Voltage-induced changes in Fluo-5N fluorescence in response to a single or to successive depolarizing pulses in the presence or absence of CPA. (A) On this representative recording, a muscle fiber was submitted to a 50-s depolarizing pulse to 0 mV in the presence of 50 µM CPA. (B) Three consecutive pulses to +30 mV of 1, 5, and 20 s in duration and separated by 2-min intervals were applied on the same fiber. Fluo-5N fluorescence images were collected at a frequency of 0.2 Hz in A and 15 Hz in B in two control fibers (OF1 mice).