NERVE-DEPENDENT REGULATION OF SUCCINATE-DEHYDROGENASE IN JUNCTIONAL AND EXTRAJUNCTIONAL COMPARTMENTS OF RAT MUSCLE-FIBERS

1. We studied the distribution of the mitochondrial enzyme succinate d ehydrogenase (SDH) within junctional and extrajunctional compartments of rat soleus muscle fibres. Using quantitative microphotometric imagi ng techniques, we showed that the motor endplate region of soleus fibr es displays SDH activity that is two- and threefold higher than in sub sarcolemmal (SS) and intermyofibrillar (IM) compartments, respectively , and that essentially all endplate SDH activity is of postsynaptic or igin. 2. In addition, we examined the influence of the motor nerve on the regulation of this enzyme within these compartments using denervat ion and tetrodotoxin (TTX)-induced blockade of nerve impulse conductio n. Both models of short-term muscle paralysis reduced SDH activity to a comparable extent (similar to 30%) in both the SS and IM compartment s, suggesting that expression of this enzyme is co-ordinately regulate d in these two regions. Alternatively, denervation and TTX inactivatio n 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 counte rparts, this reduction was significantly less (35%). 3. These findings suggest that motor activity per se is the key factor regulating expre ssion of SDH in non-innervated regions of muscle fibres and that accum ulation of SDH activity within the postsynaptic sarcoplasm is equally subject to local mechanisms involving nerve-derived trophic factors.