The rate of internalization of different receptor-ligand complexes in alveolar macrophages is receptor-specific.

To probe the mechanisms of endocytosis in alveolar macrophages, we examined the internalization rates of three different receptors. Initial rates of internalization for mannosylated BSA, diferric transferrin and alpha-macroglobulin-proteinase complexes were all different. Although the absolute rates of internalization varied depending on the cell preparation, transferrin was internalized at 10-20% and alpha-macroglobulin-proteinase complex at 40-60% of the rate of manosylated-BSA. Incubation of cells with transferrin did not affect the rate of internalization of mannosylated BSA or alpha-macroglobulin-proteinase complexes, and the rates of internalization were independent of receptor occupancy. These different internalization rates could not be ascribed to different rates of diacytosis. Altering the distribution of unoccupied surface receptors by either trypsin treatment of cells at 0 degree C or exposure to hyperosmotic solutions resulted in the absolute internalization rates being affected by the experimental condition, but the hierarchy in receptor internalization rates was maintained. The fact that a variety of conditions affect receptor internalization rates to the same degree implies the existence of co-ordinate regulation at a single rate-limiting step. Based on these results, we suggest that differences in internalization rate reflect the ability of ligand-receptor complexes to be captured by coated pits.