Uptake of a natural surfactant and increased delivery of small organic anions into type II pneumocytes

The uptake of natural lung surfactant into differentiated type II cells may be used for the targeted delivery of other molecules. The fluorescent anio n pyranine [hydroxypyren-1,3,6-trisulfonic acid, sodium salt (HPTS)] was in corporated into a bovine surfactant labeled with [H-3] dipalmitoylphosphati dylcholine ([H-3]DPPC). The uptake of [H-3]DPPC and of HPTS increased with time of incubation and concentration, decreased with the size of the vesicl es used, and was stimulated by 8-bromo-cAMP and partially inhibited by hype rtonic sucrose. However, the amount of HPTS uptake was similar to 100 times smaller than that of [H-3]DPPC. This large difference was due to a more ra pid regurgitation of some of the HPTS from the cells but not to leakage fro m the surfactant before uptake. The acidification of the internalized surfa ctant increased linearly over 90 min to 7.13, and after 24 h, a pH of 6.83 was measured. In conclusion, after internalization of a double-labeled natu ral surfactant, the lipid moieties were accumulated in relation to the anio ns, which were targeted to a compartment not very acidic and in part rapidl y expelled from the cells.