Co-operative activation of skeletal muscle thin filaments by rigor crossbridges. The effect of troponin C extraction.

When Ca2+ binds to troponin C (TnC), all 26 troponin-tropomyosin (Tn-Tm) complexes of a regulatory strand change in concert from the inactive to the active configuration. To see if the complexes respond similarly when they are activated by rigor crossbridges in the absence of Ca2+, we determined the slope (ns) of the bell-shaped pS/tension (pS = -log [MgATP], where S = MgATP2-) relationship between pS 5, where the tension is maximal, and pS 2.3, where fibers are fully relaxed. In control skinned rabbit psoas fibers the ns value is greater than 4; it progressively decreases with TnC extraction. This decrease in ns with TnC extraction is analogous to the decrease in the slope (Hill coefficient) of the pCa/tension (pCa = -log [Ca2+]) relationship with extraction. Complete TnC extraction reduces the maximum substrate-induced tension by only 25%; in contrast, it reduces the maximum Ca2+ induced tension to zero. The effects of TnC extraction on the slope of the pS/tension curve are explained by the assumptions that (1) extracted Tn-Tm complexes no longer change in concert with their neighbors but change independently of them, and (2) co-operative signals cannot cross extracted Tn-Tm complexes. The ns value, therefore, like the nH, is a direct function of the number of contiguous, intact, Tn-Tm complexes in a stretch of a regulatory strand. To describe qualitatively the bi-phasic pS/tension relationship, the mono-phasic pCa/tension relationship, and the effects of TnC extraction on them, we introduce a version of the concerted-transition formalism which includes two activating ligands, Ca2+ and rigor crossbridges.