ECC gain is decreased in S2030A^+/+ myocytes after β-adrenergic stimulation and matched SR Ca²⁺ loading. (A) Depolarization of voltage-clamped cells from −40 to −25 mV (left) or to 0 mV (right) in control conditions and during β-adrenergic stimulation with 100 nmol/liter Iso (N = 5, n = 11 for both WT and S2030A^+/+ animals). The figure shows representative line profiles of the Ca²⁺ transients (ΔF/F₀; black traces for control and red for Iso), confocal line-scan images, and I_Ca traces for both WT and S2030A^+/+ cells. (B and C) ECC gain was obtained from the ratio of the maximal Ca²⁺ transient amplitude and the peak I_Ca density (ΔF/F₀)/(pA/pF), normalized to the SR Ca²⁺ content. (D) The ECC gain was obtained in conditions of matched SR Ca²⁺ load and I_Ca between all cell groups. The gain was minimally influenced by Iso in WT cells. However, the mutation affected the ECC gain ratio in Iso, mostly because of a decreased Ca²⁺ release during β-adrenergic stimulation (see Table S2). Please note that in this experiment, the SR Ca²⁺ content was experimentally matched between control and Iso. *, P < 0.05, Iso versus control; †, P < 0.05, S2030A versus WT. The whiskers cover the range from 10% to 90%.