Regional sampling and the effects of experimental heart failure in sheep: differential responses in A, B and C-type natriuretic peptides

C-type natriuretic peptide (CNP): The cardiovascular system and beyond

C-type natriuretic peptide (CNP) represents the 'local' member of the natriuretic peptide family, functioning in an autocrine or paracrine capacity to modulate a hugely diverse portfolio of physiological processes. Whilst the best-characterised of these regulatory roles are in the cardiovascular system, akin to its predominantly endocrine siblings atrial (ANP) and brain (BNP) natriuretic peptides, CNP governs many additional, unrelated mechanisms including bone growth, gamete maturation, auditory processing, and neuronal integrity. Furthermore, there is currently great interest in mimicking the biological activity of CNP for therapeutic gain in many of these disparate organ systems. Herein, we provide an overview of the physiology, pathophysiology and pharmacology of CNP in both cardiovascular and non-cardiovascular settings.

ECG and Biomarker Profile in Patients with Acute Heart Failure: A Pilot Study

Background: Biomarkers, electrocardiogram (ECG) and Holter ECG are basic, accessible and feasible cardiac investigations. The combination of their results may lead to a more complex predictive model that may improve the clinical approach in acute heart failure (AHF). The main objective was to investigate which ECG parameters are correlated with the usual cardiac biomarkers (prohormone N-terminal proBNP, high-sensitive cardiac troponin I) in patients with acute heart failure, in a population from Romania. The relationship between certain ECG parameters and cardiac biomarkers may support future research on their combined prognostic value. Methods: In this prospective case-control study were included 49 patients with acute heart failure and 31 participants in the control group. For all patients we measured levels of prohormone N-terminal proBNP (NT-proBNP), high-sensitive cardiac troponin I (hs-cTnI) and MB isoenzyme of creatine phosphokinase (CK-MB) and evaluated the 12-lead ECG and 24 h Holter monitoring. Complete clinical and paraclinical evaluation was performed. Results: NT-proBNP level was significantly higher in patients with AHF (p < 0.001). In patients with AHF, NT-proBNP correlated with cQTi (p = 0.027), pathological Q wave (p = 0.029), complex premature ventricular contractions (PVCs) (p = 0.034) and ventricular tachycardia (p = 0.048). Hs-cTnI and CK-MB were correlated with ST-segment modification (p = 0.038; p = 0.018) and hs-cTnI alone with complex PVCs (p = 0.031). Conclusions: The statistical relationships found between cardiac biomarkers and ECG patterns support the added value of ECG in the diagnosis of AHF. We emphasize the importance of proper ECG analysis of more subtle parameters that can easily be missed. As a non-invasive technique, ECG can be used in the outpatient setting as a warning signal, announcing the acute decompensation of HF. In addition, the information provided by the ECG complements the biomarker results, supporting the diagnosis of AHF in cases of dyspnea of uncertain etiology. Further studies are needed to confirm long-term prognosis in a multi-marker approach.

CNP, the Third Natriuretic Peptide: Its Biology and Significance to the Cardiovascular System

Simple Summary

CNP is the third natriuretic peptide to be isolated and is widely expressed in the central nervous system, osteochondral system, and vascular system. The receptor that is mainly targeted by CNP is GC-B, which differs from GC-A, the receptor targeted by the other two natriuretic peptides, ANP and BNP. Consequently, the actions of CNP differ somewhat from those of ANP and BNP. Research into the actions of CNP has shown that CNP attenuates cardiac remodeling in animal models of cardiac hypertrophy, myocardial infarction, and myocarditis. Studies examining CNP/GC-B signaling showed that it contributes to the prevention of cardiac stiffness. Endogenous CNP, perhaps acting in part through CNP/NPR-C signaling, contributes to the regulation of vascular function and blood pressure. CNP regulates vascular remodeling and angiogenesis via CNP/GC-B/CGK signaling. CNP attenuates interstitial fibrosis and fibrosis-related gene expression in pressure overload and myocardial infarction models. The clinical application of CNP as a therapeutic agent for cardiovascular diseases is anticipated.

Abstract

The natriuretic peptide family consists of three biologically active peptides: ANP, BNP, and CNP. CNP is more widely expressed than the other two peptides, with significant levels in the central nervous system, osteochondral system, and vascular system. The receptor that is mainly targeted by CNP is GC-B, which differs from GC-A, the receptor targeted by ANP and BNP. Consequently, the actions of CNP differ somewhat from those of ANP and BNP. CNP knockout leads to severe dwarfism, and there has been important research into the role of CNP in the osteochondral system. As a result, a CNP analog is now available for clinical use in patients with achondroplasia. In the cardiovascular system, CNP and its downstream signaling are involved in the regulatory mechanisms underlying myocardial remodeling, cardiac function, vascular tone, angiogenesis, and fibrosis, among others. This review focuses on the roles of CNP in the cardiovascular system and considers its potential for clinical application in the treatment of cardiovascular diseases.

Large Animal Models of Heart Failure: Reduced vs. Preserved Ejection Fraction

Heart failure (HF) is the final common end point of multiple metabolic and cardiovascular diseases and imposes a significant health care burden worldwide. Despite significant improvements in clinical management and outcomes, morbidity and mortality remain high and there remains an indisputable need for improved treatment options. The pathophysiology of HF is complex and covers a spectrum of clinical presentations from HF with reduced ejection fraction (HFrEF) (≤40% EF) through to HF with preserved EF (HFpEF), with HFpEF patients demonstrating a reduced ability of the heart to relax despite an EF maintained above 50%. Prior to the last decade, the majority of clinical trials and animal models addressed HFrEF. Despite growing efforts recently to understand underlying mechanisms of HFpEF and find effective therapies for its treatment, clinical trials in patients with HFpEF have failed to demonstrate improvements in mortality. A significant obstacle to therapeutic innovation in HFpEF is the absence of preclinical models including large animal models which, unlike rodents, permit detailed instrumentation and extensive imaging and sampling protocols. Although several large animal models of HFpEF have been reported, none fulfil all the features present in human disease and few demonstrate progression to frank decompensated HF. This review summarizes well-established models of HFrEF in pigs, dogs and sheep and discusses attempts to date to model HFpEF in these species.

Circulating products of C-type natriuretic peptide and links with organ function in health and disease

Paracrine actions of CNP and rapid degradation at source severely limit study of CNP's many roles in vivo. However provided sensitive and validated assays are used, there is increasing evidence that low concentrations of bioactive CNP in plasma, and the readily detectable concentrations of the bio-inactive processed product of proCNP (aminoterminal proCNP), can be used to advance understanding of the hormone's role in pathophysiology. Provided renal function is normal, concordant changes in both CNP and NTproCNP reflect change in tissue production of proCNP whereas change in CNP alone results from altered rates of bioactive CNP degradation and are reflected in the ratio of NTproCNP to CNP. As already shown in juveniles, where plasma concentration of CNP products are higher and are associated with concurrent endochondral bone growth, measurements of plasma CNP products in mature adults have potential to clarify organ response to stress and injury. Excepting the role of CNP in fetal-maternal welfare, this review examines evidence linking plasma CNP products with function of a wide range of tissues in adults, including the impact of extraneous factors such as nutrients, hormone therapy and exercise.

Effect of statin therapy on plasma C-type Natriuretic Peptides and Endothelin-1 in males with and without symptomatic coronary artery disease

C-type Natriuretic Peptide (CNP) and Endothelin-1 (ET-1) have reciprocal roles in maintaining vascular homeostasis and are acutely modulated by statins in human cultured endothelial cells. Whether these actions of statins in vitro are reflected in studies in vivo is unknown. In a prospective study of 66 subjects with or without post- acute coronary syndrome (ACS), plasma concentrations of bioactive CNP and bio-inactive aminoterminal proCNP (NTproCNP), ET-1, B-type Natriuretic Peptide (BNP) and high sensitivity C Reactive Protein (hsCRP) were measured together with lipids before and at intervals of 1, 2 and 7 days after commencing atorvastatin 40 mg/day - and for a further period of 6months in those with ACS. Plasma lipids fell significantly in all subjects but plasma CNP, NTproCNP and ET-1 were unchanged by atorvastatin. In ACS, baseline hsCRP, BNP and CNP but not NTproCNP or ET-1 were significantly raised compared to values in age-matched controls. The ratio of NTproCNP to CNP was significantly lower in ACS throughout the study and was unaffected by statin therapy. We conclude that conventional doses of atorvastatin do not affect plasma CNP products or ET-1. Elevated CNP after cardiac injury likely results from regulated changes in clearance, not enhanced production.

Differential effects of atrial and brain natriuretic peptides on human pulmonary artery: An in vitro study

BACKGROUND

The prevalence of cardiovascular diseases, especially heart failure, continues to rise worldwide. In heart failure, increasing levels of circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are associated with a worsening of heart failure and a poor prognosis.

AIM

To test whether a high concentration of BNP would inhibit relaxation to ANP.

METHODS

Pulmonary arteries were dissected from disease-free areas of lung resection, as well as pulmonary artery rings of internal diameter 2.5–3.5 mm and 2 mm long, were prepared. Pulmonary artery rings were mounted in a multiwire myograph, and a basal tension of 1.61gf was applied. After equilibration for 60 min, rings were pre-constricted with 11.21 µmol/L PGF_2α (EC₈₀), and concentration response curves were constructed to vasodilators by cumulative addition to the myograph chambers.

RESULTS

Although both ANP and BNP were found to vasodilate the pulmonary vessels, ANP is more potent than BNP. pEC50 of ANP and BNP were 8.96 ± 0.21 and 7.54 ± 0.18, respectively, and the maximum efficacy (E_max) for ANP and BNP was -2.03 gf and -0.24 gf, respectively. After addition of BNP, the E_max of ANP reduced from -0.96gf to -0.675gf (P = 0.28).

CONCLUSION

BNP could be acting as a partial agonist in small human pulmonary arteries, and inhibits relaxation to ANP. Elevated levels of circulating BNP could be responsible for the worsening of decompensated heart failure. This finding could also explain the disappointing results seen in clinical trials of ANP and BNP analogues for the treatment of heart failure.

Natriuretic Peptides in Cardiac Anesthesia and Intensive Care

Natriuretic peptides, predominantly B-type, are widely used in cardiology as prognostic and diagnostic biomarkers or, much less often, as a substantive treatment tool. They are hormones that are produced mainly in the myocardium in response to overload and ischemia, and their level quite accurately reflects the degree of myocardial dysfunction. Although their use in cardiac anesthesia and intensive care setting seems to be very beneficial for assessing the risk of acute disturbance of myocardial function or its laboratory monitoring, the actual significance of natriuretic peptides in this area is not yet recognized. This is due to the lack of clear diagnostic and prognostic values for these biomarkers supported by high-quality researches. On the basis of the available data, main advantages, existing difficulties, and most effective ways of using natriuretic peptides for determining the risk of heart surgery and assessing the severity of sepsis, pneumonia, and other critical conditions have been discussed in this review. In addition, the expediency of using natriuretic peptides as target parameters for goal-oriented therapy and as a substantive tool for treatment is considered.

Natriuretic Peptides and Normal Body Fluid Regulation

Natriuretic peptides are structurally related, functionally diverse hormones. Circulating atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are delivered predominantly by the heart. Two C-type natriuretic peptides (CNPs) are paracrine messengers, notably in bone, brain, and vessels. Natriuretic peptides act by binding to the extracellular domains of three receptors, NPR-A, NPR-B, and NPR-C of which the first two are guanylate cyclases. NPR-C is coupled to inhibitory proteins. Atrial wall stress is the major regulator of ANP secretion; however, atrial pressure changes plasma ANP only modestly and transiently, and the relation between plasma ANP and atrial wall tension (or extracellular volume or sodium intake) is weak. Absence and overexpression of ANP-related genes are associated with modest blood pressure changes. ANP augments vascular permeability and reduces vascular contractility, renin and aldosterone secretion, sympathetic nerve activity, and renal tubular sodium transport. Within the physiological range of plasma ANP, the responses to step-up changes are unimpressive; in man, the systemic physiological effects include diminution of renin secretion, aldosterone secretion, and cardiac preload. For BNP, the available evidence does not show that cardiac release to the blood is related to sodium homeostasis or body fluid control. CNPs are not circulating hormones, but primarily paracrine messengers important to ossification, nervous system development, and endothelial function. Normally, natriuretic peptides are not powerful natriuretic/diuretic hormones; common conclusions are not consistently supported by hard data. ANP may provide fine-tuning of reno-cardiovascular relationships, but seems, together with BNP, primarily involved in the regulation of cardiac performance and remodeling. © 2017 American Physiological Society. Compr Physiol 8:1211-1249, 2018.

Effects of prenatal bisphenol-A exposure and postnatal overfeeding on cardiovascular function in female sheep

Bisphenol-A (BPA) is a widely used endocrine-disrupting chemical. Prenatal exposure to BPA is known to affect birth weight, but its impact on the cardiovascular system has not been studied in detail. In this study, we investigated the effects of prenatal BPA treatment and its interaction with postnatal overfeeding on the cardiovascular system. Pregnant sheep were given daily subcutaneous injections of corn oil (control) or BPA (0.5 mg/kg/day in corn oil) from day 30 to day 90 of gestation. A subset of female offspring of these dams were overfed to increase body weight to ~30% over that of normal fed controls. Cardiovascular function was assessed using non-invasive echocardiography and cuff blood pressure (BP) monitoring at 21 months of age. Ventricular tissue was analyzed for gene expression of cardiac markers of hypertrophy and collagen at the end of the observation period. Prenatal BPA exposure had no significant effect on BP or morphometric measures. However, it increased atrial natriuretic peptide gene expression in the ventricles and reduced collagen expression in the right ventricle. Overfeeding produced a marked increase in body weight and BP. There were compensatory increases in left ventricular area and internal diameter. Prenatal BPA treatment produced a significant increase in interventricular septal thickness when animals were overfed. However, it appeared to block the increase in BP and left ventricular area caused by overfeeding. Taken together, these results suggest that prenatal BPA produces intrinsic changes in the heart that are capable of modulating morphological and functional parameters when animals become obese in later life.

B-type natriuretic peptide and heart failure: what can we learn from clinical trials?

The B-type natriuretic peptide (BNP) may favour natriuresis and diuresis, making it an ideal drug to aid in diuresing a fluid-overloaded patient with poor or worsening renal function. Several randomized clinical trials have tested the hypothesis that infusions of pharmacological doses of BNP to acute heart failure (HF) patients may enhance decongestion and preserve renal function in this clinical setting. Unfortunately, none of these has resulted in a better outcome. The current challenge for BNP research in acute HF lies in a failure of concept and reluctance to abandon a demonstrably ineffectual research model. Future success will necessitate a detailed understanding of the mechanism of action of BNP as well as a better integration of basic and clinical science.

BNP and Heart Failure: Preclinical and Clinical Trial Data

The B-type natriuretic peptide (BNP), a member of the family of vasoactive peptides, has emerged as an important diagnostic, prognostic, and therapeutic tool in patients with heart failure (HF). The rapid incorporation into clinical practice of bioassays to BNP concentrations and pharmacological agents that augment the biological actions of this peptide such as nesiritide or vasopeptidase inhibitors has shown the potential for translational research to improve patient care. Despite the indirect evidence in support of a potential benefit from raising BNP, accumulating evidence suggests that simply increasing the amount of circulating BNP does not necessarily confer cardiovascular benefits in patient with HF. Moreover, in experimental HF, the response to treatments targeting specific natriuretic peptide receptors (NPRs) signaling seems to be attenuated. A better understanding of the NPRs signaling in HF would be clinically relevant and thus required, in order to devise strategies to develop novel agents and technologies that directly target this signaling pathway.

C-type natriuretic peptide forms in adult hyperthyroidism: correlation with thyroid hormones and markers of bone turnover

CONTEXT

Plasma C-type natriuretic peptide (CNP) forms correlate with linear growth velocity in juveniles. In hyperthyroid children, plasma CNP products fall in parallel with height velocity and thyroid hormones (TH) as euthyroidism is restored. The effect of TH on CNP forms after completion of endochondral growth is unknown.

OBJECTIVE

To determine the effect of restoring euthyroidism on plasma CNP forms and bone turnover markers (BTMs) in hyperthyroid adults.

DESIGN AND SETTING

We performed a prospective observational study in 20 adults (19 women) with acquired hyperthyroidism before and during carbimazole treatment. INTERVENTION AND MAIN OUTCOMES: Blood levels of CNP, amino-terminal propeptide of CNP (NTproCNP), TH and BTMs - bone-specific alkaline phosphatase, osteocalcin, procollagen type 1 amino-terminal propeptide and type 1 collagen C-telopeptide (CTx) - were measured before and during the first 6 months of carbimazole treatment and correlations determined.

RESULTS

Both CNP and NTproCNP were significantly correlated with TH at baseline. As in children, decreases in CNP forms were closely associated with fall in TH. Significant associations were found between CNP forms and CTx.

CONCLUSIONS

CNP production from tissues other than endochondral cartilage is responsive to TH. Strong temporal links with markers of bone resorption suggest that CNP may also participate in bone remodelling in the adult skeleton.

Regulation of C-type natriuretic peptide expression

C-type natriuretic peptide (CNP) is a member of the small family of natriuretic peptides that also includes atrial natriuretic peptide (ANP) and brain, or B-type natriuretic peptide (BNP). Unlike them, it performs its major functions in an autocrine or paracrine manner. Those functions, mediated through binding to the membrane guanylyl cyclase natriuretic peptide receptor B (NPR-B), or by signaling through the non-enzyme natriuretic peptide receptor C (NPR-C), include the regulation of endochondral ossification, reproduction, nervous system development, and the maintenance of cardiovascular health. To date, the regulation of CNP gene expression has not received the attention that has been paid to regulation of the ANP and BNP genes. CNP expression in vitro is regulated by TGF-β and receptor tyrosine kinase growth factors in a cell/tissue-specific and sometimes species-specific manner. Expression of CNP in vivo is altered in diseased organs and tissues, including atherosclerotic vessels, and the myocardium of failing hearts. Analysis of the human CNP gene has led to the identification of a number of regulatory sites in the proximal promoter, including a GC-rich region approximately 50 base pairs downstream of the Tata box, and shown to be a binding site for several putative regulatory proteins, including transforming growth factor clone 22 domain 1 (TSC22D1) and a serine threonine kinase (STK16). The purpose of this review is to summarize the current literature on the regulation of CNP expression, emphasizing in particular the putative regulatory elements in the CNP gene and the potential DNA-binding proteins that associate with them.

Skeletal contributions to plasma CNP forms: evidence from regional sampling in growing lambs

Unlike the cardiac circulating hormones, atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP), C-type natriuretic peptide (CNP) appears to be largely tissue-based and circulates at concentrations considered insufficient to affect organ function. Consistent with CNP's crucial role in regulating skeletal growth, serial studies in juveniles show that both plasma CNP and aminoterminal proCNP (NTproCNP) are highly correlated with growth velocity raising the possibility that skeletal tissues contribute to circulating concentrations of CNP forms during the growing period. Hypothesizing that venous blood draining from bone dense regions is relatively enriched in CNP, we have performed trans-organ regional blood sampling for measurement of CNP forms in 4-week-old lambs and compared the findings to simultaneous levels of ANP and BNP. Because bone growth and CNP synthesis are inhibited by glucocorticoids, identical studies were also undertaken in lambs pretreated with dexamethasone. Highly significant positive arterio-venous gradients of CNP were found across the head, heart, leg and foot. Dexamethasone significantly reduced the CNP arterio-venous gradient across the head and leg but not heart, liver or kidney. In contrast, there was no evidence of tissue secretion of ANP or BNP except across the heart, and no effect on these gradients from dexamethasone. These findings of CNP enrichment in samples from bone dense regions in growing lambs, and their selective reduction by dexamethasone, provide in vivo evidence linking plasma and skeletal tissue concentrations of CNP and further support the use of plasma CNP forms as markers of bone growth.

C-type natriuretic peptide production by the human kidney is blunted in chronic heart failure

CNP (C-type natriuretic peptide) is a vasodilatory peptide produced by vascular endothelium and the human heart with a short half-life. CNP has been identified within the human kidney; however, few results are available on whether the human kidney is a systemic source of CNP. The aim of the present study was to establish whether CNP is secreted by the human kidney and if synthesis is blunted in CHF (chronic heart failure). A total of 20 male subjects (age, 57+/-2 years; mean+/-S.E.M.) undergoing CHF assessment (n=13) or investigation of paroxysmal supraventricular arrhythmia (normal left ventricular function in sinus rhythm during procedure) (n=7) were recruited. Renal CNP production was determined from concomitant plasma concentrations in the aorta and renal vein. When considering all subjects, a significant step-up in plasma CNP was found from the aorta to renal vein (3.0+/-0.3 compared with 8.3+/-2.4 pg/ml respectively; P=0.0045). The mean increase in CNP was 5.3+/-2.4 pg/ml (range, -0.9 to +45.3 pg/ml). In patients with CHF, the aortic concentration was 3.3+/-0.4 pg/ml compared with a renal vein concentration of 4.3+/-0.6 pg/ml (P=0.11). In those with normal left ventricular function, the respective values were 2.5+/-0.5 and 15.7+/-6.0 pg/ml (P=0.01). In conclusion, CNP is synthesized and secreted into the circulation by the normal human kidney, where it may have paracrine actions. Net renal secretion of CNP appears to be blunted in patients with CHF.

Regional release and clearance of C-type natriuretic peptides in the human circulation and relation to cardiac function

Production and clearance of plasma C-type natriuretic peptide (CNP) and amino terminal (NT)-proCNP immunoreactivity in the human circulation remain poorly characterized. Accordingly, we have measured arterial and venous concentrations of CNP and NT-proCNP across multiple tissue beds during cardiac catheterization in 120 subjects (age: 64.2+/-9.0 years; 73% men) investigated for cardiovascular disorders. The heart, head and neck, and musculoskeletal tissues made the clearest contributions to both plasma CNP and NT-proCNP (P<0.05). Net release of NT-proCNP was also observed from hepatic tissue (P<0.001). Negative arteriovenous gradients for CNP were observed across renal, hepatic, and pulmonary tissue (P<0.05), indicating net clearance, whereas no tissue-specific site of NT-proCNP clearance was identified. Age, mean pulmonary artery pressure, left ventricular end diastolic pressure, Brandt score of myocardial jeopardy, and troponin I were independent predictors of circulating CNP levels in multivariable analysis. Sex and kidney function were independently predictive of arterial NT-proCNP. The proportional step-up of CNP (+60%) across the heart was less than for brain natriuretic peptide (+123%) but greater than for NT-pro-brain natriuretic peptide (NT-proBNP) (+36%) and NT-proCNP (+42%; P<0.001 for all). We conclude that cardiac and head and neck tissue are important sources of CNP. Circulating CNP but not NT-proCNP concentrations are related to cardiac hemodynamic load and ischemic burden. Although cardiac release is most evident, multiple additional tissues release NT-proCNP immunoreactivity without evidence for an organ-specific site for NT-proCNP degradation. Taken together, differences in magnitude and direction of transorgan gradients for CNP compared with NT-proCNP suggest net generalized cosecretion with differing mechanisms of clearance.

Natriuretic peptides: their structures, receptors, physiologic functions and therapeutic applications

Natriuretic peptides are a family of three structurally related hormone/ paracrine factors. Atrial natriuretic peptide (ANP) and B-type natriuretic peptide (BNP) are secreted from the cardiac atria and ventricles, respectively. ANP signals in an endocrine and paracrine manner to decrease blood pressure and cardiac hypertrophy. BNP acts locally to reduce ventricular fibrosis. C-type natriuretic peptide (CNP) primarily stimulates long bone growth but likely serves unappreciated functions as well. ANP and BNP activate the transmembrane guanylyl cyclase, natriuretic peptide receptor-A (NPR-A). CNP activates a related cyclase, natriuretic peptide receptor-B (NPR-B). Both receptors catalyze the synthesis of cGMP, which mediates most known effects of natriuretic peptides. A third natriuretic peptide receptor, natriuretic peptide receptor-C (NPR-C), clears natriuretic peptides from the circulation through receptor-mediated internalization and degradation. However, a signaling function for the receptor has been suggested as well. Targeted disruptions of the genes encoding all natriuretic peptides and their receptors have been generated in mice, which display unique physiologies. A few mutations in these proteins have been reported in humans. Synthetic analogs of ANP (anaritide and carperitide) and BNP (nesiritide) have been investigated as potential therapies for the treatment of decompensated heart failure and other diseases. Anaritide and nesiritide are approved for use in acute decompensated heart failure, but recent studies have cast doubt on their safety and effectiveness. New clinical trials are examining the effect of nesiritide and novel peptides, like CD-NP, on these critical parameters. In this review, the history, structure, function, and clinical applications of natriuretic peptides and their receptors are discussed.

C-type natriuretic peptide forms in the ovine fetal and maternal circulations: evidence for independent regulation and reciprocal response to undernutrition

C-type natriuretic peptide (CNP) has a crucial role in postnatal endochondral bone growth and is rapidly responsive to changes in nutrition. Although CNP is expressed in the placenta, little is known about the regulation and role of CNP in fetal-maternal health. We hypothesized that CNP may be similarly responsive to undernutrition in the growing fetus, in which maternal nutrition is crucial to normal growth and development. We therefore studied maternal and fetal CNP and the aminoterminal (bioinactive) fragment of proCNP (NTproCNP) in 39 chronically catheterized pregnant sheep before and after a 3-d maternal fast from 121 d gestation. Maternal CNP and NTproCNP levels were higher than in the fetus (CNP 12-fold, NTproCNP 1.5-fold, both P < 0.001). The ratio of NTproCNP to CNP was higher in the fetus than the mother (53 +/- 3 vs. 8.7 +/- 0.6, P < 0.001), suggesting enhanced synthesis and/or degradation of CNP in the fetus. As in postnatal lambs, fetal plasma CNP forms fell promptly during maternal fasting. In contrast, maternal levels exhibited reciprocal and contemporaneous increase, which was reversed by refeeding. Uteroplacental production of CNP was suggested by a high venoarterial concentration gradient across the gravid uterus, and a correlation between maternal NTproCNP levels and placental weight (r(2) = 0.26, P = 0.01). These studies provide the first evidence that CNP is regulated independently in the fetus. Reciprocal increases in maternal CNP forms may reflect the response of the uteroplacental unit to substrate deficiency. CNP may have a role in maintaining fetal welfare and provides a possible marker of uteroplacental nutrient supply.

Response of plasma CNP forms to acute anabolic and catabolic interventions in growing lambs

Using a novel marker of C-type natriuretic peptide (CNP) synthesis [amino-terminal pro-CNP (NT-proCNP)], we have recently shown that plasma NT-proCNP is strongly correlated with skeletal growth and markers of bone formation and is reversibly reduced by glucocorticoids. The effects on CNP of other catabolic or anabolic factors, known to affect skeletal growth, are unknown. Accordingly, we have studied the response of plasma CNP forms to acute catabolic (caloric restriction) and anabolic [growth hormone (GH) stimulation] interventions in lambs and related the findings to circulating IGF-I levels, growth velocity, and markers of bone formation. Lambs fed a reduced caloric intake (25% of normal) for 6 days exhibited reduced live weight, plasma urea, and IGF-I (P < 0.001 for all) compared with control lambs. Basal levels of NT-proCNP (40.1 +/- 0.9 pmol/l) fell promptly to a nadir (28.1 +/- 0.8 pmol/l, P < 0.001) on day 6, returning rapidly to basal levels upon refeeding. Although plasma alkaline phosphatase (ALP) fell (P < 0.001), reductions in metacarpal growth velocity were not significant within the 12-day period of study. In contrast to caloric restriction, long-acting bovine recombinant GH (2.5 mg/kg on days 0 and 6), as expected, increased plasma IGF-I more than twofold above control for 12 days (P < 0.001). Growth velocity did not differ during the 30 days of observation, and, consistent with unchanged growth velocity, plasma NT-proCNP and ALP were also unaffected. In conclusion, CNP synthesis and markers of bone formation are acutely sensitive to catabolism but unaffected by doses of GH that fail to stimulate skeletal growth.