DISRUPTION AND REPAIR OF THE INTERMEDIATE FILAMENT LATTICE IN RAT SKELETAL MUSCLE AFTER DAMAGING LENGTHENING EXERCISE by M.R. DROST* and M.K.C. HESSELINK (Dept. of MovementSciences, Maastricht University. P.O. Box 616, 6200 MD Maastricht, the Netherlands) ABSTRACT In the present paper the effect of repeated lengthening exercise on the expression of cytoskeletal proteins was studied in rat skeletal muscle. Tibialis anterior muscles were subjected to an exercise session consisting of 240 lengthening (L) or shortening (S) contractions. Muscles were excised 4 days post-exercise. Contralateral muscles served as controls (C). Western blotting was done with antibodies against desmin and vimentin. In C and S immunostaining yielded a regular cross-striated pattern, in L however, fibers can be detected without immunostaining. Western blots showed no major differences in the amount of desmin between C, S and L, although some proteolysis was present in L. Vimentin was absent in C and S, and prominently present in L. These data indicate that remodeling of the intermediate filament lattice after damage induced by lengthening exercise occurs in a differentiation like manner. INTRODUCTION Muscles subjected to lengthening exercise are vulnerable to structural muscle damage. It is well known that directly following lengthening ex- ercise focal disruptions in the A-band and localised dissolution of Z- lines are detectable (OGILVIE et al., 1988). In healthy mature skeletal muscle, Z-lines are kept in register by the intermediate filament desmin (GRANGER & LAZARIDES, 1979; LAZARIDES & HUBBARD, 1976). The observation that adjacent Z-lines are no longer in line following length- ening exercise is commonly attributed to loss of desmin (WATERMAN STORER, 1991). We and others have previously reported an immediate focal loss of desmin upon lengthening exercise (KOMULAINEN et al., 1998; LIEBER et al., 1996). In developing and regenerating muscle, in- tracellular expression of vimentin precedes expression of desmin, then both intermediate filaments are shortly co-expressed. Vimentin is largely * Corresponding author; e-mail: maarten.drost@bw.unimaas.n)