The proximal tubule secretes NH₄⁺ into the luminal fluid primarily through the action of the apical sodium/hydrogen ion exchanger, NHE37, 8), and to a lesser degree through an apical barium sensitive potassium ion channel8-10). The activity and abundance of the apical Na⁺/H⁺ exchanger NHE3 is increased in the proximal brush border membrane during metabolic acidosis, which could be expected to contribute to the enhanced NH₄⁺ secretion by the proximal tubule during this condition11). It was recently shown that stimulation of ammonium secretion by the proximal tubule during metabolic acidosis largely depends on Na⁺/H⁺(NH₄⁺) exchange activity and on angiotensin II12).

After ammonium is produced and secreted by the proximal tubule, it is then delivered into the renal medulla via the loop of Henle. These potential losses of luminal NH₃ are minimized because more than 75 percent of the tubular fluid NH₄⁺ is recycled within the medulla, thereby maintaining a high interstitial NH₃ concentration3, 13, 14). The primary step in this process is reabsorption in the thick ascending limb by substitution of NH₄⁺ for K⁺ both on the Na⁺/K⁺/2Cl^- carrier and, to a much lesser degree, through the K⁺ channels in the luminal membrane3, 15). The luminal NH₃ permeability in the thick ascending limb was lower compared to other nephron segments16). The low NH₃ permeability limits the backflux of NH₃ into the tubule lumen and thereby contributes to the overall efficiency of the NH₄⁺ absorptive process. Partial dissociation into NH₃ and H⁺ then occurs in the less acidic tubular cell. As a result, the NH₃ formed within the cell will diffuse out across the basolateral membrane into the medullary interstitium. The countercurrent multiplication of ammonium generates the maintenance of a high medullary interstitial NH₃ concentration which promotes secretion into the medullary collecting tubule.

Ammonium reabsorption in the thick ascending limb of Henle's loop is reduced by hyperkalemia and is enhanced by chronic metabolic acidosis due to increased NH₄⁺ production in and delivery out of the proximal tubule14, 17). In the thick ascending limb of the loop of Henle, metabolic acidosis increases ammonium reabsorption through mechanisms that appear to be involved in increasing NKCC2 expression17, 18). In vitro incubation of rat medullary thick ascending limb fragments in suspension in an acid medium strongly enhanced the BSC1/NKCC2 mRNA and protein abundance and cotransport activity18). In addition, administration of the glucocorticoid dexamethasone to adrenalectomized rats stimulated BSC1/NKCC2 expression at the mRNA and protein levels19). The acidity of the surrounding environment and glucocorticoids may account for the stimulating effect of chronic metabolic acidosis on BSC1/NKCC2 expression.

The majority of urinary ammonium is secreted into luminal fluid in the region of the nephron distal to the micropuncturable late distal tubule20). This is a heterogeneous region, and includes portions of the distal convoluted tubule (DCT), connecting segment (CNT), initial collecting tubule, cortical collecting duct (CCD), outer medullary collecting duct (OMCD) and inner medullary collecting duct (IMCD). Accordingly, understanding the mechanisms of transepithelial ammonium transport across the cells that comprise these portions of the kidney is important.

The mechanism of ammonuim secretion in OMCD and IMCD is not completely clear at present. Since the first description of the concept, the process of transepithelial transport of ammonium in the collecting duct is thought to occur primarily through passive non-ionic NH₃ diffusion21, 22). The fluid entering the collecting tubules has a relatively low NH₃ concentration, because of its removal in the loop of Henle. The net effect is that there is a relatively large gradient favoring the free diffusion of interstitial NH₃ into the tubular lumen, where it forms NH₄⁺1, 13). The cell membranes in the collecting tubules are highly permeable to NH₃ but have only a negligible permeability to NH₄⁺23). As a result, interstitial NH₃ can passively diffuse into the tubular lumen where it is then trapped as NH₄⁺. The net effect is that NH₃ is secreted into the lumen throughout the collecting tubules3).

Although in the past ammonium was believed to move across epithelia entirely by passive diffusion, an increasing number of studies demonstrate that specific proteins contribute to renal ammonium transport3, 24, 25). The basolateral Na⁺,K⁺(NH₄⁺)-ATPase pump has been demonstrated to mediate NH₄⁺ secretion in the rat IMCD isolated and perfused in vitro26). The secretory Na⁺/K⁺(NH₄⁺)/2Cl^- cotransporter BSC2/NKCC1 was recently reported to mediate K⁺- and NH₄⁺-dependent chloride secretion, and thus to be involved in transepithelial solute transport27). BSC2/NKCC1 was shown to be up-regulated by chronic metabolic acidosis in collecting ducts of the rat28).

The most recent addition to our understanding of the molecular mechanisms of ammonium metabolism is the identification of the ammonia transporter family of proteins25, 29). Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg) are expressed in the renal DCT, CNT and collecting duct, the sites where approximately 80% of urinary ammonium is secreted30-36). In conditions of increased single-nephron ammonium metabolism, such as metabolic acidosis and reduced renal mass, Rhcg expression increases, suggesting that Rhcg mediates and has an important role in renal ammonium transport32, 35, 36). However, the nature of the transported substrate (NH₄⁺ or NH₃) by Rh glycoprotein has been controversial37-39). It was recently shown that both global and collecting duct-specific Rhcg deletion altered renal urinary ammonium excretion, indicating that collecting duct ammonium secretion is, at least in part, medicated by Rhcg and not solely by lipid diffusion40, 41). In addition, in vitro microperfused collecting ducts of Rhcg-/- acid-loaded mice show reduced apical permeability to NH₃ and impaired transepithelial NH₃ transport, indicating a role of Rhcg as an ammonium transport protein mediating the net flux of NH₃ in the collecting duct.

Both the rat and the human kidney express both apical and basolateral Rhcg immunolabel30, 32, 35, 36). Basolateral Rhcg expression increases in parallel with urinary ammonium excretion in both metabolic acidosis and reduced renal mass, suggesting that changes in basolateral Rhcg expression may contribute to renal ammonium metabolism35, 36). We recently demonstrate that the mouse kidney expresses both apical and basolateral Rhcg, which is similar to findings in both the rat and human kidney, although there are strain-dependent differences in the level of basolateral Rhcg expression in the mouse kidney42). These observations suggest that basolateral Rhcg may contribute to facilitated transcellular ammonium movement. Furthermore, basolateral Rhcg is likely to be the same protein as apical Rhcg, but is trafficked to the basolateral plasma membrane. Fig. 1 summarizes the major transporters involved in renal ammonium transport.

In summary, renal ammonium handling involves intrarenal ammonium production and transepithelial transport in multiple tubular segments that results in highly regulated renal ammonium metabolism. An increasing number of studies demonstrate that specific proteins contribute to renal ammonium transport. Understanding the physiologic regulation of ammonium transporter and the contribution of other protein to renal ammonium transport, are likely to be important fields for future studies.