Noise-Free Visualization of Microscopic Calcium Signaling by Pixel-Wise Fitting

RATIONALE: Our insights into physiological and pathophysiological cardiac excitation-contraction coupling has greatly benefited from significant advancement in optical technologies such as high-speed confocal microscopy. This has pushed pixel dwell times into the time domain of nanoseconds, resulting in low signal-to-noise ratios, which have limited data analysis and interpretation. OBJECTIVE: Line scan imaging has been and still is dominant in high speed confocal recording. It allows analysis only of a small fraction of a cell's cross section (1.5%), but the appreciation of spatiotemporal fine details of excitation-contraction coupling is instrumental for the further understanding of pathological mechanisms. We aim to provide a novel analysis tool to extract otherwise hidden fine details in cardiac excitation-contraction coupling from high-speed 2-dimensional confocal image series. METHODS AND RESULTS: We demonstrate that high-speed 2-dimensional confocal data (150 frames/s) can be analyzed quantitatively by a pixel-wise fitting approach, using a mathematical formalism to phenomenologically describe local calcium transients. Such an approach produces virtually noise-free fluorescence data originating from minute volumes (0.025 femtoliter) and allows extraction of detailed and most importantly quantitative and mechanistically novel information on microscopic calcium signaling and excitation-contraction coupling in a robust manner. CONCLUSIONS: Pixel-wise fitting provides novel insights into cardiac excitation-contraction coupling. Specifically, it revealed microscopic calcium alternans on the level of individual coupling sites. Microscopic calcium alternans is an early precursor of cellular alternans and as such will shed more light onto this mechanism leading to cardiac arrhythmia.