The number and spatial distribution of IP3 receptors underlying calcium puffs in Xenopus oocytes.
Calcium puffs are local Ca(2+) release events that arise from a cluster of inositol 1,4,5-trisphosphate receptor channels (IP(3)Rs) and serve as a basic "building block" from which global Ca(2+) waves are generated. Important questions remain as to the number of IP(3)Rs that open during a puff, their spatial distribution within a cluster, and how much Ca(2+) current flows through each channel. The recent discovery of "trigger" events-small Ca(2+) signals that immediately precede puffs and are interpreted to arise through opening of single IP(3)R channels-now provides a useful yardstick by which to calibrate the Ca(2+) 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 Ca(2+) release in two sequential stages; representing the trigger by the opening of a single IP(3)R in the center of a cluster for 12 ms, followed by the concerted opening of some number of IP(3)Rs for 19 ms, representing the rising phase of the puff. The diffusion of Ca(2+) and Ca(2+)-bound indicator dye were modeled in a three-dimensional cytosolic volume in the presence of immobile and mobile Ca(2+) 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 IP(3)Rs, each carrying a Ca(2+) current of approximately 0.4 pA, with the channels distributed through a cluster 300-800 nm in diameter.
Shuai
Jianwei
J
Department of Neurobiology and Behavior, University of California, Irvine, CA 92697-4550, USA. shuaij@uci.edu
Rose
Heather J
HJ
Parker
Ian
I
D000818:Animals; D001703:Biophysics / Q000379:methods; D002118:Calcium / Q000378:metabolism*; D015544:Inositol 1,4,5-Trisphosphate / Q000737:chemistry; D053496:Inositol 1,4,5-Trisphosphate Receptors / Q000378:metabolism; D007700:Kinetics; D018613:Microscopy, Confocal; D008856:Microscopy, Fluorescence; D015233:Models, Statistical; D036103:Nanotechnology; D009865:Oocytes / Q000378:metabolism*; D015398:Signal Transduction; D013816:Thermodynamics; D013997:Time Factors; D014982:Xenopus laevis / Q000378:metabolism*
D053496:Inositol 1,4,5-Trisphosphate Receptors; D015544:Inositol 1,4,5-Trisphosphate; D002118:Calcium