[Relationships between differentiation characteristics of internal membrane systems and contractile properties in...

Relationships between differentiation characteristics of internal membrane systems (transverse (T) tubules, sarcoplasmic reticulum (SR) and T-SR junctions (triad)) and contractile properties were investigated in developing (1 to 30 days after hatching) chicken pectoralis muscle. Internal membrane systems were visualized using calcium potassium- ferricyanide and lipid soluble fluorescent dye (DiIC16(3)) and observed by electron and confocal microscopes. Location of triads at the Z-line in pectoralis muscles differentiated in three steps: an initial location of longitudinally oriented triads at the A-I junction (15 days after hatching); a subsequent movement to the Z-line to keep the longitudinal orientation (20 days after hatching); finally rotation to a transverse direction on the Z-line and finishing the differentiation (30 days after hatching). Maximum shortening velocity (+60%) and maximum tension development (+100%) increased following 30 days development after hatching. However, the time courses of the increases in contraction speed and tension development were different. It is likely that the developing pectoralis muscle transiety expresses a protein responsible for adhesion of the triads to the myofibrils at the A-I junction region, and later substitutes that for a Z-line level protein. These separate stages in the differentiation of the E-C coupling units imply distinct cellular mechanisms underlying each of these events as well as their differential regulation during myogenesis in chicken skeletal muscle.