Transport across the gastrointestinal tract of adult rats was measured for the "soluble" nitrate and/or "insoluble" oxide compounds of 228Th, z32U, z33U, 237Np, 237Pu, z4'Pu, 24'Am, ' "Cm, z52Cf and z53Es. Absorption of plutonium and americium nitrates was compared in adult rats, guinea pigs and dogs; there was little variation among species. There was much greater variation between the absorption of different elements in the rat. Other factors observed to influence absorption were particle size, compound form and mass of compound administered. 238pu, 239pu,

Absorption of 228Th, 232U, 233U, 237Np, 237Pu, 238Pu, 239Pu, z41Pu, 241Am, 2"Cm, 252Cf and 253Es from the gastrointestinal tract was measured in neonatal rats gavaged with solutions or suspensions of these actinides. Particle size, valence, chemical form, and animal age all influenced actinide absorption. The neonatal rat and guinea pig absorbed about 100 times more 238Pu than the adult: the newborn swine absorbed 20 times more plutonium than either the newborn rat or guinea pig. In addition to the fraction absorbed, a substantial fraction of radionuclides administered in "soluble" forms was retained for several days within the mucosa of the small bowel.

Absorption of isotopes of the actinide elements Np, Pu, Am and Cm from various organic media and/or in combination with plant or animal ligands or tissue is compared with their absorption from an inorganic nitrate medium. Gastrointestinal (GI) transport of 238Pu, 239Pu, 241Am and 244Cm at high concentrations from citrate medium by adult rats and/or mice was higher than from nitric acid medium. Neptunium-237 absorption, however, was not increased by citrate; probably because its oxidation state was reduced from 237Np(V) to 237Np(IV) by the medium and by the GI content. Increasing the mass of the 237Np dose resulted in increased absorption. Neither incorporation of 238Pu in rat liver nor retention of 238Pu oxide in rat lungs enhanced absorption when those Pu-containing tissues were administered intragastrically to either adult or neonatal rats. Ranking of GI absorption of the various forms of Pu gavaged in these studies suggests that transport is in the order: Pu citrate greater than Pu phytate greater than biologically incorporated Pu greater than Pu nitrate.

Groups of rats were exposed to aerosols of 237Np nitrate to determine clearance rates, retention and distribution at various intervals after inhalation. Initial lung burdens (ILB) after 237Np inhalation by three treatment groups were 0.12, 0.19 and 0.37 mu Ci/kg, respectively. Radiochemical analyses of animals killed at 4, 8, 14, 28 and 90 d, as well as data for others maintained until they became moribund, showed that their lung clearance followed a three-compartment model, clearance half-times for which were 1, 35, and 10,000 d, respectively. Only 3% of the ILB was retained after 90 d; 12% of that burden had translocated to the skeleton at 750 d; the half-time for skeletal retention was 2500 d. A single tumor was the only malignancy detected in the lungs of the 35 animals allowed to survive the early phase of the study.

Plutonium retention was measured after intragastric administration to neonatal rats, dogs and swine. At 1 week after administration, substantially more of the actinide remained in swine and dogs than in rats. The quantity of 238Pu absorbed by piglets was markedly influenced by such factors as compound solubility, mass of plutonium administered, oxidation state of the actinide, and age of the animal at gavage. Cortisone treatment reduced absorption, but was less effective in piglets than in neonatal rats. Measurements of 238Pu transport from ligated segments of the neonatal swine intestine indicated highest absorption from the duodenum, where the actinide was shown, autoradiographically, to be deposited in the epithelial region; in the ileum, deposition was predominantly in the lacteal region. Absorption of actinides by neonatal swine decreased in the order of 233U greater than 238Pu greater than 237Np greater than 244Cm greater than 241Am. Measurements at 1 yr after gavage showed a much higher retention by swine than by rats.