Relationship of renal ammonia production and potassium homeostasis

Low potassium activation of proximal mTOR/AKT signaling is mediated by Kir4.2

The renal epithelium is sensitive to changes in blood potassium (K+). We identify the basolateral K+ channel, Kir4.2, as a mediator of the proximal tubule response to K+ deficiency. Mice lacking Kir4.2 have a compensated baseline phenotype whereby they increase their distal transport burden to maintain homeostasis. Upon dietary K+ depletion, knockout animals decompensate as evidenced by increased urinary K+ excretion and development of a proximal renal tubular acidosis. Potassium wasting is not proximal in origin but is caused by higher ENaC activity and depends upon increased distal sodium delivery. Three-dimensional imaging reveals Kir4.2 knockouts fail to undergo proximal tubule expansion, while the distal convoluted tubule response is exaggerated. AKT signaling mediates the dietary K+ response, which is blunted in Kir4.2 knockouts. Lastly, we demonstrate in isolated tubules that AKT phosphorylation in response to low K+ depends upon mTORC2 activation by secondary changes in Cl- transport. Data support a proximal role for cell Cl- which, as it does along the distal nephron, responds to K+ changes to activate kinase signaling.

Sodium Zirconium Cyclosilicate in CKD, Hyperkalemia, and Metabolic Acidosis: NEUTRALIZE Randomized Study

Key Points

Sodium zirconium cyclosilicate effectively lowers serum potassium and maintains normokalemia in patients with CKD with concomitant hyperkalemia and metabolic acidosis. Despite high screen failure and small sample size, a nominally significant increase in sHCO3– was seen for sodium zirconium cyclosilicate versus placebo. Further studies on the basis of an appropriate cohort size may help validate the trend observed in sHCO3– levels, supporting these clinically relevant findings.

Background

Metabolic acidosis and hyperkalemia are common in CKD. A potential dual effect of sodium zirconium cyclosilicate (SZC), a selective binder of potassium in the gastrointestinal tract, on serum potassium (sK+) and serum bicarbonate (sHCO3−) was evaluated in patients with hyperkalemia and metabolic acidosis associated with CKD.

Methods

In the NEUTRALIZE study (NCT04727528 ), non-dialysis patients with stage 3–5 CKD, hyperkalemia (sK+>5.0 to ≤5.9 mmol/L), and metabolic acidosis (sHCO3− 16–20 mmol/L) received open-label SZC 10 g three times daily for ≤48 hours. Patients achieving normokalemia (sK+ 3.5–5.0 mmol/L) were randomized 1:1 to SZC 10 g or placebo daily for 4 weeks. The primary end point was patients (%) maintaining normokalemia at the end of treatment (EOT) without rescue. Secondary end points included mean change in sHCO3− at EOT (day 29) and patients (%) with normokalemia with a ≥3-mmol/L increase in sHCO3− without rescue.

Results

Of 229 patients screened, 37 were randomized (SZC, n =17; placebo, n =20). High screen failure led to early study termination. At EOT, 88.2% (SZC) versus 20.0% (placebo) of patients maintained normokalemia (odds ratio, 56.2; P = 0.001). Low enrollment rendered secondary end point P values nominal. SZC treatment provided nominally significant increases in sHCO3– versus placebo from day 15 onward. Patients with normokalemia with a ≥3-mmol/L increase in sHCO3− without rescue were 35.3% (SZC) and 5.0% (placebo; P < 0.05). No new safety concerns were reported.

Conclusions

SZC effectively lowered sK+ and maintained normokalemia, with nominally significant increases in sHCO3– observed for SZC versus placebo.

Acute hyperkalemia in adults

Hyperkalemia is a common, life-threatening medical situation in chronic renal disease patients in the emergency department (ED). Since hyperkalemia does not present with any specific symptom, it is difficult to diagnose clinically. Hyperkalemia causes broad and dramatic medical presentations including cardiac arrhythmia and sudden death. Hyperkalemia is generally determined through serum measurement in the laboratory. Treatment includes precautions to stabilize cardiac membranes, shift potassium from the extracellular to the intracellular, and increase potassium excretion. The present article discusses the management of hyperkalemia in the ED in the light of current evidence.

Kir4.2 mediates proximal potassium effects on glutaminase activity and kidney injury

Inadequate potassium (K+) consumption correlates with increased mortality and poor cardiovascular outcomes. Potassium effects on blood pressure have been described previously; however, whether or not low K+ independently affects kidney disease progression remains unclear. Here, we demonstrate that dietary K+ deficiency causes direct kidney injury. Effects depend on reduced blood K+ and are kidney specific. In response to reduced K+, the channel Kir4.2 mediates altered proximal tubule (PT) basolateral K+ flux, causing intracellular acidosis and activation of the enzyme glutaminase and the ammoniagenesis pathway. Deletion of either Kir4.2 or glutaminase protects from low-K+ injury. Reduced K+ also mediates injury and fibrosis in a model of aldosteronism. These results demonstrate that the PT epithelium, like the distal nephron, is K+ sensitive, with reduced blood K+ causing direct PT injury. Kir4.2 and glutaminase are essential mediators of this injury process, and we identify their potential for future targeting in the treatment of chronic kidney disease.

Neglected analytes in the 24-h urine: ammonium and sulfate

PURPOSE OF REVIEW

Evaluation of the kidney stone patient includes measurement of 24 h urine chemistries. This review summarizes the application of physiologic principles to the interpretation of urine chemistries, using sulfate and ammonium to estimate diet acid load, and the renal response.

RECENT FINDINGS

There has been increased recognition of the need to measure urine ammonium excretion in the clinical setting in order to understand renal acid excretion. Some 24 h urine kidney stone panels include ammonium measurements, providing an opportunity to apply this measurement to clinical practice. In order to better interpret ammonium excretion, one needs an estimate of dietary acid load to understand the driving forces for ammonium excretion. Sulfate is also included in some kidney stone panels and functions as an estimate of diet acid load. Combining these analytes with urine pH, the clinician can quickly estimate dietary stone risk as well as potential bowel disease, acidification disorders, and the presence of urease producing bacteria; all of which can affect stone risk.

SUMMARY

Measurement of ammonium and sulfate excretion along with urine pH provide important insights into the acid/alkali content of diet, presence and severity of bowel disease, presence of renal acidification disorders, and urinary infection.

Effects of extreme potassium stress on blood pressure and renal tubular sodium transport

We characterized mouse blood pressure and ion transport in the setting of commonly used rodent diets that drive K⁺ intake to the extremes of deficiency and excess. Male 129S2/Sv mice were fed either K⁺-deficient, control, high-K⁺ basic, or high-KCl diets for 10 days. Mice maintained on a K⁺-deficient diet exhibited no change in blood pressure, whereas K⁺-loaded mice developed an ~10-mmHg blood pressure increase. Following challenge with NaCl, K⁺-deficient mice developed a salt-sensitive 8 mmHg increase in blood pressure, whereas blood pressure was unchanged in mice fed high-K⁺ diets. Notably, 10 days of K⁺ depletion induced diabetes insipidus and upregulation of phosphorylated NaCl cotransporter, proximal Na⁺ transporters, and pendrin, likely contributing to the K⁺-deficient NaCl sensitivity. While the anionic content with high-K⁺ diets had distinct effects on transporter expression along the nephron, both K⁺ basic and KCl diets had a similar increase in blood pressure. The blood pressure elevation on high-K⁺ diets correlated with increased Na⁺-K⁺-2Cl^- cotransporter and γ-epithelial Na⁺ channel expression and increased urinary response to furosemide and amiloride. We conclude that the dietary K⁺ maneuvers used here did not recapitulate the inverse effects of K⁺ on blood pressure observed in human epidemiological studies. This may be due to the extreme degree of K⁺ stress, the low-Na⁺-to-K⁺ ratio, the duration of treatment, and the development of other coinciding events, such as diabetes insipidus. These factors must be taken into consideration when studying the physiological effects of dietary K⁺ loading and depletion.

Differences in renal ammonia metabolism in male and female kidney

Renal ammonia metabolism has a major role in the maintenance of acid-base homeostasis. Sex differences are well recognized as an important biological variable in many aspects of renal function, including fluid and electrolyte metabolism. However, sex differences in renal ammonia metabolism have not been previously reported. Therefore, the purpose of the current study was to investigate sex differences in renal ammonia metabolism. We studied 4-mo-old wild-type C57BL/6 mice fed a normal diet. Despite similar levels of food intake, and, thus, protein intake, which is the primary determinant of endogenous acid production, female mice excreted greater amounts of ammonia, but not titratable acids, than did male mice. This difference in ammonia metabolism was associated with fundamental structural differences between the female and male kidney. In the female mouse kidney, proximal tubules account for a lower percentage of the renal cortical parenchyma compared with the male kidney, whereas collecting ducts account for a greater percentage of the renal parenchyma than in male kidneys. To further investigate the mechanism(s) behind the greater ammonia excretion in female mice, we examined differences in the expression of proteins involved in renal ammonia metabolism and transport. Greater basal ammonia excretion in females was associated with greater expression of PEPCK, glutamine synthetase, NKCC2, Rhbg, and Rhcg than was observed in male mice. We conclude that there are sex differences in basal ammonia metabolism that involve both renal structural differences and differences in expression of proteins involved in ammonia metabolism.

Metabolic Acidosis and Subclinical Metabolic Acidosis in CKD

Metabolic acidosis is not uncommon in CKD and is linked with bone demineralization, muscle catabolism, and higher risks of CKD progression and mortality. Clinical practice guidelines recommend maintaining serum total CO₂ at ≥22 mEq/L to help prevent these complications. Although a definitive trial testing whether correcting metabolic acidosis improves clinical outcomes has not been conducted, results from small, single-center studies support this notion. Furthermore, biologic plausibility supports the notion that a subset of patients with CKD have acid-mediated organ injury despite having a normal serum total CO₂ and might benefit from oral alkali before overt acidosis develops. Identifying these individuals with subclinical metabolic acidosis is challenging, but recent results suggest that urinary acid excretion measurements may be helpful. The dose of alkali to provide in this setting is unknown as well. The review discusses these topics and the prevalence and risk factors of metabolic acidosis, mechanisms of acid-mediated organ injury, results from interventional studies, and potential harms of alkali therapy in CKD.

Roles of renal ammonia metabolism other than in acid-base homeostasis

The importance of renal ammonia metabolism in acid-base homeostasis is well known. However, the effects of renal ammonia metabolism other than in acid-base homeostasis are not as widely recognized. First, ammonia differs from almost all other solutes in the urine in that it does not result from arterial delivery. Instead, ammonia is produced by the kidney, and only a portion of the ammonia produced is excreted in the urine, with the remainder returned to the systemic circulation through the renal veins. In normal individuals, systemic ammonia addition is metabolized efficiently by the liver, but in patients with either acute or chronic liver disease, conditions that increase the addition of ammonia of renal origin to the systemic circulation can result in precipitation and/or worsening of hyperammonemia. Second, ammonia appears to serve as an intrarenal paracrine signaling molecule. Hypokalemia increases proximal tubule ammonia production and secretion as well as reabsorption in the thick ascending limb of the loop of Henle, thereby increasing delivery to the renal interstitium and the collecting duct. In the collecting duct, ammonia decreases potassium secretion and stimulates potassium reabsorption, thereby decreasing urinary potassium excretion and enabling feedback correction of the initiating hypokalemia. Finally, the stimulation of renal ammonia metabolism by hypokalemia may contribute to the development of metabolic alkalosis, which in turn can stimulate NaCl reabsorption and contribute to the intravascular volume expansion, increased blood pressure and diuretic resistance that can develop with hypokalemia. The evidence supporting these novel non-acid-base roles of renal ammonia metabolism is discussed in this review.

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.

Mathematical Model of Ammonia Handling in the Rat Renal Medulla

The kidney is one of the main organs that produces ammonia and release it into the circulation. Under normal conditions, between 30 and 50% of the ammonia produced in the kidney is excreted in the urine, the rest being absorbed into the systemic circulation via the renal vein. In acidosis and in some pathological conditions, the proportion of urinary excretion can increase to 70% of the ammonia produced in the kidney. Mechanisms regulating the balance between urinary excretion and renal vein release are not fully understood. We developed a mathematical model that reflects current thinking about renal ammonia handling in order to investigate the role of each tubular segment and identify some of the components which might control this balance. The model treats the movements of water, sodium chloride, urea, NH₃ and NH4+, and non-reabsorbable solute in an idealized renal medulla of the rat at steady state. A parameter study was performed to identify the transport parameters and microenvironmental conditions that most affect the rate of urinary ammonia excretion. Our results suggest that urinary ammonia excretion is mainly determined by those parameters that affect ammonia recycling in the loops of Henle. In particular, our results suggest a critical role for interstitial pH in the outer medulla and for luminal pH along the inner medullary collecting ducts.

Refining the ammonia hypothesis: a physiology-driven approach to the treatment of hepatic encephalopathy

Hepatic encephalopathy (HE) is one of the most important complications of cirrhosis and portal hypertension. Although the etiology is incompletely understood, it has been linked to ammonia directly and indirectly. Our goal is to review for the clinician the mechanisms behind hyperammonemia and the pathogenesis of HE to explain the rationale for its therapy. We reviewed articles collected through a search of MEDLINE/PubMed, Cochrane Database of Systematic Reviews, and Google Scholar between October 1, 1948, and December 8, 2014, and by a manual search of citations within retrieved articles. Search terms included hepatic encephalopathy, ammonia hypothesis, brain and ammonia, liver failure and ammonia, acute-on-chronic liver failure and ammonia, cirrhosis and ammonia, portosytemic shunt, ammonia and lactulose, rifaximin, zinc, and nutrition. Ammonia homeostatsis is a multiorgan process involving the liver, brain, kidneys, and muscle as well as the gastrointestinal tract. Indeed, hyperammonemia may be the first clue to poor functional reserves, malnutrition, and impending multiorgan dysfunction. Furthermore, the neuropathology of ammonia is critically linked to states of systemic inflammation and endotoxemia. Given the complex interplay among ammonia, inflammation, and other factors, ammonia levels have questionable utility in the staging of HE. The use of nonabsorbable disaccharides, antibiotics, and probiotics reduces gut ammoniagenesis and, in the case of antibiotics and probiotics, systemic inflammation. Nutritional support preserves urea cycle function and prevents wasting of skeletal muscle, a significant site of ammonia metabolism. Correction of hypokalemia, hypovolemia, and acidosis further assists in the reduction of ammonia production in the kidney. Finally, early and aggressive treatment of infection, avoidance of sedatives, and modification of portosystemic shunts are also helpful in reducing the neurocognitive effects of hyperammonemia. Refining the ammonia hypothesis to account for these other factors instructs a solid foundation for the effective treatment and prevention of hepatic encephalopathy.

Intercalated cell-specific Rh B glycoprotein deletion diminishes renal ammonia excretion response to hypokalemia

The ammonia transporter family member, Rh B Glycoprotein (Rhbg), is an ammonia-specific transporter heavily expressed in the kidney and is necessary for the normal increase in ammonia excretion in response to metabolic acidosis. Hypokalemia is a common clinical condition in which there is increased renal ammonia excretion despite the absence of metabolic acidosis. The purpose of this study was to examine Rhbg's role in this response through the use of mice with intercalated cell-specific Rhbg deletion (IC-Rhbg-KO). Hypokalemia induced by feeding a K(+)-free diet increased urinary ammonia excretion significantly. In mice with intact Rhbg expression, hypokalemia increased Rhbg protein expression in intercalated cells in the cortical collecting duct (CCD) and in the outer medullary collecting duct (OMCD). Deletion of Rhbg from intercalated cells inhibited hypokalemia-induced changes in urinary total ammonia excretion significantly and completely prevented hypokalemia-induced increases in urinary ammonia concentration, but did not alter urinary pH. We conclude that hypokalemia increases Rhbg expression in intercalated cells in the cortex and outer medulla and that intercalated cell Rhbg expression is necessary for the normal increase in renal ammonia excretion in response to hypokalemia.

Renal ammonia excretion in response to hypokalemia: effect of collecting duct-specific Rh C glycoprotein deletion

The Rhesus factor protein, Rh C glycoprotein (Rhcg), is an ammonia transporter whose expression in the collecting duct is necessary for normal ammonia excretion both in basal conditions and in response to metabolic acidosis. Hypokalemia is a common clinical condition associated with increased renal ammonia excretion. In contrast to basal conditions and metabolic acidosis, increased ammonia excretion during hypokalemia can lead to an acid-base disorder, metabolic alkalosis, rather than maintenance of acid-base homeostasis. The purpose of the current studies was to determine Rhcg's role in hypokalemia-stimulated renal ammonia excretion through the use of mice with collecting duct-specific Rhcg deletion (CD-Rhcg-KO). In mice with intact Rhcg expression, a K(+)-free diet increased urinary ammonia excretion and urine alkalinization and concurrently increased Rhcg expression in the collecting duct in the outer medulla. Immunohistochemistry and immunogold electron microscopy showed hypokalemia increased both apical and basolateral Rhcg expression. In CD-Rhcg-KO, a K(+)-free diet increased urinary ammonia excretion and caused urine alkalinization, and the magnitude of these changes did not differ from mice with intact Rhcg expression. In mice on a K(+)-free diet, CD-Rhcg-KO increased phosphate-dependent glutaminase (PDG) expression in the outer medulla. We conclude that hypokalemia increases collecting duct Rhcg expression, that this likely contributes to the hypokalemia-stimulated increase in urinary ammonia excretion, and that adaptive increases in PDG expression can compensate for the absence of collecting duct Rhcg.

Hyperammonemia in review: pathophysiology, diagnosis, and treatment

Ammonia is an important source of nitrogen and is required for amino acid synthesis. It is also necessary for normal acid-base balance. When present in high concentrations, ammonia is toxic. Endogenous ammonia intoxication can occur when there is impaired capacity of the body to excrete nitrogenous waste, as seen with congenital enzymatic deficiencies. A variety of environmental causes and medications may also lead to ammonia toxicity. Hyperammonemia refers to a clinical condition associated with elevated ammonia levels manifested by a variety of symptoms and signs, including significant central nervous system (CNS) abnormalities. Appropriate and timely management requires a solid understanding of the fundamental pathophysiology, differential diagnosis, and treatment approaches available. The following review discusses the etiology, pathogenesis, differential diagnosis, and treatment of hyperammonemia.

Effect of hypokalemia on renal expression of the ammonia transporter family members, Rh B Glycoprotein and Rh C Glycoprotein, in the rat kidney

Hypokalemia is a common electrolyte disorder that increases renal ammonia metabolism and can cause the development of an acid-base disorder, metabolic alkalosis. The ammonia transporter family members, Rh B glycoprotein (Rhbg) and Rh C glycoprotein (Rhcg), are expressed in the distal nephron and collecting duct and mediate critical roles in acid-base homeostasis by facilitating ammonia secretion. In the current studies, the effect of hypokalemia on renal Rhbg and Rhcg expression was examined. Normal Sprague-Dawley rats received either K(+)-free or control diets for 2 wk. Rats receiving the K(+)-deficient diet developed hypokalemia and metabolic alkalosis associated with significant increases in both urinary ammonia excretion and urine pH. Rhcg expression increased in the outer medullary collecting duct (OMCD). In OMCD intercalated cells, hypokalemia resulted in more discrete apical Rhcg expression and a marked increase in apical plasma membrane immunolabel. In principal cells, in the OMCD, hypokalemia increased both apical and basolateral Rhcg immunolabel intensity. Cortical Rhcg expression was not detectably altered by immunohistochemistry, although there was a slight decrease in total expression by immunoblot analysis. Rhbg protein expression was decreased slightly in the cortex and not detectably altered in the outer medulla. We conclude that in rat OMCD, hypokalemia increases Rhcg expression, causes more polarized apical expression in intercalated cells, and increases both apical and basolateral expression in the principal cell. Increased plasma membrane Rhcg expression in response to hypokalemia in the rat, particularly in the OMCD, likely contributes to the increased ammonia excretion and thereby to the development of metabolic alkalosis.

Effects of potassium on expression of renal sodium transporters in salt-sensitive hypertensive rats induced by uninephrectomy

Dietary potassium is an important modulator of systemic blood pressure (BP). The purpose of this study was to determine whether dietary potassium is associated with an altered abundance of major renal sodium transporters that may contribute to the modulation of systemic BP. A unilateral nephrectomy (uNx) was performed in male Sprague-Dawley rats, and the rats were fed a normal-salt diet (0.3% NaCl) for 4 wk. Thereafter, the rats were fed a high-salt (HS) diet (3% NaCl) for the entire experimental period. The potassium-repleted (HS+KCl) group was given a mixed solution of 1% KCl as a substitute for drinking water. We examined the changes in the abundance of major renal sodium transporters and the expression of mRNA of With-No-Lysine (WNK) kinases sequentially at 1 and 3 wk. The systolic BP of the HS+KCl group was decreased compared with the HS group (140.3 ± 2.97 vs. 150.9 ± 4.04 mmHg at 1 wk; 180.3 ± 1.76 vs. 207.7 ± 6.21 mmHg at 3 wk). The protein abundances of type 3 Na(+)/H(+) exchanger (NHE3) and Na(+)-Cl(-) cotransporter (NCC) in the HS+KCl group were significantly decreased (53 and 45% of the HS group at 1 wk, respectively; 19 and 8% of HS group at 3 wk). WNK4 mRNA expression was significantly increased in the HS+KCl group (1.4-fold of control at 1 wk and 1.9-fold of control at 3 wk). The downregulation of NHE3 and NCC may contribute to the BP-attenuating effect of dietary potassium associated with increased urinary sodium excretion.

The importance of carbonic anhydrase II in red blood cells during exposure of chicken embryos to CO2

The importance of carbonic anhydrase (CA) during exposure of chicken embryos to CO(2) during the second half of incubation was investigated. The protein abundance and activity of CAII in erythrocytes was significantly higher in CO(2)-exposed embryos compared to normal conditions. Daily injections of acetazolamide (ATZ), an inhibitor of CA, increased blood P(CO2) and decreased blood pH in both control and CO(2)-incubated embryos. ATZ increased blood bicarbonate concentration in embryos exposed to normal atmosphere and in day-12 embryos exposed to high CO(2). The tendency of an increased blood potassium concentration in ATZ-injected embryos under standard atmospheric conditions might indicate that protons were exchanged with intracellular potassium. However, there was no evidence for such an exchange in CO(2)-incubated ATZ-treated embryos. This study shows for the first time that chicken embryos adapt to CO(2) during the second half of incubation by increasing CAII protein expression and function in red blood cells. This response may serve to "buffer" elevated CO(2) levels.

[Two cases of infants with acute renal failure due to bilateral obstructive ureteral stones associated with rotavirus gastroenteritis]

We report on 2 infants with acute renal failure caused by bilateral obstructive ureteral stones associated with rotavirus gastroenteritis. A 28-month boy and a 13-month boy with several days history of watery diarrhea and vomiting were referred to our hospital because of anuria. They were diagnosed acute post-renal failure due to obstructive bilateral ureteral stones based on the findings of ultrasound scan and computed tomography. Immediately, percutaneous nephrostomy tubes were inserted for urinary drainage, serum levels of creatinine and uric acid returned to normal within several days. Sandy stones were excreted through the nephrostomy tubes with urine after urinary alkalization, which were proved to be mainly ammonium acid urate. Ammonium acid urate is rare in developed countries, but some cases of bilateral urolithiasis causing acute renal failure in infants with rotavirus gastroenteritis were reported in recent years. It has been known that the cause of acute renal failure is renal azotemia resulting from sustained hypovolemia, but post-renal causes due to ammonium acid urate stones should be taken into consideration.

Potassium restriction, high protein intake, and metabolic acidosis increase expression of the glutamine transporter SNAT3 (Slc38a3) in mouse kidney

Kidneys produce ammonium to buffer and excrete acids through metabolism of glutamine. Expression of the glutamine transporter Slc38a3 (SNAT3) increases in kidney during metabolic acidosis (MA), suggesting a role during ammoniagenesis. Potassium depletion and high dietary protein intake are known to elevate renal ammonium excretion. In this study, we examined SNAT3, phosphate-dependent glutaminase (PDG), and phosphoenolpyruvate carboxykinase (PEPCK) regulation during a control (0.36%) or low-K(+) (0.02%) diet for 7 or 14 days or a control (20%) or high-protein (50%) diet for 7 days. MA was induced in control and low-K(+) groups by addition of NH(4)Cl. Urinary ammonium excretion increased during MA, after 14-day K(+) restriction alone, and during high protein intake. SNAT3, PDG, and PEPCK mRNA abundance were elevated during MA and after 14-day K(+) restriction but not during high protein intake. SNAT3 protein abundance was enhanced during MA (both control and low K(+)), after 14-day low-K(+) treatment alone, and during high protein intake. Seven-day dietary K(+) depletion alone had no effect. Immunohistochemistry showed SNAT3 staining in earlier parts of the proximal tubule during 14-day K(+) restriction with and without NH(4)Cl treatment and during high protein intake. In summary, SNAT3, PDG, and PEPCK mRNA expression were congruent with urinary ammonium excretion during MA. Chronic dietary K(+) restriction, high protein intake, and MA enhance ammoniagenesis, an effect that may involve enhanced SNAT3 mRNA and protein expression. Our data suggest that SNAT3 plays an important role as the glutamine uptake mechanism in ammoniagenesis under these conditions.

Mechanisms in hyperkalemic renal tubular acidosis

The form of renal tubular acidosis associated with hyperkalemia is usually attributable to real or apparent hypoaldosteronism. It is therefore a common feature in diabetes and a number of other conditions associated with underproduction of renin or aldosterone. In addition, the close relationship between potassium levels and ammonia production dictates that hyperkalemia per se can lead to acidosis. Here I describe the modern relationship between molecular function of the distal portion of the nephron, pathways of ammoniagenesis, and hyperkalemia.

Mechanisms of NH4+and NH3transport during hypokalemia

Along the collecting duct, secretion of ammonium (NH) is thought to occur through active H+ secretion in parallel with the non-ionic diffusion of ammonia (NH3). Thus NH3 is secreted into the collecting duct lumen down its concentration gradient. Moreover, the low NH permeability and high NH3 permeability observed in collecting duct epithelia minimizes back diffusion of NH. In general, an increase in the NH3 concentration gradient between the interstitium and the collecting duct lumen correlates with increased NH secretion. However, our laboratory and others have shown an important role of direct NH transport by the Na,K-ATPase. As K+ and NH compete for a common extracellular binding site on the Na,K-ATPase, reduced interstitial K+ concentration, such as during hypokalemia, augments NH uptake. Na,K-ATPase-mediated NH uptake provides an important source of H+ for net acid secretion during hypokalemia and contributes to the increase in NH excretion and metabolic alkalosis observed in this treatment model.

The role of growth factors and ammonia in the genesis of hypokalemic nephropathy

Hypokalemia is a common electrolyte abnormality encountered in clinical practice. It can be identified in an asymptomatic patient undergoing routine electrolyte screening or can manifest itself as part of a number of functional abnormalities in a variety of organs and systems. Among the most commonly recognized complications are profound effects on the cardiovascular and neuromuscular systems, together with abnormalities in acid-base regulation. In humans, hypokalemia contributes to the development of hypertension and predisposes patients to a variety of ventricular arrhythmias, including ventricular fibrillation. Commonly recognized neuromuscular complications include weakness, cramping, and myalgia. Hypokalemia also affects systemic acid-base homeostasis by interfering with multiple components of the renal acid-base regulation and is a frequent cause of metabolic alkalosis. Less known, however, is the role of potassium deficiency in causing progressive renal failure. In animals, potassium deficiency stimulates renal enlargement because of cellular hypertrophy and hyperplasia. If potassium deficiency persists, interstitial infiltrates appear in the renal interstitial compartment, and eventually tubulointerstitial fibrosis develops. In humans, longstanding hypokalemia is associated with the development of renal cysts, chronic interstitial nephritis, and progressive loss of renal function, the so-called hypokalemic nephropathy. This review focuses on the potential mechanisms involved in the development of the hypokalemic nephropathy with emphasis on the role of ammonia and growth factors in its pathogenesis.

Effects of ammonia on acid-base transport by the B-type intercalated cell

Ammonia, in addition to its role as a constituent of urinary net acid excretion, stimulates cortical collecting duct (CCD) net bicarbonate reabsorption. The current study sought to begin determining the cellular transport processes through which ammonia regulates bicarbonate reabsorption by testing whether ammonia stimulates B-type intercalated cell bicarbonate secretion, bicarbonate reabsorption, or both. The effects of ammonia on single CCD intercalated cells was studied by use of measurements of intracellular pH taken from in vitro microperfused CCD segments after luminal loading of the pH-sensitive fluorescent dye BCECF. These results showed, first, that ammonia inhibited B-cell unidirectional bicarbonate secretion and that this occurred despite no effect of ammonia on apical Cl(-)/HCO(3)(-) exchange activity. Second, ammonia increased the contribution of a SCH28080-sensitive apical H(+)-K(+)-ATPase to basal intracellular pH regulation and it stimulated basolateral Cl(-)/HCO(3)(-) exchange activity. Thus, ammonia activated both apical proton secretion and basolateral base exit, consistent with stimulation of unidirectional bicarbonate reabsorption. It was concluded that ammonia regulates CCD net bicarbonate reabsorption, at least in part, through the coordinated regulation of the separate processes of B-cell bicarbonate reabsorption and bicarbonate secretion. These effects do not reflect a general activation of ion transport but, instead, reflect coordinated and specific regulation of ion transport.

Impact of K(+) homeostasis on net acid secretion in rat terminal inner medullary collecting duct: role of the Na,K-ATPase

For the past 50 years, the mechanism of ammonium (NH(4)(+)) transport along the collecting duct has been thought to occur through active H(+) section in parallel with the nonionic diffusion of ammonia (NH(3)). This model is supported by two basic experimental observations. First, NH(4)(+) secretion generally correlates with the NH(3) concentration gradient between the interstitium and the collecting duct lumen. This NH(3) gradient is generated through both luminal acidification, which reduces luminal NH(3) concentration, and through countercurrent multiplication, which increases interstitial NH(3) concentration. The result is secretion of NH(3) into the collecting duct lumen down its concentration gradient. Second, because NH(4)(+) permeability is low relative to that of NH(3), there is significant secretion of NH(3) into the collecting duct lumen with minimal back-diffusion of NH(4)(+). However, our laboratory, as well as others, has shown that this model is an oversimplification of the mechanism of NH(4)(+) transport along the collecting duct. NH(4)(+) is transported through a variety of K(+) transport pathways including Na,K-ATPase. K(+) and NH(4)(+) compete for a common extracellular binding site on Na, K-ATPase. During hypokalemia, interstitial K(+) concentration is reduced, which augments NH(4)(+) uptake by the Na(+) pump. In K(+) restriction, Na,K-ATPase-mediated NH(4)(+) uptake provides an important source of H(+) for net acid secretion and for the titration of luminal buffers in the terminal inner medullary collecting duct. This pathway contributes to the increase in NH(4)(+) excretion and metabolic alkalosis observed during hypokalemia.

Potassium deprivation upregulates expression of renal basolateral Na(+)-HCO(3)(-) cotransporter (NBC-1)

The purpose of the present experiments was to examine the effect of potassium deprivation on the expression of the renal basolateral Na(+)-HCO(3)(-) cotransporter (NBC-1). Rats were placed on a K(+)-free diet for various time intervals and examined. NBC-1 mRNA levels increased by about threefold in the cortex (P < 0.04) at 72 h of K(+) deprivation and remained elevated at 21 days. NBC activity increased by approximately 110% in proximal tubule suspensions, with the activity increasing from 0.091 in control to 0.205 pH/min in the K(+)-deprived group (P < 0.005). The inner stripe of outer medulla and cells of medullary thick ascending limb of Henle (mTAL) showed induction of NBC-1 mRNA and activity in K(+)-deprived rats, with the activity in mTAL increasing from 0.010 in control to 0.133 pH/min in the K(+)-deprived group (P < 0.004). K(+) deprivation also increased NBC-1 mRNA levels in the renal papilla (P < 0.02). We conclude that 1) K(+) deprivation increases NBC-1 expression and activity in proximal tubule and 2) K(+) deprivation causes induction of NBC-1 expression and activity in mTAL tubule and inner medulla. We propose that NBC-1 likely mediates enhanced HCO(3)(-) reabsorption in proximal tubule, mTAL, and inner medullary collecting duct in K(+) deprivation and contributes to the maintenance of metabolic alkalosis in this condition.

In rat tIMCD, NH4+ uptake by Na+-K+-ATPase is critical to net acid secretion during chronic hypokalemia

The purpose of this study was to determine the magnitude of Na+ pump-mediated NH4+ uptake in the terminal inner medullary collecting duct (tIMCD) at K+ and NH4+ concentrations observed in vivo in the inner medullary interstitium of normal and in K+-restricted rats. Interstitial K+ and NH4+ concentrations in the terminal half of the inner medulla were taken to be 10 and 6 mM in K+-restricted rats, but 30 and 6 mM in K+-replete rats. In tubules from K+-restricted rats, when perfused at a K+ concentration of 10 mM, addition of ouabain to the bath reduced total bicarbonate flux (JtCO2) by 40% and increased intracellular pH (pHi), indicating significant NH4+ uptake by the Na+-K+-ATPase. In tubules from K+-restricted rats, JtCO2 was reduced with increased extracellular K+. At a K+ concentration of 30 mM, ouabain addition neither reduced JtCO2 nor increased pHi in tubules from rats of either treatment group. In conclusion, in the tIMCD from hypokalemic rats, 1) acute changes in extracellular K+ concentration modulate net acid secretion, and 2) Na+ pump-mediated NH4+ uptake should be an important pathway mediating transepithelial net acid secretion in vivo.

NH4+ secretion in inner medullary collecting duct in potassium deprivation: role of colonic H+-K+-ATPase

NH4+ secretion in inner medullary collecting duct in potassium deprivation: Role of colonic H+-K+-ATPase.

BACKGROUND

In K+ deprivation (KD), gastric (g) H+-K+-ATPase (HKA) is suppressed, whereas colonic (c) HKA is induced in the terminal inner medullary collecting duct (IMCD). We hypothesized that in KD, cHKA is induced and can mediate the secretion of NH4+.

METHODS

Rats were sacrificed after 2, 3, 6, or 14 days on regular (NML) or K+-free (KD) diet. mRNA expression of HKA isoforms in terminal inner medulla was examined and correlated with NH4+ secretion in perfused IMCD in vitro.

RESULTS

Urinary NH4+ excretion increased after K+-free diet for six days. In terminal inner medulla, cHKA expression was strongly induced, whereas gHKA expression was decreased. NH4+ secretion increased by 62% in KD (JtNH4+ 0.57 vs. 0.92 pmol/min/mm tubule length, P < 0.001). Ouabain (1 mM) in perfusate inhibited NH4+ secretion in KD by 45% (P < 0.002) but not in NML. At luminal pH 7.7, which inhibits NH3 diffusion, NH4+ secretion in IMCD was 140% higher in KD (0.36 vs. 0.15, P < 0.03) and was sensitive to ouabain. ROMK-1 mRNA expression was induced in parallel with cHKA in inner medulla.

CONCLUSIONS

These data suggest that in KD, cHKA replaces gHKA and mediates enhanced secretion of NH4+ (and H+) into the lumen facilitated by K+ recycling through ROMK-1.

H+-K+-ATPases: regulation and role in pathophysiological states

Molecular cloning experiments have identified the existence of two H+-K+-ATPases (HKAs), colonic and gastric. Recent functional and molecular studies indicate the presence of both transporters in the kidney, which are presumed to mediate the exchange of intracellular H+ for extracellular K+. On the basis of these studies, a picture is evolving that indicates differential regulation of HKAs at the molecular level in acid-base and electrolyte disorders. Of the two transporters, gastric HKA is expressed constitutively along the length of the collecting duct and is responsible for H+ secretion and K+ reabsorption under normal conditions and may be stimulated with acid-base perturbations and/or K+ depletion. This regulation may be species specific. To date there are no data to indicate that the colonic HKA (HKAc) plays a role in H+ secretion or K+ reabsorption under normal conditions. However, HKAc shows adaptive regulation in pathophysiological conditions such as K+ depletion, NaCl deficiency, and proximal renal tubular acidosis, suggesting an important role for this exchanger in potassium, HCO-3, and sodium (or chloride) reabsorption in disease states. The purpose of this review is to summarize recent functional and molecular studies on the regulation of HKAs in physiological and pathophysiological states. Possible signals responsible for regulation of HKAs in these conditions will be discussed. Furthermore, the role of these transporters in acid-base and electrolyte homeostasis will be evaluated in the context of genetically altered animals deficient in HKAc.

Dietary K+ restriction upregulates total and Sch-28080-sensitive bicarbonate absorption in rat tIMCD

In tubules from the terminal segment of the inner medullary collecting duct (tIMCD) from rats with chronic metabolic acidosis, our laboratory has shown that bicarbonate absorption (JtCO2) is inhibited by removal of K+ from the luminal fluid or by the addition of Sch-28080 to the perfusate. The present study asked whether total and/or Sch-28080-sensitive JtCO2 is regulated by changes in systemic K+ homeostasis. Rat tIMCD tubules were perfused in vitro in symmetrical, HCO-3/CO2-buffered solutions containing 10 mM KCl + 6 mM NH4Cl. Total and Sch-28080-sensitive JtCO2 were measured in rats with varying K+ intake. In K+-replete rats, baseline JtCO2 was 2.1 +/- 0.3 pmol . mm-1 . min-1 (n = 6). In rats fed a K+-deficient diet for 3 days, JtCO2 was 5.4 +/- 0.7 pmol . mm-1 . min-1 (n = 16, P < 0. 05). To determine the mechanism for the increase in HCO-3 absorption observed with K+ restriction, the Sch-28080-sensitive component of JtCO2 was measured in each treatment group. Following the addition of Sch-28080 (10 microM) to the perfusate, a 40% reduction in JtCO2 was observed in K+-restricted rats. JtCO2 was not reduced following the addition of Sch-28080 in rats with normal K+ intake. Because Sch-28080-sensitive JtCO2 was increased in K+-restricted rats, Sch-28080-sensitive JtCO2 was studied further in tIMCD tubules from rats in this treatment group. In K+-restricted rats, JtCO2 decreased by 20% following the addition of 5 mM ouabain to the perfusate. This ouabain-induced decline in JtCO2 was observed both in the presence and in the absence of Sch-28080. We conclude that total and Sch-28080-sensitive net acid secretion is increased with dietary K+ restriction. However, since approximately 50% of JtCO2 is insensitive to both Sch-28080 and ouabain, future studies will be necessary to define other mechanisms of luminal acidification in the rat tIMCD.

Renal cortical mitochondrial aconitase is regulated in hypo- and hypercitraturia

BACKGROUND

Chronic metabolic acidosis and K+ deficiency increase, while alkali feeding decreases proximal tubule citrate absorption and metabolism. The present studies examined the regulation of mitochondrial aconitase (m-aconitase), the first step in mitochondrial citrate metabolism, in these conditions.

METHODS

Rats were fed appropriate diets, and m-aconitase activity and protein abundance measured.

RESULTS

In chronic metabolic acidosis and chronic K+ deficiency, renal cortical m-aconitase activity was increased 17% and 43%, respectively. This was associated with respective 90% and 221% increases in renal cortical m-aconitase protein abundance. With chronic alkali feeding, there was a 12% decrease in renal cortical m-aconitase activity, associated with a 35% decrease in m-aconitase protein abundance. Hepatic m-aconitase activity was not regulated in a similar manner. There was no regulation of citrate synthase, the enzyme responsible for mitochondrial citrate synthesis.

CONCLUSIONS

These studies demonstrate tissue specific chronic regulation of renal cortical m-aconitase activity and protein abundance, which likely contributes to the hypocitraturia and hypercitraturia seen in these conditions. As m-aconitase is the only step in citrate transport and metabolism found to be regulated in alkali feeding, its regulation likely plays a significant role in mediating the hypercitraturia seen in this condition.

Potassium depletion downregulates chloride-absorbing transporters in rat kidney

Potassium depletion (KD) causes renal chloride wasting, suggesting defect(s) in Cl- reabsorption in renal tubules. To determine whether alterations in expression of the major Cl- transporter genes might contribute to the chloride wasting, we analyzed their expression in renal cortex and medulla of animals placed on KD diet. Feeding KD diet to rats resulted in significant hypokalemia at 14 d but not at 6 d. Northern hybridization revealed that mRNA levels for the apical Na-K-2Cl cotransporter in the medulla decreased by 56 and 51% at 6 and 14 d of KD diet, respectively. Functional studies in tubular suspensions from medullary thick ascending limb demonstrated that the Na-K-2Cl cotransporter activity decreased by approximately 45 and approximately 37% at 6 and 14 d of KD diet, respectively. mRNA levels for the thiazide-sensitive Na-Cl cotransporter decreased by 57 and 64% at 6 and 14 d of KD diet. Decreased expression of the apical Na-Cl and the Na-K-2Cl cotransporters became evident at 48 and 72 h of KD, respectively. Urinary chloride excretion increased at 48 h and further increased at 72 h of KD, correlating with suppression of the Na-Cl and the Na-K-2Cl transporters. Our results indicate that increased urinary chloride loss in KD results from suppression of the chloride-absorbing transporters. Downregulation of chloride transporters in KD is an early event and can lead to hypochloremia and subsequently hypovolemia and decreased glomerular filtration rate.

Intracellular acidosis activates c-Src

The purpose of the present studies was to determine whether acidosis activates protein tyrosine kinase pathways. Incubation of MCT cells, a renal proximal tubule cell line, in acid media caused increased phosphotyrosine content of 60- to 70- and 120-kDa cytosolic proteins. Media acidification induced a twofold increase in c-Src activity that occurred within 30 s. Significant activation occurred with media pH changes as small as 0.07 pH unit accompanied by cell acidification of 0.06 pH unit. Sodium propionate addition, NH4Cl prepulse, and nigericin addition, maneuvers that decrease intracellular pH in the absence of changes in extracellular pH, activated c-Src. Significant activation by sodium propionate was seen with cell pH changes as small as 0.07 pH unit. Sodium orthovanadate, a protein tyrosine phosphatase inhibitor, prevented c-Src activation by media acidification but did not prevent protein tyrosine phosphorylation. In summary, decreased intracellular pH activates c-Src. Acid activation of c-Src represents a novel mechanism of c-Src activation that may be relevant to many cellular responses to acidosis.

Chronic hyperkalemia impairs ammonium transport and accumulation in the inner medulla of the rat

Previously we demonstrated in rats that chronic hyperkalemia had no effect on ammonium secretion by the proximal tubule in vivo but that high K+ concentrations inhibited ammonium absorption by the medullary thick ascending limb in vitro. These observations suggested that chronic hyperkalemia may reduce urinary ammonium excretion through effects on medullary transport events. To examine directly the effects of chronic hyperkalemia on medullary ammonium accumulation and collecting duct ammonium secretion, micropuncture experiments were performed in the inner medulla of Munich-Wistar rats pair fed a control or high-K+ diet for 7-13 d. In situ pH and total ammonia concentrations were measured to calculate NH3 concentrations for base and tip collecting duct and vasa recta. Chronic K+ loading was associated with significant systemic metabolic acidosis and a 40% decrease in urinary ammonium excretion. In control rats, 15% of excreted ammonium was secreted between base and tip collecting duct sites. In contrast, no net transport of ammonium was detected along the collecting duct in high-K+ rats. The decrease in collecting duct ammonium secretion in hyperkalemia was associated with a decrease in the NH3 concentration difference between vasa recta and collecting duct. The fall in the NH3 concentration difference across the collecting duct in high-K+ rats was due entirely to a decrease in [NH3] in the medullary interstitial fluid, with no change in [NH3] in the collecting duct. These results indicate that impaired accumulation of ammonium in the medullary interstitium, secondary to inhibition of ammonium absorption in the medullary thick ascending limb, may play an important role in reducing collecting duct ammonium secretion and urinary ammonium excretion during chronic hyperkalemia.

Chronic neutral phosphate supplementation induces sustained, renal metabolic alkalosis

The aim of the present study was to test whether intravenous neutral phosphate supplementation, recently shown in our laboratory to acutely stimulate proton secretion in the distal nephron, was able to induce a sustained metabolic alkalosis. Neutral Na and K phosphate supplementation for seven days, with equivalent reduction in chloride supply and unchanged intake of sodium and potassium, in ADX rats receiving fixed physiological doses of aldosterone and dexamethasone (group 1, N = 7), was responsible for a severe metabolic alkalosis (MA; delta [HCO3] 11 +/- 1.3 mM, and delta pH 0.11 +/- 0.06 unit). Metabolic alkalosis was at least in part of renal origin, since net acid excretion (NAE) transiently increased, principally due to an increment in titratable acid excretion rate. Balances were equilibrated for sodium and negative for chloride and potassium, which may have contributed to the severity of the MA. Chronic i.v. neutral Na phosphate, without change in potassium and chloride supply, in ADX rats receiving the same doses of steroids (group 2, N = 5), was responsible for a less severe MA (delta [HCO3] 7.5 +/- 0.9 mM, and delta pH 0.07 +/- 0.01 unit), also of renal origin. In this group, balances were positive for chloride and sodium and equilibrated for potassium. Finally, neutral Na and K phosphate supplementation with reduction in chloride supply in intact rats (group 3, N = 4) was also able to induce a MA (delta [HCO3] 5.5 +/- 1.8 mM, and delta pH 0.06 +/- 0.01 unit) of renal origin, with balances negative for chloride and equilibrated for potassium and sodium. In all groups, the generation and maintenance of MA probably resulted from stimulated proton secretion in the distal nephron, as suggested by the observed increase of PCO2 over HCO3 concentration ratio in the urine and a fall in urine pH despite augmented urinary buffer content throughout the phosphate infusion period. Glomerular filtration rate did not significantly vary in any group. In conclusion, chronic supplementation of neutral phosphate appears to stimulate per se proton secretion in the distal nephron, independently of sodium, chloride, and potassium balances, and adrenal steroid secretion. Thus neutral phosphate supplementation should be added to the previously known factors able to induce MA.

Renal ammoniagenesis in humans with chronic potassium depletion

Renal ammonia production and distribution and ammonia precursor utilization were evaluated in eight patients with chronic potassium depletion (CPD) and aldosterone-producing adenoma and in 20 controls. In CPD, urinary ammonia excretion and ammonia added to renal venous blood were about twofold higher than in controls; thus, total ammonia production was significantly augmented (88.0 +/- 10.3 mumol/min.1.73 m2 vs. 45.0 +/- 2.6 in controls). Total ammonia production was inversely correlated with serum potassium and directly correlated with urine flow. Stepwise multiple regression analysis showed that both factors, mainly serum potassium, significantly influence ammonia production and account for 61.4% of variations in ammonia production. Renal extraction of glutamine was significantly increased (56.6 +/- 5.9 mumol/min.1.73 m2 vs. 34.6 +/- 3.1 in controls), and this could account for ammonia production. The ratio of urinary ammonia excretion to total ammonia production, an index of the intrarenal ammonia distribution, was similar in patients and controls, and was significantly correlated with urine pH and true renal blood flow (RBF). Stepwise multiple regression analysis showed that RBF, urine pH and urine flow also significantly affected ammonia distribution. However, these factors accounted for only 41.7% of variations in intrarenal ammonia partition, urine pH having a minor role. We conclude that in patients with CPD other factors besides urine pH, urine flow and RBF intervene in the ammonia partition between urine and blood.

Distal renal tubular acidosis: the value of urinary pH, PCO2 and NH4+ measurements

Distal renal tubular acidosis (dRTA) is not a single disease. The experimental forms of the syndrome are unsatisfactory as models of the naturally occurring disease, not least because they are seldom complicated by nephrocalcinosis, which is present in the majority of patients with spontaneous disease and contributes to the renal tubular defects found in the syndrome. Impairment of minimal urine pH, reduced urine carbon dioxide tension (PCO2) during passage of alkaline urine, and reduced urinary ammonium (NH4+) excretion, have all been advocated as essential criteria for the diagnosis of dRTA. Minimal urine pH, measured during metabolic acidosis, sulphate infusion, or after oral frusemide, is the yardstick against which other criteria should be assessed. A reduced urinary PCO2 is commonly found in dRTA but is not specific for the syndrome and may be accounted for by tubular defects other than those involving reduced distal hydrogen ion secretion. NH4+ excretion is reduced in most patients with renal acidosis whatever the nature of the underlying renal disease; this function is closely related to nephron mass, and is not specifically impaired in renal tubular disease.

Effect of bath and luminal potassium concentration on ammonia production and secretion by mouse proximal tubules perfused in vitro

To determine the effects of acute changes in K+ concentration in vitro on ammonia production and secretion by the proximal tubule, we studied mouse S2 segments perfused with and bathed in Krebs-Ringer bicarbonate buffers containing various K+ concentrations. All bath solutions contained L-glutamine as the ammoniagenic substrate. High bath and luminal K+ concentrations (8 mM), but not high luminal K+ concentration alone, inhibited total ammonia production rates by 26%, while low bath and luminal K+ concentrations (2 mM), but not low luminal K+ concentration alone, stimulated total ammonia production rates by 33%. The stimulation of ammonia production by low bath K+ concentration was not observed when L-glutamine was added to the luminal perfusion solution. On the other hand, high luminal K+ concentration stimulated, while low luminal K+ concentration inhibited, net luminal secretion of total ammonia in a way that was: (a) independent of total ammonia production rates, (b) independent of Na(+)-H+ exchange activity, and (c) not due to changes in transepithelial fluxes of total ammonia. These results suggest that luminal potassium concentration has a direct effect on cell-to-lumen transport of ammonia.

Selective effect of mineralocorticoid replacement therapy on renal acid excretion in congenital adrenal hyperplasia

The renal acid excretion of eight children with salt-losing congenital adrenal hyperplasia, was studied in three different situations: before treatment (period I), under glucocorticoid therapy (period II) and when both glucocorticoid and mineralocorticoid were given as replacement treatment (period III). Although administration of glucocorticoid therapy alone allowed the correction of acidemia, normalization of urinary net acid excretion was achieved only after mineralocorticoid was added to the treatment.

Ammonium and bicarbonate homeostasis in chronic liver disease

Whereas traditionally in acid-base physiology one considers just two organs (lungs and kidneys) to be involved in the regulation of systemic acid-base homeostasis, recent developments indicate that also the liver must be viewed as an important organ for pH regulation. This is because urea synthesis is a quantitatively important bicarbonate-consuming process, which in turn underlies a feedback control by the acid-base status at least in vitro. Consequently, renal ammoniagenesis, generally accepted to be a direct bicarbonate-generating process, can be viewed as a pH-controlled ammonium homeostatic response. In view of the controversies regarding the roles of ureogenesis and renal ammoniagenesis in acid-base regulation, their relationships were studied in 28 patients with normal renal functions, but varying degrees of a well-compensated chronic liver disease. Progressive loss of urea cycle capacity (as determined by in vitro incubations of human liver tissue) was parallelled by increasing in vivo plasma bicarbonate levels (and metabolic alkalosis) and an increasing NH4+ excretion into the urine. Accordingly, renal ammoniagenesis rose with the extent of metabolic alkalosis. Neither hypokalemia, hyperaldosteronism, diuretic treatment, or volume contraction were present, and a satisfactory explanation for this unusual behavior of renal ammoniagenesis in terms of traditional acid-base physiology cannot be given. Here, it seems that renal ammoniagenesis is governed rather by the need to eliminate ammonia than by the acid-base status.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of sodium bicarbonate on nitrogen metabolism and ketone bodies during very low energy protein diets in obese subjects

This study evaluated the effects of oral sodium bicarbonate (NaHCO3) supplementation on ammonium (NH4+) nitrogen (N) and urea N excretion and on ketone bodies during the metabolic acidosis of a very low energy protein diet. Ten healthy obese female subjects (BMI, 38.4 +/- 1.5 kg/m2;weight, 100 +/- 4 kg) were given a 1.72 MJ (412 kcal) all protein (16.8 g N) liquid formula, 16 mmol KCl and a multivitamin-mineral supplement daily for 4 weeks. In addition, the five subjects in group 1 received 60 mmol Na+ daily as sodium chloride (NaCl) for 3 weeks and as NaHCO3 during week 4. The subjects in group 2 were given 40 mmol/d NaHCO3 during the first week, 60 mmol/d during weeks 2 and 3, and 60 mmol/d NaCl during week 4. Nitrogen balance was achieved in both groups by the end of week 3. The subjects in group 1 at week 2 showed an increase in blood [H+] of 0.41 +/- 0.06 x 10(-8) mol/L and a decrease in blood bicarbonate from 26.0 +/- 0.8 to 23.8 +/- 1.2 mmol/L. The subsequent NaHCO3 curtailed NH4+ N excretion by one half, without significant change in ketone body levels or excretion. Administration of NaHCO3 from the start of the diet to the subjects in group 2 prevented both the metabolic acidosis and the increase in NH4+ N excretion and attenuated the increase in blood and urine 3-hydroxybutyrate. When NaCl replaced NaHCO3 during week 4, ammonium N excretion doubled. Urea N excretion was comparable in both groups and was unaffected by bicarbonate.(ABSTRACT TRUNCATED AT 250 WORDS)