Low Magnesium levels Lead to Low Blood Calcium Levels in Gitelman’s

Evidence for Disturbed Regulation of Calciotropic Hormone Metabolism in Gitelman Syndrome

Little attention has been paid to interactions between circulating levels of calcium, PTH, and 1,25-dihydroxycholecalciferol [1,25(OH)2 D] and bone mineral density in primary renal magnesium deficiency. Plasma and urinary electrolytes, and circulating levels of calciotropic hormones were studied in 13 untreated patients with primary renal tubular hypokalemic alkalosis with hypocalciuria and magnesium deficiency. The blood ionized calcium concentration was significantly lower in patients than in controls.

Despite this fact, PTH and 1,25-(OH)2 D levels were similar in both groups of subjects. The negative linear relationships between PTH and ionized calcium, which significantly differed between Gitelman patients and healthy subjects in terms of intercept; the negative relationship between ionized calcium and 1,25-(OH)2 D, which was comparable in both groups; and the positive relationship between 1,25-(OH)2 D and PTH, which was identical in both groups, point both to a blunted secretion of PTH induced by magnesium depletion and to the lack of interference of the latter with the activation of 1alpha-hydroxylase by PTH. The similar bone mineral density at the lumbar spine by dual energy x-ray absorptiometry in 11 patients and 11 healthy subjects argues against chronically sustained negative calcium balance. ( J Clin Endocrinol Metab 80: 224-228, 1995)

MILD TO moderate renal magnesium wasting and hypocalciuria are, apart from hypokalemic alkalosis, the biochemical hallmarks of a variant of Bartter’s syndrome, sometimes referred to as Gitelman’s syndrome[1] [2] [3] [4] [5] [6] [7] [8] . It has been postulated that the defect underlying Gitelman’s syndrome is in the distal convoluted tubule[3] ,[5] [6] [7] . However, the actual site of the abnormality is unknown.

Severe magnesium deficiency secondary to low dietary intake or reduced intestinal absorption of this element impairs the secretion of PTH and causes skeletal resistance to its action (9-16); in addition, circulating 1,25-dihydroxycholecalciferol [1,25-(OH)2 D3 ] concentrations are frequently low in patients with magnesium deficiency[16] . However, in renal magnesium deficiency, little attention has been paid to the interplay between calciotropic hormones, on the one side, and bone mineral, on the other. The present study was, therefore, conducted in patients with Gitelman’s syndrome regularly seen at our two centers of pediatric nephrology to address this issue.

Subjects and Methods

Thirteen patients with Gitelman’s syndrome (seven females and six males, aged 10-21 yr) entered the study (Fig.1). They had a history of frequent tetanic episodes (n = 3), muscular weakness (n = 7), or both (n = 3). Blood pressure, plasma creatinine, urinalysis, and renal ultra-sound were normal. The diagnosis of Gitelman’s syndrome (Fig. 1) was based on the presence[5] of venous blood pH higher than 7.38 nmol/L, plasma bicarbonate concentration more than 28.0 mmol/L, plasma potassium less than 3.5 mmol/L, plasma magnesium (by colorimetric assay) less than 0.75 mmol/L, molar ratio of urinary calcium/creatinine less than 0.100, urinary chloride/creatinine ratio more than 8.23, urinary magnesium/creatinine more than 0.138, and negative urinary screen for diuretics[3] ,[5] . The z-scores ( SD from the mean for a given age and sex) for height (from -1.6 to 1.3) and body weight (from -1.6 to 1.4) were normal[17] .

The 13 patients, who had been without any medication or electrolyte supplementation for at least 4 weeks before investigation (Fig. 1), attended the out-patient clinic after overnight fasting between 0700-0900 h. They had been instructed to collect urine for determination of creatinine clearance and to bring a 3-day food record. The patients received a 1.2-L tap water loading. After voiding the bladder, a 2-h urine specimen was collected, and a midpoint blood sample was taken with minimal stasis and without movement of the forearm, according to a standard procedure[18] . Packed cell volume, blood hydrogen ion concentration, blood carbon dioxide tension, and plasma ionized calcium concentration were measured immediately after blood collection. The remaining samples were stored at -40 C. Creatinine, sodium, potassium, chloride, inorganic phosphorus, total calcium, and magnesium were measured in plasma and urine; protein, 25-hydroxycholecalciferol (25OHD3 ), and 1,25-(OH)2 D3 in plasma; and PTH in serum. The patients were subsequently asked to provide a portion of a first morning urine after overnight fasting for determination of osmolality. The aforementioned protocol was applied in a control group of 13 healthy volunteers (8 females and 5 males, aged 16-20 yr).

Mineralometry of the lumbar spine was performed in 11 of the 13 patients with Gitelman’s syndrome (6 females and 5 males, aged 10-21 yr; body surface area, 0.88-1.92 m2 ) and in a group of 11 healthy subjects matched for sex (6 females and 5 males), age (10-21 yr), and body surface area (1.00-1.95 m2 ).

The studies had been approved by the ethical committee of the participant centers, and informed consent was obtained.

All measurements were performed in duplicate. Packed cell volume was assessed by means of a microhematocrit centrifuge. Blood hydrogen ion concentration and blood carbon dioxide pressure were measured using ion-selective electrodes (IL 1306 pH/blood gas analyzer Instrumentation Laboratory S.p.Fl. Milan, Italy). The plasma bicarbonate concentration was calculated using the Henderson-Hasselbalch equation, with an acidity exponent of 6.10 and a carbon dioxide solubility coefficient of 0.0301. The plasma ionized calcium concentration was measured with an ion-selective electrode (Corning 614, Dow-Corning, Medfield, MA) at the prevailing blood pH under anaerobic conditions (coefficient of variation, 0.011). Total protein, creatinine, potassium, chloride, sodium, inorganic phosphorus, total calcium, and magnesium were measured with an autoanalyzer, using either flame photometry or colorimetric assays. Intact PTH was measured by a two-site radioimmunometric assay (Nichols Institute, San Juan Capistrano, CA), which recognizes the entire human 1-84 molecule; the limit of detection is 0.3 pmol/L, and the analytical coefficient of variation is 0.06.25OHD 3 and 1,25-(OH)2 D3 were measured with competitive protein binding assays (Nichols Institute). The lower detection limit for the 25OHD3 assay is 2.5 nmol/L; that for the 1,25-(OH)2 D 3 assay is 7.2 pmol/L. Coefficients of variation are 0.11 and 0.13, respectively. The glomerular filtration rate was estimated from creatinine clearance. Urinary excretions of electrolytes were expressed as either fractional excretion or the molar ratio over creatinine. Dietary sodium, calcium, and magnesium intakes were assessed from the 3-day food record using dietary charts.

The mineralometry of the lumbar spine was assessed in the supine position by dual energy x-ray absorptiometry using a densitometer model Hologic QDR-1000, Hologic Inc., Waltham, MA. Bone mineral density (BMD), expressed as grams per cm2 , was calculated as the ratio of hydroxyapatite content to the projected scan area of the three lumbar vertebrae L2-L4. BMD assessed in patients and controls was also compared with values reported previously for healthy Swiss children and adolescents[19] .

The groups were compared by means of multivariate Hotelling T2 statistic or logistic regression (using the Bonferroni adjustment)[20] . Values were expressed as either individual data or the mean and 95% confidence interval. P < 0.05 was regarded as statistically significant.

Results

The histories of the 13 patients with Gitelman’s syndrome are summarized in Fig. 1. Age at diagnosis ranged from 8-17 yr. In 12 of the 13 patients with Gitelman’s syndrome, supplementation with magnesium salts or with magnesium and inhibitors of prostaglandin synthesis (mostly indomethacin) had been given for 5-42 months. In these 12 patients, medication or electrolyte supplementation had been discontinued 4 weeks to 24 months before the study. The remaining patient had never been supplemented or treated.

The 13 patients with Gitelman’s syndrome and the 13 control subjects did not significantly differ with respect to dietary sodium, calcium and magnesium intake, blood pressure, heart rate, packed cell volume, plasma creatinine, creatinine clearance, plasma sodium, or plasma and urinary phosphate (Table 1). Metabolic alkalosis (blood pH 7.38-7.47; plasma bicarbonate, 28.1-35.5 mmol/L), hypokalemia (2.3-3.3 mmol/L), hypochloremia (91-101 mmol/L), hypomagnesemia (0.33-0.71 mmol/L), and hyperproteinemia (72.0-87.0 g/L) along with inappropriately high urinary potassium, sodium, chloride, and magnesium excretion rates and reduced osmolality in first morning urine all were observed in the group of Gitelman patients compared with these values in the control group (Table 1).

The relationship between urinary and circulating magnesium in patients and controls is shown in Fig. 2. In patients with Gitelman’s syndrome, a statistically significant negative linear relationship was observed between plasma magnesium, taken as the dependent variable, and the molar urinary magnesium to creatinine ratio, taken as the independent variable ( y = 0.81 – 0.58 x ; r2 = -0.64; P < 0.05). In controls, no significant correlation between the two parameters was observed. In patients and controls, no significant correlation was found between circulating magnesium and circulating potassium, bicarbonate, or hydrogen ion.

Urinary excretion of calcium, expressed as the fractional excretion of ionized calcium and the urinary calcium/creatinine ratio, was reduced in Gitelman’s syndrome patients compared with that in the healthy subjects (Table 1). The total plasma calcium concentration was higher, on the average, by 0.15 mmol/L ( P < 0.05) in Gitelman patients (2.40-2.70 mmol/L) compared with controls (2.30-2.52 mmol/L). On the other hand, the ionized calcium level was lower, on the average, by 0.061 mmol/L in Gitelman’s syndrome patients than in the control population (1.07-1.40 vs. 1.28 to 1.41 mmol/L; P < 0.05).

Blood levels of PTH, 25OHD3 , and 1,25-(OH)2 D 3 were not statistically different in patients with Gitelman’s syndrome and healthy controls; the individual values are shown in Fig. 3.

Figure 4 depicts the relationship among calcium, PTH, and 1,25-(OH) 2 D3 concentrations. The expected significant ( P < 0.05) negative relationship was observed between circulating ionized calcium (independent variable) and intact PTH (dependent variable) in both patients ( y = 29.1 – 20.7 x ; r2 = 0.84) and controls ( y = 39.5 – 27.2 x ; r 2 = 0.75). The slopes of the two regression lines were similar; both intercepts with x-axis, however, were significantly ( P < 0.05) different.

TABLE 1 — Dietary sodium, calcium, or magnesium intake; blood pressure; heart rate; and biochemical findings in 13 patients with Gitelman’s syndrome (aged 10-21 yr) and 13 control subjects (aged 16-20 yr)

Patients with Gitelman’s syndrome Control subjects
Dietary sodium intake (mmol/day) 193 (151-234) 203 (150-256)
Dietary calcium intake (mmol/day) 15.3 (9.8-20.8) 14.9 (9.9-20.5)
Dietary magnesium intake (mmol/day) 10.4 (6.7-14.1) 9.9 (5.8-14.0)
Supine blood pressure (mm Hg) 108 (103-113) /70 (65-75) 111 (107-115) /71 (67-74)
Heart rate (beats/min) 79 (69-89) 77 (63-91)
Packed cell vol 0.43 (0.40-0.46) 0.43 (0.41-0.45)
Plasma creatinine (mumol/L) 72 (63-80) 79 (62-95)
Glomerular filtration rate (ml/s. m 2 ) 1.08 (0.98-1.18) 1.02 (0.82-1.22)
Inorganic phosphorus
Plasma conc. (mmol/L) 1.25 (0.94-1.56) 1.20 (0.94-1.46)
Fractional excretion (10-2 ) 11.0 (5.32-16.6) 8.61 (4.17-13.1)
Blood pH 7.42 a (7.41-7.44) 7.34 (7.32-7.36)
Plasma bicarbonate (mmol/L) 31.5 b (30.4-32.6) 27.9 (26.2-29.6)
Potassium
Plasma conc. (mmol/L) 2.84 a (2.68-3.00) 4.10 (3.51-4.69)
Fractional excretion (10-2 ) 20.8 b (14.6-27.0) 8.74 (2.90-14.5)
Urinary osmolality (mmol/kg) 526 a (432-620) 1000 (918-1082)
Total plasma protein (g/L) 78.5 b (76.4-80.6) 73.9 (71.8-76.0)
Chloride
Plasma conc. (mmol/L) 96.9 a (95.0-98.8) 105 (103-107)
Fractional excretion (10-2 ) 1.34 b (1.20-1.51) 0.91 (0.65-1.17)
Sodium
Plasma conc. (mmol/L) 139.1 (137.4-140.7) 140.1 (138.8-141.4)
Fractional excretion (10-4 ) 95.7 b (77.6-114) 57.7 (38.9-76.5)
Magnesium
Total plasma conc. (mmol/L) 0.55 a (0.49-0.61) 0.82 (0.78-0.86)
Fractional excretion (10-2 ) 5.48 a (4.18-6.78) 1.70 (1.48-1.92)
Urinary magnesium to creatinine ratio 0.423 a (0.345-0.501) 0.178 (0.154-0.202)
Calcium
Total plasma conc. (mmol/L) 2.54 b (2.49-2.59) 2.39 (2.35-2.43)
Plasma ionized concentration (mmol/L) 1.260 b (1.216-1.304) 1.321 (1.305-1.337)
Fractional excretion (10-4 ) c 14.6 a (8.94-20.3) 112 (77.0-147)
Urinary calcium to creatinine ratio (10-2 ) 2.60 a (1.80-3.40) 19.1 (13.0-25.2)
Data are presented as the mean and 95% confidence interval.

Figure 2. Relationship between molar urinary magnesium to creatinine ratio and circulating magnesium in 13 patients with Gitelman’s syndrome (*) and 13 control subjects (). In patients, a significant ( P < 0.05) relationship was observed between urinary and circulating magnesium ( y = 0.81 – 0.58 x ; r2 = -0.64; P < 0.05). In control subjects, no significant correlation was observed.

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