Action potentials, L-type Ca²⁺ current, and intracellular Ca²⁺ transients in R4496C myocytes. (A) Representative superimposed action potentials evoked at 1 Hz in WT (n = 28 and 9) and R4496C (n = 30 and 10) myocytes. (B) Average action potential duration at 90% repolarization (APD90) at 1-, 2-, and 3-Hz pacing rates. (C) Representative superimposed L-type Ca²⁺ current (I_CaL) traces from WT and R4496C cardiomyocytes evoked with a 1.5-s depolarization at 5 mV. (Inset) Patch-clamp protocol for I_CaL current–voltage relationship. (D) Average I_CaL current–voltage relationship curves in WT (n = 28 and 9) and R4496C (n = 30 and 10) ventricular myocytes. (E) Superimposed representative Ca²⁺-transient recording under current clamp while stimulating at 1 Hz in WT and R4496C myocytes. (Inset) Representative traces of Ca²⁺ transients from current-clamped R4496C (below) and WT (above) ventricular myocytes at different frequencies of stimulation. (F) Average amplitude of Ca²⁺ transients at 1-, 2-, and 3-Hz pacing rates in WT (n = 17 and 7) and R4496C (n = 18 and 8) myocytes. *, P < 0.01 versus WT; unpaired Student’s t test. Error bars represent mean ± SEM.