Dissociation of electrical and mechanical activity caused by vibrations in the spontaneously active smooth muscle of the rat portal vein.

The effects of vibrations on the electrical membrane discharge and on the contractile force of the spontaneously active smooth muscle of isolated rat portal vein were studied. The electrical activity was recorded extracellularly and quantitatively related to the mean active force. Sinusoidal vibrations (40 Hz, 2.5--3.0% tissue length peak to peak), applied in the longitudinal direction of the smooth muscle, caused prompt and reversible reduction of active force but neither the pattern of the phasic contractions nor the electrical membrane discharge was altered. The degree of inhibition of mechanical activity increased with vibration amplitude, activity being 50% of control at a vibration amplitude of 12 +/- 4% (mean +/- S.D., n = 8). It is concluded that the induced length changes caused prompt dissociation between electrical membrane discharge and mechanical force development in the vascular smooth muscle. This finding adds support to the previously forwarded hypothesis that vibrations cause inhibition of contracting muscle by direct action on the contractile process.