Single granule pH cycling in antigen-induced mast cell secretion

The pH cycling of individual granules in secreting (serotonin-loaded) mast cells is quantitatively examined using multicolor multiphoton fluorescence microscopy. A typical exocytosis event consists of maximal calcium rise at time zero, granule alkalization a few seconds later and, finally, complete contents release at a fraction of a second after alkalization. Membrane fus ion is either transient, as indicated by subsequent granule reacidification , or 'full', as indicated by a granule disappearance with a collapse of its membrane into the plasma membrane. The relative frequency of these two coe xisting behaviors (the 'kiss-to-collapse' ratio) is approximately 2:1. A ty pical transiently fusing granule experiences multiple alkalization/acidific ation cycles after addition of exogenous antigen. Between recycling granule s, coalescence events are frequent, with 80% resulting in a collapse of the formed granule complex to the plasma membrane. The full dynamics of secret ion encompass a complex combination of these granule activities.