Intrarenal distribution of ammonia during diuresis and antidiuresis

Abstract

The intrarenal distribution of ammonia was evaluated in the dog during antidiuresis and osmotic (mannitol) diuresis at various levels of urine pH. During antidiuresis, the concentration of ammonia in renal tissue water rose progressively from the cortex to the tip of the papilla. In contrast, the corticomedullary ammonia gradient was completely obliterated by osmotic diuresis. In both experimental groups, a close relationship was observed between urine pH and the logarithm of the ratio between the urine and papillary concentrations of ammonia. The data are compatible with a proposal that a medullary countercurrent exchange system is responsible for medullary ammonia accumulation, and that diffusion equilibrium exists between the pNH3 of loop of Henle fluid, vasa recta blood, and collecting duct fluid.

Diffusion Equilibrium for Ammonia in the Kidney of the Acidotic Dog*

“…1 20 min preceded the collection of the first urine specimen to ensure attainment of steady state conditions. The ammonia pool of the kidney is no doubt small but turns over rapidly (15). The half-time of the turnover rate estimated from pool size in the dog kidney (15) and from pool turnover in our experiments is about 1 min.…”

Ammonium Handling by Superficial and Juxtamedullary Nephrons in the Rat

“…Second, it has been reported that the pH of loop fluid is more alkaline than fluid obtained at end proximal micropuncture sites (1,2). Lastly, the concentration of ammonia in renal tissue increases from cortex to medulla (20,21) and ammonium formation in collecting duct fluid seems dependent on this gradient. Thus, while the concentration in cortical tissue is not altered by an osmotic diuretic the gradient from cortex to medulla disappears (20) and ammonia entry into the collecting duct is diminished (21).…”

Effects of potassium on ammonia transport by medullary thick ascending limb of the rat.

“…It is generally recognized that transfer of ammonia from the loops of Henle to the collecting ducts in the renal medulla is an important process facilitating renal ammonium excretion (4,(14)(15)(16). This transfer process involves reabsorption of ammonia from the loops of Henle and accumulation of ammonia to high concentrations in the medullary interstitium (4,(17)(18)(19)(20). Because secretion of ammonia by the collecting ducts depends on the NH3 concentration difference between the interstitial fluid and the collecting duct lumen (4,17,21,22), the ability of the kidney to accumulate ammonia in the medulla and maintain a high medullary interstitial NH3 concentration is an important factor in the maintenance of a high rate of total ammonia excretion.…”

“…Because secretion of ammonia by the collecting ducts depends on the NH3 concentration difference between the interstitial fluid and the collecting duct lumen (4,17,21,22), the ability of the kidney to accumulate ammonia in the medulla and maintain a high medullary interstitial NH3 concentration is an important factor in the maintenance of a high rate of total ammonia excretion. The high ammonia content ofthe renal medulla has been proposed to depend on countercurrent multiplication (5,20,23). Direct evidence to support this possibility was provided when the thick ascending limb of Henle's loop was demonstrated to absorb NHZ against a concentration gradient (5).…”

“…This process is analogous to the countercurrent exchanger for NaCl in that the small concentration difference generated across the medullary thick ascending limb is multiplied by countercurrent flow between the loops, resulting in an axial gradient for total ammonia. The importance of medullary ammonia accumulation as a factor determining the rate of ammonium excretion has been demonstrated in chronic metabolic acidosis (17), in osmotic diuresis (19,20), and in the remnant kidney (26). Since ammonium absorption by the medullary thick ascending limb would provide energy for the countercurrent process, factors which affect thick ascending limb ammonia transport would be expected to alter medullary ammonia recycling and ammonium excretion.…”