Figure 1.

Assessment of renal phenotype. (A) Furosemide responsiveness testing in the patient and in 5 control subjects. Shown are maximal absolute initial changes during loop diuretic exposition over mean basal levels. The transtubular K⁺ concentration gradient (TTKG) was defined as urinary K⁺/serum K⁺ whenever serum osmolality exceeded urinary osmolality and else was calculated from the standard formula.²⁷ Overt differences between the patient and controls are observed only for fractional excretions of Mg²⁺ and K⁺ as well as for the TTKG. The loop diuretic effect on fractional Mg²⁺ excretion signifies that paracellular Mg²⁺ reabsorption in the thick ascending limb of Henle's loop is enhanced in the patient and may be involved in the development of hypermagnesemia. Loop diuretic effects on fractional K⁺ excretion can be explained by adaptive changes in the distal nephron promoting enhanced K⁺ secretion upon an increase in NaCl delivery. Median and IQR. (B) Development of urine osmolality in the patient over time during water deprivation with and without administration of KCl. Results are similar for both tests and indicate a markedly reduced urine concentrating ability with a blunted response to desmopressin, irrespective of an increase in serum K⁺ level. (C) In the patient, the increase in serum K⁺ parallels that of TTKG during water deprivation with concomitant intravenous KCl substitution. i.v., intravenously.