Objective
We sought to determine the effect of an endothelin type A receptor antagonist (ETA) on uterine artery resistive index (UARI) and mean arterial pressure (MAP) in a placental ischemia rat model of preeclampsia produced by reduction in uterine perfusion pressure (RUPP).
Study Design
UARI was assessed by Doppler velocimetry in RUPP and normal pregnant controls (NP) on gestational days (GD) 12, 15, and 18. UARI was also determined on GD 18 in NP and RUPP pregnant dams after pretreatment with ETA. MAP was recorded on GD 19.
Results
The RUPP group had a higher MAP and UARI on GD 15 and 18 than the NP group. Pretreatment with ETA attenuated both the MAP and GD-18 UARI in the RUPP group without affecting these parameters in the NP group.
Conclusion
The improvement in UARI could be one potential mechanism for the reduction in MAP in response to ETA in pregnant dams with ischemic placentas.
Preeclampsia complicates about 5-8% of pregnancies and is a significant cause of maternal-fetal morbidity. The pathophysiology of this condition is not fully understood and remains a subject under active investigation. There is evidence that suggests that endothelin 1 may cause hypertension during pregnancy by the activation of endothelin type A receptor in response to several vascular mediators of preeclampsia including tumor necrosis factor-α, agonistic autoantibodies to the angiotensin II type I receptor, and soluble fms-like tyrosine kinase 1. Endothelin is elevated in preeclamptic women and it increases uterine vascular resistance. A rise in endothelin 1 associated with hypertension was noted following a reduction of uteroplacental perfusion (RUPP) in pregnant dams. Therefore, we tested the hypothesis that pretreatment with an endothelin type A receptor antagonist (ETA) would reverse chronic placental ischemia-induced hypertension by lowering the uterine vascular resistance in pregnant dams. To test this hypothesis, the changes in uterine artery resistive index (UARI) with gestational age in RUPP-treated pregnant dams and untreated controls were first studied.
Materials and Methods
Pregnant Sprague-Dawley rats (Harlan Sprague Dawley Inc, Indianapolis, IN) were used in the study. Animals were housed in a temperature-controlled room (23°C) with a 12-/12-hour light/dark cycle. All experimental procedures executed in this study were in accordance with the National Institutes of Health guidelines for use and care of animals. The study protocol was approved by the Institutional Animal Care and Use Committee at the University of Mississippi Medical Center.
Experiments were performed in the following 4 groups of rats: normal pregnant controls (NP) (n = 12), RUPP-treated pregnant dams (n = 17), ETA-treated pregnant dams (NP/ETA; n = 8), and RUPP- and ETA-treated pregnant dams (RUPP/ETA; n = 9). Pregnant dams that underwent surgical procedures and ultrasonography were anesthetized with 2% isoflurane (WA Butler Co, Memphis, TN) delivered by an anesthesia apparatus (Vaporizer for Forane Anesthetic; Ohio Medical Products, Madison, WI).
Pregnant dams in the RUPP groups underwent laparotomy on gestational day (GD) 14, the lower abdominal aorta was isolated, and a silver clip (0.203-mm internal diameter) was placed around the aorta above the iliac bifurcation. The right and left ovarian arteries were also constricted using a silver clip (0.100 mm internal diameter), as described previously. Pregnant dams treated with ETA, ABT 627, received the agent in their drinking water (5 mg/kg/d) on GD 12-19. The concentration of the ETA in the drinking water was 0.11 mmol/L.
Power Doppler velocimetry measurements were performed on anesthetized pregnant dams at an imaging station with a Vevo 770 unit (Visual Sonics, Toronto, Ontario) using a 30-Hz transducer and an insonating angle <30 degrees. The peak systolic flow velocity (PSV) and end-diastolic flow velocity (EDV) were recorded using the uterine artery Doppler waveform. The UARI was calculated using the following formula: UARI = (PSV – EDV)/PSV. UARI was determined for the uterine artery bilaterally at 3 levels and the mean UARI was calculated. UARI was measured in the RUPP and NP on GD 12, 15, and 18. UARI was also determined on GD 18 in NP and RUPP pregnant dams after pretreatment with ETA. Rats were also surgically instrumented with a carotid catheter on GD 18 for subsequent mean arterial pressure (MAP) measurement on GD 19. Pup weight, placenta weight, litter size (live and reabsorbed pups), and proportion of live pups in the litter were recorded at harvest on GD 19.
All data are expressed as mean ± SEM. Difference between control and experimental groups were analyzed using analysis of variance with Tukey-Kramer multiple comparison tests. The t test was used when comparing 2 groups of dams. Data were considered statistically different at P values < .05.
Results
The UARI in NP dams was relatively unchanged up to GD 15 and was noted to be lower by GD 18. UARI for the RUPP dams was similar to the untreated controls on GD 12 and rose significantly by the day following treatment and continued to remain higher than the controls on GD 18 ( Figure , A). The mean UARI in NP and RUPP groups were 0.59 ± 0.02 vs 0.57 ± 0.01 ( P = .423), 0.60 ± 0.02 vs 0.71 ± 0.02 ( P < .001), and 0.54 ± 0.03 vs 0.67 ± 0.02 ( P < .001) on GD 12, 15, and 18, respectively.
When compared to NP, the MAP was significantly elevated in RUPP dams. The MAP in the NP and RUPP groups were 104 ± 1 and 129 ± 2 mm Hg, respectively ( P < .001).
Figure, B, illustrates that pretreatment with ETA attenuated both the GD-19 MAP and GD-18 UARI in the RUPP group (115 ± 1 mm Hg, P < .001, and 0.58 ± 0.02, P < .001, respectively) without affecting these parameters in the NP group (98 ± 2 mm Hg, P = .054, and 0.55 ± 0.02, P = .150, respectively).
The effect of ETA pretreatment on selected pregnancy outcomes such as pup weight, placenta weight, litter size, and proportion of live pups in the litter is summarized in the Table . A lower proportion of pups were alive in the litter from RUPP dams than NP dams (57% vs 92%, P < .05) and this was improved by ETA pretreatment (82% vs 99%). ETA pretreatment did not, however, result in any significant changes in RUPP dams’ litter size, pup weight, or placenta weight when compared to NP dams. ETA pretreated NP dams had significantly higher placenta weight (g) than the other 3 groups (0.67 ± 0.03 vs 0.54 ± 0.03 [NP], 0.51 ± 0.02 [RUPP], and 0.48 ± 0.03 [RUPP/ETA]; P < .05).
