Renal acidification in children with idiopathic hypercalciuria

Renal acidification in children with idiopathic hypercalciuria Melvin Bonilla-Felix, MD, O l g a V i l l e g a s - M e d i n a , MD, a n d V. M a t t i Vehaskari, MD From the Department of Pediatrics, Washington UniversitySchool of Medicine, St, Louis,Missouri

Distal renal tubular acidosis is frequently associated with hypercalciuria. To further investigate the cause-and-effect relationships between the two conditions, we examined 20 children (5 to 18 years of a g e ) with idiopathic hypercalciuria for evidence of renal tubular acidosis. Serum electrolytes and urine citrate levels were normal in all subjects. After a single dose of furosemide, 1 of the 20 subjects did not show a decrease Jn urine pH <5.5, which suggests an acidification defect in the cortical collecting duct. Three other patients failed to show an Jncrease in urine-minus-blood partial pressure of carbon dioxide >20 mm Hg after urine alkalinization with orally administered acetazolamide, a finding compatible with a rate-dependent distal renal tubular acidosis. These four subjects underwent acute acid loading with arginine hydrochloride. In all four subjects urine pH decreased < 5.5 but urinary ammonium excretion failed to increase normally; this supports the diagnosis of a defect in distal acidification. Four of six patients with nephrolithiasis had evidence of distal renal tubular acidosis, in contrast to none of the 14 patients without stones (p = 0.003). We conclude that distal acidification abilities seem to be intact in children with hypercalciuria in the absence of nephrolithiasis. We speculate that calcium precipitation may lead to tubular d a m a g e , including distal renal tubular acidosis. (J PEDIATR1994; 124:529-34)

The clinical manifestations of hypercalciuria range from subtle signs, such as asymptomatic hemataria l, ; and dysuria,3, 4 to more serious complications, such as nephrolithiasis or nephrocalcinosis, leading to chronic renal failure. 5 Hypercalciuria is frequently associated with distal renal tubular acidosis, but whether hypercalciuria is the cause or the result of d R T A is not well established. Metabolic acidosis may cause hypercalciuria through demineralization of bone,6, 7 as well as having a direct effect on the tubular reabsorption of calcium), 9 However, patients with incomplete d R T A , who are not acidotic, frequently have hyperSupported in part by Pediatric Clinical Research Center (National Institutes of Health) grant MO1 RR 06021. Submitted for publication Sept. 24, 1993; accepted Nov. 16, 1993. Reprint requests: Melvin Bonilla-Felix, MD, Pediatric Nephrology Division, University of Texas Medical School at Houston, 6431 Fannin St., Houston, TX 77030. Copyright 9 1994 by Mosby-Year Book, Inc. 0022-3476/94/$3.00 + 0 9/20/52956

calciuria. 1~ Moreover, in some families hypercalciuria seems to be the primary event, and d R T A develops over the years as a complication. 13' 14 Several studies have reported an incidence of renal tubular acidosis of between 6% and 50% in adult patients with renal stones. 15-22 Those with bilateral stones have the highArgHC1 CCD dRTA FE Pco2 RTA UNC UNH4 + UpH

Arginine hydrochloride Cortical collecting duct Distal renal tubular acidosis Fractional excretion Partial pressure of carbon dioxide Renal tubular acidosis Urine net charge Urine NH4 + Urine pH

est incidence, ranging between 30% and 5 0 % . 15' 21 Patients with their first renal stone have a lower incidence; only 5% have an incomplete dRTA. 23 The information on children is very limited. To better characterize the cause-and-effect

529

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The Journal of Pediatrics April 1994

Table. Response of the patients to furosemide and acetazolamide tests Acetazolamide test

Furosemide test Patient No.

History of stones

U Ca (mg/kg/day)

UpH

UpH

UHC03(mmol/L)

Upcoa (mm Hg)

U - 8pco2 (ram Hg)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

+ ---+ + + + + -

5.8 5.7 4.1 8.4 4.5 6.2 4.6 4.1 3.1 6.2 5.3 4.2 2.1 4.4 4.6 4.0 4.7 5.2 5.4 2.8

5.2 5.1 5.2 5.0 4.9 5.4 5.4 5.0 4.6 4.9 5.3 5.0 5.4 4.6 4.7 6.1 4.8 5.4 4.6 5.0

7.58 7.84 7.72 7.65 7.72 7.59 7.70 7.74 7.63 7.56 7.65 7.58 7.62 7.74 7.64 7.51 7.73 7.61 7.46 7.77

55 183 117 63 134 63 88 153 70 66 76 75 53 119 59 50 116 74 50 137

67 95 93 66 102 78 74 106 74 80 76 85 59 85 60 73 90 78 81 89

15 54 50 21 62 26 26 55 25 38 29 29 10 40 17 21 41 32 34 42

UpH, Urinary pH; UHC03-, urine bicarbonate concentration; UPco2, urine partial pressure of carbon dioxide; U - BPco2, urine-minus-bloodpartial pressure of carbon dioxide.

relationship between hypercalciuria and d R T A , we evaluated renal acidification among our pediatric patients with idiopathic hypercalciuria. METHODS Study population. All patients with idiopathic hypercalciuria attending the nephrology clinic of Saint Louis Children's Hospital between 1985 and 1991 were invited to enter the study. Twenty children (12 girls), 5 to 18 years of age, completed the study protocol. The diagnosis of hypercalciuria was based on a urine calcium/creatinine ratio >0.21 in at least two urine specimens collected at random. The diagnosis was confirmed in 17 of the 20 patients by demonstration of a 24-hour urinary calcium excretion >--4 m g / k g per day. Three patients had a urinary calcium excretion <4 m g / k g per day at the time of the 24-hour urine collection, but each had multiple random urine c a l c i u m / creatinine ratios >0.21; one also had a history of renal stones. Of the 20 patients, 6 had a history of nephrolithiasis (Table). Patients with systemic diseases or known secondary causes of hypercalciuria were excluded. All medications were stopped 1 week before the studies. The protocol was approved by the Washington University School of Medicine H u m a n Studies Committee. A written consent was obtained from the parents and patients. Study protocol. The studies were performed at the clinical research center. The patients were allowed to have a light breakfast 2 hours before the studies.

On the first day the patients underwent a urine alkalinization test with aeetazolamide, as described by Alon et al. 24 The effects of this drug on urinary Pc02 are identical to those of a bicarbonate load, and the drug offers several advantages, including shorter duration and better tolerance. 24 The patients received acetazolamide orally (15 m g / k g of body weight) after baseline urine and blood samples were collected. After drug administration, the first urine was discarded and then consecutive timed, spontaneously voided urine specimens were collected at intervals o f approximately 60 minutes. The specimens were immediately drawn into a syringe, freed of air, capped, and sent on ice to the laboratory for immediate determinations of pH, Pcoa, and creatinine level. (There are no differences in the Pc02 values obtained from urine samples collected under oil and those analyzed within 5 minutes after the collection. 25) The study Was terminated after completion of at least three urine collections with a pH >7.40. Heparinized venous blood from the antecubital vein was obtained between the second and third urine collections for determination of pH, Pco2, bicarbonate concentration, and creatinine. Urine bicarbonate concentration was calculated by means of the Henderson-Hasselbach equation as follows: pH=pK+log~

[HCOf]

where [ H C O 3 - ] was the concentration of bicarbonate in the urine, and k was the solubility coefficient (0.0309 for urine).

The Journal of Pediatrics Volume 124, Number 4

Bonilla-Felix, Villegas-Medina, and Vehaskari

53 1

6.5

r.-,

y~ s.5 o

9 (-) STONES

or-

I

t~

+

-4-

LH 03

4-

1|

Jr- (+) STONES

+

5 + 4.5

4 0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

U-BpCO2 (ACETAZOLAMIDE)

Fig. I. Res••nse•fpatientswithhypercalciuria•withandwith•utnephr••it•iasis•t•fur•semideandacetaz•lamidetests. Patients without nephrolithiasis had a normal response to both tests; in contrast, four of six patients with nephrolithiasis had an abnormal response to one of the tests.

The urine pK was calculated from the urinary ionic strength, according to the following formula: pK = 6.33 - 0 . 5 " ~ F ~

K+

where Na + and K + represent the urine concentrations in milliequivalents per liter. An abnormal response was defined as urine-minus-blood Pco2 <20 mm Hg with a urine Pr <70 mm Hg. 24 On a separate day the patients underwent an acidification test with furosemide, which in healthy persons stimulates voltage-dependent proton and potassium secretion in the cortical collecting duct. 26 The patients received an oral dose of furosemide (1 mg/kg) after baseline urine collections. Urine specimens were then collected hourly for determination of urine pH, creatinine, and electrolyte (Na +, K +, and C1-) values. The test was terminated if a UpH <5.5 was reached, or after four hourly urine collections were completed. A normal response to furosemide was defined as a decrease in UpH to <5.5. 26 Although patients with a selective defect in the medullary collecting duct can theoretically have a normal response to furosemide, they should not be able to increase urine Pc02. 27 Thus we assumed that dRTA would be detected in all patients by one of these two tests. To better assess the acidification abilities of the patients who had an abnormal response to either acetazolamide or furosemide, we subjected them to a short acid load with arginine hydrochloride, as described by Loney et al. 2s For this assessment, baseline

blood and urine samples were obtained, followed immediately by intravenous infusion of arginine hydrochloride ( 150 mEq ( H + ) / m 2) for 2 to 3 hours. Urine samples were obtained at hourly intervals and were sent immediately to the laboratory for determination of pH, electrolyte, NH4 +, and creatinine values. Venous blood for determination of pH, HCO3-, Na +, and creatinine values was obtained at hourly intervals between the urine collections. The test was terminated if UpH decreased to <5.5 or blood pH to <7.25. Urinary net charge was calculated from the formula U N C = (Na+u + K+u) - Cl-u, whereNa+~, K+u, and Cl-u are the concentrations of Na +, K +, and CI - in the urine, respectively. This test is an indirect index of the urine NH4 + concentration. A negative U N C in a patient with hyperchloremic metabolic acidosis indicates adequate UNH4 + excretion. Conversely, a positive U N C reflects a low UNH4 + concentration, which is compatible with dRTA. 29 A normal response to the acid load was defined as a decrease in UpH to <5.5, development of a negative UNC, and an increase in UNH4 + excretion >35 #mol/min per 1.73 m2. 3~ Blood and urine gases were measured with a standard blood gas analyzer (models 1301 and 1302, Instrumentation Laboratory, Lexington, Mass.). The UNH4 + was determined after 100-fold dilution in 0.9% saline solution by analysis on an Ektachem 700XR automatic analyzer (Eastman Kodak Co., Rochester, N.Y.) with use of the manufacturer's reagents and procedures. 31 Statistical analysis. Results are expressed as mean

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Bonilla-Felix, Villegas-Medina, and Vehaskari

The Journal of Pediatrics April 1994

8

70

6O ~ E

7

cO r'., 5 0 ,--

27. 0_6 E)

t''

~ PATIENT 1

E --,.....

-}- PATIENT 18

40 ~

~PATIENT 15

E

-m- PATIENT 16 -V- PATIENT 4

30 + ,,r "l-

5

Z 20 ~

4

BASE

MIN

BASE

MAX

10

Fig. 2. Urine pH and urinary ammonium excretion during arginine hydrochloride load in the four patients who had abnormal responses to furosemide or acetazolamide testing. The results of a single patient with hypercalciuria (patient 4) whose responseto furosemide and acetazolamide was normal, who had no history of nephrolithiasis, and who also was subjected to the test, are also shown.

+ SEM. Statistical significance was defined asp < 0.05, by a two-tailed Student t test. The Fisher exact analysis was used when indicated. RESULTS Acetazolamide test. Baseline serum electrolyte concentrations, including HCO3-, were normal in all subjects. None of the patients had hypocitraturia (defined as urinary citrate < 180 mg/gm of ereatinine32). Fractional excretion of HCO3-, measured at baseline serum HCO3- levels (22 to 30 mmol/L) was <1% in all subjects, which ruled out proximal RTA. After the dose of acetazolamide, UpH increased to 7.69 _+ 0.02, and FEHco3 to 12.42% _+ 0.8%. The concentration of HCO3- in the urine achieved a maximum mean value of 90 -!-_9 mmol/L. Urine Pco2 increased to a maximum of 78.6 + 4.7 mm Hg, and Urine-minusblood Pco2 to 33.5 -+ 3.3 mm Hg. Of the 20 patients, 3 failed to show an increase in urine-minus-blood Pco2 >20 mm Hg and urine Pco2 >70 mm Hg (patients 1, 13, and 15), in the face of UpH >7.5 (Table). All three patients had a history of nephrolithiasis (Fig. 1). Furosemide test. Two hours after the dose of furosemide, the FENa level reached a maximum of 8.28% _+ 0.76% (baseline 0.82% _+ 0.08%). The FEK level also increased (baseline 12.3% +_ 1.2%; maximum 45.6% _+ 5%). Most of

the patients achieved a UpH <5.5 within the first 2 hours of the test (minimum UpH 5.04 + 0.07). In one patient (No. 16) UpH failed to decrease to <5.5 during this test (minimum UpH 6.1), in the face of a normal natriuretic response (FENa 8.4%) and a normal kaliuretie response (FEK 55.1%) (Table). This patient also had a history of nephrolithiasis (Fig. 1). Intravenous acid loading test. The four subjects who had an abnormal response to one of the first two tests were subjected to an acute acid load with ArgHC1. The UpH decreased to <5.5 in all subjects (average minimum, 4.93 _+ 0.28, range 4.4 to 5.4). The UNC reached a negative value in every patient (mean, -42.5 + 12.2; range, - 17 to - 7 0 ) , and was accompanied by an increase in the excretion of UNH4 +, but to levels that were below normal29 (mean, 31.1 _+ 1.5; range, 27.6 to 34.7 t~mol/min per 1.73 m 2) (Fig. 2). Four of six patients with nephrolithiasis had evidence of dRTA, in contrast to none of the 14 patients without a history of stones (p <0.003; relative risk = 8 [Fisher Exact Test]). DISCUSSION Distal renal tubular acidosis is often implicated as a cause of hypercalciuria. It has also been speculated that calcium

The Journal of Pediatrics Volume 124, Number 4

microcrystals may produce tubular cell injury and lead to R T A . Stapleton et al. 33 demonstrated that urinary excretion of renal N-acetyl-~3-glucosaminidase, which is a marker of tubular cell injury, is elevated in children with hypercalciuria. However, the cause-and-effect relationships between these conditions are not well defined. None of our 14 hypercalciuric patients without nephrolithiasis had a defect in distal acidification. This suggests that hypercalciuria, in the absence of stones, is characterized by intact acidification abilities, an indication that, in the majority of children, hypercalciuria is not caused by an underlying primary d R T A . In contrast, four of the six patients with stones had evidence of abnormal distal acidification. This suggests that calcium precipitation may impair acidification in the distal tubule. This observation agrees with a recent report of disturbed acidification in infants with furosemide-related renal calcifications. 34 N o n e of our patients was acidotic, so the classic form of d R T A characterized by hyperchloremic acidosis was not present. Failure to increase the urine Pco2 after proper urine alkalinization seems to be the most sensitive test to establish the diagnosis of a proton secretory defect in the distal tubule.35, 36 Three of six patients with nephrolithiasis did not reach a urine-minus-blood Pco2 >20 m m Hg, consistent with a rate-dependent d R T A . In one patient the U p H value failed to decrease to <5.5 during the furosemide test. This was not the result of failure of the C C D to generate a transtubular electronegative potential, because the kaliuretic response was adequate. W e interpreted this as a partial defect in proton secretion localized primarily in the CCD. It is conceivable that during periods of systemic acidosis a compensatory increase in proton secretion by the medullary collecting duct allows this patient to acidify the urine. However, if the only stimulus for acidification relies on the abilities of the C C D to secrete protons (furosemide test), then the defect becomes unmasked. Each of our patients had a negative U N C after ArgHC1 infusion, which does not support the diagnosis of dRTA. However, the U N H 4 + excretion was low, which is considered to be diagnostic of d R T A . 29' 37 Because our patients were not chronically acidotic, we cannot assess the ability to synthesize NH4 +. However, our definition of normal U N H 4 + excretion was derived from healthy children subjected to an acute acid load. 3~ Furthermore, when one of the patients without stones was subjected to the ArgHC1 test, the U N H 4 + excretion increased to 66 # m o l / m i n per 1.73 m 2. Two studies have evaluated the association between hypercalciuria and R T A . 38, 39 One of them included eight men with a history of nephrolithiasis. 38 The other involved a limited group of children, of whom 70% had stones. 39 Although both studies failed to show an association between

Bonilla-Felix, Villegas-Medina, and Vehaskari

533

nephrolithiasis and dRTA, the significance of the studies was limited by the use of U p H as the sole indicator of proton secretory capacity. Distal renal tubular acidosis is a complex syndrome, most likely the result of different pathophysiologic mechanisms. Measurement of the U p H , which represents a minimal part of the acid excretion (free H+), is not the best test for evaluation of distal acidification. Moreover, the so-called rate-dependent defect can be missed if only the U p H is measured. 35, 36 W e conclude that, in the absence of nephrolithiasis, patients with hypercalciuria seem to have an intact capacity to acidify the urine. Because of the strong association between d R T A and nephrolithiasis, we believe that in our patients calcium precipitation played a causative role in the acidification defect. We thank all the nurses and staff of the Saint Louis Children's Hospital Clinical Research Center and Michael Landt, PhD, for their cooperation and assistance in the measurement of UpH, Pco2, and NH4 +. We also thank Barbara R. Cole, MD, for her continuous support and critical review of the experimental protocols. REFERENCES

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