Journal of Applied Physiology AJP: Endocrinology and Metabolism
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J Appl Physiol (June 30, 2002). doi:10.1152/japplphysiol.00189.2002
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Articles in PresS, published online ahead of print June 30, 2002
J Appl Physiol, 10.1152/jap.00189.2002
Submitted on March 7, 2002
Accepted on June 18, 2002

Effects of Botulinum toxin A-injection and exercise on the growth of juvenile rat gastrocnemius muscle

Chen-Ming Chen1, N. Susan Stott2, and Heather K Smith1*

1 Department of Sport and Exercise Science, University of Auckland, Auckland, New Zealand
2 Department of Surgery, University of Auckland, Auckland, New Zealand

* To whom correspondence should be addressed. E-mail: h.smith{at}auckland.ac.nz.

Botulinum toxin A (Btx) injections and supervised exercise are often used concurrently to treat calf muscle spasticity in children. This study has analysed the early effects of Btx-induced paralysis and increased activity by voluntary wheel running on cell growth-related processes in juvenile rat gastrocnemius muscle. Btx injection at 29 days of age prevented the normal increases in wet mass (50%) and fiber cross-sectional area (34%) seen by 36 days of age in control rats. Btx-injected versus contralateral muscles had 22% fewer myonuclei per fiber length, but >2-fold the number of MyoD-positive nuclei at 36 days of age. The accretion of 5'-bromo-2'-deoxyuridine-labelled newly produced myonuclei did not differ between limbs. Voluntary exercise during the 7 days increased the mass (18%) and fiber size (23%) of Btx-injected and contralateral muscles, but did not affect any other variable. Thus, Btx injection and exercise had early effects on muscle and fiber size without consistently associated changes in myonuclear production or number. This suggests the presence of non-contractile activity-dependent, growth-promoting cytoplasmic events, in juvenile muscle.




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