MUSCLE-FIBER STRESS IN RESPONSE TO EXERCISE - SYNTHESIS, ACCUMULATIONAND ISOFORM TRANSITIONS OF 70-KDA HEAT-SHOCK PROTEINS

Heat-shock or stress proteins (HSPs) are considered to play an essenti al role in protecting cells from stress and preparing them to survive new environmental challenges. This study investigates the induction ki netics of synthesis and accumulation of 70-kDa stress proteins in the soleus and extensor digitorum longus (EDL) muscles of the rat followin g exercise, as well as the isoform transitions that take place during the post-exercise period. Relative synthesis rates (referred to consti tutively expressed stress protein HSP73) of the 70-kDa heat-shock prot eins were greatly enhanced after a single bout of exercise in both mus cles. They peaked early in the post-exercise period and returned to re sting levels after approximately 5-6 h. The levels of the inducible st ress protein HSP72 in the EDL rose only transiently following exercise , while its accumulation in the soleus was more continuous and stable. The amount of HSP73 increased only transiently in both muscle types a fter exercise. The constitutive expression of the stress protein HSP72 in the soleus muscle was much higher than in the EDL and other tissue s, while that of HSP73 was relatively constant among tissues. Rat skel etal muscle HSP72 and HSP73 were made up of at least three isoforms of the same molecular mass and very close isoelectric points, although o nly one radiolabelled isoform was detected. The relative proportion of the most abundant isoforms of HSP72, isoforms 1 and 2, as well as the ir ratio (isoform 2/isoform 1), increased during the post-exercise per iod. Since isoform 2 of HSP72 partially disappeared after incubating s oleus muscle extracts of exercised rats with alkaline phosphatase, the se data indicate that phosphorylation of HSP72 is an early event in th e stress response of skeletal muscle to exercise stress.