Comparison of simulated and measured frog Δf_CaD signals elicited by 10 APs at 100 Hz. In A, the simulated troponin reaction did not include Mg²⁺ competition; in B, the reaction in Fig. 3 was used. The lowermost trace in each panel shows the Ca²⁺ release waveform used to drive the simulation. The next trace shows simulated Δ[Ca²⁺] in the release compartment (see Fig. 2), and the third trace is the simulated spatially averaged Δ[Ca²⁺] (i.e., the average of the Δ[Ca²⁺] waveforms in all 18 compartments of the model). The top traces compare the simulated and measured Δf_CaD signals (noise-free and noisy traces, respectively). The measured Δf_CaD signal (which is shown in both panels) was averaged from five fibers in which contamination of the fluorescence signal with movement artifacts was very small or negligible. The mean diameter and mean sarcomere length of these fibers were 54 ± 3 µm (mean ± SEM) and 3.7 ± 0.1 µm, respectively. The release functions for the simulations in A and B were based on the two-component release functions in Figs. 7 and 8, respectively. For convenience in the calculation, it was assumed that the onset of each subsequent release extinguished any residual release elicited by the preceding APs and that the relative amplitudes of the two release components remained unchanged. Although it is uncertain whether the relative amplitudes of the two release components remain unchanged with the second and subsequent releases, measurements in frog cut fibers indicate that virtually all of the Ca²⁺ release channels (>90%) are subject to Ca²⁺ inactivation of Ca²⁺ release (Jong et al., 1995). The simulated release due to the first AP continued to 10.6 ms, the time at which the second release became significant. The second and subsequent releases were shifted in successive 10-ms increments and their amplitudes scaled down appropriately. In A, the amount of Ca²⁺ released with the first AP is 377 µM and the amounts of the subsequent releases, expressed as a percentage of this amount, are 21.8, 17.5, 14.9, 12.6, 11.7, 10.4, 10.2, 9.5, and 10.0%, respectively. In B, the corresponding values are 343 µM and 29.2, 22.8, 16.4, 14.7, 12.7, 11.7, 10.8, 10.1, and 11.2%.