Abstract
Connectin (also called titin) is a giant fibrous elastic protein of vertebrate striated muscle with a molecular mass exceeding 3 MDa. A single molecule, more than 1 μm in length, connects the Z-line and the M-line of the striated muscle sarcomere. Because of its elasticity, connectin plays a role in maintaining the thick filament at the center of the sarcomere during contraction and relaxation of the muscle. Observations with fluorescent antibodies and immunoelectron microscopy using antibodies against connectin have demonstrated that this function occurs only in the I-band region, not in the entire sarcomere. It has been reported in human muscle that cardiac muscle connectin had fewer amino acids in the middle immunoglobulin domain and PEVK segments than psoas and soleus muscle connectins. Both segments reside in the I-band region, which is related to extension and contraction between the Z-line and the A-I junction in the muscle sarcomere. We measured passive tension generation with the stretching of mechanically skinned myofibril bundles and revealed that tension development in rabbit cardiac muscle began at shorter sarcomere length than in psoas and soleus muscles of rabbit. We infer that the cardiac muscle connectin remains in a slightly stretched state even at rest. This would explain why the cardiac sarcomere does not extend to the same degree as that of the psoas and soleus muscles under force. [J Physiol Sci. 2007;57 Suppl:S34]
