Effects of sarcomere length and Ca(2+) binding on h reactivity of myofilament bound troponin C in porcine skinned cardiac muscle fibers.

Length dependence of cardiac Ca(2+) activation is an essential component of the Frank-Starling relation. The aim of this study is to examine the length effects on the Ca(2+)-induced conformational changes of filament-bound cTnC in skinned cardiac muscle fibers. The two cysteine residues (Cys-35 and Cys-84) in the regulatory domain of cTnC allow for the attachment of conformational probes to this region. Their incorporation with the fluorescent probe, 7-diethylamino-3-[4'-maleimidylphenyl]-4-methylcoumarin (CPM), was used to determine the varying cTnC conformations in cardiac fibers. The data obtained show that the length-dependent Ca(2+)-mediated conformational changes require strong-binding cross-bridges for cardiac activation.