Flow-induced resistance artery tone: balance between constrictor and dilator mechanisms.

Infusion of physiological saline solution into the lumen of an isolated segment of a resistance artery results in constriction when the vessel is relaxed and in dilation when it is contracted. There is a level of wall tone at which infusion causes no change. In the small rabbit ear arteries used in this study (approximately 200 microns OD) this balance (null) point for infusion rates of 35 microliters/min is 69 +/- 4% of the maximum possible tone development. When tone is set at different levels by norepinephrine, infusion tends to change the level of wall force in the direction of the null point for that artery. The magnitude of flow-induced relaxation can be correlated positively (r = 0.74; P less than 0.001) and the flow-induced constriction correlated negatively (r = -0.67; P less than 0.001) with the level of wall tone. The 95% confidence prediction bands for flow relaxation and contraction against level of wall tone overlap when extrapolated to zero response. These results are consistent with the hypothesis that flow-induced change in resistance artery tone is the result of a balance between independent constrictor and dilator influences.