A Simple Numerical Model of Calcium Spark Formation and Detection in Cardiac Myocytes

Abstract

The elementary events of excitation-contraction coupling in heart muscle are Ca2+ sparks, which arise from one or more ryanodine receptors in the sarcoplasmic reticulum (SR). Here a simple numerical model is 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 (approximately 1.1 micron vs. approximately 2.0 micron). 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 subsarcomeric 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.

Imaging Ca 2+ Nanosparks in Heart With a New Targeted Biosensor

“…31 Although the targeted sensors GCaMP6f-T/J can reveal Ca 2+ release timing in the smallest domains (eg, a single dyad), we have not yet achieved the ambitious goal of directly reporting local Ca 2+ levels within the junctional space, for reasons discussed above. Nevertheless, with the highly sensitive biosensor expressed at levels that did not significantly affect junction function, we obtained a Ca 2+ signal (after temporal deblurring) that should provide an accurate measure of the local Ca 2+ release duration and semiquantitative measurement of local Ca 2+ fluxes.…”

Cooperative and Stochastic Calcium Releases from Multiple Calcium Puff Sites Generate Calcium Microdomains in Intact HeLa Cells

“…Complex shaping of Ca 2ϩ signal profiles by Ca 2ϩ buffers described by previous studies implies the importance of observing the event with minimal exogenous Ca 2ϩ buffers, e.g. cytosolic EGTA has been used often in previous studies (33). These assumptions led us to adopt a two-dimensional spatial profile analysis by Gaussian fitting in Ca 2ϩ -chelator-free, or intact HeLa cells.…”

Ca2+Syntillas, Miniature Ca2+Release Events in Terminals of Hypothalamic Neurons, Are Increased in Frequency by Depolarization in the Absence of Ca2+Influx

“…A brief summary of the signal mass method, which is described in detail in ZhuGe et al (2000) follows. During a syntilla, free Ca 2ϩ (diffusion coefficient, D ϭ 250 m 2 /sec) and Ca 2ϩ bound to fluo-3 (D ϭ 22 m 2 /sec) (Smith et al, 1998) quickly diffuse in three dimensions away from the release site as Ca 2ϩ continues to be discharged. Therefore, to quantify the increase in total fluorescence, i.e., the Ca 2ϩ signal mass (⌬CaFl) resulting from the binding of fluo-3 to the discharged Ca 2ϩ , fluorescence must be collected from a sufficiently large volume surrounding the release site.…”