Mechanical restitution in atrial trabeculae. Restitution curves show the fractional recovery (percentage of 1 Hz steady-state peak value) of force in response to the premature stimulus plotted against the premature interval. Each curve was fitted by a single exponential to obtain restitution rates (1/τ): average rates are indicated below. (A) Restitution protocol: representative traces from a WT atrial trabecula (above) and a R4496C atrial muscle (below). The subsequent trials with increasing premature interval of the extra beat are shown superimposed in different colors (black to light gray). (B) Average restitution curves from WT (gray) and R4496C (black) atrial trabeculae. Rates: 5.0 ± 0.9 s⁻¹ in WT and 9.0 ± 0.9 s⁻¹ in R4496C; P < 0.05. (Inset) Restitution curves from WT and R4496C atrial trabeculae upon superfusion with 5 × 10⁻⁷ mol/L isoproterenol. Rates: 14.6 ± 1.1 s⁻¹ in WT and 25.5 ± 4.7 s⁻¹ in R4496C; P < 0.05. (C) Post-extrasystolic potentiation. The graph displays the relative tension of the first beat after the extrasystole plotted against the premature interval. Potentiation is significantly less pronounced in R4496C atrial trabeculae (P < 0.05 for intervals between 50 and 250 ms). Error bars represent mean ± SEM.