Ca 2+ -triggered exocytosis was studied in single rat melanotrophs and bovine chromaffin cells by capacitance measurements. Sustained exocytosis required MgATP, but even in the absence of MgATP, Ca 2+ could trigger exocytosis of 2700 granules in a typical melanotroph and of 840 granules in a chromaffin cell. Granules undergoing ATP-independent exocytosis were similar in number to those appearing docked to the plasmalemma in quickly frozen unfixed sections (3300 in a melanotroph and 830 in a chromaffin cell). Most exocytosis required tens of second, but a small pool of granules was released in tens of milliseconds. Evidently, only a small subset of docked granules is rapidly releasable. We suggest that, temporally, the last ATP-dependent step in exocytosis is closely associated with docking and that docked granules reach fusion competence only after subsequent steps. © 1995 Cell Press.
Parsons, T. D., Coorssen, J. R., Horstmann, H., & Almers, W. (1995). Docked granules, the exocytic burst, and the need for ATP hydrolysis in endocrine cells. Neuron, 15(5), 1085–1096. https://doi.org/10.1016/0896-6273(95)90097-7