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1 School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, Ontario, Canada
2 School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, Ontario, Canada; Department of Chemistry and Biochemistry, Laurentian University, Sudbury, Ontario, Canada
3 School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, London, Ontario, Canada; Lawson Health Research Institute, The University of Western Ontario, London, Ontario, Canada
* To whom correspondence should be addressed. E-mail: enoble{at}uwo.ca.
Heat-shock protein 72 (Hsp70) is constitutively expressed in rat hindlimb muscles, reportedly in proportion to their content of type I myosin heavy chain (MHCI). This distribution pattern has been suggested to result from the higher recruitment and activity of such muscles and/or a specific relationship between myosin phenotype and Hsp70 content. To differentiate between these possibilities, the fiber-specific distribution of Hsp70 was examined in male Sprague-Dawley rat plantaris (PLT) under control conditions, following a fast-to-slow phenotypic shift in response to surgically-induced overload (O), and in response to O when the phenotypic shift was prevented by 3,5,3' -triiodo-DL-thyronine (T3) administration. Constitutive expression of Hsp70 was restricted to type I and IIa fibers in PLT from control rats and this fiber-specific pattern of expression was maintained following O of up to 28 days, although Hsp70 content in the O muscle doubled. When O (for 40 days) of the PLT was combined with T3 administration, despite typical hypertrophy in the overloaded PLT, prevention of the normal phenotypic transformation also blocked the increased expression of Hsp70 observed in euthyroid controls. Collectively, these data suggest that chronic changes in constitutive expression of Hsp70 with altered contractile activity appear critically dependent upon fast-to-slow phenotypic remodelling.
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