Effect of pregnancy on the uterine vasoconstrictor response to exercise in rats.

A major maternal adaptation in pregnancy is the large increase in uteroplacental blood flow that supplies the growing fetus with oxygen and nutrients. The impact of gestation on the dynamic uterine vasoconstrictor response to exercise in the rat, a common model for pathophysiological disorders in pregnancy remains unknown. We hypothesized that rats exhibit a robust uterine vasoconstrictor response to acute exercise that is attenuated in late pregnancy. Pregnant (P, N = 12) and nonpregnant (NP, N = 8) rats were instrumented chronically with a ultrasonic transit-time flowprobe and carotid arterial catheter to directly measure uterine artery blood flow (UtBF) and blood pressure (BP), respectively, at day 20 of gestation for 5 min of treadmill exercise (7 m/min; 6% grade). Preexercise UtBF [P, 2.1 (SD1.6) vs. NP, 0.5 (SD0.3) mL/min P < 0.01) and uterine artery conductance (UtC) [P, 2.1(SD1.7) vs. NP, 0.4 (SD0.2) mL/min × mmHg(-1) × 10(-2), P < 0.01] were higher in pregnant rats, whereas preexercise BP was lower in the pregnant rats [P, 111 (SD13) vs. NP, 126 (SD13) mmHg, P = 0.02]. Preexercise heart rate was similar [P, 457 (SD30) vs. NP, 454 (SD42), P = 0.3]. Exercise initiated rapid and sustained decreases in UtBF [Δ-47% (SD12)] and UtC [Δ-49% (SD12)] that were attenuated in the pregnant rats [UtBF, Δ-25% (SD20) and UtC, Δ-30% (SD20), P = 0.02]. The BP and heart rate responses to exercise were unaffected in late pregnancy (interaction term, P = 0.3). In rats, dynamic exercise induces a uterine vasoconstrictor response that is blunted during late gestation, a response that we observed previously in pregnant rabbits.