Figure 3. Ca²⁺ waves during sustained rest in the presence of isoproterenol.

A, simultaneous [Ca²⁺]_i (rhod-2, upper traces in black) and [Ca²⁺]_SR (fluo-5N, lower traces in grey) measurements following rest from electrical stimulation under control conditions (7 mm[Ca²⁺]_o, a= 1.1 Hz, b= 1.3 Hz) and in the presence of ISO (c: 2 mm[Ca²⁺]_o, 1.3 Hz). Electrical stimulation frequency was incrementally increased to elevate [Ca²⁺]_SR and determine the threshold level for Ca²⁺ waves. In the example shown, Ca²⁺ waves were only observed when [Ca²⁺]_SR was higher than the control threshold level, unless RyRs were pharmacologically sensitized using 250 μm caffeine. Dashed lines: 1, no-wave threshold control; 2, wave threshold control; 3, wave threshold ISO. B, positive correlation between electrical stimulation frequency (range 0.5–1.3 Hz) and resting [Ca²⁺]_SR measured using fluo-5N fluorescence. Frequency and [Ca²⁺]_SR values are normalized to the respective values found at the lowest pacing frequency in each experimental trial.