Comparison of mechanical properties of single intact fibres from wild-type and MLC/mIGF-1 mouse muscle.

In this study we compared the mechanical properties of single intact muscle fibres of wild-type (WT) and MLC/mIgf-1 (TG) mice, in which the localized Igf-1 transgene expression sustains hypertrophy (Musarò et al., Nat. Genet. 27, 2001). The study has been focussed on “static stiffness” (SS), a non crossbridge calcium-dependent stiffness previously identified in activated frog muscle fibres (Bagni et al., Biophys. J. 82, 2002). Single intact fibres, dissected from the flexor digitorum brevis muscle, were mounted in an experimental chamber (~23°C) between the lever arms of a force transducer and of an electromagnetic motor to apply fast stretches. Sarcomere length was measured by means of a videocamera and with laser diffraction. Tetanic tension and force-velocity relation in WT and TG mice were not significantly different, however, the maximum shortening velocity (Vmax) was faster than previously reported and comparable with frog muscle. Compared to frog fibres, the plateau of length-tension relation shifted according to the different myofilament lengths. TG fibres exhibited an increase in diameter and maximum force, but specific force was the same as for WT fibres. SS was present either in WT or in TG fibres and its time course, independent from isometric tension, was faster than in frog. A preliminary analysis suggests that the only significant mechanical difference between WT and TG fibres is in the SS properties. This may be related to a different compliance of the structure responsible of the SS that we speculated could be titin.