Contractile mechanics of venous smooth muscle

Abstract

Helically cut strips from the cat portal vein were set up in vitro for "pleotonic" stretching, whereby the tension on the vein was increased and decreased by the controlled addition and removal of Hg to a capsule hanging on the vein. Tension and length changes obtained during these dynamic stretch cycles were directly recorded on an X-Y recorder. The records were in the form of a loop, the ascending limb of which was displaced by constrictor or dilator drugs, but the peaks and descending limbs always superimposed with adequate stretches. The area of this loop was measured as the net work dissipated during the stretch cycle. This net work was found to be quantifiable in terms of the expression: a + b(c)x, where x was the unstretched muscle length. A variety of constrictor and dilator agents were tested and found to yield values in conformity with this equation with no change in the a, b, or c constants for a given vein. Large increases in K ion, used to evoke the viscous changes that have been attributed to tonoactomyosln, yielded results which departed significantly from the calculated values. It is concluded that the tono-actomyosin phenomenon does not appear to play a role in normal constrictor or dilator mechanisms.