Nephrology rounds, University of Iowa Hospitals: renal tubular acidosis

We have discussed two patients who had renal tubular acidosis complicated by hypokalemia. The first patient had a distal acidifying defect. Circumstantial evidence has been presented suggesting that exposure to toluene-diisocyanate or toluene-diamine played a role in the pathogenesis. The acidosis and the hypokalemia of this patient were easily corrected by the administration of small amounts of sodium bicarbonate without potassium supplementation. The second patient had an interstitial nephritis of unknown etiology and presented with moderate renal insufficiency, renal tubular acidosis, and proximal as well as distal acidifying defects. The proximal tubular dysfunction was associated with general aminoaciduria and glucosuria. This patient required large quantities of both alkali and potassium to correct the electrolyte abnormalities. The mechanisms of potassium wasting in proximal and distal renal tubular acidosis are reviewed. A classification is presented of cellular defects that may underlie the different renal acidifying defects. Attempts to distinguish between pump and permeability defects from urinary pCO2 levels must take into account the simultaneous HCO-3 concentration, since large pCO2 elevations require the presence of ample HCO-3 in the urine. Permeability defects may impair urinary acidification by either abnormal back flux of H+ out of the lumen or increased influx of HCO-3 into the lumen. In studies of acidification in vitro, amphotericin B causes increased H+ permeability and has little effect on HCO-3 permeability. Toluene-diamine causes a marked permeability defect which is reversible, but remains to be defined in terms of the ion species, HCO-3 or H+, affected. At times, hyperchloremic acidosis is caused by distal defects in net acid excretion that occur without impairment of the H+ gradient. In certain patients with hypoaldosteronism, for example, distal H+ secretion may be reduced without change in the force of the H+ pump.

Coupling of sodium transport to respiration in the toad bladder

Energy expenditure and transepithelial sodium transport were measured continuously and simultaneously from isolated urinary bladders of the Dominican toad, Bufo marinus. Sodium transport was measured as the short-circuit current and CO2 produced by the bladder was measured conductometrically by the method of Maffly. The rates of sodium transport and CO2 productions were linearly related. The slope of the regression of sodium transport on CO2 production, dJNa/dJCO2, was found to be quite similar in paired half bladders but to differ significantly between bladders from different toads. Thus, in this preparation there appears to be no unique stoichiometric ratio characterizing sodium transport and metabolism and past efforts to arrive at such a value by averaging results obtained from different animals do not seem warranted. The CO2 production by the isolated bladder which is unrelated to sodium transport was determined by two means: 1) extrapolating the regression of JNa on JCO2 to JNa equals O, and 2) measuring CO2 production with sodium transport suppressed by removal of all sodium from the mucosal bathing medium. The two methods gave values which were in close agreement in each preparationmthis suggests that metabolism which supports nontransport activities in this tissue cannot be recruited to support the energy requirement of sodium transport and vice versa.