A non-cross-bridge, static tension is present in permeabilized skeletal muscle fibers after active force inhibition or actin extraction

Abstract

When activated muscle fibers are stretched, there is a long-lasting increase in the force. This phenomenon, referred to as "residual force enhancement," has characteristics similar to those of the "static tension," a long-lasting increase in force observed when muscles are stretched in the presence of Ca(2+) but in the absence of myosin-actin interaction. Independent studies have suggested that these two phenomena have a common mechanism and are caused either by 1) a Ca(2+)-induced stiffening of titin or by 2) promoting titin binding to actin. In this study, we performed two sets of experiments in which activated fibers (pCa(2+) 4.5) treated with the myosin inhibitor blebbistatin were stretched from 2.7 to 2.8 μm at a speed of 40 L(o)/s, first, after partial extraction of TnC, which inhibits myosin-actin interactions, or, second, after treatment with gelsolin, which leads to the depletion of thin (actin) filaments. We observed that the static tension, directly related with the residual force enhancement, was not changed after treatments that inhibit myosin-actin interactions or that deplete fibers from troponin C and actin filaments. The results suggest that the residual force enhancement is caused by a stiffening of titin upon muscle activation but not with titin binding to actin. This finding indicates the existence of a Ca(2+)-regulated, titin-based stiffness in skeletal muscles.

Non-crossbridge stiffness in active muscle fibres

“…Another study has reported that an increase in calcium concentration reduced rather than increased titin-actin interaction (Stuyvers et al, 1998). In agreement with these results, Cornachione and Rassier (2012) observed that static tension was preserved after depletion with gelsolin of actin filaments, suggesting that titin-actin binding is not involved.…”

“…Later static stiffness was observed in intact mammalian skeletal muscle fibres (Roots et al, 2007;Colombini et al, 2009;Nocella et al, 2012Nocella et al, , 2014 and in permeabilized fibres (Campbell and Moss, 2002;Cornachione and Rassier, 2012). More recently, it has also been shown in skeletal muscle myofibrils .…”

“…1) and that calcium modulates the bending stiffness of the PEVK fragments (Labeit et al, 2003), titin seems to have all the characteristics required to explain the static stiffness properties. There are two possible mechanisms: the first is a direct effect of calcium on titin stiffness via calcium binding to the E-rich motifs (Tatsumi et al, 2001;Labeit et al, 2003;Cornachione and Rassier, 2012;Rassier et al, 2015), which could stabilize the PEVK segment, making it stiffer (Fig. 1C); the second involves an effect of calcium on titin-actin interactions that induces an increase in the binding between thin filament and titin, increasing sarcomere stiffness (Kellermayer and Granzier, 1996;Linke et al, 1997Linke et al, , 2002Bianco et al, 2007;Leonard and Herzog, 2010;Nishikawa et al, 2012).…”

The mechanisms of the residual force enhancement after stretch of skeletal muscle: non-uniformity in half-sarcomeres and stiffness of titin

“…There is one study that showed an increase in stiffness after stretch when compared to isometric contractions produced at corresponding lengths [16]. However, the increase in stiffness was attributed to non-cross bridges structures (see §3a(ii)).…”

“…While some of these studies focused on the residual force enhancement [10,11,14], others aimed to evaluate the molecular aspects of muscle contraction [15][16][17][18]. The latter studies still added important data for a better understanding of the mechanism of the force enhancement.…”