Comparison of Pulmonary Neutrophils in the Adult and Neonatal Rat after Hyperoxia

Abstract

ABSTRACT: The mechanisms of the increased tolerance to hyperoxia of neonatal animals of many species is incompletely understood. To investigate the etiology of this difference we compared neutrophil entry into the lungs of neonatal and adult rats after hyperoxic exposure. Adult rats were studied after exposure to ≥98% O₂ for 60 h and neonatal rats after 3 and 7 d. Neonatal survival was prolonged compared with that reported for adult rats (77% after 7 d of exposure). In adult rats, there were significant increases in pulmonary neutrophils after 60 h of O₂ exposure. In neonatal rats, these changes were not evident after 72 h of exposure, but pulmonary neutrophils increased after 7 d of hyperoxia. Before mortality, pulmonary neutrophils were distributed differently in the age groups. After 7 d of O₂ exposure in the neonates, total neutrophil counts in lung tissue (21.92 ± 7.29 per cm² grid) and lung myeloperoxidase (0.085 ± 0.02 U/mg protein) remained significantly lower than in adults after 60 h of O₂ exposure (41.44 ± 9.08 per cm² grid and 0.411 ± 0.085 U/mg protein, respectively). However, in histologic specimens, O₂-exposed neonates had higher percentages of neutrophils free in the alveolar air space than did adults, corresponding to a trend toward higher neutrophil counts in bronchoalveolar lavage fluid in the neonates. It appears that, in addition to delay in neutrophil influx into the lung, neonatal rats have lowered retention of neutrophils to the alveolar tissue.