Effects of norepinephrine on mechanics of arteries in vitro.

The effects of smooth muscle activation on the pressure dependence of arterial wall characteristic impedance were studied with isolated segments of canine iliac and carotid arteries. Measurements of external diameter and transmural pressure were made before and after activation of the arterial smooth muscle (SM) by norepinephrine (NE) in concentrations of 0.5 and 5 mug/ml and used to compute values of characteristic impedance (Z0). In the absence of SM tone, values of Z0 for both arterial sites increased monotonically with transmural pressure. For the larger [NE], values of ZO exhibited a minimum at pressures of the order of 125 mmHg and increased for both larger and smaller values of pressure, For the smaller [NE], values of Z0 showed a similar pressure dependence but with a broader minimum. It is concluded that the previously demonstrated constancy of vascular impedance with changes in arterial pressure is at least the result of the presence of a tonic level of SM activation in conduit arteries.