Interspecies Comparison of the Clearance of Ionic Cobalt from the Lungs

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Abstract
In a previous interspecies comparison of the clearance of inhaled Co3 O4 particles from the lungs (Bailey et al., 1989), a model was developed to explain the differences in the time-dependent rate of absorption of dissolved 57Co 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, guinea pig, two strains of rat, hamster, and dog), the retention of soluble cobalt deposited in the lung was measured directly over 100 days (more than 1000 days in the dogs). While there were significant interspecies differences in the whole-body retention curves and in the urinary and fecal excretion rates over this period, in all species studied there was a high concentration of 57Co in the trachea and in the lung tissue, relative to the concentration in the whole body. Ratios for individual animals ranged from 9.0 to 1262 for trachea and from 1.4 to 255 for lung. in the trachea the 57Co was mainly concentrated in the cartilaginous rings. The mean fraction of 57Co 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 previously. The retained fraction was highest in the HMT rat and decreased in the following order: HMT rat > guinea pig > dog > baboon > F-344 rat > hamster; this was different from the ranking predicted by the model. It is concluded that the interspecies differences in absorption rates for inhaled Co3 O4 particles are not the result of differences in the fraction of dissolved cobalt retained in lung tissue.