Constitutive expression of inducible Hsp70 is linked to natural shifts in skeletal muscle phenotype.
OBJECTIVE
Constitutive expression of heat shock protein 70 (Hsp70) is elevated in frequently recruited, metabolically efficient rodent striated muscle. We aimed to assess the relative importance of muscle phenotype vs. increased contractile activity on this pattern of expression using the rat diaphragm, which undergoes a dramatic and sustained increase in recruitment with parturition and development.


METHODS
Diaphragms were collected from rats of various ages (20 day fetus, 1 and 3 days, and 1, 3, 6 and 12 weeks postpartum; PP), and assessed for changes in oxidative capacity, Hsp70 and Type I myosin heavy chain (MHCI) (used as a marker of muscle phenotype changes).


RESULTS
Oxidative capacity of the diaphragm (as indicated by citrate synthase activity) and whole body growth rate (% increase in body weight per week), factors thought to require chaperone activity, increased rapidly, peaked at 3-6 weeks PP and declined late in development. In contrast, at 1 week PP, increased contractile activity in the diaphragm had not altered the expression of Hsp70 protein or mRNA from fetal levels. Significant increases in Hsp70 were not observed until between 1 and 3 weeks, achieving their highest levels at 12 weeks PP. Both MHC I protein (r = 0.69, P = 0.001) and mRNA (r = 0.76, P = 0.001) were significantly correlated with their Hsp70 counterparts.


CONCLUSIONS
Expression of Hsp70 in the developing diaphragm represents an adaptation associated with a shift towards a slower, more metabolically efficient adult phenotype rather than simply a response to contractile stress.
O'Neill
D E T
DE
School of Kinesiology, Faculty of Health Sciences, The University of Western Ontario, Ontario, Canada.
Noble
E G
EG
D000367:Age Factors; D000818:Animals; D002148:Calmodulin-Binding Proteins / Q000032:analysis; D002950:Citrate (si)-Synthase / Q000378:metabolism; D003964:Diaphragm / Q000737:chemistry* / Q000254:growth & development; D018840:HSP70 Heat-Shock Proteins / Q000032:analysis* / Q000235:genetics; D009119:Muscle Contraction / Q000502:physiology; D010084:Oxidation-Reduction; D010641:Phenotype; D012333:RNA, Messenger / Q000032:analysis; D051381:Rats; D017207:Rats, Sprague-Dawley; D015430:Weight Gain / Q000502:physiology
D002148:Calmodulin-Binding Proteins; D018840:HSP70 Heat-Shock Proteins; D012333:RNA, Messenger; D002950:Citrate (si)-Synthase