The purpose of this study was to ascertain the time course of change during both compensatory growth (
hypertrophy) and subsequent growth regression on
myosin isoform expression in rodent fast-twitch plantaris muscle in response to functional overload (induced by removal of synergists). Peak
hypertrophy of the plantaris muscle (92%) occurred after 9 wk of overload. After 7 wk of overload regression (induced by a model of hindlimb unweighting), muscle weight returned to within 30% of control values. Myofibril
protein content (mg/g muscle) remained relatively constant throughout the overload period but became significantly depressed relative to control values after 7 wk of regression. However, when expressed on a per muscle basis (mg/muscle) no differences existed at this time point (t = 7 wk regression). The distribution of native
myosin isoforms in the myofibril
protein pool of the overloaded plantaris muscle reflected a progressive increase (23% at t = 9 wk; P less than 0.001) in the relative proportion of slow
myosin (Sm). This change was also accompanied by increases in intermediate
myosin (Im) as well as the repression of the fast
myosin one (Fm1)
isoform (P less than 0.001). These shifts in Sm and Fm1
isoform expression were gradually reversed during the regression period, whereas Im remained elevated relative to control values. These adaptive changes in
myosin isoform expression during both
hypertrophy and regression were further supported by concomitant shifts in both
myosin adenosinetriphosphatase (
ATPase) activity (decreased during overload) and slow
myosin light chain (SLC) expression. However, during regression the changes in
myosin isoform expression and
myosin ATPase were not as synchronous as they were during overload. Estimation of the mixed
myosin heavy chain (MHC) half-life (t 1/2), using a linear model that assumes zero-order synthesis and first-order degradation kinetics, revealed t 1/2 values of approximately 19 and 10 days for the overload and regression periods, respectively. Collectively these data suggest that 1) skeletal muscle
myosin isoforms and corresponding
ATPase activity are in a dynamic state of change, although not completely synchronous, in response to altered muscle stress, and 2) the kinetics of change in the mixed MHC
protein pool are slower during compensatory growth compared with regression of growth.