Retention and Distribution of 241Am III in Neonatal Beagles

Abstract

The distribution of Am in newborn beagles was studied during the first week after birth. Seven litter mates (4 males, 3 females) weighing 250–280 g were injected intravenously with 3.0 μCi 241AmIII/kg, in citrate buffer, at 1 day of age and sacrificed 1, 3 and 5 days later. At sacrifice, livers, spleens, kidneys, other selected soft tissues and skeletons were analyzed for Am content. Cumulative excretion during the time observed was 6.7% of the injected dose, 5.5% of which occurred during the first day. Skeletal retention ranged from 76% of the injected dose at 1 day to 84% by 5 days. The skeletons of dogs given the same dose of Am at 18 months of age contained 29% of the injected Am at 1 week. The relative distribution of Am within the skeleton was dependent on the state of development of its individual bones and differed considerably from that of the young adult. In neonates, the Am content of the bones in the skull was 3 times that seen in dogs injected as young adults. Initial liver deposition of Am in neonates was only 7%, in contrast to about 50% in young adult beagles. The subcellular distribution of Am in liver also differed in the two age groups. In neonates, a greater fraction of the nuclide was associated with soluble proteins of the cytosol, and the increase in the Am content of mitochondria and lysosomes proceeded at a much slower rate than was observed in mature animals.