Prolonged passive stretch of rat soleus muscle provokes an increase in themRNA levels of the muscle regulatory factors distributed along the entire length of the fibers

The mRNA levels of the adult and the neonatal sarcoplasmic/endoplasmic reti culum Ca2+-ATPases (SERCA1a and SERCA1b, respectively) and those of the mus cle regulatory factors (MRFs: myoD, myf-5, myogenin, MRF4) have been assess ed by RT PCR in rat soleus muscles immobilized for 3 days in an extended po sition (passive stretch). The transcript level of the fast type SERCA1a Ca2 +-transport ATPase decreased to half of its normal value, whereas that of n eonatal SERCA1b isoform increased 5-fold above control in stretched muscles . Immunostaining of muscle cross sections showed that the fraction of fiber s expressing the SERCA1a protein was decreased evenly along the length of t he stretched muscles indicating that a transformation occurred of fast fibe rs to slow ones. The mRNA levels of MRFs were elevated 3- to 6-fold above t he normal level and were distributed evenly along the length of the stretch ed muscles. However in the controls these transcripts were more abundant at both ends of the muscle. The stretch increased the level of myoD and immun ocytochemistry showed the expression of myoD protein in a number of nuclei of the stretched muscles whereas it was practically undetectable by this me thod in the control muscles. Western blotting did not indicate a significan t stretch-induced increase in the level of the myogenin protein, in spite o f the fact that immunocytochemistry tended to show more myogenin-positive n uclei in stretched muscles as compared to the controls. These data indicate that after 3 days of passive stretch the central and the terminal parts of the soleus muscle adapt similarly by increasing the levels of the MRFs, by decreasing the overall levels of the fast SERCA1-type of ATPase and by par tially re-establishing a neonatal mode of alternative SERCA1 transcript spl icing resulting in an increased SERCA1b/1a ratio.