Role of atrial natriuretic peptide in sodium homeostasis in premature infants

Sodium and Growth in Preterm Infants: A Review

Aim

This article is intended to review the relationship between sodium homeostasis and growth, outline reasons why preterm infants may become sodium deficient, and share data from our group and others regarding the potential benefits of dietary sodium supplementation.

Background

Despite tremendous efforts over the past 20 years to optimize neonatal nutrition, postnatal growth failure in preterm infants remains a significant problem. Compelling associations have been identified between in-hospital growth failure and cardiometabolic and neurodevelopmental disorders, heightening the need to further identify the optimal nutritional needs of preterm infants.

Results

The impact of sodium deficiency may have on somatic growth is poorly studied and reported upon within the human literature. In contrast, animal studies dating back almost 100 years highlight the nutritional importance of dietary sodium. Sodium homeostasis during early postnatal life is understudied and underappreciated by neonatologists.

Conclusion

Insufficient sodium intake during early life is likely a critical yet underappreciated contributor to growth failure. Total body sodium depletion may be an important risk factor driving complications of premature birth.

Clinical significance

Increased awareness of sodium homeostasis in preterm infants may improve outcomes in this population. Sodium intake recommendations are provided based on the interpretation of currently available literature.

The moth and the aspen tree: sodium in early postnatal development

Over the past 25 years, our perception of the neonatal kidney has changed markedly from its being a "limited" organ compared with that of the adult to being extraordinarily well adapted in its role in maintaining homeostasis and making possible the rapid somatic growth necessary during this critical period of life. The present review focuses on the physiologic adaptations by the neonatal kidney in the maintenance of a positive sodium balance, which is necessary for normal growth not only in mammals but also in moths. There is a fine interplay between the developing brain, heart, thyroid, adrenals, and sympathetic nervous system, all converging on the kidney to conserve sodium, which is limited in the diet. The renin-angiotensin system plays a central role in this response and is balanced by developmental changes in the renal response to atrial natriuretic peptide, all of which contribute to sodium conservation. Over the next 25 years, advances in molecular genetics will doubtless elucidate many more facets of the mechanisms underlying neonatal sodium homeostasis. This will be particularly important as the survival of ever smaller preterm infants improves steadily.

Vasopressin, atrial natriuretic factor and renal water homeostasis in premature newborn infants with respiratory distress syndrome

Arginine vasopressin (AVP), human atrial natriuretic peptide (hANP), and body fluid and electrolyte balance were examined during the first five days of life in eleven premature infants (birthweight 1610 +/- 240 g, gestation 30 +/- 1 weeks) receiving mechanical ventilation for respiratory distress syndrome (RDS). Plasma hANP and urine AVP concentrations were determined by radioimmunoassay on the first, third and fifth days. Arginine vasopressin urine levels remained constantly elevated during the study period (mean +/- SD 13.5 +/- 7.8 day 1, 12.0 +/- 9.9 day 3, 13.2 +/- 5.1 ng/l day 5, p = n.s.), while plasma hANP was significantly increased on the third day (626 +/- 495 vs. 298 +/- 240 pg/ml on day 1, p < .05). Urine sodium concentration, urine osmolality and osmolality and osmolar clearance were elevated significantly as well on day 3, p < .05, and correlated to hANP levels. Body weight decreased during the study by 8.2% on the third day and by 11.3% of birthweight on the fifth day. A significant increase in creatinine clearance occurred after the third day (p < .01), while free water clearance remained essentially the same during the first five days of life. We speculate that an increase in plasma hANP concentration on day 3 of life results in a natriuresis and osmolar diuresis without correlations or temporal relationships to hypervasopressinemia of the premature neonate with RDS.

Atrial natriuretic peptide in renal development

Although discovered little more than a decade ago, atrial natriuretic peptide (ANP) has been shown to play a significant role in the maintenance of sodium homeostasis. Immediately after birth, plasma ANP concentration is very high concurrent with right atrial dilatation and a high urinary excretion of cyclic GMP (cGMP), the second messenger for ANP. Following postnatal diuresis and natriuresis, atrial volume, plasma ANP concentration, and urinary cGMP excretion decrease to baseline levels. In the ensuing suckling period, the diuretic and natriuretic response to acute saline volume expansion are attenuated, an effect which is offset by the lower hematocrit at this age. Increase in hematocrit by isovolemic exchange transfusion results in a greater rise of plasma ANP concentration following volume expansion, but a reduced excretion of cGMP. Intravenous infusion of ANP results in greater plasma ANP concentration, and greater urinary excretion of cGMP and sodium, in adult than in young rats. This increased metabolic clearance of ANP during early development is due at least in part to increased activity of clearance receptors. In addition, neutral endopeptidase contributes to removal of circulating ANP in maturing as well as adult rats. Infusion of ANP in neonatal or adult rats results in accumulation of cGMP in glomerular podocytes, with a higher threshold for activation in immature animals. Despite the similar response of intracellular generation of cGMP following exposure to ANP in neonatal and adult rats, egression of ANP out of glomeruli is low in neonates, an effect that is due to immaturity of an organic acid transporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide in the preterm infant. Lack of correlation with natriuresis and diuresis

We assessed the relation of atrial natriuretic peptide (ANP) to renal function on postnatal day 2 and day 5 in preterm infants. Plasma ANP concentration was measured by radioimmunoassay in two groups of preterm infants: group 1, gestational age less than 30 weeks, n = 10; and group 2, gestational age 30-34 weeks, n = 11. The identity of the immunoreactivity as ANP-28 was confirmed by HPLC. Plasma ANP was significantly higher in group 1 than in group 2 on day 2 and day 5 (p < 0.01) and ANP concentration decreased by day 5 in both groups (group 1, p < 0.01; group 2, p < 0.02). The results showed no correlation between plasma ANP concentration and urinary sodium excretion or creatinine clearance, which may be due to a blunted renal response to ANP, but other factors may be involved also. We conclude that preterm infants are able to release large amounts of ANP, but a high plasma ANP concentration does not correlate directly with renal regulation of sodium and water balance.

Renal function in sick very low birthweight infants: 3. Sodium, potassium, and water excretion

Renal excretion of sodium, water, and potassium was measured on 434 occasions in a sample of 40 infants of 25.5-33 weeks' gestation, birth weight 720-2000 g, between the ages of 0.5 and 36 days. Water excretion varied between 1% and 30% of the glomerular filtration rate, or 15-350 ml/kg/day, and varied widely from day to day in individual infants. Nearly all infants became hyponatraemic before or after the first postnatal week. There were a few instances of hypernatraemia in the first week caused by high insensible water loss. There were high levels of sodium excretion up to 16% of filtered sodium, or 21 mmol/kg/day, in the first two postnatal weeks. Highest levels of sodium excretion were seen in the most immature infants in the first week. In most infants sodium excretion increased either in the first week or later before a subsequent decline. Potassium excretion was often high in the first week, as much as 96% of filtered potassium, or 5 mmol/kg/day, and is associated with early hyperkalaemia.

Vasoactive factors in the immature kidney

The newborn infant is in a state of renal insufficiency with a glomerular filtration rate (GFR) as low as 20 ml/min per 1.73 m2 at term, and 10 ml/min per 1.73 m2 at 28 weeks of gestation. While the immature "insufficient" kidney can cope with most of the normal demands, its reserve is limited, and often overwhelmed by commonly occurring neonatal stresses. Various vasoactive systems such as the renin-angiotensin system, intrarenal adenosine, the prostaglandins and the atrial natriuretic peptide, are hyperactive in the neonatal period. Some of these systems appear crucial for the maintenance of GFR. Overstimulation of both angiotensin II and adenosine by an hypoxaemic stress can further impair the GFR, eventually leading to established renal failure. Inhibition of angiotensin II formation by the administration of angiotensin converting enzyme inhibitors can, on the other hand, also lead to renal failure. Prevention of these renal risks requires a precise knowledge of the newborn kidney physiology, physiopathology and pharmacology.

Salt and the newborn kidney

Renal function differs in term infants from that in adults, with lower glomerular filtration rate (GFR) and reduced proximal tubular reabsorption of sodium (Na) and water: nevertheless, it is adequate for their needs. This is not true of very preterm infants in whom hyponatraemia is common. Animal studies have shown that Na+, K(+)-ATPase and the Na+/K+ exchanger are poorly expressed at birth with rapid postnatal rises. Cell receptors for hormones that influence tubular Na transport are less numerous in the premature infant than later in life: intracellular second messenger systems may also be immature. The low GFR is due to vasoconstriction and may be necessary to prevent water and electrolyte wasting due to tubular overload. The hyponatraemia of prematurity could, in principle, be due either to Na loss or water excess and can be prevented either by giving additional Na or by restricting water intake. Na supplementation causes relative volume expansion (VE), water restriction volume contraction (VC); this is demonstrated by the effect of the two approaches on weight gain and on the levels of vasoactive hormones in the blood. We argue that moderate VE is more physiological than VC, both in attempting to simulate intrauterine conditions and in consideration of the infant's nutritional needs. The much less common complication of hypernatraemia is usually due to abnormal water loss and should be prevented by increasing water intake appropriately. The above applies to well, preterm babies: sick preterm infants are much more variable in their Na and water requirements than well infants of comparable gestation and weight and each needs an individually tailored regimen based on frequent clinical assessment and laboratory measurement.

Electrolyte balance

The kidney is structurally and biochemically immature at birth. As a consequence, renal function is low (3;10;ll;18). Glomerular filtration rate (related to body surface area or to body weight) is approximately 25% of that in adults. The capacity of several different tubular transport systems is lower in the infant than in the adult (2;5;13;21;28). A low transporting capacity of the neonatal kidney will sometimes result in undesired losses of electrolytes, amino acids, and peptides. The capacity to concentrate urine is low (7;29), and disturbances of serum tonicity, therefore, are common. The low concentrating capacity can be attributed to renal immaturity. The capacity of the newborn fullterm as well as preterm infant to release antidiuretic hormone is normal 7;31).

Changes in plasma concentrations of atrial natriuretic peptide during exchange transfusion in premature infants

Plasma concentrations of atrial natriuretic peptide (ANP) and hemodynamic parameters were investigated in five premature infants undergoing exchange transfusion. Baseline values of ANP were 51.7 +/- 21.2 fmol/ml. Volume depletion by withdrawal of 10 ml blood did not cause changes in systolic blood pressure (79.4 +/- 4.3 vs. 71.4 +/- 5.6 mmHg) and heart rate (115 +/- 5.2 vs. 115 +/- 2.4 b/min). ANP levels in plasma remained unaltered (53.4 +/- 24.9 fmol/ml). Replacement of 10 ml blood increased central venous pressure by 33% and ANP concentration in the plasma by nearly 30%, while heart rate and blood pressure remained unchanged. Our data indicate that the heart of the premature infant responded to acute blood replacement with increased ANP-release, while blood removal appeared not to influence hormone regulation.

Atrial natriuretic factor in patients with congenital heart disease: correlation with hemodynamic variables

To investigate the alpha-atrial natriuretic factor in congenital cardiac malformations, three groups of children, aged 7 months to 16 years, with different hemodynamic situations were studied during routine cardiac catheterization. Twenty-one (group I) had tetralogy of Fallot, 24 (group II) had a left to right shunt with pulmonary hypertension and 12 (control group) had a minor cardiac lesion. Alpha-atrial natriuretic factor levels were determined by a radioimmunoassay on blood samples from the inferior vena cava, right atrium, pulmonary artery, left atrium and aorta. To evaluate the effect of an acute volume load, measurements of hormone and pressures were repeated after right ventriculography. Alpha-atrial natriuretic factor levels varied over a wide range in all groups and in all chambers investigated. Nevertheless, children with pulmonary hypertension had significantly higher levels of the hormone (p less than 0.01) and were well separated from the control group, but less well from those with tetralogy of Fallot. A 50% increase of alpha-atrial natriuretic factor from the inferior vena cava to the right atrium occurred in patients with shunt lesions with pulmonary hypertension and in patients with tetralogy of Fallot (p less than 0.001) and a further 30% increase from the right atrium to the pulmonary artery (p less than 0.05). After right ventriculography, a 100% to 200% increase of alpha-atrial natriuretic factor was observed in the total sample (p less than 0.001). A positive correlation was observed between right atrial mean pressure and right atrial alpha-atrial natriuretic factor (r = 0.63) and between pulmonary artery mean pressure and pulmonary artery alpha-atrial natriuretic factor (r = 0.61).(ABSTRACT TRUNCATED AT 250 WORDS)

Endocrine control of electrolyte balance during development

The endocrine control of electrolyte balance during development is reviewed. It is suggested that the high urinary sodium excretion observed in premature infants may be secondary to the immaturity of the adrenal gland to adequately increase the secretion of aldosterone (Sulyok et al, 1979b), and to the inability of the distal tubule to respond appropriately to a rise in circulating aldosterone levels (Sulyok et al, 1979a). On the other hand, the elevated plasma aldosterone levels observed in term newborn infants may play an important role in the blunted response of the newborn kidney to saline loading (Sulyok et al, 1979a; Spitzer, 1982). The ability of ANP to induce a natriuresis and to contribute to fluid and electrolyte homeostasis during development has been investigated. It has been found that the immature kidney is less responsive to ANP than later in life (Chevalier et al, 1988; Robillard et al, 1988). On the other hand, it has been suggested that a rise in plasma ANP during the first five days of life may contribute to the physiological weight loss associated with the extracellular volume contraction occurring shortly after birth (Tulassay et al, 1987). The role of glucocorticoids, prostaglandins and the kallikrein-kinin system in regulating electrolyte balance during development is also reviewed.

Interrelationship between cortisol and atrial natriuretic factor in the immature ovine fetus

1. In chronically cannulated ovine fetuses (100-130 days of gestation) the infusion of cortisol (86.7 +/- 15 micrograms/h for 4 h) or human atrial natriuretic factor (ANF; 4.4 micrograms for 2 h) resulted in highly significant increases in the excretion of sodium, chloride, potassium and water. 2. Cortisol had no significant effect on fetal plasma ANF concentrations. All values are mean and s.e.m. Plasma immunoreactive ANF was 53 +/- 5 and 67.3 +/- 13 pmol/L in the 4 h saline infused fetuses, and 51.3 +/- 14.3 and 74 +/- 13.3 pmol/L in cortisol-infused fetuses (n = 7). A separate group of fetuses received 2 h infusions of saline or hANF (4.4 micrograms/h), and plasma IR-ANF values were measured (n = 3). The values, at 0, 60, 90 and 120 min were, respectively, 19.7 +/- 3, 17.3 +/- 0.7, 18.7 +/- 3.7 and 20.7 +/- 3.7 pmol/L in the saline infused group, and 25.3 +/- 5.3, 80.7 +/- 32.3, 123.3 +/- 4.3 and 100 +/- 15 pmol/L in the ANF-infused fetuses. 3. Blood cortisol concentrations, in fetuses infused for 4 h with 0.9% NaCl, were 3.1 +/- 0.8 nmol/L (n = 7); in fetuses infused with 0.9% NaCl for 2 h were 3.6 +/- 1 nmol/L (n = 3); in fetuses infused for 4 h with cortisol were 19.9 +/- 1.9 nmol/L (n = 7); and in fetuses infused with hANF for 2 h were 6.0 +/- 3.0 nmol/L (n = 5). 4. There was no effect of fetal hANF infusion on maternal or fetal blood aldosterone concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Atrial natriuretic peptide in heart and specific binding in organs from fetal and newborn rats

To assess the possibility that atrial natriuretic peptide plays a role in salt and water balance during early mammalian development, we examined hearts from fetal and neonatal rates for the presence of this peptide and presumed target tissues for their ability to bind the hormone. Immunohistochemistry was used to localize and radioimmunoassay to quantify this peptide in heart. Immunoreactive atrial natriuretic peptide was visualized in the fetal heart on day 17.5 post-conception. It was distributed throughout the atrial appendages and free wall and, in ventricle, in the trabeculae carnae and chordae tendineae. The concentrations of immunoreactive atrial natriuretic peptide in atria of rats on day 19.5 post-conception were one-tenth of those in the adult. Levels of this peptide in fetal ventricle were low and virtually absent from the adult tissue. Specific binding of radiolabelled atrial natriuretic peptide measured by whole organ counting occurred in several organs from 19.5-day fetal and neonatal rats. A number of these tissues, including the kidney, ileum, adrenal, lung and liver, are targets for and/or bind the peptide in adult rats. Specific binding in these tissues was localized using autoradiography at anatomical sites similar to those in adult organs. Specific binding was also seen in fetal but not neonatal skin. In the kidney, binding was associated with immature as well as mature glomeruli. These findings support the proposition that atrial natriuretic peptide may function in the perinatal rat as it does in the adult and, in addition, may play a unique role during fetal life.

Ultrastructural aspects of atrium development: demonstration of endocardial discontinuities and immunolabeling of atrial natriuretic factor in the Syrian hamster

The endocardium ultrastructure of 13 embryonic day old hamsters was examined, especially in relationship with the atrial myocytes. The endothelial morphology was described, including the junctional attachments and their relationships with subjacent atrial myocytes. Characteristic atrial myocytes organelles were identified: myofibrils, atrial granules, lipidic inclusions, and polysomes. Immunogold labeling demonstrated that atrial natriuretic factor (ANF)-containing granules were already present in the differentiating cardiomyocytes, even before the myofibrils were completely organized. At this stage of development, while the endothelium was a narrow barrier between the blood and the cardiomyocytes, it displayed fenestrations, but also epithelial discontinuities. In addition it also contains immunoreactive-ANF products. In light of the current knowledge about ANF processing it was proposed that the endocardium lining could be an obligated passageway for transport or activating proANF into ANF before its release into the blood stream. In addition the endocardial gaps could suggest that, until about 13 to 14 days of fetal development, heart atrial tissue could be more susceptible to the effects of pathogenetic compounds than in a later state of development.

Postnatal course of plasma levels of adrenocortical steroids in premature infants with and without NaCl supplementation

To assess the adrenocortical response of premature infants to alterations in sodium balance, the postnatal course of plasma progesterone, 11-deoxycorticosterone, corticosteronoe, aldosterone, 17-hydroxyprogesterone, 11-deoxycortisol, cortisol and cortisone was compared in healthy premature infants kept on low (1-2 mEq/kg per day) or high (3-5 mEq/kg per day) sodium diet. The mean birthweight (1470 g, range: 1210-1670 g vs 1410 g, range: 1130-1750 g) and mean gestational age (30.5 weeks, range: 29-32 weeks vs 30.2 weeks, range: 28-32 weeks) in the low and high sodium groups, respectively, were similar. Simultaneous steroid hormone measurements were made weekly up to the 5th week of life using mechanized Sephadex LH-20 multicolumn chromatography and standardized radioimmunoassays. It was demonstrated that in response to renal salt wasting and negative sodium balance there was a significant rise in plasma aldosterone concentration. The plasma levels of other individual corticosteroids generally declined with advancing age, the initial fall, however, was followed by a transient and insignificant but simultaneous increase in 11-deoxycortisol, cortisol, cortisone and corticosterone in prematures on low a sodium diet. This effect could be prevented by giving NaCl supplement. The NaCl-suppressible increase in adrenocortical activity may be the result of the combined effect of stress or angiotensin 11-induced adrenocorticotropic hormone (ACTH) release and/or prolactin-mediated enhanced adrenal response to ACTH.

Plasma atrial natriuretic polypeptide concentration in healthy children from birth to adolescence

We measured plasma atrial natriuretic polypeptide concentrations in the umbilical artery and vein, and peripheral veins of healthy children from birth to adolescence to establish the normal range. The plasma atrial natriuretic polypeptide concentration in the umbilical artery (mean +/- SD, 51.0 +/- 21.4 fmol/ml) was significantly higher than that in the umbilical vein (18.1 +/- 13.5 fmol/ml) in neonates after vaginal delivery. Also neonates aged 5 days or less had a significantly high concentration in the peripheral vein (60.7 +/- 29.4 fmol/ml). There was no significant difference in atrial natriuretic polypeptide concentrations in the peripheral veins between older children and adults. The concentrations in children aged more than 5 days and adults aged 20-34 years were 14.4 +/- 7.4 fmol/ml and 10.0 +/- 4.8 fmol/ml, respectively. However, the atrial natriuretic polypeptide concentration in the umbilical artery was not increased in three neonates delivered by caesarean section although they had a high concentration in the peripheral vein 24 hours after birth.

Atrial natriuretic peptide and patent ductus arteriosus in preterm infants

Preterm infants with symptomatic patent ductus arteriosus had considerably raised plasma concentrations of atrial natriuretic peptide. Surgical ligation of the patent ductus arteriosus was associated with an immediate fall in plasma atrial natriuretic peptide concentration. Thus left to right shunting and left atrial distension may cause atrial natriuretic peptide release in preterm infants with patent ductus arteriosus.