Hypocalciuria in patients with gitelman syndrome: Role of blood volume

Background: Hypocalciuria is common in patients with Gitelman syndrome (GS), and its cause primarily is enhanced renal reabsorption of calcium in the proximal tubule in response to hypovolemia, judged by recent studies in animals. Study Design: Uncontrolled trial in cases and controls to evaluate the effect of acute reexpansion of extracellular fluid volume (ECFV) on urine calcium excretion in patients with GS. Setting & Participants: 8 patients with GS and 8 sex- and age-matched healthy control subjects (CSs) were enrolled in an academic medical center. Predictor: ECFV expansion with isotonic saline at 1 L/h for 3 hours. Outcomes & Measurements: Urinary calcium excretion was measured hourly for 6 hours, and subsequent 18-hour urine was analyzed as a single collection; hormones and electrolytes were measured. Results: Patients with GS had hypokalemia, metabolic alkalosis, hypomagnesemia, severe hypocalciuria (urine calcium-creatinine ratio, 0.006 +/- 0.002 versus 0.08 +/- 0.02 mg/mg [0.02 +/- 0.01 versus 0.22 +/- 0.05 mol/mmol]; P < 0.005), and a mild degree of ECFV contraction. Sodium excretion and creatinine clearance rates were similar to those in CSs. In patients with GS, saline infusion increased ECFV, which caused a significantly greater sodium excretion rate, but there was only a small increase in calcium excretion rate, in both the first 6 hours (0.04 +/- 0.02 mg/min [1.0 +/- 0.6 mu mol/min]) and subsequent 18-hour period (0.02 +/- 0.01 mg/min [0.4 +/- 0.2 mu mol/min]), as in CSs. Notwithstanding, their calcium excretion rate was still much less than that in CSs before volume repletion (0.13 +/- 0.04 mg/min [3.2 +/- 1.0 mu mol/min]). Limitation: Patients with GS did not become euvolemic on a long-term sodium chloride supplementation because they excreted sodium chloride so rapidly. Conclusion: Hypovolemia is not the sole cause of hypocalciuria in patients with GS.