Mysterious beauty of beating heart: cardiac mechano-energetico-informatics

Large-scale models reveal the two-component mechanics of striated muscle

This paper provides a comprehensive explanation of striated muscle mechanics and contraction on the basis of filament rotations. Helical proteins, particularly the coiled-coils of tropomyosin, myosin and alpha-actinin, shorten their H-bonds cooperatively and produce torque and filament rotations when the Coulombic net-charge repulsion of their highly charged side-chains is diminished by interaction with ions. The classical "two-component model" of active muscle differentiated a "contractile component" which stretches the "series elastic component" during force production. The contractile components are the helically shaped thin filaments of muscle that shorten the sarcomeres by clockwise drilling into the myosin cross-bridges with torque decrease (= force-deficit). Muscle stretch means drawing out the thin filament helices off the cross-bridges under passive counterclockwise rotation with torque increase (= stretch activation). Since each thin filament is anchored by four elastic alpha-actinin Z-filaments (provided with force-regulating sites for Ca(2+) binding), the thin filament rotations change the torsional twist of the four Z-filaments as the "series elastic components". Large scale models simulate the changes of structure and force in the Z-band by the different Z-filament twisting stages A, B, C, D, E, F and G. Stage D corresponds to the isometric state. The basic phenomena of muscle physiology, i. e. latency relaxation, Fenn-effect, the force-velocity relation, the length-tension relation, unexplained energy, shortening heat, the Huxley-Simmons phases, etc. are explained and interpreted with the help of the model experiments.

Mechanoenergetic estimation of multiple cross-bridge steps per ATP in a beating heart

The efficiency from the ventricular O(2) consumption (VO(2)) to the total mechanical energy (TME) generated by ventricular contraction has proved relatively constant at approximately 35%, independent of the loading and contractile conditions in a canine heart. TME is the sum of the external mechanical work for ejecting a stroke volume against the afterload and of the mechanical potential energy for developing ventricular pressure in each beat. The approximately 35% VO(2)-to-TME efficiency indicates an also constant approximately 60% ATP-to-TME efficiency in a beating heart, based on the nominal approximately 60% VO(2)-to-ATP efficiency in the myocardial oxidative phosphorylation. I newly attempted to explain the load-independent approximately 60% ATP-to-TME efficiency by the recently reported approximately 7-10 nm unitary step size and approximately 0.8-1.5 pN unitary force of a cross-bridge (CB) at the molecular level in in vitro motility assays. This single CB behavior suggests that its unitary cycle could generate a mechanical energy of approximately 0.6-1.5x10(-20) J at most. From the nominal free energy of approximately 10x10(-20) J per ATP, the efficiency from one ATP to the CB unitary cycle would then be approximately 6-15%. This low efficiency is only approximately 1/10-1/4 of the approximately 60% ATP-to-TME efficiency at the heart level. This discrepancy suggests that each CB would repeat the unitary cycle at least approximately 4-10 times per ATP to achieve the high constant ATP-to-TME efficiency in a beating heart. This seems to represent a considerable mechanoenergetic advantage of the heart at the integrative heart level as compared to the molecular CB level.