Renal function in very low birth weight infants: normal maturity reached during early childhood

Long-term renal follow-up of extremely low birth weight infants

There is evidence that low birth weight caused by intrauterine growth retardation adversely affects normal renal development. Very little information on this issue is available on children born very prematurely. This investigation examined clinical and functional renal parameters in 40 children (23 boys, 17 girls) ranging in age between 6.1 and 12.4 years and weighing less than 1000 g at birth. Results were compared to those obtained in 43 healthy children of similar age and gender. Study subjects were significantly smaller and thinner than control subjects (mean height SDS: -0.36 vs. +0.70; and mean BMI SDS: -0.56 vs. +1.18). Systolic, diastolic, and mean blood pressures did not differ from those of controls. Renal sonography revealed no abnormality, and mean percentiles for renal length and volume appeared normal. In comparison with controls, plasma creatinine concentration (0.62+/-0.1 vs. 0.53+/-0.1 mg/dl) and estimated creatinine clearance (117+/-17 vs. 131+/-17 ml min(-1) 1.73 m(-2)) differed significantly. No significant differences were observed in microalbuminuria values, but five study subjects (12.5%) presented values above the upper limit of normality. A defect in tubular phosphate transport was also evident: TmP/GFR (3.6+/-0.4 vs. 4.2+/-0.8 mg/dl) and TRP (83+/-5% vs. 90+/-4%) were significantly lower, and urinary P excretion, estimated by the ratio UP/UCr, was significantly higher (1.2+/-0.4 vs. 0.9+/-0.4 mg/mg) than controls. Urinary calcium excretion, estimated by the UCa/UCr ratio, was also significantly higher (0.15+/-0.07 vs. 0.12+/-0.09 mg/mg). These data clearly demonstrate that both GFR and tubular phosphate transport are significantly diminished in school-age children born with extreme prematurity, probably as a consequence of impaired postnatal nephrogenesis.

Acute renal failure in the newborn

Acute renal failure in the newborn is a common problem and is typically classified as prerenal, intrinsic renal disease including vascular insults, and obstructive uropathy. In the newborn, renal failure may have a prenatal onset in congenital diseases such as renal dysplasia with or without obstructive uropathy and in genetic diseases such as autosomal recessive polycystic kidney disease. Acute renal failure in the newborn is also commonly acquired in the postnatal period because of hypoxic ischemic injury and toxic insults. Nephrotoxic acute renal failure in newborns is usually associated with aminoglycoside antibiotics and nonsteroidal anti-inflammatory medications used to close a patent ductus arteriosis. Alterations in renal function occur in approximately 40% of premature newborns who have received indomethacin and such alterations are usually reversible. Renal artery thrombosis and renal vein thrombosis will result in renal failure if bilateral or if either occurs in a solitary kidney. Cortical necrosis is associated with hypoxic/ischemic insults due to perinatal anoxia, placenta abruption and twin-twin or twin-maternal transfusions with resultant activation of the coagulation cascade. As in older children, hospital acquired acute renal failure is newborns is frequently multifactorial in origin. Although the precise incidence and prevalence of acute renal failure in the newborn is unknown, several studies have shown that acute renal failure is common in the neonatal intensive care unit. Recent interesting studies have demonstrated that some newborns may have genetic risks factors for acute renal failure. Once intrinsic renal failure has become established, management of the metabolic complications of acute renal failure continues to involve appropriate management of fluid balance, electrolyte status, acid-base balance, nutrition and the initiation of renal replacement therapy when appropriate. Renal replacement therapy may be provided by peritoneal dialysis, intermittent hemodialysis, or hemofiltration with or without a dialysis circuit. The preferential use of hemofiltration by pediatric nephrologists is increasing while the use of peritoneal dialysis is decreasing except for neonates and small infants. Peritoneal dialysis has been a major modality of therapy for acute renal failure in the neonate when vascular access may be difficult to maintain. In the newborn, the prognosis and recovery from acute renal failure is highly dependent upon the underlying etiology of the acute renal failure. Factors that are associated with mortality include multiorgan failure, hypotension, need for pressors, hemodynamic instability, and need for mechanical ventilation and dialysis. The mortality and morbidity of newborns with acute renal failure is much worse in neonates with multiorgan failure. Newborns who have suffered substantial loss of nephrons as may occur in cortical necrosis are at risk for late development of renal failure after apparent recovery from the initial insult. Similarly, hypoxic/ischemic and nephrotoxic injury to the developing kidney can result is decreased nephron number. Newborns with acute renal failure need life-long monitoring of their renal function, blood pressure, and urinalysis. Typically, the late development of chronic renal failure will first becomes apparent with the development of hypertension, proteinuria, and eventually an elevated blood urea nitrogen and creatinine.

Pharmacogenomics and renal drug disposition in the newborn

Genetic polymorphisms in the genes coding for drug metabolizing enzymes, drug transporters, and drug receptors are major determinants of an individual's response to drugs. The potential interactions of pharmacogenomics of renal drug transporters and drug receptors with renal drug disposition and the immature kidneys are briefly reviewed. Examples of gene polymorphisms seen in the RAAS (renin angiotensin system), beta-adrenergic receptors, dopamine receptors and cytochrome P450 and their potential clinical impact are discussed. The human newborn has deficient hepatic and renal drug metabolism and disposition. This immaturity in drug-handling capacity may potentially be superimposed to genetic polymorphisms determining drug metabolism and transport thereby substantially increasing interpatient variability in drug dose requirements and in drug responses in the newborn. Pharmacogenomics is a tool that can be used to individualize drug therapy in newborns to minimize adverse drug effects and to optimize efficacy.

Evaluation of renal glomerular and tubular functional and structural integrity in neonates

BACKGROUND

Renal cells are not fully differentiated at birth, representing a major risk in preterm infants. We evaluated glomerular and tubular functional integrity as well as structural integrity of renal tubules among healthy full-term and preterm infants as well as diseased preterm infants.

METHODS

A total of 50 newborns (10 healthy full-term, 10 healthy preterm, and 30 diseased preterm, at 38.9 +/- 1.10, 34.2 +/- 0.92, and 32 +/- 2.47 weeks gestational age, respectively) were included in the present study. Glomerular function was assessed by measuring urinary levels of both microalbumin and immunoglobulin G as well as serum creatinine levels, whereas the proximal tubular function was investigated by measuring the urinary levels of both alpha1-microglobulin and beta2-microglobulin as well as retinol-binding protein. Also, distal tubular reabsorption capacity was investigated by assessing fractional excretion of sodium. Moreover, the structural integrity of renal proximal tubules was studied by measuring the urinary activities of both the brush-border membrane enzyme leucine-aminopeptidase (LAP) and the lysosomal enzyme N-acetyl-beta-D-glucosaminidase. The preceding investigations were done on both the first and third days of life of all 50 newborns.

RESULTS

Glomerular and tubular function and structure was relatively impaired at birth among both healthy and diseased preterm as well as healthy full-term neonates and improved rapidly thereafter. The diseased preterm neonates showed worse renal function and structure with minimal improvement regardless of the underlying sickness.

CONCLUSION

Renal insufficiency and renal immaturity could be evaluated using enzymuria and low- and high-molecular-weight proteinuria as noninvasive methods.

Nutrient requirements for preterm infant formulas

Achieving appropriate growth and nutrient accretion of preterm and low birth weight (LBW) infants is often difficult during hospitalization because of metabolic and gastrointestinal immaturity and other complicating medical conditions. Advances in the care of preterm-LBW infants, including improved nutrition, have reduced mortality rates for these infants from 9.6 to 6.2% from 1983 to 1997. The Food and Drug Administration (FDA) has responsibility for ensuring the safety and nutritional quality of infant formulas based on current scientific knowledge. Consequently, under FDA contract, an ad hoc Expert Panel was convened by the Life Sciences Research Office of the American Society for Nutritional Sciences to make recommendations for the nutrient content of formulas for preterm-LBW infants based on current scientific knowledge and expert opinion. Recommendations were developed from different criteria than that used for recommendations for term infant formula. To ensure nutrient adequacy, the Panel considered intrauterine accretion rate, organ development, factorial estimates of requirements, nutrient interactions and supplemental feeding studies. Consideration was also given to long-term developmental outcome. Some recommendations were based on current use in domestic preterm formula. Included were recommendations for nutrients not required in formula for term infants such as lactose and arginine. Recommendations, examples, and sample calculations were based on a 1000 g preterm infant consuming 120 kcal/kg and 150 mL/d of an 810 kcal/L formula. A summary of recommendations for energy and 45 nutrient components of enteral formulas for preterm-LBW infants are presented. Recommendations for five nutrient:nutrient ratios are also presented. In addition, critical areas for future research on the nutritional requirements specific for preterm-LBW infants are identified.

Cloning, expression, and ontogeny of mouse organic anion-transporting polypeptide-5, a kidney-specific organic anion transporter

The full-length coding sequence of mouse organic anion-transporting polypeptide (designated mouse Oatp-5) has been cloned from mouse kidney cDNA library. Analysis of the 5'-untranslated region (5'-UTR) of Oatp-5 cDNA through capsite cloning reveals two possible transcription start sites that are 4-bp apart. The 3'-untranslated region (3'-UTR) of Oatp-5 cDNA contains an early polyadenylation signal, indicating the possibility that mRNAs with different 3'-UTR lengths may coexist. Deduced amino acid sequence of mouse Oatp-5 protein contains 670 amino acids and has 10 putative transmembrane domains, multiple potential glycosylation and phosphorylation sites. Tissue-specific expression studies indicate that mouse Oatp-5 is expressed only in kidney. Studies on the developmental expression reveal that there is no significant expression of Oatp-5 mRNA in mouse kidney for at least 3 weeks after birth, and adult levels of Oatp-5 mRNA expression are attained more than 6 weeks after birth. Phylogenetic analysis reveals that mouse Oatp-5 is an ortholog of rat Oatp-5.

Developmental changes in multispecific organic anion transporter 1 expression in the rat kidney

BACKGROUND

The cDNA of the multispecific organic anion transporter 1 (OAT1) responsible for the tubular secretion of organic anions was recently isolated. In the current study, we investigated the developmental changes in OAT1 expression in the rat kidney.

METHODS

Ontogenic expression of rat OAT1 was investigated by Northern blot, in situ hybridization, Western blot, and immunohistochemical analysis. In addition, para-aminohippurate (PAH) accumulation was measured using fetal, neonatal, and adult rat kidney slices.

RESULTS

In Northern blot analysis, OAT1 was detected as early as on embryonic day 18 in the fetal kidney. The expression level of OAT1 mRNA increased remarkably just after birth (postnatal day 0). In situ hybridization revealed OAT1 expression on embryonic day 19. In both the fetal and neonatal kidneys, OAT1 mRNA was localized in a relatively deep region in the cortex. Western blot analysis detected OAT1 protein on embryonic day 20, and the expression level increased after birth. Immunohistochemical analysis did not reveal OAT1 staining in the fetal kidneys. A faint signal of OAT1 protein was detected on postnatal day 0; thereafter, the expression level increased. In the functional study using kidney slices, low but definite probenecid-sensitive PAH accumulation was noted in fetal rat kidney on embryonic day 20. After birth, probenecid-sensitive PAH uptake was increased.

CONCLUSIONS

The present study consistently demonstrates the remarkable increase of OAT1 expression after birth, and the immature excretory capacity of the proximal tubules of the neonatal kidney can be attributed, at least in part, to the low expression level of OAT1.

Water and electrolyte metabolism of the micropremie

There are five problem schemas presented in this article that indicate potential contradictions in therapeutic goals: (1) shock and edema presenting upon premature birth; (2) the hyperosmolar state, problematic in patients less than 750 g birth weight; (3) the respiratory distress syndrome and respiratory failure, often complicated by patent ductus arteriosus; (4) bronchopulmonary dysplasia, resulting from prematurity and mechanical ventilation; and (5) late onset of hyponatremia, sometimes accompanied by growth failure in the recovering premature. These five problems considered together comprise a developmental continuum of illness and recovery, where appropriate fluid management has recently been demonstrated to benefit outcomes greatly. Clinicians over the past decade have encountered all of the fluid and electrolyte nightmares. Although there are many different formulations for treating each of these scenarios, recommending one approach for all patients is likely to be incorrect much of the time.

Intrauterine growth restriction is associated with persistent juxtamedullary expression of renin in the fetal kidney

BACKGROUND

Intrauterine growth restriction (IUGR) has been linked to impaired renal function and hypertension, suggesting that an adverse prenatal environment could alter kidney development and renin production.

METHODS

Immunohistochemistry and in situ hybridization were employed to localize renin-containing cells (RCCs) in the deep, middle, and superficial zones of autopsy kidney sections, in parallel with histologic maturation, from unexplained stillborn fetuses of normal weight (N = 26) and stillborn fetuses with IUGR (N = 17).

RESULTS

In the control group, the number of RCC per 100 glomeruli in the deep zone decreased with advancing gestation from 40 at 20 weeks gestation to five at term (P < 0.001), whereas the opposite change was found in the superficial zone (increase from 5 per 100 to 55 per 100; P < 0.001). In the IUGR group, the density of RCCs in both the superficial and deep zones was similar to the control group at 20 weeks, and no shift in renin gene expression was observed as gestation advanced. Histologic maturation was unaltered.

CONCLUSIONS

Renin gene expression persists and predominates in the deep renal cortex of the stillborn IUGR fetus, and could contribute to the pathogenesis of neonatal oliguria and/or hypertension during postnatal life.

Non-furosemide-related renal calcifications in premature infants with bronchopulmonary dysplasia

Renal calcification is a known complication of long-term furosemide therapy in infants with bronchopulmonary dysplasia (BPD). In a prospective study the clinical course and long-term renal sequelae of renal calcifications of 19 consecutive premature neonates (birthweight < 1250 g) with bronchopulmonary dysplasia who did not receive furosemide were examined. Infants were divided into two different groups on the basis of ultrasound evidence of renal calcifications (RC group) or absence of renal calcifications (NRC group). Serial examinations, performed at the age of 1, 2, 3, 6, 9 and 12 months, showed that 12 infants at the mean age of 68.5 +/- 12.8 days of life had renal calcifications (63%), and 3 of them had nephrolithiasis; 8 had bilateral renal calcifications. Among the 9 survivors, 2 had chronic renal calcifications at the age of 9 months; however, all normalized at the age of 12 months. Twelve infants received hydrochlorothiazide and spironolactone (63%), 17 had prolonged courses of xanthines and dexamethasone (89.5%), while furosemide was not part of the routine pharmacological administration. Statistical analysis showed that birthweight, gestational age, Apgar score and length of parenteral nutrition were comparable in the RC and NRC group infants. Mean serum creatinine, creatinine clearance, fractional sodium excretion and urinary calcium excretion values during the 12-month study period were comparable in the RC and NRC groups. Mechanical ventilation and hospital stay length were instead associated with renal calcification occurrence. The strongest indicator of renal calcification risk for this high-risk population is the severity of the unresolved acute lung disease, where different facets of respiratory management, other than the addition of furosemide, represent sufficient stimuli and renal injury to potentiate stone formation.

Developmental renal physiology of the low birth weight pre-term newborn

PURPOSE

The remarkable growth of the kidney and the rapid changes in renal function in the second half of gestation and early postnatal period are discussed.

MATERIALS AND METHODS

Adaptation to the extrauterine environment involves immediate postnatal natriuresis that is prolonged in the pre-term neonate, followed by the sodium retention necessary for growth. Glomerular filtration rate increases throughout the postnatal period, and it is modulated by the renin-angiotensin system and prostaglandins. Because of this, the fetus and neonate are particularly susceptible to renal injury following the administration of angiotensin converting enzyme inhibitors or nonsteroidal anti-inflammatory drugs. Renal tubular function in the neonate is characterized by reduced renal concentration and acidification ability, which can be further compromised by obstructive uropathy. Urine calcium excretion is high in the neonate, which can be aggravated by calciuric drugs, such as furosemide and glucocorticoids.

RESULTS

Reduced renal mass results in compensatory renal growth even in the fetus, a response that could prove maladaptive later in life through excessive glomerular hyperfiltration and progressive interstitial fibrosis.

CONCLUSIONS

These factors underscore the importance of attempting to maximize functional renal mass in the neonate or infant with renal impairment of any etiology.