Palmer et al. demonstrate that a stretch-induced enhancement of the myosin crossbridge detachment rate is sufficient to explain the force response of cardiac muscle to a quick stretch and would benefit diastolic function in a beating heart.
The force response of cardiac muscle undergoing a quick stretch is conventionally interpreted to represent stretching of attached myosin crossbridges (phase 1) and detachment of these stretched crossbridges at an exponential rate (phase 2), followed by crossbridges reattaching in increased numbers due to an enhanced activation of the thin filament (phases 3 and 4). We propose that, at least in mammalian cardiac muscle, phase 2 instead represents an enhanced detachment rate of myosin crossbridges due to stretch, phase 3 represents the reattachment of those same crossbridges, and phase 4 is a passive-like viscoelastic response with power-law relaxation. To test this idea, we developed a two-state model of crossbridge attachment and detachment. Unitary force was assigned when a crossbridge was attached, and an elastic force was generated when an attached crossbridge was displaced. Attachment rate, f( x), was spatially distributed with a total magnitude f 0. Detachment rate was modeled as g( x) = g 0 + g 1 x, where g 0 is a constant and g 1 indicates sensitivity to displacement. The analytical solution suggested that the exponential decay rate of phase 2 represents ( f 0 + g 0) and the exponential rise rate of phase 3 represents g 0. The depth of the nadir between phases 2 and 3 is proportional to g 1. We prepared skinned mouse myocardium and applied a 1% stretch under varying concentrations of inorganic phosphate (Pi). The resulting force responses fitted the analytical solution well. The interpretations of phases 2 and 3 were consistent with lower f 0 and higher g 0 with increasing Pi. This novel scheme of interpreting the force response to a quick stretch does not require enhanced thin-filament activation and suggests that the myosin detachment rate is sensitive to stretch. Furthermore, the enhanced detachment rate is likely not due to the typical detachment mechanism following MgATP binding, but rather before MgADP release, and may involve reversal of the myosin power stroke.