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1 Exercise Biochemistry Laboratory, Faculty of Physical Education and Recreation Physical Education and Recreation, and The Centre for Neuroscience, University of Alberta, Edmonton, AB, Canada; Faculty of Medicine and Dentistry, The Centre for Neuroscience, Edmonton, AB, Canada
2 Department of Agricultural, Food and Nutritional Sciences, University of Alberta, Edmonton, AB, Canada
3 Exercise Biochemistry Laboratory, Faculty of Physical Education and Recreation Physical Education and Recreation, and The Centre for Neuroscience, University of Alberta, Edmonton, AB, Canada
4 Biology, Univeristy of Konstanz, Konstanz, Germany
* To whom correspondence should be addressed. E-mail: tputman{at}ualberta.ca.
The purpose of this investigation was to examine the temporal changes in UCP-3 expression, as well as related adaptive changes in mitochondrial density and fast-to-slow fiber type transitions during chronically-enhanced contractile activity. We examined the effects of 1- to 42-days of chronic low-frequency electrical stimulation (CLFS), applied to rat tibialis anterior (TA) for 10 h/day, on the expression of uncoupling protein-3 (UCP-3), and concomitant changes in myosin heavy chain (MHC) protein expression and increases in oxidative capacity. UCP-3 protein content increased from 1- to 12 days, reaching 1.5-fold over control (P < 0.0005); it remained elevated for up to 42 days. In contrast, UCP-3 mRNA decreased in response to CLFS, reaching a level that was 3-fold lower than control (P < 0.0007). The activities of the mitochondrial reference enzymes citrate synthase (EC 4.1.3.7) and 3-hydroxyacyl-CoA-dehydrogenase (EC 1.1.1.35), which are known to increase in proportion to mitochondrial density, progressively increased up to an average of 2.3-fold (P < 0.00001). These changes were accompanied by fast-to-slow fiber type transitions, characterized by a shift in the pattern of MHC expression (P <0.0002): MHCI and MHCIIa expression increased by 1.7- and 4-fold, whilst MHCIIb displayed a 2.4-fold reduction. We conclude that absolute increases in UCP-3 protein content in the early adaptive phase were associated with the genesis of mitochondria containing a normal complement of UCP-3. However during exposure to long-term CLFS, mitochondria were generated with a lower complement of UCP-3, and coincided with the emergence of a growing population of oxidative type-IIA fibers.
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