The sarcolemma, sarcoplasmic reticulum (SR), and T system of the anterior (tonic) and posterior (fast twitch) latissimus dorsi muscles of the chicken have been examined by the freeze-fracture technique, and quantitative data on the P and E fracture faces have been obtained.
The fractured plasma membranes reveal (a) profiles of surface caveolae, (b) randomly distributed intramembranous particles ranging in size from 40–100 Å in diameter, and (c) orthogonal assemblies composed of groups of 60 Å particles in close association, and differences with respect to all three structures are present between the tonic (ALD) and fast twitch (PLD) muscles. In the ALD muscle, the surface caveolae are more uniformly distributed and have smaller openings than in the PLD muscle; the former muscle also has a two-fold higher caveolae density than the latter muscle. The intramembranous particles are more numerous in the ALD than in the PLD muscle in both fracture faces, but the orthogonal assemblies are fewer. The functional significance of these differences in the two fiber types are discussed.
The fractured membranes of the SR have intramembranous particles (IMP's) approximately 80 Å in diameter, with a two-fold higher packing density in the PLD than in the ALD muscle. This difference is present in both the longitudinal and cisternal components of the SR. In addition, there are collar-like expansions (CLE's) in the SR of the ALD muscle which are particularly poor in intramembranous particles. These particles are considered to represent Ca2+ transport ATP-ase, and the reduced density of IMP's could be a significant factor in the low calcium uptake and slow relaxation characteristics of the ALD muscle.