Application of the disequilibrium pH method to investigate the mechanism of urinary acidification

DuBose, Thomas D., Jr. Application of the disequilibrium pH method to investigate the mechanism of urinary acidification. Am. J. Physiol. 245 (Renal Fluid Electrolyte Physiol. 14): F535-F544, 1983.--The cellular mechanism of renal bicarbonate reabsorption has been debated for four decades. Recent technological advances have allowed distinction between primary bicarbonate reabsorption and a proton secretory mechanism. The disequilibrium pH method has been applied widely for this purpose and has supported the latter hypothesis uniformly. The demonstration of elevated values for PCO2 in tubular and vascular structures of the renal cortex has not altered this view. Indeed, by employing a newly developed method for measurement of equilibrium pH in vivo that permits contact with the environment within the tubule lumen to continue, we demonstrated an acid disequilibrium pH in the proximal tubule after carbonic anhydrase inhibition equal to -0.68 pH units. A spontaneous disequilibrium pH was not present in the distal tubule during control conditions or during metabolic alkalosis but was demonstrated during combined respiratory acidosis-metabolic alkalosis. This finding agrees qualitatively with observed rates of bicarbonate reabsorption in the perfused distal tubule in vivo. With use of similar techniques, an acid disequilibrium pH in conjunction with elevated values for PCO2 was observed in the papillary collecting duct. Thus, proton secretion appears to be the predominant mechanism of bicarbonate reabsorption in superficial nephrons and explains, as well, the means by which the urine-to-blood PCO2 gradient in alkaline urine is established.