INTERSPECIES COMPARISON OF THE CLEARANCE OF IONIC COBALT FROM THE LUNGS

In a previous interspecies comparison of the clearance of inhaled Co3O 4 particles from the lungs (Bailey et al., 1989), a model was develope d to explain the differences in the time-dependent rate of absorption of dissolved Co-57 into the blood. The model suggested marked species differences in the fraction of dissolved cobalt (q) retained long-term in the lung tissue, as well as differences in the initial absorption rates. In the present study, for many of the same species (baboon, gui nea pig, two strains of rat, hamster, and dog), the retention of solub le cobalt deposited in the lung was measured directly over 100 days (m ore than 1000 days in the dogs). While there were significant interspe cies differences in the whole-body retention curves and in the urinary and fecal excretion rates over this period, in all species studied th ere was a high concentration of Co-57 in the trachea and in the lung t issue, relative to the concentration in the whole body. Ratios for ind ividual animals ranged from 9.0 to 1262 for trachea and from 1.4 to 25 5 for lung. In the trachea the Co-57 was mainly concentrated in the ca rtilaginous rings. The mean fraction of Co-57 retained in the lung for 100 days or more ranged from 0.13% to 0.58% of the amount instilled. This was much lower than the values for q of 1-10% predicted previousl y. The retained fraction was highest in the HMT rat and decreased in t he following order: HMT rat > guinea pig > dog > baboon > F-344 rat > hamster; this was different from the ranking predicted by the model. I t is concluded that the interspecies differences in absorption rates f or inhaled Co3O4 particles are not the result of differences in the fr action of dissolved cobalt retained in lung tissue.