Figure 2.

Stochastic simulations of Ca²⁺ puffs and spikes. (A and B) show the evolution of Ca²⁺ concentration in a 1 fL volume (1.5 × 1.5 μm² in the 2D simulations, see Voorsluijs et al.⁹) centered around the cluster at two [IP₃]. For the 2 panels, one cluster, located at the centre of a 5 × 5 μm² 2D system is considered. For panels (C and D), a larger 5 × 10 μm² system containing 10 clusters is simulated. All values of parameters are the same as in A and B and listed in Table S2. Time series show the evolution of Ca²⁺ concentration averaged on the whole system. Panels (E and F) show the spatio-temporal evolutions of Ca²⁺ puffs and spikes, respectively. In the two cases, the system is 50 × 10 μm² large and 10% of the total surface is occupied by clusters (200 clusters). For panel E, [IP₃] is constant and equal to 0.075 μM everywhere in the cell. For panel F, IP₃ was released in one spot (with dimensions 0.5 × 0.5 μm²) located in the bottom center of the simulated cell. For panels E and F, times corresponding to each subpanels are: 0.1, 0.6, 2.5, 3.4 and 5.2 s after the increase in [IP₃] from 10 nM. In panels A-E, [IP₃] is increased at time 0, and Eqs. (3) and (5) are not considered. In panel F, the release of IP₃ is simulated using Eq. (3), with $\theta$ = 1000µMs⁻¹, D_I = 100 μm²s⁻¹ and t_ph = 500 ms.