Absorption of human growth hormone from the rat lung

Recombinant methionyl human growth hormone (hGH) was administered intratracheally to adult rats, and serum concentrations of immunoreactive hGH were measured for up to 24 h. The mean absolute bioavailability was approximately 36% after 18h and was similar for doses of 0.75, 1.5, and 3 mg/kg. Peak serum hGH concentrations occurred at approximately 6 h after dosing. Tritiated hGH (3H-hGH) was used to follow the clearance of hormone from the lungs. Disappearance was linear with time and by 24 h approximately 70% of the radioactivity was gone from the lungs (elimination half-life = approximately 10.5h). Monomeric and aggregated hGH appeared to account for the majority of the residual 30% of radioactivity. Immunohistochemical localization of hGH in the alveoli suggested that the hormone was concentrated in a thin layer at the air-epithelial boundary. Pulmonary macrophages, which also stained for hGH, probably degrade hGH and thus account for some loss of material in the lungs. These studies suggest that the lung may be an alternative route for systemic delivery of recombinant proteins which are currently delivered by injection.