Stimulation‐induced expression of slow muscle myosin in a fast muscle of the rat

Abstract

Fast muscles of the rat hind limb were stimulated continuously at 10 or 20 Hz for periods of 55-61 days by means of an implantable neuromuscular stimulator. Gel electrophoresis clearly demonstrated the presence in stimulated muscles of slow myosin light and heavy chains, although fast isoforms were still present in all cases. Thus, contrary to previous reports, induction of slow myosin isofonns does occur in this, as in other, mammalian species. The time course of the response to stimulation appears to be more extended than that seen in the rabbit.

Fast-to-slow transformation in stimulated rat muscle

“…We have commented before on the variability of the changes in myosin isoforrns. 22 There was a similar variability in the extent ofchangt. i n the mechanical properties, but the degree of slowing is well correlated with the degrce of biochemical change, reflected in the present study by the proportions of the different fiber types.…”

Reinnervation of Vastus lateralis is increased significantly in seniors (70-years old) with a lifelong history of high-level exercise

“…Thus the actual expression pattern of myosin isoforms in the elderly is modulated by complex factors because it depends upon the conflicting influences of both aging and reduced activity tending to shift toward slow and fast isoforms, respectively [6]. To further complicate the situation, conflicting results regarding fast to slow myosin transition arise in endurance training studies using animal models and in clinical trials of humans involving either voluntary exercise or electrical stimulation (directly to muscle or indirectly through nerve stimulation) [3,4,11,18,21,24,26]. Whether these shifts are under neural control or the direct effect of use/disuse on muscle fibers remains to be clarified.…”

Regenerated rat fast muscle transplanted to the slow muscle bed and innervated by the slow nerve, exhibits an identical myosin heavy chain repertoire to that of the slow muscle

“…Most notably, the percentage of MHC-1 in various transformed EDL muscles in these studies never exceeded 10%. A few studies arguing against this view presented very variable results in regard to the content of the slow myosin in transformed rat EDL muscles (one complete transformation per 5-9 treated muscles, and very little slow myosin in half or more of them), giving no explanation for such a variability (Hoh et al 1980;Mira et al 1992;Mayne et al 1993). The latter results are not sufficient for rejecting the general conclusion that mature non-injured EDL muscle of an adult rat is fairly resistant to nerve-induced transformation into a slow muscle, and that the EDL and SOL muscles of the rat have different adaptive ranges .…”

“…However, transformations of muscle fibres in regard to their myosin heavy chain (MHC) isoform expression and, consequently, changes of fibre types, are possible only within a limited adaptive range, characteristic for muscle type and species Pette and Vrbova 1992). Although fast muscles of the rat can change their contractile properties, histochemical profile or expression of MHC isoforms towards those of a slow muscle, it has never been proved that they can be completely and reproducibly transformed into slow muscle type, either by cross-innervation or by chronic electrical stimulation of normal or denervated muscles (Close 1969;Carraro et al 1986;Eken and Gundersen 1988;Westgaard and Lømo 1988;Termin et al 1989;Ausoni et al 1990;Mira et al 1992;Mayne et al 1993;Delp and Pette 1994). It has been assumed that the limitation of a muscle's adaptive range may stem from intrinsic properties of its myogenic stem cells Ausoni et al 1990).…”