The osmoregulatory system and the renin-angiotensin-aldosterone system in pre-eclampsia and normotensive pregnancy

G-Protein-coupled receptors as potential drug candidates in preeclampsia: targeting the relaxin/insulin-like family peptide receptor 1 for treatment and prevention

BACKGROUND

Important roles for G-protein-coupled receptors (GPCRs) have been identified in the maternal physiological adaptations to pregnancy and in the pathogenesis of preeclampsia. On this basis, GPCRs are potential therapeutic targets for preeclampsia.

OBJECTIVES AND RATIONALE

In this review, vasopressin and apelin are initially considered in this context before the focus on the hormone relaxin and its cognate receptor, the relaxin/insulin-like family peptide receptor 1 (RXFP1). Based on both compelling scientific rationale and a promising safety profile, the relaxin ligand-receptor system is comprehensively evaluated as a potential therapeutic endpoint in preeclampsia.

SEARCH METHODS

The published literature relating to the topic was searched through January 2016 using PubMed.

OUTCOMES

Relaxin is a peptide hormone secreted by the corpus luteum; it circulates in the luteal phase and during pregnancy. Activation of RXFP1 is vasodilatory; thus, relaxin supplementation is expected to at least partly restore the fundamental vasodilatory changes of normal pregnancy, thereby alleviating maternal organ hypoperfusion, which is a major pathogenic manifestation of severe preeclampsia. Specifically, by exploiting its pleiotropic hemodynamic attributes in preeclampsia, relaxin administration is predicted to (i) reverse robust arterial myogenic constriction; (ii) blunt systemic and renal vasoconstriction in response to activation of the angiotensin II receptor, type 1; (iii) mollify the action of endogenous vasoconstrictors on uterine spiral arteries with failed remodeling and retained smooth muscle; (iv) increase arterial compliance; (v) enhance insulin-mediated glucose disposal by promoting skeletal muscle vasodilation and (vi) mobilize and activate bone marrow-derived angiogenic progenitor cells, thereby repairing injured endothelium and improving maternal vascularity in organs such as breast, uterus, pancreas, skin and fat. By exploiting its pleiotropic molecular attributes in preeclampsia, relaxin supplementation is expected to (i) enhance endothelial nitric oxide synthesis and bioactivity, as well as directly reduce vascular smooth muscle cytosolic calcium, thus promoting vasodilation; (ii) improve the local angiogenic balance by augmenting arterial vascular endothelial and placental growth factor (VEGF and PLGF) activities; (iii) ameliorate vascular inflammation; (iv) enhance placental peroxisome proliferator-activated receptor gamma, coactivator 1 alpha (PCG1α) expression, and hence, peroxisome proliferator-activated receptor gamma (PPAR-γ) activity and (v) confer cytotrophoblast and endothelial cytoprotection. Insofar as impaired endometrial maturation (decidualization) predisposes to the development of preeclampsia, relaxin administration in the late secretory phase and during early pregnancy would be anticipated to improve decidualization, and hence trophoblast invasion and spiral artery remodeling, thereby reducing the risk of preeclampsia. Relaxin has a favorable safety profile both in the non-pregnant condition and during pregnancy.

WIDER IMPLICATIONS

There is a strong scientific rationale for RXFP1 activation in severe preeclampsia by administration of relaxin, relaxin analogs or small molecule mimetics, in order to mollify the disease pathogenesis for safe prolongation of pregnancy, thus allowing time for more complete fetal maturation, which is a primary therapeutic endpoint in treating the disease. In light of recent data implicating deficient or defective decidualization as a potential etiological factor in preeclampsia and the capacity of relaxin to promote endometrial maturation, the prophylactic application of relaxin to reduce the risk of preeclampsia is a plausible therapeutic approach to consider. Finally, given its pleiotropic and beneficial attributes particularly in the cardiovascular system, relaxin, although traditionally considered as a 'pregnancy' hormone, is likely to prove salutary for several disease indications in the non-pregnant population.

Effects of human pregnancy on fluid regulation responses to short-term exercise

This study tested the hypothesis that human pregnancy alters fluid and electrolyte regulation responses to acute short-term exercise. Responses of 22 healthy pregnant women (PG; gestational age, 37.0 +/- 0.2 wk) and 17 nonpregnant controls (CG) were compared at rest and during cycling at 70 and 110% of the ventilatory threshold (VT). At rest, ANG II concentration was significantly (P < 0.05) higher in PG vs. CG, whereas plasma osmolality and concentrations of AVP, sodium, and potassium were significantly lower. Atrial natriuretic peptide concentration at rest was similar between groups. ANG II and AVP concentrations increased significantly from rest to 110% VT in CG only, whereas increases in atrial natriuretic peptide concentration were similar between groups. Increases in osmolality, and total protein and albumin concentrations from rest to both work rates were similar between the two groups. PG and CG exhibited similar shifts in fluid during acute short-term exercise, but the increases in ANG II and AVP were absent and attenuated, respectively, during pregnancy. This was attributed to the significantly augmented fluid volume state already present at rest in late gestation.

Plasma osmolality and the strong ion difference predict respiratory adaptations in pregnant and nonpregnant women

This study tested the hypothesis that plasma osmolality and the strong ion difference ([SID]) predict PaCO2 during rest and during exercise in physically active pregnant (n = 22; gestational age 37.0 ± 0.2 weeks) and nonpregnant (n = 17) women. Nonpregnant subjects were in varying stages of the menstrual cycle. Arterialized blood gases, hydrogen ion concentration, plasma osmolality, [SID], and circulating levels of progesterone were measured at rest and during upright cycling at work rates corresponding to 70 and 110% of the ventilatory threshold. Pooled data from the two groups at rest revealed significant correlations (P < 0.05) between PaCO2 with plasma osmolality, [SID], and progesterone. Progesterone was also significantly correlated with [SID] and osmolality. Also, changes in PaCO2 with exercise correlated significantly with changes in [SID]. The results support the hypothesis that plasma osmolality and [SID] are important factors in the modulation of respiratory sensitivity in healthy women. Also, the effects of progesterone on PaCO2 may be expressed, at least in part, through progesterone-induced changes in [SID] and osmolality.Key words: pregnancy, strong ion difference, osmolality, respiratory control.

Maternal-fetal effects of magnesium sulfate on serum osmolality in pre-eclampsia

OBJECTIVE

To determine the effects of magnesium sulfate therapy on maternal and fetal osmolality in pre-eclampsia.

METHODS

A total of 34 pre-eclamptic women and 22 normal pregnant women participated in the study. Venous blood was drawn upon admission to the labor and delivery unit. Pre-eclamptic patients received standard magnesium sulfate therapy and had a second sample of venous blood drawn 4 h following the beginning of therapy. Fetal umbilical vein blood was obtained immediately following delivery in both groups. Osmolality was measured using a vapor pressure osmometer. Electrolyte levels were measured with a NOVA 8 biomedical analyzer. Data were analyzed via Student's t test, linear regression and correlation with significance established at p < 0.05.

RESULTS

We found no significant difference between the maternal osmolalities of the control and pre-eclamptic groups. Interestingly, fetal cord blood osmolality was significantly lower than maternal osmolality in the normal pregnant women. Sodium levels were also lower, while potassium and ionized calcium levels were higher in the fetal blood. In women with pre-eclampsia treated with magnesium sulfate, there was no difference between the osmolality of the maternal and fetal blood, while potassium and ionized calcium levels were still higher in the fetal blood. Finally, we found no correlation between maternal osmolality and blood pressure.

CONCLUSIONS

High blood pressure in pre-eclampsia is not associated with altered osmolality. An absence of the normal decrease in fetal osmolality is observed in pre-eclamptic women treated with magnesium sulfate.

Angiotensin II and its different receptor subtypes in placenta and fetal membranes

The recent discovery of a local renin-angiotensin system in trophoblastic tissues has raised many questions regarding its role in the physiology of normal gestation and its implications in the pathophysiology of hypertension during pregnancy. In this article, the authors first review the most interesting aspects of the chorioplacental renin-angiotensin system, dwelling on the tissue distribution of angiotensin II and its receptor subtypes in the placenta and fetal membranes of different species. The relationship between angiotensin II and other locally synthesized chorioplacental substances is also analysed and the therapeutic implications of phenomena observed in pregnancy-associated hypertension are discussed.

Radioimmunoassay of vasopressin during pregnancy. Use and removal of cystylaminopeptidase inhibitors

This paper describes a radioimmunoassay (RIA) for arginine-vasopressin in which o-phenanthroline effectively inhibits cystyl-amino-peptidase activity in whole blood and plasma from pregnant women but in which o-phenanthroline is removed during the extraction of vasopressin from plasma to prevent disturbance of the RIA. Cystyl-amino-peptidase causes immediate degradation of vasopressin unless cystyl-amino-peptidase enzyme inhibitors such as o-phenanthroline are applied. However, o-phenanthroline interferes with RIA. We report an extraction procedure over octadecasyl silica-packed Sep-Pak C18 columns, by which cystyl-amino-peptidase as well as most of the cystyl-amino-peptidase inhibitor is removed from plasma with chloroform. The average o-phenanthroline concentration (0.25 mmol/l) found in the assay medium after extraction appeared not to interfere with the RIA. Polyacrylamide gel isoelectric focusing of extracts of platelet-rich and platelet-poor plasma from pregnant women revealed a single vasopressin immunoreactive peak in the RIA. Recovery and between-assay coefficients of variation of 3.2 ng/l vasopressin from pregnancy whole blood were comparable with non-pregnant controls (57%/8% and 59%/13%, respectively). Results with this assay compare well with those of another assay using inhibitors in pregnant subjects and with results in non-pregnant subjects.

The blood pressure response to infusions of angiotensin II during normal pregnancy: relation to plasma angiotensin II concentration, serum progesterone level, and mean platelet volume

OBJECTIVE

This study is a survey of the determinants of refractoriness to the pressor effects of angiotensin II during normal pregnancy.

STUDY DESIGN

In 25 normal pregnant women, the effective angiotensin II pressor dose was determined 88 times from the twenty-fifth to the thirty-second week of gestation. Immediately before the angiotensin II infusion, blood samples were collected and measured for plasma angiotensin II concentration, serum progesterone level, platelet count, and mean platelet volume.

RESULTS

The effective pressor dose had a significantly positive correlation with plasma angiotensin II concentration and serum progesterone level and a negative correlation with mean platelet volume.

CONCLUSION

The pregnancy-associated refractoriness to angiotensin II is physiologically determined, at least in part, by the elevated circulating levels of endogenously produced angiotensin II and by the progesterone produced by the placenta, whereas platelet activation attenuates the relative refractoriness during normal pregnancy.

Atrial natriuretic peptide concentrations in umbilical cord plasma from pre-eclamptic women

Atrial natriuretic peptide (ANP) was measured in arterial and venous umbilical cord plasma at the time of delivery by cesarean section in pre-eclamptic (n = 7) and normal women (n = 6). In addition venous samples were obtained from pre-eclamptic (n = 7) and normal pregnant women (n = 7) near term. ANP plasma levels were higher in pregnant women with pre-eclampsia than in normal pregnant women (27.9 +/- 4.4 [mean +/- SEM] and 14.1 +/- 2.5 pmol l-1, respectively, P less than 0.05). Immediately after delivery plasma ANP in pre-eclamptic mothers was 66.7 +/- 12.8 pmol l-1 compared to 13.9 +/- 2.2 pmol l-1 in normal mothers (P less than 0.01). However, in the pre-eclamptic group the levels of ANP in arterial and venous umbilical cord plasma (19.5 +/- 4.2 and 16.7 +/- 4.3 pmol l-1, respectively) were significantly (P less than 0.01) lower than ANP levels in arterial and venous cord plasma (39.6 +/- 1.0 and 31.1 +/- 4.2 pmol l-1, respectively) from normal mothers. It is concluded that the increased ANP plasma level in pre-eclamptic women originates from a maternal source. In addition, since the ANP level is lower in cord plasma than in maternal plasma in pre-eclampsia, feto-placental volume homeostasis may also be changed in pre-eclampsia.

Angiotensin II levels in hypertensive and normotensive pregnancies

We measured circulating angiotension II by radioimmunoassay in women with pregnancy-induced hypertension (n = 54), and compared these values with those obtained in women with normal pregnancy (n = 18) and in non pregnant women (n = 20). Pregnant women had statistically significantly higher plasma angiotensin II [mean (SD): 41.3 (12.6) pg/ml] than non-pregnant women [29.2 (11.3) pg/ml; P less than 0.004]. Angiotensin II concentrations in women with pregnancy-induced hypertension [mean (SD): 31.7 (16.2) pg/ml] were, on average, 25% lower than in normal pregnancy (P less than 0.003) and resembled those obtained in non-pregnant women. The lowest angiotensin II levels were found in women with more severe forms of pregnancy-induced hypertension, such as proteinuric or superimposed pregnancy-induced hypertension. Review of the published studies on angiotensin II and our data suggest that the conflict among studies on angiotensin II levels in pregnancy-induced hypertension is largely due to the heterogeneity of the study populations in the various reports.

Volume homeostasis and osmoregulation in human pregnancy

This chapter reviews alterations in volume and sodium homeostasis and osmoregulation during human pregnancy. Pregnant women undergo extracellular and plasma volume increases of 50-70%, and these changes accompany marked cumulative sodium retention shared by both mother and fetus. Pregnancy alters several factors with opposing effects on renal salt handling; however, mechanisms by which gestational sodium accumulation and volume expansion are achieved remain obscure. Furthermore, despite substantial increases in absolute blood volume, considerable uncertainty exists as to how this volume is sensed, particularly in late pregnancy when a rapid increase in volume is associated with decreases in peripheral resistance and blood pressure. Attempts to assess 'effective' intravascular volume suggest that pregnant women sense their volume as normal. Osmoregulation is also changed. Body tonicity and the osmotic thresholds for AVP release and thirst decrease by about 10 mosm/kg. The mechanisms responsible for the osmoregulatory changes are obscure. Haemodynamic stimuli such as decrements in blood pressure and of 'effective circulating volume' do not seem to account for them. Of the many increments in hormone levels known to accompany gestation, only hCG has so far been implicated in these changes. Pregnant women experience three- to fourfold increments in AVP disposal rates between early and mid pregnancy; this may be caused by the striking rise in circulating cystine-aminopeptidase (vasopressinase) which also occurs during this period. The increments in MCR may be one reason why the hormonal response to a given osmotic stimulus appears to decrease in late pregnancy. All these alterations permit speculation on the manner in which the decrease in Posm occurs and is maintained within narrow limits. Lowering the osmotic threshold to drink stimulates a rise in water intake and dilution of body fluids. Since AVP release is not suppressed at the usual level of hypotonicity, AVP continues to circulate at levels sufficient to permit water retention. Posm continues to decline until it decreases below the new osmotic thirst threshold, when a new steady state is established. At this point water turnover, too, resembles that in the non-pregnant state. The change in MCR and the marked increment in plasma vasopressinase may explain certain observations regarding disordered water metabolism during late pregnancy. These are the transient DI syndromes due either to subclinical hypothalamic disease or to a disorder peculiar to pregnancy which is AVP-resistant but dDAVP-responsive; the latter analogue resists degradation by vasopressinase.

Muscle cell potassium, RNA and hydration in pregnancy and pre-eclampsia

Thirty four pregnant women from 26 to 38 weeks gestation and 24 pregnant women with pre-eclampsia gave samples of muscle (rectus abdominis) at caesarean section. Muscle samples were analysed for H2O, K+, Mg2+ and Na+. Cell extracellular H2O was partitioned by the use of the Cl- space. Also protein, nucleic acids and Zn2+ were determined. From 26 to 38 weeks gestation the concentration of K+ per litre of cell water ([Ki]) slowly declined. The slope was significant. Points for patients with pre-eclampsia fell below the line and analysis of covariance showed that the two populations were different (P less than 0.001). Patients A-J were regarded clinically as severe pre-eclamptics. Points for these patients, in general, fell between 1 and 2 SDs below the normal line. Since other cations per litre of muscle cell water did not change, questions are raised. is the cation gap filled by amino acids or does vascular spasm cause a leakage of K+ from muscle cells? Does hypotonicity eventually develop leading to water intoxication? The low oncotic pressure in pre-eclampsia (shown here), the negative free water clearance could all favour increased cell hydration (some evidence for this is presented here towards term). Assessment of available information concerning creatinine excretion during normal pregnancy and K40, K42 studies together with our own rodent studies leads us to believe that a significant increase in muscle mass occurs, but such may not be the case in pre-eclampsia since the reduction in RNA and Zn2+ concentrations in muscle would suggest excessive protein degradation.(ABSTRACT TRUNCATED AT 250 WORDS)