Characteristics of AP-triggered Ca²⁺ release flux in WT and R6/2 fibers. (A) Peak calcium release flux was significantly reduced in R6/2 (left). The time-to-peak for release flux was delayed by ∼1 ms (right), paralleling a similar change in the time-to-peak of the AP (middle). (B; left) The progressive decrease of peak Ca²⁺ release flux during repetitive excitation (a) was analyzed by fitting a single-exponential decay function to the peak values of the first tetanus in the sequence. Recovery within the 150-ms interval in between tetani (b) was quantified by determining the last peak of the first tetanus (A_1,last) as a fraction of the first peak of the second tetanus (A_2,first). (Right) During the tetanus, peak Ca²⁺ release flux decreased to a lower end level in R6/2 (red trace) compared with WT fibers (black trace). (C) Statistical evaluation of decay and recovery. Fractional decrease was 0.566 ± 0.009 (n = 116) in WT compared with 0.397 ± 0.011 (n = 64) in R6/2 (***, P < 0.001). The time constant of the decline (middle) was not significantly different: 21 ± 1 ms in WT and 24 ± 1 ms in R6/2 (P = 0.121). Recovery within 150 ms was larger in R6/2 fibers, corresponding to a lower A_1,last/A_2,first ratio (right): The mean ratio was 0.704 ± 0.011 in WT and 0.5 ± 0.011 in R6/2 (***, P < 0.001). Data in B are means ± SEM. For explanation of boxplots in A and B, see Materials and methods.