Calcium sensitivity of the force-pCa relationship depends strongly on sarcomere length (SL) in cardiac muscle and is considered to be the cellular basis of the Frank-Starling law of the heart. SL dependence may involve changes in myofilament lattice spacing and/or myosin crossbridge orientation to increase probability of binding to actin at longer SLs. We used the L48Q cardiac troponin C (cTnC) variant, which has enhanced Ca2+ binding affinity, to test the hypotheses that the intrinsic properties of cTnC are important in determining 1) thin filament binding site availability and responsiveness to crossbridge activation and 2) SL dependence of force in cardiac muscle. Trabeculae containing L48Q cTnC-cTn lost SL dependence of the Ca2+ sensitivity of force. This occurred despite maintaining the typical SL-dependent changes in maximal force (Fmax). Osmotic compression of preparations at SL 2.0 μm with 3% dextran increased Fmax but not pCa50 in L48Q cTnC-cTn exchanged trabeculae, whereas wild-type (WT)-cTnC-cTn exchanged trabeculae exhibited increases in both Fmax and pCa50. Furthermore, crossbridge inhibition with 2,3-butanedione monoxime at SL 2.3 μm decreased Fmax and pCa50 in WT cTnC-cTn trabeculae to levels measured at SL 2.0 μm, whereas only Fmax was decreased with L48Q cTnC-cTn. Overall, these results suggest that L48Q cTnC confers reduced crossbridge dependence of thin filament activation in cardiac muscle and that changes in the Ca2+ sensitivity of force in response to changes in SL are at least partially dependent on properties of thin filament troponin.
Contractile activation and measurements of intracellular Ca2+ concentration in cane toad twitch fibres in the presence of 2,3-butanedione monoxime
