A prospective study of plasma-renin activity in normal and toxaemic pregnancy

Responses of the renin-angiotensin-aldosterone system in pregnant chronic kidney disease patients with and without superimposed pre-eclampsia

Background

Women with chronic kidney disease (CKD) are at increased risk of superimposed pre-eclampsia (SPE). Accurate identification of SPE is challenging. We hypothesized that specific components of the renin-angiotensin-aldosterone system (RAAS) would discriminate between CKD and SPE. The aim of the study was to establish differences in circulating and intrarenal RAAS in women with CKD with and without SPE and compare these to normotensive controls (NCs) and women with pre-eclampsia (PE).

Methods

White European NC women (n = 20), women with PE (n = 9), normotensive CKD without SPE (n = 8) and with SPE (n = 11) were recruited in the third trimester. Plasma renin, plasma and urine total angiotensinogen (AGT) concentrations were quantified by enzyme-linked immunosorbent assay, urinary tetrahydroaldosterone (TH-aldo) concentration by gas chromatography-mass spectrometry and placental growth factor (PlGF) by immunoassay.

Results

Urinary TH-aldo:creatinine ratios were lower in women with PE or SPE compared with NC or women with CKD (P < 0.05 for all). The same group differences were observed for plasma active renin and PlGF concentrations (P < 0.05 for all). Urine total AGT was higher in women with PE compared with NC (P < 0.05) and urine TH-aldo:urine AGT was lower (P < 0.05). However, women with SPE had lower urinary AGT concentrations compared with women with PE (P < 0.05). No differences in plasma total AGT were observed between groups.

Conclusions

Women with SPE have a lower urinary TH-aldo:creatinine ratio, lower plasma active renin and lower PlGF concentrations than women with CKD, comparable to women with PE without pre-existing disease, suggestive of similar pathophysiology. These data suggest disruption of the RAAS pathway in SPE similar to PE. Exploration of the predictive value of RAAS components for adverse pregnancy events in women with CKD is required.

The impact of salt intake during and after pregnancy

Although high blood pressure before pregnancy is associated with a risk of gestational hypertension and preeclampsia, no convincing evidence has been produced to show that dietary salt reduction helps in the prevention and treatment of hypertension during pregnancy. Thus the current guidelines do not recommend a sodium restriction during pregnancy to prevent gestational hypertension and the development of preeclampsia. However, the long-term impact of hypertensive disorders of pregnancy for life-threatening diseases later in life is a critical issue. Gestational hypertension could contribute to the risk of developing hypertension later in life, and recent studies have suggested that gestational hypertension and preeclampsia are linked to cardiovascular complications. In this article, we provide an overview of the current perspectives on the salt intake of pregnant women and consider both the short-term influence and the impact beyond the perinatal period.

A potential pathophysiological role for galectins and the renin-angiotensin system in preeclampsia

This review discusses a potential role of galectins and the renin-angiotensin system (RAS) in the pathophysiology of preeclampsia (PE). Preeclampsia affects between 3 and 5 % of all pregnancies and is a heterogeneous disease, which may be caused by multiple factors. The only cure is the delivery of the placenta, which may result in a premature delivery and baby. Probably due to its heterogeneity, PE studies in human have hitherto only led to the identification of a limited number of factors involved in the pathogenesis of the disease. Animal models, particularly in mice and rats, have been used to gain further insight into the molecular pathology behind PE. In this review, we discuss the picture emerging from human and animal studies pointing to galectins and the RAS being associated with the PE syndrome and affecting a broad range of cellular signaling components. Moreover, we review the epidemiological evidence for PE increasing the risk of future cardiovascular disease later in life.

Predicting high blood pressure in pregnancy: a multivariate approach

OBJECTIVE

To identify predictors of pregnancy-induced hypertension and pre-eclampsia in 212 nulliparous women before 20 weeks' gestation and at approximately 28 weeks' gestation.

STUDY DESIGN

A randomized, prospective study in a teaching hospital. We performed standardized measurements of systolic and diastolic arterial blood pressures, body mass index, urinary calcium:creatinine ratio and components of the renin-angiotensin system, including platelet angiotensin II binding site density. Attending clinicians were blinded to the results. Outcome was assessed by one observer at the end of pregnancy. Discriminant function analysis was used to identify significant predictors.

RESULTS

Fifty-five women had transient, presumed 'white-coat', systolic hypertension at the time of first pregnancy visit; they were twice as likely to develop pregnancy-induced hypertension and pre-eclampsia and five times more likely to deliver prematurely. Body mass index, platelet angiotensin II binding site density and urinary calcium:creatinine ratio measured before 20 weeks gestation were also significant predictors. At 28 weeks of pregnancy, measurements of the blood pressure were significant predictors (reflecting the near clinical expression of the disease), together with the plasma angiotensinogen concentration.

CONCLUSIONS

A single systolic blood pressure reading of 140 mmHg or more before 20 weeks' gestation indicates a higher than normal risk of pregnancy-induced hypertension and pre-eclampsia and premature delivery. Discriminator biochemical variables were also identified at this time, which might allow the more rational use of prophylactic measures.

Renin and aldosterone concentrations in pregnant essential hypertensives - a prospective study

Serial blood samples were obtained throughout pregnancy from 11 women with essential hypertension (EHT). Seven were treated with labetalol (Trandate) and 4 with alpha -methyl dopa (Aldomet). Nine patients were well-controlled throughout pregnancy. Their mean plasma renin concentrations (PRC) followed the profile determined in 18 normal patients studied serially. They remained in the upper normal range until the last month, when both treatment groups showed a fall in PRC. Mean plasma aldosterone (ALD) also followed a normal profile until late gestation when it too showed a sharp fall. Of the two patients who developed superimposed PIH, one, who received labetalol, developed severe hypertension at 35 weeks, requiring delivery. Although PRC increased early in this pregnancy, ALD did not, remaining low throughout. Serum potassium [K+] measurements were also very low in this patient. The second patient only became hypertensive at 40 weeks and had PRC and ALD profiles resembling those in the successfully treated EHTs. There was a strong positive correlation throughout between serum potassium and ALD measurements (p less than 0.001) but none between PRC and ALD. This latter agrees with the known lack of correlation between PRC and ALD in normal pregnancy and may suggest that changes in electrolyte balance are more important stimuli to ALD secretion during pregnancy.

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.

Oral contraceptives--induced hypertension--nine years later

Nine years have gone by since oral contraceptive hypertension was first recognized as a clinical entity. In that time it has become apparent that what at first was thought to be a rare disorder is extremely common. Indeed, overt hypertension develops, in time, in about 5 per cent of Pill users. Increases in blood pressure, albeit within the normal range, are still more common. Pill hypertension may develop gradually. It is sometimes quite severe, and it is characteristically reversible within a few months after therapy is stopped. Oral contraceptives produce changes in the renin-angiotensin-aldosterone system, particularly a consistent marked increase in the plasma renin substrate concentration which may be associated with increases in plasma renin activity and aldosterone excretion. The estrogenic component of contraceptive pills appears to be the more important factor in producing abnormalities in the renin system. The mechanisms for the hypertensive response are not entirely clear since normotensive women using the pill may exhibit similar or even more marked changes in the renin axis. The pressor response could have both volume and vasoconstrictor components mediated by the sodium-retaining effects of the estrogen in the presence of a relatively, if not an absolutely, higher plasma renin activity, with the latter being held abnormal by the high plasma renin-substrate levels. A failure of the kidneys to fully suppress renal renin secretion could thus be an important predisposing factor. Our in vitro experiments support the idea that the increased substrate is involved in pathogenesis because they indicate that in normal subjects plasma renin substrate is not present in "excess". Thus, a doubling of the physiologic levels of substrate by oral contraceptives leads to an almost twofold increase in the capacity for angiotensin production as indicated by studies of the initial reaction velocity. These observations define certain guidelines for applying oral contraceptive therapy. At least one base-line blood pressure measurement should be obtained, and blood pressure and weight should be followed a two- or three-month intervals during treatment. Furthermore, oral contraceptive therapy may be contraindicated in women with a history of hypertension, renal disease, toxemia, or fluid retention. Others in whom oral contraceptives are relatively contraindicated include those with a positive family history of hypertension, younger women in whom a longer term commitment is likely, and groups, such as blacks, especially prone to hypertensive phenomena.

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.

A study of angiotensin II pressor response throughout primigravid pregnancy

The present study was designed to ascertain sequentially the pressor response to angiotensin II in young primigravid patients throughout pregnancy in order a) to define when in pregnancy resistance to the pressor effects of angiotensin II develops; b) to define the physiologic sequence of events leading to this resistance; and c) to ascertain whether sensitivity to infused angiotensin II could be detected before the onset of clinical signs of pregnancy-induced hypertension. With this prospective approach, two separate groups of patients were defined. The first group of patients remained normal throughout pregnancy. The second group consisted of those patients who, while clinically normotensive during the initial phase of the study, ultimately developed hypertension of pregnancy.192 patients were studied; of these, 120 patients remained normotensive and 72 developed pregnancy-induced hypertension. In both groups, vascular resistance to infused angiotensin II (more than 8 ng/kg/min required to elicit a pressor response of 20 mm Hg in diastolic pressure) was demonstrated as early as the 10th wk of pregnancy. In the group that remained normotensive, maximum mean vascular resistance occurred at 18-30 wk of pregnancy, (mean pressor dose required being 13.5 to 14.9 ng/kg/min). In those subjects who developed pregnancy-induced hypertension, the mean maximum dose required was 12.9 ng/kg/min, which was observed at the 18th wk of pregnancy. By the 22nd wk there was a clear separation of the two groups, with the mean dose requirement of the subjects destined to develop hypertension being progressively less than that of those who remained normal. The difference between the two groups became significant (P < 0.01) by 23-26 wk of pregnancy. Among patients requiring more than 8 ng/kg/min on one or more tests done between wk 28-32, 91% remained normotensive. Conversely, during the same time period among patients requiring less than 8 ng/kg/min, on at least one occasion, 90% developed pregnancy-induced hypertension.

Renin, angiotensin and aldosterone in human pregnancy and the menstrual cycle

Aldosterone secretion is frequently, although not invariably, increased above the normal non-pregnant range in normal pregnancy. Substantial increases in plasma aldosterone concentration have also been demonstrated as early as the sixteenth week. In pregnancy, aldosterone secretion rate responds in the usual way to changes in sodium intake.

Plasma renin concentration is frequently, but not invariably, raised above the normal non-pregnant range. Plasma renin-substrate is consistently raised in pregnancy. Plasma angiotensin II has also been shown usually to be raised in a series of pregnant women.

A significant positive correlation has been shown between the maternal plasma aldosterone concentration and the product of the concurrent plasma renin and renin-substrate concentrations. This suggests that the increased plasma aldosterone in pregnancy is the consequence of an increase in circulating angiotensin II, which in turn is related to the level of both renin and its substrate in maternal blood. For these reasons, estimations of renin activity in pregnancy are of dubious value.

The increased renin, angiotensin and aldosterone concentrations may represent a tendency to maternal sodium depletion, probably mainly a consequence of the increased glomerular filtration rate. It is possible that the nausea and other symptoms of early pregnancy may be a consequence of this tendency to sodium depletion, with its attendant hormonal changes. In ‘pre-eclampsia’, renin and aldosterone values are generally slightly lower than in normal pregnancy. Human chorion can apparently synthesize renin independently of the kidney. The physiological significance of this remains at present obscure, but it seems unlikely that this source contributes much, if at all, to the often elevated maternal plasma renin.

Plasma renin, renin-activity and angiotensin II concentrations, and aldosterone secretion are increased in the luteal phase of the menstrual cycle.

Fibrinolysis in pre-eclamptic toxaemia of pregnancy

Plasma fibrinolysis is much reduced in normal pregnancy but is the same in women with toxaemia or hypertension, though urinary fibrinolysis is lower in them. The evidence suggests that in toxaemia there must be in addition a slow state of intravascular coagulation. Fibrinolytic inhibitors were found to be normal in this series, though in pregnancy there are raised levels of antitrypsin, alpha(2)-macroglobulin, and beta-lipoprotein. beta-Lipoprotein levels in non-pregnant patients show a correlation with euglobulin lysis times and with inhibitor units.