Temperature effect on endocytosis and exocytosis by rabbit alveolar macrophages

Endocytosis of 125I-mannose-bovine serum albumin (BSA) and exocytosis of 125I-mannose-poly-D-lysine by rabbit alveolar macrophages were examined as a function of temperature. A plot for total ligand uptake (cell-associated ligand plus degraded ligand) versus time shows a single inflection point at 20 degrees C. Ligand degradation does not occur below 20 degrees C. Internalization of surface-bound 125I-mannose-BSA is negligible below 10 degrees C. The rate constant for internalization increases dramatically above 20 degrees C: 0.02 min-1 at 20 degrees C, 0.05 min-1 at 25 degrees C, 0.13 min-1 at 30 degrees C, and 0.29 min-1 at 35 degrees C. 125I-Mannose-N-acetyl-poly-D-lysine preloaded in lysosomes is exocytosed in a temperature and time-dependent fashion. Even at lower temperatures (2-10 degrees C), secretion of 125I-mannose-N-acetyl-poly-D-lysine was detected, indicating that movement of lysosomal content to plasma membrane and beyond cannot be suppressed at these temperatures. Thus, the temperature dependence of exocytosis of an 125I-labeled ligand is quite different from that of endocytosis, suggesting that the two processes are controlled by different mechanisms. Stimulation of secretion of preloaded 125I-mannose-N-acetyl-poly-D-lysine by mannose-BSA was more pronounced at lower temperatures with a sharp inflection point at 10 degrees C. These findings suggest that endosomes containing newly internalized mannose-BSA interact with the exocytosis pathway and enhance secretion of 125I-mannose-N-acetyl-poly-D-lysine from lysosomes.