The role of the kidney in sodium homeostasis during maturation

Hypochloremia Secondary to Diuretics in Preterm Infants: Should Clinicians Pay Close Attention?

Diuretic therapy, commonly used in the newborn intensive care unit, is associated with a variety of electrolyte abnormalities such as hyponatremia, hypokalemia, and hypochloremia. Hypochloremia, often ignored, is associated with significant morbidities and increased mortality in infants and adults. Clinicians respond in a reflex manner to hyponatremia than to hypochloremia. Hypochloremia is associated with nephrocalcinosis, hypochloremic alkalosis, and poor growth. Besides, the diuretic resistance associated with hypochloremia makes maintaining chloride levels in the physiological range even more logical. Since sodium supplementation counters the renal absorption of calcium and lack of evidence for spironolactone role in diuretic therapy for bronchopulmonary dysplasia (BPD), alternate chloride supplements such as potassium or arginine chloride may need to be considered in the management of hypochloremia due to diuretic therapy. In this review, we have summarized the current literature on hypochloremia secondary to diuretics and suggested a pragmatic approach to hypochloremia in preterm infants.

Predicting changes in renal metabolism after compound exposure with a genome-scale metabolic model

The kidneys are metabolically active organs with importance in several physiological tasks such as the secretion of soluble wastes into the urine and synthesizing glucose and oxidizing fatty acids for energy in fasting (non-fed) conditions. Once damaged, the metabolic capability of the kidneys becomes altered. Here, we define metabolic tasks in a computational modeling framework to capture kidney function in an update to the iRno network reconstruction of rat metabolism using literature-based evidence. To demonstrate the utility of iRno for predicting kidney function, we exposed primary rat renal proximal tubule epithelial cells to four compounds with varying levels of nephrotoxicity (acetaminophen, gentamicin, 2,3,7,8-tetrachlorodibenzodioxin, and trichloroethylene) for six and twenty-four hours, and collected transcriptomics and metabolomics data to measure the metabolic effects of compound exposure. For the transcriptomics data, we observed changes in fatty acid metabolism and amino acid metabolism, as well as changes in existing markers of kidney function such as Clu (clusterin). The iRno metabolic network reconstruction was used to predict alterations in these same pathways after integrating transcriptomics data and was able to distinguish between select compound-specific effects on the proximal tubule epithelial cells. Genome-scale metabolic network reconstructions with coupled omics data can be used to predict changes in metabolism as a step towards identifying novel metabolic biomarkers of kidney function and dysfunction.

Renal lesions in leptin receptor-deficient medaka (Oryzias latipes)

The aim of this study was to elucidate the renal lesions of leptin receptor-deficient medaka showing hyperglycemia and hypoinsulinemia and to evaluate the usefulness of the medaka as a model of diabetic nephropathy. Leptin receptor-deficient medaka at 20 and 30 weeks of age showed hyperglycemia and hypoinsulinemia; they also showed a higher level of plasma creatinine than the control medaka. Histopathologically, dilation of glomerular capillary lumina and of afferent/efferent arterioles was observed in leptin receptor-deficient medaka at 20 weeks of age, and then glomerular enlargement with cell proliferation and matrix expansion, formation of fibrin cap-like lesions, glomerular atrophy with Bowman’s capsule dilation, and renal tubule dilation were observed at 30 weeks of age. These histopathological characteristics of leptin receptor-deficient medaka were similar to the characteristics of kidney lesions of human and rodent models of type II diabetes mellitus, making leptin receptor-deficient medaka a useful model of diabetic nephropathy.

Age-Related Autonomous Aldosteronism

BACKGROUND

Both aging and inappropriate secretion of aldosterone increase the risk for developing cardiovascular disease; however, the influence of aging on aldosterone secretion and physiology is not well understood.

METHODS

The relationship between age and adrenal aldosterone synthase (CYP11B2) expression was evaluated in 127 normal adrenal glands from deceased kidney donors (age, 9 months to 68 years). Following immunohistochemistry, CYP11B2-expressing area and areas of abnormal foci of CYP11B2-expressing cells, called aldosterone-producing cell clusters, were analyzed. In a separate ancillary clinical study of 677 participants without primary aldosteronism, who were studied on both high and restricted sodium diets (age, 18-71 years), we used multivariable linear regression to assess the independent associations between age and renin-angiotensin-aldosterone system physiology.

RESULTS

In adrenal tissue, the total CYP11B2-expressing area was negatively correlated with age (r=-0.431, P<0.0001), whereas the total aldosterone-producing cell cluster area was positively correlated with age (r=0.390, P<0.0001). The integrated ratio of aldosterone-producing cell cluster to CYP11B2-expressing area was most strongly and positively correlated with age (r=0.587, P<0.0001). When participants in the clinical study were maintained on a high sodium balance, renin activity progressively declined with older age, whereas serum and urinary aldosterone did not significantly decline. Correspondingly, the aldosterone-to-renin ratio was positively and independently associated with older age (adjusted β=+5.54 ng/dL per ng/mL per hour per 10 years, P<0.001). In contrast, when participants were assessed under sodium-restricted conditions, physiological stimulation of aldosterone was blunted with older age (β=-4.6 ng/dL per 10 years, P<0.0001).

CONCLUSIONS

Aging is associated with a pattern of decreased normal zona glomerulosa CYP11B2 expression and increased aldosterone-producing cell cluster expression. This histopathologic finding parallels an age-related autonomous aldosteronism and abnormal aldosterone physiology that provides 1 potential explanation for age-related cardiovascular risk.

Characterization of Hydronephrosis in Neonatal Rats from Dams Receiving Candesartan Cilexetil (TCV-116), an Angiotensin II Type 1 Receptor Antagonist

The characteristics and mechanisms of hydronephrosis in neonatal rats induced by candesartan cilexetil (TCV-116), a potent angiotensin II (AngII) type 1 receptor antagonist, were examined. TCV-116 (300 mg/kg/day) was orally administered to dams for 4 weeks from gestation day 15 through lactation day 21. On lactation days 0, 4, 7, 14, and 22, the kidneys of the pups were examined. Hydronephrosis was observed starting on lactation day 14 accompanied by other histological changes, atrophy of the renal papillary tubules, dilatation of the renal tubules, and basophilic renal tubules in the cortex. These changes could also be observed at 10 weeks of age, 7 weeks after the last dose was administered. These renal structural abnormalities were consistent with that seen in other renin-angiotensin system antagonists. TCV-116 (300 mg/kg/day) was then administered to dams for four separate 1-week periods: gestation days 15 through 21, lactation days 0 to 6, lactation days 7 to 13, and lactation days 14 to 21. Pups were most susceptible to the induction of hydronephrosis when TCV-116 was administered from lactation days 0 to 6 and lactation days 7 to 13. The increased incidence of hydronephrosis and renal histological changes in the pups was prevented by administering mineralocorticoid, deoxycorticosterone acetate (10 mg/kg/day), subcutaneously to the pups from lactation days 7 to 13. Also, plasma aldosterone concentration in the pups was decreased after three daily treatments of TCV-116, accompanied by the increased plasma potassium concentration and urine Na/K ratio and the decreased urine osmolality. Therefore, we considered that the development of hydronephrosis in pups is closely related to the AngII blockade for the first 2 weeks after birth, and the reduction of aldosterone secretion by the inhibition of AngII leads to the disorder of the sodium and potassium homeostasis in neonates, and subsequent increase in urine volume may be involved in the mechanisms of hydronephrosis. We conclude that the hydronephrosis was caused by the sodium imbalance resulted from the pharmacological action of TCV-116 during the neonatal period.

Renal effects of angiotensin II in the newborn period: role of type 1 and type 2 receptors

BACKGROUND

Evidence suggests a critical role for the renin-angiotensin system in regulating renal function during postnatal development. However, the physiological relevance of a highly elevated renin-angiotensin system early in life is not well understood, nor which angiotensin receptors might be involved. This study was designed to investigate the roles of angiotensin receptors type 1 (AT1R) and type 2 (AT2R) in regulating glomerular and tubular function during postnatal development.

METHODS

The renal effects of the selective antagonist to AT1R, ZD 7155 and to AT2R, PD 1233319 were evaluated in two groups of conscious chronically instrumented lambs aged ~ one week (N = 8) and ~ six weeks (N = 10). Two experiments were carried out in each animal and consisted of the assessment of renal variables including glomerular and tubular function, for 30 min before (Control) and 60 min after infusion of ZD 7155 and PD 123319, respectively. Statistical significance was determined using parametric testing (Student t-test, analysis of variance ANOVA) as appropriate.

RESULTS

ZD 7155 infusion was associated with a significant decrease in glomerular filtration rate and filtration fraction at one but not six weeks; urinary flow rate decreased significantly in older animals, whereas sodium excretion and free water clearance were not altered. There was an age-dependent effect on potassium handling along the nephron, potassium excretion decreasing after ZD 7155 infusion in younger but not in older lambs. PD 123319 had no significant effects on glomerular filtration rate and tubular function in either age group.

CONCLUSIONS

These results provide evidence to support an important role for AT1Rs in mediating the renal effects of angiotensin II during postnatal maturation in conscious developing animals. In contrast to a role for AT2Rs later in life, there appears to be no role for AT2Rs in influencing the renal effects of Angiotensin II in the postnatal period.

Pericytes synthesize renin

AIM: To investigate renin expression in pericytes during normal kidney development and after deletion of angiotensinogen, the precursor for all angiotensins.

METHODS: We examined the distribution of renin expressing cells by immunoshistochemistry in the interstitial compartment of wild type (WT) and angiotensinogen deficient (AGT -/-) mice at different developmental stages from embryonic day 18 (E18: WT, n = 4; AGT -/-, n = 5) and at day 1 (P1: WT, n = 5; AGT -/-, n = 5), 5 (P5: WT, n = 7; AGT -/-, n = 8), 10 (P10: WT, n = 3; AGT -/-, n = 5), 21 (P21: WT, n = 7; AGT -/-, n = 5), 45 (P45: WT, n = 3; AGT -/-, n = 3), and 70 (P70: WT, n = 2; AGT -/-, n = 2) of postnatal life. We quantified the number of pericytes positive for renin at all the developmental stages mentioned above and compared the results of AGT -/- mice to their WT counterparts.

RESULTS: In WT mice, renal interstitial pericytes synthesize renin in early life supporting a lineage relationship with renin cells in the vasculature. The number of pericytes positive for renin per area of 0.32 mm² (density) in WT mice was maintained from fetal life till weaning age (E18 = 4.25 ± 0.63, P1 = 3.75 ± 0.48, P5 = 3.75 ± 0.48, P10 = 4 ± 0.71, P21 = 3.8 ± 0.58) and markedly decreased in adult life (P45 = 1.2 ± 0.37, P70 = 0.8 ± 0.20). On the other hand, in AGT -/- mice the density of pericytes expressing renin was not significantly different from WT mice at E18 and P1: E18 = 5.75 ± 0.50 vs 4.25 ± 0.63 (P = 0.106), P1 = 9.25 ± 3.50 vs 3.75 ± 0.48 (P = 0.175) but significantly increased from P5 till P70: P5 = 38.25 ± 5 vs 3.75 ± 0.48 (P = 0.0004), P10 = 173 ± 7.50 vs 4 ± 0.70 (P = 5.24567 × 10^-7), P21 = 83 ± 6.70 vs 3.8 ± 0.58 (P = 2.97358 × 10^-6), P45 = 49 ± 3.50 vs 1.2 ± 0.37 (P = 8.18274 x 10^-7) and P70 = 17.8 ± 2.30 vs 0.8 ± 0.20 (P = 3.51151 × 10^-5). The AGT -/- mice showed a marked increase in the number of pericytes per field studied starting from P5, reaching its peak at P10, and then a gradually decreasing until P70.

CONCLUSION: Interstitial pericytes synthesize renin during development and the number of renin-expressing pericytes increases in response to a homeostatic threat imposed early in life such as lack of angiotensinogen.

Does salt intake in the first two years of life affect the development of cardiovascular disorders in adulthood?

Excess salt intake impacts on blood pressure (BP) and the pathogenetic mechanisms of atherosclerosis predisposing to stroke and other cardiovascular disorders. The influence of salt starts early in life. Two randomized controlled trials in newborn infants suggest a direct association between dietary sodium intake and BP since the first few months of life. Newborn infants display aversion to the salt taste to develop a "preference" for salt only at age 2-3 years, in part in relation to post-natal events: this preference might be associated with later development of hypertension. The amount of sodium to be retained by an infant for proper physiological growth is largely covered by breast feeding (or low sodium formula milk) in the first six months, and later on by the gradual implementation of complementary feeding, without the need for any added salt upon food preparation. Given the lack of dose-dependence data, reference nutrient intakes (RNI) or adequate intakes (AI) for sodium have been established by national health institutions in various countries. The U.K. RNI was set at 242 mg a day for infants 0-6 months with gradual increase to 0.5 g up until age 3. The U.S. AI is somewhat lower for age 0-6 months but larger for age 1-3 years. According to a recent report, the average sodium intake in U.S. children is close to the AI up to age 2 years, to become progressively greater exceeding the Institute of Medicine recommendation later on.

Sodium and potassium clearances by the maturing kidney: clinical-molecular correlates

A temporal dissociation exists between the early appearance of sodium absorptive and later detection of potassium secretory processes in the maturing rabbit collecting duct. To extend the latter findings to the human, we sought to correlate developmental changes in renal sodium and potassium clearances with the molecular expression of corresponding ion channels in kidneys of premature infants. In a longitudinal prospective study of 23- to 31-week gestational age (GA) infants, sodium, potassium, and creatinine clearances were measured weekly for 5 weeks and the absolute and fractional excretions of sodium (FE(Na)) and potassium (FE(K)) calculated. Gene-specific probes were used to assess steady-state abundance of mRNA encoding the sodium channel ENaC and potassium channel ROMK in homogenates of human kidneys (obtained from the Anatomic Gift Foundation). Although urinary losses of sodium in infants <approximately 28 weeks GA exceeded intake, leading to a state of negative sodium balance, most infants >/=28 weeks and all infants >approximately 32 weeks GA achieved a state of positive balance, a maturational process associated with a decrease in FE(Na )and increase in ENaC. Infants >approximately 30 weeks GA maintained a state of positive potassium balance. We noted a twofold reduction in FE(K )after approximately 26 weeks GA and no change in ROMK abundance during the developmental window studied. We speculate that the developmental regulation of renal ENaC expression contributes, at least in part, to the decrease in FE(Na )observed with advancing GA, and that in the human, as in the rabbit, there is a delay between the maturation of sodium absorptive and potassium secretory pathways.

Basic principles and practical steps in the management of fluid balance in the newborn

The fluid management of newborn babies can pose many problems. This article discusses the factors that affect fluid balance in the newborn infant, both term and preterm, and the special circumstances of the surgical neonate. The main determinants of management are: (1) an estimation of transepidermal water losses; (2) an awareness of glomerular filtration rate and how this is influenced by age, respiratory distress and medical intervention; and (3) knowledge of tubular function and its maturation and the processes of postnatal adaptation. This knowledge and appropriate monitoring are the mainstay of management of neonatal fluid balance.

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.

Randomised controlled trial of postnatal sodium supplementation on body composition in 25 to 30 week gestational age infants

AIMS

To compare the effects of early and delayed sodium supplementation on body composition and body water compartments during the first two weeks of postnatal life.

METHODS

Preterm infants of 25-30 weeks' gestation were stratified and randomly assigned according to gender and gestational age, to receive a sodium intake of 4 mmol/kg/day beginning either on the second day after birth or when weight loss of 6% of birthweight had been achieved. Daily sodium intake, total fluid intake, energy intake, urine volume, and urinary sodium excretion were recorded. Total body water was measured by H(2)(18)O dilution on days 1, 7, and 14, and extracellular fluid volume by sodium bromide dilution on days 1 and 14.

RESULTS

Twenty four infants received early, and 22 delayed, sodium supplementation. There were no significant differences between the groups in body water compartments on day 1. In the delayed group, but not the early group, there was a significant loss of total body water during the first week (delayed -44 ml/kg, p=0. 048; early 6 ml/kg, p=0.970). By day 14 the delayed, but not the early group, also had a significant reduction in extracellular fluid volume (delayed -53 ml/kg, p=0.01; early -37 ml/kg, p=0.2). These changes resulted in a significant alteration in body composition at the end of the first week (total body weight: delayed 791 ml/kg; early 849 ml/kg, p=0.013). By day 14 there were once again no significant differences in body composition between the two groups.

CONCLUSIONS

Body composition after preterm birth is influenced by the timing of introduction of routine sodium supplements. Early sodium supplementation can delay the physiological loss of body water that is part of normal postnatal adaptation. This is likely to be of particular relevance to babies with respiratory distress syndrome. A tailored approach to clinical management, delaying the introduction of routine sodium supplements until there has been postnatal loss of body water, is recommended.

Sodium potassium adenosine triphosphatase activity in preterm and term infants and its possible role in sodium homeostasis during maturation

AIM

To investigate sodium (NA(+)) potassium (K(+)) adenosine triphosphatase (ATPase) activity in newborn infants at different gestational ages, to elucidate the mechanism underlying poor renal sodium conservation in preterm infants.

METHODS

Fifty three healthy newborn infants, gestational age 30-42 weeks, were studied. Umbilical cord red blood cell Na(+) K(+)ATPase activity, plasma renin activity, and plasma aldosterone activities were measured in all of them. Red blood cell Na(+) K(+)ATPase activity was re-examined in eight preterm infants, one and two weeks after birth. Total and ouabain sensitive ATPase activity was measured spectrophotometrically using a method that couples ATP hydrolysis with NADH oxidation.

RESULTS

Red blood cell Na(+) K(+)ATPase activity was significantly lower (p<0.01) in preterm babies with a gestational age below 35 weeks, compared with those with aged 35 weeks and above: 2.3 (0.8) and 6.7 (1.3) nmol NADH/minute/mg protein, respectively. There was no correlation between gestational age, Na(+) K(+)ATPase, plasma renin activity and aldosterone values either in the preterm or term babies. Two weeks after birth, irrespective of gestational age, the enzyme activity of the preterm babies increased to values similar to those observed in the term neonates at birth.

CONCLUSION

The differences in sodium homeostasis between term and preterm babies are modulated via changes in Na(+) K(+)ATPase activity.

Mechanism of renal phosphate retention during growth

We have previously demonstrated that the retention of phosphate required for growth is due to a a high Vmax of the Na(+)-Pi cotransport system located in the brush border membrane of the proximal tubule. Because of this and other similarities between adaptation of the kidney to a high Pi demand (growth) and that to low Pi supply, we measured the levels of NaPi-2 mRNA and cDNA present in kidney cortex of 3- and > 12-week-old rats. Like in Pi depletion, Western blots revealed that a 80 to 85 kDa protein recognized by a polyclonal antibody directed against the N-terminal region of the NaPi-2 protein was 2.3-fold more abundant in renal microvilli of the young than of adult animals. However, unlike in Pi depletion, Northern blot analysis failed to reveal a significant difference between mRNA levels at the two ages. Furthermore, suppression of NaPi-2 mRNA activity by annealing with antisense oligomers, or removal of the NaPi-2 transcripts by subtractive hybridization did not affect the rate of Na(+)-Pi cotransport induced in oocytes by polyA RNA of rapidly growing animals, while abolishing the ability of the renal cortical polyA RNA of adult rats to encode for Na(+)-Pi cotransport. RT-PCR of subtracted polyA RNA using primers specific for a region conserved in NaPi type II (Pi modulated) cotransporters yielded a product that was 98% homologous with that region, despite the absence of NaPi-2 cDNA. The results of these experiments demonstrate that the polyA RNA from kidneys of young animals contains unique mRNA transcripts able to encode for a NaPi protein homologous to, but distinct from NaPi-2.

Developmental consequences of the renin-angiotensin system

Molecular, cellular, and physiological studies indicate that the renin-angiotensin system (RAS) is highly expressed during early kidney development. We propose that a major function of the RAS during early embryonic development is the modulation of growth processes that lead the primitive kidney into a properly differentiated and architecturally organized organ suited for independent extrauterine life. As development progresses, the RAS acquires new and overlapping functions such as the endocrine and paracrine regulation of blood pressure and renal hemodynamics. Disease states in adult mammals often result in expression of RAS genes and phenotypic changes resembling the embryonic pattern, emphasizing the importance of undertaking developmental studies. Because of their importance in health and disease, the immediate challenge is to identify the mechanisms that regulate the unique development of the RAS and its role(s) in normal and abnormal growth processes.

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.

Developmental changes in rat renal 11 beta-hydroxysteroid dehydrogenase

11 beta-hydroxysteroid dehydrogenase (11 beta-OHSD) transforms endogenous glucocorticoids to their respective "biologically inert" 11-dehydro derivatives. A decrease in enzyme activity allows glucocorticoids to induce mineralocorticoid-like renal sodium retention. Since positive sodium balance is required for optimum growth in the newborn, we hypothesized that renal 11 beta-OHSD activity would be low in the postnatal period, a time of active growth. To test this, incubations with corticosterone were carried out using minces or homogenates prepared from kidneys of newborn, 8-day-old, and mature Sprague-Dawley rats. 11 beta-OHSD activity in renal minces, assessed by the percent of corticosterone (10(-8) M) transformed to 11-dehydrocorticosterone (compound A), was significantly lower in the newborn kidney (newborn 45.7 +/- 3.8%, 8 day 70.2 +/- 3.8%, and adult 73.4 +/- 3.1%, P < 0.001 1 vs. 8 day). Parallel studies were conducted using an antibody directed against liver 11 beta-OHSD counter stained with immunofluorescent labeled IgG. Kidneys from mature rats were brightly stained at S2 and S3 segments of proximal tubules. In contrast, staining was barely detectable in kidneys from the newborn and 8-day-old rats. When enzyme kinetics were examined in kidney homogenates (average protein concentration 2.5 mg/ml) in the presence of 200 microM NADP+, the apparent Km for corticosterone in the adult was 4.42 x 10(-6) M with a corresponding Vmax of 1.33 x 10(-9) mol/min/mg protein, while the apparent Km for corticosterone in the newborn was calculated to be 12.8 x 10(-8) M with a Vmax of 2.08 x 10(-11) mol/min/mg protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Scanning and transmission electron-microscopic study of peripolar cells in the newborn lamb kidney

Scanning and transmission electron microscopy were used to study the ultrastructural characteristics and positions of granulated peripolar cells in newborn lamb kidney. Following tissue fixation by vascular perfusion in situ, the vascular pole region of the glomerulus was exposed for examination by scanning electron microscopy following removal of the glomerular tuft. Peripolar cells were recognized by their surface morphology enabling their quantification and an assessment of the relationship of their position in the renal cortex. The prominent expression of peripolar cells in this species was confirmed. Almost every vascular pole examined revealed peripolar cells (405 out of 407; 99.5%) and thus, throughout the cortex, the distribution of peripolar cells was the same as the distribution of renal corpuscles. Larger, more protruding peripolar cells were observed in the outer cortical renal corpuscles. The numbers of peripolar cells encircling each vascular pole ranged from 1 to 10. There was no correlation between number of granulated peripolar cells at the vascular pole and the position of the renal corpuscle within the renal cortex. As viewed by transmission electron microscopy, organelles of protein synthesis were abundant in the cytoplasm of peripolar cells. Exocytosis of cytoplasmic granules was observed by both scanning and transmission electron microscopy implying that a process of regulative secretion occurs from these cells. The use of ultrastructural techniques has provided evidence supporting the concept that peripolar cells are prominent in the cuff region of each renal corpuscle of the newborn lamb and furthermore that peripolar cells in this species most likely have a secretory function.

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)

Regulation of extracellular fluid volume in neonates

This review summarises mechanisms of control of extracellular fluid volume in the neonatal period. 'Normal' body fluid distribution and methods of its measurement are discussed as well as regulatory mechanisms with particular emphasis on hormonal and renal aspects.

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.

Abnormal sodium pump distribution during renal tubulogenesis in congenital murine polycystic kidney disease

Congenital polycystic kidney disease is characterized by the formation of large fluid-filled cysts in kidney tubules. It has been postulated that increased epithelial cell proliferation and altered transtubular fluid transport are necessary for cyst formation. To address the latter problem, we have studied the plasma membrane distribution of the alpha 1 and beta 1 subunits of Na+/K(+)-ATPase during progressive stages of proximal and collecting tubular cyst formation in the CPK mouse, a murine model of autosomal recessive polycystic kidney disease. In both control and cystic proximal tubules, Na+/K(+)-ATPase distribution was restricted to the basal-lateral membrane of cells. However, in newborn through day 5 kidney tissue, 16% of control vs. 47% of cystic outer cortical, 6% of control vs. 46% of cystic inner cortical, and 2% of control vs. 63% of cystic medullary collecting tubules demonstrated apical and lateral membrane distribution of Na+/K(+)-ATPase. In all nephrogenic zones, the percentage of control or cystic collecting tubules demonstrating apical membrane distribution of Na+/K(+)-ATPase decreased over time, but the percentage of cystic collecting tubules with apical membrane Na+/K(+)-ATPase remained significantly greater than in developmentally matched controls. No alterations in the normal distributions of other apical or basal-lateral membrane marker proteins were noted at any stage of control or cystic proximal or collecting tubule development. We conclude that apical-lateral membrane Na+/K(+)-ATPase expression is a normal transient feature of early collecting tubule development. However, apical membrane Na+/K(+)-ATPase persists in cystic kidneys, suggesting that such expression may be a manifestation of the relatively undifferentiated phenotype of epithelial cells lining collecting tubule cysts. The persistence of apical membrane Na+/K(+)-ATPase, if the enzyme is functional, may have pathogenic important in abnormal transtubular fluid transport in polycystic kidney disease.

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.

Sodium restriction versus daily maintenance replacement in very low birth weight premature neonates: a randomized, blind therapeutic trial

To test the hypothesis that restriction of sodium intake during the first 3 to 5 days of life will prevent the occurrence of hypernatremia and the need for administration of large fluid volumes, we prospectively and randomly assigned 17 babies (mean +/- SD: 850 +/- 120 gm; 27 +/- 1 weeks of gestation) to receive in blind fashion either daily maintenance sodium or salt restriction with physician-prescribed parenteral fluid intake. Maintenance-group infants received 3 to 4 mEq of sodium per kilogram per day; restricted infants received no sodium supplement other than with such treatments as transfusion. Sodium balance studies conducted for 5 days demonstrated that maintenance salt intake resulted in a daily sodium balance near zero, whereas sodium-restricted infants continued to excrete urinary sodium at a high rate, which promoted a more negative balance (average daily sodium balance -0.30 +/- 1.78 SD in maintenance group vs -3.71 +/- 1.47 mEq/kg per day in restriction group; p less than 0.001). Care givers tended to prescribe daily increases in parenteral fluids for the salt-supplemented infants, perhaps because serum sodium concentrations were elevated in these infants after the first day of the study (p less than 0.001). Hypernatremia developed in two sodium-supplemented infants (greater than 150 mEq/L), and hyponatremia developed in two sodium-restricted infants (less than 130 mEq/L); however, the restricted infants were more likely to have normal serum osmolality (p less than 0.05). Both groups of infants produced urine that was neither concentrated nor dilute, with a high fractional excretion of sodium; renal failure was not observed. The mortality rate was not affected, but the incidence of bronchopulmonary dysplasia was significantly less in the sodium-restricted babies (p less than 0.02). We conclude that in tiny premature infants, a fluid regimen that restricts sodium may simplify parenteral fluid therapy targeted to prevent hypernatremia and excessive administration of parenteral fluids.

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.

Effect of chronic ethanol consumption on postnatal development of renal (Na + K)-ATPase in the rat

Renal (Na + K)-ATPase was studied to ascertain whether it follows the pattern of adaptation of membrane-bound enzymes that are inhibited by acute ethanol exposure and develop greater activity after chronic ethanol treatment. A colony of rats was given 20 per cent (v/v) ethanol as sole drinking solution throughout gestation, lactation and following weaning. (Na + K)-ATPase and ouabain-insensitive Ca(2+)-ATPase activities were determined; regional distribution of these enzymes was assessed in renal cortex and outer medulla. Control rats drank tap water. (Na + K)-ATPase in whole homogenate of kidney increased with age in controls and ethanol-fed rats, but the latter showed higher values at every age studied. Between 15 and 60 days of age, the control group showed 2-fold increases in cortex and 5-fold in outer medulla, whereas ethanol-fed rats reached a 3-fold increase in the enzyme activity in both renal regions. Ca(2+)-ATPase showed the same time course in developing kidney of both groups. Chronic ethanol treatment of adult rats resulted in an increase of (Na + K)-ATPase activity in cortex and outer medulla, but no change in other ATPases. Since an earlier maturational development of renal (Na + K)-ATPase was displayed by ethanol-fed rats, underlying mechanisms that may account for these results are discussed.

Molecular and cellular aspects of renin during kidney ontogeny

The activity of the renin-angiotensin system is subjected to remarkable developmental changes. Circulating as well as renal concentrations of renin are high in early life, decreasing progressively as maturation evolves. This review summarizes the current molecular framework underlying those changes during kidney development. Evidence is presented demonstrating that expression of the renin gene is developmentally regulated. Renin messenger ribonucleic acid (mRNA) levels are higher in fetuses and newborns than in adult mammals. As maturation progresses, the distribution of renin and its mRNA shifts from large intrarenal arteries in the fetus to the classic juxtaglomerular localization in the adult. Potential explanations for these changes as well as the cytosolic events mediating renin release and gene expression are discussed. Evidence is also presented demonstrating that under diverse physiologic and pathologic conditions the adult kidney vasculature has the capability to recruit renin gene expressing and/or containing cells. Throughout, an effort has been made to identify gaps in our knowledge. Not without bias, we hope that future research in this area will lead to a better understanding of the biology of renin in the developing as well as the adult individual.

Special needs of the newborn infant in fluid therapy

The preterm baby, although less able to tolerate high fluid and solute loads, has a remarkable ability to adapt to a rather broad range of environmental challenges, and thus renal function in the preterm infant should no longer be considered impaired. Lower rather than higher fluid administration is the current trend in neonatal care because it is safer and more physiologic. Although changes in body weight over time provide a good indicator of water balance, determination of plasma and urine electrolytes and tonicity are also essential in the monitoring of fluid therapy in preterm infants.

Deoxycorticosterone hypertension in the intact weanling rat without salt loading

Deoxycorticosterone (DOC) hypertension in the rat is generally induced in rats at an age of approximately 3 months. Both uninephrectomy and a high sodium diet are necessary, however, to induce DOC hypertension. Considering the inability of the developing kidney to adequately excrete a sodium load, we studied the possibility that DOC alone might induce hypertension when treatment is initiated in rats at the age of 21 days. The contribution of volume expansion as a factor mediating the pressor response to DOC was assessed in rats given a high sodium diet instead of DOC. Systolic blood pressure increased in DOC-treated rats within 3 weeks. Although systolic blood pressure also increased in rats on a high sodium diet, the increase was transient and of a lesser magnitude than that observed in DOC-treated rats. The rise in blood pressure in both groups of rats was associated with suppression of plasma renin activity and aldosterone concentration. Furthermore, extracellular fluid volume was similarly increased in DOC-treated rats and rats given a high sodium diet. Consistent with these data, DOC-treated rats showed an exaggerated natriuretic response to acute saline loading as compared with a vehicle-treated control group. Discontinuation of DOC treatment after 5 weeks led to normalization of all variables studied including blood pressure. Yet, when DOC was continued for 8 weeks, stopping treatment did not lower blood pressure despite normalization of the renin-angiotensin-aldosterone system and the natriuretic response to saline loading. In contrast, discontinuation of the high sodium diet after 8 weeks normalized blood pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Age differences in renal response to atrial natriuretic peptide in rabbits

Plasma levels of atrial natriuretic peptide (ANP) and the effect of exogenous ANP on renal function have been studied in newborn and adult rabbits. In order to investigate an age difference in responsiveness to ANP, we studied the renal effects of alpha-human ANP (1-28) administered at the same dose per kg body weight in adult and neonatal rabbits. Plasma basal ANP levels were similar in 18 newborn (4- to 11-day-old) compared to 7 adult rabbits (150 +/- 16 and 151 +/- 28 pg/ml, resp.). Eleven newborn and 11 adult rabbits were anesthetized and mechanically ventilated. After a control period, each animal received an hANP loading dose (3 micrograms/kg i.v.), followed by an infusion of 0.3 micrograms/kg/min. Blood gases remained stable throughout the experiment in both groups. Mean blood pressure decreased in newborn (28.5 +/- 0.8 to 26.2 +/- 1.0 mmHg) and adult (92 +/- 3 to 84 +/- 3 mmHg) animals. Percent hANP-induced changes in renal functions in newborn and adult rabbits were, respectively: urine flow rate: -21 +/- 4% and +57 +/- 8%; urinary sodium excretion: +4 +/- 7% and +81 +/- 11%; glomerular filtration rate (GFR): -19 +/- 4% and -4 +/- 6%; renal blood flow (RBF): -22 +/- 4% and -11 +/- 5%. As expected, diuresis and natriuresis increased in adult rabbits. Failure of hANP to increase natriuresis and diuresis in newborn rabbits could be related to the marked decrease in GFR, receptor immaturity and/or interactions with other hormonal systems.

The influence of postnatal respiratory adaptation on sodium handling in preterm neonates

The relationship between sodium handling and alveolar-arterial oxygen gradient (AaDO2) was studied in 26 preterm babies requiring ventilation from birth. The difference between sodium intake and urinary sodium excretion was determined in sequential, four hourly periods throughout the course of the respiratory illness. Sodium retention changed from positive (mean 1.42 mmol kg-1 day-1) during the phase of deteriorating respiratory function to negative (mean - 1.99 mmol kg-1 day-1) during improving respiratory function. This difference is highly significant (P less than 0.001). A temporal relationship is also shown, in the form of individual time plots of AaDO2 and sodium retention for each baby; changes in sodium handling parallel changes in AaDO2. The diuresis that accompanies improving respiratory function is characterised as a natriuresis, with free water clearance remaining unchanged before and after the point of improvement. We suggest that a primary change in sodium handling, triggered by postnatal respiratory adaptation, may initiate the contraction in extracellular fluid volume that characterises early postnatal existence.

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.

Pharmacology of diuretics in the newborn

Diuretics are used in various conditions with fluid overload. Their efficacy in the management of congestive heart failure is well documented. In contrast, the indication of diuretics in chronic lung disease and central nervous system disorders of the newborn have not been clearly established. Substantial pharmacologic knowledge of diuretics in the young infant remains to be described. Most investigations on diuretics in the sick newborn infant have examined furosemide. In contrast, the pharmaco-dynamics, pharmacokinetics, clinical indications, and toxicity of other diuretics used in the newborn require considerable further evaluation. Future studies using a combination of diuretics, acting at different segments of the nephron, also may provide newer therapeutic modalities to overcome or prevent the development of frequently observed tolerance to diuretics, as well as to treat refractory edema.

Sodium balance and extracellular volume regulation in very low birth weight infants

We assessed sodium balance and extracellular volume regulation in very low birth weight infants by examining the effect of differences in sodium intake on postnatal sodium homeostasis and body water composition. Twenty infants (mean birth weight 1103 +/- 216 gm, mean gestation 28.5 +/- 1.7 weeks) were randomly assigned to receive sodium in doses of either 1 or 3 mmol.kg-1.day-1 for the first 10 postnatal days. Extracellular volume (estimated by the bromide dilution method), sodium excretion, creatinine clearance, fractional sodium excretion, plasma atrial natriuretic factor level, urine aldosterone concentration, and vasopressin excretion were measured on postnatal days 1, 5, 10, 20, and 30. The corrected bromide space was large at birth and decreased in both groups during the first 5 days of observation, concomitant with a negative sodium balance. After 5 days of age, sodium excretion decreased in both groups so that sodium balance became positive and the corrected bromide space increased in proportion to increasing body weight. Differences in sodium intake were associated with differences in tubular sodium reabsorption; corrected bromide space and net sodium balance were similar in the two groups. Serum sodium concentration was significantly lower in the low-sodium intake group. Creatinine clearance, plasma atrial natriuretic factor level, and excretion of aldosterone and vasopressin were not significantly different between the two groups. We conclude that very low birth weight infants are able to regulate sodium balance by altering renal sodium excretion. However, the renal response to sodium intake may be insufficient to prevent changes in serum sodium concentration. The roles of specific renal and hormonal mechanisms regulating sodium excretion in very low birth weight infants remain incompletely defined.

Nonoliguric hyperkalemia in the premature infant weighing less than 1000 grams

Eighteen very low birth weight premature infants born before 28 weeks gestation and weighing less than 1000 gm were evaluated prospectively for disturbances in serum electrolyte concentrations and for renal glomerular and tubular functions. Clinically symptomatic hyperkalemia resulting in significant electrocardiographic dysrhythmias developed in eight of these infants; 10 babies remained normokalemic. Peak serum potassium concentration ranged from 6.9 to 9.2 mEq/L in the hyperkalemic group; all potassium values in the normokalemic group were less than 6.6 mEq/L. Indices of renal glomerular function and urine output were similar in both groups; no infant had oliguria. Serum creatinine concentrations were the same in both groups (1.04 +/- 0.16 SD mg/dl in normokalemic vs 1.19 +/- 0.24 mg/dl in hyperkalemic infants, beta less than 0.2 at alpha = 0.05), and glomerular filtration rates did not differ significantly (6.29 +/- 1.78 ml/min/1.73 m2 in normokalemic vs 5.70 +/- 1.94 ml/min/1.73 m2 in hyperkalemic infants, beta less than 0.2 at alpha = 0.05). In contrast, indicators of tubular function revealed a significantly larger fractional excretion of sodium in hyperkalemic infants: 13.9 +/- 5.4% versus 5.6 +/- 0.9% in normokalemic control subjects (p less than 0.001). Hyperkalemic infants also had a tendency toward lower urine concentrations of potassium, although there was no significant difference in their net potassium excretion in comparison with that in the normokalemic group. We speculate that hyperkalemia in the tiny baby is in part the result of immature distal tubule function with a compromise in ability to regulate potassium balance.

Renal reabsorption of phosphate during development: tubular events

Studies performed in our laboratory on the isolated perfused kidney of the guinea pig have demonstrated that the rate of Pi reabsorption is substantially greater in the newborn than in the adult, when appropriate corrections are being made either for differences in glomerular filtration rate (GFR) or in renal tubular mass. In order to determine the location of this enhanced reabsorption along the nephron, micropuncture experiments were performed on euvolemic, non-fasted guinea pigs 5-14 and 42-49 days of age, maintained on standard guinea-pig chow diet (0.76% Pi). Concomitant measurements of overall kidney function were also obtained. The results confirmed that fractional reabsorption of Pi (TRPi%) across the entire kidney was significantly higher (P less than 0.01) in the newborn (89.93 +/- 2.55%) than in the adult (78.25 +/- 2.89%) animals. The difference was also significant (P less than 0.05) when TRPi was expressed in mol/ml GFR (1.87 +/- 0.14 vs 1.53 +/- 0.12, respectively). At comparable locations along the proximal tubule (TF/Pin of 1.90 +/- 0.16 in the newborn, and 1.79 +/- 0.15 in the adult, P greater than 0.70), the fraction of the filtered load of Pi reabsorbed was significantly higher (P less than 0.001) in the immature (76.66 +/- 2.74%) than in the mature (67.21 +/- 2.74%) guinea pigs. Estimates based on the differences between proximal Pi reabsorption and the urinary excretion of Pi indicate that the reabsorption of Pi in tubular segments located beyond the proximal tubule is also enhanced in the newborn when compared with the adult (15.62 +/- 2.11% vs 10.51 +/- 1.83%, respectively, P less than 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

Role of atrial natriuretic peptide in sodium homeostasis in premature infants

To examine the possible involvement of atrial natriuretic peptide (ANP) in sodium homeostasis in premature infants, two groups of low birth weight infants with different dietary sodium regimens were studied. Sodium balance and plasma concentration of ANP were measured at weekly intervals for 5 weeks. At 1 week of age the study was started by dividing infants into two groups, group 1 with low and group 2 with increased sodium intake. Mean plasma concentrations of ANP were 47.7 +/- 7.6 and 51.4 +/- 9.5 fmol/ml, respectively. A steady decrease in plasma ANP concentration to 18.8 +/- 2.9 fmol/ml was observed in infants with sodium intake 1.5 mmol/kg/d (group 1), which was related to the decrease in serum sodium concentration in this group. In contrast, supplementation with NaCl 4.6 mmol/kg/d (group 2) was associated with a 30% increase in plasma ANP concentration, significantly different (P less than 0.025) from that in infants not given supplement, and was also higher than the values in full-term neonates. Our data suggest that altered sodium homeostasis induces regulatory changes in plasma ANP levels. ANP may provide a sensitive and important hormonal system for the control of sodium balance, even in premature neonates.

The ontogeny of renal alpha 1- and alpha 2-adrenoceptors in the Dahl rat model of experimental hypertension

[3H]prazosin (PRAZ) and [3H]rauwolscine (RAUW) were used to examine the ontogeny of renal alpha 1- and alpha 2-adrenoceptors in the inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rat. PRAZ and RAUW each bound to a single population of non-interacting sites. The binding of each ligand was saturable and reversible. The greatest proliferation of each receptor subtype occurred between 5 and 25 days of age. During this period, a 4 to 5-fold increase in the density of each was observed. Adult levels of each were reached by 50 days of age. The alpha 2-adrenoceptor was the predominant subtype present in renal tissue. However, its ratio to the alpha 1 subtype was influenced by strain and age: the ratio was greatest in the S/JR strain and decreased with age in both strains. The profile of alpha 1-adrenoceptor development was similar in S/JR and R/JR rats. In contrast, the density of alpha 2-adrenoceptors was similar in S/JR and R/JR rats at 5 and 15 days of age but significantly greater in the S/JR rat between 25 and 150 days of age. The elevated density of alpha 2-adrenoceptors could not be explained by strain-related differences in blood pressure or alterations in the affinity of the receptor. The results suggest that a relationship may exist between elevated renal alpha 2-adrenoceptor density and the genetic predisposition to hypertension in the S/JR rat. However, because this relationship is not apparent during the neonatal period of development, the possibility that the elevated density of sites may be secondary to some other event should also be considered.

Modern fluid and electrolyte management of the critically ill premature infant

In this article, the authors introduce the concept of a transitional physiology which governs fluid and electrolyte balance in the immediate postnatal period. The important impact of the extrauterine environment on fluid balance is also discussed. Finally, the pathophysiology of diuresis in RDS, and fluid shifts in the VLBW infant with therapeutic recommendations are presented.

Effect of variations in dietary sodium intake on sodium excretion in mature rats

Sprague-Dawley rats weighing 400 g or more were studied to determine whether their continued weight gain affects renal sodium handling. Rats maintained on a wide range of sodium intakes gained 3.9 +/- 0.4 g/day. The intercept of a linear regression of intake against urinary excretion provided an estimate of the minimum daily requirement for sodium intake of 247 +/- 33 microEq/day. When more than this required amount was ingested, the animals excreted the excess quantitatively in the urine. When less was ingested they continued to gain weight at a slower rate, 1.6 +/- 0.6 g/day, and remained in positive sodium balance. Nonetheless, they developed a sodium deficit manifested as retention of a sodium challenge. Thus, on an adequate dietary intake the normal physiological state of Sprague-Dawley rats of this size is one of chronic sodium retention rather than neutral sodium balance. In contrast, when inadequate sodium is ingested a deficit develops in the absence of external losses. These observations have important implications for the interpretation of studies of renal sodium handling in these animals.

Peripolar cell hypertrophy in the renal juxtaglomerular region of newborn sheep

In the renal juxtaglomerular region of newborn sheep, it was found that glomerular peripolar cells and their granules were very much larger than those found in fetal lambs or adult sheep. Similar peripolar cell hypertrophy was triggered in fetal lambs treated in utero with intraperitoneal injections of dexamethasone. Ultrastructurally, granules of peripolar cells from newborn lambs resembled closely the enlarged zymogen granules described in the pancreas of newborn rats. Such peripolar cell hypertrophy may reflect a functional adaptation of the kidney to immediate postnatal life.

Fetal blood volume responses to intravenous saline solution and dextran

The purpose of this study was to determine to what extent the volume of blood circulating in the fetus can be increased by intravascular infusions of either isotonic saline solution or high-molecular weight dextran in saline solution. Unanesthetized, chronically catheterized fetal sheep that averaged 130 days' gestation were studied. In seven fetuses, 50 to 100 ml of warm saline solution (i.e., an average of 19.5% of the initial fetal blood volume) was infused into a vein over 5 minutes, and changes in blood volume were determined 10, 20, and 30 minutes after the infusion was begun. At these times, an average of only 15%, 9%, and 7%, respectively, of the infused volume remained in the fetal circulation. A second infusion of saline solution into these fetuses gave intravascular retentions that averaged 17%, 6%, and 6% at 10, 20, and 30 minutes, respectively. In five fetuses, a 6% solution of dextran 70 that averaged 14.9% of the initial fetal blood volume was infused over 5 minutes. The intravascular retentions averaged 80%, 63%, and 55%, respectively, at 10, 20, and 30 minutes after the infusions were started. Under comparable conditions in the adult, blood volume expansion 30 minutes after rapid infusions averages 20% to 40% of the infused volume for saline solution and 150% to 200% for 6% dextran 70. Thus, the present study suggests that intravascular infusions of crystalloid solutions have very little effect on fetal blood volume, whereas oncotic solutions cause only moderate blood volume expansion.

Transient pseudohypoaldosteronism secondary to obstructive uropathy in infancy

A syndrome of renal tubular resistance to aldosterone has been identified in infants with obstructive uropathy and urinary tract infection. Six infants (ages 9 days to 7 months) were seen with fever, vomiting, polyuria, dehydration, or failure to thrive. Urine cultures were positive for Escherichia coli. Radiologic studies demonstrated bilateral ureterohydronephrosis (four patients), left ureteral duplication with upper pole hydronephrosis (one), and left vesicoureteral reflux (one). The infants had hyponatremia, hyperkalemia, and metabolic acidosis. Plasma aldosterone concentration was markedly elevated, and plasma renin activity was similar to or higher than that reported in normal infants of comparable age. Fractional excretion of potassium was not significantly different from control values, both in absolute terms or when related to glomerular filtration rate, but fractional sodium excretion was significantly increased. The UK/UNa ratio was significantly lower in the patients. After medical or surgical therapy (when appropriate), all blood and urine determinations returned to normal, except for UK/UNa values, which although higher, remained significantly diminished. Our data indicate that a hyperkalemic salt-losing state can arise in infants with obstructive uropathy and urinary tract infection as a consequence of tubular unresponsiveness to aldosterone, and that the clinician should rule out such cause before establishing the diagnosis of primary pseudohypoaldosteronism.