[Secretion of ammonium ions in the collecting tubules of mammalian kidney]

Basolateral membrane sodium-independent Cl-/HCO3- exchanger in rat inner medullary collecting duct cell

Previous studies have shown that the middle third of the rat inner medullary collecting duct (IMCD-2) secretes protons despite the absence of intercalated cells, the cell thought to secrete protons in other portions of the collecting duct. A new cell, the IMCD cell, is the predominant cell in IMCD-2. The mechanism responsible for base exit in the IMCD cell was characterized by measuring cell pH of isolated perfused tubules with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein. Reduction of bath HCO3- caused a significant and reversible decrease in cell pH, whereas a similar change in luminal HCO3- had a significantly smaller effect, indicating that the HCO3-/H+ permeability of the basolateral membrane is much larger than the apical membrane. The rate of cell acidification induced by reduction in bath HCO3-, a measure of basolateral HCO3- transport, was significantly decreased in the absence of bath and lumen Cl. Decreases in bath Cl caused a significant and reversible increase in cell pH, which was not changed significantly by complete removal of Na from perfusate and bath, but was significantly inhibited by basolateral 4',5'-diisothiocyanostilbene-2,2'-disulfonic acid. A chemical voltage clamp did not inhibit the rate of cell alkalinization after bath Cl removal, indicating that Cl-/HCO3- exchange is not via parallel Cl and HCO3- conductances. Cell pH was measured in single cells by low-light-level imaging to show that most cells contain the chloride-dependent HCO3- pathway. We conclude that the rat IMCD cell possesses a basolateral Na-independent CL-/HCO3- exchanger which may serve as the base exit step for transepithelial proton secretion.

Ammonium handling by superficial and juxtamedullary nephrons in the rat. Evidence for an ammonia shunt between the loop of Henle and the collecting duct

Papillary and surface micropuncture was used to assess the effects of a chronic metabolic acidosis on the renal tubular handling of ammonium by surface nephrons, juxtamedullary nephrons, and the terminal segment of collecting duct. Rats chronically fed ammonium chloride had an expected decline in arterial pH and bicarbonate concentration associated with a doubling in the amount of ammonium excreted and a decline in urine pH. The glomerular filtration rate and absolute delivery of water and sodium to micropuncture sites of surface and deep nephrons was not measurably altered. Ammonium delivery to the end of the proximal tubule increased from 853+/-102% to 1,197+/-142% (SE) of the filtered load of ammonium after the induction of metabolic acidosis. This increase was due to a rise in tubular fluid ammonium content from 2.31+/-0.23 to 4.06+/-0.28 mM/liter. After the induction of acidosis, absolute and fractional delivery of ammonium ion to the end of the distal tubule was less than to the end of the accessible portion of the proximal tubule. These findings indicate that ammonium is lost in the intervening segment.Ammonium handling by deep nephrons was profoundly affected by acid loading. Absolute delivery to the bend of the loop of Henle increased twofold while fractional delivery rose from 1,222+/-108% to 1,780+/-132% of the filtered ammonium. This was due to a marked increase in ammonia entry. During acidosis, ammonium delivery to the terminal segment of the collecting duct was doubled (709+/-137% in controls vs. 1,415+/-150% in acidosis, P < 0.005) but did not change between proximal and tip collecting duct sites. In both groups of animals delivery of ammonium to the terminal segment of the collecting duct was greater than to end distal tubular micropuncture sites suggesting that ammonia entry occurred between these two sites. The differences in delivery was greater after the induction of a metabolic acidosis (887+/-140% vs. 384+/-144%, P < 0.05). Thus, the present study indicates that deep nephrons contribute to the adaptive increase in ammonium excretion seen during the induction of metabolic acidosis. The data also suggest that ammonia leaves the nephrons at a site(s) along the loop of Henle to enter the collecting duct and that the induction of a metabolic acidosis enhances this reentry.

Sites of ammonia addition to tubular fluid in rats with chronic metabolic acidosis

The purpose of this investigation was to determine in which nephron segments ammonia was added to or removed from the lumenal fluid of the rat. Ammonium was measured in proximal and distal tubular fluid samples obtained by micropuncture and in collecting duct fluid samples obtained by microcatheterization. Water abstraction was assessed by examining the tubular fluid-to-plasma inulin concentration, (TF/P)In. In normal or acidotic rats, the vast bulk of the final urine ammonium appeared in the proximal tubular fluid samples. Most of this ammonia was lost, however, in transit from the proximal to the distal tubule so that only 20 to 30% of the excreted ammonium was present at the distal site. Ammonia reentered the lumenal fluid primarily in the cortical collecting duct in acidotic rats and in the medullary collecting duct in normal rats. Although the pattern was qualitatively similar in both groups of rats, the absolute quantity of ammonium in each nephron segment of normal rats was about 10 to 20% of that in acidotic animals.

Ammonia addition into the medullary collecting duct of the rat

The purpose of these studies was to determine if ammonia is added directly to the medullary collecting duct of the rat, and if so, to estimate its quantitative contribution to ammonium excretion. Samples of fluid were obtained along the length of the medullary collecting duct by retrograde microcatheterization. To document net addition of ammonium, we measured the ammonium concentration in tubular fluid by an enzymatic isotope technique, and we calculated the degree of fluid reabsorption from the ratio of the inulin concentration in tubular fluid and plasma. The ammonium concentration corrected for water reabsorption ([NH4+]/[TF/P]In) rose progressively from the beginning to the end of the medullary collecting duct, indicating net ammonia addition to this nephron segment. By calculation, it appears that about 40% of the excreted ammonium reached the urine by direct addition of ammonia to the duct fluid.