Paradoxical relaxation of arterial strips induced by vasoconstrictive agents.

Longitudinal strips isolated from canine distributing muscular arteries were relaxed by noradrenaline and contracted by papaverine. This paradoxical relaxation and contraction may be due to a helical and circular arrangement of smooth muscle in the wall which is incompressible. Various vasoconstrictive agents, including noradrenaline, produced a circumferential contraction and longitudinal relaxation in muscular arteries. Vasodilator agents produced the opposite effect. The direction and magnitude of response of a helical strip to a vasoactive substance depended upon its sectioning angle from the transverse axis of the vessel.