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Calcium release from intracellular stores plays a key role in the regulation of a variety of cellular activities.In various cell types this release occurs through inositol-triphosphate IP3 receptors which are Ca2+ channelswhose open probability is modulated by the cytosolic Ca2+ concentration itself. Thus, the combination of Ca2+release and Ca2+ diffusion evokes a variety of Ca2+ signals depending on the number and relative location ofthe channels that participate of them. In fact, a hierarchy of Ca2+ signals has been observed in Xenopus laevisoocytes, ranging from very localized events puffs and blips to waves that propagate throughout the cell. Inthis cell type channels are organized in clusters. The behavior of individual channels within a cluster cannot beresolved with current optical techniques. Therefore, a combination of experiments and mathematical modelingis unavoidable to understand these signals. However, the numerical simulation of a detailed mathematicalmodel of the problem is very hard given the large range of spatial and temporal scales that must be covered.In this paper we present an alternative model in which the cluster region is modeled using a relatively fine gridbut where several approximations are made to compute the cytosolic Ca2+ concentration Ca2+ distribution.The inner-cluster Ca2+ distribution is used to determine the openings and closings of the channels of thecluster. The spatiotemporal Ca2+ distribution outside the cluster is determined using a coarser grid in whicheach active cluster is represented by a point source whose current is proportional to the number of openchannels determined before. A full reaction-diffusion system is solved on this coarser grid.

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