Ca ²⁺  influx dependence of contractile force during repetitive high-frequency stimulation of intact EDL muscles . (A, C, and E) Time course of peak specific force in EDL muscles from WT (A and B, n = 11; C and D, n = 10; E and F, n = 10 muscles), CASQ1-null (A and B, n = 7; C and D, n = 9; E and F, n = 9 muscles), and dCASQ-null (A and B, n = 7; C and D, n = 7; E and F, n = 9 muscles) mice during 60 consecutive 500-ms-duration stimulus trains in the presence of either standard Ringer’s solution containing 2.5 mM Ca²⁺ (A), nominally Ca²⁺-free Ringer’s solution (C), or standard Ringer’s solution supplemented with 10 μM BTP2 (E). (B, D, and F) Quantitative analyses of the relative decay during the second stimulus train (ST_2nd/ST_1st) and the peak of the rebound phase (ST_peak/ST_2nd) from the corresponding data shown in A, C, and E. Data are shown as mean ± SEM. Statistical analysis: B, decay phase: *, P = 0.039, WT vs. CASQ1-null; *, P = 0.044, WT vs. dCASQ-null; rebound phase: *, P = 0.018, WT vs. CASQ1-null; *, P = 0.02, WT vs. dCASQ-null. D, decay phase: *, P = 0.000000065, WT vs. CASQ1-null; *, P = 0.00000021, WT vs. dCASQ-null; rebound phase: *, P = 0.000000000027, WT vs. CASQ1-null; *, P = 0.0000000025, WT vs. dCASQ-null. F, decay phase: *, P = 0.022, WT vs. CASQ1-null; *, P = 0.046, WT vs. dCASQ-null; rebound phase: *, P = 0.00015, WT vs. CASQ1-null; *, P = 0.0000031, WT vs. dCASQ-null. Number of mice used: WT (A and B, n = 8; C and D, n = 8; E and F, n = 7), CASQ1-null (A and B, n = 4; C and D, n = 5; E and F, n = 7); dCASQ-null (A and B, n = 4; C and D, n = 5; E and F, n = 6).