Rk. Josephson et Kap. Edman, CHANGES IN THE MAXIMUM SPEED OF SHORTENING OF FROG-MUSCLE FIBERS EARLY IN A TETANIC CONTRACTION AND DURING RELAXATION, Journal of physiology, 507(2), 1998, pp. 511-525
1. Isotonic shortening velocities at very light loads were examined in
single fibres of the anterior tibialis muscle of the frog, Rana tempo
raria, using load-clamp recording and slack tests (temperature, 1-3 de
grees C; initial sarcomere length, 2.25 mu m). 2. Shortening velocitie
s at very light loads (for ce-clamp recording) were found to be higher
early in the rise of a tetanic contraction than during the plateau of
the contraction. The upper limit of the load at which there was eleva
ted shortening velocity early in the contraction was 1.5-5.4% of the m
aximum tetanic tension (F-0) depending on the particular fibre. 3. The
maximum shortening velocity determined using the slack test method (V
-0) was as much as 30% greater early in a contraction than at the teta
nic plateau. V-0 was elevated above the plateau level up to about 30 m
s after the end of the latent period, which is equivalent to the time
required for the force in an isometric contraction to rise to about 30
% of F-0. V-0 is depressed below the plateau value during relaxation a
t the cessation of stimulation. 4. Simulation studies show that the cr
oss-bridge model of Huxley (1957) predicts the maximum shortening velo
city to be greater early in a contraction, when new actin binding site
s are becoming activated and new croos-bridge connections are being fo
rmed rapidly than during steady-state contraction. The elevated shorte
ning velocity in the model is a consequence of new cross-bridges being
formed in the pulling configuration, and there being a delay before t
he newly added bridges are dragged beyond their equilibrium position s
o they begin to retard shortening. The model also predicts that maximu
m shortening velocity should be depressed below the plateau level duri
ng early relaxation as cross-bridge binding sites are rapidly removed
from the active population.