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

In a previous study, it was shown that a combined treatment of hyperthyroid ism and hindlimb suspension effectively converted the slow-twitch soleus mu scle to a fast-twitch muscle. The objective of this study was to test the h ypothesis that hypothyroidism [absence of triiodothyronine (-T-3)] and mech anical overload (OV) would convert the plantaris (Plan) muscle from a fast- to a slow-twitch muscle. Single-fiber analyses demonstrated that the norma l rodent Plan muscle was composed of similar to 13 different fiber types as defined by myosin heavy chain (MHC) isoform content. The largest proportio n of fibers (similar to 35%) coexpressed the fast type IIX and IIB MHC isof orms (i.e., type IIX/IIB fibers). In this context, the combined interventio n of -T3 and OV produced a significant reduction in the relative proportion of the fast type TIE MHC isoform and a concomitant increase in the slow ty pe I MHC isoform. These transitions were manifested by a large decrease in the proportion of type IIX/IIB fibers and a large increase in fibers coexpr essing all four MHC protein isoforms. The mechanical consequences of these transitions, however, were modest, producing a 15% decrease in maximal shor tening velocity. The findings of this study demonstrate that -T-3 + OV does produce a partial shift toward a slower phenotype; however, the high degre e of polymorphism found in the Plan muscle represents a unique design that appears to minimize the functional consequences of these significant MHC tr ansitions.