Fig. 1. Menadione induces cytosolic Ca²⁺ spikes that are similar to those elicited by ACh. Cells were loaded with Fura Red AM. (A) Addition of 20 µM menadione to cells in the presence (upper trace) and absence (lower trace) of 1 mM Ca²⁺ induces repetitive Ca²⁺ spikes. (B) Addition of 100 nM ACh to cells in medium with 1 mM Ca²⁺ induces repetitive Ca²⁺ spikes. (C) Ca²⁺ responses to menadione following application of 100 µM ACh in the presence (upper trace) and absence (lower trace) of extracellular Ca²⁺ are shown.