Modeling the permeation experiments. (A) Geometry of the simulated cell and bathing medium. Spherical shells are placed every 0.5 µm from the cell center out for 150 µm. The cell membrane (red) is the boundary between the 16th and 17th shells. The arrow symbolizes diffusion of molecules between shells. (B) Simulation of the experiment in Fig. 4 D showing ΔF, the difference in melatonin fluorescence, with and without cells in the chamber while melatonin was perfused in the bath for 20 s. The axis was rescaled to match the experiment. (C) Simulation of the experiment in Fig. 5 B showing the transient decrease in Fura-2 fluorescence as melatonin is perfused in the bath. The trace is actually the calculated time course of intracellular melatonin rescaled to match the presentation of the experiment. Two dashed traces labeled 5 and 0.2 show the effect of increasing the assumed membrane permeability by 5 or decreasing it to 0.2 times the standard melatonin value. (D) Geometry of the simulated pipette. See section “Adding the pipette.” The dashed circle represents the assumed shape of the Ω-shaped on-cell membrane patch when it was needed. It has an area of 14.7 µm² chosen to match the increase of extracellular amperometric signal upon breaking the patch to go to whole-cell configuration. (E) Schematic of the diffusion regimen in the model simulation. (F) Decay of melatonin concentration with distance in the model. Negative distances are inside the pipette. The pipette, cell, and extracellular domains are marked Pip, C, and Ex, respectively. (G) Decay with distance of molecules of different assumed membrane permeability in the model. Numbers are multipliers, so that that curve marked 10 assumes a membrane permeability of 10× the standard melatonin value of 1.7 µm/s, etc.