NEURONAL PEPTIDE RELEASE IS LIMITED BY SECRETORY GRANULE MOBILITY

Neuropeptides are slowly released from a limited pool of secretory gra nules. To visualize this process, GFP-tagged preproatrial natriuretic factor (ANF) was expressed in nerve growth factor-treated PC12 cells. Biochemical and microfluorimetric experiments demonstrate that proANF- EGFP is packaged in granules that accumulate at neurite endings and is released in a Ca2+-dependent manner by secretagogs. Confocal microsco py shows that secretion is associated with depletion of granules distr ibuted throughout the terminal. Fluorescence recovery after photobleac hing and time-lapse particle tracking reveal that only a subpopulation of cytoplasmic secretory granules, similar in size to the releasable pool, can move quickly enough (D = 6 x 10(-11) cm(2)/s) to support rel ease. Therefore, sustained secretory responses are limited by the numb er of mobile granules and their slow rate of diffusion.