cTn reconstitution and its impact on length-dependent activation in rabbit psoas muscle. (A, left) 15% gel. Cont., control fibers; cTn, cTn-reconstituted fibers; cTn-sTn, cTn-reconstituted fibers treated with sTn. cTn reconstitution ratio: cTnT, 59.2 ± 5.2%; cTnI, 52.7 ± 3.7% (n = 6) (residual skeletal subunits: sTnT, 54.6 ± 7.6%; sTnI, 56.2 ± 4.0% [n = 6]). sTn reconstitution ratio after cTn reconstitution: sTnT, 83.6 ± 5.4%; sTnI, 74.9 ± 9.2% (n = 6) (residual cardiac subunits: cTnT, 4.3 ± 2.9%; cTnI, 4.9 ± 2.7% [n = 6]). For abbreviations, see Fig. 1. (A, right) 2–7% gel. T1, intact fast skeletal titin. T2, titin's degradation product(s). (B) Typical chart recording showing force–pCa protocols. Arrows indicate percentage compared with the maximum obtained at the end of experiment (pCa 6.0). Gain, percent increase in Ca²⁺ sensitivity of force upon stretch (pCa 6.0). PF = 0, zero passive force (PF). (C), Force–pCa curves (top) and passive force (bottom) in control (n = 7) and cTn-reconstituted (n = 10) rabbit psoas muscle at SL 1.9 and 2.3 μm. Dashed black and solid blue lines indicate control and cTn-reconstituted fibers, respectively. Inset, ΔpCa₅₀ (Psoas, control rabbit psoas muscle). *, P < 0.05. (D) Comparison of SL-dependent increase in maximal force (ΔMaximal force). *, P < 0.05.