The maximum velocity of shortening during the early phases of the contraction in frog single muscle fibres

Abstract

The maximum velocity of shortening (Vmax) was determined at preset times during the development and the plateau of isometric tetani in single fibres isolated from the tibialis anterior muscle of the frog. Experiments were performed at low temperature (3.6-6 degrees C) and at about 2.25 micron sarcomere length. The controlled velocity release method was used. Vmax was measured by determining the lowest velocity of release required to keep the tension at zero. Extreme care was taken in dissection and mounting of the fibres in order to make the passive series compliance very small. The value of Vmax at the end of the latent period for the development of isometric tension (at 4.5 degrees C about 10 ms after the beginning of the stimulus volley) was already the same as later during either the tension rise or at the plateau of isometric tetani. These results show that the value of Vmax of intact fibres is independent of time and activation subsequent to the latent period, and suggest that the cycling rate of the crossbridges may thus attain its steady-state value just at the end of the isometric latent period.

Structural changes in the myosin filament and cross‐bridges during active force development in single intact frog muscle fibres: stiffness and X‐ray diffraction measurements

“…The end of the latent period has been shown to coincide with the decrease in the resting viscosity of the fibre (Ford et al 1977; Lombardi & Menchetti, 1984). This resting viscosity was first described as a short‐range elastic component and attributed to a small number of cross‐bridges attached at rest (Hill, 1968), but its nature is still unclear (Bagni et al 1995; Proske & Morgan, 1999) When shortening at V 0 is imposed at rest or during the latent period, this viscosity appears as a small compressive force (∼0.005 T 0 ), corresponding to a viscous coefficient of 7 × 10 8 N s m −3 , and vanishes abruptly at the end of the latent period (Lombardi & Menchetti, 1984). This viscous coefficient is similar to that found by stretching resting fibres at a wide range of velocities (Ford et al 1977; Bagni et al 1998).…”

“…The high gain of the mechanical signals reveals that shortening at V 0 applied 5 ms after the start of stimulation (∼5 ms before t s ) generates a small compressive force (∼0.005 T 0 ; marked with an asterisk in Fig. 5) that vanishes at t s (see also Lombardi & Menchetti, 1984). This transient compressive force is due to a resting viscosity that disappears abruptly at the end of the latent period (Ford et al 1977; Lombardi & Menchetti, 1984).…”

“…This transient compressive force is due to a resting viscosity that disappears abruptly at the end of the latent period (Ford et al 1977; Lombardi & Menchetti, 1984). In these fibres T 0 was 230 ± 26 kPa and V 0 was 1.65 ± 0.3 μm s −1 (half‐sarcomere) −1 , so the viscous coefficient is 1.15 kPa/(1650 nm s −1 (half‐sarco‐mere) −1 ) = 7 × 10 8 N s m −3 , similar to values reported for a large range of velocities (Ford et al 1977; Lombardi & Menchetti, 1984). This resistance to shortening also appears in the sarcomere length trace as a period of slower shortening (marked with an asterisk in Fig.…”

“…Before that time, the only mechanical sign of activation in isometric conditions is a small reduction of resting tension (latency relaxation) that occurs just before the end of the latent period and becomes more evident at sarcomere lengths longer than the 2.15 μm used in this work (Lannergren, 1971; Haugen & Sten‐Knudsen, 1976). The end of the latent period has been shown to coincide with the decrease in the resting viscosity of the fibre (Ford et al 1977; Lombardi & Menchetti, 1984). This resting viscosity was first described as a short‐range elastic component and attributed to a small number of cross‐bridges attached at rest (Hill, 1968), but its nature is still unclear (Bagni et al 1995; Proske & Morgan, 1999) When shortening at V 0 is imposed at rest or during the latent period, this viscosity appears as a small compressive force (∼0.005 T 0 ), corresponding to a viscous coefficient of 7 × 10 8 N s m −3 , and vanishes abruptly at the end of the latent period (Lombardi & Menchetti, 1984).…”

The effect of hypertonicity on force generation in tetanized single fibres from frog skeletal muscle.

“…At the same time, it may well be that, due to the larger resting viscosity in hypertonic medium (Edman & Hwang, 1977), some degree of correction is necessary for fibres activated in hypertonic solution. In fact the resting viscosity, determined by imposing steady lengthening at a speed similar to the maximum velocity of shortening of the fibre (Lombardi & Menchetti, 1984), is 5-8 times larger in hypertonic Ringer solution than in normal Ringer solution (Fig. 4).…”