Measurement of plasma renin activity by semi-micro radioimmunoassay of generated angiotensin I

Tailored Assays for Pharmacokinetic and Pharmacodynamic Investigations of Aliskiren and Enalapril in Children: An Application in Serum, Urine, and Saliva

OBJECTIVES

Drugs that are effectively used to treat hypertension in adults (e.g., enalapril) have not been sufficiently investigated in children. Studies required for pediatric approval require special consideration regarding ethics, study design, and conduct and are also associated with special demands for the bioanalytic method. Pediatric-appropriate assays can overcome these burdens and enable systematic investigations of pharmacokinetics and pharmacodynamic in all pediatric age groups.

METHODS

Tailored assays were developed for pharmacokinetic investigation of a drug in 100 μL of serum, saliva, and urine. All assays were applied in a proof-of-concept study to 22 healthy volunteers who had been given 300 mg aliskiren hemifumarate or 20 mg enalapril maleate and allowed for dense sampling. Changes in humoral parameters of the renin-angiotensin-aldosterone system were also evaluated with 6 parameters in 2.1 mL blood per time point.

RESULTS

The pharmacokinetic results of aliskiren and enalapril obtained by low-volume assays in serum and urine were comparable to that noted in the literature. The dense sampling enabled very detailed concentration-time profiles that showed high intersubject variability and biphasic absorption behavior of aliskiren. The replacement of invasive sampling by saliva collection appears inappropriate for both drugs because the correlations of drug concentrations in both fluids were low. A low-volume assay was also used to determine values for in the renin-angiotensin-aldosterone system and to compare those results with the published literature.

CONCLUSION

These results support both the use of low-volume assays in pediatric research and the systematic investigation of their use in neonates and infants. Use of this assay methodology will increase information about drug pharmacokinetics and pharmacodynamics in this vulnerable population and might contribute to safe and effective use of pharmacotherapy.

Severe loss-of-function mutations in the adrenocorticotropin receptor (ACTHR, MC2R) can be found in patients diagnosed with salt-losing adrenal hypoplasia

OBJECTIVE

Familial glucocorticoid deficiency type I (FGD1) is a rare form of primary adrenal insufficiency resulting from recessive mutations in the ACTH receptor (MC2R, MC2R). Individuals with this condition typically present in infancy or childhood with signs and symptoms of cortisol insufficiency, but disturbances in the renin-angiotensin system, aldosterone synthesis or sodium homeostasis are not a well-documented association of FGD1. As ACTH stimulation has been shown to stimulate aldosterone release in normal controls, and other causes of hyponatraemia can occur in children with cortisol deficiency, we investigated whether MC2R changes might be identified in children with primary adrenal failure who were being treated for mineralocorticoid insufficiency.

DESIGN

Mutational analysis of MC2R by direct sequencing.

PATIENTS

Children (n = 22) who had been diagnosed with salt-losing forms of adrenal hypoplasia (19 isolated cases, 3 familial), and who were negative for mutations in DAX1 (NR0B1) and SF1 (NR5A1).

RESULTS

MC2R mutations were found in three individuals or kindred (I: homozygous S74I; II: novel compound heterozygous R146H/560delT; III: novel homozygous 579-581delTGT). These changes represent severely disruptive loss-of-function mutations in this G-protein coupled receptor, including the first reported homozygous frameshift mutation. The apparent disturbances in sodium homeostasis were mild, manifest at times of stress (e.g. infection, salt-restriction, heat), and likely resolved with time.

CONCLUSIONS

MC2R mutations should be considered in children who have primary adrenal failure with apparent mild disturbances in renin-sodium homeostasis. These children may have been misdiagnosed as having salt-losing adrenal hypoplasia. Making this diagnosis has important implications for treatment, counselling and long-term prognosis.

Nitric oxide synthase inhibitors and hypertension in children and adolescents

OBJECTIVE

To establish the role played by the circulating nitric oxide synthase inhibitors N(G)-monomethyl-L-arginine (L-NMMA), asymmetrical dimethyl arginine (ADMA) and symmetric dimethyl arginine (SDMA) and its association with hypertension of children and adolescents.

DESIGN

We measured plasma concentrations of L-NMMA, ADMA and SDMA in 38 hypertensives (median age 7.7 years) and in nine healthy normotensive controls (median age 8.2 years) using high-performance liquid chromatography. In addition, their plasma renin activity was determined. The subjects' glomerular filtration rates were calculated from plasma creatinine and height measurements. To determine the vasoactive potency of the arginine analogues, concentration-response curves were plotted for the responses in isolated endothelium-intact and endothelium-denuded mouse aortic rings that had been pre-contracted by administration of a threshold concentration of phenylephrine.

RESULTS

Plasma ADMA and SDMA concentrations in members of the hypertensive group [0.23 +/- 0.03 and 1.37 +/- 0.06 micromol/l, respectively (means +/- SEM)] were significantly higher than those in members of the control group (ADMA 0.10 +/- 0.01 micromol/l and SDMA 1.18 +/- 0.06 micromol/l). Plasma concentrations of L-NMMA were similar in members of the hypertensive (0.21 +/- 0.01 micromol/l) and control (0.18 +/- 0.02 micromol/l) groups. The glomerular filtration rate of the hypertensive group was below normal [70.4 +/- 5.4 ml/min per 1.73 m2 (mean +/- SEM)] and was significantly associated with elevated plasma concentrations of ADMA (r = -0.77, P < 0.001), SDMA (r = -0.38, P = 0.02) and L-NMMA (r = 0.35, P = 0.03). Higher plasma ADMA concentrations were associated with a lower plasma renin activity (r = -0.36, P = 0.04). The vasoactive potencies of ADMA (concentration for half-maximal effect with the endothelium intact 25.4 +/- 7.1 micromol/l) and L-NMMA (concentration for half-maximal effect with the endothelium intact 8.2 +/- 2.9 micromol/l) was significantly (P < 0.05) greater than that of SDMA. Both ADMA and L-NMMA (at 3 micromol/l concentrations) initiated a significant vasocontractile response from baseline (P = 0.03 and P < 0.001, respectively). These effects were absent after the endothelium had been removed. SDMA had no effect.

CONCLUSIONS

Plasma ADMA and SDMA levels are increased in hypertensive children. By inference from in-vitro data, ADMA appears to attain sufficient concentrations to produce a significant change in vascular tone and hence might play a role in the pathophysiology of childhood hypertension.

Tubular proteinuria in reflux nephropathy: post ureteric re-implantation

We studied urine protein excretion in 55 adults with reflux nephropathy (median age 26.9 years) who had had normal blood pressure, renal function and ureteric reimplantation in childhood. Urine retinol binding protein (RBP), N-acetyl-beta-D-glucosaminidase (NAG), albumin, bacteriuria, systolic blood pressure, glomerular filtration rate (GFR), peripheral plasma renin activity (PRA) and the degree of renal scarring were measured in each subject; 20 had bilateral and 35 unilateral renal scarring; 5 were hypertensive and none were in renal failure. Urinary NAG and RBP excretions were significantly greater in the study group than in 34 healthy controls (median age 29.7 years). Within the study group, NAG excretion significantly correlated with PRA (P = 0.02). RBP excretion correlated with PRA, systolic blood pressure and the laterality (bilateral vs. unilateral) of scarring (P < 0.01). Urinary albumin excretion correlated with systolic blood pressure (P = 0.03). We conclude that increased urinary protein, especially NAG and RBP excretion, occur late after ureteric re-implantation in reflux nephropathy independent of GFR. Its association with PRA supports the concept of segmental perfusion and filtration as an important mechanism that may explain the above findings.

15-year follow-up of renin and blood pressure in reflux nephropathy

BACKGROUND

Beginning in 1978 a cohort of patients with reflux nephropathy first seen at a London Childrens hospital have had 5-yearly follow-ups. This is the fourth (15-year) report from that series.

METHODS

Of the original 100 normotensive children with reflux nephropathy 78 were traced for the 15-year study in 1994. Five patients were excluded because of nephrectomy, ten for other reasons, and eight refused to take part, leaving 55. 26 were on oral contraceptives. Supine blood pressure and plasma renin activity (PRA) were measured, and daily sodium excretion was assessed on a sample of overnight urine.

FINDINGS

Of the 55 patients (15 male, 40 female, median age 27 years, range 20-31), five had systolic and two had diastolic hypertension. Compared with the 10-year (1988) follow-up there was no change in blood pressure standard deviation scores (SDS) in this cohort. PRA showed an increasing dissociation from controls after 15 years of age and was significantly above that of controls by age 25. Exclusion of the patients on oral contraceptives did not significantly alter the results. The PRA values in 1988 were not individually predictive of the development of hypertension over the ensuing 5 years.

INTERPRETATION

Previously, in the long-term study of reflux nephropathy, blood pressure SDS had progressively increased with age. By 15 years blood pressure had levelled out and the PRA, though raised, did not predict the development of hypertension. Oral contraceptive use did not significantly modify the results.

Atypical (non-diarrhea-associated) hemolytic-uremic syndrome in childhood

We describe the clinical and laboratory features of 20 children who were seen during the past 20 years with idiopathic nondiarrhea-associated hemolytic-uremic syndrome. There was no seasonal variation in time of onset; a genetic pre-disposition seemed likely in two of the cases. The prodromal illness was nonspecific and by definition did not include diarrhea. Hypertension was a major problem in the majority of the patients. Five died, three during the initial illness; four are in end-stage renal failure, and all but two of the survivors have residual nephropathy. Eleven patients had a "relapsing" course; up to eight additional documented episodes of hemolytic-uremic syndrome occurred in individual patients. Of the nine children treated before 1980, three died shortly after onset, two never recovered function after the initial illness, one had a relapsing course and died later, and one had residual nephropathy. Plasma exchange was introduced for the management of non-diarrhea-associated hemolytic-uremic syndrome in 1980; since then, all of the 11 patients have recovered function after the initial episode, but 10 of them had relapses. It appears that with the introduction of plasma exchange there has been an improved outcome in the initial phase, but the survivors tend to have relapses. Atypical (non-diarrhea-associated) hemolytic-uremic syndrome is a heterogeneous yet distinct subgroup of hemolytic-uremic syndrome that differs from diarrhea-associated hemolytic-uremic syndrome on epidemiologic, clinical, laboratory, histologic, and prognostic grounds.

Renal size and function after cure of Wilms' tumour

Now that most patients with Wilms' tumour are cured, it is practicable to study the long-term morbidity of their treatment and use this information to reduce treatment sequelae in the future. In this study we evaluate the size and function of the remaining kidney in 53 survivors of Wilms' tumour with a mean off treatment follow-up of 13 years. There was evidence of renal dysfunction in 17 (32%), including ten (19%) with a low GFR (< 80 ml/min/1.73 m2SA), six (11%) with hypertension and five (9%) with increased urinary albumin excretion. Measurements of renal size showed 'good' renal compensatory hypertrophy in only 55% of patients. 'Good' refers to renal size of more than 2 s.d. above the mean renal length for children with two kidneys. There were no correlations between GFR, renal size, blood pressure, microalbuminuria or type of treatment. However, children less than 24 months at diagnosis and children receiving chemotherapy with radiation doses to remaining kidney of more than 1200 cGy had a worse renal prognosis. Patients whose Wilms' tumour is diagnosed in infancy should have careful long-term follow-up of renal function and size. Older patients may safely be followed up less often, unless their remaining kidney was received > 1200 cGy.

Prediction of blood pressure from plasma renin activity in reflux nephropathy

As there is a 10% risk of hypertension developing in children with reflux nephropathy and the renin-angiotensin system has been implicated in its aetiology, a long term prospective study has been undertaken to explore the relationship between plasma renin activity (PRA) and blood pressure in such patients. In 1978, of 100 normotensive children with reflux nephropathy 8% were shown to have PRA above normal. Five years later of 85 subjects suitable for analysis 13% had increased PRA and it was shown that PRA and blood pressure SD scores significantly increased. The present study refers to the 10 year follow up in which 95 of the original group were traced but eight of these were unavailable for study and 28 others were excluded from analysis because of extraneous factors that might influence blood pressure or PRA. Results therefore on 59 have been analysed. PRA was above normal in 13/59 (20%) subjects, and PRA and blood pressure SD scores had further increased. The data continue to support the role of the renin-angiotensin system in the observed rise of blood pressure in reflux nephropathy, but individual PRA measurements do not appear so far to predict reliably the onset of hypertension in affected patients.

Renal tubular dysfunction in methylmalonic acidaemia

Renal tubular function was assessed in seven patients with methylmalonic acidaemia not responsive to vitamin B12. Five patients failed to concentrate their urine normally and in these patients the glomerular filtration rate was also reduced. Fractional excretion of sodium was increased in four patients, fractional excretion of potassium in one patient and in three there was a decreased tubular reabsorption of phosphate. Although possibly representing primary tubular damage these findings were thought to be consistent with adaptive changes secondary to the reduced glomerular filtration rate. Two patients had evidence of a defect of urinary acidification and several had a degree of hyporeninaemic hypoaldosteronism suggesting type 4 renal tubular acidosis. In one patient with a mild variant no renal disease was detected. Decreased renal function and tubular abnormalities were common in patients with methylmalonic acidaemia. It is likely that they are linked and essentially secondary to the tubulo-interstitial nephritis that is histologically demonstrable on renal biopsy. The failure of urinary concentrating ability and the disturbed urine acidification will contribute to the metabolic derangement during episodes of decompensation.

Antidiuretic hormone following surgery in children

We studied 13 children subjected to elective tonsillectomy, 6 of whom (study patients) received supplemental intravenous isotonic saline during and after operation, and 7 of whom (controls) did not. Clinical and biochemical evidence of hypovolaemia was present in the control but not in the study patients. Plasma antidiuretic hormone (ADH) and urine osmolality were higher in controls (p less than 0.005 and p less than 0.05 respectively). Plasma sodium concentration and osmolality were similar in the two groups. We conclude that hypovolaemia is the principal stimulus to ADH release following surgery and that, in addition to replacement of observed losses of blood and other fluids by fluids of appropriate composition, hypovolaemia should be prevented by the administration of maintenance quantities of isotonic fluid, rather than exacerbated by fluid restriction, in patients in whom oral fluid intake is interrupted for more than a brief period. Hypotonic and sodium free fluids should be avoided because of the risk of hyponatraemia.

Plasma renin activity in fetal disease

Fetal plasma renin activity (PRA) was measured in 42 pregnancies. Compared to control fetuses, PRA was elevated in three of four hypoxemic fetuses, in two of five with hydrops and in two of five with uropathies. PRA did not change with transfusion in seven alloimmunized fetuses. This study demonstrates PRA in human fetuses and suggests that the renin-angiotensin system can respond to stimuli in fetal life.

Congenital mesoblastic nephroma, renin production, and hypertension

Between 1964 and 1987, 12 patients with mesoblastic nephroma were treated. The BP was measured preoperatively in five patients, four of whom were hypertensive. Following nephrectomy, the BP returned to normal. Plasma renin levels were measured in one patient; they were markedly elevated pre-operatively, but returned to normal following tumour excision. Immunoreactive renin staining, using a polyclonal antibody to human renin, was performed in the 12 patients. Staining was positive in ten patients. The most intense staining was noted in the areas of cortex entrapped within the tumour, and in perivascular spaces not associated with entrapped cortex. These findings suggest that hypertension secondary to increased tumour associated renin production is a feature of congenital mesoblastic nephroma.

The captopril test: an aid to investigation of hypertension

Twenty three children aged from 5 to 16 with mild to moderate hypertension were investigated using the orally active angiotensin converting enzyme inhibitor captopril. Falls in both systolic and diastolic blood pressure after a single dose of captopril were significantly correlated with initial plasma renin activity. In addition, some information about the aetiology of hypertension was deduced from the renin response to captopril. The blood pressure response to captopril is a useful screening test for renin dependent hypertension in childhood.

Renin in mesoblastic nephroma: an immunohistochemical study

Congenital mesoblastic nephroma (CMN) is the most common renal tumor of early infancy. It is usually evident at birth as an abdominal mass and is composed of spindle-shaped cells resembling smooth muscle cells and fibroblasts. There is macroscopic and microscopic infiltration of the surrounding kidney and entrapped tubules and glomeruli are common at the edge of the tumor. In this report, we describe a case of CMN associated with hyperreninaemia and hypertension. We examined the tumor from this case and 11 other cases for the presence of renin using a polyclonal antibody to human kidney renin. Hypertension was present in three of four additional cases for which records were available. Immunoreactive renin was present in ten of the 12 cases studied. In all of these cases, intense staining was present in vessels within the areas of the trapped cortex. In seven cases, renin was identified in the walls of vessels within the tumor itself without identifiable adjacent cortical structures. These findings indicate that CMN may often be associated with primary reninism in early infancy.

Hyponatraemia in premature babies and following surgery in older children

Hyponatraemia implies water retention in excess of sodium with or without increased loss of sodium from the body; extracellular fluid volume may be increased, normal or reduced. It has many causes which are briefly reviewed. Among these is the rare syndrome of inappropriate secretion of antidiuretic hormone (SIADH). It is suggested that SIADH is often diagnosed incorrectly because the raised ADH levels are appropriate for the volume status of the child. Precision in the diagnosis is important because whilst water restriction is necessary for the treatment of SIADH, other measures including the administration of extra fluid are often required if the raised ADH is appropriate. Hyponatraemia in the newborn may be caused by prerenal failure, renal failure or renal sodium wasting which is common in premature infants. Careful control of sodium intake as well as water intake is vital in this age group. Surgery is associated with water retention, but recent studies suggest that ADH levels are raised post-operatively because of volume depletion and that present recommendations for fluid therapy during and following surgery are inadequate. The use of electrolyte-free dextrose solutions should be abandoned and more liberal use of physiological saline or colloid is recommended.

Renal vein renin measurement and arteriography in the investigation and management of severe childhood hypertension

Forty-two children aged one to sixteen years with persistent and severe hypertension were investigated by renal vein renin measurements. There were no serious complications in the 49 procedures performed and technical failure occurred on three occasions. Arteriography was performed in 35. Asymmetrical renin release was found in 22 patients and of these 15 underwent surgery. This was successful in 12 patients (80%) who became normotensive. Ten had unilateral disease (100% cure rate) but only 2 (40%) with bilateral disease became normotensive. Renal vein renin studies combined with arteriography have a useful role in the investigation and management of childhood hypertension.

Comparison of aortography, renal vein renin sampling, radionuclide scans, ultrasound and the IVU in the investigation of childhood renovascular hypertension

The results of the investigations of all patients who underwent renal arteriography for hypertension due to renovascular disease over a three and a quarter year period prior to January 1st 1981 are reviewed and discussed. The experience gained has helped demonstrate the usefulness of DMSA scans, segmental renal vein renin estimations, and AP and oblique selective renal arteriograms with macroradiography.

Pseudohypoaldosteronism

10 infants are described with pseudohypoaldosteronism, 5 in detail and a further 5 briefly. They all presented with hyperkalaemia, urinary salt-wasting disease, and ostensibly normal renal and adrenocortical function. Diagnosis was established by demonstrating the greatly increased values of plasma renin activity and plasma aldosterone concentration, plus the increased excretion of aldosterone and its metabolites on gas chromatographic and mass spectrometric analyses of urine. The children were treated with sodium chloride supplements, up to 60 mmol/day, but by the time most of the infants were about a year old these could be stopped. Exogenous mineralocorticoids were without effect in those to whom they were administered. The precise aetiology of the condition remains conjectural; lack of renal tubular response to aldosterone seems probable. Pseudohypoaldosteronism may be more common than has been thought and new techniques for investigating salt-wasting disorders may show its true incidence.

Renal vein renin measurements in normotensive children

Renal venous PRA was measured in 49 normotensive children without renal disease undergoing routine cardiac catheterization. PRA levels did not differ significantly between both renal veins and were significantly higher in the renal veins than in the IVC. There was a constant mean ratio of 1.21 between the renal veins and the IVC at low, intermediate, and high absolute PRA levels. Three patients had a renal venous PRA ratio greater than 1.4 and the highest ratio observed was 1.55. This fidning supports 1.5 as an acceptable upper limit of normality for the interpretation of renal vein PRA ratios in the investigation of patients with suspected renal hypertension. In four patients, PRA in the renal veins was significantly lower than in the IVC. The possibility of renin removal by these kidneys is discussed.

Renin and blood-pressure in children with renal scarring and vesicoureteric reflux

Plasma-renin-activity (P.R.A.) was raised in 9 of 15 hypertensive children with pyelonephritic scarring secondary to urinary-tract infection and vesicoureteric reflux and also in 8 of 100 normotensive children with such scars. P.R.A. was much less likely to fall with age in normotensive children with renal scarring than in normal children. The identification of hyperreninaemic normotensive children with renal scarring is important, since P.R.A. may prove to be of value in early identification of children at risk of developing hypertension. A longitudinal follow-up is proposed to establish this hypothesis.

Renin and aldosterone response in human newborns to acute change in blood volume

Increased activity of the renin/aldosterone system in the neonatal period is now well established in both animals and man but the control mechanisms are poorly understood. We have monitored the plasma renin activity (PRA) and plasma aldosterone concentration (PAldo) in 14 infants undergoing 21 exchange transfusions. PRA and PAldo were measured before and at 5, 10, 15, 30, 45, and 60 minutes after the injection, and 5, 10, 15, and 30 minutes after the withdrawal of 7 ml blood/kg birthweight immediately before exchange transfusions. PRA increased to a maximum of 53% and decreased to a maximum of 39% of the resting values after withdrawal or injection of blood respectively. PAldo values did not change significantly during the same period. Thus the renin-angiotensin system in the newborn infant is responsive to changes in blood volume.

Congenital adrenal hyperplasia: renin and steroid values during treatment

Plasma renin activity (PRA), aldosterone (Aldo), 17alpha-hydroxyprogesterone (17-OHP) and testosterone (T), together with urine sodium, pregnanetriol, 17-oxosteroids and the 11-oxygenation index (11-OH) were estimated in 23 patients (age 5.7--18 yrs.) with congenital adrenal hyperplasia (CAH) due to 21-hydroxylase deficiency during glucocorticoid treatment. Elevated PRA levels (1400--17200 ng Al/l/hr) were found in 13 out of 15 patients with a history of salt loss. Three non-salt losers showed high PRA levels and in the remaining 5 the levels were in the upper normal range (540--900 ng Al/l/hr). Plasma Aldo levels were normal (25--620 pmol/l) in 18 patients and slightly elevated (690--2360 pmol/l) in 5. While these results indicate persistent impairment of sodium homeostasis in CAH patients, no significant correlations between log. PRA, log. Aldo and urinary sodium excretion were found. Mid-day 17-OHP levels ranged from 9 to 117 nmol/l and T from 0.3 to 18.0 nmol/l. Neither the 17-OHP nor the T results correlated well with the clinical assessment of therapeutic control. The results of the urinary steroid determinations showed better agreement with the clinical assessment of treatment and the 17-oxosteroid, pregnanetriol and 11-OH index results appeared to be better discriminants between good and poor control. Twelve of the patients with a history of early salt loss were reinvestigated after one month's treatment with oral 9 alpha-flurohydrocortisone (0.05 mg/day). PRA was reduced in 7 patients and 17-OHP fell in 10 patients. No consistent changes were found in Aldo, T, or urinary sodium and steroid excretion during this low-dose mineralocorticoid treatment.

Salt-losing syndrome in 2 infants with defective 18-dehydrogenation in aldosterone biosynthesis

Two infants presented with a salt-losing syndrome, the presenting features of which were subtle. One case appeared to be transient. Deficient production of aldosterone was shown by plasma renin activity and plasma aldosterone profile. Gas chromatography-mass spectrometry of urine indicated a defect in 18-dehydrogenation of 18-hydroxycorticosterone. Treatment with salt supplements and 9alpha-fludrocortisone reversed the salt-losing state and in one case treatment was later stopped. Although the disease may appear transient, the biochemical defect is persistent and for adequate growth a positive salt-balance is necessary.

Aldosterone and sodium HOMEOSTASIS in preterm infants

A specific mass spectrometric method was used for tetrahydroaldosterone determination in urine of preterm infants (26-34 weeks gestational age) up to 9 weeks of age. Hyponatraemia during the first 2 weeks of life was associated with an excretion of tetrahydroaldosterone (5-50 mug/24 h) comparable with full-term infants. Excretion of tetrahydroaldosterone was significantly elevated in all infants studied during the third week of life (80-350 mug/24 h) and this was associated with establishment of positive sodium balance. The excretion of tetrahydro aldosterone remained high for 2 or 3 weeks. The results are discussed in relation to the development of renal tubules and control mechanism for sodium homeostasis.

Plasma renin activity and aldosterone concentration in the human newborn

Twenty-one healthy newborn infants whose mothers had normal pregnancies and deliveries were studied. Plasma renin activity (PRA) and plasma aldosterone concentration (PAldo) were measured by semimicro methods in cord blood and on the 6th day of life. PRA and PAldo were increased in cord blood compared with values previously found in healthy infants between 1 month and 1 year of age. There was a twofold reduction in both values by the 6th day. PRA and PAldo were positively correlated when cord blood and 6th-day values were considered as a whole, but no significant correlations were found between either cord blood PRA or cord blood PAldo and their respective values on the 6th day. The PRA and PAldo findings were not related to birthweight, the infant's sex, weight change, 6th-day urinary sodium, or blood pressure measurements. The type of milk feed did not influence PRA but PAldo was slightly greater in bottle-fed babies on the 6 day. These observations confirmed that the inverse relationship between PRA, PAldo, and age extends into the neonatal period but they suggest that the balance of factors controlling these variables at birth is different to that operating at the age of 6 days.