Effects of hypobaric hypoxia on the oxidative capacity of the extensor digitorum longus motor units in the rat.

The fiber number, fiber type distribution, and succinate dehydrogenase activity were investigated from the fast-twitch extensor digitorum longus muscle of male rats exposed to 7 weeks of hypobaric hypoxia. The oxidative metabolic capacity of the motoneurons in the extensor digitorum longus neuron pool was also determined from quantitative histochemical analyses. The fiber number and oxidative enzyme activity of the muscle were not changed by hypoxia. An increase in the percentage of fast-twitch oxidative (FO) fibers and a concomitant decrease in the percentage of fast-twitch (F) fibers were observed in the hypoxic muscle. On the other hand, the oxidative capacity of small- to medium-sized alpha motoneurons (24-45 microns average soma diameter) was increased. The increase in the oxidative capacity of small- to medium-sized motoneurons and the type shift of muscle fibers from F (low-oxidative) to FO (high-oxidative) indicate that hypoxia enhances the oxidative capacity of particular motor units in the neuron pool.