n the mechanism of inhibitory action of vibrations as studied in a molluscan catch muscle and in vertebrate vascular smooth muscle.

In previous studies longitudinal vibrations have been found to reduce active force development in smooth muscle, possibly due to a direct action on the contractile mechanism. In the present experiments the inhibitory effect of vibrations on isometric tension was studied in isolated preparations of the rat portal vein, the rabbit thoracic aorta and the anterior byssus retractor muscle (ABRM) of the Mytilus edulis. The results demonstrate that vibrations of appropriate frequency and amplitude caused prompt inhibition of contractile tension and that complete recovery of active force normally occurred after cessation of vibration in vertebrate smooth muscle as well as during the phasic contraction of ABRM. However, in the "catch" of the ABRM there was no regain in force following the vibration induced inhibition. The contractile proteins are considered to be a locked state during the catch situation. Thus, this contracted state seems to be released by vibrations. It is therefore concluded that vibrations do interfere with the interrelationship between the myofilaments. This conclusion supports the previously forwarded hypothesis that vibrations act by increasing the rate of detachment of actin-myosin crosslinks in vertebrate smooth muscle.