(A) In situ measurement of gastrocnemius-developed force upon an electrical stimulus in wild-type (Nr1d1^+/+) and Nr1d1^–/– mice. ***P = 0.0004 vs. Nr1d1^+/+, unpaired t test; n = 9–10. (B) Representative curves of SERCA-inhibitable Ca²⁺ uptake in microsomal fractions prepared from muscle from Nr1d1^+/+ and Nr1d1^–/– mice. Decrease in fluorescence indicates Ca²⁺ uptake by microsomes. Arrows indicate Ca²⁺ or thapsigargin (TG) injections. The red rectangle was used for (C) slope calculation of fluorescence decrease, indicative of specific SERCA Ca²⁺ uptake. *P = 0.0203 vs. Nr1d1^+/+, unpaired t test; n = 6. (D) Representative curves of SERCA-inhibitable Ca²⁺ uptake in microsomes prepared from muscle from vehicle- or SR9009-treated wild-type mice. (E) Slopes of the decreasing fluorescence over time. *P = 0.0408 vs. vehicle, unpaired t test; n = 4. (F) TG-induced SR Ca²⁺ release in control pBabe- and NR1D1-overexpressing C2C12 myotubes. Cells were loaded with Fluo4-AM to detect cytosolic Ca²⁺. SR Ca²⁺ content depletion was induced by TG (1 μM) (n = 10). (G) Delta F/F0 ratio normalized to pBabe values, obtained 5 minutes after TG-induced Ca²⁺ release. **P = 0.007 vs. pBabe, unpaired t test; n = 10. (H) Representative Fura-2/AM experiments (ratio F340/F380) in pBabe- and NR1D1-overexpressing cells. (I) Normalized SR calcium concentration released upon TG treatment in pBabe- and NR1D1-overexpressing cells. (J) Normalized basal cytosolic calcium concentration (mean of the 100 first seconds) in pBabe- and NR1D1-overexpressing cells. (I and J) Box-and-whisker plots show all points, with minimums and maximums (n > 300 in each group). ***P < 0.0001 vs. pBabe, unpaired t test. (K) TG-induced SR Ca²⁺ release in C2C12 cells transfected by control (siCTRL) or Nr1d1 siRNA (siNr1d1). Results are shown as Delta F/F0 ratio (n = 17). (L) Delta F/F0 ratio normalized to siCTRL values obtained 5 minutes after TG-induced Ca²⁺ release. **P = 0.0044 vs. siCTRL, unpaired t test; n = 17. Data are shown as the mean ± SEM.