The interaction of phosphatidylcholine with alveolar type II pneumocytes is dependent on its physical state

Dipalmitoylphosphatidylcholine, the principal phospholipid component of surfactant, inhibits agonist stimulated surfactant secretion whereas dioleoylphosphatidylcholine does not. As knowledge of the type of interaction of phosphatidylcholines is important for the detailed analysis of surfactant homeostasis, this was examined in isolated rat type II pneumocytes in primary culture. Solid state [3H]-dipalmitoylphosphatidylcholine liposomes associated with the cells rapidly. No effect of blockade of endocytosis on the cellular association was observed, whereas that of the fluid phase marker [14C]-sucrose was reduced. No evidence for the fusion of the dipalmitoylphosphatidylcholine liposomes or for phospholipid exchange with the cells was detected, suggesting that the cells primarily bound the dipalmitoylphosphatidylcholine liposomes. Although a "specific" site with a saturable binding capacity (20 nmol dipalmitoylphosphatidylcholine/mg protein, KD 25 microM) was demonstrated, the interaction did not exhibit all the characteristics of a typical pharmacological receptor. The preincubation with nonlabeled dipalmitoylphosphatidylcholine almost completely inhibited binding to the cells. In accordance with their effects on stimulated surfactant secretion, various other phosphatidylcholine liposomes inhibited binding that was very much dependent on their physical state, as only those in a solid state were inhibitory by more than 50%. These results support the view that the binding of dipalmitoylphosphatidylcholine may be involved in the feedback regulation of surfactant secretion in type II pneumocytes and that these processes are dependent on the physical state of the interacting liposomes.