Activation of β-adrenergic signaling decreases the duration of [Ca ²⁺ ] _i transients in S3A cardiomyocytes treated with Gap19. (A) Line scan images from representative WT (top) and S3A (bottom) cardiomyocytes under field stimulation (1 Hz) under control conditions (left) and after isoproterenol stimulation (right). (B) Line scan images from representative cardiomyocytes previously loaded with the Gap19 peptide (200 µM) before (left) and after (right) Iso stimulation (green). The traces above each line scan represent the spatially averaged time curse of [Ca²⁺]_i for each image. The number in parentheses indicates biological replicates. (C and D) We analyzed three to five cells per mouse. C and D show scatter plots of the amplitude and time to 90% decay, respectively. P = 0.0203 WT vs. S3A; P = 0.0164 S3A vs. S3A + Iso + Gap19. The number in parentheses indicates the n value. Comparisons between groups were made using nested-ANOVA. The number in parentheses indicates biological replicate. We analyzed three to five cells per mice.