Fig. 4. The intracellular basal Ca²⁺ concentration is affected by extracellular Ca²⁺.

Wide-field confocal images of native HEK cells loaded with Fluo4-AM indicator, displaying resting [Ca²⁺]_i in different levels of extracellular Ca²⁺, as indicated above (color coding indicates higher Ca²⁺ levels towards the warm colors). Note how the intracellular Ca²⁺ concentration decreases significantly, upon lowering of the extracellular Ca²⁺concentration, in a reversible manner (middle panel). This is most likely due to the activity of the Na- Ca²⁺ exchanger. Scale bar 20 μm.