Figure 6. Estimates of nanodomain Ca²⁺ amplitude from Ca_V2.2/TN-XL responses.

a, Average Ca_V2.2/TN-XL responses to nanodomain Ca²⁺ signals, corrected to reflect signal of active channels alone. Top, black trace plots average whole-current, rescaled to open probability P_O by normalizing peak current during 30-mV pulse to 0.69. Gray trace, exponential fit to P_O waveform. Middle, cyan trace plots average S̄_C, corrected CFP signal. Red trace, forward transform prediction of experimental S̄_C using Ca_spike = 60 μM. Bottom, Green trace plots average R̄_F/C, corrected FRET-ratio signal. Red trace, forward transform prediction of experimental R̄_F/C using Ca_spike = 60 μM. Gray traces, transform prediction with Ca_spike as labeled. b, Nanodomain Ca²⁺ signals input to forward transform for prediction of Ca_V2.2/TN-XL responses in a. Top, P_O waveform reproduced from a. Middle, Schematic of stochastic Ca²⁺ pulses in nanodomain. Bottom, Forward transform box with parameters in Table 1. c, Ca_spike estimation criteria. Presumed Ca_spike values plotted versus fold-fitting error (sum of squared deviations between fit and data, normalized to sum with Ca_spike = 60 μM). Cusp of plot denotes Ca_spike = 60 μM as best fit. Shaded zone specifies approximate confidence region, where fits yield errors within 25% of minimum.