SCW without or before stimulation and during stimulation. (A and B) The percentage of fibers per mouse displaying SCW without or before stimulation (A) and during stimulation (between transients) (B). (C) Representative spontaneous Ca²⁺ transient after the stimulation. (D) Representative slow Ca²⁺ wave after stimulation. Representative long duration Ca²⁺ sparks after 1 Hz pacing. (E) Representative Ca²⁺ sparks were detected in the 20-s period after 1 Hz pacing of cardiomyocytes. (F) Representative long duration Ca²⁺ sparks after 1 Hz pacing. Scale bars are 1 s.