Disposition of 3H-fenoterol after inhalation in rats

The time course of radioactivity in blood was compared following oral administration, intratracheal instillation and inhalation, respectively, of 3H-fenoterol hydrobromide in rats. Also the tissue levels after inhalation were measured. A small peak of the blood level appeared within 10 min after inhalation followed by a second larger peak. The peak time in the first phase corresponded with that after intratracheal instillation, suggesting that this phase should be attributed to local absorption in the lung. The second peak corresponded with that after oral administration, suggesting that a large proportion of the drug taken in by the animal after inhalation was swallowed and absorbed in the gastrointestinal tract. A direct measurement of radioactivity in tissue samples of the lung, trachea, oesophagus, stomach and small intestine demonstrated clearly the above accounts.