IP mediated global Ca signals arise through two temporally and spatially distinct modes of Ca release.

The 'building-block' model of inositol trisphosphate (IP)-mediated Ca liberation posits that cell-wide cytosolic Ca signals arise through coordinated activation of localized Ca puffs generated by stationary clusters of IP receptors (IPRs). Here, we revise this hypothesis, applying fluctuation analysis to resolve Ca signals otherwise obscured during large Ca elevations. We find the rising phase of global Ca signals is punctuated by a flurry of puffs, which terminate before the peak by a mechanism involving partial ER Ca depletion. The continuing rise in Ca, and persistence of global signals even when puffs are absent, reveal a second mode of spatiotemporally diffuse Ca signaling. Puffs make only small, transient contributions to global Ca signals, which are sustained by diffuse release of Ca through a functionally distinct process. These two modes of IP-mediated Ca liberation have important implications for downstream signaling, imparting spatial and kinetic specificity to Ca-dependent effector functions and Ca transport.