Regulation of succinate dehydrogenase within muscle fiber compartments by nerve-mediated activity and CNTF

We investigated the regulatory effects of neural activation and trophic factors on the selective accumulation of succinate dehydrogenase (SDH; EC 1.3.99.1) activity at the endplate, as well as within subsarcolemmal and intermyofibrillar regions of rat soleus muscle fibers. The role of activation was assessed by stimulation of tetrodotoxin (TTX)-inactivated nerves distal to the site of drug application. We also studied whether ciliary neurotrophic factor (CNTF) is involved in regulating the postsynaptic accumulation of SDH. Using quantitative microphotometry, we found that daily stimulation of quiescent but intact nerves prevented the TTX-induced decrease in SDH activity within extrajunctional regions, whereas, at the endplate, the counteraction was partial (30%). Thus it appears that endplate levels of this enzyme are regulated by a complex mechanism involving an interaction between neuromuscular activation and trophic factors. We also found that daily CNTF administration counteracted the denervation-associated loss of SDH activity exclusively within the intermyofibrillar compartment, suggesting that CNTF treatment mimics the effects of activity on SDH levels within the core region of denervated fibers, but under these conditions does not influence the endplate accumulation of this enzyme.