Blink nadir measurements of sarcoplasmic reticulum are consistent with strong local Ca2+ depletion.

Ca²⁺ blinks measure the exit of Ca²⁺ from the junctional sarcoplasmic reticulum (JSR) in a cardiac myocyte during a Ca²⁺ spark. Here, the relationship between experimental blink fluorescence measurements and the [Ca²⁺] in the JSR is explored using long 3D simulations of diastolic Ca²⁺ release. For a fast intra-SR Ca²⁺-activated fluorophore like Fluo-5N, we show that a simple mathematical formula relates the two for an ideal blink (i.e., when fluorescence signals come only from the JSR). The formula shows that normalized JSR [Ca²⁺] is much lower than the normalized fluorescence and that JSR Ca²⁺ depletes ∼40-50% more than previously inferred from blink fluorescence measurements. In addition, we show that stray fluorescence signals (e.g., from other parts of the sarcoplasmic reticulum network) can mask even deeper Ca²⁺ depletion. Overall, the simulations show that strong JSR Ca²⁺ depletion like that seen in many simulations is consistent with the relatively moderate fluorescence changes seen in experiments.