Isoform switching in myofibrillar and excitation-contraction coupling proteins contributes significantly to diminished contractile function in regenerating rat soleus muscle.

Postnatal development of skeletal muscle occurs through the progressive transformation of diverse biochemical, metabolic, morphological, and functional characteristics from the embryonic to the adult phenotype. Since muscle regeneration recapitulates postnatal development of muscle fiber, it offers an appropriate experimental model to investigate the existing relationships between diverse muscle functions and the expression of key protein isoforms, particularly at the single-fiber level. This study was carried out in regenerating soleus muscle 14 days after injury. At this intermediate stage, the regenerating muscle exhibited a recovery of mass greater than its force generation capacity. The lower specific tension of regenerating muscle suggested intrinsic defective excitation-contraction coupling and/or contractility processes. The presence of developmental isoforms of both the voltage-gated Ca2+ channel (alpha1C) and of ryanodine receptor 3, paralleled by an abnormal caffeine contracture development, confirms the immature excitation-contraction coupling of the regenerating muscle. The defective Ca2 handling could also be confirmed by the lower sarcoplasmic reticulum caffeine sensitivity of regenerating single fibers. Also, regenerating single fibers revealed a lower maximal specific tension, which was associated with the residual presence of embryonic myosin heavy chains. Moreover, the fibers showed a reduced Ca2+ sensitivity of myofibrillar proteins, particularly those simultaneously expressing the slow and fast isoforms of troponin C. The present results indicate that the expression of developmental proteins determines the incomplete functional recovery of regenerating soleus.