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Fig. 4. Spatially heterogeneous Ca²⁺ signalling during atrial myocyte EC-coupling. Panel A shows the development of a Ca²⁺ signal following electrical depolarisation in a single atrial myocyte. The left-hand montage shows pseudocolour-coded images of the entire cell. To indicate the initiation of discrete Ca²⁺ sparks (designated by the arrowheads) and their lateral spreading, the right-hand column of images depicts part of the cell (the region bounded by the dashed box) at a higher magnification. The time at which the electrical pulse was applied is shown by the open arrow. Three obvious Ca²⁺ spark sites are marked by black arrowheads. The surface plots in panel B depict the spatial and temporal development of a Ca²⁺ signal from a different atrial cell. The Ca²⁺ concentration is indicated by both the colour and the height of the peaks (blue/green indicates low [Ca²⁺] and yellow/red depicts high [Ca²⁺]). A clear peripheral ring of Ca²⁺ is evident with a steep `valley' of low Ca²⁺ concentration in the centre of the cell. Panel C illustrates the consistency of the spatial gradient of Ca²⁺ signalling in atrial myocytes during electrical pacing. The traces show the Ca²⁺ concentration at the two regions depicted in panel A. The peripheral region (red circle and red trace) displays a large Ca²⁺ rise with each depolarisation (denoted by the black arrowheads) whereas the central region has a lower response (black circle and black trace). This figure was reproduced with permission from Blackwell Publishing and The Physiological Society (Mackenzie et al., 2001).

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