Differential reactivity of the gravid uterine vasculatures: effects of norepinephrine.

Norepinephrine dose-conductance-response relationships were determined for the placental and nonplacental vasculatures of ewes between 95 and 130 days of gestation with the use of radioactive-labeled microspheres injected under flow meter guidance. The regression line for the placental circulation differed significantly from that for the nonplacental circulation (P less than 0.01). The decrease in nonplacental conductance was 30 to 45% of control greater than the decrease in placental conductance. These responses are consistent with the hypothesis that the site of placental vascular resistance is in vessels of larger caliber than those which are usually the primary site of resistance to blood flow.