Tropomyosin Transformation Accompanying Muscle Fibre Adaptations With Compensatory Hypertrophy

Tropomyosin (TM), a myofibrillar protein of the thin filament exists in several isoforms and the complement of these isoforms has been demonstrated to be specific to the muscle fibre type. By 30 days of overload, the compensatory overload model induces in the rat plantaris muscle a fibre type transformation which demonstrates an increase in the histochemically determined type 1 fibre content. Overload of the plantaris muscle was induced in male Sprague-Dawley rats by surgical ablation of the ipsilateral gastrocnemius. The animals were sacrificed at various times post-ablation (2, 5, 7, 15, and 30 days). Both control (CP) and hypertrophied plantaris (HP) muscle samples (homogenates) were subjected to electrophoretic analysis. Separation of contractile proteins was achieved by employing single dimension SDS-PAGE. Analysis of the Coomassie Blue stained protein bands by laser densitometry revealed a rapid shift in the TM composition of overloaded muscles such that a significant decrease in the ratio of the {dollar}\alpha{dollar}:{dollar}\beta{dollar} isoforms was observed as early as 5 days and persisted up to 30 days post-surgery. Muscle homogenates from animals sacrificed at 2, 5, 7, 15, 21, 30 and 77 days post-ablation were subjected to two-dimensional IEF/SDS-PAGE. Significant shifts in the {dollar}\alpha{dollar}:{dollar}\beta{dollar} TM ratio were again detected following only 5 days of overload (1.46 {dollar}\pm{dollar}.33 vs 1.96 {dollar}\pm{dollar}.22 (p {dollar}\u3c{dollar} 0.05) in the control muscles). These significant differences persisted through the 30 day overload period. The TM profile was not transformed to that typical of slow muscle. Moreover, by 77 days post-ablation, the HP muscle revealed an {dollar}\alpha{dollar}:{dollar}\beta{dollar} ratio (1.53 {dollar}\pm{dollar}.25) resembling that of a FT muscle and differed significantly from 30 day HP muscle (.97 {dollar}\pm{dollar}.24). The importance of this initial rapid, but transitory shift in the TM isoform ratio in the overloaded plantaris would appear to be associated with muscle fibre degeneration-regeneration. Muscle histology in HP demonstrated focal disruption to muscle fibres and accumulation of macrophages in the interstitium which appeared by 2 days and peaked by 7 days. Furthermore, histological evidence may indicate that the damage to HP muscle was confined to the type II fibre population. In addition, muscle regeneration, first observed at 7 days, was nearly complete by 30 days. (Abstract shortened with permission of author.