Activation-induced reduction of SDH activity in diaphragm muscle fibers

In this study, we examined whether exhaustive activation reduces succinate dehydrogenase (SDH) activity in diaphragm muscle fibers. In adult male rats (~300 g), the costal diaphragm was excised and positioned in a chamber perfused with mammalian Ringer solution kept at 26°C and oxygenated with 95% O₂-5% CO₂. The muscle was stimulated directly at 10 or 75 Hz in trains of 500 ms duration (1/s) for 8 min. An adjacent unstimulated segment of muscle served as control. The two muscle segments were frozen, and serial sections were stained for myofibrillar adenosinetriphosphatase activity after alkaline and acid preincubation to classify type I, IIa, and IIb fibers. The extent of glycogen utilization was also examined histochemically to confirm exhaustive activation of muscle fibers. SDH activity was quantified using a microdensitometric procedure implemented on an image-processing system. Exhaustive activation at both 10 and 75 Hz caused a significant decrease in SDH activity of all fiber types, with the decrease after 10-Hz stimulation being greater than that after 75-Hz stimulation. At both stimulation frequencies, type IIb fibers demonstrated the greatest decrease in SDH activity (36% after 10-Hz and 27% after 75-Hz stimulation), whereas type I and IIa fibers both displayed reductions of ~27 and ~19% after 10- and 75- Hz stimulation, respectively. The greater reduction of SDH activity in type IIb fibers indicates an inverse relationship between activation-induced reductions in SDH activity and fiber oxidative capacity. On the basis of these results, we hypothesize that oxygen free radicals, known inhibitors of mitochondrial respiration, may be important in regulating the reduction of SDH activity during exhaustive activation.