Ca²⁺ spark-to-spark delay shortening is blunted during β-adrenergic stimulation in S2030A^+/+ cells. (A) Representative line-scan image in control conditions (i). A very low concentration of ryanodine (50 nmol/liter) entrains repetitive sparks after binding to only a single monomeric RyR channel subunit in an entire couplon (ii). The spark-to-spark delay and the relative spark amplitudes were measured to investigate Ca²⁺ spark restitution (iii). (B) Histograms of spark-to-spark delays for control WT (328.8 ms, 95% CI, 313.8–348.3 ms; N = 10, n = 36, 937 spark pairs; i), Iso WT (261.8 ms, 95% CI, 254.5–269.4 ms; N = 11, n = 44, 1,783 spark pairs; ii), control S2030A^+/+ (308.3 ms, CI 95%: 291.6–329.2 ms; N = 6, n = 20, 488 spark pairs; iii), and Iso S2030A^+/+ cells (299.6 ms, CI 95%: 286.0–310.9 ms; N = 7, n = 27, 929 spark pairs; iv). Red lines identify the median values.