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1225 PHOSPHATIDYLCHOLINE SECRETION FROM A CELL LINE (A549) WHICH RESEMBLES TYPE II PNEUMOCYTES

The A549 cell line resembles Type II pneumocytes morphologically, synthesizes disaturated phosphatidylcholine and stores it in lamellar bodies. The phospholipid secreting properties of this cell line were studied. The cells were pulsed with either (32P)- or (3H)-choline and release of phospholipid into the medium measured. In the absence of a stimulating agent, small amounts of labelled phospholipid were recoverable in the medium. However, removal of serum from the medium caused a marked (10 fold after 2 hours) reduction in the rate of phospholipid release, probably due to the elimination of exchange between cellular membranes and serum lipoprotein. Albumin can mimic the effect of serum. The calcium ionophore A23187 stimulates exocytosis in many secretory systems and it produced a 30 fold increase in the rate of secretion of phosphatidylcholine (45% disaturated) from A549 cells. The effect was inhibited by the removal of calcium or the addition of EDTA to the medium. Incubation of cells with some potential physiologic secretagogues, including isoproterenol, norepinephrine, carbamyl choline, and dopamine did not produce a significant increase in the rate of phospholipid secretion. The A549 cell line secretes phosphatidylcholine by calcium dependent exocytosis and may be a useful experimental system for elucidating factors which regulate surfactant secretion from Type II pneumocytes.