Urinary N-acetyl-beta-D-glucosaminidase excretion in term and preterm neonates

Tubular dysfunction in extremely low birth weight survivors

BACKGROUND

Extremely low birth weight (ELBW) survivors may develop glomerulosclerosis due to low nephron number, whereas their tubular function remains unknown except for hypercalciuria and phosphaturia.

METHODS

Fifty-three subjects (30 boys and 23 girls, aged 7 months-19 years, median 36 months) were studied retrospectively. The median gestational age and birth weight were 26 weeks (range 22-32) and 745 g (range 316-999), respectively. Urine calcium-to-creatinine ratio (Ca/Cr), N-acetyl-β-D-glucosaminidase-to-creatinine ratio (NAG/Cr), β2 microglobulin-to-creatinine ratio (β2m/Cr), uric acid-to-creatinine ratio (UA/Cr), glucose-to-creatinine ratio (glu/Cr), and microalbumin-to-creatinine ratio (malb/Cr) were examined. We also assessed the association between urine parameters and current age, gestational age, birth weight, and predictors of renal injury. Follow-up data were analyzed in 43 subjects 4-6 years later.

RESULTS

Ninety percent of subjects had at least one tubular dysfunction. Frequency of elevated values was NAG/Cr 77.5%, UA/Cr 54.1%, β2m/Cr 38.2%, malb/Cr 30.4%, Ca/Cr 21.5%, and glu/Cr 20.5%. There were significant negative correlations between the current age and Ca/Cr, NAG/Cr, glu/Cr, and UA/Cr, suggesting tubular function maturation. Urine β2M/Cr and glu/Cr were negatively correlated with the gestational age. There were significant associations between elevated glu/Cr and asphyxia or neonatal acute kidney injury, and elevated NAG/Cr and indomethacin use, although these were not confirmed by multivariate analysis. At follow-up, the frequency of elevated NAG/Cr, glu/Cr, UA/Cr, and malb/Cr was reduced but that of elevated Ca/Cr, IgG/Cr, and β2m/Cr remained similar or increased.

CONCLUSION

Tubular dysfunction is common in ELBW survivors. Some abnormalities resolved with age while some remained persistent or even increased.

The Diagnostic Role of Urinary N-Acetyl-β-D-glucosaminidase (NAG) Activity in the Detection of Renal Tubular Impairment

The kidney function can be assessed by a number of methods. The urinary excretion of enzymes, in particular N-acetyl-β-D-glucosaminidase (NAG), is considered a relatively simple, cheap, fast and non-invasive method in the detection and follow-up of renal tubular function under various conditions. The determination of urinary NAG provides a very sensitive and reliable indicator of renal damage, such as injury or dysfunction due to diabetes mellitus, nephrotic syndrome, inflammation, vesicoureteral reflux, urinary tract infection, hypercalciuria, urolithiasis, nephrocalcinosis, perinatal asphyxia, hypoxia, hypertension, heavy metals poisoning, treatment with aminoglycosides, valproate, or other nephrotoxic drugs. This paper gives an overview of the current use of urinary NAG in the detection of renal injury.

Biochemical parameters of renal impairment/injury and surrogate markers of nephron number in intrauterine growth-restricted and preterm neonates at 30-40 days of postnatal corrected age

BACKGROUND

Premature and/or intrauterine growth-restricted neonates have an increased risk of developing postnatal renal injuries in later life. Studies on renal physiology in these neonates at a corrected age of 30-40 days are scarce and mostly relate to preterm infants. The data from these studies often lack the results of correlation analyses between biochemical parameters and nephron number-data which could provide additional insight and/or improve recognition of individuals at higher risk of renal failure.

METHODS

Urinary total protein and albumin levels and N-acetyl-β-D-glucosaminidase and cathepsin B activity were evaluated in preterm and intrauterine growth-restricted infants at a corrected age of 30-40 days and compared to data from a healthy control neonate population. The data were then associated with predominant susceptibility factors of renal damage related to low nephron number, such as gestational age, birth weight, total renal volume and renal cortex volume.

RESULTS

Compared to the control neonate population, we found significantly increased levels of all biochemical parameters tested in the intrauterine growth-restricted neonates, whereas in the preterm infants we observed a significant increase in cathepsin B activity, total protein level and, to a lesser extent, albumin level. Cathepsin B activity showed a significant, strong and inverse correlation with all surrogate markers of nephron number and was also strongly and positively correlated with urinary albumin level.

CONCLUSIONS

At this postnatal age, we found that lower nephron number in low birth weight neonates was associated to tubular impairment/injury that could be concurrent with a dysfunction of glomerular permeability. Urinary cathepsin B activity may be a candidate marker for the early prediction of renal susceptibility to damage in low birth weight neonates.

Evaluation of renal functions in preterm infants with respiratory distress syndrome

BACKGROUND

The aim of this prospective study was to evaluate urinary glutathione S transferases π (GST-π), beta-2-microglobulin (B2-MG), and N-acetyl-β-d-glucosaminidase (NAG) levels as markers revealing the effect of respiratory distress syndrome (RDS) on renal function in preterm infants.

METHODS

The study was performed with 76 preterm infants whose gestational ages were between 28 and 32 weeks. Twenty-six preterm infants with RDS (cases) and 50 preterm infants without RDS (controls) enrolled in the study. Blood and urine samples were obtained on postnatal (PN) day 3 and 30. Urinary GST-π levels were measured by enzyme-linked immunosorbent assay (ELISA), and urinary B2-MG levels were determined by nephelometric method.

RESULTS

There was no significant difference in urinary B2-MG and GST-π levels between RDS and non-RDS groups on PN day 3 (P > 0.05 for each). However, preterm infants with RDS had significantly higher urinary B2-MG and GST-π levels than the control group on PN day 30 (P = 0.0001 and P = 0.031, respectively). Urinary NAG levels were higher in RDS group than those of the controls on both PN day 3 and 30, but these findings were not statistically significant (P > 0.05, for each).

CONCLUSION

Preterm infants with RDS had increased levels of both GST-π and B2-MG levels on PN day 30, suggesting subclinical tubular dysfunction, probably secondary to hypoxic stress.

Quantitation of glutathione S transferase-pi in the urine of preterm neonates

BACKGROUND

Glutathione S transferases (GSTs) are widely distributed enzymes found in highly varying amounts in tissues of the human body. The enzyme GST-pi in urine has been used as a marker of renal distal tubular cell damage. The present study was intended to evaluate urinary excretion of GST-pi and its relationship to other renal markers and to the status of oxidative stress in preterm neonates.

METHODS

Levels of urinary GST-pi, N-acetyl-beta-glucosaminidase (a marker of proximal tubular damage), albumin (a marker of glomerular damage) and 8-hydroxy-2'-deoxyguanosine (a marker of oxidative stress) and serum creatinine were measured in preterm neonates at 1 and 4 weeks of age.

RESULTS

The results showed that urinary excretion of GST-pi is increased in preterm neonates compared with reported values for healthy adults. No significant relationship was detected between urinary GST-pi and other markers for renal function. Urinary GST-pi showed significantly positive correlation with urinary 8-hydroxy-2'-deoxyguanosine at 1 and 4 weeks. Sick neonates treated with supplemental oxygen and mechanical ventilation showed significantly higher levels of GST-pi as well as 8-hydroxy-2'-deoxyguanosine than clinically stable neonates did at 4 weeks.

CONCLUSIONS

These results indicate the potential effect of systemic oxidative stress on urinary excretion of GST-pi. Further studies are necessary to explore the effect of oxidative conditions on expression of GST-pi in distal tubules in the human kidney.

Quantification of L-type fatty acid binding protein in the urine of preterm neonates

We measured urinary excretion of L-type fatty acid binding protein (L-FABP) in preterm neonates on days 1, 5-10, and 25-30 of life. Urinary L-FABP levels (expressed as the ratio to creatinine) in preterm neonates were considerably higher than those of healthy adults. They did not change significantly during the study period. Urinary L-FABP levels showed significant positive correlation with those of urinary N-acetyl-beta-D-glucosaminidase activity on day 1, and with those of 8-hydroxy-2'-deoxyguanosine on days 25-30. These results suggest that L-FABP is expressed in the neonatal kidney. Our results may also point to potential effects of proximal tubular damage and oxidative stress on urinary excretion of L-FABP.

Renal water channel expression in newborns: measurement of urinary excretion of aquaporin-2

Aquaporin-2 (AQP-2) encodes the vasopressin-regulated "water channels" of the renal collecting duct and is excreted in human urine. We measured urinary excretion of AQP-2 by radioimmunoassay in 15 term and 10 preterm infants on day 1 and day 4 of life to determine the molecular basis of water balance during the newborn period. AQP-2 was detectable in the urine of term and preterm newborns, but AQP-2 excretion was severalfold less than the reported level in normal adults. Urinary excretion of AQP-2 significantly decreased postnatally, in parallel with a reduction in urine osmolality and arginine vasopressin (AVP) excretion. Urinary AQP-2 correlated positively and significantly with urine osmolality on days 1 and 4 and with AVP on day 1 in both groups. No significant differences were detected in AQP-2 levels between term and preterm newborns. Our findings suggest that vasopressin-regulated water channels are expressed in the renal collecting duct of both term and preterm newborns, although to a lesser extent as compared with adults, and these channels encoded by AQP-2 contribute to the urine concentrating power of the newborn kidney.

High-turnover osteopenia in preterm infants: determination of urinary pyridinium cross-links of collagen

Osteopenia is a frequent condition in preterm infants, but its pathogenesis is uncertain. In the present study, we measured longitudinal changes in the excretion of pyridinium cross-links of collagen (specific markers of bone resorption) and evaluated the relationship between collagen cross-links and other indexes of bone and renal function in preterm infants. In these infants, urinary collagen cross-links were markedly increased on day 7 and day 30 of life and at estimated full-term gestation. The values were several times higher than those of older children and almost comparable to those of healthy full-term infants. Cross-link excretion did not correlate with beta2-microglobulin (B2M) or N-acetyl-beta-D-glucosaminidase (NAG) activity (markers of renal function), indicating that cross-link excretion is not influenced directly by infantile renal function. High serum osteocalcin and low bone mineral density (BMD) in the lumbar spine were also observed at estimated full-term gestation. There was no significant correlation between collagen cross-link excretion and either serum osteocalcin or spine BMD. We conclude that a state of high bone turnover underlies the development of osteopenia in preterm infants.

Renal functional measurements in young rats with chronic inhibition of nitric oxide synthase

The purpose of the present study was to examine renal functional changes caused by chronic blockade of nitric oxide (NO) synthesis in young rats. Two types of NO synthase inhibitor were used: NG-nitro-L-arginine methyl ester (L-NAME) as a non-selective inhibitor and aminoguanidine (AG) as a selective inhibitor of the inducible isoform. Oral administration of L-NAME (20-80 mg/dL of drinking water), not AG (400 mg/dL), for 4 weeks induced systemic hypertension in the treated rats. Both inhibitors caused a significant reduction in urinary excretion of NO2-/NO3-. Rats treated with L-NAME developed proteinuria and tubular enzymuria (high excretion of N-acetyl-beta-D-glucosaminidase) in a dose-dependent fashion, with normal serum levels of creatinine, albumin and cholesterol. Chronic AG administration did not alter the urinary levels of protein and N-acetyl-beta-D-glucosaminidase or serum laboratory values. Overall, these observations highlight the importance of the continuous generation of NO by the constitutive isoform in the control of vascular tone and the maintenance of renal glomerular and tubular function. Oral administration of L-NAME may serve as a model of chronic NO-deficient hypertension with renal injury in young rats.