Clearance of lead-212 ions from rabbit bronchial epithelium to blood

The absorption of 212Pb ions from bronchial epithelium to blood has been investigated in anaesthetized rabbits. The 212Pb ions were introduced by intubation either into the trachea or into smaller, more distal bronchi. Removal from lung was followed by external gamma-counting. Mucociliary clearance to the GI tract was blocked by tracheostomy. Two distinct phases of clearance from bronchial epithelium to blood were observed. Approximately 20% of deposited 212Pb is rapidly absorbed with a half-time of about 4 min, the remainder with a biological half-time of about 9 h, irrespective of the site of instillation in the bronchial tree. Two hours after deposition, the 212Pb remaining in lung was found to be partitioned between mucus and the bronchial epithelium, with a substantial but minor fraction in the epithelium. Uptake of 212Pb in the skeleton was estimated to be about 20% of the 212Pb entering the blood circulation. Removal by the kidneys, at 25%, was comparable with skeletal uptake. These results are compared with previously published work using rodents, dogs and man which demonstrated either rapid or slow absorption but not both phases occuring together.