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1 Department of Pharmacology, The Milton S. Hershey Medical Center, College of Medicine, The Pennsylvania State University, Hershey, PA, USA
2 Department of Integrative Physiology, University of Colorado, Boulder, CO, USA
* To whom correspondence should be addressed. E-mail: ycn1{at}psu.edu.
Our previous study demonstrated age-associated alterations in expression of the Na+,K+-ATPase subunit isoforms in rat skeletal muscles (J. Appl. Physiol. 87(3): 1132-1140, 1999). The present study tests the hypothesis that endurance exercise training (ETr) reverses these age-related changes. Expression of the subunit isoforms (
1, 2, 
1, 2, 3) in skeletal muscles of F-344/BN rats was examined by western blotting in three groups of rats: 16-month-old sedentary middle-aged rats (Ms), 29-month-old sedentary senescent rats (Ss), and 29-month-old treadmill exercise trained senescent rats (St). Levels of 1 increased with age in red gastrocnemius (GR), white gastrocnemius (GW), and extensor digitorum longus (EDL), and ETr further increased its levels. Levels of 2 were unchanged in GR, had a strong trend for a decrease in GW, while that of EDL decreased significantly. ETr increased the expression of 2 in all three muscle groups in senescent rats. There was no significant increase in 1 levels in GR, GW, or EDL with age, while ETr markedly increased levels of 1 in the three muscle types in senescent rats. There was a marked decrease with age in levels of 2 in the three muscle types, which was not reversed by ETr. By contrast, levels of 3 increased with age in GR and GW, and ETr was able to reverse this increase in senescent rats. Na+,K+-ATPase enzyme activity was unchanged with age in GR and GW and slightly, but significantly, increased in EDL. ETr increased enzyme activity in GR and GW, and did not change in EDL. Myosin heavy chain isoforms in the three muscle groups did not change significantly with age; ETr caused a general shift toward more oxidative fibers. Thus, ETr differentially modifies age-associated alterations in expression of the Na+,K+-ATPase subunit isoforms. Since ETr was able to reverse only some of the age-associated changes, mechanism(s) other than physical inactivity appears to play significant role in those changes.
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