Cross-bridge Detachment and Sarcomere 'give' During Stretch of Active Frog's Muscle

1. A study has been made of the tension responses and sarcomere length changes produced by servo-controlled stretches applied to isometrically contracting frog muscle. Sarcomere lengths were monitored by cine-photography of diffiraction spectra obtained by illuminating a small area of muscle with a laser. 2. The tension increment produced by a ramp-and-hold stretch of approximately 1 mm (ca. 4% of the muscle length) comprises three phases whose limits are defined by two points, S1 and S2, where the slope of the response decreases abruptly. S1 and S2 correspond to extensions of 0.13 and 1.2% of the muscle length. 3. Movements of the first order spectra relative to the zero order recorded during stretch reveal that S2 coincides with an abrupt elongation of the sarcomeres. This is termed sarcomere 'give' and it occurs when the filaments are displaced by 11-12 nm from their steady-state (isometric) position. 4. The stiffness of the sarcomeres, Es, up to S2 decreases with increasing sarcomere length. The maximum force sustained by the muscle at S2, PS2, also shows an inverse dependence on sarcomere length. Both Es and PS2 fall to zero at an extrapolated sarcomere spacing of 3.6-3.7 micrometer, coinciding with the length at which the actin and myosin filaments no longer overlap. 5. The ratio PS2/P0 (where P0 = maximum isometric tension) varies with temperature and speed of stretch. It increases with increasing speeds of stretch until a certain critical velocity, Vc, is reached, beyond which it is almost independent of any further increase. Vc has a positive temperature coefficient, increasing 5-6 in the range 0-30 degrees C (Q10 = 1.8). There is a positive correlation between the maximum speed of isotonic shortening (Vmax.) and Vc in different muscles. 6. Sarcomere 'give' during stretch is considered to be due to forcible detachment of cross-bridges between the actin and myosin filaments. This results in recoil of the extended series elastic elements in the muscle at the expense of the sarcomers. The amount of filament displacement required to induce sarcomere 'give' (11-12 nm) is thought to represent the range of movement over which a cross-bridge can remain attached to actin during a stretch.