CHANGES IN MESSENGER-RNA LEVELS OF THE SARCOPLASMIC ENDOPLASMIC-RETICULUM CA2+-ATPASE ISOFORMS IN THE RAT SOLEUS MUSCLE REGENERATING FROM NOTEXIN-INDUCED NECROSIS/

The relative mRNA levels corresponding to the different sarcoplasmic/e ndoplasmic-reticulum Ca2+-ATPase isoforms (SERCA1a, SERCA1b, SERCA2a, SERCA2b and SERCA3) were measured by reverse transcriptase-PCR in rat soleus muscles regenerating after notexin-induced necrosis. The succes sion of appearance of the different types of SERCA mRNA species in reg enerating muscle largely recapitulates those observed during normal on togenesis. The mRNA levels of the muscle-specific isoforms SERCA1a and SERCA2a became very low on the first and third days after injection o f the snake venom. It was only on the fifth day of regeneration that t he mRNA of the neonatal variant of the fast-twitch skeletal SERCA1b is oform began to rise, well before the other SERCA transcripts. At 7 and 10 days, i.e. at a time when the new myofibres normally become reinne rvated, the mRNA level of SERCA1a and SERCA2a increased markedly, but the fast-twitch skeletal SERCA1a isoform was still the most prominent. On day 21, in the advanced stage of regeneration, a switch in the rel ative expression levels of SERCA1a and SERCA2a mRNA was observed and t he ratio of both isoforms became similar to that found in the normal s oleus muscles. This was followed by a decline in the level of all SERC A mRNA species, so that on day 28 the levels of the sarcoplasmic/endop lasmatic-reticulum Ca2+-pump RNAs was again lower but their ratio rema ined similar to that of the untreated control soleus.