HPTS fluorescence signal from the t-system of a rat skinned fiber. HPTS was trapped in the skinned fiber as described in Materials and methods. The long axis of the fiber was positioned to run in parallel with that of the image. Note that continuous imaging of the t-system in this manner allowed sampling of the proton-dependent changes in HPTS fluorescence from more than one hundred planes of transverse tubules. Inset: Magnified region of the preparation showing the structure of the t-system.