MHC polymorphism in rodent plantaris muscle: effects of mechanical overload and hypothyroidism

In a previous study, it was shown that acombined treatment of hyperthyroidism and hindlimb suspensioneffectively converted the slow-twitch soleus muscle to a fast-twitchmuscle. The objective of this study was to test the hypothesis thathypothyroidism [absence of triiodothyronine(T₃)] and mechanical overload (OV) would convert the plantaris (Plan) muscle from a fast- to a slow-twitch muscle. Single-fiber analyses demonstrated that the normal rodent Planmuscle was composed of ~13 different fiber types as defined by myosinheavy chain (MHC) isoform content. The largest proportion of fibers(~35%) coexpressed the fast type IIX and IIB MHC isoforms (i.e.,type IIX/IIB fibers). In this context, the combined intervention ofT₃ and OV produced a significant reduction in therelative proportion of the fast type IIB MHC isoform and a concomitant increase in the slow type I MHC isoform. These transitions were manifested by a large decrease in the proportion of type IIX/IIB fibersand a large increase in fibers coexpressing all four MHC proteinisoforms. The mechanical consequences of these transitions, however,were modest, producing a 15% decrease in maximal shortening velocity.The findings of this study demonstrate that T₃ + OV does produce a partial shift toward a slower phenotype;however, the high degree of polymorphism found in the Plan musclerepresents a unique design that appears to minimize the functionalconsequences of these significant MHC transitions.