Publication Date:
2014-12-16
Description:
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.
Print ISSN:
0363-6143
Electronic ISSN:
1522-1563
Topics:
Medicine
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