Fig. 5. Decrease of sarcoplasmic reticulum (SR) Ca²⁺levels during Ca²⁺depletion due to Ca²⁺leak via  cardiac ryanodine receptor. (A ) Time course of changes in mag-fluo-4 fluorescence during Ca²⁺-free superfusion in Control and in the presence of tetracaine (1 mM) or thapsigargin (1 μM). Mag-fluo-4 fluorescence intensity was normalized with reference to its initial value measured just before Ca²⁺depletion (F/F⁰). (B ) Summarized data for mag-fluo-4 F/F⁰measured at the end of 15 min of Ca²⁺-depletion, in Control (0.80, [0.77–0.83], n = 21, N = 5) and in the presence of tetracaine (0.95, [0.89– 1.01]; n = 8, N = 3) or thapsigargin (0.62, [0.53–0.70]; n = 8, N = 3). *P < 0.05 (P = 0.038), ***P < 0.005 (P = 0.0047) compared with Control. Note that the time-matched control experiments showed that the mag-fluo-4 fluorescence intensity (F/F⁰) was only slightly decreased (0.96, [0.91–1.01]; n = 3, N = 2) during repetitive excitation at 10-s intervals for 15 min in myocytes superfused with normal Tyrode solution supplemented with 1 mM tetracaine, a condition where SR Ca²⁺content was expected to be maintained (data not shown).