ARE SKELETAL MUSCLE FIBERS OBLIGATED TO FOLLOW THE IlrarrIIAlrarrIIXlrarrIIB SCHEME OF MyHC ISOFORM PLASTICITY?

Abstract

219 It has been proposed that transitions in myosin heavy chain (MyHC) isoform expression must follow an obligatory scheme summarized as follows: IlrarrIIAlrarrIIXlrarrIIB. This scheme stipulates, for instance, that slow muscle fibers can express the fast Type IIB MyHC but only after the sequential expression of the fast Type IIA and IIX MyHC isoforms. This concept was tested by using a combined treatment of hyperthyroidism and hindlimb suspension(T3+HS; n=24 animals), and comparing the MyHC isoform composition of single fibers taken from the soleus muscles in these animals with that of single fibers obtained from control animals (CON; n=24). Muscles (n=8 per group and time point) were harvested at 1, 2, and 4 wks of treatment. Approximately 40 fibers/muscle (total = 1,870 fibers) were individually isolated by microdissection, and the MyHC isoform composition of each fiber determined by electrophoresis. Combinations of MyHC isoforms found to be inconsistent with the IlrarrIIAlrarrIIXlrarrIIB scheme are shown below, and reported as a percent of the entire population of fibers for that group and time point. The major findings of this study demonstrate that: 1) the combined treatment of T3+HS creates transitions in single fiber MyHC isoform composition that are inconsistent with IlrarrIIAlrarrIIXlrarrIIB scheme of MyHC isoform plasticity; and 2) as exemplified by the I/IIB and I/IIX/IIB hybrid fibers, time-course studies are essential for accurately identifying schemes of MyHC isoform transitions at the single fiber level. TableTable