Atrial natriuretic peptide in the fetus

Influence of natriuretic peptide receptor-1 on survival and cardiac hypertrophy during development

The heart adapts to an increased workload through the activation of a hypertrophic response within the cardiac ventricles. This response is characterized by both an increase in the size of the individual cardiomyocytes and an induction of a panel of genes normally expressed in the embryonic and neonatal ventricle, such as atrial natriuretic peptide (ANP). ANP and brain natriuretic peptide (BNP) exert their biological actions through activation of the natriuretic peptide receptor-1 (Npr1). The current study examined mice lacking Npr1 (Npr1(-/-)) activity and investigated the effects of the absence of Npr1 signaling during cardiac development on embryo viability, cardiac structure and gene and protein expression. Npr1(-/-)embryos were collected at embryonic day (ED) 12.5, 15.5 and neonatal day 1 (ND 1). Npr1(-/-)embryos occurred at the expected Mendelian frequency at ED 12.5, but knockout numbers were significantly decreased at ED 15.5 and ND 1. There was no indication of cardiac structural abnormalities in surviving embryos. However, Npr1(-/-)embryos exhibited cardiac enlargement (without fibrosis) from ED 15.5 as well as significantly increased ANP mRNA and protein expression compared to wild-type (WT) mice, but no concomitant increase in expression of the hypertrophy-related transcription factors, Mef2A, Mef2C, GATA-4, GATA-6 or serum response factor (SRF). However, there was a significant decrease in Connexin-43 (Cx43) gene and protein expression at mid-gestation in Npr1(-/-)embryos. Our findings suggest that the mechanism by which natriuretic peptide signaling influences cardiac development in Npr1(-/-) mice is distinct from that seen during the development of pathological cardiac hypertrophy and fibrosis. The decreased viability of Npr1(-/-)embryos may result from a combination of cardiomegaly and dysregulated Cx43 protein affecting cardiac contractility.

Relation of ANP and BNP to their N-terminal fragments in fetal circulation: evidence for enhanced neutral endopeptidase activity and resistance of BNP to neutral endopeptidase in the fetus

OBJECTIVES

To investigate the role of neutral endopeptidase in the turnover of atrial (ANP) and brain (BNP) natriuretic peptides and their N-terminal fragments in human fetal circulation.

DESIGN

Retrospective case-control study.

SETTING

Department of Obstetrics and Gynaecology, University of Leipzig, Germany.

SAMPLE

Nine control pregnancies and nine pregnancies with rhesus isoimmunisation before and after intravascular transfusion.

METHODS

Natriuretic peptides and N-terminal fragments in maternal and fetal blood were measured by radio-immunoassay. Neutral endopeptidase activity was determined by HPLC.

MAIN OUTCOME MEASURES

Maternal and fetal plasma concentrations of ANP, NT-proANP, BNP, NT-proBNP as well as neutral endopeptidase activity. Ratios between mature peptide and N-terminal fragment. Feto-maternal ratio.

RESULTS

Plasma NT-proANP concentrations are 11.7 times higher in fetal than in maternal circulation. The ANP concentration is only 1.8 times higher, probably due to doubled neutral endopeptidase activity. In contrast, both NT-proBNP and BNP are doubled in fetal plasma. Fetuses with Rh isoimmunisation had significantly higher NT-proBNP but not NT-proANP and neutral endopeptidase activity than controls. An additional volume load by intravascular transfusion did not influence N-terminal fragments or neutral endopeptidase activity.

CONCLUSIONS

Our study is the first to determine NT-pro natriuretic peptide concentrations and neutral endopeptidase activity in human fetuses. The results show that increased fetal neutral endopeptidase activity shifts the ANP/NT-proANP but not the BNP/NT-proBNP ratio and that the shifted BNP/NT-proBNP ratio in fetuses with Rh isoimmunisation does not involve increased neutral endopeptidase activity. These findings point to a BNP degradation that is not dependent on neutral endopeptidase.

Detection of C-type natriuretic peptide in fetal circulation

C-type natriuretic peptide (CNP) belongs to the natriuretic peptide family that plays an important role in the control of blood pressure, renal function and volume homeostasis. In contrast to the atrial natriuretic peptide and brain natriuretic peptide, CNP acts in an autocrine/paracrine fashion and is considered to be the endothelial component of the natriuretic peptide system. CNP has a high expression and tissue-specific regulation in reproductive organs. Using a radio-immunoassy for CNP-22 we measured for the first time CNP in fetal blood. Samples were taken by cordocentesis in a group of fetuses with rhesus isoimmunisation (10.74 +/- 2.81 pg/ml), fetuses with rhesus isoimmunisation after intravascular transfusion (10.03 +/- 4.01 pg/ml) and a group with structural anomalies (12.9 +/- 5.67 pg/ml). A group of healthy fetuses was used as controls (11.64 +/- 4.32 pg/ml). In contrast to ANP, the fetal CNP-plasma concentrations remain stable in the investigated fetal diseases and after volume load during intravascular transfusion. Moreover, fetal CNP-plasma levels are higher than previously measured maternal concentrations in normal pregnancies. Therefore, the fetus expresses CNP independently of the maternal circulation.

Alpha-atrial natriuretic peptide-induced attenuation of vasoconstriction in the fetal circulation of the human isolated perfused placenta

In order to examine the effect of alpha-ANP on fetal placental vascular tone, single placental lobules were bilaterally perfused and fetal inflow pressure recorded. The placental vasculature was sub-maximally pre-constricted by infusion of the nitric oxide synthase inhibitor N omega-nitro-L-arginine (NOLA) or the thromboxane A2-mimetic U46619. In the presence of continuous infusion of 59.3 mumol/l NOLA, producing a mean pressure increase of 43.7 +/- 1.7 mmHg (n = 8, mean +/- SEM), alpha-ANP (10.7 to 325 nmol/l) produced significant pressure decreases (P < 0.05). In separate experiments (U46619 was either infused at concentrations (4.8 to 21.4 nmol/l) to produce a mean pressure increase (50.1 +/- 2.6 mmHg, n = 10) similar to that produced by NOLA infusion or was infused at a concentration (28.5 nmol/l) that produced a significantly higher pressure increase (104 +/- 15 mmHg), infusion of 1 mumol/l alpha-ANP significantly reduced perfusion pressure. However, 100 nmol/l alpha-ANP or less had no significant effect (n = 4-7). These results indicate that alpha-ANP attenuates NOLA-induced and U46619-induced vasoconstriction in the human placenta, but at concentrations higher than those in fetal or maternal plasma.

Atrial natriuretic peptide: a vasodilator of the fetoplacental circulation?

Paired maternal and fetal atrial natriuretic peptide concentrations were measured in 62 percutaneous umbilical blood samplings performed principally for the assessment and treatment of rhesus isoimmunization. Pretransfusion fetal atrial natriuretic peptide levels were significantly higher than maternal atrial natriuretic peptide levels (median 117 pg/ml vs median 32 pg/ml; p less than 0.001); paired pretransfusion fetal and maternal atrial natriuretic peptide samples showed a weak correlation with each other (R2 = 17%; p = 0.002). Fetal atrial natriuretic peptide levels correlated inversely with hematocrit (R2 = 14%; p = 0.003), but not with albumin or gestational age. Paired pretransfusion and posttransfusion (median = 134 pg/ml) fetal atrial natriuretic peptide levels (n = 38) showed a significant rise after transfusion (p less than 0.001); this rise was related to the percentage of fetoplacental blood volume transfused (R2 = 33%; p = 0.035). In a subgroup of 26 procedures, change in fetal atrial natriuretic peptide levels was weakly correlated with transient reductions in the Doppler systolic/diastolic ratio of the umbilical artery (R2 = 14%; p = 0.07). These data support work in animals that indicate a role for atrial natriuretic peptide in the human fetus, but these data do not confirm that atrial natriuretic peptide modulates fetoplacental vascular impedance in the human fetus.

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.

[Atrial natriuretic factor in men]

The atrial natriuretic peptide (ANP) is rapidly secreted in case of acute changes in atrial volume and heart rate. Its effects are mainly natriuretic and vasodilator. This hormone is of interest to the anaesthetist because induction of anaesthesia, epidural anaesthesia and administration of morphine all result in changes in ANP plasma concentration.

[Atrial natriuretic factor]

Although ANF research started 30 years ago, the atrial natriuretic factor (ANF) was only discovered recently (1981). The presence of such a factor has been suspected for many years because of histological and physiological arguments. In 1956, Kish found "dense granules" in the atrial walls of guinea pigs. Gauer and Henry could explain some of their experimental results on diuresis and natriuresis only by suggesting the presence of a third hormonal factor, but neither by the renin-angiotensin system, nor the anti-diuretic hormone. Hall et al. were the first to recognize a link between the granules and water and sodium metabolism. But it was De Bold who published the crucial experiment in 1981: injecting right atrial extracts to anaesthetized rats rapidly induced intense and transitory diuresis and natriuresis. ANF was born, and, at the same time, the concept of the heart as an endocrine gland. Indeed, ANF corresponds to the strict definition of a hormone. It has the following properties: natriuresis and diuresis via an increase in glomerular filtration fraction without any major changes in renal plasma flow; direct vasodilation of the large arteries with only few effects on small arterioles and veins. The stimuli for ANF secretion are mechanical and pharmacological, especially drugs currently used by anaesthetists. Atrial distension is the main mechanical stimulus. An increase in atrial transmural pressure is always followed by a release in ANF, but this effect is not constant for increases in intra-luminal pressure. It is the former pressure gradient alone that reflects the volume of the right atrium, the mechanical stimulus for ANF secretion. Tachycardia, or, more precisely, an increase in the atrial contraction rate, also leads to an important release of ANF. Cardiac nerves are not necessary for this, as demonstrated by studies in heart transplant patients. Only few pharmacological agents have been shown to really stimulate ANF secretion. In rats, morphine has a direct secretory effect, whereas ketamine hydrochloride, diethylether and chloral hydrate do so by increasing the release of catecholamines. The effects of alpha, beta adrenergic agonists and calcium agonists remain controversial. ANF, which has diuretic and vasodilator effects, plays a part, together with the renin-angiotensin system and the anti-diuretic hormone, in blood volume control in mammals. However, it has a special role to play, because it is a rapid release hormone: rapid vascular filling leads to an increase in ANF in less than 1 minute, with a parallel increase in diuresis.

Atrial natriuretic peptide: water and electrolyte homeostasis

In the few years since its identification, a clear role for ANP in the regulation of water and electrolyte balance has emerged (Figure 3). The peptide is released in response to blood volume expansion, both acutely and gradually during changes in dietary sodium intake. Similarly, plasma levels are elevated in pathophysiological conditions such as cardiac and renal failure. It has become apparent that ANP has natriuretic, diuretic and vasorelaxant properties. Many of the original studies employed what we now know to be pharmacological doses of the peptide. However, recent reports have confirmed that small, sustained elevations in plasma ANP within or marginally above the 'normal' physiological range produce similar effects. A number of recent studies have tried to specifically address the physiological relevance of ANP. Although undoubtedly release by atrial distension and effective when infused to similar concentrations, atrial distension also has other effects via neural pathways. Thus, the demonstration that excretion of saline is impaired by atrial appendectomy (Benjamin et al, 1988) does not imply that this is only due to the absence of an atrial hormone. Goetz et al (1986) demonstrated that in the denervated heart, although ANP is still released, the excretion of a saline load is impaired. Similarly, in man, Richards et al (1988a) needed to infuse ANP to much higher plasma levels than those achieved by a saline load in order to reproduce the natriuresis. Although these experiments can be criticized, they confirm that ANP is not the sole mechanism for excreting a volume load, or for the natriuresis following atrial distension, but that these effects are likely to reflect the balance between ANP, AVP, the renin-angiotensin and autonomic nervous systems. In rats immunized against ANP (Greenwald et al, 1988), although the ability to excrete an acute saline load was impaired, long-term sodium balance was normal, suggesting that the rats were able to compensate for the absence of ANP. Many of the actions of ANP can be explained by antagonism of the renin-angiotensin-aldosterone system. Teleologically, it seems appropriate that a natriuretic hormone should counterbalance the major pressor and antinatriuretic hormones within the body. There is good evidence for cellular interactions between angiotensin, AVP, aldosterone and ANP at a number of discrete sites which are additional to the straightforward physiological antagonism of systems with opposing actions. ANP inhibits aldosterone secretion directly and may also reduce renal renin release. In the vascular tree there is evidence that ANP specifically blocks the vasoconstrictor actions of angiotensin II and possibly AVP.(ABSTRACT TRUNCATED AT 400 WORDS)

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.