The calcium clock couples to membrane currents to drive faster beating rate. (A) In the absence of the SR (no uptake or release), there is slow beating. (B) When RyR sensitivity is low (20 µM IC50), systolic release occurs, altering the pattern of I_NCX; however, there is minimal diastolic release, and rate is not affected. (C) When RyRs have normal sensitivity, there is extensive diastolic release. This produces a diastolic inward NCX current, driving a substantial increase in beating rate. The “funny” current I_f was omitted from these simulations for illustrative purposes.