Figure 4.

BTP2 does not compromise SR Ca²⁺ loading. (A and B) Normalized spatially averaged SR fluo-5N fluorescence during exposure to a thorough depletion of SR calcium with 30 mM caffeine (shown in green). The SR was then loaded in increasing concentrations of free cytoplasmic Ca²⁺ (28, 67, and 200 nM). This procedure was repeated on the same fiber, depleting the SR of Ca²⁺ with 30 mM caffeine and re-exposing the fiber to the increasing [Ca²⁺], this time adding vehicle (A) or 10 µM BTP2 (B). The fluo-5N steady states of the two rounds were compared. F₀ was established as the fluo-5N fluorescence intensity at 67 nM [Ca²⁺]_cyto. The horizontal broken lines on A and B provide a guide to the effect of vehicle or BTP2 on the steady-state SR fluo-5N fluorescence signal in the same [Ca²⁺]_cyto. (C) Summary of ΔFluo-5N values ± SEM obtained from seven to nine experiments. Multiple t tests by using the Holm-Sidak method revealed significant differences between vehicle and 10 µM BTP2 across different [Ca²⁺]_cyto. *, P < 0.05.