Renin-angiotensin system in normal and in hypertensive disease of pregnancy

Mineralocorticoid Dysfunction during Critical Illness: A Review of the Evidence

The recent demonstration of the significant reduction in mortality in patients with septic shock treated with adjunctive glucocorticoids combined with fludrocortisone and the effectiveness of angiotensin II in treating vasodilatory shock have renewed interest in the role of the mineralocorticoid axis in critical illness. Glucocorticoids have variable interactions at the mineralocorticoid receptor. Similarly, mineralocorticoid receptor-aldosterone interactions differ from mineralocorticoid receptor-glucocorticoid interactions and predicate receptor-ligand interactions that differ with respect to cellular effects. Hyperreninemic hypoaldosteronism or selective hypoaldosteronism, an impaired adrenal response to increasing renin levels, occurs in a subgroup of hemodynamically unstable critically ill patients. The suggestion is that there is a defect at the level of the adrenal zona glomerulosa associated with a high mortality rate that may represent an adaptive response aimed at increasing cortisol levels. Furthermore, cross-talk exists between angiotensin II and aldosterone, which needs to be considered when employing therapeutic strategies.

Salt, aldosterone and extrarenal Na+ - sensitive responses in pregnancy

Outside of pregnancy excessive salt consumption is known to be harmful being linked to increased blood pressure and cardiovascular disease. However, pregnancy represents a major change to a woman's physiology resulting in an intimate adaptation to environmental conditions. It is now becoming apparent that salt is essential for a number of these changes during pregnancy including haematological, cardiac adaptations as well as directly influencing placental development and the uteroplacental immune environment. The present review discusses the important role that salt has during normal pregnancy and evidence will also be presented to show how the placenta may act as a salt sensing organ temporarily, yet substantially regulating maternal blood pressure.

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The placenta may function as an extrarenal regulator of maternal blood pressure.

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Na⁺handling in pregnancy is completely different to the non-pregnant situation.

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Na⁺may actually lower blood pressure in pregnancy affected with pre-eclampsia.

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Aldosterone is an important regulator of placental and fetal development.

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Na⁺ may compensate for aldosterone deficiency in pregnancy.

A brief overview of preeclampsia

Preeclampsia (PE) is a leading cause of maternal mortality and morbidity worldwide. It occurs in women with first or multiple pregnancies and is characterized by new onset hypertension and proteinuria. Improper placentation is mainly responsible for the disease. If PE remains untreated, it moves towards more serious condition known as eclampsia. Hypertension, diabetes mellitus, proteinuria, obesity, family history, nulliparity, multiple pregnancies and thrombotic vascular disease contribute as the risk factors for PE. PE triggered metabolic stress causes vascular injury, thus contributing to the development of cardiovascular disease (CVD) and/or chronic kidney disease (CKD) in future. This risk appears to be increased especially in women with a history of recurrent PE and eclampsia. Clinically increased serum levels of sFlt-1 and decreased placental growth factor (PIGF) and vascular endothelial growth factor (VEGF) represent the severe condition of PE. The clinical findings of sever PE are assorted by the presence of systemic endothelial dysfunction, microangiopathy, the liver (hemolysis, elevated liver function tests and low platelet count, namely HELLP syndrome) and the kidney (proteinuria). The early detection of PE is one of the most important goals in obstetrics.

Longitudinal study of the renin angiotensin aldosterone system in purebred Spanish broodmares during pregnancy

During pregnancy, the coordinated interaction of the components of the renin-angiotensin-aldosterone system (RAAS) plays a vital role in accommodating the cardiovascular, haemodynamic and haematological needs imposed by foetal development and the placenta. This significantly influences the birth weight of the neonate and foetal viability. Although the evolution of each of the components of this system has been widely described in various species, it has not yet been clarified in the mare. Thus, the objectives of the present research were: 1) to establish reference values for renin (REN), angiotensin II (ANG-II) and aldosterone (ALD) concentrations in Spanish broodmares, and 2) to analyse the evolution of the aforementioned components during pregnancy. Thirty-one Purebred Spanish broodmares aged between 5 and 15 years old were studied for 11 months of pregnancy and compared to a control group composed of 11 non-pregnant Spanish mares. Morning venous blood samples were drawn on a monthly basis during pregnancy and pre-treated to prevent degradation until subsequent analysis. Serum REN, ANG-II and ALD concentrations were analysed by competitive immunoassay. This study found that pregnancy in Purebred Spanish broodmares is characterised by a gradual increase in REN concentrations, variable fluctuations in ALD concentrations, and no significant modifications in ANG-II concentrations. These results could provide potentially valuable information in understanding the physiological basis of the RAAS in mares, since we have been able to establish specific reference ranges for these components, as well as obtaining information on their evolution during pregnancy. As is often the case in other animal species, the increase in RAAS activity is a natural physiological process that occurs during pregnancy in Spanish broodmares. This may also be related to certain metabolic and hormone responses that contribute to the control of homeostasis in pregnant mares.

Preeclampsia: a renal perspective

Preeclampsia is a syndrome that affects 5% of all pregnancies, producing substantial maternal and perinatal morbidity and mortality. The aim of this review is to summarize our current understanding of the pathogenesis of preeclampsia with special emphasis on the recent discovery that circulating anti-angiogenic proteins of placental origin may play an important role in the pathogenesis of proteinuria and hypertension of preeclampsia.

Reproductive tissue renin gene expression in preeclampsia

OBJECTIVE

Using an analogy with renin gene overexpression, low-renin hypertension animal models, we wished to test the hypothesis that renin gene expression is increased in decidua basalis in human gestation with preeclampsia.

METHODS

Human placentas were obtained immediately after delivery from 11 control (C) and 11 preeclamptics (PE). Tissue samples were microdissected and renin gene expression in decidua basalis (DB), chorionic villi (CV), and decidua vera (DV) was measured using dot-blot hybridization.

RESULTS

Overall renin gene expression is highest in decidua basalis (mean +/- SEM, 2.66 +/- 0.69 densitometry area units) compared to chorionic villi (mean +/- SEM, 1.85 +/- 0.5) or compared to decidua vera (mean +/- SEM, 1.63 +/- 0.9) (both t-tests p = 0.001 two-tailed and analysis of variance p = 0.0001). Renin gene expression in DB and in CV was similar in both preeclamptic and normal pregnancies (DB mean +/- SEM C 2.79 +/- 0.96 versus PE 2.54 +/- 1.04, and CV mean +/- SEM C 2.11 +/- 0.91 versus PE, 1.59 +/- 0. 44). Renin gene expression in DV was approximately threefold higher in tissues from preeclamptics compared to control (mean +/- SEM PE 2. 44 +/- 1.76 versus C 0.82 +/- 0.42). Using the median value of 0.5 units for DV as cutoff, the preeclamptics displayed higher renin gene expression (chi square p = 0.033, two tailed).

CONCLUSION

Our data suggest that renin gene expression is increased in preeclampsia in decidua vera. This may explain previously reported increased renin secretion in uterine circulation in preeclampsia.

Increased peripheral resistance during reduced uterine perfusion pressure hypertension in pregnant rabbits

The role of an increase in total peripheral resistance (TPR) and the contribution of angiotensin II (ANG II) to the hypertension induced by reduced uterine perfusion pressure (RUPP) was explored in pregnant rabbits. On the 22nd day of gestation, a catheter and a microthermocouple were placed in the aorta to measure mean arterial pressure (MAP) and cardiac output (CO), respectively. Three days later, RUPP was induced by a clip on the aorta proximal to the ovarian and distal to the renal arteries. Mean arterial pressure distal to the clip (uterine perfusion pressure) was reduced to 56 +/- 8% (mean +/- SD) of the initial level. Twenty-four hours later, MAP rose from 65 +/- 3 to 84 +/- 11 mm Hg; CO index decreased from 207 +/- 18 to 169 +/- 27 ml/min/kg; and TPR index increased from 0.32 +/- 0.03 to 0.51 +/- 0.08 mm Hg kg/ml/min, respectively (n = 7, all p less than 0.01). Sham-operated pregnant rabbits (n = 7) and non-P rabbits (n = 5) with a comparable distal aortic pressure reduction experienced no change in MAP or CO. Infusion of a receptor antagonist of angiotensin II (Sar1,Ile8-Ang II, 1 microgram/kg/min for 20 min) decreased MAP in sham-operated pregnant rabbits from 64 +/- 6 to 54 +/- 6 mm Hg (p less than 0.01) but did not change MAP in RUPP hypertensive rabbits (86 +/- 9 mm Hg before and 87 +/- 8 at the end of infusion, n = 6). These data indicate that RUPP in pregnant rabbits leads to a high resistance form of hypertension in which the formation of Ang II is not increased.

Fetal and maternal plasma atrial natriuretic factor responses to angiotensin II infusion

Plasma atrial natriuretic factor and angiotensin II have opposing actions in the regulation of body fluid homeostasis and systemic blood pressure. Angiotensin II infusions stimulate atrial natriuretic factor release in some, but not all, studies in adult mammals. To examine the response during the perinatal period, graded intravenous angiotensin II infusions were administered to four chronically instrumented pregnant ewes and fetuses (132 +/- 1 days of gestation). Fetuses received successive 20-minute intravenous angiotensin II infusions at 25, 50, and 100 ng/kg/min. After a 90-minute recovery, maternal ewes received successive 20-minute angiotensin II infusions (5, 10, and 25 ng/kg/min). During the fetal infusions, mean (+/- SEM) fetal arterial blood pressure (47 +/- 2 to 61 +/- 2 mm Hg, p less than 0.05) and heart rate (155 +/- 16 to 196 +/- 36 beats/min, p less than 0.05) increased, although there was no change in maternal measured parameters. In response to the maternal infusion, maternal mean arterial blood pressure increased (92 +/- 7 to 110 +/- 8 mm Hg, p less than 0.05) and maternal heart rate decreased (136 +/- 4 to 125 +/- 2 beats/min, p less than 0.05) without change in fetal parameters. Fetal plasma atrial natriuretic factor levels (210 +/- 27 to 664 +/- 250 pg/ml, p less than 0.05) significantly increased during the fetal angiotensin II infusions in spite of no change in maternal plasma atrial natriuretic factor (85 +/- 21 to 124 +/- 22 pg/ml) during the maternal angiotensin II infusions. These findings indicate that similar increases in systemic blood pressure in response to angiotensin II infusions stimulate increased fetal, but not maternal, plasma atrial natriuretic factor.

Effect of progesterone on renin secretion in endometrial stromal, chorionic trophoblast, and mesenchymal monolayer cultures

Renin messenger ribonucleic acid expression has been shown in decidual tissue and endometrium, but weakly in chorion laeve, suggesting decidua as a primary source of uteroplacental renin. We proposed to confirm active renin secretion by endometrial stromal cells and examined the effect of progesterone-induced decidualization on secretion. Separate monolayer cultures of endometrial stroma, trophoblast, and mesenchymal cells were established and maintained for 10 to 15 days. Active renin concentration was measured with radioimmunoassay for angiotensin I generation and expressed as picograms per milliliter of angiotensin I generation per 10(5) cells. Active renin concentration was high in control secretory endometrial stromal cells (513 pg/ml) compared with trophoblast (none) and mesenchymal cell cultures (74 pg/ml). Marked decrease in active renin secretion occurred in control endometrial stromal cells (days 13 to 15, 86 pg/ml), whereas progesterone-induced decidualization sustained and increased secretion (days 13 to 15, 1017 pg/ml). This renin activity was quantitatively inhibited in culture fluid assays by specific human renal renin antibody in serial dilutions. Renin activity measurements before and after trypsin activation showed the majority of renin (91.15%) in the inactive form and a smaller fraction (8.85%) in the active form. Immunohistochemistry with the use of specific human renal renin antibody confirmed the presence of renin and its stimulation by progesterone in endometrial stromal cells. Progesterone had minimal effect on mesenchymal cells. These data confirmed endometrial stromal cells as a significant source of active renin and showed that progesterone induced a marked increase in this production.

Serum aminopeptidase A (AAP) in normal pregnancy and pregnancy complicated by pre-eclampsia

Serum aminopeptidase A (AAP) activity was measured in normal pregnancy and pre-eclampsia. The AAP activity in normal pregnancy increased progressively with advancing gestation, reaching the highest value at the end of pregnancy. The AAP activity in pregnancy complicated by pre-eclampsia was lower than in normal pregnancy.

Plasma angiotensin I and serum placental leucine aminopeptidase (P-LAP) in pre-eclampsia

A study was undertaken on serial measurements of plasma angiotensin I (A-I) and serum placental leucine aminopeptidase (P-LAP) activities in normal and pre-eclamptic pregnancy. There was the significant difference in A-I levels between normal and mild pre-eclamptic pregnancy at weeks 30, 35, 37, between normal and severe pre-eclamptic pregnancy at week 37. There were no differences in serum P-LAP between normal and mild pre-eclamptic pregnancy up to week 33, but thereafter the levels for the mild pre-eclampsia were significantly higher than for the normal pregnancy. The P-LAP activity for the severe pre-eclampsia reached its maximum level at week 31. Around this week, the levels for severe pre-eclampsia were significantly higher than in the normal pregnancy. After week 35, the activities decreased precipitously to week 40; the activities for severe pre-eclampsia in late pregnancy at weeks 39 and 40 were significantly lower than in normal pregnancy. The above data support the idea that P-LAP test is useful for prediction or diagnosis of pre-eclampsia.

Purification and partial characterization of aminopeptidase A from the serum of pregnant and non-pregnant women

Aminopeptidase A (L-alpha-aspartyl(L-alpha-glutamyl)-peptide hydrolase, EC 3.4.11.7) was purified from human maternal and control sera using CM-cellulose chromatography, DEAE-Sephacel chromatography, Sephacryl S-300 gel filtration and hydroxyapatite chromatography. The purification coefficients were 3069 and 5210 and the yields 6.3 and 6.1% for the maternal and control serum, respectively. The purified enzymes appeared free from other serum aminopeptidases in polyacrylamide gel electrophoresis. The biochemical and physical characteristics of the enzymes from maternal and control sera were similar. A molecular weight of 260 000, an optimum at pH 6.75-7.25 and a fairly good stability of the enzymes at 4 and -18 degrees C were recorded. The alkaline earth metals (Ca2+, Ba2+, Sr2+) were the activators of alpha-L-glutamyl-beta-naphthylamide hydrolysis, while alpha-L-aspartyl-beta-naphthylamide hydrolysis was markedly potentiated with Ca2+ but not with Ba2+ at all. The most rapid hydrolysis was shown with GluNA (Km with Ba2+ 0.156 +/- 0.014 mM and 0.136 +/- 0.009 mM in maternal and control serum, respectively), while only minimal hydrolysis of some neutral and basic amino-acid-beta-naphthylamides were observed. The contribution of the placenta to the elevated aminopeptidase A levels in the pregnancy plasma could not be solved on the basis of the present observations.

Changes in the renin-angiotensin-aldosterone system in normotensive primigravidae in the four days after normal spontaneous delivery

Plasma renin activity, plasma renin concentration, plasma renin substrate, plasma angiotensin II and plasma aldosterone were measured in 10 true primiparae, normotensive throughout pregnancy, for 4 days after a spontaneous vaginal delivery. The concentration of all the measured variables, except plasma renin substrate, fell steeply over the first 3 days post partum. In four of the 10 patients, all components of the renin-angiotensin system increased in concentration somewhat thereafter, although not to the levels of the first day post partum. There are multiple potential sources of renin in the fetoplacental unit and it is possible that this 'pregnancy' renin may be capable of the partial suppression of the maternal renin production during pregnancy. The slight rebound observed in some patients may represent a hunting for re-establishment of non-pregnant values.

Hormonal adaptation to the stresses imposed upon sodium balance by pregnancy and lactation in the Yanomama Indians, a culture without salt

The Yanomama Indians of northern Brazil and southern Venezuela have been identified as a "no-salt" culture. In this study, data were obtained to determine in this population the adjustments of sodium-related hormones to the stresses imposed upon sodium balance by pregnancy and prolonged lactation. Controls against the possibility that findings in the Yanomama were ethnic rather than dietary were provided by similar observations in the Guaymi Indians of Panama, who have free access to salt. Urinary concentrations of sodium were approximately 1 mEq/l in male and female Yanomama, with 24-hour excretion rates in the males averaging 1 mEq, similar to our prior observation. The pregnant Yanomama had exceeding high urinary concentrations of aldosterone. These were associated with higher plasma renin activities and serum aldosterone concentrations than in all other subjects. Although pregnant Guaymi had elevations of serum and urinary aldosterone, these were significantly lower (p < 0.001) than those of the Yanomama. Prolonged lactation in the Yanomama was associated with elevation of plasma renin activity and serum and urinary aldosterone concentration compared with the Guaymi, but were not higher than those in nonlactating Yanomama females. The findings suggest that pregnancy in a salt-poor environment is associated with an exaggerated augmentation of hormonal responses that enhance positive sodium balance.

Identification of vasopressor constituents of amniotic fluid in pregnancy toxaemia

The vasopressor activity of human amniotic fluid was assessed in 46 primigravidas during the third trimester of pregnancy. The subjects included 20 normal and 26 cases of pregnancy toxaemia. For identification of the vasopressor activity of the amniotic fluid, blood pressure measurements were made in the intact rat, and the smooth muscle contraction of the rat ascending colon preparation was determined in vitro. The samples of amniotic fluid showing vasopressor activity were lyophilized and subjected to gel filtration using Sephadex G-100. The various fractions thus obtained were monitored at 280 nm and the pressor activity was re-estimated. It was observed that most of the pressor activity of the amniotic fluid of the toxaemic patients was capable of causing contraction of the rat colon as well as a significant rise in the systolic blood pressure of the intact rat. Furthermore, this activity was found to be heat-labile and was completely blocked by cinnarizine. Fractionation of the amniotic fluid showing vasopressor activity by gel filtration further substantiated the findings reported above. On the basis of these results it was concluded that the pressor activity of the amniotic fluid was due to the presence of an angiotensin II-like substance. Its concentration was found to be higher in the toxaemic group than in normotensive subjects.

Pregnancy hypertension, parity, and the renin-angiotensin system

Factors which influence levels of plasma AII in late pregnancy have been studied in 50 primigravidas and 53 multigravidas. A highly significant relationship has been shown between diastolic blood pressure and plasma AII (r = 0.4190 p less than 0.005) in primigravidas but not in multigravidas (r = 0.205; p less than 0.3). Multiple regression analysis and analysis of covariance have been applied to a series of independent variables with plasma AII as the dependent variable. The single most important variable related to AII levels in primigravidas was diastolic blood pressure whereas in multigravidas it was proteinuria. Rhesus blood group was shown to have a significant effect in both parity groups, Rh-negative primigravid women exhibiting higher values of AII.

Sequential changes in the human renin-angiotensin system following delivery

Plasma renin activity and concentration and angiotensin II concentration have been measured serially in six normotensive primigravidae and in four mildly hypertensive patients in the four immediately following normal delivery. In normotensive patients, both renin and angiotensin II levels fell sharply over the first two hours to levels within the non-pregnant range, with a subsequent rise to levels, similar to those found in late pregnancy. Changes in the mildly hypertensive patients were smaller, and somewhat slower. It is suggested that the initial fall represents clearance of a renin of fetoplacental origin and that the subsequent rise may be a release from feedback suppression of renal renin synthesis.

Total immunoreactive angiotensin ii, its hepta-octapeptide fraction, and its hexapeptide in patients with liver disease

Arterial and venous plasma concentrations of total immunoreactive angiotensin II (AT II), its bioactive hepta-octapeptide fraction and its inactive hexapeptide were measured in normal subjects (n=16), in patients with acute viral hepatitis (n=12), and in treated (n=16) and untreated (n=17) patients with cirrhosis of the liver and ascites. Independent of normal or increased values of total immunoreactive AT II, the ratio between the hepta-octapeptides and the hexapeptide remained unchanged. This might indicate continuous octapeptide generation and balanced metabolite turnover throughout the systemic circulation. Moreover, a significant arterio-venous peptide gradient was lacking. It has to be concluded that total venous plasma AT II sufficiently reflects both the arterial hormone concentration and its major fraction of hepta-octapeptides in arterial (79%) and venous (76%) blood.

Prospective study of angiotensin II response to positional change in pregnancy-induced hypertension

The response of endogenous angiotensin II levels to positional change, lateral to supine recumbency, was investigated in a prospective study of 55 primigravid patients during the last half of pregnancy. Blood samples were obtained in the lateral and supine recumbent positions. The mean supine angiotensin II level was significantly higher between 29 and 34 weeks' gestation in those patients destined to develop pregnancy-induced hypertension than in those who remained normotensive (P less than 0.05). As gestation advanced, the mean per cent relative change of angiotensin II from the lateral to the supine position altered from negative to positive in those patients destined to develop pregnancy-induced hypertension, whereas it remained negative in those patients who remained normotensive. These findings are discussed in relation to pathophysiologic alterations in the development of pregnancy-induced hypertension.

Changes in the renin-angiotensin system in primigravidae with hypertensive disease of pregnancy

Plasma renin concentration, renin activity and angiotensin II (AII) have been studied in 35 primigravidae at or near term immediately prior to surgical induction of labour. A highly significant positive correlation was established between diastolic blood pressure at the time of sample collection and plasma AII. This relationship was observed over the range of diastolic pressure studied irrespective of the diagnosis made at the time of admission to hospital. In patients with hypertension, a significant negative relationship was established between plasma renin concentration and renin activity and AII. Furthermore, plasma AII levels were higher for a given plasma renin activity or plasma renin concentration in the hypertensive women than in the normotensive group. These findings suggest that the renin-angiotensin system is actively involved in hypertensive disease of pregnancy and that suppressed levels of renin concentration and activity may result from elevated angiotensin levels. It is suggested that the utero-placental complex may be the source of the elevated AII levels.

Plasma renin and aldosterone in an abdominal pregnancy with toxemia

The first reported measurement of plasma renin and aldosterone in toxemia with an abdominal pregnancy is presented. In contrast to toxemia where plasma renin and aldosterone are either normal or low, extraordinarily high levels of both were found in this patient which returned to normal after delivery. The role of extrarenal renin in the hypertension of toxemia is discussed and the possibility raised that the elevated renin in this case was of placental origin.

Plasma renin activity in maternal and umbilical cord blood during parturition

Plasma renin activity (PRA) in umbilical cord blood exceeded that of maternal blood in 14 out of 20 cases. Fetal PRA was not significantly correlated to maternal PRA. Significantly higher fetal renin values than the corresponding maternal ones were restricted to shorter labor (N equals 10). In cases with uterine inertia (N equals 8) higher values than in normals (N equals 7) were observed, whereas suppressed maternal and cord blood levels were found in maternal fluid retention with edema (N equals 3). The data suggest that fetal production of renin may be largely independent of maternal renin secretion.