Half Sarcomere Research Articles

X-ray diffraction of strained muscle fibers in rigor.

The effect of strain on the equatorial x-ray diffraction pattern of glycerinated rabbit psoas fibers was studied in the rigor (ATP free) state. Strains between 30 and 100 A per half sarcomere, measured directly by laser diffraction, did not change the intensity ratio, (10)/ . (11). Because the intensity ratio depends on the distribution of mass within the myofilament lattice, the negative result indicates that strain does not change the angle of attachment of the subfragment 1 (S1) moiety of the myosin molecule to the actin filament. The effect of strain on the ordering of the actin filaments also was considered and judged to be negligible.

Coupling between the enzymatic site of myosin and the mechanical output of muscle

When Mg-imido-ATP binds to the active site of myosin in glycerol-extracted insect muscle fibres it causes a rapid, fully reversible and stress-independent increase in rest length of 2 nm per half sarcomere, whilst the isotonic stiffness remains within 2% of the rigor value. Mg-ADP and H-imido-ATP bind but have less mechanical effect and also less effect on the equatorial X-ray diffraction pattern. Coupling of binding to length change is quantitatively reversible, since stress of 200 nm per fibre doubles the amount of either Mg-imido-ATP or Mg-ATP bound at low concentrations but has no effect on Mg-ADP or H-imido-ATP binding. Linkage between the nucleotide and actin binding sites is shown by studies with subfragment-1 in solution. By fluorometric titration the nucleotide affinity for subfragment-1 was shown to increase in the order of H-imido-ATP, Mg-ADP and Mg-imido-ATP, while by a sedimentation technique the affinity of actin for subfragment-1-nucleotide was shown to decrease in the same order. The data are interpreted in terms of a simple cross-bridge model and their relevance to the mechanism of muscle contraction is discussed.

Muscle Compliance and the Longitudinal Transmission of Mechanical Impulses

The time required for a mechanical impulse to propagate from one end to the other was measured directly in frog sartorius muscles and in fiber bundles from the semitendinosus muscle. When the fibers were fully activated, the transmission velocity was 170 mm/ms. In resting fibers the transmission time was three to four times greater than in activated fibers. Control experiments indicated that the transmission time across the tendons was negligible. A muscle compliance of 55-80 A per half sarcomere was estimated from these data. The "measurement time" of the method was calculated to be about 15 micros. This relatively short measurement time makes the method potentially useful for detecting changes in cross-bridge compliance.