Fig. 4. Increasing SR Ca²⁺ load promotes the recruitment of central RyRs during atrial EC-coupling. (A) Cell image showing the locations of the subsarcolemmal site (black circle) and the central non-regenerative region (grey circle) that were analysed. (Ba) The black traces illustrates the response of the subsarcolemmal site before (Bai) and after (Baii) increasing SR Ca²⁺ load. (Bb) The grey traces in i and ii illustrate the corresponding responses from the central region. The SR Ca²⁺ load was increased by incubating cells for 1 minute in 10 mM extracellular Ca²⁺ at the time indicated between the dashed lines. The black arrowheads indicate when the cell was depolarised. Please note, the cell was not paced when the extracellular Ca²⁺ was increased. Apart from the 1 minute period in 10 mM extracellular Ca²⁺, the whole experiment was conducted using 1 mM Ca²⁺. (C) The increase in twitch amplitude during the transients, marked by stars in Bai and Baii. The data shown in this figure are from a single cell representative of seven others (from three rats).