation. It was concluded that conditioning doses of 150 to 300 R initiate cyclically fluctuating radiosensitivity changes which are characterized by a period of approximately 8 hrs. during the first 48 postconditioning hrs. Evidence from subsequent experiments indicates the presence of a higher frequency cycle of a period of approximately 4.6 hrs. during the 3rd and 4th postconditioning days. Other aspects of the data of these experiments include demonstration of "overrecovery", consistent with the observations of others. The findings may be interpreted on a cellular basis: The observed radio-sensitivity differences result from synchronization in the postirradiation growth of surviving hemopoietic stem cells in the bone marrow. In addition to killing some cells immediately, it is suggested that conditioning irradiation imposes temporary delays on the progress of surviving cells, causing them to be grouped in relatively synchronous pulses or cohorts which then assume different radiosensitivities according to their generation, stage-sensitivity dependence. Thus, the approximately 8-hr. cycle may be regarded as a reflection of the normal generation time of hemopoietic stem cells, and might represent half the .generation cycle time of a cell regarded as having 2 resistant peaks, or, alternatively, the total mean generation time of cells having but one resistant peak.