Figure 5. Possible pathways of FACE.

(A) Ca²⁺ entry into the cytoplasm through channels located on the plasma membrane. (B) Ca²⁺ entry into the cytoplasm through channels located on the LB membrane. Here Ca²⁺ entry occurs in series with the fusion pore, with the consequence that the amount of Ca²⁺ entering the cytosol could be a function of fusion pore permeability, LB membrane permeability and amount of Ca²⁺ ions stored inside the LB. In both cases, A and B, channel activation may occur mechanically (membrane stress) or chemically (membrane merger; see also Discussion). The relative contribution of each model to FACE is yet unknown. In addition, intracellular amplification of the FACE-induced Ca²⁺ signal by Ca²⁺-induced Ca²⁺ release may take place.