INTERMEDIATE FILAMENT PROTEINS INCREASE DURING CHRONIC STIMULATION OFSKELETAL-MUSCLE

Chronic low-frequency electrical stimulation of rabbit fast-twitch ske letal muscle induces increased levels of two intermediate filament pro teins, desmin and vimentin, during the first 3 weeks of stimulation. T hese increases occur over the same timecourse as reported shifts in al pha-adinin expression and increased Z-disc width, but precede the fast -to-slow shifts in contractile proteins, which have been described by others. Desmin and vimentin levels increase during the first 2 weeks o f stimulation, at which time the increase in desmin appears to plateau while vimentin continues to increase significantly through 3 weeks of stimulation. Absolute amounts of vimentin are lower than desmin at al l time points, however increases in desmin and vimentin levels are str ongly correlated during the stimulation period, suggesting that the tw o proteins are coordinately increased during the initial phases of mus cle transformation. We suggest that rapid increases in the expression of intermediate filament proteins, which coincide with alterations in Z-disc structure, may indicate a fortification of the force-bearing ul trastructure of the muscle fibre in response to the increased activity that is induced by stimulation. The presence of vimentin and elevated levels of desmin expression suggest that mature skeletal muscle rever ts toward a developmental program of intermediate filament protein exp ression during fast-to-slow transformation.