Figure 6.

Simulation of paired trigger-puff events considering 3-channel triggers. A: Geometry of the cytosolic space: a cuboid of dimensions 2.7 × 2.7 × 1.5 μm. The plane z=0 represents the surface of the ER and the circle at the center a cluster of IP₃R’s. B: Three examples of IP₃R distributions within clusters (from top to bottom, N_p=5, 30 and 50). The width of the cluster is constant (0.5 □m). In the figure, the triggering channels are surrounded by boxes. C. Results of the simulation of four paired trigger-puff events. The upper plot shows the number of open channels as a function of time. Three trigger channels open at t=0. At t=0.012s a puff begins with a total of N_p channels simultaneously open. We show results with N_p=10 (solid lines) and N_p=15 (dotted lines). The lower panel shows the corresponding time evolution of the fluorescence signal, F_R. The upper traces correspond to simulations in which the single channel current was I_ch=I_o=0.086pA, independent of N_p, while the other two correspond to N_pI_ch=I, with I given by Eq. 13. We also show the puff and trigger amplitudes obtained in these simulations in the lower panel. D. Comparison of the distributions of puff-to-trigger amplitude ratios obtained from the simulations and from the experiments. The bar plot shows the experimental distribution reported by Rose et al in [2] (the data points were directly captured from their Fig.4F). The circles correspond to simulations in which the distribution of open channels was assumed to be given by Eq. 11 with m=4 (white circles) or was assumed to be given by the superposition of two Poisson distributions that eventually leads to Eq. 16 (black circles).