Comparative Metabolism of Radionuclides in Mammals-X. Retention of Tracer-level Cobalt in the Mouse, Rat, Monkey and Dog

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Abstract
An interspecies comparison of 60Co metabolism was made among mice, rats, monkeys and dogs following intravenous, intragastric and oral administration. Results of these studies have been compared with human data available in the literature. Long-term whole-body retention appeared to be longer in the mouse (495 days) than in the rat (309 days), monkey (183 days) or dog (180 days). This pattern is quite different than normally encountered in the retention of trace metals and may be attributed to differences in vitamin B12 behavior among species. The possibility of a carrier effect in the amount of stable cobalt administered between species may explain the differences observed. The human data indicate a long retention half-life up to 800 days which was not seen in the experimental species studied. Tissue distribution of 60Co, administered here as the chloride, favors the liver and kidney at short times after single administration and during chronic (daily) dosage in the drinking water. Data from a few longer term animals following intravenous injection indicate that bone may be the organ of highest concentration. The data on cobalt chloride absorption from the rat gastrointestinal tract to blood indicate that less than 10% enters the circulation after single dosage but that daily administration leads to an equilibrium body burden of about 20%, a value about two-thirds that used by the International Commission on Radiological Protection for humans.