COMPARISON OF PULMONARY NEUTROPHILS IN THE ADULT AND NEONATAL RAT AFTER HYPEROXIA

The mechanisms of the increased tolerance to hyperoxia of neonatal ani mals of many species is incompletely understood. To investigate the et iology of this difference we compared neutrophil entry into the lungs of neonatal and adult rats after hyperoxic exposure. Adult rats were s tudied after exposure to greater than or equal to 98% O-2 for 60 h and neonatal rats after 3 and 7 d. Neonatal survival was prolonged compar ed with that reported for adult rats (77% after 7 d of exposure). In a dult rats, there were significant increases in pulmonary neutrophils a fter 60 h of O-2 exposure. In neonatal rats, these changes were not ev ident after 72 h of exposure, but pulmonary neutrophils increased afte r 7 d of hyperoxia. Before mortality, pulmonary neutrophils were distr ibuted differently in the age groups. After 7 d of O-2 exposure in the neonates, total neutrophil counts in lung tissue (21.92 +/- 7.29 per cm(2) grid) and lung myeloperoxidase (0.085 +/- 0.02 U/mg protein) rem ained significantly lower than in adults after 60 h of O-2 exposure (4 1.44 +/- 9.08 per cm(2) grid and 0.411 +/- 0.085 U/mg protein, respect ively). However, in histologic specimens, O-2-exposed neonates had hig her percentages of neutrophils free in the alveolar air space than did adults, corresponding to a trend toward higher neutrophil counts in b ronchoalveolar lavage fluid in the neonates. It appears that, in addit ion to delay in neutrophil influx into the lung, neonatal rats have lo wered retention of neutrophils to the alveolar tissue.