Expression of developmental myosin and morphological characteristics in adult rat skeletal muscle following exercise-induced injury

The extent and stability of the expression of developmental isoforms of myo sin heavy chain (MHCd), and their association with cellular morphology, wer e determined in adult rat skeletal muscle fibres following injury induced b y eccentrically-biased exercise. Adult female Wistar rats [274 (10) g] were either assigned as non-exercised controls or subjected to 30 min of treadm ill exercise (grade, -16 degrees, speed, 15 m . min(-1)), and then sacrific ed following 1, 2, 4, 7, or 12 days of recovery (n = 5-6 per group). Histol ogically and immunohistologically stained serial, transverse cryosections o f the soleus (S), vastus intermedius (VI), and tibialis anterior (TA) muscl es were examined using light microscopy and digital imaging. Fibres stainin g positively for MHCd (MHCd+) were seldom detected in the TA. In the VI and S. higher proportions of MHCd+ fibres (0.8% and 2.5%, respectively) were o bserved in rats at 4 and. 7 days post-exercise, in comparison to all other groups combined (0.2%, 1.2%; P less than or equal to 0.01). In S, MHCd+ fib res were observed less frequently by 12 days (0.7%) than at 7 days (2.6%) f ollowing exercise. The majority (85.1%) of the MHCd+ fibres had morphologic al characteristics indicative of either damage, degeneration, repair or reg eneration. Most of the MHCd+ fibres also expressed adult slow, and/or fast myosin heavy chain. Quantitatively, the MHCd+ fibres were smaller (<2500 mu m(2)) and more angular than fibres not expressing MHCd. Thus, there was a transient increase in a small, but distinct population of MHCd+ fibres foll owing unaccustomed, functional exercise in adult rat S and VI muscles. The observed close coupling of MHCd expression with morphological changes withi n muscle fibres suggests that these characteristics have a common, initial exercise-induced injury-related stimulus.