FRET response of T306-Rv-Mermaid during Ca²⁺ release. (A) Patchy expression and triadic confocal pattern (bottom image and associated trace showing the longitudinal fluorescence profile along the white box) of T306-Rv-Mermaid in muscle fibers. (B) Changes in T306-Rv-Mermaid FRET ratio in response to the indicated voltage-clamp depolarizing pulses. (C) Mean voltage dependence of the peak amplitude of the drop in FRET ratio measured in response to single 0.5-s-long pulses from −80 mV. For this, successive single depolarizing pulses of increasing amplitude were applied, separated by a time interval of 30 s. Data points are mean values from six fibers. Error bars represent ± SEM.