Effects of FKBP12 deficiency on Ca ²⁺ handling. (A) Representative Ca²⁺ transients in FL (top, black) and MHC-FKD (bottom, blue) cardiomyocytes. 1 Hz stimulation (E.S.) and spontaneous Ca²⁺ waves (SCW) are indicated with arrows. (B) Average amplitude with 1 Hz stimulation. (C) Time to the peak of the Ca²⁺ transient. (D) Rate of decay of the Ca²⁺ transient. (E) Percent of fibers per mouse with spontaneous Ca²⁺ transients or waves (SCW) occurring after electrical stimulation. (F) Velocity of the fast spontaneous Ca²⁺ transients. (G) Velocity of slow Ca²⁺ waves. (H) Caffeine-induced transients. FL: n = 4 mice, 13 cardiomyocytes, MHC-FKD: 5 mice, 11 cardiomyocytes, MHC-FKD: n = 4 mice, 14 cardiomyocytes. (I) Cytosolic Ca²⁺ levels were assessed with Fura 2. FL: n = 4 mice, 20 cardiomyocytes, MCK-FKD: n = 4 mice, 15 myocytes, MHC-FKD, n = 3 mice, 21 cardiomyocytes. (I and J) Ca²⁺ spark frequency. (K) Ca²⁺ spark amplitude. (L) Time to peak. (M) Decay rate (Tau). (N) Full-width half maximum (FWHM). (O) Full-duration half maximum (FDHM). (P) Calculated spark mass. Ca²⁺ transient and spark data: FL, n = 6 mice, 42 cardiomyocytes. MCK-FKD, n = 5 mice, 42 cardiomyocytes. MHC-Cre⁺, n = 5, 36 cardiomyocytes. MHC-FKD, n = 6 mice, 47 cardiomyocytes. Data were analyzed by a nested one-way ANOVA. Data were obtained from cardiomyocytes from male mice.