Fig. 4. Characteristics of SCTs observed during control pacing. (A) Average frequency (mean ± s.e.m.; n=28 cells) of SCTs during control pacing of isolated ventricular myocytes. To calculate the SCT frequency, the number of events occurring within defined stimulation periods of 30-seconds was counted. (B) Time of occurrence of the SCTs relative to the previous field-stimulation-evoked Ca²⁺ signal. Since the cells were electrically paced at 0.33 Hz, the maximal time of occurrence was 3 seconds. (C,D) Number of spontaneous Ca²⁺ sparks recorded using confocal imaging of electrically (C) paced or (D) unpaced cells.