The origin of passive force enhancement in skeletal muscle

Abstract

The aim of the present study was to test whether titin is a calcium-dependent spring and whether it is the source of the passive force enhancement observed in muscle and single fiber preparations. We measured passive force enhancement in troponin C (TnC)-depleted myofibrils in which active force production was completely eliminated. The TnC-depleted construct allowed for the investigation of the effect of calcium concentration on passive force, without the confounding effects of actin-myosin cross-bridge formation and active force production. Passive forces in TnC-depleted myofibrils ( n = 6) were 35.0 ± 2.9 nN/ μm2 when stretched to an average sarcomere length of 3.4 μm in a solution with low calcium concentration (pCa 8.0). Passive forces in the same myofibrils increased by 25% to 30% when stretches were performed in a solution with high calcium concentration (pCa 3.5). Since it is well accepted that titin is the primary source for passive force in rabbit psoas myofibrils and since the increase in passive force in TnC-depleted myofibrils was abolished after trypsin treatment, our results suggest that increasing calcium concentration is associated with increased titin stiffness. However, this calcium-induced titin stiffness accounted for only ∼25% of the passive force enhancement observed in intact myofibrils. Therefore, ∼75% of the normally occurring passive force enhancement remains unexplained. The findings of the present study suggest that passive force enhancement is partly caused by a calcium-induced increase in titin stiffness but also requires cross-bridge formation and/or active force production for full manifestation.

Force produced by isolated sarcomeres and half-sarcomeres after an imposed stretch

“…muscle fibers (10,12,17,46) and myofibrils (41) and well described by the predictions of overlap between myosin and actin filaments (32,40). The forces produced by single sarcomeres in the current study are in the range of those observed with myofibrils (23,35,36), single fibers (6), and single sarcomeres (32). Of high importance for this study, sarcomeres underwent three to four consecutive activation-relaxation cycles (with or without stretches), without a significant decrease in force (Ͼ5%), which gives us confidence that forces measured during and/or after stretch are reliable; isometric contractions were always performed after the stretches to assure the preparations were not damaged.…”

“…Furthermore, in several preparations, half-sarcomere nonuniformities were not observed and still showed a residual force enhancement after stretch, suggesting that it contains the participation of a sarcomeric structure. In recent years, it has been suggested that titin, responsible for most of the passive forces in muscle fibers and myofibrils, could be responsible for the residual force enhancement (23,28,29,33). Conceptually, there are three mechanisms by which titin could be independently tuned by Ca 2ϩ .…”

“…The force remains elevated after the stretch to reach a steady-state level that is higher than that produced during isometric contractions at corresponding lengths (8,24,38). The mechanisms responsible for this "residual force enhancement" remain elusive and there is substantial disagreement in the literature.Recent studies with isolated myofibrils, preparations in which individual sarcomeres (22,23,36,37,41,43) and half-sarcomeres in series (44, 45) can be evaluated, have shown intriguing results. Whereas a few studies suggest that force enhancement could be linked to sarcomeres and half-SL nonuniformity, as half-sarcomeres change their length continuously during activation and relaxation (36,44,45), some others show that sarcomeres may be stable (22,37), and that the increase in force after stretch are caused by contractile and/or passive structures.…”

Titin force is enhanced in actively stretched skeletal muscle

“…Residual force enhancement increases with increased magnitude of stretch and is greater as the muscle is stretched further on the descending limb of the force-length relationship (Edman et al, 1982). Additional studies have observed enhanced passive force in muscles following active stretch (Herzog and Leonard, 2002;Labeit et al, 2003;Joumaa et al, 2008), alluding to a passive element in the property of force enhancement.…”

Variation in isometric force after active shortening and lengthening and their mechanisms: a review

“…This theory is based on the scanning mechanism of the elastic force in parallel, with the titin filament assuming the main role (32,39,66,79,80,89,90,91,92,93). Such filaments would be responsible for producing the voltage required to maintain the centering, the length and the ideal overlap between the actin and myosin.…”