Yoshiyuki Shiraishi

ABSORPTION AND RETENTION OF

The effect of age on the absorption and retention of 144Ce and 95Zr-95Nb in rats was investigated with special emphasis on sucklings. Although poorly absorbed by adult rats, these nuclides were absorbed to a considerable extent by suckling rats resulting in a ten to a thousand times higher retention than in adults. The initial rapid loss of the nuclides from suckling rats disappeared at around weaning age and was followed by a considerably slower exponential decrease. Though the adult rats showed a much steeper and larger initial loss, a similar rate of exponential decrease was observed thereafter. The whole-body retention curves of these nuclides for suckling rats previously treated with cortisone acetate showed much steeper initial loss and significantly lower retention than those for controls. The ratio of 95Zr-95Nb in bone of sucklings was found to be approximately 4-5, whereas that in most of their soft tissues was almost unity. The substantial absorption of these nuclides by suckling rats is mainly due to the pinocytosis which is more active in younger animals.