Initiation of IP₃-mediated Ca²⁺ waves in Xenopus oocytes

Inositol (1,4,5)-trisphosphate (IP₃) evokes Ca²⁺ liberation in Xenopus oocytes as elementary events (Ca²⁺ puffs) that become coupled to propagate Ca²⁺ waves with increasing [IP₃]. To investigate this transition between local and global Ca²⁺ signaling, we developed an optical method for evoking rapid subcellular Ca²⁺ elevations, while independently photoreleasing IP₃ and simultaneously recording confocal Ca²⁺ images. Focal Ca²⁺ elevations triggered waves within 100 ms of photoreleasing IP₃, compared with latencies of seconds following photorelease of IP₃ alone. Wave velocity varied with [IP₃] but was independent of time after photorelease of IP₃, indicating that delayed wave initiation did not involve slow binding of IP₃ to its receptors. The amount of Ca²⁺ required to trigger a wave was ~ 10-fold greater than the average size of puffs, and puffs showed no progressive increase in magnitude before waves initiated. Instead, Ca²⁺ puffs contributed to a slow rise in basal free [Ca²⁺], which further increased puff frequency and sensitized IP₃ receptors so that individual events then triggered waves. Because the wave threshold is much greater than the size of the elementary puff, cells can employ both local and global signaling mechanisms, and the summation of stochastic behavior of elementary events allows generation of reproducible periodic waves.