AJP - Heart Calcium Transients and Cell-Sarcomere
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Am J Physiol Heart Circ Physiol 287: H126-H134, 2004. First published February 19, 2004; doi:10.1152/ajpheart.00046.2003
0363-6135/04 $5.00
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow All Versions of this Article:
287/1/H126    most recent
00046.2003v1
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Right arrow Citation Map
Services
Right arrow Email this article to a friend
Right arrow Similar articles in this journal
Right arrow Similar articles in ISI Web of Science
Right arrow Similar articles in PubMed
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Citing Articles
Right arrow Citing Articles via HighWire
Right arrow Citing Articles via ISI Web of Science (4)
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Cox, B. E.
Right arrow Articles by Rosenfeld, C. R.
Right arrow Search for Related Content
PubMed
Right arrow PubMed Citation
Right arrow Articles by Cox, B. E.
Right arrow Articles by Rosenfeld, C. R.

Angiotensin II mediates uterine vasoconstriction through {alpha}-stimulation

Blair E. Cox, Timothy A. Roy, and Charles R. Rosenfeld

Department of Pediatrics, The University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75390

Submitted 17 January 2003 ; accepted in final form 17 February 2004


   ABSTRACT
TOP
 ABSTRACT
METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Intravenous angiotensin II (ANG II) increases uterine vascular resistance (UVR), whereas uterine intra-arterial infusions do not. Type 2 ANG II (AT2) receptors predominate in uterine vascular smooth muscle; this may reflect involvement of systemic type 1 ANG II (AT1) receptor-mediated {alpha}-adrenergic activation. To examine this, we compared systemic pressor and UVR responses to intravenous phenylephrine and ANG II without and with systemic or uterine {alpha}-receptor blockade and in the absence or presence of AT1 receptor blockade in pregnant and nonpregnant ewes. Systemic {alpha}-receptor blockade inhibited phenylephrine-mediated increases in mean arterial pressure (MAP) and UVR, whereas uterine {alpha}-receptor blockade alone did not alter pressor responses and resulted in proportionate increases in UVR and MAP. Although neither systemic nor uterine {alpha}-receptor blockade affected ANG II-mediated pressor responses, UVR responses decreased >65% and also were proportionate to increases in MAP. Systemic AT1 receptor blockade inhibited all responses to intravenous ANG II. In contrast, uterine AT1 receptor blockade + systemic {alpha}-receptor blockade resulted in persistent proportionate increases in MAP and UVR. Uterine AT2 receptor blockade had no effects. We have shown that ANG II-mediated pressor responses reflect activation of systemic vascular AT1 receptors, whereas increases in UVR reflect AT1 receptor-mediated release of an {alpha}-agonist and uterine autoregulatory responses.

angiotensin receptors; autoregulation; pregnancy; uterine blood flow; blood pressure

MAMMALIAN PREGNANCY IS ASSOCIATED with numerous cardiovascular changes (14, 28, 46, 49), including attenuated pressor responses to systemic infusions of angiotensin II (ANG II) and catecholamines, suggesting decreased peripheral vascular sensitivity (1, 8, 22, 31, 42, 53). As in the kidney (28), the uterine vascular bed also becomes refractory to these agents (18, 32, 42, 49, 52, 53); however, there is greater uterine sensitivity to {alpha}-agonists than to ANG II (10, 13, 31, 48, 49). In contrast, systemic pressor responses to ANG II generally exceed those observed with {alpha}-agonists (42, 48, 49). These differences in systemic and uteroplacental vascular responsiveness to these vasoconstrictors are unclear, but they may be due to inherent mechanisms necessary for maintaining uterine blood flow (UBF) during pregnancy, when circulating levels of ANG II are normally increased. This theory is supported by reports that systemic (22, 58) and uteroplacental (17) refractoriness to ANG II and catecholamines is lost in women with pregnancy-induced hypertension, which may contribute to increases in fetal growth restriction and fetal/neonatal mortality and morbidity (15, 28, 46). Thus it is important to understand the mechanisms that mediate uterine vascular refractoriness in normal mammalian pregnancy.

The uterine refractoriness to systemic ANG II infusions is not due to increases in ANG II clearance or decreases in ANG II (AT) receptor binding density in uterine vascular smooth muscle (VSM) (29, 40, 48). However, as in the renal vasculature (21, 28), basal synthesis of uterine artery endothelium-derived prostacyclin (PGI2) and nitric oxide (NO) increases substantially in normal ovine pregnancy, and ANG II further augments their synthesis (30, 32), providing local mechanisms whereby responses to infused ANG II or other agonists may be attenuated. Alternatively, differences in AT receptor expression in uterine and systemic VSM could contribute to the attenuated uterine responses to infused ANG II. In women and sheep, uterine VSM predominantly expresses AT2 receptors, which do not mediate VSM contractions (3, 11, 13) but may be associated with VSM relaxation (27, 33, 59). In contrast, AT1 receptors predominate in systemic VSM and are responsible for VSM contraction and, probably, systemic pressor responses (3, 11). If AT2 receptors predominate in uterine VSM but do not mediate vasoconstriction, this could contribute to the differences in uterine and systemic responses to intravenous ANG II infusions. If this is the case, it is unclear how intravenous ANG II infusions increase uterine vascular resistance (UVR) in women and sheep.

Uterine vascular responses to intravenous ANG II infusions are well described (10, 18, 42, 48, 53), whereas responses to uterine intra-arterial ANG II infusions are not as well studied (10, 12). In a recent report, we (12) observed that intravenous ANG II dose dependently increased mean arterial pressure (MAP) and UVR and decreased UBF in the expected manner. In contrast, infusions via a uterine artery catheter had no pressor effect and did not alter UVR or UBF, suggesting that intravenous ANG II increases UVR via secondary mechanisms that could include increases in endothelin release (7), sympathetic outflow (4, 45), or adrenal catecholamine secretion (5, 44). In the present studies, we determined whether systemic and uterine vascular responses to intravenous ANG II infusions are due to different mechanisms and whether this is altered in the final month of ovine pregnancy. We hypothesized that systemic pressor responses reflect peripheral VSM AT1 receptor activation, whereas increases in UVR are due to ANG II-mediated release of {alpha}-agonists into the systemic circulation.


   METHODS
TOP
 ABSTRACT
 METHODS
RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 
Animal preparation. Twenty pregnant ewes ≥115 days gestation (full term = 145 ± 5 days) and 13 nonpregnant oophorectomized ewes of mixed Western breed were included in these studies. The chronically instrumented animal preparations used in these studies have been described in detail elsewhere (12, 42, 52). Briefly, with the animals under inhalation anesthesia, electromagnetic flow probes (Carolina Medical, King, NC) were implanted on both main uterine arteries (6.0–7.0 and 3.0–3.5 mm ID for pregnant and nonpregnant animals, respectively). Polyvinyl catheters filled with heparinized saline (250 U/ml) were inserted retrograde 1–2.5 cm into a distal branch of the uterine artery supplying each uterine horn for local drug infusions and into the lower abdominal aorta (the tip lying at the trifurcation) and inferior vena cava (the tip lying just below the diaphragm) via a femoral artery and vein, respectively, to monitor arterial pressure and infuse drugs systemically, respectively. The ovaries of nonpregnant ewes were surgically removed. The flow probes and catheters were externalized to the flank through a subcutaneous tunnel and maintained in a canvas pouch attached to the skin with steel pins. The catheters were flushed daily with heparinized saline (250 U/ml) and closed with sterile pins. Intramuscular penicillin (600,000 U) and gentamicin (40 mg) were given on the day of surgery and on the following 2 postoperative days. Intravenous benamine was given for postoperative discomfort and pain. Each animal recovered for ≥5 days before the studies were initiated. Ovariectomized nonpregnant ewes received daily estrogen replacement with estradiol-17{beta} (Sigma Chemical, St. Louis, MO; 1.0 g·kg–1·day–1) beginning on postoperative days 4–5, but not within 24 h of a study, as previously described (12, 31). Castrated nonpregnant ewes were studied to control for cyclic changes in ovarian estrogen and/or progesterone synthesis, which are known to modify vascular sensitivity to ANG II and {alpha}-agonists (49). These studies were approved by the Institutional Animal Care and Research Advisory Committee at the University of Texas Southwestern Medical Center.

Experimental protocols. Five protocols were designed to assess the mechanisms whereby intravenous ANG II infusions mediate systemic pressor and uterine vascular responses. When possible, animals were included in several protocols; however, malfunction of infusion catheters or flow probes often precluded achieving this goal. Pregnant animals were studied at several gestational ages in each protocol to determine whether changes in the maternal hormonal milieu and/or vasculature in the last month of pregnancy modified the pressor and/or uterine vascular responses to the agonists and/or antagonists used (49, 52), precluding their inclusion in all protocols. All pregnant ewes delivered a live-born lamb at completion of these studies. Nonpregnant animals were studied once in each protocol. Baseline hemodynamic data were obtained in each protocol before infusion of a drug, and experimental values were obtained at 5–7 min of a constant infusion, when steady-state responses for MAP and UBF were established (13, 32, 48).

In protocol 1, we characterized the systemic pressor and uterine vascular responses to the {alpha}1-agonist phenylephrine (PE) and the efficacy of systemic infusion of an {alpha}-receptor antagonist in inhibiting these responses. This permitted us to subsequently compare responses to infused ANG II. PE (Sigma Chemical) was diluted in sterile isotonic saline and infused through a femoral venous catheter with a constant-infusion pump (Harvard Apparatus, S. Natick, MA) at 0.75–1.5 µg·min–1·kg–1 for 7 min to establish steady-state responses. This dose decreases UBF ~30% in pregnant and nonpregnant ewes and is comparable to responses seen with systemic infusions of ANG II at 0.046 µg·min–1·kg–1, which result in physiological blood levels of the peptide (38). This was repeated in the presence of a continuous intravenous infusion of the {alpha}-receptor antagonist phentolamine (Phen; Sigma Chemical) at 180 µg·min–1·kg–1, which was initiated 5 min before and continued throughout the PE infusion. This dose of Phen was observed in preliminary studies to inhibit PE-induced decreases in UBF in pregnant and nonpregnant ewes. Nine experiments were performed in four pregnant ewes between 132 and 144 days gestation and one each in seven nonpregnant sheep.

Protocol 2 was designed to determine whether {alpha}-receptor activation mediated systemic pressor and uterine vascular responses to intravenous ANG II infusions. ANG II (human; Sigma Chemical) was diluted in sterile isotonic saline to 3 µg/ml and infused via a femoral venous catheter at 0.046 µg·min–1·kg–1 in the absence of systemic {alpha}-receptor blockade. As noted above, this dose of ANG II decreases UBF ~30% and results in physiological plasma levels of the peptide (38); therefore, responses are more likely to have physiological relevance. This procedure was repeated in the presence of systemic {alpha}-receptor blockade with Phen at 32–180 µg·min–1·kg–1, permitting the establishment of dose-inhibition curves that included a Phen dose that abolished ANG II-induced decreases in UBF. Experiments were performed in eight pregnant ewes between 124 and 149 days gestation that were not included in protocol 1 and in the seven nonpregnant sheep included in protocol 1. Studies were initiated in the latter 30 min after completion of protocol 1, allowing for the clearance of {alpha}-receptor blockade and for return of hemodynamic parameters to baseline. In preliminary studies, we observed that responses to PE at that time were similar to those seen before {alpha}-receptor blockade with Phen.

Protocol 3 was designed to determine the effects of local uterine artery infusions of the vascular {alpha}-receptor antagonist Phen on systemic pressor and uterine vascular responses to intravenous PE and ANG II infusions in pregnant ewes. PE was infused through a femoral venous catheter at 0.75–1.5 µg·min–1·kg–1 for 7 min in the absence or presence of a continuous infusion of Phen via a uterine artery catheter calculated to achieve an arterial concentration of ~10 ng/ml, which in preliminary studies abolished PE-induced decreases in UBF. The rate of infusion in nanograms per minute was calculated for each experiment from the desired arterial concentration (ng/ml) x UBF (ml/min), which was continuously measured with flow probes (12). The study was repeated using intravenous ANG II (0.046 µg·min–1·kg–1) in the absence and then presence of uterine {alpha}-receptor blockade with Phen. All but one of the animals included in these studies were studied in protocol 1. Five received PE + local Phen between 132 and 144 days gestation, and three received ANG II + local Phen between 120 and 142 days gestation.

During the course of these studies, increases in UVR during systemic ANG II infusions were not completely blocked by systemic (protocol 2) or uterine (protocol 3) {alpha}-receptor blockade. Because AT1 receptors account for 15–20% of uterine VSM AT receptor binding (11, 13), their activation might account for the residual effects of ANG II on UVR. Therefore, we examined the vascular responses to intravenous infusions of ANG II + Phen in the absence and presence of uterine AT1 receptor blockade in pregnant and nonpregnant ewes (protocol 4). L-158809, a specific AT1 receptor antagonist (kindly provided by Merck Pharmaceuticals, Rahway, NJ), was reconstituted in isotonic saline (1 mg/ml) and infused over 1 min via a uterine artery catheter in 0.1-mg incremental bolus doses while MAP, heart rate, and UBF were continuously monitored. At 5 min after completion of each infusion, systemic {alpha}-receptor blockade was induced with Phen (180 µg·min–1·kg–1) followed by the intravenous infusion of ANG II (0.046 µg·min–1·kg–1). This sequence was repeated until the rise in MAP decreased from that observed with intravenous ANG II alone. At this point, L-158809 was considered to have saturated uterine VSM AT1 receptors and "spilled over" into the systemic circulation, blocking systemic VSM AT1 receptors and altering pressor responses, an approach previously used in studies of ANG II (12). The total cumulative intra-arterial dose of L-158809 was 0.3–0.6 mg in pregnant and nonpregnant ewes. Studies were performed in three pregnant ewes between 134 and 146 days gestation and four nonpregnant animals. Because of the ever-present question of the role of the AT2 receptor in modulating UBF and vascular responses (27, 33), the effect of uterine VSM AT2 receptor blockade was similarly studied using the AT2 receptor antagonist PD-123319 (kindly provided by Parke-Davis Pharmaceutical, Ann Arbor, MI). Cumulative intra-arterial bolus doses were 0.3–12.0 and 0.15–0.3 mg in pregnant and nonpregnant animals, respectively. The dose range was expanded in the former because of high basal UBF. Studies were performed in three pregnant sheep between 133 and 145 days gestation and in four nonpregnant ewes.

Having demonstrated in protocols 1–4 that ANG II mediates 65–70% of the rise in UVR through a secondary mechanism involving an {alpha}-agonist that does not facilitate the pressor response (see RESULTS) and knowing that adrenal catecholamine secretion occurs via an AT1 receptor-mediated mechanism (57), we examined the effects of systemic AT1 receptor inhibition on ANG II-mediated systemic pressor responses and increases in UVR (protocol 5). Systemic and uterine responses to ANG II (0.046 µg·min–1·kg–1) were determined in the absence and presence of systemic AT1 receptor blockade with L-158809, which was infused intravenously (0.5 mg) over 1 min in three pregnant ewes studied between 130 and 145 days gestation.

Hemodynamic measurements. During all studies, MAP, heart rate, and UBF were continuously monitored with a six-channel pen recorder (Gould, Cleveland, OH), and data were recorded using the PONEMAH data acquisition system (PNM-P3-P-010, Gould, Valley View, OH). MAP and heart rate were monitored through a femoral artery catheter attached to a pressure transducer (type 4-327-0109, Bell and Howell, Pasadena, CA) connected to an amplifier (model N-4307-04, Gould); heart rate was obtained from the integrated phasic signal. UBF was monitored with electromagnetic flowmeters (model 501, Carolina Medical) with linear responses to flows in the range studied and a flow signal and zero-flow calibration. Flow probes are routinely calibrated in vitro and have a measurement error of 5–10%. Baseline UBF in nonpregnant ewes is 15–30 ml/min in each uterine horn; thus the sensitivity of the recording system was increased to quantify the changes in UBF in nonpregnant sheep as previously reported (12, 31). UVR (mmHg·min·l–1) was calculated as MAP (mmHg) ÷ UBF (ml/min). No animal was studied on consecutive days. Measurements of UBF and UVR reflect the uterine horn being studied, because in several animals there was a nonfunctional flow probe during the course of these studies.

Statistical methods. Experimental observations derived from multiple experiments in a ewe were averaged, resulting in an n value that reflects averaged data from a single animal. Because basal measurements of hemodynamic variables differ in pregnant and nonpregnant ewes (Table 1) and have different units of measurement and we wished to compare responses between groups and variables, we examined the relative changes in each variable, i.e., the percent change from baseline, which takes these differences into account (12, 42). Student's t-test was used to determine changes from baseline, with Welch's approximation applied where applicable. Repeated-measures ANOVA was used to examine changes across doses. Two-way ANOVA and all pairwise multiple comparison procedures (Tukey's test) were used to determine differences between responses to systemic and local ANG II infusions. Linear regression analysis with the least squares method was employed to determine whether there were differences in responses to agonists and/or antagonists across gestational ages. Values are means ± SE.


View this table:
[in this window]
[in a new window]
  		Table 1. Basal hemodynamic measurements

 

   RESULTS
TOP
 ABSTRACT
 METHODS
 RESULTS
DISCUSSION
 GRANTS
 REFERENCES
 
Baseline hemodynamic data. Cardiovascular measurements were obtained from 9 pregnant ewes between 124 and 149 days gestation and 11 nonpregnant ewes before infusion of PE, ANG II, or Phen to examine the differences in basal hemodynamics. As expected (47), basal heart rate and UBF were greater (Table 1) and UVR was decreased 95% (P < 0.001) in pregnant animals. MAP was lower, but this was not statistically significant. There were no detectable changes in these variables in the last 3 wk of pregnancy (P > 0.2 by regression analysis), likely reflecting the relatively short period of pregnancy studied. Because significant differences exist in basal hemodynamic values in pregnant and nonpregnant animals, comparisons should be made using the relative changes (12, 42).

Next, we analyzed data collected before and after systemic and/or local uterine vascular {alpha}-blockade with Phen to examine the effects of {alpha}-adrenergic blockade on basal hemodynamics (Table 2). Basal MAP, UBF, and UVR were unaffected by systemic Phen infusions of 32–180 µg·min–1·kg–1 in pregnant and nonpregnant ewes (P ≥ 0.1). There was, however, a modest rise in heart rate in both groups (Table 2). Uterine artery infusions of Phen calculated to achieve a concentration of ~10 ng/ml also had no effect on basal UBF or UVR in pregnant ewes and did not affect MAP (P ≥ 0.8; Table 2).


View this table:
[in this window]
[in a new window]
  		Table 2. Effects of systemic and local uterine artery {alpha}-blockade on basal MAP, HR, and UBF in pregnant and nonpregnant ewes

 
Effects of systemic {alpha}-receptor blockade on responses to intravenous PE and ANG II. The effects of systemic {alpha}-blockade on responses to intravenous PE were examined in four pregnant ewes between 124 and 149 days gestation and seven nonpregnant sheep (protocol 1). In this and subsequent studies, there was no significant effect of gestational age (P > 0.1 by regression analysis) on the responses by any hemodynamic variable to {alpha}-receptor stimulation, ANG II, or {alpha}-receptor inhibition in the last 3 wk of gestation; thus studies performed at different gestational ages were averaged to provide a single value for each pregnant animal included in the analysis. Intravenous PE (0.75–1.5 µg·min–1·kg–1) consistently increased (P < 0.05) MAP and UVR and decreased (P < 0.05) UBF in the pregnant animals (Fig. 1A). However, the rise in UVR was nearly threefold greater than the rise in MAP. These responses were abolished during continuous intravenous infusions of Phen (180 µg·min–1·kg–1). Nonpregnant ewes responded similarly (Fig. 1B); i.e., intravenous PE (1.0 µg·min–1·kg–1) increased (P < 0.05) MAP and UVR while decreasing (P < 0.05) UBF, and the rise in UVR was more than threefold greater than the rise in MAP. As in the pregnant group, systemic {alpha}-receptor blockade with Phen abolished (P < 0.005) all the hemodynamic responses to PE (Fig. 1B).



View larger version (26K):
[in this window]
[in a new window]
  		Fig. 1. Effects of phenylephrine (PE) infusions (0.75–1.5 µg·min–1·kg–1 iv) in 4 pregnant (A) and 7 nonpregnant (B) ewes in the absence (solid bars) and presence (open bars) of a continuous systemic infusion of the {alpha}-receptor antagonist phentolamine (Phen, 180 µg·min–1·kg–1; protocol 1). Relative changes in mean arterial pressure (MAP), uterine vascular resistance (UVR), and uterine blood flow (UBF) are presented to permit comparisons between groups and hemodynamic variables. *Significantly different from baseline (P < 0.05 by t-test). {dagger}Significantly different from absence of Phen (P < 0.05 by paired t-test).

 
To determine whether the vascular responses to systemic ANG II infusions involve {alpha}-receptor activation, intravenous ANG II was infused before and after systemic {alpha}-receptor blockade with Phen (protocol 2) in eight pregnant ewes between 124 and 149 days gestation. As noted earlier, there was no effect of gestational age on the responses. Systemic ANG II (0.046 µg·min–1·kg–1) alone increased (P < 0.005) MAP 35 ± 3% (Fig. 2A) and UVR 79 ± 7.7% (Fig. 2B) while decreasing (P < 0.001) UBF 22 ± 2% (Fig. 2C). Systemic {alpha}-receptor blockade with Phen dose dependently inhibited the ANG II-mediated rise in UVR (Fig. 2B; P = 0.006 by ANOVA) and fall in UBF (Fig. 2C; P < 0.001 by ANOVA) but did not affect the rise in MAP (Fig. 2A; P = 0.5 by ANOVA). Although the highest dose of Phen completely inhibited (P = 0.1 by paired t-test) the fall in UBF, UVR rose >32%, demonstrating a residual effect of ANG II on UVR that was proportionate to the rise in MAP.



View larger version (16K):
[in this window]
[in a new window]
  		Fig. 2. Effects of ANG II infusions (0.046 µg·min–1·kg–1 iv) in 8 pregnant ewes (124–149 days gestation) on relative changes in MAP (A), UVR (B), and UBF (C) in the absence and presence of intravenous infusions of Phen at 32–64 and 180 µg·min–1·kg–1 (protocol 2). *Significantly different from baseline (P < 0.05 by paired t-test). Different letters (a, b) within and across a hemodynamic parameter (i.e., MAP, UVR, and UBF) represent significant differences between doses (P < 0.03 by ANOVA).

 
A similar pattern of responses was observed in nonpregnant ewes (n = 6; Fig. 3). In the absence of {alpha}-receptor blockade, intravenous ANG II (0.046 µg·min–1·kg–1) increased MAP (Fig. 3A; P < 0.001) and UVR (Fig. 3B; P < 0.001) and decreased UBF (Fig. 3C; P < 0.04); however, the rise in MAP and UVR was twofold greater (P < 0.04) than in the pregnant animals. As in the pregnant group, systemic {alpha}-receptor blockade had no significant effect on the ANG II-induced rise in MAP (P = 0.4 by ANOVA; Fig. 3A), but the rise in UVR (Fig. 3B; P < 0.001 by ANOVA) and fall in UBF (Fig. 3C; P < 0.04 by ANOVA) were inhibited dose dependently. Although the highest dose of Phen abolished the fall in UBF, there again was a ~40% rise in UVR that was parallel to and proportionate with the rise in MAP.



View larger version (18K):
[in this window]
[in a new window]
  		Fig. 3. Effects of ANG II infusions (0.046 µg·min–1·kg–1 iv) in 6 nonpregnant sheep on relative changes in MAP (A), UVR (B), and UBF (C) in the absence and presence of intravenous infusions of Phen (protocol 3). *Significantly different from baseline (P < 0.05 by paired t-test). Different letters (a, b, and c) within and across a hemodynamic parameter (i.e., MAP, UVR, and UBF) represent significant differences between doses (P < 0.04 by ANOVA).

 
Effects of uterine vascular {alpha}-receptor blockade on responses to intravenous PE and ANG II. Having demonstrated that systemic {alpha}-receptor blockade abolished all the responses to intravenous PE but only inhibited the changes in UVR and UBF during intravenous ANG II, we examined the effects of local uterine artery {alpha}-receptor blockade on responses to intravenous PE and ANG II in three pregnant ewes studied between 132 and 144 days gestation (protocol 3). PE was infused in the absence and presence of uterine artery infusions of Phen. There were no gestational effects on the responses to PE or uterine {alpha}-receptor blockade. Uterine {alpha}-receptor blockade with Phen inhibited PE-induced increases in UVR 72% (P = 0.009) and decreases in UBF >82% (P < 0.001) but did not alter the 25–30% rise in MAP, demonstrating selective uterine vascular {alpha}-adrenergic blockade and maintenance of the systemic pressor response. The rise in MAP was paralleled by a proportionate rise in UVR.

To further delineate the contribution of {alpha}-adrenergic activation in the effects of intravenous ANG II on UVR and UBF and remove the effects of systemic {alpha}-receptor blockade, Phen was directly infused into the uterine vascular bed of one uterine horn (protocol 3) to achieve and maintain an arterial concentration of ~10 ng/ml, which, as noted above, inhibited the uterine responses to PE >70%. A representative experiment is illustrated in Fig. 4, where the effects of systemic ANG II alone (Fig. 4A) and the effects of systemic ANG II + Phen infused into the left uterine horn (Fig. 4B) are shown. The right uterine horn, which served as an internal control, demonstrates an intact response to intravenous ANG II; i.e., UBF falls soon after initiation of the ANG II infusion in the absence and presence of the contralateral arterial infusion of Phen. In contrast, the infused uterine horn demonstrates a markedly attenuated UBF response. Although the ANG II-mediated rise in MAP was unaffected by uterine {alpha}-receptor blockade (Fig. 5; P = 0.9), the rise in UVR was inhibited >63% (P < 0.001) and the fall in UBF >90%. Thus local uterine vascular {alpha}-receptor blockade resulted in a proportionate rise in MAP and UVR (Fig. 5).



View larger version (25K):
[in this window]
[in a new window]
  		Fig. 4. Representative traces of effects of ANG II infusions (0.046 µg·min–1·kg–1 iv) on MAP, heart rate (beats/min), and left and right UBF in a pregnant ewe at 147 days gestation in the absence (A) and presence (B) of a left uterine arterial infusion of Phen. Note absence of an effect of {alpha}-receptor blockade on baseline hemodynamic variables.

 


View larger version (14K):
[in this window]
[in a new window]
  		Fig. 5. Effects of ANG II infusions (0.046 µg·min–1·kg–1 iv) in 3 pregnant ewes (120–142 days gestation) in the absence (solid bars) and presence (open bars) of continuous uterine intra-arterial infusions of Phen (protocol 3) on relative changes in MAP, UVR, and UBF. *Significantly different from baseline (P < 0.005 by paired t-test). {dagger}Significantly different from absence of Phen (P ≤ 0.01 by ANOVA).

 
Effect of systemic {alpha}-receptor blockade + local uterine vascular AT1 receptor blockade on responses to intravenous ANG II. Although systemic infusions of Phen in pregnant and nonpregnant ewes and local uterine artery infusions in pregnant ewes decreased the rise in UVR during intravenous ANG II infusions, we observed a persistent 25–35% increase in UVR (Figs. 2B, 3B, and 5). This suggests that another mechanism is involved in the ANG II-mediated increases in UVR. Because AT1 receptors account for ~15% of AT receptor binding in uterine VSM (11, 13), this might explain the residual rise in UVR. Thus we incrementally blocked AT1 receptors in the uterine vascular bed of one horn with cumulative intra-arterial doses of L-158809 (protocol 4). Studies were performed in three pregnant ewes between 134 and 146 days gestation and four nonpregnant ewes. Systemic infusions of Phen + uterine AT1 receptor blockade did not alter the ANG II-mediated rise in UVR in pregnant or nonpregnant sheep, which was proportional to the rise in MAP (Fig. 6) and resembled the responses seen with local uterine artery {alpha}-receptor blockade in the presence of intravenous ANG II (Fig. 5; protocol 3). The fall in UBF was completely inhibited. The residual rise in UVR tended to be greater in nonpregnant animals: 53 ± 15% vs. 33 ± 4.6% (P = 0.3).



View larger version (16K):
[in this window]
[in a new window]
  		Fig. 6. Relative changes in MAP, UVR, and UBF in 5 pregnant (A; 134–146 days gestation) and 6 nonpregnant (B) ewes during ANG II infusions (0.046 µg·min–1·kg–1 iv) plus systemic {alpha}-receptor blockade with Phen (180 µg·min–1·kg–1) in the absence (solid bars) and presence (open bars) of uterine vascular AT1 receptor blockade with L-158809 (protocol 4). *Significantly different from baseline (P < 0.001 by paired t-test). There were no significant differences in responses to ANG II in the presence and absence of Phen (P ≥ 0.1), and AT1 receptor blockade did not alter systemic or uterine responses (P ≥ 0.1 by ANOVA).

 
AT2 receptors predominate in uterine VSM and may attenuate UVR responses to infused ANG II (11, 13, 27, 33). To determine whether AT2 receptor activation played a role, we infused cumulative intra-arterial doses of the AT2 receptor antagonist PD-123319 for a total dose of 0.3–12.0 mg in pregnant sheep (n = 3, 133–145 days gestation). Uterine AT2 receptor blockade did not alter basal MAP (90 ± 1 vs. 88 ± 1 mmHg, P > 0.2), UVR (109 ± 26 vs. 109 ± 29 mmHg·min·l–1, P > 0.6), or UBF (1,102 ± 52 vs. 1,103 ± 48 ml/min, P > 0.9). Responses by MAP (48 ± 5.7% vs. 37 ± 2.4%, P > 0.1), UVR (60 ± 10% vs. 45 ± 6%, P > 0.2), and UBF (–5.7 ± 4.5% vs. –5.7 ± 2.8%, P > 0.9) to intravenous ANG II also were unaffected. Nonpregnant sheep (n = 4) similarly treated with cumulative doses of 0.15–0.3 mg demonstrated no effects (P > 0.1) on basal MAP (92 ± 2 vs. 91 ± 3 mmHg), UVR (3,455 ± 104 vs. 3,450 ± 106 mmHg·min·l–1), or UBF (20 ± 4.1 vs. 20 ± 4.0 ml/min) or on ANG II-induced increases in MAP (57 ± 7.2% vs. 41 ± 6.2%) and UVR (75 ± 11% vs. 53 ± 9%) and decreases in UBF (–14 ± 3.5% vs. –9 ± 2.6%).

Although {alpha}-receptor blockade modified uterine responses to systemic ANG II infusions, the mechanism for ANG II-mediated release of {alpha}-receptor agonist was unclear. This appears to occur via an AT1 receptor-mediated event (57). Therefore, we examined the effects of systemic AT1 receptor blockade on the responses to intravenous ANG II (protocol 5) in three pregnant ewes between 130 and 145 days gestation. Systemic AT1 receptor blockade with L-158809 (0.5 mg) did not alter basal MAP (86 ± 11 vs. 86 ± 11 mmHg), UVR (227 ± 17 vs. 226 ± 16 mmHg·min·l–1), or UBF (384 ± 27 vs. 339 ± 50 ml/min). However, the hemodynamic responses to intravenous ANG II were abolished (Fig. 7A). Similar results were observed in three nonpregnant ewes (i.e., 0.5 mg of L-158809 did not alter basal MAP, UVR, and UBF, with values averaging 82 ± 5 vs. 81 ± 5 mmHg, 4,039 ± 73 vs. 4,140 ± 73 mmHg·min·l–1, and 26 ± 5.1 vs. 24 ± 4.6 ml/min, respectively), but there was complete inhibition (P < 0.01) of ANG II-mediated increases in MAP and UVR and decreases in UBF (Fig. 7B).



View larger version (24K):
[in this window]
[in a new window]
  		Fig. 7. Relative changes in MAP, UVR, and UBF in 3 pregnant (A; 130–145 days gestation) and 3 nonpregnant (B) ewes during ANG II infusions (0.046 µg·min–1·kg–1 iv) in the absence (solid bars) and presence (open bars) of systemic AT1 receptor blockade with 0.5 mg of L-158809. *Significantly different from baseline (P < 0.005 by paired t-test). {dagger}Significantly different from absence of systemic AT1 blockade (P ≤ 0.01 by paired t-test).

 

   DISCUSSION
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
GRANTS
 REFERENCES
 
The uterine and systemic vasculature of pregnant women and sheep are refractory to ANG II and {alpha}-receptor agonists (11, 18, 31, 42, 48, 53), but the responses to these agents differ in these vascular beds (10, 32, 42, 48). Hypertensive pregnant women lose this refractoriness (17), demonstrating increases in UVR and fetal morbidity (14, 15, 28, 46). Thus understanding the molecular, cellular, and physiological mechanisms responsible for these vascular changes in normotensive and hypertensive pregnancies is medically important. We report novel observations demonstrating that 1) the differences in the uterine and systemic responses to ANG II and {alpha}-receptor agonists are inherent to these vascular beds and unchanged by pregnancy, 2) the effects of ANG II on uterine and systemic vasculature occur through entirely different mechanisms, and 3) the uterine vascular bed in pregnancy is sensitive to increases in perfusion pressure, suggesting that local autoregulatory mechanisms contribute to the regulation of UBF. These findings not only extend our understanding of uterine vascular changes in pregnancy but also provide new insights into the mechanisms whereby maternal hypertension may alter fetal growth and well-being through changes in UVR.

The ovine uteroplacental vascular bed, similar to the kidney (28), is refractory to the vasoconstricting effects of intravenous physiological doses of ANG II during normotensive pregnancy (12, 38, 48, 49, 53). Furthermore, the ANG II-mediated increases in UVR are generally less than simultaneous rises in systemic vascular resistance (SVR) and MAP; therefore, increases in perfusion pressure contribute to the maintenance of UBF (42, 48). In contrast, intravenous physiological doses of {alpha}-receptor agonists increase UVR in the absence of simultaneous changes in SVR or MAP, and UBF falls (31, 49, 54). In the present study, the pattern of the responses to systemic ANG II and PE were strikingly similar in pregnant and nonestrogenized nonpregnant sheep, confirming our (12) recent suggestion that mechanisms inherent within the uterine and systemic vascular beds account for these differences. In pregnancy, the attenuated uterine responses to ANG II could be explained by the lack of responsiveness by the placental vasculature (53), which accounts for ≥80% of total UBF at full term (49), or the increased synthesis of local vasodilators (30, 32). However, both are specific to pregnancy and, therefore, do not explain the similarity in uterine responses in pregnant and nonpregnant ewes. In contrast, responses to {alpha}-receptor agonists by the placental and myoendometrial vasculature in intact animals (31, 49, 54) and uterine arteries in vitro (2, 23, 43) exceed the response in the systemic vasculature. Although this may reflect a predominance of {alpha}1-receptors in uterine vs. systemic arteries (20, 25), this remains controversial and requires further study. Physiologically, the insensitivity of the uterine vasculature to ANG II would protect it from elevated plasma levels normally seen in pregnancy as well as further increases after orthostatic changes. On the other hand, the greater sensitivity to {alpha}-receptor agonists permits the rapid redistribution of 20–25% of cardiac output from the gravid uterus to essential organs and tissues, thereby ensuring maternal well-being during stresses. In the absence of such stresses, the overall uterine refractoriness associated with pregnancy provides a large margin of safety for the fetus (49).

The differences in uterine and systemic vascular responses to intravenous ANG II are unlikely to be due to the additive effects of increases in basal uterine artery endothelial synthesis of NO- and PGI2 + ANG II-mediated increases, because both are specific to pregnancy (30, 32, 48). In addition, AT receptor binding density is similar in ovine uterine and mesenteric VSM, and neither changes in pregnancy (29), excluding this as well. However, AT2 receptors, which do not mediate ANG II-induced contractions (3), account for >80% of binding in uterine VSM from women and sheep and are unchanged in pregnancy, whereas AT1 receptors predominate in peripheral VSM (11, 13). This difference in AT receptor subtype expression could explain the similar differences in uterine and systemic responses to ANG II in pregnant and nonpregnant ewes but does not explain how intravenous ANG II increases UVR. One possibility is that intravenous ANG II increases the synthesis or release of another vasoconstrictor. This is supported by recent observations that intra-arterial infusions of physiological doses of ANG II do not alter UVR or UBF in pregnant and nonpregnant sheep (12). ANG II is known to elicit {alpha}-receptor-mediated responses and adrenal catecholamine release (5, 44, 45, 57). In the present study, uterine and systemic {alpha}-receptor blockade caused a 70–75% reduction in the rise in UVR during systemic ANG II infusions, suggesting that ANG II increases adrenal catecholamine secretion (5, 44, 45). Although plasma catecholamines were not measured, inhibition of uterine responses to ANG II after uterine and systemic {alpha}-receptor blockade is convincing. Furthermore, complete inhibition of these responses occurred after systemic AT1 blockade, consistent with AT1 receptor-mediated secretion of adrenal catecholamines (57). In contrast, systemic {alpha}-receptor blockade did not modify ANG II-induced increases in MAP, demonstrating that {alpha}-receptor stimulation does not contribute to the systemic pressor responses, whereas the converse occurs in the uterine vasculature (31, 49). This is the first report demonstrating that uterine and systemic responses to intravenous ANG II occur via AT1 receptor activation, but through different mechanisms, the former predominantly via the release of an {alpha}-receptor agonist, very likely of adrenal medullary origin (5, 44, 57).

Surprisingly, there were persistent increases in UVR during intravenous ANG II after {alpha}-receptor blockade that were greater in nonpregnant ewes. AT1 receptors account for ~15% of binding in uterine VSM and could mediate this residual rise in UVR (11, 13). However, uterine AT1 receptor blockade did not affect responses in either group of sheep, suggesting that VSM AT1 receptors do not contribute to ANG II-induced rises in UVR or that local vasodilators effectively inhibit their effects (30, 32). ANG II-mediated increases in local PGI2 require AT1 receptor activation (11); thus this is an unlikely explanation. It does not exclude local NO synthesis, which, as in the kidney (21, 28), increases in pregnancy (32) and could attenuate responses to ANG II. AT2 receptor blockade also had no effect. Notably, the residual rise in UVR was consistently proportionate to the rise in MAP or perfusion pressure and also was observed during systemic PE infusions in the presence of uterine {alpha}-receptor blockade. Thus a sensitive autoregulatory mechanism appears to exist in the uterine vascular bed (34, 56) and is attenuated in normotensive pregnancy. Furthermore, it appears to contribute to the rise in UVR seen with intravenous PE and ANG II. Myogenic responses occur in the renal and mesenteric artery and are attenuated in pregnancy (21, 28, 35, 56). They also have been observed in the uterine vasculature (9, 37, 43, 60). Thus autoregulation, possibly via local myogenic mechanisms, appears to contribute to uterine responses to ANG II and {alpha}-receptor agonists. If uterine vascular refractoriness is absent in hypertensive pregnant women (58), this may contribute to increases in UVR (17, 26, 56), VSM remodeling, decreases in UBF, and, subsequently, fetal growth restriction.

SVR and UVR fall in pregnancy, but it is unclear how vascular tone is maintained. Increases in cardiac output occur early in pregnancy and facilitate the maintenance of MAP (14, 47, 49). Sympathetic nervous activity and the renin-angiotensin system are normally upregulated (15, 24, 28) and may contribute to peripheral vascular tone. However, neither systemic {alpha}-adrenergic nor AT1 receptor blockade alone altered basal MAP in the last 3 wk of ovine pregnancy. In estrogenized nonpregnant ewes, vascular tone also is maintained after isolated receptor blockade, whereas simultaneous {alpha}-receptor and AT receptor inhibition cause dramatic falls in MAP (16). Therefore, both systems contribute to vascular tone after estrogen-induced vasodilation. In the present study, we have not excluded increases in sympathetic outflow, but existing data suggest that increases in cardiac output are primarily responsible for maintaining MAP in pregnancy (47). Uterine {alpha}-adrenergic and AT1/AT2 receptor blockade also had no effect on basal UVR or UBF in nonpregnant or pregnant ewes; the former is consistent with earlier observations (41). These factors, therefore, do not independently contribute to basal systemic or uterine vascular tone in nonpregnant or pregnant ewes. Acute NO synthase and cyclooxygenase inhibition also minimally alter basal UVR and UBF (36, 39, 51). However, inhibition of the large-conductance calcium-dependent potassium (BKCa) channel in the uterine vasculature decreases UBF in pregnant ewes (50), but it is unclear to what extent the BKCa channel modulates basal UVR and UBF in pregnancy.

We have demonstrated for the first time that intravenous ANG II increases UVR primarily through release of an {alpha}-receptor agonist, most likely adrenal catecholamines (4, 44, 57), in nonpregnant and pregnant sheep and that local autoregulatory mechanisms contribute. In contrast, the resulting rise in circulating {alpha}-receptor agonists does not contribute to ANG II-mediated pressor responses, again demonstrating the enhanced sensitivity of the uterine vascular bed to {alpha}-receptor agonists, which we now report is an inherent property of this vascular bed. We now believe that ANG II-mediated increases in uterine artery endothelial PGI2 and NO synthesis in pregnancy modify uterine autoregulation and responses to {alpha}-receptor agonists similar to that seen in the kidney (21). If pregnancy-induced hypertension is associated with increased sympathetic outflow or {alpha}-adrenergic receptor activity and the loss of vascular refractoriness (17, 22, 24, 55), prolonged increases in MAP may modify VSM remodeling, further increase UVR (26, 56), decrease UBF, and increase the risk for abnormalities of uteroplacental oxygen delivery and, thus, fetal growth restriction (46, 49). It may now be possible to develop strategies to address this problem.


   GRANTS
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
REFERENCES
 
This work was supported by National Institute of Child Health and Human Development Grant HD-08783-26.


   ACKNOWLEDGMENTS
 
This study was presented in part at the 45th Annual Meeting of the Society for Gynecologic Investigation, Atlanta, GA, March 1998.


   FOOTNOTES
 

Address for reprint requests and other correspondence: B. E. Cox, Dept. of Pediatrics, The Univ. of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75390-9063 (E-mail: blair.cox{at}utsouthwestern.edu).

The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.


   REFERENCES
TOP
 ABSTRACT
 METHODS
 RESULTS
 DISCUSSION
 GRANTS
 REFERENCES
 

  1. Adbul-Karim R and Assali NS. Pressor response to angiotensin in pregnant and nonpregnant women. Am J Obstet Gynecol 82: 246–251, 1961.[ISI][Medline]
  2. Annibal DJ, Rosenfeld CR, and Kamm KE. Alterations in vascular smooth muscle contractility during ovine pregnancy. Am J Physiol Heart Circ Physiol 256: H1282–H1288, 1989.[Abstract/Free Full Text]
  3. Bottari SP, de Gasparo M, Steckelings UM, and Levens NR. Angiotensin II receptor subtypes: characterization, signaling mechanisms, and possible physiologic implications. Front Neuroendocrinol 14: 128–171, 1993.
  4. Buckley JP. Actions of angiotensin on the central nervous system. Fed Proc 31: 1332–1337, 1972.[ISI][Medline]
  5. Bunn SJ and Marley PD. Effects of angiotensin II on cultured, bovine adrenal medullary cells. Neuropeptides 13: 121–132, 1989.[CrossRef][ISI][Medline]
  6. Campbell WB and Jackson EK. Modulation of adrenergic transmission by angiotensin in the perfused rat mesentery. Am J Physiol Heart Circ Physiol 236: H211–H217, 1979.[Abstract/Free Full Text]
  7. Chen L, McNeill JR, Wilson TW, and Gopalakrishnan V. Heterogeneity in vascular smooth muscle responsiveness to angiotensin II. Role of endothelin. Hypertension 26: 83–88, 1995.[Abstract/Free Full Text]
  8. Chesley LC, Talledo E, Bohler CS, and Zuspan FP. Vascular reactivity to angiotensin II and norepinephrine in pregnant and nonpregnant women. Am J Obstet Gynecol 91: 837–842, 1965.[ISI][Medline]
  9. Cipolla MJ, Binder ND, and Osol G. Myoendometrial versus placental uterine arteries: structural, mechanical, and functional differences in late-pregnant rabbits. Am J Obstet Gynecol 177: 215–221, 1997.[CrossRef][ISI][Medline]
  10. Clark KE, Irion GL, and Mack CE. Differential responses of uterine and umbilical vasculatures to angiotensin II and norepinephrine. Am J Physiol Heart Circ Physiol 259: H197–H203, 1990.[Abstract/Free Full Text]
  11. Cox BE, Rosenfeld CR, Kalinyak JE, Magness RR, and Shaul PW. Tissue-specific expression of vascular smooth muscle angiotensin II receptor subtypes during ovine pregnancy. Am J Physiol Heart Circ Physiol 271: H212–H221, 1996.[Abstract/Free Full Text]
  12. Cox BE, Williams CE, and Rosenfeld CR. Angiotensin II indirectly vasoconstricts the ovine uterine circulation. Am J Physiol Regul Integr Comp Physiol 278: R337–R344, 2000.[Abstract/Free Full Text]
  13. Cox BE, Word RA, and Rosenfeld CR. Angiotensin II receptor characterization and subtype expression in uterine arteries and myometrium during pregnancy. J Clin Endocrinol Metab 81: 49–58, 1995.
  14. Cunningham FG, Gant NF, Leveno KJ, Gilstrap LC III, Hauth JC, and Wenstrom KD (Editors). Maternal adaptations to pregnancy. In: Williams Obstetrics (21st ed.). New York: McGraw-Hill, 2001, p. 167–200.
  15. Cunningham FG, Gant NF, Leveno KJ, Gilstrap LC III, Hauth JC, and Wenstrom KD. (Editors). Fetal growth disorders. In: Williams Obstetrics (21st ed.). New York: McGraw-Hill, 2001, p. 743–764.
  16. Davis LE, Magness RR, and Rosenfeld CR. Role of angiotensin II and {alpha}-receptors during estrogen-induced vasodilation in ewes. Am J Physiol Endocrinol Metab 263: E837–E843, 1992.[Abstract/Free Full Text]
  17. Erkkola RU and Pirhonen JP. Uterine and umbilical flow velocity waveforms in normotensive and hypertensive subjects during the angiotensin II sensitivity test. Am J Obstet Gynecol 166: 910–916, 1992.[ISI][Medline]
  18. Erkkola RU and Pirhonen JP. Flow velocity waveforms in uterine and umbilical arteries during angiotensin II sensitivity test. Am J Obstet Gynecol 162: 1193–1197, 1990.[ISI][Medline]
  19. Fallgren B, Bjellin L, and Edvinsson L. Effect of pregnancy and sex steroids on {alpha}1-adrenoreceptor mechanisms in the guinea-pig uterine vascular bed. Pharmacol Toxicol 63: 375–381, 1988.[ISI][Medline]
  20. Farley DB, Ford SP, Reynolds LP, Bhatnagar RK, and Van Orden DE. Quantitation of {alpha}1-adrenergic receptors in porcine uterine and mesenteric arteries. Am J Obstet Gynecol 150: 485–491, 1984.[ISI][Medline]
  21. Gandley RE, Conrad KP, and McLaughlin MK. Endothelin and nitric oxide mediate reduced myogenic reactivity of small renal arteries from pregnant rats. Am J Physiol Regul Integr Comp Physiol 280: R1–R7, 2001.[Abstract/Free Full Text]
  22. Gant NF Jr, Daley GL, Chand S, Whalley PJ, and MacDonald PC. A study of angiotensin II pressor responses throughout primigravid pregnancy. J Clin Invest 52: 2682–2689, 1973.[ISI][Medline]
  23. Gough ED and Dyer DC. Responses of isolated human uterine arteries to vasoactive drugs. Am J Obstet Gynecol 110: 625–629, 1971.[ISI][Medline]
  24. Greenwood JP, Stoker JB, Walker JJ, and Mary DASG. Sympathetic nerve discharge in normal pregnancy and pregnancy-induced hypertension. J Hypertens 16: 617–624, 1998.[CrossRef][ISI][Medline]
  25. Isla M and Dyer DC. Characterization of {alpha}-adrenoreceptors in the late pregnant ovine uterine artery. Eur J Pharmacol 27: 321–331, 1990.
  26. Izzard AS and Heagerty AM. Hypertension and the vasculature: arterioles and the myogenic response. J Hypertens 13: 1–4, 1995.[ISI][Medline]
  27. Lambers DS, Greenberg SG, and Clark KE. Functional role of angiotensin II type 1 and 2 receptors in regulation of uterine blood flow in nonpregnant sheep. Am J Physiol Heart Circ Physiol 278: H353–H359, 2000.[Abstract/Free Full Text]
  28. Lindheimer MD, Roberts JM, and Cunningham FG. (Editors). Chesley's Hypertensive Disorders in Pregnancy (2nd ed.). Stamford, CT: Appleton and Lange, 1999.
  29. Mackanjee HR, Shaul PW, Magness RR, and Rosenfeld CR. Angiotensin II vascular smooth muscle receptors are not down-regulated in near-term pregnant sheep. Am J Obstet Gynecol 165: 1641–1648, 1991.[ISI][Medline]
  30. Magness RR, Osei-Boaten K, Mitchell MD, and Rosenfeld CR. In vitro prostacyclin production by ovine uterine and systemic arteries: effects of angiotensin II. J Clin Invest 76: 2206–2212, 1985.[ISI][Medline]
  31. Magness RR and Rosenfeld CR. Systemic and uterine responses to {alpha}-adrenergic stimulation in pregnant and nonpregnant ewes. Am J Obstet Gynecol 155: 897–904, 1986.[ISI][Medline]
  32. Magness RR, Rosenfeld CR, Hassan A, and Shaul PW. Endothelial vasodilator production by uterine and systemic arteries. I. Effects of ANG II on PGI2 and NO in pregnancy. Am J Physiol Heart Circ Physiol 270: H1914–H1923, 1996.[Abstract/Free Full Text]
  33. McMullen JR, Gibson KJ, Lumbers ER, Burrell LH, and Wu J. Interactions between AT1 and AT2 receptors in uterine arteries from pregnant ewes. Eur J Pharmacol 378: 195–202, 1999.[CrossRef][ISI][Medline]
  34. Meininger GA and Davis MJ. Cellular mechanisms involved in the vascular myogenic response. Am J Physiol Heart Circ Physiol 263: H647–H659, 1992.[Abstract/Free Full Text]
  35. Meyer MC, Osol G, and McLaughlin M. Flow decreases myogenic activity of mesenteric arteries from pregnant rats. J Soc Gynecol Investig 4: 293–297, 1997.[CrossRef][ISI][Medline]
  36. Miller SL, Jenkins G, and Walker DW. Effect of nitric oxide synthase inhibition on the uterine vasculature of the late-pregnant ewe. Am J Obstet Gynecol 180: 1138–1145, 1999.[CrossRef][ISI][Medline]
  37. Moll W and Gotz R. Pressure-diameter curves of mesometrial arteries of guinea pigs demonstrate a non-muscular, oestrogen-inducible mechanism of lumen regulation. Pflügers Arch 404: 332–336, 1985.[CrossRef][ISI][Medline]
  38. Naden RP, Coultrup S, Arant JBS, and Rosenfeld CR. Metabolic clearance of angiotensin II in pregnant and nonpregnant sheep. Am J Physiol Endocrinol Metab 249: E49–E55, 1985.[Abstract/Free Full Text]
  39. Naden RP, Iliya CA, Arant BS Jr, Gant NF Jr, and Rosenfeld CR. Hemodynamic effects of indomethacin in chronically instrumented pregnant sheep. Am J Obstet Gynecol 151: 484–494, 1985.[ISI][Medline]
  40. Naden RP and Rosenfeld CR. Systemic and uterine responsiveness to angiotensin II and norepinephrine in estrogen-treated sheep. Am J Obstet Gynecol 153: 417–425, 1985.[ISI][Medline]
  41. Naden RP and Rosenfeld CR. The role of {alpha}-receptors in estrogen-induced vasodilation in nonpregnant sheep. Am J Physiol Heart Circ Physiol 248: H339–H344, 1985.[Abstract/Free Full Text]
  42. Naden RP and Rosenfeld CR. Effect of angiotensin II on uterine and systemic vasculature in pregnant sheep. J Clin Invest 68: 468–474, 1981.[ISI][Medline]
  43. Osol G and Cipolla M. Interaction of myogenic responses and adrenergic mechanism in isolated, pressurized uterine radial arteries from late-pregnant and nonpregnant rats. Am J Obstet Gynecol 168: 696–705, 1993.
  44. Peach MJ, Cline WH Jr, and Watts DT. Release of adrenal catecholamines by angiotensin II. Circ Res 19: 571–575, 1966.[Abstract/Free Full Text]
  45. Reid IA. Interactions between ANG II, sympathetic nervous system, and baroreceptor reflexes in regulation of blood pressure. Am J Physiol Endocrinol Metab 262: E763–E778, 1992.[Abstract/Free Full Text]
  46. Rosenfeld CR. Consideration of the uteroplacental circulation in uterine growth. Semin Perinatol 8: 42–51, 1984.[ISI][Medline]
  47. Rosenfeld CR. Distribution of cardiac output in ovine pregnancy. Am J Physiol Heart Circ Physiol 232: H231–H235, 1977.[Abstract/Free Full Text]
  48. Rosenfeld CR. Mechanisms regulating angiotensin II responsiveness by the uteroplacental circulation. Am J Physiol Regul Integr Comp Physiol 281: R1025–R1040, 2001.[Abstract/Free Full Text]
  49. Rosenfeld CR. The Uterine Circulation. Ithaca, NY: Perinatology, 1989.
  50. Rosenfeld CR, Cornfield DN, and Roy T. Ca2+-activated K+ channels modulate basal and E2{beta}-induced rises in uterine blood flow in ovine pregnancy. Am J Physiol Heart Circ Physiol 281: H22–H31, 2001.[Abstract/Free Full Text]
  51. Rosenfeld CR, Cox BE, Roy T, and Magness RR. Nitric oxide contributes to estrogen-induced vasodilation of the ovine uterine circulation. J Clin Invest 98: 2158–2166, 1996.[ISI][Medline]
  52. Rosenfeld CR and Gant NF Jr. The chronically instrumented ewe: a model for studying vascular reactivity to angiotensin II in pregnancy. J Clin Invest 67: 486–492, 1981.[ISI][Medline]
  53. Rosenfeld CR and Naden RP. Uterine and nonuterine vascular responses to angiotensin II in ovine pregnancy. Am J Physiol Heart Circ Physiol 257: H17–H24, 1989.[Abstract/Free Full Text]
  54. Rosenfeld CR and West JA. Circulatory responses to systemic infusions of norepinephrine in the pregnant ewe. Am J Obstet Gynecol 127: 376–383, 1984.
  55. Schobel HP, Fischer T, Heuszer K, Geiger H, and Schmieder RE. Preeclampsia—a state of sympathetic overreactivity. N Engl J Med 335: 1480–1485, 1996.[Abstract/Free Full Text]
  56. Schubert R and Mulvany MJ. The myogenic response: established facts and attractive hypotheses. Clin Sci (Lond) 96: 313–326, 1999.[Medline]
  57. Smith RD and Timmermans PBMWM. Human angiotensin receptor subtypes. Curr Opin Nephrol Hypertens 3: 112–122, 1994.[CrossRef][Medline]
  58. Talledo OE, Chesley LC, and Zuspan FP. Renin-angiotensin system in normal and toxemic pregnancies. III. Differential sensitivity to angiotensin II and norepinephrine in toxemia of pregnancy. Am J Obstet Gynecol 100: 218–221, 1968.
  59. Tsutsumi Y, Matsubara H, Masaki H, Kurihara H, Murasawa S, Takai S, Miyazaki M, Nozawa Y, Ozono R, Nakagawa K, Miwa T, Kawada N, Mori Y, Shibasaki Y, Tanaka Y, Fujiyama S, Koyama Y, Fujiyama A, Takahashi H, and Iwasaka T. Angiotensin II type 2 receptor overexpression activates the vascular kinin system and causes vasodilation. J Clin Invest 104: 925–935, 1999.[ISI][Medline]
  60. Venuto RC, Cox JW, Stein JH, and Ferris TF. The effect of changes in perfusion pressure on uteroplacental blood flow in the pregnant rabbit. J Clin Invest 57: 938–944, 1976.[ISI][Medline]



This article has been cited by other articles:


Home page
 Reproductive SciencesHome page
C. R. Rosenfeld, R. A. Word, K. DeSpain, and X.-t. Liu
Large Conductance Ca2+--Activated K+ Channels Contribute to Vascular Function in Nonpregnant Human Uterine Arteries
Reproductive Sciences, September 1, 2008; 15(7): 651 - 660.
[Abstract] [PDF]

Home page
 Biol. Reprod.Home page
H. Zhang and L. Zhang
Role of Protein Kinase C Isozymes in the Regulation of alpha1-Adrenergic Receptor-Mediated Contractions in Ovine Uterine Arteries
Biol Reprod, January 1, 2008; 78(1): 35 - 42.
[Abstract] [Full Text]