DOCKED GRANULES, THE EXOCYTIC BURST, AND THE NEED FOR ATP HYDROLYSIS IN ENDOCRINE-CELLS

Ca2+-triggered exocytosis was studied in single rat melanotrophs and b ovine chromaffin cells by capacitance measurements. Sustained exocytos is required MgATP, but even in the absence of MgATP, Ca2+ could trigge r exocytosis of 2700 granules in a typical melanotroph and of 840 gran ules in a chromaffin cell. Granules undergoing ATP-independent exocyto sis were similar in number to those appearing docked to the plasmalemm a in quickly frozen unfixed sections (3300 in a melanotroph and 830 in a chromaffin cell). Most exocytosis required tens of seconds, but a s mall pool of granules was released in tens of milliseconds. Evidently, only a small subset of docked granules is rapidly releasable. We sugg est that, temporally, the last ATP-dependent step in exocytosis is clo sely associated with docking and that docked granules reach fusion com petence only after subsequent steps.