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1 Biomedical Engineering, University of Michigan, Ann Arbor, MI, USA
2 Biomedical Engineering, University of North Carolina, Chapel Hill, NC, USA; Mechanical Engineering, University of Michigan, Ann Arbor, MI, USA; HST, Harvard MIT, Cambridge, MA, USA
3 Molecular Physiology, University of Dundee, Dundee, United Kingdom
* To whom correspondence should be addressed. E-mail: k.baar{at}dundee.ac.uk.
In vitro studies have used protein markers to distinguish between myogenic cells isolated from fast and slow skeletal muscles. The protein markers provide some support for the hypothesis that satellite cells from fast and slow muscles are different, but the data are equivocal. To test this hypothesis directly, 3-dimensional skeletal muscle constructs were engineered from myogenic cells isolated from fast tibialis anterior (TA) and slow soleus (SOL) muscles of rats and functionality was tested. Time to peak twitch tension (TPT) and 1/2 relaxation times (1/2RT) were ~ 30% slower in constructs from the SOL. The slower contraction and relaxation times for the SOL constructs resulted in left shift of the force-frequency curve compared to those from the TA. Western blot analysis showed a 60% greater quantity of fast myosin heavy chain in the TA constructs. 14 days of chronic low frequency electrical stimulation (CLFS) resulted in a 15% slower TPT and a 14% slower 1/2RT, but no change in absolute force production in the TA constructs. In SOL constructs, slow electrical stimulation resulted in an 80% increase in absolute force production with no change in TPT or 1/2RT. The addition of Cyclosporine A did not prevent the increase in force in SOL constructs following CLFS suggesting that calcineurin is not responsible for the increase in force. We conclude that myogenic cells associated with a slow muscle are imprinted to produce muscle that contracts and relaxes slowly and that calcineurin activity cannot explain the response to a slow pattern of electrical stimulation.
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