Calcium dependence of action potential-induced endocytosis in chromaffin cells

Exocytosis occurs via fusion of transmitter-containing granules with the cell membrane, whereupon the granule contents are released and the cell membrane surface area increases. Exocytosis is followed by endocytosis to maintain proper cell membrane surface area and composition. We have shown that adrenal chromaffin cells internalize membrane in a biphasic manner following action potential stimulation. At basal firing rates (single - 0.5 Hz trains) endocytosis occurs by a rapid retrieval of membrane (termed Phase I) that is independent of the activity of the protein phosphatase calcineurin and wanes in efficiency with cell activity. At intermediate firing frequencies (> 6 Hz) a second, calcineurin-sensitive, form of activity-enhanced endocytosis emerges (Phase II). In this study, we employ electrophysiological, electrochemical, and computational techniques to estimate intracellular Ca2+ at the site of endocytosis by measuring secretion rates. The measured rates of secretion yield estimates of [Ca2+](i) based on a kinetic scheme for exocytosis calibrated under highly controlled [Ca2+](i). Based on this analysis, we propose that Phase I endocytosis is inhibited by cytosolic Ca2+ with a K-inh = 605 nM, while Phase II endocytosis is activated by Ca2+ with a K-act = 1.46 muM. Molecular processes that may be consistent with the measured behaviors are discussed.