The relationship between rise time and a simple estimator of Ca²⁺ release flux in sparks. (A) Examples of spark profiles used to estimate release flux. Thick trace, F/F₀ vs. t at the center of the average of the 88 sparks with T between 4 and 5 ms in the group represented in Fig. 1 C. Thin trace, F/F₀ vs. t for the average of the 18 sparks with T between 9 and 11 ms in the same group. Release flux was estimated by the average rate of rise of normalized fluorescence, m₁, calculated on bin averages as the ratio a/T, which for the averages shown is respectively 0.234 and 0.069 ms⁻¹. Note that the time-varying rate of rise of fluorescence may diverge substantially from m₁. (B–D) Bin averages of T plotted against m₁ for the sets of sparks in Fig. 1, B–D. A similar, strictly monotonic dependence is obtained in all cases.