pH sensitivity of the FRET response from T306-Rv-Mermaid during Ca²⁺ release. (A) Changes in fluorescence and in the corresponding FRET ratio in response to extracellular changes in pH in the presence of nigericin, in a muscle fiber expressing T306-Rv-Mermaid^D129E/Y235R. (B) Change in FRET ratio from five nigericin-treated muscle fibers expressing T306-Rv-Mermaid^D129E/Y235R, upon changes in pH. Each symbol corresponds to a different fiber. The continuous line shows the result from fitting a Hill equation to the data shown as triangles, inverted triangles, and diamonds.