Contribution of individual superficial nephron segments to ammonium handling in chronic metabolic acidosis in the rat. Evidence for ammonia disequilibrium in the renal cortex.

Abstract

Ammonia entry along surface nephron segments of rats was studied with micropuncture techniques under control and chronic metabolic acidosis conditions. Tubule fluid was collected successively from sites at the end and beginning of the distal tubule and at the end of the proximal tubule of the same nephron. During chronic metabolic acidosis, ammonium excretion doubled. As anticipated, the ammonium concentration (TFNHO3 was significantly higher in proximal tubule fluid during acidosis, and ammonium delivery to end proximal sites increased from 19.4±2.3 to 34.0±3.2 pmol/min (P < 0.001). Although chronic acidosis did not affect TFNW at the beginning of the distal tubule, ammonium delivery to the end of the distal tubule increased from 5.72±0.97 to 9.88±0.97 pmol/min. In both control and acidotic groups ammonium delivery was lower (P < 0.001) to end distal sites than to end proximal sites, indicating net loss in the intervening segment. This loss was greater during chronic metabolic acidosis (23.9±33 vs. 13.6±2.0 pmol/min in controls, P < 0.025). In both groups net entry of ammonia, in similar amounts, occurred along the distal tubule (P < 0.05). In situ pH averaged 6.80±0.05 at end proximal tubule sites and fell to 6.54±0.08 at the beginning of the distal tubule (P < 0.005). Chronic metabolic acidosis did not affect these measurements. The calculated free ammonia at the end of the proximal tubule rose from 93±2.2 to 21±9 iM (P < 0.005) during chronic metabolic acidosis, and was also higher at beginning distal sites during acidosis (8.8±2.4 vs.2.7±0.7 uM in controls, P < 0.05). In both groups ammonia values for the beginning distal tubule fluid were lower than for end proximal tubule fluid. Thus, loss of ammonium in the loop segment is enhanced by chronic metabolic acidosis. Distal entry of ammonia is markedly less than along the proximal tubule and does not change in chronic metabolic acidosis, and ammonia permeabilities for the proximal and distal segments of surface nephrons seem different.

Acid pH Increases the Stability of BSC1/NKCC2 mRNA in the Medullary Thick Ascending Limb

“…Indeed, BSC1 mRNA abundance significantly increased 3 h after the creation of metabolic acidosis by peritoneal dialysis, which was followed within 24 h by an augmentation of the BSC1 protein abundance (16). The augmentation of both mRNA and protein abundance persisted after 6 d of CMA induced by administration of NH 4 Cl in drinking water (16). Most important, we showed that incubation of freshly harvested MTAL fragments in an acid medium enhanced BSC1 mRNA and protein abundance (16).…”

“…Chronic metabolic acidosis enhances the ability of the medullary thick ascending limb (MTAL) to absorb NH 4 ϩ at least in part by stimulating the mRNA and protein expression of BSC1/NKCC2, the MTAL apical Na ϩ -K ϩ (NH 4 ϩ )-2Cl Ϫ co-transporter. For assessing the mechanism by which an acid pH enhances the BSC1 mRNA abundance, MTAL were harvested from adrenalectomized rats and incubated in control (pH 7.35) and acid (pH 7.10) 1:1 mixtures of Ham's nutrient mixture F-12 and DME.…”

“…Increased urinary NH 4 ϩ excretion, which augments acid excretion, has long been recognized to be quantitatively the major compensatory response of the kidney against chronic metabolic acidosis (CMA) (1). It is established that most of ammonia that leaves the proximal tubule is absorbed by the medullary thick ascending limb (MTAL), which causes ammonia accumulation in the medullary interstitium followed by its secretion in adjacent medullary collecting tubule (2).…”

“…It is established that most of ammonia that leaves the proximal tubule is absorbed by the medullary thick ascending limb (MTAL), which causes ammonia accumulation in the medullary interstitium followed by its secretion in adjacent medullary collecting tubule (2). The role of MTAL ammonia absorption may be particularly important in states of CMA because micropuncture experiments have shown that the NH 4 ϩ amount absorbed by the loop of Henle is increased under the latter condition (3,4). In this regard, Good (5) has shown that rat MTAL isolated and perfused in vitro have an increased ability to absorb NH 4 ϩ in response to CMA.…”

“…The role of MTAL ammonia absorption may be particularly important in states of CMA because micropuncture experiments have shown that the NH 4 ϩ amount absorbed by the loop of Henle is increased under the latter condition (3,4). In this regard, Good (5) has shown that rat MTAL isolated and perfused in vitro have an increased ability to absorb NH 4 ϩ in response to CMA. This adaptive response in MTAL would substantially augment NH 4 ϩ excretion during metabolic acidosis.…”