Key result
Sarcomere strain increased A-band lateral stiffness significantly more in myofibrils with stiff titin (psoas) than in those with compliant titin (diaphragm), independent of lattice spacing effects.
Titin stiffness directly modulates the lateral stiffness of the actin-myosin overlap zone independently of lattice spacing, suggesting a mechanosensory role for titin in length-dependent activation of striated muscle.
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Titin isoform effects on sarcomere stiffness warrant inclusion in muscle models; extends basic mechanics research while leaving cardiac translation open.
The contractile units of striated muscle, the sarcomeres, comprise the thick (myosin) and thin (actin) filaments mediating active contraction and the titin filaments determining "passive" elasticity. We hypothesized that titin may be more active in muscle contraction by directly modulating thick-filament properties. We used single-myofibril mechanical measurements and atomic force microscopy of individual sarcomeres to quantify the effects of sarcomere strain and titin spring length on both the inter-filament lattice spacing and the lateral stiffness of the actin-myosin overlap zone (A-band). We found that strain reduced the lattice spacing similarly in sarcomeres with stiff (rabbit psoas) or compliant titin (rabbit diaphragm), but increased A-band lateral stiffness much more in psoas than in diaphragm. The strain-induced alterations in A-band stiffness that occur independently of lattice spacing effects may be due to titin stiffness-sensing by A-band proteins. This mechanosensitivity could play a role in the physiologically important phenomenon of length-dependent activation of striated muscle.
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Li et al. (2016) studied Muscle contraction mechanics (basic science). Sarcomere strain and osmotic compression vs. Slack sarcomere length, no dextran was evaluated on A-band lateral stiffness (pointwise apparent modulus, Eapp). Sarcomere strain increased A-band lateral stiffness significantly more in myofibrils with stiff titin (psoas) than in those with compliant titin (diaphragm), independent of lattice spacing effects.
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