[Sodium excretion in children with lithogenic disorders]

INTRODUCTION

The causes of nephrolithisis are multifactorial and have not yet been enough investigated [1]. Hypercalciuria is the most common cause of metabolic nephrolithiasis [2-4]. Close relationship between urinary calcium and urinary sodium has been a subject of reported observations in the past, showing that high urinary sodium is associated with high urinary calcium [5-7]. Hyperoxaluria, hyperuricosuria and cystinuria are also metabolic disorders that can lead to nephrolithiasis. Recent studies have indicated that urinary elimination of cystine is influenced by urinary sodium excretion. Based on these observations it has been hypothesised that patients with high urinary sodium excretion are at high risk of urinary stone disease. The purpose of the study was to investigate sodium excretion in a 24-hour urine and first morning urine collected from children with lithogenic metabolic abnormalities (hypercalciuria, hyperoxaluria, hyperuricosuria, cystinuria), both with nephrolithiasis and without it, in order to determine its significance in urinary calculi formation.

PATIENTS AND METHODS

Urinary sodium excretion was investigated in 2 groups of children: patients with lithogenic metabolic abnormalities, but without urinary stone disease (L group) and patients with nephrolithiasis (C group). Both groups were divided into 2 subgroups: patients with hypercalciuria and without it. There were 22 patients in group L (mean age 11.97 +/- 4.13 years), of whom 17 formed a hypercalciuric subgroup and 5 formed a non-hypercalciuric subgroup (3 patients with hyperuricosuria and 2 patients with hyperoxaluria). Group C consisted of 21 patients with nephrolithiasis (mean age 12.67 +/- 3.44 years), of whom 6 formed a hypercalciuric subgroup and 15 formed a non-hypercalciuric group (2 patients with cystinuria and 13 patients without lithogenic metabolic abnormalities). Control group consisted of 42 healthy age-matched children. All subjects had a normal renal function. A detailed history and clinical examination were done, and ultrasonography was performed in all patients. A 24-hour urine, first morning urine and serum specimen were analysed for sodium, potassium, calcium, uric acid, urea and creatinine. Fractional excretion of sodium, as well as urinary sodium to creatinin ratio and urinary sodium to potassium ratio, were calculated from the findings. Sodium and potassium levels were determined by flame photometry, calcium was measured by atomic absorption technique (Beckman Atomic Spectrophotometer, Synchron CX-5 model, USA), uric acid by carbonate method and creatinine by Jaffe technique. Cystine and dibasic amino acids were quantified by ion chromatography. Urinary oxalate excretion was determined by enzyme spectrophotometry. Hypercalciuria was defined by 24-hour calcium excretion greater than 3.5 mg/kg per day and/or calcium to creatinine ratio greater than 0.20 [8]. Uric acid excretion was expressed as uric acid excretion factored for glomerular filtration, according to Stapleton's and Nash's formula [9]. Normal values were lower than 0.57 mg/dl of glomerular filtration rate in 24-hour samples. Mean values were statistically analyzed by Pearson's linear correlation and analysis of variance (ANOVA).

RESULTS

Urinary sodium concentration values including urinary sodium to potassium ratios, are shown in Table 1. We found that urinary sodium excretion was significantly increased in patients of both L and C groups when compared with controls (p < 0.05). Further analysis of the subgroups showed that urinary sodium excretion was significantly higher only in patients with hypercalciuria of both L and C groups in comparison to controls (p < 0.05) (Table 2). A significant positive correlation was found between 24-hour urinary sodium to creatinine ratio and urinary calcium to creatinine ratio (r = 0.31; p < 0.001) (Graph 1), as well as between urinary sodium to potassium ratio in 24-hour and first morning urine (r = 0.69; p < 0.001) (Graph 2). (A

[Renal tubular function in children with hypercalciuria]

INTRODUCTION

Renal stone disease is commonly due to hypercalciuria [1, 2], which may be assessed either from a 24-hour urinary collection or from the fasting first morning urine. Hypercalciuria during childhood has been defined by a 24-hour calcium excretion greater than 3.5 mg/kg per day and/or calcium to creatinine ratio greater than 0.20 [3]. The alteration in the calcium transporting systems plays a pathogenetic role in promoting hypercalciuria [4, 5]. Since calcium reabsorption along the nephron is intimately related to that of other electrolytes and substances, it can be hypothesized that patients with hypercalciuria may have other renal tubular defects. The aim of the study was to investigate proximal tubular function (tubular reabsorption of sodium, potassium, phosphate and glucose) and distal tubular function (urinary concentrating capacity and acidifying capacity) in children with hypercalciuria.

PATIENTS AND METHODS

Two groups of children were studied: hypercalciuric group included 23 children with hypercalciuria (10 males, aged 11.9 +/- 4.1 years), of whom 6 with nephrolithiasis, and control group included 42 healthy children (20 males, aged 11.2 +/- 3.8 years). All subjects had normal serum values for calcium, sodium, potassium, phosphate and glucose, as well as normal renal function. The urinary excretion of calcium, sodium, potassium, phosphate, glucose and creatinine was measured in a 24-hour urine specimen by standard laboratory methods. Urine osmolality and urinary specific gravity were measured following 12-hour water-deprivation test. A short ammonium chloride loading test was performed in 3 patients with urinary pH above 5.5. The fractional excretion of sodium, tubular phosphate reabsorption and renal threshold phosphate concentration were calculated according to standard formula. Statistical analysis was performed using the t-test and analysis of variance (ANOVA). Kruskal-Wallis method was used to compare urinary phosphate excretion between two groups.

RESULTS

Table 1 summarizes urinary excretion of electrolytes in children with hypercalciuria compared with healthy controls. We found that urinary sodium excretion was significantly increased in patients with hypercalciuria when compared with controls (p < 0.05). Urinary phosphate excretion was significantly higher in patients with hypercalciuria in comparison to controls, and this was accompanied by a significant lowering of the tubular phosphate reabsorptive threshold (p < 0.05). Urinary potassium excretion tended to be lower, although not significantly, in the hypercalciuric children than in normal subjects. Table 2 shows the mean values +/- standard deviation of urinary specific gravity, urinary osmolality and urinary pH. Urinary specific gravity mean value was significantly lower in patients with hypercalciuria in comparison to controls (p < 0.05). Urinary pH was found below 5.5 in all patients. Glycosuria was detected in 3 patients (13.3%). As shown in Graph. 1, a significant correlation between the urinary excretion of calcium and sodium was demonstrated in both groups of children (r = 0.29; p < 0.01).

DISCUSSION

The present study shows that children with hypercalciuria have significantly higher urinary sodium and urinary phosphate excretion in comparison to controls, while urinary potassium excretion is normal in both groups of children. According to some recent reports [6-9], these findings may indicated defects of the renal tubular transport of sodium and phosphate which may be interpreted as a cause or a consequence of the alteration of the calcium transporting system. Defects in both proximal and distal renal tubular functions have been demonstrated in patients with nephrolithiasis, particularly those with hypercalciuria. Proximal renal tubular defects include defects in sodium, fluid, phosphate and glucose reabsorption, which were evident also in our patients. (ABSTRACT TRUNCATED)