Studies on the pathogenesis of hypokalemia in Gitelman's syndrome: role of bicarbonaturia and hypomagnesemia.

OBJECTIVE: Hypokalemia and renal potassium (K) wasting are hallmarks of the group of disorders called Bartter's syndrome. The presence of hypomagnesemia and a low rate of excretion of calcium are currently used to characterize a subgroup of these patients as having Gitelman's syndrome (GS) in which the molecular lesion is a defect in the thiazide-sensitive NaCl cotransporter in the distal convoluted tubule. This study was undertaken to examine whether bicarbonaturia or hypomagnesemia exacerbates the kaliuresis in patients with GS.

METHODS: Six patients with most of the diagnostic features of GS were examined. To examine the role of bicarbonaturia, the transtubular K concentration gradient (TTKG) was assessed before and after an oral load of NH4Cl which caused the urine pH to be < 6. To evaluate the role of hypomagnesemia, the TTKG was examined after an infusion of enough magnesium (Mg) to achieve normal levels of Mg in plasma for close to 24 h.

RESULTS: The TTKG remained very high even when the pH of the urine was < 6.0. An infusion of Mg caused the TTKG to approach expected values for hypokalemia in 4 of 6 patients. The infusion of Mg was extended in 1 patient who had a sustained high TTKG for 24 h; the TTKG remained elevated for 96 h despite normal plasma Mg levels.

CONCLUSIONS: Bicarbonaturia does not play a critical role in maintaining the very high TTKG in these patients. The K wasting in 4 of 6 of these patients could largely be attributed to hypomagnesemia and/or Mg depletion. The plasma aldosterone level tended to be higher in patients who did not respond to the infusion of Mg. Therefore, these patients may not represent a homogeneous group with regard to the pathophysiology of their renal K wasting.