On the mechanism of renal potassium wasting in renal tubular acidosis associated with the Fanconi syndrome (type 2 RTA)

Anthony Sebastian; Elisabeth McSherry; R. Curtis Morris

doi:10.1172/JCI106479

The mechanism of renal potassium wasting in renal tubular acidosis associated with the Fanconi syndrome (type 2 RTA) was investigated in 10 patients, each of whom had impaired proximal renal tubular reabsorption of bicarbonate as judged from a greater than 15-20% reduction of renal tubular bicarbonate reabsorption (THCO₃^-) at normal plasma bicarbonate concentrations. When the plasma bicarbonate concentration ([HCO₃^-]p) was experimentally increased to normal levels in three patients with a fractional potassium excretion (C_K/C_in) of less than 1.0 during acidosis, C_K/C_in and urinary potassium excretion (U_KV/C_in) increased strikingly and concurrently with a striking increase in urinary sodium (U_NaV/C_in) and bicarbonate (U_HCO₃^-V/C_in) excretion. When [HCO₃^-]p was increased to normal levels in two patients with a C_K/C_in of greater than 1.0 during acidosis and in whom U_NaV/C_in and U_HCO₃^-V/C_in were already markedly increased, C_K/C_in did not increase further. When [HCO₃^-]p was decreased to subnormal levels in a patient given ammonium chloride, U_KV/C_in, C_K/C_in, and U_HCO₃^-V/C_in decreased concurrently. In the six patients in whom [HCO₃^-]p was maintained at normal levels (oral alkali therapy) for 2 months or longer, C_K/C_in was directly related to the urinary excretion rates of sodium and bicarbonate, hence was directly related to the magnitude of reduction of THCO₃^- at normal [HCO₃^-]p; C_K/C_in was greater than 0.55 in all six patients and greater than 1.0 in four.

In eight patients with classic RTA (type 1 RTA), proximal renal tubular reabsorption of bicarbonate was largely intact as judged from a trivial reduction of THCO₃^- at normal [HCO₃^-]p. When [HCO₃^-]p was either increased from subnormal to normal levels, or decreased from normal to subnormal levels, U_HCO₃^-V/C_in remained essentially constant, and U_KV/C_in did not change significantly. When correction of acidosis was sustained, U_HCO₃^-V/C_in remained a trivial fraction of that filtered, and C_K/C_in was consistently less than 0.55.

These results provide evidence that renal potassium wasting in type 2 RTA is physiologically separable from that in type 1 RTA and in part the result of a reduction in the rate at which the proximal tubule reabsorbs bicarbonate and the distal delivery of supernormal amounts of sodium bicarbonate. With an increased stimulus to distal sodium reabsorption, indicated by the finding of hyperaldosteronism, delivery to the distal nephron of supernormal amounts of sodium with the relatively impermeant bicarbonate anion would be expected to increase intraluminal negativity in the distal nephron, and as a consequence, increase potassium secretion and promote renal potassium wasting.