MYOENDOMETRIAL VERSUS PLACENTAL UTERINE ARTERIES - STRUCTURAL, MECHANICAL, AND FUNCTIONAL DIFFERENCES IN LATE-PREGNANT RABBITS

OBJECTIVE: This study compared late-pregnant radial uterine arteries t hat supplied the placenta versus the myoendometrium to evaluate differ ences in active and passive mechanical properties. STUDY DESIGN: Press urized segments of placental versus myoendometrial radial uterine arte ries from late-pregnant (day 28 to 30) New Zealand White rabbits (n = 12) were compared in vitro for differences in luminal diameter, wall t hickness, distensibility, and intrinsic tone as a function of transmur al pressure. RESULTS: Both types of arteries responded to increased tr ansmural pressure with active vasoconstriction; however, the amount of tone present in myoendometrial arteries was significantly greater tha n in placental arteries (percent tone at 75 mm Hg = 39% +/- 3% for myo endometrial versus 31% +/- 2% for placental arteries, p < 0.01). Measu rements of unpressurized, fully relaxed arteries revealed that placent al arteries were 38% larger in diameter and had thicker walls than myo endometrial arteries did. However, myoendometrial arteries were signif icantly more distensible al transmural pressures >5 mm Hg. CONCLUSIONS : The increased size and diminished tone of placental compared with ad jacent myoendometrial arteries would favor increased blood flow to the placenta; differences in size and passive mechanical properties sugge st that a localized factor(s) originating from the fetus or placenta c ontributes to the gestational enlargement of those arteries that perfu se the placenta.