Effect of gestational stage on uterine artery blood flow during exercise in rabbits.

We tested the hypothesis that the uterine artery vasoconstrictor response to graded exercise during early gestation would be similar to the nonpregnant (NP) state and would be attenuated at mid and term gestation. Responses to graded treadmill exercise were measured in six female New Zealand White rabbits in the NP state and at day 10, day 20, and day 28 (term) of gestation. Uterine artery blood flow (UtBF) was measured continuously with a Transonic flow probe. Rabbits performed a graded exercise test to voluntary exhaustion (maximal exercise) starting at 7 m/min, 7% grade. UtBF and uterine artery conductance (UtC) decreased similarly during graded exercise (P < 0.01) in the NP state [at maximal exercise: -40% (SD 20) for UtBF and -45% (SD 14) for UtC] and at day 10 of gestation [at maximal exercise: -48% (SD 17) for UtBF; -56% (SD 14) for UtC]. In contrast, there was little change in UtBF or UtC during graded exercise at day 20 [at maximal exercise: -4% (SD 17) for UtBF, P < 0.05 vs. NP; -16% (SD 12) for UtC, P < 0.01 vs. NP] and at day 28 [At maximal exercise: +7% (SD 15) for UtBF, P < 0.01 vs. NP; -2% (SD 24) for UtC, P < 0.01 vs. NP], indicating substantial attenuation of the uterine artery vasoconstrictor response to exercise. At rest, UtC responses to graded doses of intravenous phenylephrine were unaffected by the stage of gestation, which suggests that uterine artery responsiveness to alpha(1)-adrenoreceptor stimulation is preserved through gestation. Normal pregnancy in the rabbit is associated with attenuation of the uterine artery vasoconstrictor response to graded exercise that develops by mid gestation.