Role of elementary Ca²⁺ puffs in generating repetitive Ca²⁺ oscillations

Inositol (1,4,5)-trisphosphate (IP₃) liberates intracellular Ca²⁺ both as localized 'puffs' and as repetitive waves that encode information in a frequency-dependent manner. Using video-rate confocal imaging, together with photorelease of IP₃ in Xenopus oocytes, we investigated the roles of puffs in determining the periodicity of global Ca²⁺ waves. Wave frequency is not delimited solely by cyclical recovery of the cell's ability to support wave propagation, but further involves sensitization of Ca²⁺-induced Ca²⁺ release by progressive increases in puff frequency and amplitude at numerous sites during the interwave period, and accumulation of pacemaker Ca²⁺, allowing a puff at a 'focal' site to trigger a subsequent wave. These specific 'focal' sites, distinguished by their higher sensitivity to IP₃ and close apposition to neighboring puff sites, preferentially entrain both the temporal frequency and spatial directionality of Ca²⁺ waves. Although summation of activity from many stochastic puff sites promotes the generation of regularly periodic global Ca²⁺ signals, the properties of individual Ca²⁺ puffs control the kinetics of Ca²⁺ spiking and the (higher) frequency of subcellular spikes in their local microdomain.