title: Force enhancement after stretch in mammalian muscle fiber: no evidence of cross-bridge involvement.
abstract: Stretching of activated skeletal muscles induces a force increase above the isometric level persisting after stretch, known as residual force enhancement (RFE). RFE has been extensively studied; nevertheless, its mechanism remains debated. Unlike previous RFE studies, here the excess of force after stretch, termed static tension (ST), was investigated with fast stretches (amplitude: 3-4% sarcomere length; duration: 0.6 ms) applied at low tension during the tetanus rise in fiber bundles from flexor digitorum brevis (FDB) mouse muscle at 30°C. ST was measured at sarcomere length between 2.6 and 4.4 μm in normal and N-benzyl-p-toluene sulphonamide (BTS)-added (10 μM) Tyrode solution. The results showed that ST has the same characteristics and it is equivalent to RFE. ST increased with sarcomere length, reached a peak at 3.5 μm, and decreased to zero at ∼4.5 μm. At 4 μm, where active force was zero, ST was still 50% of maximum. BTS reduced force by ∼75% but had almost no effect on ST. Following stimulation, ST developed earlier than force, with a time course similar to internal Ca(2+) concentration: it was present 1 ms after the stimulus, at zero active force, and peaked at ∼3-ms delay. At 2.7 μm, activation increased the passive sarcomere stiffness by a factor of ∼7 compared with the relaxed state All our data indicate that ST, or RFE, is independent of the cross-bridge presence and it is due to the Ca(2+)-induced stiffening of a sarcomeric structure identifiable with titin.
authors.affiliation: Department of Experimental and Clinical Medicine, University of Florence, Florence, Italy; and Interuniversity Institute of Myology, Italy.
authors.forename: Marta
authors.initials: M
authors.lastname: Nocella
authors.affiliation: Department of Experimental and Clinical Medicine, University of Florence, Florence, Italy; and Interuniversity Institute of Myology, Italy.
authors.forename: Giovanni
authors.initials: G
authors.lastname: Cecchi
authors.affiliation: Department of Experimental and Clinical Medicine, University of Florence, Florence, Italy; and Interuniversity Institute of Myology, Italy.
authors.forename: Maria Angela
authors.initials: MA
authors.lastname: Bagni
authors.affiliation: Department of Experimental and Clinical Medicine, University of Florence, Florence, Italy; and Interuniversity Institute of Myology, Italy barbara.colombini@unifi.it.
authors.forename: Barbara
authors.initials: B
authors.lastname: Colombini
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism; D004548:Elasticity; D056966:Excitation Contraction Coupling; D008297:Male; D008810:Mice, Inbred C57BL; D009119:Muscle Contraction; D018485:Muscle Fibers, Skeletal / Q000378:metabolism / Q000502:physiology*; D053580:Muscle Strength; D011494:Protein Kinases / Q000378:metabolism; D012026:Reflex, Stretch; D012518:Sarcomeres / Q000502:physiology; D013997:Time Factors
chemical_list: D011494:Protein Kinases; C579730:titin protein, mouse; D002118:Calcium
keywords: force enhancement; non-cross-bridge force; static tension; stretch; titin