Update on renal acidification: A physiological view

The kidney plays a major role in the regulation of acid-base balance. This process is mainly dependent on H+ secretion in the tubular lumen. Two acid extruder proteins are involved: the Na+/H+ exchanger and H+-ATPase. Studies using in vivo and in vitro microperfusion and isolated membrane vesicles have clearly demonstrated that the Na+/H+ exchanger is the main mechanism regulating H+ secretion/HCO3- reabsorption along the proximal nephron. Moreover, several reports indicate that this protein is involved in intracellular pH (pH(i)) regulation. Newer studies using molecular biology techniques have identified at least five isoforms of the Na+/H+ exchanger: NHE-1 is the housekeeping isoform, while NHE-3 seems to be implicated in transepithelial acid-base transport, although other isoforms could be involved too. H+-ATPase is the major acid extruder protein along the distal nephron, but it is also expressed along the proximal tubule, where a Na+-independent bicarbonate reabsorption has been described. There are a few studies indicating that the proton pump participates in pH(i) regulation, particularly in the presence of a large acid load. Its absence along the distal nephron may be one of the causes of distal tubular acidosis.