sTn reconstitution and its impact on length-dependent activation in PLV. (A, left) 15% gel. Cont., control PLV; sTn, sTn-reconstituted PLV; sTn-cTn, sTn-reconstituted PLV treated with cTn. cTnT (sTnT), cardiac (skeletal) troponin T; cTnI (sTnI), cardiac (skeletal) troponin I. sTn reconstitution ratio: sTnT, 107.0 ± 8.6%; sTnI, 117.7 ± 6.7% (n = 6) (residual cardiac subunits: ∼0% for both [n = 6]). cTn reconstitution ratio after sTn reconstitution: cTnT, 40.6 ± 4.4%; cTnI, 37.9 ± 2.9% (n = 5) (residual skeletal subunits: sTnT, 34.9 ± 6.0%; sTnI, 41.3 ± 11.5% [n = 5]). Tm, LC-1 and LC-2 denote tropomyosin, myosin light chain 1 and myosin light chain 2, respectively. (A, right) 2–7% gel. N2BA and N2B indicate large and small cardiac titins, respectively (Fukuda et al., 2003; Udaka et al., 2008). T2, titin's degradation product(s). MHC, myosin heavy chain. (B) Typical chart recording showing force–pCa protocols. Arrows indicate percentage compared with the maximum obtained at the end of experiment (pCa 5.75). Gain, percent increase in Ca²⁺ sensitivity of force upon stretch (pCa 5.75). PF = 0, zero passive force (PF). (C, left) Force–pCa curves (top) and passive force (bottom) in control (n = 7) and sTn-reconstituted (n = 7) PLV at SL 1.9 and 2.3 μm. Black and red lines indicate control and sTn-reconstituted PLV, respectively. Inset, ΔpCa₅₀ (C, control PLV). *, P < 0.05. (C, right) Force–pCa curves (top) and passive force (bottom) in rabbit psoas muscle (n = 7) and sTn-reconstituted PLV (same as in left) at SL 1.9 and 2.3 μm. Dashed black lines, rabbit psoas muscle. *, P < 0.05 compared with corresponding passive force in sTn-reconstituted PLV. Inset, ΔpCa₅₀. (D) Comparison of SL-dependent increase in maximal force (ΔMaximal force). *, P < 0.05.