Mechanisms Underlying Calcium Sparks in Cardiac Muscle

Calcium sparks were discovered in single isolated rat cardiac myocytes by Cheng et al. (1993) while imaging fluorescence from the calcium indicator fluo-3 (Minta et al., 1989) with a confocal microscope. Cardiac muscle calcium sparks are associated with an approximate doubling of the resting fluo-3 fluorescence (ΔF/F = 1.0) and occupy a tiny region of the cell ∼2 μm in diameter. A simple equilibrium calculation of the likely change in calcium underlying the spark suggested that the local calcium peaked at ∼300 nM in 10 ms (Cheng et al., 1993). However, this figure underestimates the true change in calcium because of the limited kinetics and dynamic range of fluo-3, as well as blurring by the microscope. From a recent paper by Smith et al. (1998), we can estimate that the true change in calcium underlying the spark (averaged by microscope blurring...