Intra-SR Ca sensitization signal during atrial ECC. (A) [Ca]_i (rhod-2) and [Ca]_SR (fluo-5N) fluorescence signals (F/F₀; averaged over 0.6-µm-wide regions of interest) elicited by AP depolarization and recorded from individual peripheral (a) and central (b) release sites in a ventricular myocyte. The dashed vertical line marks the simultaneous onset of the cytosolic CaT and SR depletion in both subcellular regions. (B) [Ca]_i and [Ca]_SR signals recorded from an atrial cell. In the cell periphery (a), the rise of [Ca]_i and [Ca]_SR decline is simultaneous (vertical dashed line 1). In the cell center (b), the rise of [Ca]_i is delayed, and [Ca]_SR depletion lags behind the rise of [Ca]_i. The decline of [Ca]_SR is preceded by the Ca sensitization signal, a transient elevation of [Ca]_SR (arrow). Vertical dashed lines 2 and 3 mark the duration of the Ca sensitization signal, and the time interval 1–3 is defined as latency. Reproduced from Maxwell and Blatter (2017).