Force enhancement after stretch in mammalian muscle fibre: no evidence of crossbridge involvement.

Force enhancement after stretch in mammalian muscle fiber: no evidence of cross-bridge involvement. Am J Physiol Cell Physiol 307: C1123–C1129, 2014. First published October 8, 2014; doi:10.1152/ajpcell.00290.2014.—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 um in normal and N-benzylp- toluene sulphonamide (BTS)-added (10 uM) 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 um, and decreased to zero at u4.5 um. At 4 um, where active force was zero, ST was still 50% of maximum. BTS reduced force by about 75% but had almost no effect on ST. Following stimulation, ST developed earlier than force, with a time course similar to internal Ca2+ concentration: it was present 1 ms after the stimulus, at zero active force, and peaked at about 3-ms delay. At 2.7 um, activation increased the passive sarcomere stiffness by a factor of about 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 Ca2+- induced stiffening of a sarcomeric structure identifiable with titin