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The Number and Spatial Distribution of IP₃ Receptors Underlying Calcium Puffs in Xenopus Oocytes

Department of Neurobiology and Behavior, University of California, Irvine, California

Correspondence: Address reprint requests to Dr. Jianwei Shuai, Dept. of Neurobiology and Behavior, University of California, Irvine, CA 92697-4550. Tel.: 949-824-7833; Fax: 949-824-2447; E-mail: shuaij{at}uci.edu.

Calcium puffs are local Ca²⁺ release events that arise from a cluster of inositol 1,4,5-trisphosphate receptor channels (IP₃Rs) and serve as a basic "building block" from which global Ca²⁺ waves are generated. Important questions remain as to the number of IP₃Rs that open during a puff, their spatial distribution within a cluster, and how much Ca²⁺ current flows through each channel. The recent discovery of "trigger" events—small Ca²⁺ signals that immediately precede puffs and are interpreted to arise through opening of single IP₃R channels—now provides a useful yardstick by which to calibrate the Ca²⁺ flux underlying puffs. Here, we describe a deterministic numerical model to simulate puffs and trigger events. Based on confocal linescan imaging in Xenopus oocytes, we simulated Ca²⁺ release in two sequential stages; representing the trigger by the opening of a single IP₃R in the center of a cluster for 12 ms, followed by the concerted opening of some number of IP₃Rs for 19 ms, representing the rising phase of the puff. The diffusion of Ca²⁺ and Ca²⁺-bound indicator dye were modeled in a three-dimensional cytosolic volume in the presence of immobile and mobile Ca²⁺ buffers, and were used to predict the observed fluorescence signal after blurring by the microscope point-spread function. Optimal correspondence with experimental measurements of puff spatial width and puff/trigger amplitude ratio was obtained assuming that puffs arise from the synchronous opening of 25–35 IP₃Rs, each carrying a Ca²⁺ current of 0.4 pA, with the channels distributed through a cluster 300–800 nm in diameter.

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