Maturation of renal potassium transport

Congenital urinary tract obstruction: defining markers of developmental kidney injury

Congenital urinary tract obstruction (diagnosed antenatally by ultrasound screening) is one of the main causes of end-stage kidney disease in children. The extent of kidney injury in early gestation and the resultant abnormality in kidney development determine fetal outcome and postnatal renal function. Unfortunately, the current approach to diagnostic evaluation of the severity of injury has inherently poor diagnostic and prognostic value because it is based on the assessment of fetal tubular function from fetal urine samples rather than on estimates of the dysplastic changes in the injured developing kidney. To improve the outcome in children with congenital urinary tract obstruction, new biomarkers reflecting these structural changes are needed. Genomic and proteomic techniques that have emerged in the past decade can help identify the key genes and proteins from biological fluids, including amniotic fluid, that might reflect the extent of injury to the developing kidney.

Hypernatraemia induced by sodium polystyrene sulphonate (Kayexalate) in two extremely low birth weight newborns

Hyperkalaemia is a life-threatening electrolyte disorder that can occur in the first week of life in almost 50% of preterm infants with a birth weight less than 1000 g [extremely low birth weight (ELBW)]. Serum potassium values higher than 7 mmol x l-1 are associated with cardiac arrhythmias and an increased incidence of intraventricular haemorrhage and periventricular leucomalacia. Therapeutic options to treat this dangerous imbalance comprise calcium gluconate, insulin plus glucose, albuterol/salbutamol inhalation. Administration of cation-exchange resin such as sodium polystyrene sulphonate (Kayexalate) is effective in lowering plasma potassium, although complications following oral or rectal administration are reported in newborns. We describe two ELBW infants affected by hyperkalaemia, treated with Kayexalate, who developed serious hypernatraemia, that has never been reported before in preterm infants.

Biochemical profile of amniotic fluid for the assessment of fetal and renal development

Creatinine plays a key role in the function and maturation of fetal kidneys throughout pregnancy. It is important to identify other markers that may help in the diagnosis of renal dysfunction. Our aim was to determine the profile of and the correlation between biochemical markers to be used to assess renal function and maturation of the fetus in the amniotic fluid during pregnancy and to determine the distribution of normal values for creatinine, N-acetyl-beta-D-glucosaminidase (NAG), beta2-microglobulin, glucose, urea, sodium, potassium, phosphorus, calcium, uric acid, albumin, and osmolality in three gestational age groups. This was a cross-section study that assessed 115 samples of amniotic fluid during three different periods of pregnancy, i.e., 13 to 20, 27 to 34, and 36 to 42 weeks. Concentrations of creatinine, NAG, urea, potassium and uric acid increased during pregnancy (P<0.05). Beta2-microglobulin, glucose, sodium, phosphorus, calcium, and albumin concentration and osmolality decreased (P<0.05), whereas beta2-microglobulin, glucose and uric acid presented significant correlations with gestational age and creatinine, respectively (r>0.6, P<0.05). Urea, potassium and phosphorus showed mild correlations with both (r>0.5, P<0.05). NAG, sodium, albumin and osmolality did not show significant correlations (r<0.5, P<0.05). These tests confirmed the important role of creatinine in terms of correlation with gestational age. beta2-Microglobulin, glucose and uric acid were significant as markers of function and maturation of fetal kidneys, whereas NAG did not demonstrate a useful role for the assessment of renal maturation.

Low systemic blood flow and hyperkalemia in preterm infants

OBJECTIVE

Early low systemic blood flow is common in preterm infants. This study examines the relationship among low flow, renal function, and early changes in blood potassium (K(+)).

METHODS

Preterm infants (n = 119) born before 30 weeks' gestational age underwent serial Doppler echocardiographic studies. Superior vena cava flow (SVC flow) was assessed as a measure of upper body systemic blood flow uncorrupted by systemic to pulmonary shunts. Serial whole blood K(+) concentrations on each arterial blood gas sample and urinary output in the first 48 hours were recorded.

RESULTS

Most infants had a variable degree of rise in K(+) during the first 24 hours of life. The mean rate of rise was 0.17 mmol/L/h, the mean peak K(+) was 5.54 mmol/L, and the mean time of peak K(+) was 20 hours. The peak K(+) occurred after the lowest measured SVC flow in 84% of infants. A significant positive relationship was found between the lowest measured SVC flow and the mean (r = 0.31, P =.001) and peak (r = 0.31, P =.001) K(+) in the first 24 hours. Low SVC flow at 5 hours best predicted the rate of K(+) rise (r = 0.28, P =.002) and at 12 hours best predicted the peak K(+) concentration (r = 0.47, P <.001). The mean minimum SVC flow in the 17 babies who became hyperkalemic was 29.5 mL/kg/min versus 46.2 mL/kg/min in the 102 infants with normokalemia. Urine output in the first 24 hours was significantly lower in the hyperkalemic infants. A K(+) rate rise exceeding 0.12 mmol/L/h in the first 12 hours predicted low SVC flow with 93% accuracy.

CONCLUSIONS

The data are consistent with a role for low systemic blood flow leading to reduced urinary output and subsequent hyperkalemia in preterm infants.

Effect of prenatal steroids on potassium balance in extremely low birth weight neonates

OBJECTIVE

Potassium is the most abundant intracellular cation and plays an important role in a variety of cell functions. Potassium homeostasis and regulation are important aspects of fluid and electrolyte homeostasis in extremely low birth weight (ELBW) infants. Because prenatal steroid (PNS) treatment promotes maturation of many epithelial cell systems, we sought to determine whether PNS affects potassium homeostasis in ELBW infants (<1000 g) during the first week of life.

METHOD

Serum potassium (SK) concentration, potassium intake and output, and renal clearance were collected prospectively each day during the first week of life. Infants whose mothers received a full course of steroids before delivery (PNS group: n = 16) were compared with those infants whose mothers did not receive steroids (nonsteroid group [NSG]: n = 14). The decision to treat with PNS was made entirely by the obstetric staff in a nonrandomized manner. Potassium intake and excretion and serum and urine electrolytes were measured every 12 hours, and urine output was monitored every 2 to 3 hours. Hyperkalemia was defined as SK >6. 5 mmol/L in a nonhemolyzed sample on at least 1 measurement from a central line.

RESULTS

There were no significant differences between the groups in gestational age, Apgar score, and birth weight. SK increased initially after birth in the absence of exogenous K intake in all infants, then subsequently decreased and stabilized by day 4 of life. The peak SK was significantly lower in the PNS group than in the NSG group (5.2 +/-.2 mmol/L vs 6.2 +/-.4 mmol/L). Moreover, the peak SK was higher than 6.5 mmol/L in 70% of the NSG infants and in none of the PNS group. Hyperkalemia occurred in the NSG infants within the first 2 days when urine output was significantly lower than in PNS infants. SK peaked in the absence of potassium intake with similar potassium excretion in both groups. PNS infants had similar cumulative potassium intake with a lower cumulative potassium excretion than did NSG infants. PNS infants had a significantly less negative potassium balance than did NSG infants by day 7 of life (-1.0 mmol/kg vs -7.0 mmol/kg). There was no statistical difference in the daily serum creatinine levels, fractional excretion of potassium, and in the daily creatinine clearance between the 2 groups.

CONCLUSION

We conclude that treatment with PNS prevents the nonoliguric hyperkalemia known to occur in ELBW neonates. We speculate that PNS induces upregulation of cell membrane sodium, potassium-adenosinetriphosphatase activity in the fetus. The differences in negative potassium balance may be accounted for by stabilization of cell membranes that may result in a decrease in potassium shift from intracellular to extracellular compartments.

Plasma and urine riboflavin during riboflavin-free nutrition in very-low-birth-weight infants

BACKGROUND

Very-low-birth-weight (VLBW; birth weight <1500 g) infants receive enteral and parenteral nutriture that provides greater daily riboflavin (vitamin B2) than does term infant nutriture, and elevated plasma riboflavin develops in these infants after birth. The purpose of this study was to measure plasma and urine riboflavin concentrations in VLBW infants during riboflavin-free nutrition. Our hypothesis was that elevated plasma riboflavin develops in VLBW infants because of high daily intake and immature renal riboflavin elimination.

METHODS

Eighteen clinically healthy VLBW infants received parenteral nutrition and preterm infant formula during the first postnatal month. On postnatal days 10 and 28, the infants received specially prepared riboflavin-free enteral and parenteral nutrition for the 24-hour study period. Serial collections of plasma were made at time 0 and at 12 and 24 hours. Urine was collected continuously for the 24-hour period in 4-hour aliquots. Samples were analyzed for riboflavin concentration.

RESULTS

During the 24-hour riboflavin-free study period on postnatal day 10, plasma riboflavin decreased 56% from 185 +/- 37 ng/mL (mean +/- SEM), and urine riboflavin decreased 75% from 3112 +/- 960 mg/mL. Similarly, on postnatal day 28, plasma riboflavin decreased 79% from 184 +/- 32 ng/mL, and urine riboflavin concentration decreased 91% from 5092 +/- 743 ng/mL during the 24-hour riboflavin-free study period. Riboflavin half-life (t(1/2)) was 18.5 hours on postnatal day 10 and decreased 48% by postnatal day 28. Riboflavin elimination was 145.1 +/- 20.6 mg/kg per day on postnatal day 10 and increased 40% by postnatal day 28.

CONCLUSION

The VLBW infants who received parenteral nutrition and preterm infant formula had elevated plasma riboflavin on postnatal days 10 and 28. Plasma riboflavin t(1,2) was shorter and renal riboflavin elimination was greater on postnatal day 28 than on postnatal day 10. Plasma riboflavin was normal after 24 hours of riboflavin-free nutrition. The pattern of plasma and urine riboflavin in VLBW infants suggests a lower daily intake would maintain plasma riboflavin close to normal.

Pregnancy after liver transplantation under tacrolimus

BACKGROUND

The maternal and fetal risk of pregnancy after organ transplantation under tacrolimus has not been reported. This was prospectively studied in 27 pregnancies by 21 female liver recipients who were treated with tacrolimus before and throughout gestation.

METHOD

Twenty-seven babies were born between October 1990 and April 1996. In 15 cases, samples were obtained at or after delivery and stored (-40 degrees C) for comparison of tacrolimus concentration in the maternal blood with different combinations of cord and infant venous blood, breast milk, or a section of the placenta.

RESULTS

The 21 mothers had surprisingly few serious complications of pregnancy and no mortality. Two infants with 23 and 24 weeks gestation died shortly after birth. The mean birth weight of the other 25 was 2638+/-781 g after a gestational period of 36.6+/-3.3 weeks. Mean birth weight percentile for gestational age was 50.2+/-26.2 (median 40). On the day of delivery, the mean tacrolimus concentrations (ng/ml) were 4.3 in placenta versus 1.5, 0.7, and 0.5 in maternal, cord, and child plasma, and 0.6 in the first breast milk specimens. The infants had a 36% incidence of transient perinatal hyperkalemia (K+>7.0 meq/L) and a mild reversible renal impairment, which were thought to reflect in part maternal homeostasis. One newborn had unilateral polycystic renal disease (the only anomaly). All 25 babies have had satisfactory postnatal growth and development with a current mean weight percentile of 62+/-37 (median 80).

CONCLUSIONS

Pregnancy by postliver transplant mothers under tacrolimus was possible with a surprisingly low incidence of the hypertension, preeclampsia, and other maternal complications historically associated with such gestations. As in previous experience with other immunosuppressive regimens, preterm deliveries were common. However, prenatal growth for gestational age and postnatal infant growth for postpartum age were normal.

Potassium metabolism in extremely low birth weight infants in the first week of life

OBJECTIVE

Nonoliguric hyperkalemia has been reported to occur in the first week of life in as many as 50% of extremely low birth weight (ELBW) infants. We studied potassium balance and renal function in the first 5 days of life to characterize potassium metabolism during the three phases of fluid and electrolyte homeostasis that we have described in ELBW infants and to elucidate the factors that contribute to the development of nonoliguric hyperkalemia.

STUDY DESIGN

Plasma potassium concentration (PK), potassium intake and output, and renal clearances were obtained for the first 6 days of life in 31 infants with a birth weight of 1000 gm or less. Collection periods in which urine flow rate was greater than or equal to 3 ml/kg per hour and weight loss was greater than or equal to 0.8 gm/kg per hour were denoted to be diuretic. Prediuresis includes all collection periods before the first diuretic period; diuresis includes all collection periods between the first and last diuretic periods; postdiuresis includes all collection periods after the last diuretic period. Infants with a PK greater than 6.7 mmol/L on at least one measurement were denoted to have hyperkalemia.

RESULTS

PK increased initially after birth--despite the absence of potassium intake- and then decreased and stabilized by the fourth day of life. Diuresis occurred in 27 of 31 infants. The age at which PK peaked was closely related to the onset of diuresis. PK decreased significantly during diuresis as the result of a more negative potassium balance, despite a significant increase in potassium intake. In fact, PK fell to less than 4 mmol/L in 13 of 27 infants during diuresis. After the cessation of diuresis, potassium excretion decreased even though there was a significant increase in potassium intake, potassium balance was zero, and PK stabilized. Hyperkalemia developed in 11 of 31 infants. The pattern of change in PK with age was similar in infants with normokalemia and hyperkalemia: PK initially increased (essentially in the absence of potassium intake) and then decreased and stabilized by the fourth day of life. However, the rise in PK after birth was greater in infants with hyperkalemia than in those with normokalemia: 0.7 +/- 0.2 versus 1.8 +/- 0.2 mmol/L (p < 0.001). No differences in fluid and electrolyte homeostasis or renal function were identified as associated with hyperkalemia.

CONCLUSIONS

PK increases in most ELBW infants in the first few days after birth as a result of a shift of potassium from the intracellular to the extracellular compartment. The increase in the glomerular filtration rate and in the fractional excretion of sodium, with the onset of diuresis, facilitates potassium excretion, and PK almost invariably decreases. Hyperkalemia seems to be principally the result of a greater intracellular to extracellular potassium shift immediately after birth in some ELBW infants.

Plasma and urine riboflavin and pyridoxine concentrations in enterally fed very-low-birth-weight neonates

Preterm infant formulas (PIFs) for very-low-birth-weight (VLBW) infants (birth weight, < 1,500 g) are augmented to provide daily riboflavin and pyridoxine at levels up to five-fold greater than in term infant formula and 18-fold greater than in human milk. We evaluated plasma riboflavin and pyridoxine concentrations in VLBW infants who received PIF during their first postnatal month. Eighty-eight plasma and 124 urine samples were collected for riboflavin- and pyridoxine-concentration measurements from 57 clinically healthy VLBW infants weekly during their first postnatal month. Concentrations were measured using high-performance liquid chromatography. At the time of the sample, patients were receiving > or = 80% of their total calories via enteral feedings. Plasma riboflavin concentrations rose from 45.3 +/- 7.3 ng/ml at baseline (mean +/- SEM) to 173.5 +/- 20.3 ng/ml by 1 week of age and remained at 177.3-199.7 ng/ml during the following three weekly measurements; values were up to 14-fold above baseline concentration. Urine riboflavin concentration increased from 534 +/- 137 ng/ml at baseline to 3,521 +/- 423 ng/ml by 1 week of age and remained at 4,451-5,216 ng/ml during the next 3 weeks. In a similar pattern, baseline plasma (69.4 +/- 10.4 ng/ml) and urine (145 +/- 30 ng/ml) pyridoxine concentrations were significantly increased by 1 week postnatal age; they remained at 163-248 ng/ml (plasma) and 1,573-2,394 ng/ml (urine) through the first postnatal month. Plasma and urine riboflavin and pyridoxine concentrations in enterally fed VLBW infants increased from baseline concentrations by 1 week of postnatal age and remained elevated for the first postnatal month. High daily intake and immature renal development are probable contributing causes of the elevated plasma riboflavin and pyridoxine concentrations. We suggest that lower daily enteral administration of riboflavin and pyridoxine should maintain adequate blood concentrations and minimize potential toxicity.

Potassium homeostasis and its disturbances in children

Although only 2% of the body potassium is present in the extracellular space, its concentration is finely regulated by the internal balance, or distribution of potassium between the intracellular and extracellular compartments, and by the external balance, or difference between intake and output of potassium. Internal balance is modulated by a host of factors, including insulin, epinephrine, extracellular pH and plasma tonicity. Potassium output from the body is mainly determined by renal excretion. Renal secretion of potassium takes place predominantly in the principal cells of late distal and cortical collecting tubules, by a process involving the accumulation of potassium in the cell by the activity of the basolateral Na+,K(+)-ATPase and its exit through luminal conductive channels. The factors regulating renal potassium secretion are potassium intake, rate of tubular fluid flow, distal sodium delivery, acid-base status and aldosterone. Hypokalaemia may result from a low potassium intake, excessive gastrointestinal, cutaneous or renal losses and altered body distribution. Aetiological diagnosis and therapy are best accomplished when the acid-base status is assessed at the same time. Before establishing the diagnosis of hyperkalaemia, spurious hyperkalaemia due to haemolysis or release of potassium from cells during clot retraction (pseudohyperkalaemia) should be ruled out. Hyperkalaemia may result from exogenous or endogenous loading, decreased renal output and altered body distribution. Acute hyperkalaemia represents an emergency situation which requires immediate therapy.

Use of calcium excretion values to distinguish two forms of primary renal tubular hypokalemic alkalosis: Bartter and Gitelman syndromes

Clinical or biochemical findings were reevaluated in 34 pediatric patients with primary renal tubular hypokalemic metabolic alkalosis. The patients were subdivided into two groups. Bartter syndrome (primary renal tubular hypokalemic metabolic alkalosis with normocalciuria or hypercalciuria) was diagnosed in 18 patients with molar urinary calcium/creatinine ratios greater than 0.20, and Gitelman syndrome (primary renal tubular hypokalemic metabolic alkalosis with magnesium deficiency and hypocalciuria) was diagnosed in 16 patients with molar urinary calcium/creatinine ratios less than or equal to 0.20 and plasma magnesium levels less than 0.75 mmol/L. Some clinically important differences between the groups were observed. Patients with Bartter syndrome were often born after pregnancies complicated by polyhydramnios (8/18) or premature delivery (7/18) and had short stature (11/18) or polyuria, polydipsia, and a tendency to dehydration (16/18) during infancy (12/18) or before school age (18/18). Patients with Gitelman syndrome had tetanic episodes (12/16) or short stature (3/16) at school age (14/16). We conclude that the Bartter and Gitelman syndromes represent two distinct variants of primary renal tubular hypokalemic metabolic alkalosis and are easily distinguished on the basis of urinary calcium levels.