Natriuretic peptide system in fetal heart and circulation (2002)

It is 20 years since de Bold et al. first demonstrated that extracts of atrial tissue injected into rats elicited a potent natriuresis in recipient animals [1]. Subsequently, a family of natriuretic peptides has been identified, and their physiological roles have been shown to extend beyond that of hormones regulating blood pressure and body fluid homeostasis. The natriuretic peptides serve a vital function in cardiovascular regulation through their diuretic, natriuretic and vasorelaxant effects [2,3]. In addition, these peptides have multiple roles as antiproliferative factors in cardiovascular tissues, neuropeptides in the central nervous system and paracrine regulators of bone growth. Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac peptides, produced predominantly by the atria and ventricles, respectively, in the normal adult heart. A number of hormones have been shown to regulate secretion of ANP and BNP from the heart into the circulation, but the primary acute stimulus for release is increased stretch of the myocardium [2]. ANP and BNP exert their biological actions by binding to the natriuretic peptide receptor-A (NPR-A), resulting in the generation of the second messenger cyclic GMP. A third member of the family, C-type natriuretic peptide (CNP), is expressed primarily in the brain, pituitary, vascular endothelium, kidney [4] and female reproductive tract [5]. In contrast to ANP and BNP, very little CNP is produced by cardiac tissue. A second guanylyl cyclase-linked receptor, NPR-B, binds CNP with high affinity [4], and although CNP also has vasodilatory effects, concentrations of CNP in the circulation are very low. It is thought that CNP acts as a local, paracrine hormone in a number of tissues, including blood vessels, lung, brain and bone [4]. In this issue of the journal, Walther et al. review the literature on the expression of the natriuretic peptides in the fetal heart and circulation, and discuss their physiological roles during embryological development [6]. Some of their conclusions call for further comment. In discussing the cardiac distribution of ANP and BNP in the fetus, Walther et al. conclude that ‘atrial ANP is significantly higher than ventricular ANP at all investigated time points during fetal development, indicating that, in mammal fetuses, the atrium is the predominant tissue of ANP production'. This conclusion appears to be at odds with some of the cited literature. In rodent development, many studies report higher levels in ventricles compared to atria of both ANP mRNA expression [7,8] and ANP peptide [9]. In mouse embryos, we have shown predominant expression in the ventricles of both ANP and BNP mRNA until full-term at 20 days [7]. By contrast, in fetal sheep, both ANP mRNA and ANP peptide were clearly greater in atria than ventricles throughout gestation [10,11], but levels of ventricular ANP expression were still higher in fetal than in adult sheep [10]. In the hearts of human fetuses, ventricular ANP mRNA was higher than that of control adult ventricles [12,13]. Ventricular ANP mRNA levels were one-fifth of those in the atria compared to the normal adult ventricle in which ANP mRNA levels are 1% of those in the atria [3]. These studies suggest that the relative contribution of ANP from embryo ventricles is significantly greater than from adult ventricles in all species studied and, at certain stages of development in some species, the ventricle is the predominant site of ANP and BNP expression during development. Walther et al. also refer to evidence from immunohistochemistry of tissue extracts to draw their conclusion about the relative contributions of ANP from atria and ventricles. However, tissue peptide content of atria and ventricles cannot be easily compared, as ANP is stored in granules in the atria whereas secretion is primarily constitutive in the ventricles. Irons and Gelmbotski have demonstrated that the transit time of newly synthesized ANP is significantly shorter in ventricular myocytes (t1/2 = 0.5–1.5 h) than in atrial myocytes (t1/2 = 4 h) [14]. Hence, it would be expected that more ANP peptide would accumulate in atrial than ventricular tissue, and conclusions regarding the relative contribution of atria and ventricles based on comparing peptide content of the two tissues are of dubious validity. The issue is important because the currently held contention that regulation of ANP expression in the developing ventricle is different from the adult ventricle is central to current theories regarding the molecular processes underlying cardiac hypertrophy [15–17]. Cardiac gene expression of both ANP and BNP is increased in animal models of myocardial infarction and heart failure [18–20] and in human heart disease [21,22], particularly in hypertrophic ventricles [23]. The appearance of increased ANP expression in adult ventricles has become established as a marker for the induction of the embryonic gene program during the development of ventricular hypertrophy [16]. It has been reported that ANP inhibits cardiac hypertrophy in cultured cardiac myocytes [24], and that ANP affects apoptosis in cardiac myocytes in culture [25]. In addition to inhibiting cardiac hypertrophy, all three natriuretic peptides, ANP, BNP and CNP, suppress cardiac fibroblast growth [26]. This raises the possibility that these peptides may function in a paracrine manner to modulate the development of cardiac hypertrophy and fibrosis during remodelling of the cardiac ventricle. Further support for a role of natriuretic peptides in regulating cardiac hypertrophy is provided by the phenotype of the NPR-A knockout (Npr1−/−) mouse [27]. These Npr1−/− mice lack the receptor which mediates the biological actions of both ANP and BNP, and have elevated blood pressure and marked cardiac hypertrophy and fibrosis [27,28]. The cardiac hypertrophy observed in Npr1−/− mice is greater than that seen in other mouse models of hypertension and is disproportionate to their mildly increased blood pressure. This suggests that the NPR-A pathway directly modulates the hypertrophic response, independent of blood pressure. Furthermore, when the blood pressure of Npr1−/− mice was maintained within the normal range by chronic treatment with antihypertensive drugs, Npr1−/− mice had significantly greater cardiac hypertrophy than wild-type mice [28]. Significantly, Npr1−/− mice had enlarged hearts at birth, with Npr1−/− neonates demonstrating heart weight to body weight ratios that were 140% greater than wild-type littermates [28]. This suggests that the ANP/BNP system is an important regulator of myocyte growth during development. Knockout mice in which either the ANP [29] or BNP [30] gene was disrupted showed no such change in the hearts of neonates. Therefore, it would appear that if either of the ANP or BNP genes are disrupted, the other can compensate and no effect on cardiac organogenesis occurs. However, if the signalling pathway for both ANP and BNP is disrupted, as in the Npr1−/− mice, cardiac hypertrophy results, and the hearts become enlarged during development. It should be noted that BNP over-expressing mice [31] and natriuretic peptide clearance receptor (NPR-C) knockout mice [32] show skeletal over-growth, whereas CNP knockout mice show dwarfism and bone malformation [33]. Thus, the natriuretic peptides have effects on the development of other tissues besides the heart. However, none of the mouse models involving NPR-A, ANP and BNP gene knockout or over-expression were found to be embryonic lethal. Hence, it is reasonable to conclude that the natriuretic peptides may modify physiological parameters, but are not essential for survival during embryogenesis. As discussed by Walther et al., the natriuretic peptides appear to respond to similar stimuli and to perform similar cardiovascular functions during development as they do in adults. Some excellent studies in fetal sheep have demonstrated that ANP secretion is stimulated in response to volume loading, hyperosmolality, hypoxia [34,35] and to vasoconstrictors such as angiotensin II, phenyleprine [36] and endothelin [34]. Furthermore, infusion of ANP into the circulation of fetal sheep, albeit at supraphysiological concentrations (increasing plasma levels from 163 to > 2000 pg/ml), decreased arterial blood pressure and elicited diuresis but not natriuresis [34]. One area of research not referred to in the review by Walther et al. [6] is the role of natriuretic peptides in the placenta. Expression of BNP and CNP has been demonstrated within the mouse placenta [7], and expression of ANP was demonstrated in the human placenta, particularly by cytotrophoblast cells [37,38]. Furthermore, ANP or BNP administered into the fetal placental circulation have been shown to inhibit the effects of vasoconstrictor agents [39,40]. These studies suggest that the natriuretic peptides act as vasodilators in the fetal–placental vasculature, regulating the blood supply to the fetus. In summary, the review by Walther et al. summarizes current knowledge about the pre-natal expression, regulation and actions of the natriuretic peptides during development. However, this information does not yet establish the natriuretic peptide system to be ‘of major importance under pathophysiological conditions serving as a second-line, compensatory mechanism', as claimed by the authors. The research to date suggests that, during development, the natriuretic peptides participate in the regulation of blood pressure and body fluid homeostasis and modify growth and development of cardiovascular tissues and bone. Further research will be needed before we will understand fully all the roles played by the natriuretic peptides during embryogenesis.