SURFACTANT LIPID UPTAKE AND SECRETION IN TYPE-II CELLS IN RESPONSE TO LECTINS AND SECRETAGOGUES

Secretion of surfactant phospholipids can be inhibited by the surfactant-associated protein SP-A. SP-A was reported to stimulate lipid uptake by type II cells, whereas surfactant secretagogues were reported to have the same effect in isolated perfused lungs. We examined the effect of such secretagogues on uptake of liposomes containing L-alpha-[2-palmitoyl-9,10-H-3(N)]-dipalmitoyl phosphatidylcholine in primary cultures of type II cells. As SP-A contains a lectinlike domain and other lectins were reported to inhibit surfactant secretion, we also examined the effect of such lectins on lipid uptake. At concentrations at which they maximally stimulate phosphatidylcholine secretion in type II cells, several secretagogues had no effect on liposome uptake. Maclura pomifera agglutinin (MPA) stimulated uptake approximately 10-fold with a concentration eliciting 50% maximum stimulation (EC50) of 17-mu-g/ml. The effect of MPA on uptake was considerably greater than that of SP-A. However, although the stimulatory effect of ATP on phosphatidylcholine secretion was almost completely antagonized by SP-A, it was maximally inhibited only 75% by MPA. The concentration elicing 50% maximum inhibition (IC50) for MPA inhibition of secretion was 0.5-mu-g/ml. Concanavalin A, another lectin, had no effect on lipid uptake but completely inhibited secretion. These data show that a lectin other than SP-A can stimulate phospholipid uptake by type II cells cultured on plastic and suggest that surfactant secretion and reuptake are independently regulated processes.