Figure 3

Spontaneous Ca²⁺ transients of HES-2 and iPS(Foreskin) C1-derived contracting cell clusters. A. Transmission light images of two beating cell aggregates derived from ES and iPS cells that were loaded with Fura-2 AM (upper row) and the corresponding emitted fluorescence light presented in pseudo color. (Scale bar = 100 μm). B. Response of spontaneously beating aggregates derived by both cell lines to caffeine (10 mM) treatment indicates the existence of caffeine-releasable Ca²⁺ stores in this stage of development (up to 2 weeks after onset of beating). Two representative experiments are shown: 30 s tyrode perfusion followed by 'puff' perfusion of tyrode directly in front of the cells for the same time period with subsequent caffeine application. Both cell types temporally show a caffeine evoked increase of basal, systolic and diastolic [Ca²⁺]_i. However the Ca²⁺ amplitude is reduced, accompanied by an increase in beating frequency. C. Representative single tracings. The transients show statistically identical characteristics regarding basal Ca²⁺ level, amplitude, maximum upstroke velocity (V_{max, upstroke}) and maximum decay velocity (V_{max, decay}) with p > 0.05 (data not shown). D+E Caffeine response (n = 7 for iPS-CMs; n = 7 for ES-CMs). Beating frequency is increased by 'puff'-perfusion and subsequent caffeine application in both celltypes (D). Basal calcium levels and the maximum value (A_max) are raised significantly after caffeine treatment (E). * = p < 0.05; ** = p < 0.01. Additional analyses of these measurements are provided in Additional file 1, Tables S3 and S4.