Figure 1.

Intracellular calcium homeostasis in cardiomyocytes from 1-month-old mice. (A) Intracellular Ca²⁺ responses to mild hypo-osmotic shock in WT and mdx cells. Left panels are XY images of cardiac myocytes after returning to isotonic solution and line-scan representations of series of images acquired from the cells on the left upon application of an osmotic challenge. Averaged fluorescence was determined within each cell and converted to a two-dimensional X,t image (as in Martins et al.³²). Bars under the line-scans depict the protocol of extracellular solution changes. Right panel represents pooled data of mean values of normalized fluorescence during 60 s after the osmotic shock. The averaged response to the osmotic shock was extremely small in WT cells compared with mdx. (B) Left panels show representative traces of Ca²⁺ currents, line-scan images of Ca²⁺-related fluorescence and normalized cytosolic transients elicited by a 400 ms test pulse to −25 mV in WT and mdx cells superfused with either 1.8 or 0.5 mM Ca²⁺. Line plot on the top represents the voltage protocol used for the experiments. Right panel shows the statistical comparison of EC-coupling gain in 0.5 mM Ca²⁺ in WT and mdx cells. For each group, data were normalized to the value of the gain obtained in 1.8 mM Ca²⁺. SR Ca²⁺ release was much more resistant to the reduction in I_Ca trigger in mdx myocytes. (C) Left panels illustrate intracellular Ca²⁺ signals during the protocol designed to estimate SR Ca²⁺ leak (as in Shannon et al.¹⁴): line-scan images of fluo-3 fluorescence and normalized cytosolic transients. Bars on the bottom depict the protocol of extracellular solution changes. Averaged values of estimated SR Ca²⁺ leak and SR Ca²⁺ load in WT and mdx cells are shown at the right. The SR Ca²⁺ leak was determined as a reduction in the resting fluo-3 fluorescence following tetracaine application, expressed as a per cent of SR Ca²⁺ content estimated from the amplitude of the caffeine-induced SR Ca²⁺ transient. There was no significant difference in the values obtained in WT and mdx cells. See Supplementary material online, Table S1 for details.