Reduced cross-bridge dependent stiffness of skinned myocardium from mice lacking cardiac myosin binding protein-C

The role of cardiac myosin binding protein-C (MyBP-C) on myocardial stiffness was examined in skinned papillary muscles of wild-type (WT+/+) and homozygous truncated cardiac MyBP-C (MyBP-C-t/t) male mice. No MyBP-C was detected by gel electrophoresis or by Western blots in the MyBP-C-t/t myocardium. Rigor-bridge dependent myofilament stiffness, i.e., rigor minus relaxed stiffness, in the MyBP-Ct/t myocardium (281 +/- 44 kN/m(2)) was 44% that in WT+/+ (633 +/- 141 kN/m(2)). The center-to-center spacing between thick filaments as determined by X-ray diffraction in MyBP-C-t/t (45.0 +/- 1.2 nm) was not significantly different from that in WT+/+ (43.2 +/- 0.9 nm). The fraction of cross-sectional area comprised of myofibrils, as determined by electron microscopy, was reduced in the MyBP-Ct/t (39.9%) by 10% compared to WT+/+ (44.5%). These data suggest that the 56% reduction in rigor-bridge dependent stiffness of the skinned MyBP-C-t/t myocardium could not be due solely to a 10% reduction in the number of thick filaments per cross-sectional area and must also be due to approximately 50% reduction in the stiffness of the rigor-bridge attached thick filaments lacking MyBP-C.