Transient changes in the force-velocity relationship during tetanic contractions of frog sartorius muscles, normal or poisoned with 1-fluoro-2,4-dinitrobenzene.

Dumoulin, C. [UCL] Maréchal, Georges [UCL]

1.Frog sartorius muscles were tetanized for two seconds six times at two minute intervals, at 0°C; after the tension was fully developed at constant length, the muscles were released for two millimeters at constant velocity with an ergometer. 2.As a result of the repetition, the isometric force Pm decreases by about 3-5 % per tetanus for the normal muscles. This effect is enhanced by about 50°/, after poisoning with 1-fluoro-2,4-dinitrobenzene (FDNB). 3.The force maintained during the release (Pr) decrease still more than the isometric tension (Pm). The ratio Pr/Pm decreases by approximately 1°/0 per tetanus. This second effect of the repetition is not affected by the speed of shortening, but is enhanced about four-fold by FDBN. 4.These results seem to require that the externally recorded isometric force Pm is equal to the force produced internally by the contractile protein, but that the force recorded during the release Pr is less than that produced by the contracting proteins by an amount equal to the friction between myofilaments which must be overcome in order that the muscle can shorten.