Muscle fibre stress in response to exercise: synthesis, accumulation and isoform transitions of 70-kDa heat-shock proteins.

Heat-shock or stress proteins (HSPs) are considered to play an essential role in protecting cells from stress and preparing them to survive new environmental challenges. This study investigates the induction kinetics of synthesis and accumulation of 70-kDa stress proteins in the soleus and extensor digitorum longus (EDL) muscles of the rat following exercise, as well as the isoform transitions that take place during the post-exercise period. Relative synthesis rates (referred to constitutively expressed stress protein HSP73) of the 70-kDa heat-shock proteins were greatly enhanced after a single bout of exercise in both muscles. They peaked early in the post-exercise period and returned to resting levels after approximately 5-6 h. The levels of the inducible stress 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 after exercise. The constitutive expression of the stress protein HSP72 in the soleus muscle was much higher than in the EDL and other tissues, while that of HSP73 was relatively constant among tissues. Rat skeletal muscle HSP72 and HSP73 were made up of at least three isoforms of the same molecular mass and very close isoelectric points, although only one radiolabelled isoform was detected. The relative proportion of the most abundant isoforms of HSP72, isoforms 1 and 2, as well as their ratio (isoform 2/isoform 1), increased during the post-exercise period. Since isoform 2 of HSP72 partially disappeared after incubating soleus muscle extracts of exercised rats with alkaline phosphatase, these data indicate that phosphorylation of HSP72 is an early event in the stress response of skeletal muscle to exercise stress.