Substrate and enzyme profile of fast and slow skeletal muscle fibers in rhesus monkeys

Results from the Russian Cosmos program suggest that the rhesus monkey is a n excellent model for studying weightlessness-induced changes in muscle fun ction. Consequently, the purpose of this investigation was to establish the resting levels of selected substrate and enzymes in individual slow- and f ast-twitch muscle fibers of the rhesus monkey. A second objective was to de termine the effect of an 18-day sit in the Spacelab experiment-support prim ate facility [Experimental System for the Orbiting Primate (ESOP)]. Muscle biopsies of the soleus and medial gastrocnemius muscles were obtained I mo before and immediately after an 18-day ESOP sit. The biopsies were freeze-d ried, and individual fibers were isolated and assayed for the substrates gl ycogen and lactate and for the high-energy phosphates ATP and phosphocreati ne. Fiber enzyme activity was also determined for the glycolytic enzymes ph osphofructokinase and lactate dehydrogenase (LDH) and for the oxidative mar kers 3-hydroxyacyl-CoA dehydrogenase (beta-OAC) and citrate synthase. Consi stent with other species, the fast type II fibers contained higher glycogen content than did the slow type I fibers. The ESOP sit had no significant e ffects on the metabolic profile of the slow fibers of either muscle or the fast fibers of the soleus. However, the fast gastrocnemius fibers showed a significant decline in phosphocreatine and an increase in lactate. Also, si milar to other species, the fast fibers contained significantly higher LDH activities and lower 3-hydroxyacyl-CoA dehydrogenase activities. For the mu scle enzymes, the quantitatively most important effect of the ESOP sit occu rred with LDH where activities increased in all fiber types postsit except the slow type I fiber of the medial gastrocnemius.