EXPRESSION OF SARCOPLASMIC ENDOPLASMIC RETICULUM CA2+ ATPASES IN THE RAT EXTENSOR DIGITORUM LONGUS (EDL) MUSCLE REGENERATING FROM NOTEXIN-INDUCED NECROSIS/

The level of sarcoplasmic/endoplasmic reticulum Ca2+ ATPase (SERCA) mR NAs and proteins have been assessed by RT-PCR, immunoblotting and immu nocytochemistry in the rat extensor digitorum longus (EDL) muscles dur ing regeneration from notexin-induced necrosis. As a result of the nec rosis, SERCA1 and SERCA2 declined on days 1 and 3 after administration of the toxin. Thereupon the mRNA of the fast isoform SERCA1 rapidly i ncreased between days 5 and 10 to the normal level. The mRNA level of the ''housekeeping'' SERCA2b isoform increased markedly during the act ual necrosis at days 1 and 5, probably due to invading cells. Then the mRNA level of the neonatal SERCA1b splice variant increased first, an d exceeded the level of the adult SERCA1a transcript on day 5. At late r stages of regeneration the neonatal form was gradually replaced by t he adult SERCA1a form, thus recapitulating similar changes known to oc cur during normal ontogenesis. Along with SERCA1, the levels of the sl ow isoform (SERCA2a) mRNA and protein increased on day 5, but the SERC A2a mRNA levels never rose above 10% of SERCA1 and after 10 days gradu ally declined again. In the normal and regenerated muscles, SERCA1 was expressed in 97% of the fibres which accounted for the population of fast-twitch fibres (type IIa, type IIb and probably type IIx/d). SERCA 2a was present in 6% of the fibres of normal muscle (mostly in the slo w-twitch type I fibres). At the end of regeneration the number of fibr es expressing SERCA2a was twice as high and were found in small groups , unlike in normal EDL, but about 50% of these clustered fibres also e xpressed SERCA1. (C) Kluwer Academic Publishers.