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2-agonist
administration and exercise on contractile activation of skeletal
muscle fibers
Muscle and Cell Physiology Laboratory, Department of Physiology, The University of Melbourne, Parkville, Victoria 3052, Australia
Received 23 January 1995; accepted in final form 3 May 1996.
Lynch, Gordon S., Alan Hayes, Siun P. Campbell, and David A. Williams. Effects of
2-agonist administration and
exercise on contractile activation of skeletal muscle fibers.
J. Appl. Physiol. 81(4):
1610-1618, 1996.
Clenbuterol, a
2-adrenoceptor agonist, has
therapeutic potential for the treatment of muscle-wasting diseases, yet
its effects, especially at the single-fiber level, have not been fully
characterized. Male C57BL/10 mice were allocated to three groups:
Control-Treated mice were administered clenbuterol (2 mg · kg
1 · day1)
via their drinking water for 15 wk; Trained-Treated mice underwent low-intensity training (unweighted swimming, 5 days/wk, 1 h/day) in
addition to receiving clenbuterol; and Control mice were sedentary and
untreated. Contractile characteristics were determined on membrane-permeabilized fibers from the extensor digitorum longus (EDL)
and soleus muscles. Fast fibers from the EDL and soleus muscles of
Treated mice exhibited decreases in
Ca2+ sensitivity. Endurance
exercise offset clenbuterol's effects, demonstrated by similar
Ca2+ sensitivities in the
Trained-Treated and Control groups. Long-term clenbuterol treatment did
not affect the normalized maximal tension of fast or slow fibers but
increased the proportion of fast fibers in the soleus muscle. Training
increased the proportion of fibers with high and intermediate succinate
dehydrogenase activity in the EDL and soleus muscles, respectively. If
clenbuterol is to be used for treating muscle-wasting disorders, some
form of low-intensity exercise might be encouraged such that
potentially deleterious slow-to-fast fiber type transformations are
minimized. Indeed, in the mouse, low-intensity exercise appears to
prevent these effects.
fiber types; mouse; muscle contraction; skinned fibers
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