Natriuretic peptide system in fetal heart and circulation

Metabolism of atrial and brain natriuretic peptides in the fetoplacental circulation of fetuses with congenital heart diseases

INTRODUCTION

Natriuretic peptides (NPs) play a pivotal role in maintaining fetal circulation; however, little is known about their metabolism. The aim of the present study was to elucidate the metabolism of plasma NPs in the fetoplacental circulation.

METHODS

Plasma NP concentrations in maternal vein and umbilical artery (UA) and vein (UV) samples from fetuses with congenital heart defect (n = 86) or arrhythmia (n = 31) and controls (n = 127) were analyzed.

RESULTS

Levels of plasma atrial NP (ANP) and brain NP (BNP) showed good correlation between UV versus UA samples (p < 0.01). In all three fetus groups, the regression coefficients between UV and UA plasma ANP levels were close to 0.5, while those between UV and UA plasma BNP levels were close to 1. The molecular forms of immunoreactive ANP in UA plasma showed a single peak corresponding to mature ANP, while those of immunoreactive BNP in UA plasma showed two major peaks and several minor peaks corresponding to mature BNP-32 and its partially digested peptides, as well as glycosylated and non-glycosylated BNP precursors (proBNP). No correlation was found between fetuses and mothers in terms of either plasma ANP or BNP levels.

CONCLUSIONS

The mother and fetus independently secrete and metabolize both ANP and BNP. Fetal plasma ANP consists exclusively of the mature form, and the placenta and umbilical vessels are possible major sites of ANP metabolism. In contrast, fetal plasma BNP consists predominantly of the precursor forms, which may contribute to protecting BNP from metabolism in the fetoplacental circulation.

Chimeric natriuretic peptide ACNP stimulates both natriuretic peptide receptors, the NPRA and NPRB

Here, we investigated the receptor profile of the newly designed natriuretic peptide (NP) ACNP consisting of the N- and C-terminus of human ANP and the ring structure of CNP, its potency/efficacy in stimulating cGMP generation in primary cells, and its stability towards peptidase activity. ACNP stimulated both human natriuretic peptide receptors (NPRs), NPRA and NPRB, as potent as their native ligands in receptor transfected cells. Consequently, ACNP was more efficient in generating cGMP compared to ANP, BNP, and CNP, in primary cells expressing both NPRs. All NPs have been similarly degraded by neprilysin, except the neprilysin-resistant BNP. However, ACNP was fastest degraded in serum, while CNP was most stable. Congruently, CNP but not ACNP reduced blood pressure most significantly after acute peptide infusion in normotensive mice. Our data identify ACNP as the first compound being able to stimulate both natriuretic receptors with similar potency and efficacy as their respective ligands.

Changes in atrial natriuretic peptide levels during cardiac bypass in the fetal goat

The aim of this study is to evaluate the effect of fetal cardiac bypass on the production and secretion of fetal atrial natriuretic peptide (ANP) in the goat. Eighteen pregnant goats, at days 120 to 140 of gestation, were randomly divided into control (n = 8) and bypass (n = 10) groups. The control group underwent a sham procedure involving fetal sternotomy and cannulation. The bypass group underwent fetal cardiac bypass using a centrifugal pump for 30 min. Fetuses in the bypass group exhibited hypoxia, hypercapnia, and acidosis during and after cardiac bypass. The pulse index (PI) of the umbilical artery in the bypass group increased significantly after cardiac bypass compared with the control group. Tei indices of the left and right ventricles in the bypass group increased remarkably after cardiac bypass. Plasma troponin I levels in the bypass group increased significantly compared with that of the control group. Plasma ANP levels increased markedly in the bypass group after cardiac bypass, and the difference between two groups was significant. Transcriptional levels of ANP mRNA in the fetal heart elevated remarkably in the bypass group compared with the control group at 2 h after the bypass. A significant positive correlation between plasma ANP levels and Tei indices of the ventricles, plasma troponin I was observed (left ventricular Tei index, r = 0.606, P < 0.01; right ventricular Tei index, r = 0.581, P < 0.01; plasma troponin I, r = 0.275, P < 0.05). In conclusion, fetal cardiac bypass promoted the production and secretion of ANP and was associated with fetal cardiac dysfunction.

Relationship between Cardiofemoral Index and the plasma concentration of brain natriuretic peptide in anemic fetuses associated with Rh alloimmunization

BACKGROUND

Fetal anemia is a common result of alloimmunization and is an important cause of fetal congestive heart failure resulting in heart dilation. Fetuses suspected of having heart failure present a higher Cardiofemoral Index and an increase in plasma BNP levels, because the cardiac hormonal system is activated by increased heart wall stretch due to increased left ventricular volume and pressure overload.

METHODS

Our group studied 33 pregnant women (22-31 weeks' gestation at the first cordocentesis) referred for intrauterine fetal transfusion due to severe red blood cells isoimmunization. Up to 6 h prior to each cordocentesis, all fetuses were submitted to ultrasonography measurements where the Cardiofemoral Index was calculated. Samples of blood from the umbilical vein were collected for hemoglobin concentration and blood gas measurements. Plasma levels of BNP were determined with the use of plasma that had previously been frozen and thawed once. Plasma BNP was measured by radioimmunoassay. Pearson's correlation test and regression analysis were used to determine the association between the plasma concentration of BNP and the Cardiofemoral Index of the anemic fetuses with RH alloimmunization.

RESULTS

Anemia was severe in 17 (50%) fetuses, mild in 9 (26.47%) and in 8 patients (23.53%) the fetuses were not anemic; hemoglobin ranged from 3.10 to 15.70 g/dl. The Cardiofemoral Index ranged from 0.43 to 0.87 and it was altered (>or=0.59) in 23 fetuses. A significant positive correlation was observed between BNP plasma fetal concentration and Cardiofemoral Index (Pearson r=0.61, P<0.0001).

CONCLUSIONS

These results suggest that fetal plasma concentration of BNP may increase in fetuses with heart dilation leading to a positive association between Cardiofemoral Index and plasma concentration of BNP.

Chronic hypoxemia increases ventricular brain natriuretic peptide precursors in neonatal swine

BACKGROUND

Circulating levels of atrial natriuretic peptide and brain natriuretic peptide (BNP) are elevated in patients with cyanotic congenital heart disease and associated with the severity of ventricular dysfunction. We evaluated the effect of chronic hypoxemia on left ventricle pro-atrial natriuretic peptide and pro-BNP, the cytoplasmic precursors of the plasma hormones.

METHODS

Forty newborn piglets were randomized to placement of a pulmonary artery to left atrium shunt to create hypoxemia or sham thoracotomy. Animals were studied at 1 or 2 weeks after the procedure (four groups, n = 10 per group). Arterial oxygen tension and hematocrit were obtained. Left ventricular shortening fraction was measured by echocardiography. Left ventricular tissue was harvested and cytoplasm was extracted. Pro-BNP levels were determined by Western blot analysis. Pro-atrial natriuretic peptide levels were determined using enzyme-linked immunosorbent assay.

RESULTS

Significant differences among treatment groups were observed for arterial oxygen tension (p < 0.001) and hematocrit (p < 0.001). Pairwise comparisons indicated lower arterial oxygen tension and higher hematocrit for hypoxemic piglets compared with control piglets at 1 and 2 weeks. Left ventricular shortening fraction was not decreased in the hypoxemic animals at any time (p = 0.638). Left ventricular pro-atrial natriuretic peptide decreased in hypoxemic piglets (p = 0.029), whereas left ventricular pro-BNP increased in hypoxemic piglets at 2 weeks (p = 0.002).

CONCLUSIONS

Chronic hypoxemia alone, even in the absence of cardiac dysfunction, is sufficient to increase ventricular levels of pro-BNP. This finding may have implications for the interpretation of BNP levels in the clinical management of patients with cyanotic congenital heart disease.

Fetal aortic stenosis and changes in amniotic fluid natriuretic peptides

OBJECTIVE

Natriuretic peptides, especially brain natriuretic peptide (BNP), have demonstrated great usefulness in pediatric and adult cardiology. We studied their usefulness, based on amniotic fluid concentrations, in an ovine model of fetal aortic stenosis and in response to fetal cardiac intervention.

STUDY DESIGN

After their natural history was established with gestation (n = 18 fetuses), natriuretic peptide levels were measured in a fetal model of aortic stenosis (50-60 days; term, 148 days; n = 9) and were correlated to the severity of fetal heart disease. Response to fetal cardiac intervention in 3 hydropic fetuses was also assessed. Significance was established with 2-sided paired t-tests at a probability value of <.05.

RESULTS

Amniotic fluid BNP (but not atrial natriuretic peptide) concentrations were elevated significantly with aortic stenosis (181.9 +/- 109.9 pg/mL vs 50.0 +/- 40.5 pg/mL in control fetuses), especially if complicated with hydrops (283 +/- 74.4 pg/mL), and were correlated positively with the severity of stenosis and left ventricle hypertrophy. In the 1 animal surviving fetal intervention, BNP levels normalized.

CONCLUSION

Amniotic fluid BNP concentrations correlate with the severity of fetal aortic stenosis.

Cardiac troponin T and amino-terminal pro-natriuretic peptide concentrations in fetuses in the second trimester and in healthy neonates

We measured the concentrations of cardiac troponin T (cTnT) and amino-terminal pro-natriuretic peptide (NT-proBNP) in umbilical cord blood during the second trimester (20-25 weeks of gestation) and at delivery in 109 uncomplicated pregnancies to define reference values. Using the 97.5th percentile, the upper reference limits for cTnT and NT-proBNP were 0.308 ng/mL and 5402 pg/mL in fetuses in the second trimester, and 0.038 ng/mL and 1690 pg/mL in healthy neonates, respectively. We also evaluated whether labor and delivery mode affected the concentrations of the two analytes. We found significantly higher (p<0.001) umbilical blood NT-proBNP and cTnT levels at 20-25 weeks of gestation than at term. In addition, within the term group, NT-proBNP concentrations were significantly higher in women delivering by elective cesarean section than in women delivering spontaneously (p<0.001), and higher than in women delivering by cesarean section during active labor. This indicates that the decrease in NT-proBNP levels is probably due to labor rather than delivery mode. Finally, we confirmed the at-birth reference values previously established for cTnT and NT-proBNP.

Biology of natriuretic peptides and their receptors

Increasing evidence suggests that natriuretic peptides (NPs) play diverse roles in mammals, including renal hemodynamics, neuroendocrine, and cardiovascular functions. Collectively, NPs are classified as hypotensive hormones; the main actions of NPs are implicated in eliciting natriuretic, diuretic, steroidogenic, antiproliferative, and vasorelaxant effects, important factors in the control of body fluid volume and blood pressure homeostasis. One of the principal loci involved in the regulatory actions of NPs is their cognate plasma membrane receptor molecules, which are activated by binding with specific NPs. Interaction of NPs with their receptors plays a central role in physiology and pathophysiology of hypertension and cardiovascular disorders. Gaining insight into the intricacies of NPs-specific receptor signaling pathways is of pivotal importance for understanding both hormone-receptor biology and the disease states arising from abnormal hormone receptor interplay. During the last decade there has been a surge in interest in NP receptors; consequently, a wealth of information has emerged concerning molecular structure and function, signaling mechanisms, and use of transgenics and gene-targeted mouse models. The objective of this present review is to summarize and document the previous findings and recent discoveries in the field of the natriuretic peptide hormone family and receptor systems with emphasis on the structure-function relationship, signaling mechanisms, and the physiological and pathophysiological significance in health and disease.

Prenatal cardiovascular manifestations in the twin-to-twin transfusion syndrome recipients and the impact of therapeutic amnioreduction

OBJECTIVE

We evaluated the cardiovascular pathologic condition in the recipient twin in twin-to-twin transfusion syndrome and the influence of amnioreduction.

STUDY DESIGN

Fetal echocardiograms and medical records of 54 pregnancies that were complicated by twin-to-twin transfusion syndrome were reviewed. Recipient twin right and left ventricular wall thickness, diameters, systolic and diastolic function, valve regurgitation, and structural cardiac defects were assessed at examination and after amnioreduction.

RESULTS

At examination (n = 28 pregnancies), cardiomegaly because of right ventricular and/or left ventricular hypertrophy was observed in 58% of recipient twins, and biventricular hypertrophy was observed in 33% of recipient twins, without ventricular dilation. Biventricular diastolic dysfunction was present in two thirds of recipient twins, and right ventricular systolic dysfunction and significant atrioventricular valve regurgitation was observed in one third of recipient twins. Serial assessment (n = 21 pregnancies) revealed progressive biventricular hypertrophy and right ventricular systolic and biventricular diastolic dysfunction in most recipient twins. Steeper progression of hypertrophy, diastolic dysfunction, and structural or functional right ventricular outflow disease (20% incidence) were associated with an increased perinatal mortality rate.

CONCLUSION

In twin-to-twin transfusion syndrome, the recipient twin has progressive biventricular hypertrophy with predominant right ventricular systolic and biventricular diastolic dysfunction. Despite amnioreduction, the cardiovascular disease persists and even progresses in many recipient twins.

Tbx2 is essential for patterning the atrioventricular canal and for morphogenesis of the outflow tract during heart development

Tbx2 is a member of the T-box transcription factor gene family, and is expressed in a variety of tissues and organs during embryogenesis. In the developing heart, Tbx2 is expressed in the outflow tract, inner curvature, atrioventricular canal and inflow tract, corresponding to a myocardial zone that is excluded from chamber differentiation at 9.5 days post coitus (dpc). We have used targeted mutagenesis in mice to investigate Tbx2 function. Mice heterozygous for a Tbx2 null mutation appear normal but homozygous embryos reveal a crucial role for Tbx2 during cardiac development. Morphological defects are observed in development of the atrioventricular canal and septation of the outflow tract. Molecular analysis reveals that Tbx2 is required to repress chamber differentiation in the atrioventricular canal at 9.5 dpc. Analysis of homozygous mutants also highlights a role for Tbx2 during hindlimb digit development. Despite evidence that TBX2 negatively regulates the cell cycle control genes Cdkn2a, Cdkn2b and Cdkn1a in cultured cells, there is no evidence that loss of Tbx2 function during mouse development results in increased levels of p19(ARF), p16(INK4a), p15(INK4b) or p21 expression in vivo, nor is there evidence for a genetic interaction between Tbx2 and p53.

Single Transmembrane Spanning Heterotrimeric G Protein-Coupled Receptors and Their Signaling Cascades

Heptahelical of serpentine receptors such as the adrenergic receptors are well known to mediate their actions via heterotrimeric GTP-binding proteins. Likewise, receptors that traverse the cell membrane once have been shown to mediate their biological actions by activating several different mechanisms including stimulation of their intrinsic tyrosine kinase activities or the kinase activities of other proteins. Some of these single transmembrane receptors have an intrinsic guanylyl cyclase activity and can stimulate the cyclic GMP second messenger system; however, over the last few years, several studies have shown the involvement of heterotrimeric GTP-binding proteins in mediating signals that eventually culminate in the biological actions of single transmembrane spanning receptors and proteins. These receptors include the receptor tyrosine kinases that mediate the actions of growth factors such as epidermal growth factor, insulin, insulin-like growth factor as well as receptors for atrial natiuretic hormone or the zona pellucida protein (ZP3) and integrins. In this review, the significance of the coupling of the single transmembrane spanning receptors to G proteins has been highlighted by providing several examples of the concept that signaling via these receptors may involve the activation of multiple signaling cascades.

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.

Differential effects of atrial natriuretic peptide on the brain water and sodium after experimental cortical contusion in the rat

Atrial natriuretic peptide (ANP) plays an important role in the regulation of water and sodium in the body via cyclic GMP (cGMP) pathway. Although ANP has been shown to be protective in cerebral ischemia or intracerebral hemorrhage, its role in traumatic brain injury (TBI) has yet to be elucidated. We herein assessed ANP effects on brain water and sodium in TBI. Controlled cortical impact (3 mm depth, 6 m/sec) was used to induce an experimental cortical contusion in rats. Continuous administration of ANP 0.2 (n = 6) or 0.7 microg/kg/24 h (n = 6), cGMP analogue (8-Bromo-cGMP) 0.1 (n = 5) or 0.3 mg/kg/24 h (n = 5), or vehicle (n = 6) was begun 15 minutes after injury, using a mini-osmotic pump implanted into the peritoneal cavity. At 24 hours after injury, ANP significantly exacerbated brain edema in the injured hemisphere in a dose-dependent manner while it reduced brain sodium concentrations in both hemispheres. These ANP effects could be mimicked by a cGMP analogue. In the second series (n = 20), BBB integrity was assessed by evaluating the extravasation of Evans blue dye. ANP or cGMP analogue significantly worsened BBB disruption in the injured hemisphere at 24 hours after injury. These findings suggest that ANP administration exacerbates brain edema after the experimental cortical contusion in rats, possibly because of an increase in the BBB permeability via cGMP pathway, whereas it reduces brain sodium levels.

Atrial natriuretic peptide levels in children with habitual epistaxis

OBJECTIVE

By virtue of no identifiable causes in the majority of children with habitual epistaxis, it continues to be problematic in pediatric clinical practice. The purpose of this study is to explore the possible change of atrial natriuretic peptide (ANP) levels in the children with epistaxis.

METHODS

Both the plasma and nasal mucus ANP levels have been determined in 30 sick children by a sensitive radioimmunoassay (RIA) technique.

RESULTS

Our results revealed that the plasma and nasal mucus ANP levels were considerably decreased in 24 children with habitual epistaxis when compared with control group (P<0.05), making up 80%, and amongst the interest of these are the nasal mucus ANP levels changing inversely as the times bled from the nose.

CONCLUSION

Although the plasma and nasal mucus ANP levels will not establish the diagnosis of its etiology, it is helpful for us to know the cardiovascular status compensating for chronic blood loss in the children with habitual epistaxis.

Functional contribution of voltage-dependent and Ca2+ activated K+ (BK(Ca)) channels to the relaxation of guinea-pig aorta in response to natriuretic peptides

We examined the relaxant effects of natriuretic peptide family on the isolated guinea-pig aorta to determine the receptor subtype which primarily mediates this vascular relaxation, with particular attention to the apparent contribution of voltage-dependent and Ca2+-activated KS (BK(Ca)) channels to the response. Three endogenous natriuretic peptide ligands (natriuretic peptide, ANP; brain natriuretic peptide, BNP; C-type natriuretic peptide, CNP) produced a concentration-dependent relaxation in de-endothelialized guinea-pig aorta pre-contracted by noradrenaline (NA), with a potency order of ANP > or = BNP >> CNP. Although the relaxations elicited by these three natriuretic peptide ligands were significantly diminished by iberiotoxin (IbTx, 10(-7) M), a selective BK(Ca) channel blocker, the inhibitory effect of IbTx was most pronounced for the CNP-induced relaxation; when estimated at 10(-7) M of each peptide, the apparent extent of BK(Ca) channel contribution to the total relaxant response was approximately 60% for CNP > approximately 20% for either ANP or BNP. Supporting the substantial role of BK(Ca) channels in the vascular responses, high-KCl (80 mM) potently suppressed the relaxations induced by these natriuretic peptide ligands. The relaxant response to 8-Bromo-cyclic GMP, a membrane permeable cyclic GMP analogue, was also diminished by IbTx (10(-7) M) and high-KCl (80 mM), which indicates the key role of cyclic GMP in the BK(Ca) channel-mediated, natriuretic peptide-elicited vascular relaxation. These results indicate that the A-type receptor (NPR-A, which is more selective for ANP and BNP) rather than the B-type receptor (NPR-B, which is more selective for CNP) predominates in the guinea-pig aorta as the natriuretic peptide receptor which mediates this vascular smooth muscle relaxation. Although activation of BK(Ca) channels substantially contributes to both NPR-A- and NPR-B-activated relaxations, particularly in the NPR-B-activated relaxation, this K channel may function as a primary relaxant mediator in this conduit artery.