Ca ²⁺ influx dependence of contractile force during repetitive high-frequency stimulation of intact soleus muscles. (A, C, and E) Time course of peak specific force recorded in soleus muscles from WT (A and B, n = 13; C and D, n = 5; E and F, n = 10 muscles), CASQ1-null (A and B, n = 21; C and D, n = 6; E and F, n = 7 muscles), and dCASQ-null (A and B, n = 16; C and D, n = 6; E and F, n = 6 muscles) mice during 60 consecutive 1-s duration repetitive 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 fourth stimulus train (ST_4th/ST_1st) and the peak of the rebound phase (ST_peak/ST_4th) from the corresponding data shown in A, C, and E. Data are shown as mean ± SEM. Statistical significance: *, P = 0.023, WT vs. CASQ1-null; *, P = 0.00000055, WT vs. dCASQ-null; *, P = 0.00000047, CASQ1-null vs. dCASQ-null; rebound phase: *, P = 0.0026, WT vs. CASQ1-null; *, P = 0.0000025, WT vs. dCASQ-null; *, P = 0.00003, CASQ1-null vs. dCASQ-null. D: decay phase: *, P = 0.027, WT vs. dCASQ-null. F: decay phase: *, P = 0.004, WT vs. CASQ1-null; *, P = 0.00008, WT vs. dCASQ-null. Number of mice used: WT (A and B, n = 7; C and D, n = 3; E and F, n = 7), CASQ1-null (A and B, n = 12; C and D, n = 4; E and F, n = 4); dCASQ-null (A and B, n = 12; C and D, n = 4; E and F, n = 4).