TRANSFORMATION OF SLOW-TWITCH OR FAST-TWITCH RABBIT MUSCLES AFTER CROSS-REINNERVATION OR LOW-FREQUENCY STIMULATION DOES NOT ALTER THE IN-VITRO PROPERTIES OF THEIR SATELLITE CELLS

We previously showed that satellite cells isolated from rabbit fast-tw itch and slow-twitch muscles presented different behaviours in culture ; cells from slow muscle differentiated more quickly and fused into mo re numerous myotubes than those from fast muscle. Moreover, only slow- muscle derived satellite cells expressed in vitro the slow type I myos in heavy chain isoform (MyHC). We wanted to investigate whether the pr operties of satellite cells originating from different muscles were un der the influence of the adult fibre type on which they were located. For this purpose, we transformed the properties of the adult rabbit fa st-twitch semimembranosus accessorius (SMa; similar to 100% type II fi bres) and the slow-twitch semimembranosus proprius (SMp; 100% type I f ibre) muscles by (1) cross-reinnervating the SMp with the main branch of the fast SMa nerve; or (2) electrical stimulation at 10 Hz of the S Ma muscle. We studied their satellite cells in vitro. Five-month cross -reinnervation of the SMp induced a large shift of its MyHC type chara cteristics towards those of a fast muscle, and three-month electrical stimulation at low frequency transformed the fast-twitch SMa into a sl ow-twitch muscle, as shown by SDS-PAGE of MyHC. In spite of the transf ormation of their muscle characteristics, satellite cells in culture k ept their original properties. Indeed, as shown by MyoD and myogenin g ene expression as markers of fusion, satellite cells isolated from cro ss-reinnervated and from control SMp began to fuse by eight days of cu lture, and expressed MyoD and myogenin at that stage. Later they diffe rentiated into numerous myotubes. Satellite cells isolated from electr ically stimulated and control SMa presented a similar behaviour in cul ture: they did not express MyoD and myogenin at eight days, and fused by ten days into only a few myotubes. Moreover, MyHC gene expression s howed that, in contrast with slow-muscle derived satellite cells, the type I MyHC gene was not expressed by satellite cells isolated from th e stimulated SMa in spite of its homogeneous type I fibre composition. Taken together, these data support the idea that once constituted, mu scle fibre types per se do not influence the properties of their assoc iated satellite cells.