SR Ca²⁺ load at various stimulation frequencies. (A) Representative examples of SR Ca²⁺ load assessed during rapid application of 20 mM caffeine and calculated from the integration of the inward NCX current at 1 Hz (top) and 3 Hz (bottom). (B) Mean ± SE data of SR Ca²⁺ content at 1-, 2-, and 3-Hz stimulation frequencies. SR Ca²⁺ content in R4496C (n = 34 and 5) myocytes is significantly below WT (n = 27 and 4) at all stimulation rates investigated. The difference is more pronounced at high rates (e.g., 3 Hz). *, P < 0.01 versus WT at all three frequencies; unpaired Student’s t test.