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 (⌬ F / F ϭ 1.0) and occupy a tiny region of the cell ‫ف‬ 2 ␮ m in diameter. A simple equilibrium calculation of the likely change in calcium underlying the spark suggested that the local calcium peaked at ‫ف‬ 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 in a region 0.5 ␮ m across) may be Ͼ 10 ␮ M. Calcium sparks occur at a frequency of 1.6 s Ϫ 1 in line scan images (Cheng et al., 1993), which survey ‫ف‬ 70–100 sarcomeres. However, calcium sparks can also be produced by membrane depolarization and therefore reflect the process of excitation–contraction (E–C) coupling at individual junctions between the t-tubular system and the sarco-plasmic reticulum that occur at the z-line (Shacklock et al., 1995; Cheng et al., 1996). Spark sites are therefore separated by ‫ف‬ 1.8 ␮ m longitudinally in resting cardiac cells. In the transverse direction, calcium spark sites have a more variable spacing and sites that are closer to each other are more likely to coactivate (Parker et al., 1996). The whole-cell calcium transient can be explained by the spatio-temporal summation of calcium sparks (Can-nell et al., 1994, 1995). Hence, the whole-cell sarcoplas-mic reticulum (SR) calcium release flux should be the average probability of SR release (P Spark) multiplied by the average local SR release flux (J Rel), but these variables are not separable in conventional whole-cell pho-tometric measurements. However, recording calcium sparks overcomes this problem because this method provides a direct measure of P Spark (which is proportional to the number of sparks detected in the confocal imaging volume per unit time) as well as J Rel (which can be estimated from the spatio-temporal properties of the calcium spark). An initial estimate of J Rel by Cheng et al. (1993) suggested that the calcium spark could be explained …