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Background  Ca 2+  is essential for vesicle fusion with the plasma membrane in virtually all types of regulated exocytoses. However, in contrast to the well-known effects of a high cytoplasmic Ca 2+  concentration ([Ca 2+ ] c ) in the prefusion phase, the occurrence and significance of Ca 2+  signals in the postfusion phase have not been described before.    Methodology/Principal Findings  We studied isolated rat alveolar type II cells using previously developed imaging techniques. These cells release pulmonary surfactant, a complex of lipids and proteins, from secretory vesicles (lamellar bodies) in an exceptionally slow, Ca 2+ - and actin-dependent process. Measurements of fusion pore formation by darkfield scattered light intensity decrease or FM 1-43 fluorescence intensity increase were combined with analysis of [Ca 2+ ] c  by ratiometric Fura-2 or Fluo-4 fluorescence measurements. We found that the majority of single lamellar body fusion events were followed by a transient (t 1/2  of decay = 3.2 s) rise of localized [Ca 2+ ] c  originating at the site of lamellar body fusion. [Ca 2+ ] c  increase followed with a delay of ∼0.2–0.5 s (method-dependent) and in the majority of cases this signal propagated throughout the cell (at ∼10 µm/s). Removal of Ca 2+  from, or addition of Ni 2+  to the extracellular solution, strongly inhibited these [Ca 2+ ] c  transients, whereas Ca 2+  store depletion with thapsigargin had no effect. Actin-GFP fluorescence around fused LBs increased several seconds after the rise of [Ca 2+ ] c . Both effects were reduced by the non-specific Ca 2+  channel blocker SKF96365.    Conclusions/Significance    F usion- a ctivated  C a 2+   e ntry (FACE) is a new mechanism that leads to [Ca 2+ ] c  transients at the site of vesicle fusion. Substantial evidence from this and previous studies indicates that fusion-activated Ca 2+  entry enhances localized surfactant release from type II cells, but it may also play a role for compensatory endocytosis and other cellular functions.

Fusion-Activated Ca2+ Entry: An “Active Zone” of Elevated Ca2+ during the Postfusion Stage of Lamellar Body Exocytosis in Rat Type II Pneumocytes