Force response to stretches in activated frog muscle fibres at low tension.

Abstract

It is well known that tension development in skeletal muscle fibres upon electrical activation is preceded by an increase of muscle stiffness that begins during the latent period and continues throughout the whole tension rise in both twitch or tetanic contractions (Bressler and Clinch, 1974; Cecchi et al., 1982; Ford et al., 1986). At moderate and high tensions, fibre stiffness increase is essentially due to crossbridge attachment. However, as shown previously (Bagni et al., 1994; Bagni et al., 2002), at zero tension during the latent period and at very low tension during force development, a substantial contribution to fibre stiffness cames from some (unknown) sarcomere structure(s), outside the crossbridges, whose stiffness increases upon stimulation. The presence of this stiffness was inferred from the force response to stretches (and hold) applied to a single muscle fibre during force generation. It was found that the fast force transient produced by the stretch was followed by a period during which the tension settled to a consistent level greater than the isometric tension at the time of the stretch, until relaxation or until the fibre returned to the original length at end of the stretch. Because of this characteristic the excess of tension respect to isometric was referred to as static tension while the ratio between static tension and stretch amplitude, measured at sarcomere level, was termed static stiffness. Experiments made on tetanic contractions in Ringer containing 1-6 mM of 2,3-butanedione monoxime (BDM), an agent which strongly inhibits crossbridge formation (Horiuti et al., 1988; Bagni et al., 1992) without altering static stiffness (Bagni et al., 1994), showed that the structure responsible for static stiffness behaves with Hookean elasticity located in parallel with the crossbridges (Bagni et al., 2002). Interestingly, in both twitch or tetanic contractions, static stiffness development followed a characteristic time course distinct from that of tension and roughly similar to that of internal calcium concentration.