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Departments of Physiology and Biophysics and Orthopaedics, College of Medicine, University of California, Irvine, California 92717
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
(
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 Plan
muscle was composed of ~13 different fiber types as defined by myosin
heavy 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 of
T3 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 T3 + 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.
single fiber; messenger RNA; rat; hybrid fibers; protein
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