We studied the distribution of the mitochondrial enzyme succinate dehydrogenase (SDH) within junctional and extrajunctional compartments of rat soleus muscle fibres. Using quantitative microphotometric imaging techniques, we showed that the motor endplate region of soleus fibres displays SDH activity that is two- and threefold higher than in subsarcolemmal (SS) and intermyofibrillar (IM) compartments, respectively, and that essentially all endplate SDH activity is of postsynaptic origin. 2. In addition, we examined the influence of the motor nerve on the regulation of this enzyme within these compartments using denervation and tetrodotoxin (TTX)-induced blockade of nerve impulse conduction. Both models of short-term muscle paralysis reduced SDH activity to a comparable extent (approximately 30%) in both the SS and IM compartments, suggesting that expression of this enzyme is co-ordinately regulated in these two regions. Alternatively, denervation and TTX inactivation led to distinct alterations at the level of the motor endplate. SDH activity at denervated endplates was dramatically reduced (by 60%) in comparison to controls, whereas at endplates of TTX-inactivated counterparts, this reduction was significantly less (35%). 3. These findings suggest that motor activity per se is the key factor regulating expression of SDH in non-innervated regions of muscle fibres and that accumulation of SDH activity within the postsynaptic sarcoplasm is equally subject to local mechanisms involving nerve-derived trophic factors.
