Light and x-ray diffraction studies on chick skeletal muscle under controlled physiological conditions.

1. The sarcomere length (s) of ehick slow and fast muscles (anterior and posterior latissimus dorsi (ant. lat. dorsi and post. lat. dorsi)) was measured by the method of light diffraction. In resting ant. lat. dorsi, s changed from 1.76 to 2.30 mum during stretch from minimum to maximum muscle lengths in situ, and in resting post. lat. dorsi from 2.18 to 2.63 mum.2. Resting tension started to rise in ant. lat. dorsi when s exceeded 1.7-1.8 mum, but in post. lat. dorsi not until s exceeded 2.6-2.7 mum.3. X-ray diffraction patterns showed that ant. lat. dorsi contains collagen filaments; collagen reflexions were not seen in patterns obtained from post. lat. dorsi with the same exposure time.4. The relation between active tension and sarcomere length was similar for ant. and post. lat. dorsi. The maximum active tension was observed when s = 2.05-2.15 mum in ant. lat. dorsi, and when s = 2.10-2.25 mum in post. lat. dorsi.5. X-ray diffraction patterns from both muscles showed that the periodic structures of the thick and thin filaments are similar to those in frog and rabbit skeletal muscles.6. The volume of the myofilament lattice in resting ant. lat. dorsi was 3.06 (+/- 0.14) x 10(9) A(3), in resting post. lat. dorsi 2.98 (+/- 0.09) x 10(9) A(3). These values are close to that of frog skeletal muscle. The lattice volume remained constant in ant. lat. dorsi and post. lat. dorsi over the range of sarcomere lengths found in situ.7. The equatorial diffraction patterns from the ant. lat. dorsi in rigor (glycerol extracted) were different from that of the resting muscle, and suggested that a large number of cross-bridges were attached to the thin filaments during rigor. During potassium contracture, however, the diffraction pattern remained similar to that from the resting ant. lat. dorsi.