Initiation of IP3-mediated Ca2+ waves in Xenopus oocytes

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Abstract
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.