Ca2+-induced deprotonation of peptide hormones inside secretory vesicles in preparation for release

The acidic environment inside secretory vesicles ensures that neuropeptides and peptide hormones are packaged in a concentrated condensed form. Althou gh this is optimal for storage, decondensation limits release. Thus, it wou ld be advantageous to alter the physical state of peptides in preparation f or exocytosis. Here, we report that depolarization of the plasma membrane r apidly increases enhanced green fluorescent protein (EGFP)-tagged hormone f luorescence inside secretory vesicles. This effect requires Ca2+ influx and persists when exocytosis is inhibited by N-ethylmaleimide. Peptide deproto nation appears to produce this response, because it is not seen when the ve sicle pH gradient is collapsed or when a pH-insensitive GFP variant is used . These data demonstrate that Ca2+ evokes alkalinization of the inside of s ecretory vesicles before exocytosis. Thus, Ca2+ influx into the cytoplasm a lters the physical state of intravesicular contents in preparation for rele ase.