The type II epithelial cells of the lung. VI. Incorporation of 3H-choline and 3H-palmitate into lipids of cultured type II cells.
We measured incorporation of 3H-labeled lipid precursors into surfactant-associated and nonsurfactant associated phospholipids in 2- to 7-day primary cultures of rabbit type II alveolar epithelial cells, in order to assess the degree to which cells retain metabolic features related to surfactant production in vitro. Unsaturated phosphatidyl choline label increased progressively in type II cell monolayers grown in the continuous presence of 3H-choline, but label in saturated phosphatidyl choline, the surfactant-associated fraction, remained constant after 2 days in culture. Although type II cell cultures could be stimulated to increase saturated phosphatidyl choline production by brief exposure to 8.5 x 10(-5) M palmitic acid, this response proved to be transient. Type II cells synthesized the other major surfactant-associated lipid, phosphatidyl glycerol, from 3H-palmitic acid in vitro, in proportions decreasing from 6.3 per cent of total lipid synthesis in fresh cell isolates to 1.8 per cent in 2-day cultures and 1.4 per cent in 4-day cultures. Aging of the cell cultures was also associated with a proportionate decrease in synthesis of neutral lipids and increases in synthesis of phosphatidyl choline, sphingomyelin, and phosphatidyl inositol, from palmitic acid. We conclude that: (1) isolated type II alveolar cells continue synthesis of surfactant-related lipid species, a major index of differentiated function, for at least 4 days after introduction into cell culture; (2) production of non-surfactant related lipids occupies a progressively larger portion of type II cell lipid synthetic activity during this period, probably reflecting increased synthesis of cell membranes.