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

In a previous study, it was shown that a combined treatment of hyperthyroidism and hindlimb suspension effectively converted the slow-twitch soleus muscle to a fast-twitch muscle. The objective of this study was to test the hypothesis that hypothyroidism [absence of triiodothyronine (-T(3))] 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 Plan muscle was composed of approximately 13 different fiber types as defined by myosin heavy chain (MHC) isoform content. The largest proportion of fibers ( approximately 35%) coexpressed the fast type IIX and IIB MHC isoforms (i.e., type IIX/IIB fibers). In this context, the combined intervention of -T(3) and OV produced a significant reduction in the relative 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 fibers and a large increase in fibers coexpressing all four MHC protein isoforms. The mechanical consequences of these transitions, however, were modest, producing a 15% decrease in maximal shortening velocity. The findings of this study demonstrate that -T(3) + OV does produce a partial shift toward a slower phenotype; however, the high degree of polymorphism found in the Plan muscle represents a unique design that appears to minimize the functional consequences of these significant MHC transitions.