Angiotensin II indirectly vasoconstricts the ovine uterine circulation

Alterations in the Maternal Peripheral Microvascular Response in Pregnancies Complicated by Preeclampsia and the Impact of Fetal Sex

OBJECTIVE

Peripheral microvascular function is altered in preeclampsia (PE). Recent studies suggest that maternal physiology varies with fetal sex. We wanted to examine if there were sex-specific differences in maternal peripheral microvascular function in normal pregnancy and pregnancy complicated by PE.

METHODS

Peripheral microvascular responses were examined using the noninvasive technique of laser Doppler flowmetry in normotensive healthy pregnant women and in women diagnosed with PE. We measured baseline perfusion, response to thermal hyperemia, post-occlusive reperfusion, and vasodilatation in response to corticotropin-releasing hormone (CRH), a potent vasodilator in human skin.

RESULTS

At 31 to 40 weeks' gestation those women with a male fetus exhibited increased vasodilatation in response to CRH (P <.05) and greater baseline perfusion (P <.05) than those pregnant with a female fetus. PE women pregnant with a male fetus demonstrated a significantly reduced vasodilatation in response to CRH (P <.05), reduced baseline perfusion (P <.05), and reduced response to thermal hyperemia (P <.05) compared to normotensive women pregnant with a male fetus. Microvascular function was not significantly different between preeclamptic and normotensive women with a female fetus.

CONCLUSION

These data show that there are differences in maternal peripheral microvascular function in relation to fetal sex.

Differential sensitivity to angiotensin II and norepinephrine in human uterine arteries

BACKGROUND

During pregnancy, uteroplacental responses to norepinephrine (NE) exceed systemic responses. In contrast, uteroplacental responses to angiotensin II (ANG II) are less than systemic. The explanation for these differences in uteroplacental sensitivity remain unclear but may reflect type 2 ANG II receptor (AT(2)R) predominance in uterine artery (UA) vascular smooth muscle (VSM).

OBJECTIVE

The objective of the study was to examine VSM sensitivity to KCl, NE, and ANG II in UA from nonpregnant (NP) and pregnant (P) women and determine VSM ANG II receptor subtype expression.

METHODS

Responses to KCl, NE, and ANG II were examined in endothelium-denuded UA rings from NP (n = 28) and P (n = 13; 34-40 wk gestation) women, and ANG II receptor subtype, α(1)-receptor and contractile proteins were measured.

RESULTS

KCl and NE dose dependently contracted UA (P < 0.001), P exceeding NP 2-fold or greater; but α(1)-receptor expression was unchanged. ANG II did not elicit dose effects in NP or P UA; however, P responses exceeded NP approximately 2-fold (P < 0.001) and were approximately 2.5-fold less than NE (P < 0.001). AT(2)R and AT(1)R expression were similar (P > 0.1) in VSM from NP and term P women. AT(1)R blockade abolished ANG II contractions (P < 0.001); AT(2)R blockade did not enhance ANG II sensitivity in UA with or without endothelium. Actin contents increased approximately 2-fold in term UA.

CONCLUSIONS

Sensitivity to α-stimulation exceeds ANG II in NP and P UA, explaining the differential uteroplacental sensitivity in pregnancy. Because AT(2)R predominate in UA VSM throughout reproduction, this contributes to the inherent refractoriness to ANG II in the uterine vasculature. The increase in UA contractile proteins at term P suggests remodeling, explaining the enhanced contractility seen.

Defining the differential sensitivity to norepinephrine and angiotensin II in the ovine uterine vasculature

The intact ovine uterine vascular bed (UVB) is sensitive to α-agonists and refractory to angiotensin II (ANG II) during pregnancy; the converse occurs in the systemic circulation. The mechanism(s) responsible for these differences in uterine sensitivity are unclear and may reflect predominance of nonconstricting AT(2) receptors (AT(2)R) in uterine vascular smooth muscle (UVSM). The contribution of the placental vasculature also is unclear. Third generation and precaruncular/placental arteries from nonpregnant (n = 16) and term pregnant (n = 23) sheep were used to study contraction responses to KCl, norepinephrine (NE), and ANG II (with/without ATR specific inhibitors) and determine UVSM ATR subtype expression and contractile protein content. KCl and NE increased third generation and precaruncular/placental UVSM contractions in a dose- and pregnancy-dependent manner (P ≤ 0.001). ANG II only elicited modest contractions in third generation pregnant UVSM (P = 0.04) and none in precaruncular/placental UVSM. Moreover, compared with KCl and NE, ANG II contractions were diminished ≥ 5-fold. Whereas KCl and ANG II contracted third generation>>precaruncular/placental UVSM, NE-induced contractions were similar throughout the UVB. However, each agonist increased third generation contractions ≥ 2-fold at term, paralleling increased actin/myosin and cellular protein content (P ≤ 0.01). UVSM AT(1)R and AT(2)R expression was similar throughout the UVB and unchanged during pregnancy (P > 0.1). AT(1)R inhibition blocked ANG II-mediated contractions; AT(2)R blockade, however, did not enhance contractions. AT(2)R predominate throughout the UVB of nonpregnant and pregnant sheep, contributing to an inherent refractoriness to ANG II. In contrast, NE elicits enhanced contractility throughout the ovine UVB that exceeds ANG II and increases further at term pregnancy.

Involvement of neuronal nitric oxide synthase (nNOS) in the regulation of migrating motor complex (MMC) in sheep

The objectives of this study were to evaluate the role of nitric oxide (NO) synthase isoforms (nNOS, eNOS, and iNOS) in the regulation of the migrating motor complex (MMC) in sheep using electromyography and their expression in the gastrointestinal (GI) tract by Western blot (WB) and immunohistochemistry. Intravenous administration of L-NAME or the nNOS inhibitor 7-nitroindazole (7-NI) decreased the MMC interval. Myoelectric activity of intestinal phase II was increased, whereas antral activity was reduced. These effects were blocked by L-arginine. Inhibitors of either iNOS (aminoguanidine and S-methylisothiourea) or eNOS (L-NIO) were ineffective. The NO donor sodium nitroprusside decreased GI myoelectric activity, inhibited the MMC pattern, and prevented the effects induced by L-NAME and 7-NI in the intestine. Intracerebroventricular administration of these agents did not modify GI motility. In the rumen, abomasal antrum, duodenum, and jejunum, WB showed three bands at about 155, 145, and 135kDa corresponding to nNOS, and a 140-kDa band (eNOS); however iNOS was not detected. Positive nNOS immunostaining was observed in neurons of the myenteric and submucous plexus of all GI tissues, while eNOS was found in the endothelial cells, ruminal and intestinal epithelium, as well as in some enteric neurons and in endocrine-like cells of the duodenal Brunner's glands. In contrast, only weak iNOS immunoreactivity was found in ruminal epithelium. Taken together, our results suggest that NO, synthesized at a peripheral level by nNOS, is tonically inhibiting the MMC pattern and intestinal motility in sheep.

Physiological changes in maternal cortisol do not alter expression of growth-related genes in the ovine placenta

OBJECTIVES

The aim of this study was to investigate the effect of cortisol on growth-related genes in the ovine placenta.

STUDY DESIGN

Ewes carrying singleton pregnancies were operated on between 112 and 116 days of gestation (115 ± 0.4, term = 147 days) and randomly assigned into three groups: six control animals, five ewes that were administered cortisol by continuous intravenous infusion (1 mg/kg/day, high cortisol), and five ewes that were adrenalectomized and replaced with 0.5-0.6 mg cortisol/kg/day and 3 μg aldosterone/kg/day to produce cortisol concentrations equivalent to pre-pregnancy values (low cortisol). At necropsy (130 ± 0.2 days of gestation), placental tissue was frozen and stored at -80 °C for mRNA analysis.

MAIN OUTCOME MEASURES

To assess potential molecular mechanisms by which cortisol alters placental structure and function and fetal growth.

RESULTS

Cortisol levels did not significantly affect 11β-hydroxysteroid dehydrogenase 1 and 2 enzymes, glucocorticoid receptor, mineralocorticoid receptor and angiotensin II receptor, type 1 (AT1R) expression levels. Gene expression levels of AT2R were increased in the high cortisol group for type B placentomes. There was little effect of cortisol on the insulin-like growth factor (IGF) axis. There was significantly more IGF-I mRNA in B versus A type and more IGFBP-2 mRNA in B and C type versus A type placentomes regardless of treatment (p < 0.05).

CONCLUSIONS

These data suggest that cortisol increases placental AT2R expression at high concentrations whereas it has little effect on the placental IGF axis.

Pregnancy and Ovarian Steroid Regulation of Angiotensin II Type 1 and Type 2 Receptor Expression in Ovine Uterine Artery Endothelium and Vascular Smooth Muscle

Although pregnancy is clearly associated with refractoriness to infused angiotensin II (AII) in the uteroplacental unit, there is still dispute over the mechanism by which angiotensin type 1 and type 2 receptors (AT1R and AT2R) may mediate this response in the uterine artery. This is in large part due to incomplete knowledge of levels of AT1R and AT2R expression and function in uterine artery endothelium (UA Endo) in the nonpregnant (NP) and pregnant (P) states, combined with the disagreement on whether AII may act through release of adrenomedullary catecholamines. The authors have previously described an increase in AT1R in UA Endo but not UA vascular smooth muscle (VSM) during pregnancy as compared to the nonpregnant intact ewe. Herein they report that the pregnancy-associated increase in AT(1)R expression in UA Endo is regulated by ovarian steroids. Using a recently developed antibody to AT2R, the authors now show there is no change in AT2R in UA Endo or VSM associated with ovarian function, and although AT2R is not changed in UA Endo by pregnancy, there is a significant decrease observed in UA VSM at that time. The authors also examined changes in receptors in UA Endo and VSM in estrogen (E2beta)-primed ewes in view of the common use of this model as a control for physiologic studies. In contrast to their findings in nonprimed nonpregnant or pregnant animals, the authors observed a significant increase in both AT1R and AT2R in UA Endo in response to the supraphysiologic priming with E2beta. In order to address the possible functionality of AT1R or AT2R in UA Endo, the authors used the uterine artery endothelial cell (UAEC) model of UA endothelial cells maintained in culture to passage 4. Differences in expression of AT1R or AT2R were normalized at passage 4 in P-UAECs and NP-UAECs. Treatment with AII activated phospholipase C (PLC) in both NP- and P-UAECs but signaling through the extracellular signal-regulated kinase (ERK) pathway was dramatically enhanced in P-UAECs compared to NP-UAECs. Surprisingly, both phosphoinositol turnover and ERK2 phosphorylation responses failed to display the expected dose-responses. Inhibition of AII-stimulated ERK2 phosphorylation with antagonists DUP 753 (AT1R, 10 microM) and PD 123319 (AT2R, 10 microM) failed to selectively inhibit ERK2 phosphorylation. The authors conclude that (a) the net effect of pregnancy may be an increase in the AT1R/AT2R ratio in both UA Endo and VSM but through apparently distinct mechanisms, (b) the ovariectomized animal model is similar to the luteal state for AT1R and AT2R expression, while the E2beta-primed model does not resemble the nonpregnant or pregnant state, and (c) there is a real possibility that AII may mediate its effects either through a complex AT1R-AT2R interaction or via an as-yet unidentified non-AT1, non-AT2 receptor.

Effect of morphology on placentome size, vascularity, and vasoreactivity in late pregnant sheep

Ovine placentomes vary in shape, with type A placentomes being concave, type D convex, and types B and C intermediate in morphology. It has been speculated that as placentomes advance in type they differ in vascularity and nutrient transport capacity. Our objective was to determine cellularity and vascularity measurements, angiogenic factor expression, and arterial vasoactivity within different morphologic types of placentomes. On Day 130 of gestation, placentomes were collected from multiparous ewes (n = 38) and were evaluated for size, cellularity estimates, angiogenic factor mRNA expression, capillary vascularity (capillary size, capillary surface density [CSD], capillary number density [CND], and capillary area density [CAD]), and vasoreactivity to potassium chloride and angiotensin II. The average weight and size of type A and B placentomes were less (P < 0.01) than those of type C and D placentomes. Placentome morphology did not affect (P > or = 0.24) cotyledonary or caruncular cellularity estimates or percentage of cellular proliferation. Placentome morphology affected (P > or = 0.41) neither caruncular CAD, CND, CSD, or capillary size nor cotyledonary CND, CSD, or capillary size. Cotyledonary CAD was increased (P < 0.01) in type B and D placentomes compared with type A placentomes. Furthermore, placentome type did not affect (P > or = 0.06) angiogenic factor gene expression in the cotyledon or the caruncle. Size, but not morphologic type of placentome, was associated with greater caruncular artery contractility to potassium chloride and angiotensin II (P < 0.01 for both). Placentome size, but not morphologic type, may be important for vascularity and nutrient transfer in the placenta of the pregnant ewe.

The role of RAS in the pathogenesis of preeclampsia

Preeclampsia is a hypertensive disorder that is unique to pregnancy, with consistent involvement of the kidney. The renin-angiotensin system (RAS) has been implicated in the pathogenesis of preeclampsia. In the gravid state, in addition to the RAS in the kidney, there is a tissue-based RAS in the uteroplacental unit. Increased renin expression in human preeclampsia and in transgenic mouse models with a human preeclampsia-like syndrome shows that activation of the uteroplacental RAS, with angiotensin II entering the systemic circulation, may mediate the pathogenesis of preeclampsia. Vascular maladaptation in preeclampsia with increased vasomotor tone, endothelial dysfunction, and increased sensitivity to angiotensin II and norepinephrine in manifest preeclampsia may be explained on the basis of angiotensin II-mediated mechanisms through angiotensin receptor type I (AT1) activation. Recently, novel angiotensin II-related biomolecular mechanisms have been described in preeclampsia. These include AT1 and bradykinin B2 receptor heterodimerization and the production of autoantibody against AT1. Various organ systems with predilection for involvement in preeclampsia are sites of tissue-based RAS. Angiotensin II-mediated mechanisms may explain the primary clinicopathologic features of preeclampsia. In this review, these various aspects are critically examined and an integrated concept on the role of RAS in preeclampsia is presented.

Effects of systemic and local phenylephrine and arginine vasopressin infusions in conscious postnatal sheep

Mean arterial pressure (MAP) increases after birth, however, the mechanisms remain unclear. Systemic angiotensin II (ANG II) infusions increase MAP in newborn sheep, but the direct effects of ANG II on peripheral vascular resistance (PVR) are minimal. Thus, its systemic pressor effects may reflect release of other pressor agents, e.g. alpha-agonists and/or AVP, suggesting they contribute to postnatal regulation of MAP and PVR. To address this, we performed studies in conscious sheep at 7-14, 15-21, and 22-35 d postnatal, infusing phenylephrine (PE) or AVP systemically or intra-arterially into the hindlimb while measuring MAP, heart rate (HR), and femoral blood flow (FmBF). Basal MAP and FmBF rose, whereas HR and femoral vascular resistance (FmVR) fell (p < or = 0.03) during the first month postnatal. Although systemic PE and AVP dose dependently increased MAP and FmVR and decreased FmBF and HR (p < 0.001, ANOVA) at all ages, responses were not age dependent. Notably, increases in FmVR exceeded increases in MAP, and responses to PE appeared to exceed AVP (p < 0.05). Hindlimb infusions of both agents decreased FmBF and increased FmVR dose dependently (p < 0.001, ANOVA) at all ages without altering MAP or HR. These responses also were not age dependent. Unlike ANG II, PE and AVP directly increase PVR in newborn sheep. Moreover, FmVR increases more than MAP at all doses, suggesting these agonists may contribute to postnatal MAP regulation and could mediate the effects of systemic ANG II on postnatal MAP.

Role of the renin-angiotensin system in the pathogenesis of preeclampsia

Preeclampsia is a hypertensive disorder unique to pregnancy with consistent involvement of the kidney. The renin-angiotensin system (RAS) has been implicated in the pathogenesis of preeclampsia. In the gravid state, in addition to the RAS in the kidney, there is a tissue-based RAS in the uteroplacental unit. Increased renin expression observed both in human preeclampsia and in a transgenic mouse model with a human preeclampsia-like syndrome supports the concept that activation of the uteroplacental RAS, with angiotensin II entering the systemic circulation, may mediate the pathogenesis of preeclampsia. A novel disease paradigm of the two-kidney one-clip (2K-1C) Goldblatt model is presented for preeclampsia, wherein the gravid uterus is the clipped “kidney” and the two maternal kidneys represent the unclipped kidney. Validation of the 2K-1C Goldblatt model analogy requires evidence of elevated angiotensin II in the peripheral circulation before vascular maladaptation in preeclampsia. Convincing evidence of the elevation of angiotensin II in preeclampsia does not exist despite the fact that much of vascular pathogenesis appears to be due to angiotensin type I (AT1) receptor activation. Vascular maladaptation with increased vasomotor tone, endothelial dysfunction, and increased sensitivity to angiotensin II and norepinephrine in manifest preeclampsia may be explained on the basis of angiotensin II-mediated mechanisms. Recently, novel angiotensin II-related biomolecular mechanisms have been described in preeclampsia. These include AT1and bradykinin B2receptor heterodimerization and the production of an autoantibody against AT1. Various organ systems with a predilection for involvement in preeclampsia are each a site of a tissue-based RAS. How angiotensin II-mediated mechanisms may explain the primary clinical-pathological features of preeclampsia is described. Future investigations are proposed to more precisely define the role of activation of the uteroplacental RAS in the mechanisms underlying preeclampsia.

Angiotensin II mediates uterine vasoconstriction through alpha-stimulation

Intravenous angiotensin II (ANG II) increases uterine vascular resistance (UVR), whereas uterine intra-arterial infusions do not. Type 2 ANG II (AT(2)) receptors predominate in uterine vascular smooth muscle; this may reflect involvement of systemic type 1 ANG II (AT(1)) 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 AT(1) 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 AT(1) receptor blockade inhibited all responses to intravenous ANG II. In contrast, uterine AT(1) receptor blockade + systemic alpha-receptor blockade resulted in persistent proportionate increases in MAP and UVR. Uterine AT(2) receptor blockade had no effects. We have shown that ANG II-mediated pressor responses reflect activation of systemic vascular AT(1) receptors, whereas increases in UVR reflect AT(1) receptor-mediated release of an alpha-agonist and uterine autoregulatory responses.

Central angiotensin II-induced pressor responses and neural activity in utero and hypothalamic angiotensin receptors in preterm ovine fetus

The central renin-angiotensin system is important in the control of blood pressure in the adult. However, few data exist about the in utero development of central angiotensin-mediated pressor responses. Our recent studies have shown that the application of ANG II into the fetal brain can increase blood pressure at near term. The present study determined fetal blood pressure and heart rate in response to a central application of ANG II in the chronically prepared preterm ovine fetus, determined the action sites marked by c-Fos expression in the fetal central pathways after intracerebroventricular injection of ANG II in utero, and determined angiotensin subtype 1 receptors in the fetal hypothalamus. Central injection of ANG II significantly increased fetal mean arterial pressure (MAP). Adjusted fetal MAP against amniotic pressure was also increased by ANG II. Fetal heart rate was subsequently decreased after the central administration of ANG II and/or the increase of blood pressure. ANG II induced c-Fos expression in the central putative cardiovascular area, the paraventricular nuclei in the brain sympathetic pathway. Application of ANG II also caused intense Fos immunoreactivity in the tractus solitarius nuclei in the hindbrain. In addition, intense angiotensin subtype 1 receptors were expressed in the hypothalamus at preterm. These data demonstrate that central ANG II-related pressor centers start to function as early as at preterm and suggest that the central angiotensin-related sympathetic pathway is likely intact in the control of blood pressure in utero.

Intravenous angiotensin induces brain c-fos expression and vasopressin release in the near-term ovine fetus

The effect of intravenous angiotensin II (ANG II) on fetal brain c-fos expression and arginine vasopressin (AVP) release was studied in the near-term ovine fetus. Fetuses with chronically implanted catheters received an intravenous infusion of ANG II or saline. Fetal plasma AVP concentrations were significantly increased after the peripheral administration of ANG II, with peak levels (3-fold) at 30 min after the intravenous infusion. There was no change in fetal plasma osmolality, sodium, and hematocrit levels between the control and experimental groups or between the periods before and after the infusion of ANG II. Intravenous ANG II administration induced Fos immunoreactivity (Fos-IR) in the circumventricular organs and the median preoptic nucleus of the fetal brain. Fos-IR was also demonstrated in the fetal supraoptic nuclei (SON). Double labeling demonstrated that the AVP-containing neurons in the SON were expressing c-fos in response to intravenous ANG II. These results indicate that the peripheral ANG II in the fetus may play a significant role in stimulating the central hypothalamic-neurohypophysial system during late gestation. It supports the hypothesis that circulating ANG II may act at the fetal AVP neurons in the hypothalamus in body fluid balance via the circumventricular organs, which are situated outside the blood-brain barrier, and the central neural pathway between these two brain structures has been relatively established in utero, at least at near-term.

Mechanisms regulating angiotensin II responsiveness by the uteroplacental circulation

Pregnancy is associated with increases in cardiac output and uterine blood flow (UBF) and a fall in systemic vascular resistance. In ovine pregnancy, UBF rises from approximately 3% of cardiac output to approximately 25% at term gestation, reflecting a >30-fold rise in UBF by term. This increase in UBF supports exponential fetal growth during the last trimester and maintains fetal well-being by providing excess oxygen and nutrient delivery. These hemodynamic changes are associated with numerous hormonal changes, including increases in placental steroid hormones and enhanced activation of the renin-angiotensin and sympathetic nervous systems, all of which are believed to modulate systemic and uterine vascular adaptation and vascular reactivity. Systemic pressor responses to infused ANG II are attenuated in normotensive pregnancies and the uteroplacental vasculature is even less sensitive, suggesting development of mechanisms to maintain basal UBF and permit the rise in UBF necessary for fetal growth and well-being. The effects of ANG II on the uteroplacental vasculature are reviewed, and the mechanisms that may account for attenuated vascular sensitivity are examined, including ANG II metabolism, vascular production of antagonists, ANG II-receptor subtype expression, and the role of indirect mechanisms.