Large currents generate cardiac Ca²⁺ sparks

Previous models of cardiac Ca²⁺ sparks have assumed that Ca²⁺ currents through the Ca²⁺ release units (CRUs) were ∼1-2 pA, producing sparks with peak fluorescence ratio (F/F_o) of ∼2.0 and a full-width at half maximum (FWHM) of ∼1 μm. Here, we present actual Ca²⁺ sparks with peak F/F_o of >6 and a FWHM of ∼2 μm, and a mathematical model of such sparks, the main feature of which is a much larger underlying Ca²⁺ current. Assuming infinite reaction rates and no endogenous buffers, we obtain a lower bound of ∼11 pA needed to generate a Ca²⁺ spark with FWHM of 2 μm. Under realistic conditions, the CRU current must be ∼20 pA to generate a 2-μm Ca²⁺ spark. For currents ≥5 pA, the computed spark amplitudes (F/F_o) are large (∼6-12 depending on buffer model). We considered several factors that might produce sparks with FWHM ∼ 2 μm without using large currents. Possible protein-dye interactions increased the FWHM slightly. Hypothetical Ca²⁺ "quarks" had little effect, as did blurring of sparks by the confocal microscope. A clusters of CRUs, each producing 10 pA simultaneously, can produce sparks with FWHM ∼ 2 μm. We conclude that cardiac Ca²⁺ sparks are significantly larger in peak amplitude than previously thought, that such large Ca²⁺ sparks are consistent with the measured FWHM of ∼2 μm, and that the underlying Ca²⁺ current is in the range of 10-20 pA.