Succinate dehydrogenase activity within synaptic and extrasynaptic compartments of functionally-overloaded rat skeletal muscle fibers.

Activity of the mitochondrial enzyme succinate dehydrogenase (SDH) was assessed using quantitative microphotometric techniques within postsynaptic, subsarcolemmal and intermyofibrillar compartments of overloaded soleus muscle fibers. Six weeks of overload, induced via synergist tenotomy, significantly increased soleus muscle mass (23%) and mean fiber cross-sectional area (17%). Despite these increases in cell size, SDH activity within all three intracellular compartments of overloaded muscle fibers was not different from levels in corresponding regions of control fibers. Thus, we show for the first time that activity-related increases in muscle cell volume, and specifically motor endplate area, appear to be coordinated with increased levels of oxidative enzymes within distinct subcellular compartments, including the postsynaptic sarcoplasm.