Ca²⁺ sparks, which arise from one or more ryanodine receptors in the sarcoplasmic reticulum (SR), are the elementary events of excitation-contraction coupling in heart muscle. I present a simple numerical model constructed to explore Ca²⁺ spark formation, detection, and interpretation in cardiac myocytes. This model includes Ca²⁺ release, cytosolic diffusion, resequestration by SR Ca²⁺-ATPases, and the association and dissociation of Ca²⁺ with endogenous Ca²⁺-binding sites and a diffusible indicator dye (fluo-3). Simulations in a homogeneous, isotropic cytosol reproduce the brightness and the time course of a typical cardiac Ca²⁺ spark, but underestimate its spatial size (\~1.1 mm vs. \~2.0 mm). Back-calculating [Ca²⁺]_i by assuming equilibrium with indicator fails to provide a good estimate of the free Ca²⁺ concentration even when using blur-free fluorescence data. A parameter sensitivity study reveals that the mobility, kinetics, and concentration of the indicator are essential determinants of the shape of Ca²⁺ sparks, whereas the stationary buffers and pumps are less influential. Using a geometrically more complex version of the model, we show that the asymmetric shape of Ca²⁺ sparks is better explained by anisotropic diffusion of Ca²⁺ ions and indicator dye rather than by sub-sarcomeric inhomogeneities of the Ca²⁺ buffer and transport system. In addition, we examine the contribution of off-center confocal sampling to the variance of spark statistics.

This is joint work with Joel E. Keizer, Michael D. Stern, W. Jonathan Lederer, and Heping Cheng.

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