CO-EXISTENCE OF DISTAL RENAL TUBULAR ACIDOSIS (dRTA) AND NEPHROGENIC DIABETES INSIPIDUS (NDI) IN TWO PATIENTS: IMPLICATIONS FOR THE PATHOGENESIS OF DISTAL RENAL TUBULAR ACIDOSIS

Abstract

dRTA was documented in 2 females (Case 1 - 14 yrs, Case 2 - 8 mo). The diagnosis of dRTA was based upon a urine pH of 6.9 for each case during an acute metabolic acidosis (blood pH 7.01 and 7.14), a normal HCO3 threshold (fractional HCO3 excretion 4.7 and 3.4%) and a low urine minus blood PCO2 gradient in alkaline urine (U-B PCO2 2 and 3). The diagnosis of NDI was suspected because of polyuria, persistent hypotonic urine and confirmed when investigated in detail. Maximum spontaneous urine osmolality (U Osm) for Case 1 and 2 were 139 and 93 mOsm/kg and increased slightly after infusion of antidiuretic hormone to 180 and 122 mOsm/kg respectively. 2 controls given a similar infusion achieved U Osm's >800 mOsm/kg. Thus our cases have a distal nephron which is impermeable to water and also dRTA with a low U-B PCO2. The low U-B PCO2 can be due to an increased permeability of the distal nephron to H+ in acid urine and H2CO3 in alkaline urine. However our cases had a distal nephron with reduced permeability to H2O and dRTA. Therefore unless there is a selective increased permeability to H2CO3 and H+ but decreased permeability to water, the low U-B PCO2 observed in these cases is more compatible with a failure of H+ secretion in the distal nephron. Animal studies supporting this will be presented.