Triggered exocytosis and endocytosis have different requirements for calcium and nucleotides in permeabilized bovine chromaffin cells.

The intracellular requirements for membrane recapture in permeabilized chromaffin cells were compared to the requirements for exocytosis from the same cells. In permeabilized bovine chromaffin cells, calcium-driven exocytosis also triggers, with a short delay, uptake of extracellular horseradish peroxidase (HRP). This internalized HRP remains compartmentalized within the cell and migrates to a low density band on a Percoll gradient which is distinct from the heavier chromaffin granules. The amount of horseradish peroxidase internalized is similar in intact and leaky cells and is approximately equivalent to the volumes secreted. Endocytosis in both preparations is blocked by botulinum toxin, operates in a collapsed membrane potential, and is inhibited by low temperature. In permeabilized cells, exocytosis and coupled endocytosis are activated by the same concentrations of Ca2+ and MgATP. Although secretion requires Ca2+ and MgATP, once exocytosis has occurred the subsequent endocytosis can proceed in the virtual absence of Ca2+ or MgATP, and is largely unaffected by a variety of nucleotide triphosphates (including nonhydrolyzable analogues), and cyclic nucleotides. These data suggest that endocytosis can proceed, once exocytosis has been triggered, under conditions that are quite different from those necessary to support exocytosis, and that the specific requirements for Ca2+ and MgATP in secretion are for the exocytotic limb of the secretory cycle rather than for the associated endocytotic pathway.