Effect of repetitive exposures to gamma rays on the hematopoietic system of the rat.

By SIEGMUND J. BAUM, DONALD J. I IMELDORF AND ELLY M. ,JACOBSEN T HE HEMATOLOGIC EFFECTS of acute ionizing radiations have beemi described extensively in recent review articles.’ ‘ 2 , 4 The effects of prolonged exposure to low intensity radiations and the effects of dose fractionat.ion or periodic radiation exposures are somewhatmore difficult to interpret because of the complex interaction of destructive amid regenem’ative factors inolved. When rats were exposed daily to low (7.7 r) doses of x-rays m’elatively few changes were observed in the number of circulating blood cells, although the exposure was continued for 250 days.5 Mice subjected to 1.1, 4.4 on’ 8.8 r of gamma rays during eight hours daily6 for a one year period exhibited rio significamit erythrocyte changes. However, daily doses of 33.5 r of x-rays5 have been demonstrated to be sufficient to cause a slight but distinct decrease in eryt.hrocyte count-s in rats. The dose-response relationship is somewhat species dependent. For example, a significant decrease in the erythrocyte estimate imi guinea pigs exposed to 8.8 r gamma radiation for eight hours daily was ol)served by Lorenz et al.7 Leukocytes are much more semisitive to periodic low dose radiation. Daily exposures to 7.7 r x-radiation caused a distinct decrease in the miuniber of leukocytes in the rat.5 In the present experiment male rat-s were subjected to a series of one or mimore doses of 75 r gamma radiation at seven day intervals between exposures. Herniatologic examinations were made followimig the last exposure imi each series of irradiations. This experimental design was selected imi order to detem’mimie the effects of periodic exposure to sublethal doses of low int.emisity ioniziimg radiat.iomis upomi recovery in the hematopoietic system as manifested by changes iii the circulating blood cells. Presumably, observable effects would be the resultant of interaction between radiation damage caused by the periodic exposures and the forces of regeneration active during the interval.