The equilibria associated with the Mn ²⁺ quench. (A) Recordings of Fura-2 fluorescence changes induced by a depolarizing pulse to 0 mV for 1 s. The grey trace shows [Ca²⁺]_I with a decrease of fluorescence corresponding to an increase of [Ca²⁺]_i. The dark trace is the Mn²⁺ quench signal taken from Fig. 5 A of Jacque-Fernandez et al. (2025). (B) The network of important equilibria involving Mn²⁺, Ca²⁺, Mg²⁺, Fura-2, and parvalbumin (Pv). Note that Fura has a relatively low affinity for Mg²⁺; therefore, this reaction can be ignored. The species Mn-Fura (in red) represents the form of Fura that does not fluoresce and is therefore the basis of the Mn²⁺ quench transient shown in panel A.