Angiotensin-converting enzyme activity and contractile effects of angiotensin I and II in human uteroplacental arteries

Role of the Uteroplacental Renin-Angiotensin System in Placental Development and Function, and Its Implication in the Preeclampsia Pathogenesis

Placental development and function implicate important morphological and physiological adaptations to thereby ensure efficient maternal–fetal exchanges, as well as pregnancy-specific hormone secretion and immune modulation. Incorrect placental development can lead to severe pregnancy disorders, such as preeclampsia (PE), which endangers both the mother and the infant. The implication of the systemic renin–angiotensin system (RAS) in the pregnancy-related physiological changes is now well established. However, despite the fact that the local uteroplacental RAS has been described for several decades, its role in placental development and function seems to have been underestimated. In this review, we provide an overview of the multiple roles of the uteroplacental RAS in several cellular processes of placental development, its implication in the regulation of placental function during pregnancy, and the consequences of its dysregulation in PE pathogenesis.

A novel mechanism of angiotensin II-regulated placental vascular tone in the development of hypertension in preeclampsia

The present study tested the hypothesis that angiotensin II plays a role in the regulation of placental vascular tone, which contributes to hypertension in preeclampsia. Functional and molecular assays were performed in large and micro placental and non-placental vessels from humans and animals. In human placental vessels, angiotensin II induced vasoconstrictions in 78.7% vessels in 155 tests, as referenced to KCl-induced contractions. In contrast, phenylephrine only produced contractions in 3.0% of 133 tests. In non-placental vessels, phenylephrine induced contractions in 76.0% of 67 tests, whereas angiotensin II failed to produce contractions in 75 tests. Similar results were obtained in animal placental and non-placental vessels. Compared with non-placental vessels, angiotensin II receptors and β -adrenoceptors were significantly increased in placental vessels. Compared to the vessels from normal pregnancy, angiotensin II-induced vasoconstrictions were significantly reduced in preeclamptic placentas, which was associated with a decrease in angiotensin II receptors. In addition, angiotensin II and angiotensin converting enzyme in the maternal-placenta circulation in preeclampsia were increased, whereas angiotensin I and angiotensin1-7 concentrations were unchanged. The study demonstrates a selective effect of angiotensin II in maintaining placental vessel tension, which may play an important role in development of hypertension in preeclampsia.

Lack of change in serum angiotensin-converting enzyme activity during the menstrual cycle

Introduction. The Deletion (D) rather than Insertion (I) variant of the angiotensin-converting enzyme (ACE) gene is associated with higher circulating ACE activity. Meanwhile, coronary risk rises with the menstrual nadir in oestrogen levels, exogenous oestrogen reduces serum ACE activity (with a greater reduction the higher the baseline ACE activity), and pharmacological reduction in ACE activity is cardioprotective. Alterations in coronary risk associated with the menstrual cycle may thus be mediated through (genotype-dependent) changes in ACE activity. We have examined this hypothesis.

Materials and methods. Twenty-three healthy female subjects (12 II, 11 DD genotype) were studied. None were taking oral contraceptive agents. Blood was assayed for oestrogen, follicle stimulating hormone (FSH), luteinising hormone (LH), progesterone and ACE activity every three days throughout their menstrual cycle. Results ACE activity was unrelated to oestrogen, FSH or LH during the menstrual cycle, irrespective of ACE genotype.

Conclusions. The increase in myocardial ischaemia during low oestrogen phases of the menstrual cycle does not appear mediated through a fall in serum ACE activity.

Vasoconstriction of small arteries isolated from the human placental chorionic plate in normal and compromised pregnancy

Our aim was to compare placental chorionic plate small artery function in normal pregnancies and those complicated by preeclampsia and intrauterine growth restriction. In particular we wished to test the hypothesis that the constrictive potential of placental small arteries is modified in compromised pregnancy. Biopsies were obtained from term placentas from uncomplicated pregnancies and those affected by preeclampsia and intrauterine growth restriction. Small arteries from the chorionic plate were dissected free from surrounding tissue and studied using parallel wire myography. Placental small arteries developed maintained constrictions to arginine vasopressin and the thromboxane-mimetic U46619. Arterial maximal constriction was reduced in both preeclampsia and intrauterine growth restriction. This effect was agonist independent. In intrauterine growth restriction, placental small arteries showed decreased sensitivity to U46619 but not to arginine vasopressin. Human placental chorionic plate small artery vasoconstriction is significantly reduced in compromised pregnancy, a factor that may lead to altered blood flow within the fetoplacental circulation in these conditions.

CAPTOPRIL REVERSES THE REDUCED VASODILATOR RESPONSE TO BRADYKININ IN HYPERTENSIVE PREGNANT RATS

1. Pregnancy in rats is characterized by a reduction in arterial pressure that is associated with a decreased response to vasoconstrictors. However, the responses to vasodilators in isolated vessels remain controversial and are not well established in hypertensive pregnant rats. 2. In the present study, we investigated the effect of pregnancy on the bradykinin (BK)-induced vasodilator responses of the isolated mesenteric arterial bed (MAB) from Wistar normotensive and spontaneously hypertensive rats (SHR) and determined the role of nitric oxide (NO), endothelium-derived hyperpolarizing factor (EDHF) and angiotensin-converting enzyme (ACE) in these responses. 3. Mean arterial pressure (MAP) in pregnant normotensive and pregnant hypertensive rats (93 +/- 1 and 122 +/- 2 mmHg, respectively) was lower than in non-pregnant controls (128 +/- 1 and 163 +/- 2 mmHg, respectively; P < 0.05). In MAB isolated from normotensive rats and precontracted with phenylephrine, the effects of bradykinin, acetylcholine (ACh) and nitroglycerine (NG) were not influenced by pregnancy. In contrast, the vasodilator responses to BK were significantly reduced in pregnant compared with non-pregnant SHR and seemed to be specific to BK. 4. The ACE inhibitor captopril potentiated BK vasodilator responses and abolished the differences between pregnant and non-pregnant SHR. Inhibition of nitric oxide (NO) synthase by N(G)-nitro-L-arginine methyl ester (l-NAME) significantly reduced the vasodilator effect of BK in all groups. In the presence of l-NAME plus high K+ solution (47 mmol/L), BK-induced vasodilation was completely blocked. The NO-dependent component of the responses seems to be more important in hypertensive rats and pregnancy does not modify this profile. 5. Our results suggest that increased ACE activity may be involved in the pregnancy associated reduction in vasodilator responses to BK in the MAB of hypertensive rats. Pregnancy does not modify the relative contribution of the EDHF and NO to the vasodilator effect of BK.

Vasoactive responses of veins isolated from the human placental chorionic plate

The control of the blood flow within the fetoplacental circulation is poorly understood despite the essential role of the placenta in pregnancy. Our aim was to assess the vasoactive responses of veins from the placental chorionic plate. Biopsies were obtained from term placentae from uncomplicated pregnancies. Small veins from the chorionic plate were dissected free from surrounding tissue and studied using parallel wire myography. Human placental chorionic plate veins developed maintained constrictions to the thromboxane-mimetic U46619. Endothelium-dependent agonists did not promote venous relaxation. However, NO donation with the endothelial-independent agent, sodium nitroprusside, elicited significant relaxation. Venous constriction to U46619 and relaxation to sodium nitroprusside were modified by adjustment of media oxygen tension and normalization parameters. Human placental chorionic plate veins respond to vasoactive agents and may play a role in the control of the blood flow in the fetoplacental circulation.

Caffeine enhances the expression of the angiotensin II Type 2 receptor mRNA in BeWo cell culture and in the rat placenta

Although chronic caffeine exposure during pregnancy has been shown to affect fetal growth, adverse effects of caffeine on embryogenesis are not only well understood, but also controversial. We have used gene chip technology in an attempt to identify to what extent, if any, caffeine could possibly alter gene expressions in the cytotrophoblast-like cell line BeWo. Few down-regulated genes were found; most of the genes were up-regulated, suggesting that chronic caffeine exposure during the gestational period could exert certain influences on embryogenesis. The highest up-regulated gene expression of BeWo cells by caffeine was angiotensin II type 2 (AT(2)) receptor gene. We focused the genes of the renin-angiotensin system (RAS), angiotensin II type 1 (AT(1)) and AT(2)receptors and angiotensin I converting enzyme, for study on caffeine's responsive gene expression in BeWo cells and in the placentae of pregnant rats that were fed a diet supplemented with caffeine (2 mg/100 g body weight) during gestation, and analysed the gene expressions using RT-PCR and LightCycler system. A significantly increased AT(2)receptor gene expression and a slight decreased AT(1)receptor gene expression demonstrated the caffeine's effect to the placental RAS.

Angiotensin-converting enzyme activity in the bovine uteroplacental unit changes in relation to the cycle and pregnancy

The expression of the angiotensin-forming enzymes, renin and angiotensin-converting enzyme (ACE), were examined in the bovine uteroplacental unit. The ACE activity was determined in cell membrane fractions, and ACE and renin were localized by autoradiography and immunohistochemistry, respectively. In the myometrium, the ACE activity was significantly higher in dioestrous than in oestrous. ACE activity correlated negatively with the day of gestation in the endometrium and myometrium but positively in the placentome and allantoamniotic membrane. Autoradiography showed, that ACE was localized in vascular endothelial cells in all compartments. ACE was also expressed in the endometrial stroma and uterine glands, most pronounced in the outer part of the basal zone. In the intercotyledonary membrane and the placentome, the mesenchymal cells located near the trophoblast cells expressed ACE. Solitary macrophage- or monocyte-like cells showing intense renin immunoreactivity were found in the uterus, while the uterine and the glandular epithelial cells displayed inconsistent reactivity. No renin was observed in the placentomes or in the fetal membranes. The findings demonstrate a regulated expression of angiotensin-forming enzymes throughout the bovine uteroplacental unit. Whether this local renin-angiotensin system contributes to the highly regulated morphological and functional changes throughout the oestrous cycle and gestation remains to be established.

Functional role of local angiotensin-converting enzyme (ACE) in adrenal catecholamine secretion in vivo

The aim of the present study was to investigate whether exogenous angiotensin I (AngI) is locally converted to angiotensin II (AngII), which in turn results in an increase in the adrenal catecholamine (CA) secretion in the adrenal gland in anesthetized dogs. Plasma CA concentrations in adrenal venous and aortic blood were determined by an HPLC-electrochemical method. Adrenal venous blood flow was measured by gravimetry. Local administration of AngI (0.0062 to 6.2 µg, 0.0096 to 9.6 µM) to the left adrenal gland resulted in significant increases in CA output in a dose-dependent manner. Following administration of 0.62 µg (0.96 µM) of AngI, adrenal epinephrine and norepinephrine outputs increased from 20.8 ± 13.6 to 250.9 ± 96.4 ng·min-1·g-1 (p < 0.05, n = 5) and from 2.8 ± 1.7 to 29.6 ± 11.1 ng·min-1·g-1 (p < 0.05, n = 5), respectively. From the same left adrenal gland, the output of AngII increased from -0.02 ± 0.04 to 26.39 ± 11.38 ng·min-1·g-1 (p < 0.05, n = 5), while plasma concentrations of AngII in aortic blood remained unchanged. In dogs receiving captopril (12.5 µg, 0.5 mM) 10 min prior to AngI, the net amounts of CA and AngII secreted during the first 3 min after AngI were diminished by about 80% (p < 0.05, n = 5) compared with those obtained from the control group. There was a close correlation (r2 = 0.91, n = 6) between the net increases in AngII and CA outputs induced by AngI. The results indicate that the local angiotensin converting enzyme is functionally involved in regional AngII formation in the canine adrenal gland in vivo. The study suggests that AngII thus generated may play a role in the local regulation of adrenal CA secretion.Key words: angiotensin I, angiotensin II, captopril, adrenal gland, anesthetized dog.

Interactions between AT1 and AT2 receptors in uterine arteries from pregnant ewes

This study was performed to investigate the roles of angiotensin receptors (AT1 and AT2) in the contractility of uterine arteries during normal pregnancy and after angiotensin II levels have been elevated. Pregnant ewes were given intravenous infusions of saline for 24 h (control) or angiotensin II (30 ng kg(-1) min(-1)) for 2 or 24 h. The contractile responses of uterine arterial rings to angiotensin II (4 microM) and antagonists were then examined in vitro. Most uterine arteries were relatively insensitive to the vasoconstrictor effects of angiotensin II. In rings from control ewes an angiotensin AT2 antagonist enhanced (P < 0.05) the contractile responses to angiotensin II, suggesting that angiotensin AT2 receptors inhibited the angiotensin AT1 receptor mediated contractions. Uterine arterial rings from ewes given intravenous infusions of angiotensin II displayed greater (P < 0.05) contractile responses to angiotensin II in vitro compared to rings from control ewes. This was in part due to down regulation of angiotensin AT2 receptors. Surprisingly, while performing these experiments a small number of ewes had uterine arteries which were "hyperreactive" to angiotensin II (contractile responses 6-fold greater). These ewes also had abnormal renin angiotensin systems and had some features which are characteristic of those seen in preeclampsia. The "hyperreactivity" of these arteries could only in part be explained by down regulation of angiotensin AT2 receptors. It is concluded that in normal pregnancy angiotensin AT2 receptors play a role in maintaining an adequate uterine blood flow for the fetus. When angiotensin II levels are elevated for a prolonged period this protective effect is lost partly because angiotensin AT1 receptors are down regulated.

Angiotensin II stimulates DNA and protein synthesis in vascular smooth muscle cells from human arteries: role of extracellular signal-regulated kinases

OBJECTIVE

This study investigates the growth effects and associated signaling pathways of angiotensin II (Ang II) in human vascular smooth muscle cells.

METHODS

Cultured vascular smooth muscle cells derived from resistance arteries (< 300 microm diameter) from subcutaneous gluteal biopsies of healthy subjects (n = 6) and human aortic vascular smooth muscle cells were used. Cells were studied between passages 3 and 6. Both 3H-thymidine and 3H-leucine incorporation were measured as indices of vascular smooth muscle cell hyperplasia (DNA synthesis) and cell hypertrophy (protein synthesis), respectively. Growth effects of Ang II (10(-12) - 10(-6) mol/l), in the absence and presence of 10(-5) mol/l losartan (AT1 antagonist) and PD123319 (AT2 antagonist), were determined. Ang II-induced effects were compared to those of endothelin-1. To determine whether extracellular signal-regulated kinase (ERK)-dependent pathways play a role in Ang II-mediated growth, cells were pretreated with the selective ERK kinase (MEK) inhibitor, PD98059 (10(-5) mol/l). ERK activation was determined by Western blot in the absence and presence of PD98059.

RESULTS

Ang II dose-dependently increased 3H-thymidine incorporation in cells from aorta (Emax = 276 +/- 10.4% of control) and resistance arteries (Emax = 284 +/- 5.1% of control). Ang II also stimulated 3H-leucine incorporation in cells from aorta (Emax = 162 +/- 11.6 of control) and resistance arteries (Emax 175 +/- 10% of control). Unlike Ang II, endothelin-1 failed to significantly alter cellular growth, except at high concentrations (> 10(-7) mol/l), where it had a weak stimulatory effect Losartan, but not PD123319, blocked Ang II-stimulated growth responses. Ang II significantly increased phosphorylation of ERK-1 and ERK-2, with maximum responses obtained at 5 min. PD98059 inhibited Ang II-stimulated ERK activity and abrogated agonist-induced DNA and protein synthesis. Losartan, but not PD123319 inhibited Ang II-induced phosphorylation of ERK-1 and ERK-2.

CONCLUSIONS

Ang II stimulates both hyperplasia and hypertrophy in vascular smooth muscle cells from human arteries. These growth effects are mediated via Ang II receptors of the AT1 subtype that are linked to ERK-dependent signaling pathways.

Role of extracellular signal-regulated kinases in angiotensin II-stimulated contraction of smooth muscle cells from human resistance arteries

BACKGROUND

We assessed the role of extracellular signal-regulated kinases (ERKs) in Ang II-stimulated contraction and associated signaling pathways in vascular smooth muscle cells (VSMCs) from human small arteries.

METHODS AND RESULTS

VSMCs derived from resistance arteries (<300 microm in diameter) from subcutaneous gluteal biopsies of healthy subjects (n=8) were used to assess Ang II-stimulated [Ca2+]i, pHi, and contractile responses. [Ca2+]i and pHi were measured with fura 2-AM and BCECF-AM, respectively, and contraction was measured photomicroscopically in cells grown on Matrigel matrix. To determine whether tyrosine kinases and ERKs influence Ang II-stimulated responses, cells were pretreated with 10(-5) mol/L tyrphostin A-23 (tyrosine kinase inhibitor) and PD98059 (MEK inhibitor). Ang II-stimulated MEK activity was determined by tyrosine phosphorylation of ERKs. The angiotensin receptor subtypes (AT1 and AT2) were assessed with [Sar1,Ile8]Ang II (a nonselective subtype antagonist), losartan (a selective AT1 antagonist), and PD123319 (a selective AT2 antagonist). Ang II dose-dependently increased [Ca2+]i (pD2=8.4+/-0.36, Emax=541+/-55 nmol/L), pHi (pD2=9. 4+/-0.29, Emax=7.19+/-0.01), and contraction (pD2=9.2+/-0.21, Emax=36+/-2.2%). Ang II induced rapid tyrosine phosphorylation of ERKs, which was inhibited by PD98059. Tyrphostin A-23 and PD98059 attenuated (P<0.05) Ang II-stimulated second messengers, and PD98059 reduced Ang II-induced contraction by >50%. [Sar1,Ile8]Ang II and losartan, but not PD123319, blocked Ang II-stimulated responses.

CONCLUSIONS

These data demonstrate that in VSMCs from human peripheral resistance arteries, functional Ang II receptors of the AT1 subtype are coupled to signaling cascades involving Ca2+ and pHi pathways that are partially dependent on tyrosine kinases and ERKs. ERKs, the signaling cascades characteristically associated with cell growth, may play an important role in Ang II-stimulated contraction of human VSMCs.

Autoradiographic localization and characterization of angiotensin II receptors in the bovine placenta and fetal membranes

Autoradiography and angiotensin (Ang) II receptor binding studies showed that all parts of the bovine placenta and fetal membranes contained high densities of Ang II receptors throughout gestation. The receptors were predominantly subtype 2 (AT2) receptors in the fetal and subtype 1 (AT1) receptors in the maternal compartment. In the allantoamnionic membrane, Ang II receptors were evenly distributed in the mesenchymal tissue, with the highest expression around the few arteries. In the intercotyledonary and cotyledonary allantochorionic membrane, AT2 receptors as well as the less-expressed AT1 receptors were located on mesenchymal cells, especially adjacent to the allantoic endoderm, trophoblast cell layer, and arteries. In the mesenchymal tissue of the placentome, Ang II receptors were mostly expressed at the main branches of the fetal villi of the cotyledons. In the maternal part of the placentome, mainly AT1 receptors but also low densities of AT2 receptors and non-AT1/non-AT2 Ang II binding sites were found close to the stalk and at the main branches of the maternal crypts. Autoradiography revealed no changes in the pattern of distribution of the Ang II receptors throughout gestation. It is suggested that Ang II has an effect on regulatory as well as growth processes in these tissues.

The human placental renin-angiotensin system

The human placenta and related tissues are considered to be examples of the recently accepted local renin-angiotensin systems (RAS). The brain is another example of a system that is thought to be regulated independently of the kidney and the role of angiotensin within the CNS as a neural mediator has drawn considerable attention. It has been known for a long time that many of the neuroendocrine mediators and receptors are expressed in the placenta and it has been suggested that there are many parallels between the classical neuroendocrine system and the placental one. The present review summarizes information that components of the RAS are expressed in uteroplacental tissues, are regulated by endogenous substances, and have important biological functions within this reproductive system. A comparison of similarities and differences between the classical and the placental RAS may provide clues to functions in other endocrine and neuroendocrine systems. The major components of the placental RAS that are considered are renin, prorenin, angiotensin I, angiotensin II, angiotensin converting enzyme (ACE), angiotensin receptors, and angiotensinogen (renin substrate). The factors that regulate these components at the cellular and the nuclear level are described. It is concluded that prorenin via angiotensin-dependent and angiotensin-independent mechanisms influences functions within uteroplacental tissues. Some of these actions are direct and others are mediated by the release of different signalling molecules. These features are similar to many neuroendocrine systems and utilize some of the same messengers.

Comparative vasoconstrictor effects of angiotensin II, III, and IV in human isolated saphenous vein

Elevated levels of angiotensin (Ang II) and its degradation products angiotensin III (Ang III) and angiotensin IV (Ang IV) may contribute to the regulation of vascular tone under various clinical circumstances. We investigated the contractile effects of Ang III and Ang IV in endothelium-denuded human saphenous vein (SV) preparations and compared them with those of Ang II. The veins were suspended in organ chambers, and changes in isometric force were recorded. Ang II (0.1-100 nM), Ang III (1 nM-3 microM), and Ang IV (0.3 microM-0.1 mM) caused concentration-dependent contractions with comparable maximal responses (Emax). Ang III was 16 times less active than Ang II, whereas Ang IV was approximately 2,700-fold less potent than Ang II. In the presence of the aminopeptidase-A and -M inhibitor amastatin (10 microM), the potencies of Ang III and Ang IV were increased by approximately 16 and 12 times, respectively, although no changes of Ang II potency were observed. The AT1-selective Ang II receptor antagonist losartan (10 and 100 nM) but not the AT2-selective antagonist PD123177 (1 microM), shifted the concentration-response curves (CRC) for the angiotensin peptides to the right in a parallel manner. Preincubation with indomethacin (10 microM), a cyclooxygenase inhibitor, did not influence the CRCs for any of the angiotensin peptides studied. Tachyphylaxis was investigated by constructing a second series of CRCs for the angiotensin peptides after an interval of 60 min. Ang II showed strong tachyphylaxis (the Emax value of the second Ang II CRC was approximately 50% of the first), whereas Ang III and Ang IV did not. Our results indicate that in endothelium-denuded human SV, both Ang III and Ang IV are less potent but similarly efficacious vasoconstrictor agents compared with Ang II. Endogenous aminopeptidase activity may counteract the effects of the angiotensin peptides. The contractile responses to all three peptides are mediated via AT1-receptors but not AT2-receptors.