Assaying sarcoplasmic reticulum Ca²⁺-leak in mouse atrial myocytes

More and more studies have suggested an essential role of sarcoplasmic reticulum (SR) Ca²⁺ leak of atrial myocytes in atrial diseases such as atrial fibrillation (AF). The increasing interest in atrial Ca²⁺ signaling makes it necessary to develop a more accurate approach for Ca²⁺ measurement in atrial myocytes due to obvious differences between atrial and ventricular Ca²⁺ handling. In the present study, we proposed a new approach for quantifying total SR Ca²⁺ leak in atrial myocytes with confocal line-scan Ca²⁺ images. With a very precious approximation of the histogram of normalized line-scan Ca²⁺ images by using a modified Gaussian distribution, we separated the signal pixel components from noisy pixels and extracted two new dimensionless parameters, F_signals and R_signals, to reflect the summation of signal pixels and their release components, respectively. In the presence of tetracaine blocking SR Ca²⁺ leak, the two parameters were very close to 0, and in atrial myocytes under normal conditions, the two parameters are well positive correlative with Ca²⁺ spark frequency and total signal mass, the two classic readouts for SR Ca²⁺ leak. Consistent with Ca²⁺ Spark readouts, the two parameters quantified a significant increase of SR Ca²⁺ leak in atrial myocytes from mice harboring a leaky type 2 ryanodine receptor mutation (RyR2-R2474S^+/−) compared to the WT group. Collectively, this study proposed a simple and effective approach to quantify SR Ca²⁺ leak in atrial myocytes, which may benefit research on calcium signaling in atrial physiology and diseases.