SURFACTANT PROTEIN-A INHIBITS LAVAGE-INDUCED SURFACTANT SECRETION IN NEWBORN RABBITS

In vitro experiments with granular pneumocytes suggest that surfactant protein-A (SP-A) inhibits secretion of pulmonary surfactant. We examined whether SP-A inhibits surfactant secretion induced by lung distention during lung lavage. Human SP-A was obtained by lung lavage in a patient with pulmonary alveolar proteinosis. After centrifugation of the lavagate, the pellet was repeatedly washed with saline and then extracted with chloroform:methanol. The methanol:saline phase was separated and lyophilized to yield the SP-A product. SDS-PAGE and immunoblot analysis indicated that our preparation of SP-A had only minor contamination with human plasma proteins. To examine secretion, we used freshly killed newborn rabbit pups of 29.5 days gestation and lavaged the lungs by 10 sequential, fresh saline washes. Littermate neighbor pairs were lavaged with SP-A or human plasma protein at concentrations of 1, 6, 10, and 50 mug/ml, and disaturated phosphatidylcholine (DSPC) was analyzed as a marker for surfactant. The inhibition of surfactant secretion was maximal at a concentration of 10 mug/ml; the average yield by the last two pairs of washes, an index of surfactant secretion, was 286 +/- 41 mug/g dry lung weight for SP-A, compared to 405 +/- 37 mug/g dry lung weight for controls, an inhibition of 30% (p < 0.005). There were no changes in the volumes of returned lavage or in the concentrations of lactate dehydrogenase or DNA. To test whether SP-A increased cellular uptake of DSPC in the lungs, we prepared radioactive exogenous surfactant, lavaged it into the lungs, and monitored recovery of radioactivity by continued lavage. Recovery was the same in treated and control lungs. We conclude that SP-A may inhibit surfactant secretion stimulated by lung distention.