Ammonia transport by early and late proximal convoluted tubule of the rat

State of knowledge on ammonia handling by the kidney

The disposal of ammonia, the main proton buffer in the urine, is important for acid-base homeostasis. Renal ammonia excretion is the predominant contributor to renal net acid excretion, both under basal condition and in response to acidosis. New insights into the mechanisms of renal ammonia production and transport have been gained in the past decades. Ammonia is the only urinary solute known to be produced in the kidney and selectively transported through the different parts of the nephron. Both molecular forms of total ammonia, NH3 and NH4+, are transported by specific proteins. Proximal tubular ammoniagenesis and the activity of these transport processes determine the eventual fate of total ammonia produced and excreted by the kidney. In this review, we summarized the state of the art of ammonia handling by the kidney and highlighted the newest processes described in the last decade.

Teleostean fishes may have developed an efficient Na+ uptake for adaptation to the freshwater system

Understanding Na+ uptake mechanisms in vertebrates has been a research priority since vertebrate ancestors were thought to originate from hyperosmotic marine habitats to the hypoosmotic freshwater system. Given the evolutionary success of osmoregulator teleosts, these freshwater conquerors from the marine habitats are reasonably considered to develop the traits of absorbing Na+ from the Na+-poor circumstances for ionic homeostasis. However, in teleosts, the loss of epithelial Na+ channel (ENaC) has long been a mystery and an issue under debate in the evolution of vertebrates. In this study, we evaluate the idea that energetic efficiency in teleosts may have been improved by selection for ENaC loss and an evolved energy-saving alternative, the Na+/H+ exchangers (NHE3)-mediated Na+ uptake/NH4 + excretion machinery. The present study approaches this question from the lamprey, a pioneer invader of freshwater habitats, initially developed ENaC-mediated Na+ uptake driven by energy-consuming apical H+-ATPase (VHA) in the gills, similar to amphibian skin and external gills. Later, teleosts may have intensified ammonotelism to generate larger NH4 + outward gradients that facilitate NHE3-mediated Na+ uptake against an unfavorable Na+ gradient in freshwater without consuming additional ATP. Therefore, this study provides a fresh starting point for expanding our understanding of vertebrate ion regulation and environmental adaptation within the framework of the energy constraint concept.

A mathematical model of the rat kidney. III. Ammonia transport

Ammonia generated within the kidney is partitioned into a urinary fraction (the key buffer for net acid excretion) and an aliquot delivered to the systemic circulation. The physiology of this partitioning has yet to be examined in a kidney model, and that was undertaken in this work. This involves explicit representation of the cortical labyrinth, so that cortical interstitial solute concentrations are computed rather than assigned. A detailed representation of cortical vasculature has been avoided by making the assumption that solute concentrations within the interstitium and peritubular capillaries are likely to be identical and that there is little to no modification of venous composition as blood flows to the renal vein. The model medullary ray has also been revised to include a segment of proximal straight tubule, which supplies ammonia to this region. The principal finding of this work is that cortical labyrinth interstitial ammonia concentration is likely to be several fold higher than systemic arterial ammonia. This elevation of interstitial ammonia enhances ammonia secretion in both the proximal convoluted tubule and distal convoluted tubule, with uptake by Na⁺-K⁺-ATPases of both segments. Model prediction of urinary ammonia excretion was concordant with measured values, but at the expense of greater ammoniagenesis, with high rates of renal venous ammonia flux. This derives from a limited capability of the model medulla to replicate the high interstitial ammonia concentrations that are required to drive collecting duct ammonia secretion. Thus, renal medullary ammonia trapping appears key to diverting ammonia from the renal vein to urine, but capturing the underlying physiology remains a challenge.NEW & NOTEWORTHY This is the first mathematical model to estimate solute concentrations within the kidney cortex. The model predicts cortical ammonia to be several fold greater than in the systemic circulation. This higher concentration drives ammonia secretion in proximal and distal tubules. The model reveals a gap in our understanding of how ammonia generated within the cortex is channeled efficiently into the final urine.

Ammonia Transporters and Their Role in Acid-Base Balance

Acid-base homeostasis is critical to maintenance of normal health. Renal ammonia excretion is the quantitatively predominant component of renal net acid excretion, both under basal conditions and in response to acid-base disturbances. Although titratable acid excretion also contributes to renal net acid excretion, the quantitative contribution of titratable acid excretion is less than that of ammonia under basal conditions and is only a minor component of the adaptive response to acid-base disturbances. In contrast to other urinary solutes, ammonia is produced in the kidney and then is selectively transported either into the urine or the renal vein. The proportion of ammonia that the kidney produces that is excreted in the urine varies dramatically in response to physiological stimuli, and only urinary ammonia excretion contributes to acid-base homeostasis. As a result, selective and regulated renal ammonia transport by renal epithelial cells is central to acid-base homeostasis. Both molecular forms of ammonia, NH₃ and NH₄⁺, are transported by specific proteins, and regulation of these transport processes determines the eventual fate of the ammonia produced. In this review, we discuss these issues, and then discuss in detail the specific proteins involved in renal epithelial cell ammonia transport.

A mathematical model of the rat nephron: glucose transport

Mathematical models of the proximal tubule (PT), loop of Henle (LOH), and distal nephron have been combined to simulate transport by rat renal tubules. The ensemble is composed of 24,000 superficial (SF) nephrons and 12,000 juxtamedullary (JM) nephrons in 5 classes (according to LOH length); all coalesce into 7,200 connecting tubules (CNT). Medullary interstitial solute concentrations are specified. The model equations require that each nephron glomerular filtration rate (GFR) satisfies a tubuloglomerular feedback (TGF) relationship, and each initial hydrostatic pressure yields a common CNT pressure; that common CNT pressure is determined from an overall distal hydraulic resistance to flow. By virtue of the greater GFR for JM nephrons, fluid delivery to SF and JM tubules is comparable. Glucose reabsorption is restricted to the PT, cotransported with one Na in the convoluted tubule (SGLT2), and two Na in the straight tubule (SGLT1). Increasing ambient glucose from 5 to 10 mM increases proximal Na reabsorption and decreases distal delivery. This is mitigated by a TGF-mediated increase in GFR, and may thus be an etiology for TGF-mediated glomerular hyperfiltration. With SGLT2 inhibition by 95%, the model predicts that under normoglycemic conditions about 60% of filtered glucose will still be reabsorbed, so that profound glycosuria is not to be expected. Compared with glucose-driven osmotic diuresis, SGLT2 inhibition provokes greater natriuresis. When hyperglycemia is superimposed on SGLT2 inhibition, the model suggests that natriuresis may be severe, reflecting synergy of a proximal diuretic and osmotic diuresis. In sum, the model captures TGF-mediated diabetic hyperfiltration and predicts glomerular protection with SGLT2 inhibition.

Liver-specific Aquaporin 11 knockout mice show rapid vacuolization of the rough endoplasmic reticulum in periportal hepatocytes after amino acid feeding

Aquaporin 11 (AQP11) is a protein channel expressed intracellularly in multiple organs, yet its physiological function is unclear. Aqp11 knockout (KO) mice die early due to malfunction of the kidney, a result of hydropic degeneration of proximal tubule cells. Here we report the generation of liver-specific Aqp11 KO mice, allowing us to study the role of AQP11 protein in liver of mice with normal kidney function. The unchallenged liver-specific Aqp11 KO mice have normal longevity, their livers appeared normal, and the plasma biochemistries revealed only a minor defect in lipid handling. Fasting of the mice (24 h) induced modest dilatation of the rough endoplasmic reticulum (RER) in the periportal hepatocytes. Refeeding with standard mouse chow induced rapid generation of large RER-derived vacuoles in Aqp11 KO mice hepatocytes. Similar effects were observed following oral administration of pure protein or larger doses of various amino acids. The fasting/refeeding challenge is associated with increased expression of markers of ER stress Grp78 and GADD153 and decreased glutathione levels, suggesting that ER stress may play role in the development of vacuoles in the AQP11-deficient hepatocytes. NMR-based metabolome analysis of livers from mice subject to amino acid challenge showed decreased amount of extractable metabolites in the AQP11-deficient livers and particularly a decrease in glucose levels. In conclusion, in the liver, deletion of AQP11 results in disrupted RER homeostasis and increased sensitivity to RER injury upon metabolic challenge with amino acids.

The hyperpolarization-activated cyclic nucleotide-gated HCN2 channel transports ammonium in the distal nephron

Recent studies have identified Rhesus proteins as important molecules for ammonia transport in acid-secreting intercalated cells in the distal nephron. Here, we provide evidence for an additional molecule that can mediate NH3/NH4 excretion, the subtype 2 of the hyperpolarization-activated cyclic nucleotide-gated channel family (HCN2), in collecting ducts in rat renal cortex and medulla. Chronic metabolic acidosis in rats did not alter HCN2 protein expression but downregulated the relative abundance of HCN2 mRNA. Its cDNA was identical to the homolog from the brain and the protein was post-translationally modified by N-type glycosylation. Electrophysiological recordings in Xenopus oocytes injected with HCN2 cRNA found that potassium was transported better than ammonium, each of which was transported significantly better than sodium, criteria that are compatible with a role for HCN2 in ammonium transport. In microperfused rat outer medullary collecting duct segments, the initial rate of acidification, upon exposure to a basolateral ammonium chloride pulse, was higher in intercalated than in principal cells. A specific inhibitor of HCN2 (ZD7288) decreased acidification only in intercalated cells from control rats. In rats with chronic metabolic acidosis, the rate of acidification doubled in both intercalated and principal cells; however, ZD7288 had no significant inhibitory effect. Thus, HCN2 is a basolateral ammonium transport pathway of intercalated cells and may contribute to the renal regulation of body pH under basal conditions.

Role of the Rhesus glycoprotein, Rh B glycoprotein, in renal ammonia excretion

Rh B glycoprotein (Rhbg) is a member of the Rh glycoprotein family of ammonia transporters. In the current study, we examine Rhbg's role in basal and acidosis-stimulated acid-base homeostasis. Metabolic acidosis induced by HCl administration increased Rhbg expression in both the cortex and outer medulla. To test the functional significance of increased Rhbg expression, we used a Cre-loxP approach to generate mice with intercalated cell-specific Rhbg knockout (IC-Rhbg-KO). On normal diet, intercalated cell-specific Rhbg deletion did not alter urine ammonia excretion, pH, or titratable acid excretion significantly, but it did decrease glutamine synthetase expression in the outer medulla significantly. After metabolic acidosis was induced, urinary ammonia excretion was significantly less in IC-Rhbg-KO than in control (C) mice on days 2-4 of acid loading, but not on day 5. Urine pH and titratable acid excretion and dietary acid intake did not differ significantly between acid-loaded IC-Rhcg-KO and C mice. In IC-Rhbg-KO mice, acid loading increased connecting segment (CNT) cell and outer medullary collecting duct principal cell Rhbg expression. In both C and IC-Rhbg-KO mice, acid loading decreased glutamine synthetase in both the cortex and outer medulla; the decrease on day 3 was similar in IC-Rhbg-KO and C mice, but on day 5 it was significantly greater in IC-Rhbg-KO than in C mice. We conclude 1) intercalated cell Rhbg contributes to acidosis-stimulated renal ammonia excretion, 2) Rhbg in CNT and principal cells may contribute to renal ammonia excretion, and 3) decreased glutamine synthetase expression may enable normal rates of ammonia excretion under both basal conditions and on day 5 of acid loading in IC-Rhbg-KO mice.

Superoxide enhances Na-K-2Cl cotransporter activity in the thick ascending limb

Superoxide (O2-) enhances Na reabsorption by the thick ascending limb (THAL). Na absorption in this segment involves the Na-K-2Cl cotransporter, K channel, and Na-K-ATPase. We hypothesized that O2- stimulates NaCl absorption primarily by enhancing Na-K-2Cl cotransport. First, we measured steady-state intracellular Na (Nai) and chloride (Cli). Xanthine oxidase (XO; 0.75 mU/ml) and hypoxanthine (HX; 0.125 mM) were added to the bath to increase O2-. During the control period, Nai was 12.2 +/- 1.9 mM. After treatment with O2-, it rose to 20.9 +/- 3.3 mM, a 71% increase (P < 0.01). Cli also increased (P < 0.01). Neither XO nor HX alone had a significant effect on Nai or Cli. Next, we tested cotransport activity by measuring the initial rate of increase in Nai caused by changing luminal Na-Cl-K from 50/0/0 to 140/134/4 mM. During the control period, the initial rate of increase was 0.13 +/- 0.02 arbitrary units (AU)/min. After treatment with O2-, it was 0.22 +/- 0.04 AU/min (P < 0.025), a 69% increase. Neither XO nor HX alone had a significant effect. Furosemide completely blocked the increase in intracellular Na in the control and O2- treatment periods. Next, we studied K channel activity by measuring the depolarization caused by increasing luminal K from 1 to 25 mM using a voltage-sensitive dye. During the control period, maximum depolarization was 7.31 +/- 0.77 AU. After O2- treatment, it was 6.18 +/- 0.90 AU (P < 0.05), a 15% decrease. Finally, we assessed the effects of O2- on Na-K-ATPase activity in THAL suspensions by measuring ATP hydrolysis. Vmax and K1/2 for Na were not affected by O2-. We concluded that O2- stimulates THAL NaCl absorption primarily by enhancing Na entry via Na-K-2Cl cotransport.

Enhanced renal ammonia excretion following volume expansion in patients with well compensated cirrhosis of the liver

BACKGROUND AND AIMS

In patients with cirrhosis, hepatic encephalopathy is often precipitated by dehydration. This study tests the hypothesis that volume expansion in cirrhotic patients increases renal ammonia excretion.

PATIENTS AND METHODS

Sixteen well compensated cirrhotic patients (mean Pugh score 6.7 (SEM 0.4)) were studied after an overnight fast. One litre of 0.9% saline was administered to patients intravenously over one hour. Plasma and urinary ammonia and sodium, renal plasma flow (RPF), glomerular filtration rate (GFR), plasma renin activity (PRA), and angiotensin II (ANG II) were measured before, during, and two hours after saline infusion.

RESULTS

Saline infusion resulted in a significant reduction in plasma ammonia (93 (SEM 7) to 56 (4) micromol/l; p<0.05) and RPF and GFR increased (p<0.05). Urinary ammonia excretion increased (p<0.05) significantly. There was a significant reduction in ANG II and PRA (p<0.05 for each) and the change in ammonia excretion correlated directly with the change in urinary sodium excretion (p<0.007), ANG II (p<0.002), and PRA (p<0.01). The mean increase in urinary ammonia excretion during the observation period was 1.08 mmol. Assuming a volume of distribution of 45 litres, the corresponding change in whole body ammonia during the same period was 1.67 mmol.

CONCLUSION

The results of this study suggest that volume expansion reduces plasma ammonia concentration by increasing ammonia excretion and reducing ammoniagenesis.

Effect of polyamines on glutamate dehydrogenase within permeabilized kidney-cortex mitochondria and isolated renal tubules of rabbit

The effect of polyamines on glutamate dehydrogenase [L-glutamate: NAD(P) oxidoreductase (deaminating) [EC 1.4.1.3]) activity has been studied in both permeabilized kidney-cortex mitochondria and isolated renal tubules of rabbit. Spermidine was the most potent inhibitor of glutamate synthesis in permeabilized mitochondria resulting in about 80% decrease of the enzyme activity at 5 mM concentration. Putrescine, alpha-monofluoromethylputrescine (MFMP) and (R,R)-delta-methyl-alpha-acetylenic-putrescine (MAP) were more efficient than spermine. The inhibitory action of polyamines was potentiated by an elevated NADH content in the reaction mixture. Increasing concentrations of either NH4Cl, KCl or NaCl in the incubation medium resulted in a decrease of polyamine-induced inhibition of the enzyme activity, indicating that monovalent cations can compete with polyamines for the binding site at glutamate dehydrogenase. The inhibitory action of spermidine on glutamate synthesis was abolished by 2 mM ADP or 10 mM L-leucine, allosteric activators of the enzyme, as well as on the addition of either oxalate or sulphate at 20 mM concentrations. Spermidine did not affect glutamate formation when NADH was substituted by NADPH, suggesting an importance of the NADH binding to the inhibitory site of the enzyme for a decrease of reductive amination of 2-oxoglutarate by polyamine. Although spermidine did not influence glutamate deamination in the presence of NAD+, it stimulated this process by about 70% when NAD+ was substituted by NADP+. In the presence of ADP the stimulatory effect of polyamine was not significant. The data indicate that in permeabilized rabbit kidney-cortex mitochondria the effect of polyamines on both glutamate formation and glutamate deamination via the reaction catalysed by glutamate dehydrogenase is dependent upon the coenzyme utilized by the enzyme. In the presence of NADH their inhibitory effect on the glutamate formation may be alleviated by allosteric activators of the enzyme, and concentrations of potassium, sodium, sulphate and oxalate. In isolated rabbit renal tubules incubated with 5 mM methionine sulfoximine and aminooxyacetate, in order to inhibit glutamine synthetase and aminotransferases, respectively, 5 mM spermidine decreased glutamate formation by about 30%, while putrescine and spermine did not significantly diminish the enzyme activity. In the presence of octanoate glutamate formation was reduced by about 30% by naturally occurring polyamines as well as MFMP and MAP, indicating that under these conditions NADH rather than NADPH is utilized as the coenzyme. In view of these data it is possible to suggest that polyamines may be of importance to control glutamate dehydrogenase activity under physiological conditions.

Influence of ammonia and pH on protein and amino acid metabolism in LLC-PK1 cells

Metabolic acidosis inhibits protein synthesis (PS) and stimulates protein degradation (PD) in muscle and cultured myocytes but causes hypertrophy of the proximal tubule. The reason for this tissue-specific difference in response to acidosis is unknown, but it might be related to stimulation of renal ammonia production since ammonia reportedly increases PS and inhibits PD in cultured kidney cells. We examined how ammonia and pH could interact to change protein turnover in confluent LLC-PK1 cells. Varying extracellular pH from 6.95 to 7.60 did not alter PS or PD even though intracellular pH changed predictably. Six millimolar NH4Cl did not change PS while 20 mM inhibited PS; there was no interaction with pH. This unexpected difference from the reported stimulation of PS by NH4Cl could be explained by our use of L-[U-14C]phenylalanine rather than radiolabelled leucine to measure PS. NH4Cl was found to inhibit leucine degradation which would increase radiolabelled leucine available for incorporation into protein. Either 6 mM or 20 mM NH4Cl inhibited PD measured as the release of L-[14C]phenylalanine from prelabelled protein. Experiments with an inhibitor of lysosomal function, chloroquine, suggest that NH4Cl inhibits lysosomal proteolysis. There was no interaction of cell pH and ammonia-induced changes in PD. Thus, the response of renal cells to acidification differs markedly from myocytes and ammonia changes protein turnover primarily by suppressing PD.

Might distal renal tubular acidosis be a proximal tubular cell disorder?

Incomplete renal tubular acidosis (RTA) and overt distal RTA may be different stages of the same underlying pathophysiology in certain individuals. The rationale that draws these conditions together is the relatively alkaline pH of the urine, hypocitraturia, and a possible familial association. The rate of excretion of ammonium (NH4+), on the other hand, suggests that these conditions stem from fundamentally different lesions. To explain this difference, we suggest that two possible disorders may result in the evolution from incomplete RTA to overt distal RTA. One subgroup could have gradient-limited distal RTA, while the other subgroup may have a lower pH of the intracellular fluid of the proximal convoluted tubular epithelium. Indices of proximal intracellular pH (rates of excretion of NH4+, NH3, and citrate) were culled from the literature spanning the years 1959 to 1991 on patients with incomplete RTA and overt distal RTA. Three points emerge: (1) the rate of excretion of NH4+ was lower in patients with overt distal RTA than in normals following an acute acid load (23 +/- 1 v 49 +/- 3 mumol/min); (2) the concentration of NH3 in the urine was almost 25-fold higher in incomplete RTA than in normals (69 +/- 14 v 3 +/- 0.4 nmol/min); and (3) in incomplete RTA, the pH of the urine fell to very low values (4.9 +/- 0.1) when high urine flows were induced with furosemide. The low pH of the urine would therefore suggest that many of these patients do not gradient-limited distal RTA, but more likely have proximal renal epithelial cell acidosis. We hypothesize that this high rate of excretion of NH4+ and low rate of excretion of citrate in the absence of acidosis or hypokalemia is consistent with proximal cell acidosis. To explain a transition from incomplete RTA to overt distal RTA, we speculate that toxicity of high concentrations of NH3 in the medullary interstitium as well as nephrolithiasis and nephrocalcinosis due to low urinary citrate and possibly an alkaline medullary interstitium may lead to damage of structures in this region.

Expression of rat renal Na/H antiporter mRNA levels in response to respiratory and metabolic acidosis

The mammalian proximal tubule is an important mediator of the renal adaptive response to systemic acidosis. In chronic metabolic and respiratory acidosis the bicarbonate reabsorptive (or proton secretory) capacity is increased. This increase is mediated, at least in part, by an increase in Vmax of the luminal Na/H antiporter. To determine whether this adaptation involves increased mRNA expression, Na/H antiporter mRNA levels were measured by Northern analysis in renal cortex of rats with metabolic (6 mmol/kg body wt NH4Cl for 2 or 5 d) and respiratory (10% CO2/air balanced for 2 or 5 d) acidosis and of normal, pair-fed rats. Na/H antiporter mRNA levels were unchanged after 2 d of both metabolic and respiratory acidosis. After 5 d, however, Na/H antiporter mRNA expression was increased 1.76 +/- 0.12-fold in response to metabolic acidosis (P less than 0.005, n = 8), but was not different from normal in response to respiratory acidosis: 1.1 +/- 0.2 (NS, n = 8). Thus, the renal adaptive response to metabolic acidosis involves increased cortical Na/H antiporter mRNA levels. In contrast, the enhanced proximal tubule Na/H antiporter activity and bicarbonate reabsorption in respiratory acidosis seem to involve mechanisms other than increased Na/H antiporter gene expression.

Induction of hypertrophy in cultured proximal tubule cells by extracellular NH4Cl

Ammonia production increases in several models of renal hypertrophy in vivo. The present study was designed to determine whether ammonia can directly modulate the growth of renal cells in the absence of a change in extracellular acidity. In serum-free media NH4Cl (0-20 mM) caused JTC cells and a primary culture of rabbit proximal tubule cells to hypertrophy (increase in cell protein content) in a dose-dependent fashion without a change in DNA synthesis. Studies in JTC cells revealed that the cell protein content increased as a result of both an increase in protein synthesis and a decrease in protein degradation. Total cell RNA content and ribosome number increased after NH4Cl exposure and the cell content of the lysosomal enzymes cathepsin B and L decreased. Inhibition of the Na+/H+ antiporter with amiloride did not prevent the hypertrophic response induced by NH4Cl. The results indicate that ammonia is an important modulator of renal cell growth and that hypertrophy can occur in the absence of functioning Na+/H+ antiport activity.

Ammonia transport in the proximal tubule in vivo

Studies were performed to characterize the determinants of proximal tubule ammonia entry (and retention) in vivo. Rat proximal tubules were studied in vivo using in situ microperfusion. In both normal animals and animals with metabolic acidosis, increasing luminal flow rate significantly enhanced luminal ammonia entry. In contrast, luminal pH was not as important in determining ammonia entry. Analysis of the levels of luminal NH3 in these studies was not consistent with simple diffusion equilibrium of NH3. In animals with chronic metabolic acidosis, additional studies demonstrated that inhibition of the Na+-H+ exchanger had no direct effect on luminal ammonia entry. However, studies of ammonia efflux from tubules perfused with 10 mmol/L ammonia demonstrated significant transport of both NH3 and NH4+. Studies of luminal glutamine deamidation via gamma-glutamyltransferase in control conditions did not indicate a significant role for luminal ammoniagenesis in the superficial proximal tubule in vivo. These and other recent studies of proximal tubule ammonia transport significantly modify the traditional diffusion equilibrium (of NH3) model of ammonia transport. Luminal flow rate is an important determinant of luminal ammonia entry. Transport of NH4+, both into and out of the tubule lumen, represents a major component of total ammonia transport.

Physiology and molecular biology of the renal Na/H antiporter

The renal Na/H antiporter is involved in cell pH regulation and, predominantly in the proximal tubule, also mediates transcellular NaCl and NaHCO3 transport. The transporter is regulated by systemic acid-base factors and hormones such as angiotensin II, PTH, glucocorticoids and alpha2-adrenergics. The human gene of the Na/H antiporter has been cloned and sequenced recently.

Effect of selective aldosterone deficiency on acidification in nephron segments of the rat inner medulla

Mineralocorticoid plays a role in urinary acidification and acid-base balance, but the response of the inner medulla to aldosterone has not been elucidated. A model of selective aldosterone deficiency (SAD) with hyperkalemia and hyperchloremic metabolic acidosis was employed to assess segmental acidification by measuring in situ pH, titratable acidity (TA) and total ammonia (Am). Hydrogen ion secretion was also examined as a function of the increment in in situ PCO2 in the collecting duct during bicarbonate loading. SAD rats were compared to ADX controls that received adrenalectomy and chronic replacement of gluco- and mineralocorticoid and to rats with chronic metabolic acidosis induced by oral NH4Cl (CMA). Both fractional and absolute delivery of Am to the loop of Henle was lower in SAD vs. CMA rats (1.34 to 3.63 mM, P less than 0.01). Delivery of Am to the base and tip collecting duct (BCD and TCD) was also markedly lower in SAD (1.50 vs. 0.52 and 1.77 vs. 0.47 mM, respectively, P less than 0.01). Net addition of Am and net acid between BCD and TCD, observed in CMA rats, was not observed in SAD despite equivalent degrees of systemic metabolic acidosis. Similarly, the concentration gradient favoring transfer of NH3 between loop of Henle and CD was reduced in SAD. During bicarbonate loading the increment in PCO2 at BCD, TCD and in final urine was significantly lower in SAD rats than in adrenal intact bicarbonate-loaded rats. Therefore, the acidification defect in this model of SAD appears to be a result of a decrease in ammonia production and delivery to the loop of Henle, impaired transfer from loop to collecting duct and reduction in the rate of H+ secretion by the collecting duct.

Chronic metabolic acidosis causes an adaptation in the apical membrane Na/H antiporter and basolateral membrane Na(HCO3)3 symporter in the rat proximal convoluted tubule

The effect of chronic dietary acid on the apical membrane Na/H antiporter and basolateral membrane Na(HCO3)3 symporter was examined in the in vivo microperfused rat proximal tubule. Transporter activity was assayed with the epifluorescent measurement of cell pH using the intracellular, pH-sensitive fluorescent dye, (2'7')-bis(carboxyethyl)-(5,6)-carboxy-fluorescein (BCECF). BCECF was calibrated intracellularly, demonstrating similar pH-sensitivity of the dye in control and acidotic animals. In subsequent studies, lumen and peritubular capillaries were perfused to examine Na/H and Na(HCO3)3 transporter activity in the absence of contact with native fluid. The initial rate of change in cell pH (dpHi/dt) was 97, 50, and 44% faster in tubules from acidotic animals when peritubular [HCO3] was changed from 25 to 10 mM in the presence or absence of chloride, or peritubular [Na] was changed from 147 to 50 mM, respectively. dpHi/dt was 57% faster in tubules from acidotic animals when luminal [Na] was changed from 152 to 0 mM. Buffer capacities, measured using NH3/NH+4 addition, were similar in the two groups. The results demonstrate that chronic metabolic acidosis causes an adaptation in the intrinsic properties of both the apical membrane Na/H antiporter and basolateral membrane Na(HCO3)3 symporter.