A study of the renin-angiotensin system in newborn infants

Normative values of renin and aldosterone in clinically stable preterm neonates

BACKGROUND

There is a paucity of literature on the normative levels of plasma renin concentration (PRC) and serum aldosterone (SA) in premature neonates. This study aims to provide normative data on PRC and SA levels in preterm neonates in the first 2 weeks after birth and explore associations with maternal, perinatal, or postnatal factors.

METHODS

Neonates born at 26- to 34-week gestation were recruited from two neonatal intensive care units in Canada and Australia. The direct renin assay PRC and SA were analyzed on day 1 and days 14-21 after birth to compare across categorical variables and to produce normative values.

RESULTS

A total of 262 subjects were enrolled from the Canadian (29%) and Australian (71%) sites. The mean gestational age was 30 weeks, with a mean birth weight of 1457 g. The normative values of PRC and SA for neonates born between 26 + 0 and 29 + 6 weeks and 30 + 0 and 34 + 0 weeks of gestation were produced for day 1 and day 14-21 after birth. Both PRC and SA increased from day 1 to day 14-21. The more premature neonates reached a higher PRC on days 14-21 after birth but exhibited lower SA levels on day 1 after birth. When comparing gender, birth weight, and maternal risk factor categories, no statistical differences in PRC or SA were found. A small but significant decrease in PRC, but not SA, was noted for neonates with placental pathology.

CONCLUSIONS

This study produced normative values of PRA and SA in clinically stable preterm neonates that can be referenced for use in clinical practice. A higher resolution version of the Graphical abstract is available as Supplementary information.

Phthalates cause a low-renin phenotype commonly found in premature infants with idiopathic neonatal hypertension

Since the 1970s, when the initial reports of neonatal hypertension related to renal artery thromboembolism were published, other secondary causes of neonatal hypertension have been reported. Those infants with no identifiable cause of hypertension were labeled with a variety of terms. Herein, we describe such infants as having idiopathic neonatal hypertension (INH). Most, but not all, of these hypertensive infants were noted to have bronchopulmonary dysplasia (BPD). More recently, reports described common clinical characteristics seen in INH patients, whether or not they had BPD. This phenotype includes low plasma renin activity, presentation near 40 weeks postmenstrual age, and a favorable response to treatment with spironolactone. A small prospective study in INH patents showed evidence of mineralocorticoid receptor activation due to inhibition of 11β-HSD2, the enzyme that converts cortisol to the less potent mineralocorticoid-cortisone. Meanwhile, phthalate metabolites have been shown to inhibit 11β-HSD2 in human microsomes. Premature infants can come in contact with exceptionally large phthalate exposures, especially those infants with BPD. This work describes a common low-renin phenotype, commonly seen in patients categorized as having INH. Further, we review the evidence that hypertension in INH patients with the low-renin phenotype may be mediated by phthalate-associated inhibition of 11β-HSD2. Lastly, we review the implications of these findings regarding identification, treatment, and prevention of the low-renin hypertension phenotype seen in premature infants categorized as having INH.

Characteristics of hypertension in premature infants with and without chronic lung disease: a long-term multi-center study

BACKGROUND

Many causes for neonatal hypertension in premature infants have been described; however in some cases no etiology can be attributed. Our objectives are to describe such cases of unexplained hypertension and to compare hypertensive infants with and without chronic lung disease (CLD).

METHODS

We reviewed all cases of hypertension in premature infants referred from 18 hospitals over 16 years. Inclusion criteria were hypertension occurring at <6 months of age and birth at <37 weeks gestation; the main exclusion criterion was known secondary hypertension. Continuous variables were compared using analysis of variance. Nominal variables were compared using chi-square tests.

RESULTS

A total of 97 infants met the inclusion criteria, of whom 37 had CLD. Among these infants, hypertension presented at a mean of 11.3 ± 3.2 chronological weeks of age and a postmenstrual age of 39.6 ± 3.6 weeks. Diagnostic testing was notable for plasma renin activity (PRA) being <11 ng/mL/h in 98% of hypertensive infants. Spironolactone was effective monotherapy in 51 of 56 cases of hypertension. Hypertension resolved in all infants, with an average treatment duration of 25 weeks. Significant differences between the two groups of infants were a 0.4 kg lower birthweight and a 2.5 weeks younger gestational age at birth in those with CLD (p < 0.01, p < 0.01, respectively). Hypertension presented in those with CLD 1.8 weeks later, but at the same postmenstrual age as those without CLD (p < 0.01, p = 0.45, respectively).

CONCLUSION

Premature infants with unexplained hypertension, with and without CLD, presented at a postmenstrual age of 40 weeks with low PRA, transient time course, and a favorable response to spironolactone treatment.

Neonatal fluid management

Perioperative fluid management in paediatrics has been the subject of many controversies in recent years, but fluid management in the neonatal period has not been considered in most reviews and guidelines. The literature regarding neonatal fluid management mainly appears in the paediatric textbooks and few recent data are available, except for resuscitation and fluid loading during shock and major surgery. In the context of anaesthesia, many neonates requiring surgery within the first month of life have organ malformation and/or dysfunction. This article aims at reviewing basic physiological considerations important for neonatal fluid management and mainly focusses on fluid maintenance and replacement during surgery.

Blood pressure and kidney size in term newborns with intrauterine growth restriction

CONTEXT AND OBJECTIVE

Low birth weight is associated with higher blood pressure in childhood and adulthood. The aim of this study was to investigate the influence of intrauterine growth restriction (IUGR) on newborn systolic blood pressure (SBP).

DESIGN AND SETTING

Prospective comparative study at Neonatal and Intensive in Clinical Pediatrics Division, Maternity Hospital in Hospital das Clínicas, Faculdade de Medicina da Universidade de São Paulo.

METHODS

35 newborns with IUGR and 35 without IUGR were compared. Healthy term newborns without malformations, with Apgar score at fifth minute > 6 were included. Birth weight, kidney weight/birth weight ratio, kidney weight (ultrasound scan), plasma renin activity (PRA) and SBP evolution were analyzed during the first month of life (on 1st, 3rd, 7th and 30th days).

RESULTS

SBP evolution, kidney weight/birth weight ratio and PRA did not differ between the two groups. In newborns with IUGR, SBP presented positive correlations with birth weight (r = 0.387 p = 0.026) and BMI (r = 0.412 p = 0.017) on the 7th day of life. Positive correlations with birth weight (r = 0.440 p = 0.01) and birth length (r = 0.386 p = 0.026) were also seen on the 30th day. There was an inverse correlation on the 7th day between SBP and kidney weight/birth weight ratio (r = -0.420 p = 0.014), but this did not persist to the end of the month.

CONCLUSIONS

IUGR seems not to have any influence on SBP, PRA or kidney weight among term newborns during their first month of life.

Ontogeny of renal renin in congenital hypothyroid rats

In the present study we have found age-related differences between the renal renin activity (RRA) and the immunoreactive renal renin (IRR) profiles during the neonatal development of the rat. RRA was markedly greater in newborn rats than in adult ones, while IRR was low at birth and progressively increased until adulthood. These observations suggest the existence of a control mechanism operating either at the level of the translation of the template or at a post-translational level which varies throughout the development. Since thyroid hormones have been demonstrated to affect several renin-angiotensin components, the neonatal RRA and IRR profiles in congenital hypothyroid rats were evaluated to determine whether renal renin activity or its synthesis could be endogenously regulated by thyroid hormones in the early stages of life. Although significant differences were observed in the RRA profiles of congenital hypothyroid and control rats, no changes were found in the relative amount of immunoreactive protein. These findings indicate that thyroid hormone deficiency does not directly affect renal renin expression during the critical period of the normal morpho-functional development of the newborn.

The renal lesions that develop in neonatal mice during angiotensin inhibition mimic obstructive nephropathy

BACKGROUND

Inhibition of angiotensin action, pharmacologically or genetically, during the neonatal period leads to renal anomalies involving hypoplastic papilla and dilated calyx. Recently, we documented that angiotensinogen (Agt -/-) or angiotensin type 1 receptor nullizygotes (Agtr1 -/-) do not develop renal pelvis nor ureteral peristaltic movement, both of which are essential for isolating the kidney from the high downstream ureteral pressure. We therefore examined whether these renal anomalies could be characterized as "obstructive" nephropathy.

METHODS

Agtr1 -/- neonatal mice were compared with wild-type neonates, the latter subjected to surgical complete unilateral ureteral ligation (UUO), by analyzing morphometrical, immunohistochemical, and molecular indices. Agtr1 -/- mice were also subjected to a complete UUO and were compared with wild-type UUO mice by quantitative analysis. To assess the function of the urinary tract, baseline pelvic and ureteral pressures were measured.

RESULTS

The structural anomalies were qualitatively indistinguishable between the Agtr1 -/- without surgical obstruction versus the wild type with complete UUO. Thus, in both kidneys, the calyx was enlarged, whereas the papilla was atrophic; tubulointerstitial cells underwent proliferation and also apoptosis. Both were also characterized by interstitial macrophage infiltration and fibrosis, and within the local lesion, transforming growth factor-beta 1, platelet-derived growth factor-A and insulin-like growth factor-1 were up-regulated, whereas epidermal growth factor was down-regulated. Moreover, quantitative differences that exist between mutant kidneys without surgical obstruction and wild-type kidneys with surgical UUO were abolished when both underwent the same complete surgical UUO. The hydraulic baseline pressure was always lower in the pelvis than that in the ureter in the wild type, whereas this pressure gradient was reversed in the mutant.

CONCLUSION

The abnormal kidney structure that develops in neonates during angiotensin inhibition is attributed largely to "functional obstruction" of the urinary tract caused by the defective development of peristaltic machinery.

Angiotensin induces the urinary peristaltic machinery during the perinatal period

The embryonic development of mammalian kidneys is completed during the perinatal period with a dramatic increase in urine production, as the burden of eliminating nitrogenous metabolic waste shifts from the placenta to the kidney. This urine is normally removed by peristaltic contraction of the renal pelvis, a smooth muscle structure unique to placental mammals. Mutant mice completely lacking angiotensin type 1 receptor genes do not develop a renal pelvis, resulting in the buildup of urine and progressive kidney damage. In mutants the ureteral smooth muscle layer is hypoplastic and lacks peristaltic movements. We show that angiotensin can induce the ureteral smooth muscles in organ cultures of wild-type, but not mutant, ureteral tissues and that, in wild-type mice, expression of both renal angiotensin and the receptor are transiently upregulated at the renal outlet at birth. These results reveal a new role for angiotensin in the unique cellular adaptations of the mammalian kidney to the physiological stresses of postnatal life.

Use of enalapril in neonatal hypertension

Treatment of hypertension with enalapril in a preterm infant is described. Enalapril is a new converting enzyme blocker with a longer plasma half-life and less side effects than captopril. Oral administration of enalapril 0.1 mg/kg caused severe hypotension and renal failure in a preterm infant. We recommend an oral starting dose of 0.01 mg/kg in preterm infants and extremely close monitoring of infants receiving the first dose of enalapril.

Age-related changes in the renin-aldosterone system. Physiological effects and clinical implications

Age-related changes in the renin-aldosterone system in normal humans are well documented. The most pronounced changes are observed at the extremes of life: plasma renin activity and plasma aldosterone levels are highest in the newborn, and lowest in the elderly population. There is a close temporal and directional relationship between the age-related decrease in plasma renin activity and the age-related decrease in plasma aldosterone. The renin-aldosterone system is also influenced by sex and race. The activation of the renin-aldosterone system in newborns and infants probably represents an important physiological mechanism designed to maintain positive sodium balance. The decreases in plasma renin activity and plasma aldosterone levels observed in elderly persons are usually only modest, and are not associated with clinical alterations in fluid or electrolyte metabolism. The superimposition of a disease process, or the injudicious prescription of a drug, inhibiting renin release or angiotensin II production, could theoretically facilitate sodium wasting in newborns or infants, or precipitate hyporeninaemic hypoaldosteronism in older adults. The primary clinical importance of age-related changes in the renin-aldosterone system relates to its impact on the proper classification of an individual's renin-aldosterone profile when attempting to diagnose a clinical condition (e.g. low, normal or high renin hypertension). This is particularly true for newborns, infants and children to age 4, and for adults entering the sixth decade of life.

Hypertension in infancy

Hypertension in infants is rare and requires a thorough evaluation. The incidence of hypertension in infancy has risen in recent years, reflecting both better monitoring methods and increasingly successful salvage of smaller and smaller newborns. Overall mortality and morbidity rates for uncontrolled hypertension in infants are unknown. With appropriate treatment, the prognosis for resolution of hypertension is good. In most cases, hypertension is short-lived and blood pressures return to normal even when medication is discontinued. Recent experience with improved antihypertensive agents in infancy has meant that nephrectomy for renovascular hypertension is rarely required. There is still much to learn about the indications for treatment of elevated blood pressures in infancy and the potential adverse effects of therapy. Infants with a history of neonatal hypertension should be followed closely because the long-term prognosis is not known and recurrence of hypertension remains a possibility. Because hypertension can develop in high-risk newborns following discharge from the nursery, these infants deserve routine blood pressure measurements as part of their outpatient follow-up.

Mechanisms regulating renal sodium excretion during development

The present review focuses on the ontogeny of mechanisms involved in renal sodium excretion during renal maturation. The effect of birth on renal excretion of sodium and the role played by the different tubular segments in the regulation of sodium excretion during maturation are discussed. The influence of circulating catecholamines and renal sympathetic innervation in regulating sodium excretion during renal development is reviewed. The effects of aldosterone, atrial natriuretic factor, and prostaglandins on sodium regulation during renal maturation are discussed. Special emphasis is given to the potential role of glucocorticoids in modulating sodium excretion early in life.

Effects of the mode of delivery on ANP and renin-aldosterone system in the fetus and the neonate

Plasma ANP and aldosterone levels, plasma renin activity (PRA), haematocrit, systolic and diastolic blood pressure (BP), were evaluated in 15 full-term infants delivered by elective Caesarean section (CS group) and in 15 full-term infants delivered vaginally (vaginal group). The mode of delivery did not influence the cord blood levels of ANP and their increase at the 24th hour of life. Instead, PRA was lower and plasma aldosterone levels were higher in the CS group than in the vaginal group. Also haematocrit and BP were influenced by mode of delivery. The haematocrit values were lower in the CS group than in the vaginal group at birth as well at the 24th hour of life. The values of systolic and diastolic BP were the same in both groups, at birth, but at the 24th hour of life increases were observed only in the vaginal group. On the fourth day of life weight loss was the same in both groups. Our findings suggest that the mode of delivery has more influence on neonatal BP adaptation than on neonatal volume homeostasis.

Atrial natriuretic peptide and renin-aldosterone relationship in healthy newborn infants

Plasma atrial natriuretic peptide (ANP) and aldosterone concentrations, and plasma renin activity (PRA) were measured by radioimmunoassay concurrently in 20 healthy full term infants, in cord blood, at 24 hours after birth and on the 4th day of life. ANP and aldosterone increased significantly at 24 hours and was persistently elevated on the 4th day of life. PRA remained unchanged during the first four days of life. In cord blood, ANP concentrations were correlated with aldosterone concentrations (r = 0.49, p less than 0.05) and hematocrit (r = 0.58, p less than 0.02). At the 24 hours of life, plasma ANP concentrations were correlated with weight loss observed on the 4th day of life (r = -0.70, p less than 0.005), while the percentage changes in plasma aldosterone concentrations were correlated with percentage changes in systolic blood pressure (BP) (r = 0.49, p less than 0.05). These findings suggest that during the early newborn period ANP and aldosterone act as an integrated system which has a role in regulation blood pressure and intravascular volume homeostasis.

Factors influencing plasma renin and renin substrate in premature infants

Concentrations of plasma renin (PRC) and plasma renin substrate (PRS) were measured during the first week of life in 52 infants born at less than 37 weeks' gestation (mean (SEM) gestation 30 (0.4) weeks, mean (SEM) birth weight 1.35 (0.08) kg). Both PRC (median 35, interquartiles 16.3, 94.5 ng/ml/hour) and PRS (median 2.3, interquartiles 1.3, 5.0 micrograms/ml) were raised compared with adults. The proportional rise in PRC was much greater than that in PRS, suggesting that PRS may be rate limiting in the generation of angiotensin I. Log10 PRC was inversely proportional to gestational age and a high urinary sodium loss was associated with a significantly raised log10 PRC. In hypoxaemic infants, there was a strong correlation between log10 PRS and haemoglobin concentration; this is a new observation in human infants but consistent with data available from other species.

Ontogeny of angiotensinogen mRNA and angiotensin II receptors in rat brain and liver

The renin-angiotensin-system (RAS) is active in fetal and neonatal life. This study was undertaken to examine the ontogenic regulation of angiotensinogen (AT) gene expression and angiotensin II (A II) receptors in liver and brain. AT gene expression was studied in fetal, neonatal, adult and aged rats, using slot blot hybridization to quantify AT mRNA levels. During fetal life (gestational days 15-20), AT mRNA was more abundant in brain than in liver. Soon after birth, brain AT mRNA levels increased to a concentration 3 fold above fetal levels. In contrast, liver AT mRNA abundance increased 30-fold within 12 h of birth. Aging (3-20 months) resulted in a gradual decrease in AT mRNA in both the brain and liver. Liver A II receptors in the neonate were 2-fold higher than in the fetus, but returned to fetal levels by 8 weeks of age. In the brain, A II receptor abundance increased to a level 75% above fetal levels in 7 days old animals, but returned to fetal levels by 14 days of age. These studies suggest than in the fetus, the liver is not the primary source of AT but that unknown factors at parturition result in a dramatic increase in liver AT mRNA. In contrast, the more modest increases in brain AT mRNA parallel the gradual maturation of the CNS. In both tissues, further aging resulted in a gradual decrease in AT mRNA, reflecting either increased sensitivity to feedback downregulation by A II or age related increases in other extrahepatic sites of AT synthesis. Age related changes were also found in the A II receptor in both the liver and brain.

Importance of angiogenic action of angiotensin II in the glomerular growth of maturing kidneys

We studied the effect of three antihypertensive drugs on the growth of glomeruli in four- to five-week-old Munich-Wistar rats (N = 24), which were undergoing rapid maturation processes. Young rats were given an angiotensin converting enzyme inhibitor (ACEI, enalapril, 50 mg/liter drinking water), verapamil (50 mg/liter) or hydralazine (80 mg/liter) or no treatment for six weeks. Body weight increased comparably in the treatment groups and age-matched controls, reaching on average 197 +/- 11, 214 +/- 12 and 198 +/- 3 g in ACEI-, verapamil- and hydralazine-treated rats, respectively, versus 218 +/- 6 g in control rats. Glomerular hemodynamic patterns, including glomerular capillary pressure, measured in maturing rats after one and six weeks of ACEI treatment were unaffected by ACEI. Mean planar area of glomeruli (PAmean) achieved was smaller than control in ACEI rats (6.60 +/- 0.20 x 10(-3) mm2 vs. 5.37 +/- 0.22, respectively, P less than 0.005), but not in rats treated with other antihypertensive drugs. Furthermore, the maturational PAmean increase in rats given ACEI for six weeks was, on average, only half of that achieved by age-matched controls not given ACEI, in contrast to normal maturational growth with hydralazine or verapamil (29% increase in PAmean from normal baseline in ACEI vs. 52%, 53% and 59% increases in verapamil, hydralazine and control, respectively). In contrast, comparable PAmean values were found in adults with (7.08 +/- 0.22 x 10(-3)mm2, N = 6) and without (6.98 +/- 0.33 x 10(-3)mm2, N = 6) ACEI treatment given for six weeks. Therefore, ACEI, but not verapamil and hydralazine, causes growth retardation in maturing glomeruli.(ABSTRACT TRUNCATED AT 250 WORDS)

Renin and angiotensin converting enzyme concentrations in the fetal and neonatal guinea-pig

1. Plasma renin concentration (PRC) and plasma and pulmonary angiotensin converting enzyme (ACE) concentration were measured in fetal and neonatal guinea-pigs from 45 days gestational age (GA) until 21 days post-partum. 2. Fetal PRC increased towards term to reach values greater than those measured in normal adult males. Pentobarbitone anaesthesia of the mother resulted in significant elevation of fetal PRC after 66 days GA but not before this time. 3. PRC were very high in the newborn guinea-pig, decreased rapidly during the first 24 h after birth and then more gradually, to reach approximately adult values by day 21. 4. Fetal plasma ACE concentration increased towards term to reach values greater than those measured in adult males and decreased subsequently. 5. Pulmonary ACE concentrations were very low throughout gestation but increased considerably between days 3 and 14 post-partum. Low concentrations of ACE were measured in other fetal tissues but placental concentrations were relatively high. 6. Propranolol (0.1 mg I.P.) or saline was administered (under halothane-nitrous oxide anaesthesia) to fetuses of litters of various GA from 55 days to term. Fetal PRC were measured 3 h later. Propranolol treatment resulted in significantly lower fetal PRC than saline treatment in litters aged 63 days to term but not in younger litters. 7. These data indicate that the renin-angiotensin system is functional in the fetal guinea-pig during the last third of gestation. Fetal plasma renin concentrations near term are greater than those measured in normal adult males. This may, in part, reflect an increased influence of the fetal sympathetic nervous system.

Comparison of plasma renin and prorenin in healthy infants and children as determined with an enzymatic method and a new direct immunoradiometric assay

We compared plasma active and inactive renin (prorenin) concentrations and activities in infants and children, as measured with a newly available direct immunoradiometric assay kit for active renin and a widely used enzymatic renin assay (plasma renin activity). The study was performed in 57 healthy infants and children under steady-state conditions and in eight subjects after orthostatic stimulation. Our study; (i) reports concentrations for active and inactive renin determined by the immunoradiometric assay in normal infants and children; (ii) confirms, by use of the immunoradiometric assay, the previously described decrease in plasma active and inactive renin, as measured by the enzymatic assay, with increasing age; and (iii) shows close correlations between the results obtained by immunoradiometric and enzymatic assays for active and inactive renin in normal infants and children.

Plasma renin activity and albumin excretion in teenage type I diabetic subjects. A prospective study

Plasma renin activity (PRA) may be high among teenage and young adult insulin-dependent diabetic subjects. Supine PRA and stimulated PRA were therefore measured in 50 female and 50 male diabetic subjects, 13-20 years old, diagnosed before the age of 16. Fifty percent have been restudied after 4.6 +/- 0.2 (mean +/- SEM) years. Initially, 43% had high PRA (supine 4.0 +/- 0.37, stimulated 12.02 +/- 0.8 ng/ml/hr angiotensin I), 45% had normal activity (supine 2.89 +/- 0.26, stimulated 6.47 +/- 0.34 ng/ml/hr/angiotensin I), and 12% had low activity (supine 1.57 +/- 0.05, stimulated 3.09 +/- 0.08 ng/ml/hr/angiotensin I). Levels were directly associated with prepubertal duration of diabetes and were inversely associated with duration of diabetes after onset of puberty but not with total duration or patient age. Within 4.6 +/- 0.2 years the percentage of subjects with high PRA fell to 13%, and the percentage of those with low PRA rose to 35%. Initially 51% of the cohort had normal albumin excretion rates (AER) at rest and during exercise equal to or less than 10 micrograms/min/m2; 32% had elevated rates only during exercise of 39 +/- 5 micrograms/min/m2; 13% had elevated rates at rest of 41 +/- 8 micrograms/min/m2 and during exercise of 116 +/- 21 micrograms/min/m2; and 4% had clinical proteinuria at rest and during each exercise period equal to or greater than 150 micrograms/min/m2. After 5 years, 58% continued to have normal AER, or their AER improved.(ABSTRACT TRUNCATED AT 250 WORDS)

Plasma renin activity in infants with congenital heart disease

Plasma renin activity was estimated in 11 infants with severe congestive heart failure. The infants had congenital heart disease with left to right shunts and were receiving diuretic treatment. Plasma renin activity was measured by radioimmunoassay of generated concentrations of angiotensin I. The mean (SD) plasma renin activity was 84 (21) ng angiotensin I/ml/hour, which is considerably above normal infant values. A hyperactive renin-angiotensin system may be detrimental in these patients. Angiotensin converting enzyme inhibitors may be of value in treating infants with severe congestive heart failure.

The renin-angiotensin-aldosterone system in infancy and childhood in basal conditions and after stimulation

Plasma renin activity (PRA), aldosterone (PA), sodium and potassium concentration were measured in 107 healthy infants and children under basal conditions of normal diet and recumbency. Urinary aldosterone (UAldo), sodium and potassium were also measured (n = 51). A significant (P less than 0.001) age-related decrease in PRA (r = -0.67), PA (r = -0.67), UAldo (r = -0.56) was observed, with a striking scatter of values especially in infancy. The reninangiotensin-aldosterone system (RAAS) was also studied after stimulation by standardised sodium restriction during 4 days, followed by acute postural change (n = 40). After salt restriction a rise of PRA and UAldo was noted, but a rise in PA could not be demonstrated in children aged 0-6 months. The influence of postural change on the RAAS seems more important in older children. The reported values not only in basal but also in stimulated conditions allow study of the RAAS in diseases such as salt loss and hypertension.

Effects of maternal ritodrine administration on neonatal renal function

Because of its effects on the cardiovascular and renin-angiotensin systems and on fluid and electrolyte homeostasis, maternal administration of ritodrine to inhibit preterm labor may cause significant alterations in renal function in the newborn infant. We determined inulin clearance, plasma renin activity, urinary arginine vasopressin excretion, and serum and urine electrolyte concentrations and osmolalities at 12 to 36 hours of life and at 6 days of life in 15 infants whose mothers had received ritodrine and in 15 infants whose mothers did not (control infants). At the time of each study, plasma ritodrine concentrations were obtained in the infants whose mothers received ritodrine. The infants whose mothers had received ritodrine had significantly lower inulin clearances and higher plasma renin activity and urinary arginine vasopressin excretion on day 1 but not on day 6. Gestational age was inversely correlated with plasma ritodrine concentration, plasma renin activity, and urinary arginine vasopressin excretion. There were no overt clinical signs of renal failure in any of the infants, and no differences in serum and urine electrolyte values, osmolality, fractional sodium excretion, or urine flow rate were observed between the groups.

Studies on the effect of mode of delivery on the renin-angiotensin system in mother and fetus at term

Plasma renin substrate (PRS) and renin (PRC) concentrations were measured in the paired umbilical artery and vein blood of 54 babies and in the venous blood of 23 of their mothers at vaginal delivery. Similar samples were collected from 31 babies and their mothers at elective caesarean section. Fetal renin concentrations in both umbilical artery and vein blood were markedly increased at vaginal delivery compared with values at operative delivery. Renin substrate concentration was correlated with the length of second stage of labour. Umbilical vein PRC and PRS were greater than corresponding umbilical artery values at vaginal, but not at operative delivery, suggesting that some factor associated with delivery might stimulate the placenta to produce renin and renin substrate during vaginal delivery. Maternal and umbilical concentrations of both renin and renin substrate were unrelated, supporting the view that the maternal and fetal compartments are independent. It is suggested that hyperactivity of the renin-angiotensin system may be an integral part of the marked changes in cardiovascular homeostasis which occur at birth.

Plasma and adrenal aldosterone levels in premature mice at birth and during neonatal development

The concentrations of aldosterone in the plasma and adrenal glands, the concentrations of sodium and potassium in the plasma and the hematocrit were estimated from birth to day 6 after birth in premature mice removed by Caesarean section on day 19 of pregnancy in comparison with newborn mice delivered spontaneously vaginally on day 20 of pregnancy. In premature mice, the plasma aldosterone concentrations increased twice: at birth after reanimation, then at 6 h after birth. The first increase at birth resulted probably from ACTH stimulation. Several factors could be involved in the peak at 6 h after birth: ACTH stimulation, the decrease in the level of sodium in the plasma and the increase in the hematocrit due to kidney immaturity of premature mice. The results suggest that the renin-angiotensin-aldosterone system is able to respond to stimulations in the first 6 h after birth in premature mice. The rise in the level of plasma aldosterone which has been found at birth in newborns delivered spontaneously vaginally on day 20 of pregnancy (control animals) did not result from variations of plasma electrolytes, plasma volume and ACTH; this rise has been induced by labor of the parturition which caused the aldosterone release from adrenal glands.

The role of the kidney in sodium homeostasis during maturation

Evidence is presented that the retention of sodium observed during development is consequent primarily to enhanced tubular reabsorption rather than to low rates of glomerular filtration. The enhanced transport of sodium occurs in nephron segments located beyond the proximal tubule, apparently under the stimulation of the high plasma concentration of aldosterone. This adaptive mechanism may account for the fact that the infant thrives on a rather low intake of sodium, as prevails during the period of breast-feeding. The renin-angiotensin-aldosterone system cannot be fully inhibited even by intravascular volume expansion and this may account for the blunted natriuretic response of the developing animal and human to the acute infusion of saline or albumin solutions. Conversely, the renal sodium loss and the hyponatremia often encountered in premature babies appear to be due to an insufficient rise in aldosterone secretion or to a limited responsiveness of the distal tubule to aldosterone stimulation.

Importance of plasma angiotensin concentrations in a comparative study of responses to angiotensin in the maturing newborn lamb

Plasma angiotensin concentrations were measured in a longitudinal study of the vascular, renal, and adrenal responses to infusions of angiotensin II (AII) in the maturing newborn lamb. Basal plasma concentrations of angiotensin increased with age and correlated with the rising arterial pressure that occurred with maturation. However, age was a stronger determinant of arterial pressure than was plasma angiotensin concentration. For any given dose of AII per kilogram of body weight, the actual plasma angiotensin concentration achieved increased as the lambs matured and gained weight. Therefore, a comparative study of biologic responses to AII in maturing animals must be based on actual plasma angiotensin concentrations achieved rather than on dose of AII infused per kilogram of body weight. When analyzed on the basis of actual plasma angiotensin concentration, the increase in arterial pressure and the suppression of plasma renin activity in response to increasing plasma angiotensin concentrations did not differ significantly as the lambs matured. However, the increment in plasma aldosterone concentrations in response to increasing plasma angiotensin concentrations was diminished in immature lambs (less than 18 days) when compared to the aldosterone responses in the same lambs at older ages.

Sodium, potassium and age: possible determinants of plasma renin activity and aldosterone during childhood (age 4-16)

The renin-angiotensin-aldosterone system was studied in fifty healthy children aged 4-16 years under normal sodium and potassium intake. The plasma renin activity (PRA) and plasma aldosterone (PA) decreased with age: r = -0.30, P less than 0.05 for plasma renin activity and r = -0.33, P less than 0.05 for plasma aldosterone. Significant negative correlation was obtained between plasma renin activity and the 24-h urinary sodium excretion; r = -0.40, P less than 0.01. This relationship remained significant when the daily urinary sodium excretion was corrected for 1.73 m2 body surface area (BSA); r = -0.40, P less than 0.01. Using the multivariance analysis, plotting the plasma renin activity against the two combined parameters (24-h urinary sodium excretion and age), no improvement was obtained (r = 0.38, P greater than 0.05). This finding suggests that during childhood, sodium rather than age has a major modulatory role on plasma renin activity. With advancing age the plasma aldosterone showed a significant positive correlation coefficient with plasma renin activity(r = 0.29, P less than 0.05). Multivariance analysis between plasma aldosterone and the two combined parameters, Plasma renin activity and age, significantly improved the correlation coefficient (r = 0.42, P less than 0.05) suggesting that both plasma renin activity and age play a dominant modulatory role in the control of plasma aldosterone during childhood. Neither 24-h urinary sodium excretion, nor 24-h urinary potassium excretion, improved the multiple correlation coefficient with plasma aldosterone when added to plasma renin activity and age.

Renin immunocytochemistry of the differentiating juxtaglomerular apparatus

The differentiation of the juxtaglomerular apparatus in fetuses and newborn mice was investigated by renin immunocytochemistry and electron microscopy. Three to four days before delivery and prior to other organs renin was found in the fetal kidney. At this early time immunoreactivity was preferentially located in cells of the media of interlobular arteries. In newborn mice the formation of new nephrons and maturation of their glomeruli was accompanied by a shift in renin localization from the interlobular arteries to the afferent arterioles. At the same time, kidney renin content and concentration increased rapidly. Synchronously with renin immunoreactivity, during the capillary loop stage of glomerular development, granulated epitheloid cells became visible in the afferent arteriole.

Renin and angiotensin levels in children

Plasma renin activity, plasma renin concentration, and angiotensin II levels were measured in 63 normal children aged between 2 months and 12 years. The results showed that the high levels of renin and angiotensin II present in infancy remained above adult levels throughout the first decade of life but that there was a decline with age. Boys less than 8 years old had lower plasma renin activity and angiotensin II levels than girls of a similar age; this may be due to a relative substrate deficiency. Serum urea levels were inversely correlated with plasma renin activity in both sexes. A significant inverse relationship was found between both plasma renin activity and angiotensin II, and serum sodium in the girls; a similar, although not statistically significant, relationship was seen with plasma renin activity in the boys. An inverse correlation was found between plasma renin concentration and diastolic blood pressure for the group as a whole.

Decreased vascular and increased adrenal and renal sensitivity to angiotensin II in the newborn lamb

The effects of two doses of angiotensin II (0.025 micrograms/kg per min and 0.25 micrograms/kg per min) on plasma renin activity (PRA), aldosterone concentration, creatinine clearance, and sodium and potassium excretion were studied in two groups of 2- to 3-week-old lambs. The 0.025 micrograms/kg per min dose of angiotensin II also was studied in six adult ewes. In response to the angiotensin II infusions, PRA decreased in the newborns from baselines of 24.5 +/- 2.3 ng/ml per hr (mean +/- SE) and 22.9 +/- 3.5 to 13.6 +/- 0.8 and 13.0 +/- 2.2 at 30 minutes (P < 0.01), respectively, and returned to baseline at 60 minutes; PRA decreased from 1.7 +/- 0.1 to 0.95 +/- 0.3 ng/ml per hr at 30 minutes (P < 0.01) in the ewe and returned to baseline at 60 minutes. Plasma aldosterone increased in the newborns from baselines of 17.4 +/- 5.0 ng/dl and 14.7 +/0 3.9 to 33.1 +/- 6.9 and 32.5 +/- 6.3, respectively, at 15 minutes (P < 0.01) and returned to baseline at 60 minutes. Plasma aldosterone increased from 4.3 +/- 0.7 to 9.2 +/- 2.0 ng/dl in the ewe, and returned to baseline at 60 minutes. The change in the PRA and aldosterone responses from baseline to peak for the low and high angiotensin dose was similar in the newborn lambs and greater than in the ewe (P < 0.01). There was no change in the creatinine clearance, plasma sodium, or hematocrit. Urine sodium excretion increased from 0.16 +/- 0.04 total mEq/30 min to 0.87 +/- 0.27 (P < 0.05) in the newborns during the 0.25 micrograms/kg per min angiotensin II infusion. We conclude that, under basal conditions, the newborn lamb has high PRA and aldosterone levels with decreased pressor and increased aldosterone and renin release responsiveness to angiotensin II compared to the adult.

Renin-angiotensin-aldosterone system

There is increased activity of the renin, angiotensin, aldosterone (RAA) system in infancy and childhood. An inverse relationship between plasma renin, aldosterone and age has been demonstrated. In childhood hypertension due to renovascular disease or pyelonephritic scarring peripheral plasma renin is increased. Renal vein renin measurements in children with renal hypertension have proved valuable in predicting surgical curability of the underlying lesion. The upper limit of normal for the renal venous renin ratio in normotensive children without renal disease is 1.5. Pharmacological blockade of the RAA system has a place in diagnosis and treatment of hypertensive children. The plasma renin aldosterone profile is diagnostically useful in the investigation of salt-wasting disease and can easily distinguish between aldosterone biosynthetic defects and pseudohypoaldosteronism.

Postnatal development of renal sodium handling in premature infants

To estimate the contribution of the specific defect in proximal and distal tubular reabsorption of sodium to renal salt wasting, fractional sodium excretion, distal tubular sodium delivery, and distal tubular sodium reabsorption were determined in 11 healthy premature infants. The study was performed on the seventh day and at weekly intervals thereafter up to the sixth week of life. Sodium clearance and fractional sodium excretion decreased significantly with increasing postnatal age (P less than 0.001). There was no significant alteration in either osmolar or free-water clearances. Distal tubular sodium delivery steadily decreased from 4.96 +/- 0.66 (mean +/- SE) in the first week to 3.3 +/- 0.41 ml/minute/dl GFR in the sixth week of life (P less than 0.05). Distal tubular sodium reabsorption was 69.5 +/- 2.36% in the first week, then rose significantly to reach a value of 83.7 +/- 1.85% in the second week (P less than 0.001) and remained practically unchanged thereafter. It is suggested that the rapid improvement of distal tubular sodium reabsorption in premature infants might result from forced stimulation by the excessively activated renin-angiotensin-aldosterone system.

Plasma renin activity and plasma aldosterone concentration in infants with various sodium intakes

Plasma renin activity (PRA) and plasma aldosterone concentration (PA) were measured in 20 healthy infants 2-12 months of age, after taking a commercial modified cow's milk formula with 8.2 mmol/l sodium for at least five days, and after at least five days on a home-made, undiluted acidified cow's milk formula with 12.6 mmol/l sodium. The mean sodium intakes were 1.20 and 1.87 mmol/kg of body weight per day respectively. The arithmetic means for the corresponding PRA were 6.16 and 1.08 ng/ml . h (difference not significant) and the corresponding arithmetic means for PA were 603 and 355 pg/ml, the difference being highly significant (P less than 0.01). It is concluded that one has to consider the sodium intake in the milk formula when studying plasma aldosterone in infancy.