The release of surfactant in rat lung by brief periods of hyperventilation

We investigated the release of surfactant-type phospholipids (S) using the isolated perfused rat lung (IPL). Following a 20 min equilibration period the lungs were hyperventilated for up to 15 min and then lavaged. Changing the peak inspired pressure (PIP) from 10 to 20 cm H₂O rapidly increased the rate of release of S; this rate declined after 2 min. In contrast, doubling frequency of ventilation while maintaining the control tidal volume had no effect. The increase in alveolar S reflected release, rather than redistribution, and after 2 min amounted to about 8% of total S in lung tissue. Equivalent hyperventilation in an open-chested intact rat released significantly less S, suggesting possible tonic neurohumoral suppression in vivo. The release of S in the IPL was depressed by reducing temperature, but was not affected by hypoxia, 2,4 dinitrophenol, phenylephrine or dibutyrylguanosine 3′,5′-cyclic monophosphate. We suggest that increasing tidal volume may directly distort the alveolar type II cell; each cell reacts to its own threshold distortion by releasing a pool of S in an all-or-none fashion.