I describe recent work on the synchronization of IP3R calcium channels in the interior of cells. Hybrid models of calcium release couple deterministic equations for diffusion and reactions of calcium ions to stochastic gating transitions of channels. I discuss the validity of such models as well as numerical methods.Hybrid models were used to simulate cooperative release events for clusters of channels. I show that for these so-called puffs the mixing assumption for reactants does not hold. Consequently, useful definitions of averaged calcium concentrations in the cluster are not obvious. Effective reaction kinetics can be derived, however, by separating concentrations for self-coupling of channels and coupling to different channels.
Based on the spatial approach, a Markovian model can be inferred, representing well calcium puffs in neuronal cells. I then describe further reduction of the stochastic model and the synchronization arising for small channel numbers. Finally, the effects of calcium binding proteins on duration of release is discussed.