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Figure S2. Model calculations of local Ca²⁺ depletion. Simulations from a planar compartmental diffusion model of the spatial profile of free Ca²⁺ as described in the second part of Appendix. The full computer program is given in this supplementary material. The "cell" starts with 1 µM free Ca²⁺ everywhere, and a transport process removes Ca²⁺ steadily from the left side. (A) Spatial profiles after 0.3, 1, 3, and 10 ms of pumping. Other parameters: D_Ca = 0.2 × 10⁻⁵ cm²/s and buffer reaction time constant 1 ms. By 10 ms, the Ca²⁺ buffer resupplies free Ca²⁺ so the drop of free Ca²⁺ slows considerably. (B) Spatial profiles at 3 ms as the diffusion coefficient is changed. Parameters: D_Ca = 0.1, 0.2, and 0.3 × 10⁻⁵ cm²/s; buffer reaction time constant 1 ms. The fastest diffusion allows the Ca²⁺ depletion to reach the center of the cell. (C) Spatial profiles at 3 ms as the buffer reaction time constant is changed. Parameters: buffer reaction time constant, 0.1, 1, and 10 ms; D_Ca = 0.2 × 10⁻⁵ cm²/s. The slowest buffer releases little Ca²⁺ in 3 ms, so the diffusion depletion of free Ca²⁺ extends to the center of the cell. The green curve in each panel is the same reference spatial profile (at 3 ms, D_Ca = 0.2 × 10⁻⁵ cm²/s, and buffer reaction time constant 1 ms).