SARCO(ENDO)PLASMIC RETICULUM CA2-TYPE AND SLOW-TYPE DENERVATED SKELETAL-MUSCLES - A TIME-COURSE STUDY( PUMP AND METABOLIC ENZYME EXPRESSIONIN RABBIT FAST)

Recent reports by d'Albis et al. have shown that denervation of 8-day- old rabbit fast-twitch muscle (gastrocnemius) leads to the transformat ion of the muscle towards a slow phenotype but the changes towards slo w-type myosin isoforms and contractile properties of the muscle were t emporally uncoordinated. We analysed the time course of the effects of denervation of the gastrocnemius on the expression of the sarcoplasmi c reticulum calcium pump isoforms (SERCA) and on the metabolic state o f the muscle. Northern-blot analysis showed a rapid loss of the fast C a2+ pump isoform (SERCA 1) mRNA from the denervated gastrocnemius whic h became of the oxidative type. The changes observed were complete as early as 35 days post-natal, i.e at the time when changes in contracti le properties were previously observed. Denervation of the slow-twitch soleus led to a 50% decrease in the level of the slow Ca2+ pump isofo rm (SERCA 2) mRNA and was without effect on the metabolic state of the muscle. These findings extend previous results suggesting that in rab bit, continuous innervation is required for differentiation of fast-tw itch muscles but is not an absolute requirement for differentiation of the slow-twitch muscle.