Paradoxical effects of endurance training and chronic hypoxia on myofibrillar ATPase activity

• Published in: American Journal of Physiology-Regulatory, Integrative and Comparative Physiology
• Main Authors: Roels, B., Reggiani, C., Reboul, C., Lionne, C., Iorga, B., Obert, P., Tanguy, S., Gibault, A, Jougla, A., Travers, F., Millet, G.P., Candau, Robin
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2008
• Subjects: Médecine humaine et pathologie; Human health and pathology; MYOSINE ATPASE ACTIVITY;MYOSIN HEAVY CHAIN ISOFORM;MYOSIN LIGHT;CHAIN ISOFORM;EXERCISE;FAST KINETICS ;RAT;BIOCHIMIE; rattus rattus; muscle; in vivo; activité enzymatique; Rattus rattus
• Online Access: http://prodinra.inra.fr/ft/9D748EC3-FCBB-48E0-8814-FD67E5DE3262; http://prodinra.inra.fr/record/30452; https://doi.org/10.1152/ajpregu.00210.2006

CHRONIC HYPOXIC EXPOSURE INDUCES several structural modifications in skeletal muscles, that is, a decrease in fiber crosssectional area and in muscle mass, mainly due to an anorexic effect. Even when the anorexic effect is controlled by using a pair-fed experimental design, the composition of myosin heavy chain (MHC) is changed, and an increase in the fast-type isoforms is observed. Indeed, a shift toward glycolytic-oxidative fibers (MHC-IIA) in muscles of rats exposed to hypoxia is generally reported, except in the study of Sillau and Banchero (35), in which no difference was shown in the fiber type composition of rat soleus after hypoxic exposure. In addition, Bigard et al. (7) showed that 4 wk in hypobaric hypoxia resulted in a decrease in MHC-I in the soleus of young rats. Thus, hypoxia seems to affect the structural and biochemical properties of skeletal muscles by inducing a transformation from type I to type II fibers. Opposite to chronic hypoxia, endurance training is likely to induce a decrease in fast fibers and an increase in slow-fiber proportion. For instance, Green et al. (18) demonstrated that a prolonged (15 wk) endurance training induced a fiber transformation from fast-type IIa to slow type I on the basis of enzyme histochemistry and MLC distribution. Endurance training (10 wk) also induced changes in the distribution patterns of fast alkali MLC isoforms detectable within FMb (triplet of isomyosins with distinct MLC complement, FM1b, FM2b, and FM3b) composed of the fastest isoform MHC-IIB (3). Surprisingly, endurance training seems to induce an increase in the actomyosin ATPase activity and maximal shortening velocity in the slow-twitch soleus muscle and a decrease in the fast region of the vastus lateralis (34). These observations of changes in ATPase activity and myosin isoform shifts appear to be paradoxical, as ATP hydrolyzed by myosin represents the main component of ATP consumption during contraction. These changes in myosin isoform expression probably regulate ATP consumption. Actually, there is strong evidence that myosin isoforms are the main determinant of ATPase activity, at least at low temperature in vitro. In this study, we analyzed myosin isoform composition and myofibrillar ATPase activity in soleus muscles of rats exposed to normoxia or hypoxia with or without endurance training to assess the variations of soleus myofibrillar ATPase (m-ATPase) activity that accompany the changes in MHC isoform expression under training and/or chronic hypoxia conditions.