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

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, bicarbonate reclamation, and net acid formation was assessed under hydropenic conditions and after a chronic acid load. In these two settings the proximal segment of both kinds of nephrons is the major site of ammonium production and bicarbonate reclamation. However, this segment's contribution to net acid formation was only significant during acidosis. On the other hand, 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. It is postulated that this difference in pH allows ammonium to dissociate, permitting the movement of ammonia out of the tubule lumen and into collecting duct fluid where it is protonated and, therefore, reentrapped. This process is enhanced by the ingestion of a chronic acid load.