Hypermagnesuria in Humans Following Acute Intravenous Administration of Digoxin

Hypomagnesemia is a known predisposing condition for the appearance of digitalis toxicity. The detection of a genetic form of Mg urinary wasting with hypomagnesemia being caused by a mutation in the γ subunit (FXYD2) of the Na,K-ATPase, the pharmacological target of Digoxin, prompted us to investigate whether Digoxin administration increases urinary Mg excretion.

Two groups of subjects, with rapid atrial fibrillation, received intravenous Digoxin (n = 9) or verapamil (n = 8), for heart rate control. During the following 4 h, blood and urinary creatinine, sodium, potassium, calcium, and magnesium levels were determined, and fractional excretion (Fex) values for Na, K, Ca, and Mg were calculated.

In the Digoxin group, at 60 min Fex Mg rose from 3.07 ± 1.21 to 7.58 ± 2.51% (an increase of 269 ± 107% of baseline, p < 0.001), and at 240 min to 6.05 ± 2.30% (204 ± 56% of baseline, p < 0.01). No significant change was observed for Fex Na, Fex K, and Fex Ca. A striking correlation was found between individual values of Fex Mg and serum Digoxin concentration (r = 0.678, p < 0.0001). No significant correlation was found between Fex Na or Fex K and serum Digoxin. A correlation of borderline significance was found between Fex Ca and serum Digoxin (r = 0.349, p = 0.073).

The hypermagnesuric effect of acute Digoxin treatment is reminiscent of the effect of the missense mutation in FXYD2, which assumes that FXYD2 is a positive regulator of Na,K-ATPase in the distal convoluted tubule (DCT). The borderline calciuric effect of Digoxin may point to an additional site of action, more proximal to the DCT, that is, the thick ascending limb.