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

Ca blinks measure the exit of Ca from the junctional sarcoplasmic reticulum (JSR) in a cardiac myocyte during a Ca spark. Here, the relationship between experimental blink fluorescence measurements and the [Ca] in the JSR is explored using long 3D simulations of diastolic Ca release. For a fast intra-SR Ca-activated fluorophore such as 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 [Ca] is much lower than the normalized fluorescence and that JSR Ca 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 Ca depletion. Overall, the simulations show that strong JSR Ca depletion such as that seen in many simulations is consistent with the relatively moderate fluorescence changes seen in experiments.