Quantitation of phosphatidylcholine secretion in lung slices and primary cultures of rat lung cells

Rat lung slices and isolated rat lung cells were used to study the secretion of phosphatidylcholine by the lung in vitro. The rate of incorporation of [(3)H]choline by lung slices was 20-fold greater than by liver slices and 4-fold greater in lung cells compared to confluent skin fibroblasts. Labeling lung slices or cells with [(3)H]choline for up to 8 hr failed to reveal a significant amount of labeled phosphatidylcholine in the medium of either system compared to the medium from liver slice or fibroblast controls. Labeling of isolated lung cells for up to 24 hr, with or without 10% fetal calf serum, also showed no significant difference in the amount of labeled phosphatidylcholine in the medium compared to control fibroblast cultures. Washing labeled lung slices or cells with a nonlysing concentration of Triton X-100 (0.05%) did not selectively release labeled phosphatidylcholine, indicating that any secreted phosphatidylcholine did not adhere to the surface of the lung slices or cells. Experiments were performed to determine whether the small amount of phosphatidylcholine in the medium and detergent-released phosphatidylcholine was similar to the tissue and cell phosphatidylcholine. The saturated fatty acid composition of the phosphatidylcholine released by Triton X-100 and in the medium (from lung slices) was identical to that of the tissue phosphatidylcholine. In addition, the relative labeling rates of the phospholipids released by Triton X-100 and in the medium (labeled with [(14)C]glycerol) were identical to those of the tissue and cell phospholipids. Based on these results, we conclude that phosphatidylcholine is not secreted by lung slices and lung cells in large amounts compared to controls. The implication of these data is that pulmonary surfactant material may actually not be secreted by the lung in vitro, and perhaps in vivo, in the manner that is currently generally accepted.