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ENVIRONMENTAL, EXERCISE AND RESPIRATORY PHYSIOLOGY

Parallel and divergent adaptations of rat soleus and plantaris to chronic exercise and hypergravity

¹Section of Neurobiology, Physiology and Behavior, University of California, Davis; and ²Department of Biophysics and Physiology, University of California, Irvine, California

Submitted 9 August 2005 ; accepted in final form 15 September 2005

It has been demonstrated that endurance exercise and chronic acceleration, i.e., hypergravity, produce comparable adaptations in a variety of physiological systems, including decreased adiposity, increased energy metabolism, and altered intermediary metabolism. Similar adaptations have not been demonstrated for skeletal muscle per se. To further differentiate between these general responses with respect to gravity and exercise, this study tested the hypothesis that chronic exercise (voluntary wheel running) and chronic acceleration (2 G via centrifugation) will induce similar changes in muscle myosin heavy chain (MHC) isoform expression in rat plantaris, a fast extensor, and in rat soleus, a slow "antigravity" extensor. The experimental design involved four groups of mature male rats (n = 8/group): 1 G and 2 G with running wheels, and 1 G and 2 G controls without running wheels. The primary observations from the study were as follows: 1) 8 wk of 2 G are an adequate stimulus for MHC compositional changes in rat plantaris and soleus muscle; 2) both exercise and +G caused an increase in the slow MHC₁ isoform in soleus muscle, suggesting that loading is a primary stimulus for this shift; and 3) 2 G and exercise appeared to have differential effects on the plantaris muscle MHC isoforms, with 2 G causing an increase in MHC_2b, and exercise causing a decrease in MHC_2b with a concomitant increase in MHC₁, suggesting that factors other than enhanced loading, possibly locomotor activity levels, are the primary stimulus for this shift.

centrifugation; wheel running; activity