Submitted on December 11, 2006
Accepted on January 10, 2007
Isoform switching in myofibrillar and excitation-contraction coupling proteins contributes to diminished contractile function in regenerating rat soleus muscle
Alessandra Esposito1, Elena Germinario2, Marika Zanin1, Philip T. Palade3, Romeo Betto4, and Daniela Danieli-Betto5*
1 Human Anatomy and Physiology, University of Padova, Padova, Italy
2 Interuniversity Institute of Myology, Italy; Human Anatomy and Physiology, University of Padova, Padova, Italy
3 Department of Pharmacology and Toxicology, University of Arkansas for Medical Sciences, Little Rock, Arkansas, United States;
4 Muscle Biology and Physiopathology, C.N.R. Institute of Neuroscience, Padova, Italy; Interuniversity Institute of Myology, Italy
5 Padova, Italy; Human Anatomy and Physiology, University of Padova, Padova, Italy; Interuniversity Institute of Myology, Italy
* To whom correspondence should be addressed. E-mail: daniela.danieli{at}unipd.it.
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 (EC) coupling and/or contractility processes. The presence of developmental isoforms of both the voltage-gated Ca2+ channel (
1C) and of the ryanodine receptor (RyR3), paralleled by an abnormal caffeine contracture development, confirms the immature EC 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 to the residual presence of embryonic myosin heavy chains (MyHC). 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.
