Type II alveolar cells in organotypic culture: A model system for the study of surfactant synthesis

Abstract

1. 1. Type II cells, maintained in an organotypic system where in vivo morphologic characteristics are retained, were utilized to study surfactant phospholipid and protein synthesis. Cellular components were separated into a surfactant and a residual fraction by sucrose density centrifugation. The major phospholipid of the surfactant fraction is phosphatidylcholine (70%). Phosphatidylglycerol accounts for 4.0% of the total surfactant phospholipids. Phosphatidylcholine is also the major phospholipid of the residual fraction, constituting 50% of this fraction's phospholipids; phosphatidylglycerol is also present (4.2%). 2. 2. Type II cells in organotypic culture are capable of metabolizing exogenous glucose. The uptake of glucose from the incubation medium is concentration dependent. Glucose can be utilized by the type II cell for surfactant and residual phosphatidylcholine synthesis. The rate of incorporation of glucose into residual phosphatidylcholine is three times the incorporation rate into surfactant phosphatidylcholine. 3. 3. Palmitate and choline can also be utilized for surfactant and residual phosphatidylcholine synthesis. Palmitate incorporation into residual phosphatidylcholine is five times greater than the incorporation into surfactant phosphatidylcholine. Surfactant phosphatidylcholine is labelled with choline at a rate one-fourth that of residual phosphatidylcholine. In contrast to glucose and palmitate, choline incorporation into surfactant phosphatidylcholine is linear only after a 15 min lag period. The labelling of residual phosphatidylcholine with choline is biphasic. 4. 4. Type II cells in organotypic culture can also synthesize the protein moiety of surfactant. Leucine was found to incorporate linearly into surfactant proteins for approx. 8 h.