Correction

2011. In situ calibration of nucleoplasmic versus cytoplasmic Ca²⁺ concentration in adult cardiomyocytes. Biophys. J. 100:2356–2366.

In the Discussion, on page 2363, the following two sentences are incorrect:

“Conversely, results calculated from [Ca²⁺]_rest, assuming that F_rest ≈ 0 (34):

$$\left[\mathrm{C}{\mathrm{a}}^{2+}\right]=\frac{{\mathrm{K}}_{\mathrm{d}}\mathrm{R}}{\left(\frac{{\mathrm{K}}_{\mathrm{d}}}{{\left[\mathrm{C}{\mathrm{a}}^{2+}\right]}_{\mathrm{rest}}}-\mathrm{R}+1\right)}$$ (7)

yielded significantly lower values. The reason for this important difference is that cellular [Ca²⁺]_rest levels are not negligible for indicators with very bright fluorescence like Fluo-4, typically making F_rest considerably larger than F_min.”

It was assumed in Reference 34 that F_min ≈ 0, not F_rest ≈ 0.

Therefore, the paragraph

“We further compared [Ca²⁺] values obtained using different forms of the Grynkiewicz equation. No significant difference was observed using either Eqs. 1 or 4. Conversely, results calculated from [Ca²⁺]_rest, assuming that F_rest ≈ 0 (34):

$$\left[\mathrm{C}{\mathrm{a}}^{2+}\right]=\frac{{\mathrm{K}}_{\mathrm{d}}\mathrm{R}}{\left(\frac{{\mathrm{K}}_{\mathrm{d}}}{{\left[\mathrm{C}{\mathrm{a}}^{2+}\right]}_{\mathrm{rest}}}-\mathrm{R}+1\right)}$$ (7)

yielded significantly lower values. The reason for this important difference is that cellular [Ca²⁺]_rest levels are not negligible for indicators with very bright fluorescence like Fluo-4, typically making F_rest considerably larger than F_min.”

has to be deleted and replaced by the following:

“The frequently used self ratio equation (34):

$$\left[\mathrm{C}{\mathrm{a}}^{2+}\right]=\frac{{\mathrm{K}}_{\mathrm{d}}\mathrm{R}}{\left(\frac{{\mathrm{K}}_{\mathrm{d}}}{{\left[\mathrm{C}{\mathrm{a}}^{2+}\right]}_{\mathrm{rest}}}-\mathrm{R}+1\right)}$$ (7)

is simpler and provides a good estimate of [Ca²⁺] changes without aiming at precise calibration. It assumes that F_min is 0. With bright indicators like Fluo-4, however, we found that F_min is considerably larger than 0, thus necessitating the use of a different approach for calibration of [Ca²⁺].”