Sarcomere Length Dependent Contractile Activation is Reduced in Rat Trabeculae Exchanged with cTn Containing the L48Q cTnC Variant Independently of Strong Binding Cross-Bridges

Calcium sensitivity of the force-pCa relationship depends strongly on sarcomere length (SL) in cardiac muscle. It can also be influenced by maneuvers that alter the distribution of cross-bridges within the cross-bridge cycle. We have demonstrated that cardiac trabeculae have left-shifted and virtually eliminated SL dependence of force-pCa relationship following passive exchange with cTn containing a mutant (L48Q) cTnC (with enhanced TnC-TnI interaction). Here we designed experiments to investigate the importance of strongly bound crossbridges and lattice spacing in modulating the force-pCa relationship of WT and L48Q cTnC-cTn exchanged trabeculae. Using 3% dextran at SL = 2.0 µm to osmotically compress preparations to widths ∼SL = 2.3 µm, we observed increased maximal force but not increased pCa50 in L48Q cTnC-cTn exchanged trabeculae. Conversely, crossbridge inhibition with of 2,3-butanedione monoxime (BDM, 7 mM) at SL 2.3 µm decreased maximal force and Ca2+ sensitivity in native and WT-cTn exchanged trabeculae to levels measured at ∼SL = 2.0 µm. L48QcTnC-cTn exchanged preparations treated with BDM also decreased maximal force to that seen at SL = 2.0 µm, but demonstrated no shift in Ca2+ sensitivity. This result is similar to decrease in maximal force but no shift in Ca2+ sensitivity for L48Q cTnC-cTn exchanged preparations at SL 2.0 vs. 2.3 µm. The combined results further support the idea that L48Q cTnC confers crossbridge independence on thin filament activation. It may also imply that native thin filaments are dependent on strong crossbridge binding for full activation because of relatively weak cTnC-cTnI interaction. Finally, the relative strength of cTnC-cTnI interaction may be an important determinant in length dependent activation of cardiac muscle. Support provided by NIH HL65497 (MR) and T32 HL07828 (FSK).