The effects of changes in acid-base balance on urinary citrate in the rat

A review of renal tubular acidosis

OBJECTIVE

To review the current scientific literature on renal tubular acidosis (RTA) in people and small animals, focusing on diseases in veterinary medicine that result in secondary RTA.

DATA SOURCES

Scientific reviews and original research publications on people and small animals focusing on RTA.

SUMMARY

RTA is characterized by defective renal acid-base regulation that results in normal anion gap hyperchloremic metabolic acidosis. Renal acid-base regulation includes the reabsorption and regeneration of bicarbonate in the renal proximal tubule and collecting ducts and the process of ammoniagenesis. RTA occurs as a primary genetic disorder or secondary to disease conditions. Based on pathophysiology, RTA is classified as distal or type 1 RTA, proximal or type 2 RTA, type 3 RTA or carbonic anhydrase II mutation, and type 4 or hyperkalemic RTA. Fanconi syndrome comprises proximal RTA with additional defects in proximal tubular function. Extensive research elucidating the genetic basis of RTA in people exists. RTA is a genetic disorder in the Basenji breed of dogs, where the mutation is known. Secondary RTA in human and veterinary medicine is the sequela of diseases that include immune-mediated, toxic, and infectious causes. Diagnosis and characterization of RTA include the measurement of urine pH and the evaluation of renal handling of substances that should affect acid or bicarbonate excretion.

CONCLUSIONS

Commonality exists between human and veterinary medicine among the types of RTA. Many genetic defects causing primary RTA are identified in people, but those in companion animals other than in the Basenji are unknown. Critically ill veterinary patients are often admitted to the ICU for diseases associated with secondary RTA, or they may develop RTA while hospitalized. Recognition and treatment of RTA may reverse tubular dysfunction and promote recovery by correcting metabolic acidosis.

Distal Renal Tubular Acidosis: ERKNet/ESPN Clinical Practice Points

Distal renal tubular acidosis (dRTA) is characterised by an impaired ability of the distal tubule to excrete acid, leading to metabolic acidosis. Associated complications include bone disease, growth failure, urolithiasis and hypokalaemia. Due to its rarity, there is a limited evidence to guide diagnosis and management, however, available data strongly suggest that metabolic control of the acidosis by alkali supplementation can halt or revert almost all complications. Despite this, cohort studies show that adequate metabolic control is present in only about half of patients, highlighting problems with treatment provision or adherence. With these clinical practice points the authors, part of the working groups tubulopathies in the European Rare Kidney Disease Reference network (ERKnet) and inherited kidney diseases of the European Society for Paediatric Nephrology (ESPN) aim to provide guidance for the management of patients with dRTA to facilitate adequate treatment and establish an initial best practice standard against which treatment of patients can be audited.

Regulation of renal NaDC1 expression and citrate excretion by NBCe1-A

Citrate is critical for acid-base homeostasis and to prevent calcium nephrolithiasis. Both metabolic acidosis and hypokalemia decrease citrate excretion and increase expression of Na+-dicarboxylate cotransporter 1 (NaDC1; SLC13A2), the primary protein involved in citrate reabsorption. However, the mechanisms transducing extracellular signals and mediating these responses are incompletely understood. The purpose of the present study was to determine the role of the Na+-coupled electrogenic bicarbonate cotransporter (NBCe1) A variant (NBCe1-A) in citrate metabolism under basal conditions and in response to acid loading and hypokalemia. NBCe1-A deletion increased citrate excretion and decreased NaDC1 expression in the proximal convoluted tubules (PCT) and proximal straight tubules (PST) in the medullary ray (PST-MR) but not in the PST in the outer medulla (PST-OM). Acid loading wild-type (WT) mice decreased citrate excretion. NaDC1 expression increased only in the PCT and PST-MR and not in the PST-MR. In NBCe1-A knockout (KO) mice, the acid loading change in citrate excretion was unaffected, changes in PCT NaDC1 expression were blocked, and there was an adaptive increase in PST-MR. Hypokalemia in WT mice decreased citrate excretion; NaDC1 expression increased only in the PCT and PST-MR. NBCe1-A KO blocked both the citrate and NaDC1 changes. We conclude that 1) adaptive changes in NaDC1 expression in response to metabolic acidosis and hypokalemia occur specifically in the PCT and PST-MR, i.e., in cortical proximal tubule segments; 2) NBCe1-A is necessary for normal basal, metabolic acidosis and hypokalemia-stimulated citrate metabolism and does so by regulating NaDC1 expression in cortical proximal tubule segments; and 3) adaptive increases in PST-OM NaDC1 expression occur in NBCe1-A KO mice in response to acid loading that do not occur in WT mice.

Effect of NBCe1 deletion on renal citrate and 2-oxoglutarate handling

The bicarbonate transporter, NBCe1 (SLC4A4), is necessary for at least two components of the proximal tubule contribution to acid-base homeostasis, filtered bicarbonate reabsorption, and ammonia metabolism. This study's purpose was to determine NBCe1's role in a third component of acid-base homeostasis, organic anion metabolism, by studying mice with NBCe1 deletion. Because NBCe1 deletion causes metabolic acidosis, we also examined acid-loaded wild-type adult mice to determine if the effects of NBCe1 deletion were specific to NBCe1 deletion or were a non-specific effect of the associated metabolic acidosis. Both NBCe1 KO and acid-loading decreased citrate excretion, but in contrast to metabolic acidosis alone, NBCe1 KO decreased expression of the apical citrate transporter, NaDC-1. Thus, NBCe1 expression is necessary for normal NaDC-1 expression, and NBCe1 deletion induces a novel citrate reabsorptive pathway. Second, NBCe1 KO increased 2-oxoglutarate excretion. This could not be attributed to the metabolic acidosis as experimental acidosis decreased excretion. Increased 2-oxoglutarate excretion could not be explained by changes in plasma 2-oxoglutarate levels, the glutaminase I or the glutaminase II generation pathways, 2-oxoglutarate metabolism, its putative apical 2-oxoglutarate transporter, OAT10, or its basolateral transporter, NaDC-3.

IN SUMMARY

(1) NBCe1 is necessary for normal proximal tubule NaDC-1 expression; (2) NBCe1 deletion results in stimulation of a novel citrate reabsorptive pathway; and (3) NBCe1 is necessary for normal 2-oxoglutarate metabolism through mechanisms independent of expression of known transport and metabolic pathways.

Hypocitraturia: pathophysiology and medical management

Low urinary citrate excretion is a known risk factor for the development of kidney stones. Citrate inhibits stone formation by complexing with calcium in the urine, inhibiting spontaneous nucleation, and preventing growth and agglomeration of crystals. Hypocitraturia is a common metabolic abnormality found in 20% to 60% of stone formers. It is most commonly idiopathic in origin but may be caused by distal renal tubular acidosis, hypokalemia, bowel dysfunction, and a high-protein, low-alkali diet. Genetic factors, medications, and other comorbid disorders also play a role. Hypocitraturia should be managed through a combination of dietary modifications, oral alkali, and possibly lemonade or other citrus juice-based therapy. This review concerns the pathophysiology of hypocitraturia and the management of stone formers afflicted with this abnormality.

Long-term lemonade based dietary manipulation in patients with hypocitraturic nephrolithiasis

PURPOSE

Citrus fruits and juices are a known natural source of dietary citrate. Of all the citrus juices, lemon juice appears to have the highest concentration of citrate. Therefore, lemonade therapy has been proposed as a potential treatment for patients with hypocitraturia. We retrospectively evaluated the impact of long-term lemonade therapy on urinary metabolic parameters and stone formation in patients with hypocitraturic nephrolithiasis.

MATERIALS AND METHODS

A total of 32 patients were identified as being on long-term lemonade therapy for hypocitraturic nephrolithiasis. The 11 patients on lemonade therapy who met the entrance criteria for evaluation were compared to an age and sex matched control group of patients treated with oral slow release potassium citrate. Pre-therapy and post-therapy urinary parameters were recorded for both groups. The effect of lemonade therapy on stone burden and stone formation rate was calculated. New stone formation was defined as passage, surgical removal or appearance of new stones, or an increase in the size of existing stones on radiographic imaging.

RESULTS

Four males and 7 females (mean age 52.7 years) were treated with lemonade therapy for a mean of 44.4 months. The control group consisted of 4 males and 7 females (mean age 54.5 years) treated with potassium citrate for a mean of 42.5 months. Of the 11 patients on lemonade 10 demonstrated increased urinary citrate levels (mean increase +383 mg per day, p <0.05). All potassium citrate therapy subjects demonstrated an increase in urinary citrate (mean increase +482 mg per day, p <0.0001). Mean pretreatment and posttreatment stone burden in the lemonade group was 37.2 and 30.4 mm(2), respectively (p >0.05). During lemonade therapy the stone formation rate decreased from 1.00 to 0.13 stones per patient per year (p >0.05).

CONCLUSIONS

Due to its significant citraturic effect, lemonade therapy appears to be a reasonable alternative for patients with hypocitraturia who cannot tolerate first line therapy. Future study in the form of a prospective, randomized trial is needed to validate these findings.

Comparative value of orange juice versus lemonade in reducing stone-forming risk

Foods that are high in citrate content generally are assumed to deliver alkali load when consumed irrespective of the accompanying cation. The object of this randomized, crossover study was to compare the effects of orange juice with those of lemonade on acid-base profile and urinary stone risks under controlled metabolic conditions. Thirteen volunteers (nine healthy subjects and four stone formers) sequentially received distilled water, orange juice, or lemonade while on constant metabolic diet. Twenty-four-hour urine samples were collected for acid-base parameters and stone risk analysis. Orange juice but not lemonade provided alkali as evidenced by higher net gastrointestinal alkali absorption and higher urinary pH and citrate compared with control. Urinary calcium was not significantly different, but urinary oxalate was higher during the orange juice phase. The calculated supersaturation of calcium oxalate was lower in the orange juice phase compared with control. Calculated undissociated uric acid was lower in the orange juice phase compared with both control and lemonade phases. The calculated supersaturation of brushite was significantly higher in the orange juice phase compared with both control and lemonade phases. Despite comparable citrate content, this study showed that orange juice has greater alkalinizing and citraturic effects than lemonade. Consumption of orange juice was associated with lower calculated calcium oxalate supersaturation and lower calculated undissociated uric acid. This short-term study suggests that orange juice consumption could result in biochemical modification of stone risk factors; however, additional studies are needed to evaluate its role in long-term prevention of recurrent nephrolithiasis.

Citrate therapy for polycystic kidney disease in rats

BACKGROUND

Few treatments are available to slow the progression to renal failure in autosomal dominant polycystic kidney disease (PKD). In an animal model of PKD, the male heterozygous Han:SPRD rat, intake of a solution of potassium citrate plus citric acid (KCitr) from age one to three months prevented a decline in glomerular filtration rate (GFR). The present study tested whether this beneficial effect is sustained and explored handling of citrate and ammonia in normal and cystic kidneys.

METHODS

Rats were provided with tap water or citrate solutions to drink, and clearance and survival studies were performed.

RESULTS

The GFRs of rats with PKD that consumed KCitr from one month of age were normal at six months of age, while those of their counterparts on water were about one third of normal. Long-term KCitr treatment extended the average life span of rats with PKD from 10 to 17 months. Compared with normal rats, water-drinking rats with PKD had higher plasma [citrate], renal cortical [citrate], and fractional excretion of citrate, and lower rates of renal citrate consumption, ammonia synthesis, and ammonia excretion. Cortical PNH3 was not elevated in cystic kidneys. Intake of Na3 citrate/citric acid solution or K3 citrate solution, but not ammonium citrate/citric acid solution, prevented a decline in GFR in three-month-old rats with PKD.

CONCLUSIONS

Rats with PKD show abnormal renal handling of citrate and ammonia. Citrate salts that have an alkalinizing effect preserve GFR and extend survival.

Converting enzyme inhibition causes hypocitraturia independent of acidosis or hypokalemia

BACKGROUND

Angiotensin II stimulates the proximal tubular Na/H antiporter and increases proximal tubular cell pH. Because intracellular pH may affect urinary citrate excretion and enzymes responsible for renal citrate metabolism, the present studies examined the effect of enalapril, an angiotensin converting enzyme inhibitor, on the activity of renal cortical ATP citrate lyase and urinary citrate excretion.

METHODS

Enalapril was given to rats (15 mg/kg/day) for seven days and to humans (10 mg twice daily) for 10 days. Blood and 24-hour urine samples were obtained in both groups. Renal cortical tissue from rats was analyzed for enzyme activity.

RESULTS

In rats, enalapril decreased urinary citrate excretion by 88%. The change in urinary citrate was not associated with a difference in plasma pH, bicarbonate nor potassium concentration. However, similar to metabolic acidosis and hypokalemia, enalapril caused a 42% increase in renal cortical ATP citrate lyase activity. When given to humans, enalapril significantly decreased urinary citrate excretion and urine citrate concentration by 12% and 16%, respectively, without affecting plasma pH or electrolytes.

CONCLUSIONS

Enalapril decreases urinary citrate in rats and humans. This is due, at least in part, to increases in cytosolic citrate metabolism through ATP citrate lyase in rats similar to that seen with chronic metabolic acidosis and hypokalemia. The effects of enalapril on urinary citrate and renal cortical ATP citrate lyase occur independently of acidosis or hypokalemia but may be due to intracellular acidosis that is common to all three conditions.

Relationship between supersaturation and crystal inhibition in hypercalciuric rats

Calcium oxalate (CaOx) and calcium phosphate (CaP) crystals do not precipitate in large amounts in normal urine despite considerable supersaturation (SS), partly because urine inhibits crystal nucleation, aggregation, and growth. In normal rats and rats bred for hypercalciuria (GHS), we varied SS by varying calcium intake to test the hypothesis that increased SS might deplete inhibitors and reduce inhibition of crystal formation. In normal rats when compared to a low calcium diet (0.02% Ca), a high calcium diet (1.2% Ca) raised the SS of CaOx from 0.8 to 8.2. The high calcium diet also raised the upper limit of metastability (ULM) of CaOx (the SS at which crystals form in urine) from 11.8 to 36. In GHS rats, diet change altered CaOx SS from 1.5 to 12, and ULM from 17 to 50 (all differences, P < 0.001). Because ULM rose with SS, the increased SS had little potential to increase CaOx stone risk. For CaP, however, SS rose from 0.6 to 2.4 and 1.1 to 8 in normal and GHS rats (P < 0.001 for both), respectively, whereas ULM for CaP did not increase significantly (8 vs. 7 and 7 vs. 11; P = NS, both changes). Therefore, CaP SS rose close to the ULM, posing a high stone risk. The stones formed by these rats are composed of CaP. Increasing CaOx SS by diet raises ULM for CaOx thereby offsetting the risk of CaOx stones in rats.

Biochemical markers of renal disease in primary Sjögren's syndrome

Primary Sjögren's syndrome (SS) is characterized by an inflammatory process in the salivary and lacrimal glands, but the kidneys may also be involved. Renal tubular functions were studied in 27 patients with SS, all females, age 37-78. Both SS-patients with and without known distal renal tubular acidosis (dRTA) were included, dRTA was found in 18/27 (67%), impaired urine concentrating ability in 13/27 (48%). Hypocitraturia was identified in 20/27 (74%) and reduced tubular reabsorption of phosphate (TRP%) in 18/27 (67%). Tubular proteinuria (alpha 1-mikroglobulin) was present in 11/24 (46%), and tubular enzymuria (NAG) in 7/24 (29%). Hypocitraturia and/or dRTA were found in all patients with any kind of abnormal renal tubular function test. All except one of the patients with dRTA not treated with sodium bicarbonate had hypocitraturia. We conclude that distal tubular dysfunction was common in our SS-patients, but a concommitant proximal dysfunction was also seen. Determination of urinary citrate represents a valuable test for detection of renal disease in SS.

Increased urinary saturation and kidney calcium content in genetic hypercalciuric rats

We have established a colony of genetic hypercalciuric (IH) rats as a model of idiopathic hypercalciuria in humans. To test the hypothesis that hypercalciuria can cause crystallization in kidneys through increased supersaturation, in the absence of confounding effects of diet and whatever complex inhibitor disorders underlay stone disease, we fed males and females of the 21st generation of IH rats 13 g per day of a low calcium (LCD, 0.02% Ca), followed by a normal calcium (NCD, 0.6% Ca) and then a high calcium (HCD, 1.2% Ca) diet, each for seven days. During the last 24 hours of each period complete urine collections were obtained and analyzed for all substances known to affect urinary calcium oxalate (CaOx) and brushite (CaHPO4) supersaturation. Relative supersaturation with respect to the solid phases of CaOx and CaHPO4 were then calculated. Compared to same gender controls (Ctl) urine calcium excretion was higher in the female IH rats on all diets and in the male IH rats on NCD and HCD. The female and male IH rats on NCD and HCD were supersaturated with respect to CaOx; however, the male and female Ctl were supersaturated with respect CaOx only on HCD. The female IH rats on NCD and HCD and the male IH rats on NCD were supersaturated with respect to CaHPO4; however, neither the male nor female Ctl rats were supersaturated with respect to CaHPO4 on any diet. On NCD and HCD urine supersaturation with respect to CaHPO4 by females exceeded that of males.(ABSTRACT TRUNCATED AT 250 WORDS)

Citraturic response to oral citric acid load

It is possible that some orally administered citrate may appear in urine by escaping oxidation in vivo. To determine whether this mechanism contributes to the citraturic response to potassium citrate, we measured serum and urinary citrate for 4 hours after a single oral load of citric acid (40 mEq.) in 6 normal subjects. Since citric acid does not alter acid-base balance, the effect of absorbed citrate could be isolated from that of alkali load. Serum citrate concentration increased significantly (p less than 0.05) 30 minutes after a single oral dose of citric acid and remained significantly elevated for 3 hours after citric acid load. Commensurate with this change, urinary citrate excretion peaked at 2 hours and gradually decreased during the next 2 hours after citric acid load. In contrast, serum and urinary citrate remained unaltered following the control load (no drug). Differences of the citratemic and citraturic effects between phases were significant (p less than 0.05) at 2 and 3 hours. Urinary pH, carbon dioxide pressure, bicarbonate, total carbon dioxide and ammonium did not change at any time after citric acid load, and did not differ between the 2 phases. No significant difference was noted in serum electrolytes, arterialized venous pH and carbon dioxide pressure at any time after citric acid load and between the 2 phases. Thus, the citraturic and citratemic effects of oral citric acid are largely accountable by provision of absorbed citrate, which has escaped in vivo degradation.

Potassium bicarbonate supplementation blocks the hypocalciuric response to hydrochlorothiazide in rats

Bicarbonate salts and thiazide diuretics each lower urinary calcium excretion. This study was undertaken to compare the effects of administering oral sodium or potassium bicarbonate supplements (4.5 mmol./day of each salt) each with or without hydrochlorothiazide (10 mg./kg. body weight/day) on urinary calcium excretion in rats. Urinary calcium decreased (p less than 0.01) by about 50 per cent after sodium bicarbonate supplementation and potassium bicarbonate supplementation. The hypocalciuric response to each bicarbonate salt was similar. However, although hydrochlorothiazide depressed urinary calcium in rats consuming sodium bicarbonate, rats receiving equimolar supplements of potassium bicarbonate did not lower urinary calcium when given hydrochlorothiazide, despite evidence of the expected thiazide-mediated diuresis. It is concluded that in the rat potassium bicarbonate loading blocks the ability of hydrochlorothiazide to lower urinary calcium excretion.

Sodium-dependent dicarboxylate transport in rat renal basolateral membrane vesicles

Dicarboxylate transport in basolateral membrane vesicles prepared from rat kidney cortex was studied using 3H-methylsuccinate as a substrate. A sodium gradient (out greater than in) simulated methylsuccinate uptake and led to a transient overshoot. Lithium inhibited methylsuccinate uptake in the presence of sodium. The dependence of methylsuccinate uptake on sodium concentration indicated the interaction of more than one sodium ion with the transporter. Half-maximal stimulation was observed at 24 mmol/l sodium. Sodium-driven methylsuccinate uptake was electrogenic carrying a net positive charge. The basolateral dicarboxylate transport system exhibited an optimum at pH 7.0-7.5. In contrast, the sodium-dependent dicarboxylate transport system of brush border membranes depended much less on pH and had no optimum in the tested range. Cis-inhibition studies showed a preference of the system for dicarboxylates in the trans-configuration (fumarate) over cis-dicarboxylates (maleate). Citrate was accepted but oxalate and L-glutamate were not. DIDS exhibited a small inhibition. Among the monocarboxylates, gluconate and pyruvate inhibited methylsuccinate uptake whereas probenecid and p-aminohippurate (1 mmol/l) were without effect. The data indicate the presence of a sodium-dependent transport system in the basolateral membrane which accepts tricarboxylic acid cycle intermediates. This system is most likely not identical to the transport system responsible for organic anion secretion.

Secretion and contraluminal uptake of dicarboxylic acids in the proximal convolution of rat kidney

The transport of dicarboxylic acids in the proximal convolution was investigated by measuring: a) the zero net flux transtubular concentration difference of DL-methyl-succinate, b) its 2-s influx from the interstitium into tubular cells, and c) its 3.5-s efflux from the tubular lumen. With the first method a luminal concentration exceeding the peritubular concentration was observed, thus indicating a net active transtubular secretion of this slowly metabolized substance. All transport steps, luminal and contraluminal , as well as the overall transport, were Na+-dependent and inhibited by lithium (apparent Ki approximately equal to 1.8 mmol/l). The overall transport of methylsuccinate , as well as the contraluminal influx into proximal tubular cells, could be inhibited by paraaminohippurate and H2-DIDS with an apparent Ki of approximately equal to 1.8 mmol/l, by taurocholate with an apparent Ki approximately equal to 3.1 mmol/l and by pyruvate with an apparent Ki approximately equal to 5 mmol/l, but not by sulfate, thiosulfate, L-lactate, oxalate and urate. As judged from the inhibition of contraluminal methylsuccinate influx by 48 dicarboxylic acids (aliphatic and aromatic), a specificity pattern was observed similar to that of inhibition of luminal efflux of 2-oxoglutarate [22]: a preference of dicarboxylates in the transconfiguration with a chain length of 4-5 carbons; little change in the inhibitory potency with CH3-, OH-, SH- and O=, but strong reduction with a NH3+ in the 2 position; only a small reduction of inhibitory potency with 2,3 disubstituted SH and OH analogs; preference of the dicarboxylic benzene in the 1,4 position and of the diacetyl benzene in the 1,2 position. The data indicate a Na+-dependent dicarboxylic transport system at the contraluminal cell side of the proximal tubule which is very similar to the luminal transport system for dicarboxylic acids.

Effect of diet on plasma acid-base composition in normal humans

Steady-state plasma and urine acid-base composition was assessed in 19 studies of 16 normal subjects who ingested constant amounts of one of three diets that resulted in different rates of endogenous noncarbonic acid production (EAP) within the normal range. Renal net acid excretion (NAE) was used to quantify EAP since the two variables are positively correlated in normal subjects. A significant positive correlation was observed between plasma [H+] and plasma PCO2, and between plasma [HCO3-] and plasma PCO2, among the subjects. Multiple correlation analysis revealed a significant interrelationship among plasma [H+], plasma PCO2, and NAE (r = 0.71, P less than 0.001), and among plasma [HCO3-], plasma PCO2, and NAE (r = 0.77, P less than 0.001). The partial correlation coefficients indicated a significant positive correlation between plasma [H+] and NAE, and a significant negative correlation between plasma [HCO3-] and NAE, when plasma PCO2 was held constant. These findings indicate that two factors influence the level at which plasma [H+] is maintained in normal subjects: (1) the steady-state rate of endogenous noncarbonic acid production, and (2) the setpoint at which plasma PCO2 is regulated by the respiratory system. Plasma [HCO3-] is also co-determined by these two factors. In disease states, therefore, both factors must be known before a disturbance in acid-base homeostasis can be excluded.

Contrasting effects of potassium citrate and sodium citrate therapies on urinary chemistries and crystallization of stone-forming salts

Effects of potassium citrate therapy (60 mEq/day) on urinary chemistries and crystallization were compared to those of sodium citrate treatment in five patients with uric acid lithiasis. Both alkali treatments significantly increased urinary pH (P less than 0.001), from 5.35 +/- 0.18 SD to 6.68 +/- 0.14 for potassium citrate and 6.73 +/- 0.20 for sodium citrate. During potassium citrate therapy, urinary calcium significantly declined from 154 +/- 47 mg/day to 99 +/- 23 mg/day (P less than 0.01) and urinary citrate rose from 398 +/- 119 mg/day to 856 +/- 103 mg/day (P less than 0.001). The urinary saturation (activity product ratio) of calcium oxalate decreased from 3.21-fold to 1.69-fold saturation (P less than 0.01), and the inhibitor activity against calcium oxalate precipitation (formation product ratio) significantly increased. However, sodium citrate therapy did not significantly decrease urinary calcium (to 139 +/- 24 mg/day), although it increased urinary citrate substantially (to 799 +/- 89 mg/day, P less than 0.01). Urinary environment became supersaturated with respect to brushite (calcium phosphate) and monosodium urate. The inhibitor activity against calcium oxalate precipitation was not significantly altered for the whole group; in two patients, it decreased by more than 30%. The results indicate that (1) both alkali therapies are equally effective in preventing uric acid stone formation because of their ability to increase urinary pH, and (2) potassium citrate may prevent the complication of calcium nephrolithiasis in patients with uric acid stones, whereas sodium citrate may not.

Mechanism of volume expansion on citrate, ammonia, and acid excretion in the rat

Potassium-depleted rats receiving sodium chloride or i.v. mannitol decrease their blood bicarbonate concentration isohydrically, reduce urinary citrate, and increase urinary ammonia excretion per milligram of urinary creatinine. To determine the mechanisms of these renal changes, we volume expanded normal rats with mannitol, normal saline, or saline bicarbonate solutions. Blood pH in each group remained constant, but blood bicarbonate fell significantly in the mannitol and saline-infused rats. In these two groups, urinary citrate per unit GFR decreased 27% and 25% (P less than 0.01) but urinary ammonia excretion per unit GFR increased 120% and 90% (P less than 0.01). By contrast, in saline bicarbonate rats, citrate excretion increased 27% and urinary ammonia excretion rose 29% (P greater than 0.2). Rats with metabolic acidosis given saline did not alter blood pH or bicarbonate. Citrate excretion per unit GFR increased 150% (P less than 0.02) but urinary ammonia excretion rose insignificantly by 11%. Calculated net tubular reabsorption of citrate increased in the mannitol and saline-infused rats. The rise in urinary pH in these two groups during expansion suggest that renal ammonia production was also increased. We conclude that volume expansion changes renal citrate and ammonia metabolism by isohydrically reducing blood bicarbonate concentration, thereby demonstrating another effect of bicarbonate, independent of pH, on renal metabolism.

Hypocitraturia in patients with gastrointestinal malabsorption

We measured serum and urinary citrate, oxalate, calcium, and magnesium in 22 normal subjects and in 16 patients with malabsorption. The patients had subnormal levels of serum citrate and magnesium during fasting, subnormal 24-hour levels of urinary citrate, magnesium, and calcium, and excessive levels of urinary oxalate. Daily citrate excretion averaged only 15 per cent of normal. The hypocitraturia in the patients resulted from a subnormal filtered load of citrate and abnormally high net tubular reabsorption of the anion. An oral citrate supplement raised both the serum concentration and the filtered load of citrate to normal fasting values, but net tubular reabsorption remained abnormally high and urinary excretion abnormally low. Intramuscular magnesium sulfate, which corrected the hypomagnesemia and hypomagnesuria, had no effect on serum citrate or its filtered load. Nevertheless the injection restored net tubular reabsorption of citrate to normal and partially improved the hypocitraturia. Full correction of the hypocitraturia was achieved by combined treatment with oral citrate and intramuscular magnesium sulfate. Hypocitraturia may contribute to the formation of oxalate stones in these patients, and therefore our treatment may help to prevent this complication.

pH and bicarbonate effects on mitochondrial anion accumulation. Proposed mechanism for changes in renal metabolite levels in acute acid-base disturbances

Mitochondria from rabbit and dog renal cortex were incubated with 1 mM (14)C-weak acid anions in media containing low (10 mM) or high (40 mM) concentrations of bicarbonate and the steady-state accumulation of labeled anion in the matrix was measured. In the absence of an energy source, no concentration of (14)C-anion in the mitochondrial matrix space was present, but the anion concentration was significantly higher at low- than at high-bicarbonate concentration. Addition of an energy source, usually ascorbate plus tetramethyl-p-phenylenediamine, led to increases in matrix space anion levels and to accentuation of the difference in anion uptake between low- and high-bicarbonate media, so that two to four times as much anion was present at low- than at high-bicarbonate concentrations. The anions affected included substrates for which inner membrane carriers are present in mitochondria, such as citrate, alpha-ketoglutarate, malate, and glutamate, as well as substances which diffuse passively across the inner membrane such as acetate and formate. When a nonbicarbonate medium buffered with Hepes was used, pH change did not alter anion uptake although anion concentrations exceeding those in the medium still developed when an energy source was present. The difference in mitochondrial anion accumulation between low- and high-bicarbonate levels diminished with decreasing temperature or with increasing anion concentration in the medium. Estimation of intramitochondrial pH with [(14)C]5,5-dimethyl-oxazolidine-2,4-dione showed that the pH gradient across the inner mitochondrial membrane was significantly greater with 10 than with 40 mM bicarbonate in the medium.A hypothesis is described that relates this effect of pH and bicarbonate on mitochondrial anion accumulation to the very rapid changes in substrate levels in renal cortex, which develop when acute metabolic acidosis or alkalosis is produced in the intact animal. It is suggested that an abrupt fall in systemic pH and bicarbonate is associated with a shift in substrate in renal cortex out of the cytoplasm and into mitochondria, where some of the added substrate is metabolized. Reduction in the size of the cytoplasmic pool of substrate occurs with relatively little accompanying change in the size of the mitochondrial pool, thus causing a net reduction in the total tissue pool. This mechanism accounts for the reduction in tissue levels of many mitochondrial substrates observed acutely in metabolic acidosis. In metabolic alkalosis, reversal of these effects leads to expansion of the cytoplasmic pool, thereby resulting in the rise in tissue levels of substrates which occurs in this condition.

Effects of acetazolamide on proximal tubule C1, Na, and HCO3 transport in normal and acidotic dogs during distal blockade

It has been suggested that the establishment of a tubular fluid to plasma chloride gradient in the late proximal tubule by the reabsorption of bicarbonate (and other anions) in the early proximal tubule is responsible for a significant part of sodium chloride and water reabsorption in the proximal tubule. In the present study the effects of acetazolamide on proximal tubule water and electrolyte excretion were examined in 6 normal dogs and 10 chronic ammonium chloride-loaded dogs during distal blockade produced by ethacrynic acid and chlorothiazide administration. During distal blockade control urine/plasma osmolality and urine/plasma sodium were close to unity in all experiments. Urine/plasma chloride and urine/plasma bicarbonate were 1.21+/-0.02 and 0.75+/-0.07 in normal and 1.24+/-0.01 and 0.04+/-0.01 in acidotic dogs, respectively. After the administration of acetazolamide (20 mg/kg i.v.), there was a significant increase in urine flow, absolute and fractional excretion of sodium, bicarbonate, and chloride in all animals. Associated with these effects, urine/plasma osmolality and urine/plasma sodium remained unchanged but urine/plasma chloride decreased significantly to 1.15+/-0.01 in normal and to 1.19+/-0.01 in acidotic dogs. In acidotic dogs there was a significant correlation between the increase in bicarbonate, sodium, or chloride excretion after acetazolamide and the plasma bicarbonate level (range 6.8-12.5 meq/liter). These data demonstrate a significant effect of acetazolamide on bicarbonate, sodium, and chloride reabsorption in the proximal tubule even in the face of severe acidosis. Moreover, the data suggest that the decrease in chloride reabsorption (and accompanying sodium) after acetazolamide is related to the decrease in bicarbonate reabsorption and the associated decrease in the transtubular chloride gradient.

Effect of bicarbonate feeding on immobilization osteoporosis in the rat

A time course study of immobilization osteoporosis was carried out in an experimental model in which the hind half of the rat was paralyzed by spinal section between the 3rd and 4th lumbar vertebrae. The earliest changes in the femure were a reduction in water content and in bone volume (3 days after operation). A significant reduction in both hydroxyproline and phosphate content was observed 8 days after immobilization. Calcium, magnesium, sodium, citrate, and carbonate were found significantly decreased at 15 days. Bone potassium remained unchanged. The rate of calcium accretion and the size of the exchangeable pools, when expressed per unit of bone mass, were not significantly affected by treatment. Sham-operated rats fed a bicarbonate-enriched diet, showed increased rates of calcium accretion and bone calcium content. The latter was not accompanied by increases in phosphate and carbonate contents. Operated animals fed the same diet showed an increased rate of bone resorption attributed to an assumed higher rate of parathyroid hormone secretion induced by bicarbonate feeding.

Renal metabolism of citrate

Our studies on renal handling of citrate have shown that: (1) citrate enters renal tubular cells from luminal fluid (reabsorption) and peritubular blood; (2) reabsorption becomes maximal, i.e., Tm-limited, at filtered loads 7 to 8 times the normal; (3) administration of malate stimulates net renal production of citrate, leading to release into urine (net secretion) and into peritubular blood; (4) acute metabolic alkalosis, induced while plasma citrate levels are above normal, depressess net citrate reabsorption, stimulates citrate release into peritubular blood and abolishes overall renal uptake of citrate; (5) essentially all citrate extracted by the kidney is converted to CO2 at endogenous circulating levels. This contribution is 15 per cent of the total renal CO2 production and is independent of chronic alterations in acid-base balance.

The effect of lithium and related metal ions on the urinary excretion of 2-oxoglutarate and citrate in the rat

1 Administration of lithium ions to rats, either acutely by intraperitoneal injection or chronically in food, causes increased excretion of 2-oxoglutarate and citrate.2 Chronic administration in food of rubidium and caesium causes decreased excretion of 2-oxoglutarate and citrate.3 The effects described are not due to changes in urine volume, nor pH, nor are they simply related to the excretion of the injected ion.4 Acute administration of lithium caused an increased level of 2-oxoglutarate in kidney and reduced the ratio of glutamate to 2-oxoglutarate.5 Renal gluconeogenesis in slices was only slightly affected by either acute administration of lithium to the animals or by its presence in the incubation medium of renal slices.

The effect of lithium salts on the urinary excretion of -oxoglutarate in man

1. Lithium ions in therapeutic doses cause an increase in the renal excretion of alpha-oxoglutarate and glutaric acid.2. The excretion is probably due to reduced renal tubular reabsorption.3. Neither citrate, lactate nor pyruvate excretion rises.

Regulation of glutamine metabolism in vitro by bicarbonate ion and pH

The effect of variations of medium pH and bicarbonate concentration on glutamine oxidation was studied in slices and mitochondria from dog renal cortex. Decreasing pH and bicarbonate concentration increased the rate of oxidation of glutamine-U-(14)C to (14)CO(2) in both slices and mitochondria, an effect comparable to the acute stimulation of glutamine utilization produced by metabolic acidosis. Decreases in the concentration of glutamate and alpha-ketoglutarate, which accompany metabolic acidosis in the intact animal, also occurred in tissue slices when pH and [HCO(3) (-)] were lowered; decrease in alpha-ketoglutarate but not in glutamate content occurred in mitochondria under these conditions. Study of independent variations of medium pH and [HCO(3) (-)] showed that simultaneous changes in both pH and [HCO(3) (-)] produced a greater effect on glutamine metabolism than did change in either of these parameters alone. The rate of glutamine oxidation was also compared in tissue preparations from pairs of litter-mate dogs with chronic metabolic acidosis and alkalosis. No significant difference in the rate of glutamine oxidation was present in mitochondria from the two sets of animals. Slices from animals with chronic metabolic acidosis consistently oxidized glutamine at a more rapid rate than slices from alkalotic dogs both at high and at low concentrations of bicarbonate in the medium. We believe this difference is a result of the same mechanism which leads to the delayed increase in ammonium excretion during induction of metabolic acidosis. The close parallel between the effects demonstrated here and the changes in ammonium production and glutamine utilization in the intact animal with metabolic acidosis suggest that the observed in vitro changes accurately represent the operation of the physiologic mechanism by which acid-base changes regulate ammonium excretion. The similarity between the changes in glutamine oxidation observed in this study and those described previously for citrate suggests that one control mechanism affects the metabolism of both citrate and glutamine. Thus, we believe that the increase in citrate clearance in metabolic alkalosis and the increase in glutamine utilization and ammonium production in metabolic acidosis reflect the operation of the same underlying biochemical mechanism. This mechanism permits changes in pH and [HCO(3) (-)] in the cellular environment to regulate the rate of mitochondrial uptake and oxidation of several physiologically important substrates.