Segmental analysis of the renal tubule in buffer production and net acid formation

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Abstract
Papillary and surface micropuncture in Munich-Wistar rats was used to assess the role of proximal segments of superficial and juxtamedullary (JM) nephrons, the distal tubule of superficial nephrons, and the terminal collecting duct in acid excretion. The relative role of these segments in ammonium production, HCO3- reclamation, and net acid formation was assessed under hydropenic conditions and after a chronic acid load. In these 2 settings the proximal segment of both kinds of nephrons is the major site of ammonium production and HCO3- reclamation. This segment''s contribution to net acid formation was only significant during acidosis. Segments beyond the distal tubule appear to be the major site of acid formation. In situ pH measurements were lower in these nephron segments and fell even more after the induction of an acidosis. Ammonia appears to enter fluid between the end of the distal tubule and the base of the collecting duct. In vivo pH measurements made near the bend of Henle''s loop of JM nephrons were more alkaline than near the end of the proximal tubule of superficial nephrons. This difference in pH may allow ammonium to dissociate, permitting the movement of ammonia out of the tubule lumen and into collecting duct fluid where it is protonated and reentrapped. This process is enhanced by the ingestion of a chronic acid load.

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