Effect of cardiac myosin binding protein-C on mechanoenergetics in mouse myocardium

We examined the effect of cardiac myosin binding protein-C (cMyBP-C) on contractile efficiency in isovolumically contracting left ventricle (LV) and on internal viscosity and oscillatory work production in skinned myocardial strips. A 6-week diet of 0.15% 6-n-propyl-2-thiouracil (PTU) was fed to wild-type (+/+(PTU)) and homozygous-truncated cMyBP-C (t/tPTU) mice starting at age approximate to8 weeks and leading to a myosin heavy chain (MHC) isoform profile of 10% alpha-MHC and 90% beta-MHC in both groups. Western blot analysis confirmed that cMyBP-C was present in the +/+(PTU) and effectively absent in the t/t(PTU). Total LV mechanical energy per beat was quantified as pressure-volume area (PVA). O-2 consumption (VO2) per beat was plotted against PVA at varying LV volumes. The reciprocal of the slope of the linear VO2-PVA relation represents the contractile efficiency of converting O-2 to mechanical energy. Contractile efficiency was significantly enhanced in t/t(PTU) (26.1 +/- 2.6%) compared with +/+(PTU) (17.1 +/- 1.6%). In skinned myocardial strips, maximum isometric tension was similar in t/t(PTU) (18.7 +/- 2.1 mN . mm(-2)) and +/+(PTU) (21.9 +/- 4.0 mN . mm(-2)), but maximum oscillatory work induced by sinusoidal length perturbations occurred at higher frequencies in t/t(PTU) (7.31 +/- 1.17 Hz) compared with +/+ (PTU) (4.48 +/- 0.60 Hz) and was significantly more sensitive to phosphate concentration in the t/t(PTU). Under rigor conditions, the internal viscous load was significantly lower in the t/t(PTU) compared with +/+(PTU), ie, approximate to40% lower at 1 Hz. These results collectively suggest that contractile efficiency is enhanced in the t/t(PTU), probably through a reduced loss of mechanical energy by a viscous load normally provided by cMyBP-C and through a gain of phosphate-dependent oscillatory work normally inhibited by cMyBP-C.