Biophysics Department, “Carol Davila” University of Medicine and Pharmaceutics, 8, Eroilor Sanitari Blvd, 050474 Bucharest, Romania
Abstract. A possible way to explain the difference between theory and experiment with regard to the slow kinetics of elementary calcium events visualized by lateral scanning in Xenopus oocytes is to assume that the Ca2+ source is located on the surface of tubules of the endoplasmic reticulum and the image is not captured frontally, but on the opposite side of a tubule. Our results obtained with this release-detection configuration concur with a distribution of functional inositol 1,4,5-trisphosphate receptors disposed as clusters at depths of either 2.5 or 6.5 µm under the plasma membrane. This arrangement can explain why fast transients are detected on radial but not lateral direction and reproduces accurately the time scale, spatial size, fluorescence amplitude and other features of Ca2+ transients. The model suggests that the radial distribution of operative receptors should be estimated by selecting fast calcium events detected by axial scanning. Geometry that breaks spherical symmetry determines large differences in the estimation of release fluxes. Isotropic diffusion-based methods may overestimate with one order of magnitude both the duration and the amount of released Ca2+ but the current amplitude may be correct. Contrary to previous interpretation, fluorescence intensity does not begin to decay until the calcium current turns off.
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