URINARY AMMONIUM EXCRETION IN PRIMARY GOUT

Impact of Hyper- and Hypo-Uricemia on Kidney Function

Uric acid (UA) forms monosodium urate (MSU) crystals to exert proinflammatory actions, thus causing gout arthritis, urolithiasis, kidney disease, and cardiovascular disease. UA is also one of the most potent antioxidants that suppresses oxidative stress. Hyper andhypouricemia are caused by genetic mutations or polymorphism. Hyperuricemia increases urinary UA concentration and is frequently associated with urolithiasis, which is augmented by low urinary pH. Renal hypouricemia (RHU) is associated with renal stones by increased level of urinary UA, which correlates with the impaired tubular reabsorption of UA. Hyperuricemia causes gout nephropathy, characterized by renal interstitium and tubular damage because MSU precipitates in the tubules. RHU is also frequently associated with tubular damage with elevated urinary beta2-microglobulin due to increased urinary UA concentration, which is related to impaired tubular UA reabsorption through URAT1. Hyperuricemia could induce renal arteriopathy and reduce renal blood flow, while increasing urinary albumin excretion, which is correlated with plasma xanthine oxidoreductase (XOR) activity. RHU is associated with exercise-induced kidney injury, since low levels of SUA could induce the vasoconstriction of the kidney and the enhanced urinary UA excretion could form intratubular precipitation. A U-shaped association of SUA with organ damage is observed in patients with kidney diseases related to impaired endothelial function. Under hyperuricemia, intracellular UA, MSU crystals, and XOR could reduce NO and activate several proinflammatory signals, impairing endothelial functions. Under hypouricemia, the genetic and pharmacological depletion of UA could impair the NO-dependent and independent endothelial functions, suggesting that RHU and secondary hypouricemia might be a risk factor for the loss of kidney functions. In order to protect kidney functions in hyperuricemic patients, the use of urate lowering agents could be recommended to target SUA below 6 mg/dL. In order to protect the kidney functions in RHU patients, hydration and urinary alkalization may be recommended, and in some cases an XOR inhibitor might be recommended in order to reduce oxidative stress.

A hypothesis of bone joint defects

The work presented proposes origins of the extensive range of observations concerning changes in bone joints associated with rheumatoid arthritis, osteoarthritis and arthritis urica. These changes are shown to originate with alterations of cell reactions involving four basic cell biochemicals. The proposals allow explanations of, for example, the link between rheumatoid arthritis and anaemia. It is also shown that performic acid is a presently unrecognised cause of degradation of tissue in bone joints. Suggestions are made as to means of control of these changes.

Medical therapy for nephrolithiasis: State of the art

The prevalence of nephrolithiasis is increasing worldwide. Understanding and implementing medical therapies for kidney stone prevention are critical to prevent recurrences and decrease the economic burden of this condition. Dietary and pharmacologic therapies require understanding on the part of the patient and the prescribing practitioner in order to promote compliance. Insights into occupational exposures and antibiotic use may help uncover individual risk factors. Follow-up is essential to assess response to treatment and to modify treatment plans to maximize therapeutic benefit.

Uric acid nephrolithiasis: An update

Uric acid nephrolithiasis appears to increase in prevalence. While a relationship between uric acid stones and low urinary pH has been for long known, additional association with various metabolic conditions and pathophysiological basis has recently been elucidated. Some conditions such as diabetes and metabolic syndrome disease, excessive dietary intake, and increased endogenous uric acid production and/or defect in ammoniagenesis are associated with low urinary pH. In addition, the phenomenon of global warming could result in an increase in areas with greater climate risk for uric acid stone formation. There are three therapeutic steps to be taken for management of uric acid stones: identification of urinary pH profiles, assessment of urinary volume status, and identification of disorders leading to excessive uric acid production. However, the most important factor for uric acid stone formation is acid urinary pH, which is a prerequisite for uric acid precipitation. This article reviews recent insights into the pathophysiology of uric acid stones and their management.

Proteomic study of renal uric acid stone

OBJECTIVE

To analyze urinary uric acid stone matrix proteins (SMP) with mass spectrometry (MS) to evaluate the mechanisms of uric acid stone formation. SMP plays an important role in urinary stone formation. Several proteomic studies apply to calcium-containing stones have been reported; however no proteomic study for urinary uric acid stone has been reported.

METHODS

Pure kidney uric acid stones from 5 individuals were demineralized, and SMPs were isolated. The obtained proteins were analyzed with reverse-phase liquid chromatography-tandem MS. The acquired data were searched against a Swiss Prot human protein database using Matrix Science, Mascot. The identified proteins were submitted to the AmiGO Web site for gene ontology analysis. They were also sumitted to Metacore software and Kyoto Encyclopedia of Genes and Genomes website (KEGG) for pathway analysis. MS-determined protein expressions were verified by immunoblot.

RESULTS

MS analysis identified 242 proteins from 5 proteomic results and the number of the identified protein of each result ranged from 52 to 156. Metacore software analysis suggested that inflammation may play an important role for kidney uric acid stone formation. Endogenous metabolic pathways were also analyzed and submitted to KEGG Web site, which revealed that these proteins may participate in fat metabolism. Five identified proteins were selected for immunoblot validation, and 3 proteins were confirmed.

CONCLUSION

Our results suggest that inflammatory process may play a role in kidney uric acid stone formation. Our endogenous metabolic pathway analysis data revealed that these proteins may participate in lipid metabolism. Whether this finding implies a relation between lipotoxicity and kidney uric acid stone former requires further investigation.

Diabetes and nephrolithiasis

Type 2 diabetes is associated with an increased risk of nephrolithiasis, specifically in the form of uric acid (UA) nephrolithiasis. Diabetic patients who produce uric stones exhibit a low urine pH, the key factor of UA crystallization. Production of such acidic urine appears to result from the insulin-resistant state characteristic of diabetes mellitus. Insulin resistance is also involved in the pathogenesis of primary UA nephrolithiasis observed in overweight subjects with the metabolic syndrome. Therefore, UA nephrolithiasis should be considered a possible manifestation of insulin resistance, as it already is for hyperuricemia. Occurrence of UA stones in a patient, especially if overweight or hypertensive, should prompt a search for components of the metabolic syndrome in order to implement therapeutic intervention aimed at preventing the development of type 2 diabetes and atherosclerotic complications. Reciprocally, diabetologists should be aware of the risk of UA stones in their patients.

Influence of body size on urinary stone composition in men and women

A larger body size has been shown to be associated with increased excretion of urinary lithogenic solutes, and an increased risk of nephrolithiasis has been reported in overweight patients. However, the type of stones produced in these subjects has not been ascertained. Based on a large series of calculi, we examined the relationship between body size and the composition of stones, in order to assess which type of stone is predominantly favoured by overweight. Among 18,845 consecutive calculi referred to our laboratory, 2,100 came from adults with recorded body height and weight. Excluding calculi from patients with diabetes mellitus, as well as struvite and cystine stones, the study material consisted of 1,931 calcium or uric acid calculi. All calculi were analysed by infrared spectroscopy and categorized according to their main component. Body mass index (BMI) values were stratified as normal BMI (< 25 kg/m2), overweight (BMI 25-29.9) or obese (BMI > or = 30). Overall, 27.1% of male and 19.6% of female stone formers were overweight, and 8.4 and 13.5% were obese, respectively. In males, the proportion of calcium stones was lower in overweight and obese groups than in normal BMI group, whereas the proportion of uric acid stones gradually increased with BMI, from 7.1% in normal BMI to 28.7% in obese subjects (P<0.0001). The same was true in females, with a proportion of uric acid stones rising from 6.1% in normal BMI to 17.1% in obese patients (P=0.003). In addition, the proportion of uric acid stones markedly rose with age in both genders (P<0.0001). The average BMI value was significantly higher in uric acid stone formers aged < 60 years than in all other groups, whereas it did not differ from other groups in those aged > or = 60 years. Stepwise regression analysis identified BMI and age as significant, independent covariates associated with the risk of uric acid stones. Our data provide evidence that overweight is associated with a high proportion of uric acid stones in patients less than 60 years of age, whereas beyond this limit, advancing age is the main risk factor.

The association between gout and nephrolithiasis in men: The Health Professionals' Follow-Up Study

Approximately 10 million adults in the United States have experienced the passage of a kidney stone, and up to 5 million have been diagnosed with gout by a physician. Previous reports have suggested that gout increases the risk for the development of kidney stones, but there are no prospective data. We used data from a cohort of 51,529 male health care professionals to examine the independent association between gout and kidney stone disease. In a cross-sectional analysis of gout and kidney stone disease reported on the 1986 baseline questionnaire, the prevalence of kidney stone disease was almost twofold higher in men with history of gout compared to those without (15% vs. 8%). After adjusting for age and body mass index (BMI), a history of gout remained significantly associated with kidney stone disease (OR 1.88; 95% CI 1.68 to 2.11). We then prospectively examined the risk of incident kidney stones in men with and without a confirmed diagnosis of gout after excluding men who reported a history of kidney stone disease or gout on the baseline questionnaire. A confirmed diagnosis of gout increased the multivariate relative risk of incident kidney stones (RR 2.12; 95% CI 1.22 to 3.68). In contrast, a history of kidney stone disease was not associated with increased risk of gout (RR 1.05; 95% CI 0.54 to 2.07). In conclusion, a history of gout independently increases the risk for incident kidney stones in men. Physicians should provide dietary counseling, such as increasing fluid intake and decreasing salt consumption, to subjects with gout in addition to other risk factors, such as family history of kidney stones, in order to decrease the likelihood of stone formation.

Pathophysiology of uric acid nephrolithiasis

Humans although a predominantly ureotylic organism, has preserved the ability to excrete nitrogen as uric acid and ammonia. An imbalance between these two secondary modes of nitrogen excretion has resulted in uric acid precipitation in human urine. Uric acid nephrolithiasis can arise from diverse etiologies all with distinct underlying defects converging to one or more of three defects of hyperuricosuria, acidic urine pH, and low urinary volume, originating from secondary, genetic or heretofore undefined (idiopathic) causes. A subset of idiopathic uric acid nephrolithiasis (gouty diathesis) may be the "tip of the icebergp" of a broader systemic illness characterized by insulin resistance. A novel renal manifestation of insulin resistance is a mild defect in ammonium excretion, which is not severe enough to disturb acid-base homeostasis, but is sufficient to set up the chemical milieu for uric acid nephrolithiasis.

Uric acid nephrolithiasis: current concepts and controversies

PURPOSE

Uric acid calculi with or without a calcium component comprise a significant proportion of urinary stones. Knowledge of the pathophysiology of stone formation is important to direct medical treatment. The aim of this review is to provide an update on the epidemiology, pathophysiology and management of uric acid renal stones.

MATERIALS AND METHODS

A MEDLINE search was performed on the topic of uric acid stones. Current literature was reviewed with regard to the epidemiology, pathophysiology, associated medical conditions and management of uric acid stones.

RESULTS

The incidence of uric acid stones varies between countries and accounts for 5% to 40% of all urinary calculi. Hyperuricuria, low urinary output and acidic urine are well known contributing factors. However, the most important factor for uric acid stone formation is persistently acidic urine. Gout and myeloproliferative disorders are associated with uric acid stones. Why most patients with gout present with acidic urine yet only 20% have uric acid stone formation remains unclear. The pathophysiological basis for persistent urine acidity also remains unclear although various mechanisms have been proposed. Urinary alkalization with potassium citrate or sodium bicarbonate is a highly effective treatment, resulting in dissolution of existing stones and prevention of recurrence.

CONCLUSIONS

Acidic urine is a prerequisite for uric acid stone formation and growth. Medical management with urinary alkalization for stone dissolution and prevention of recurrence is effective and should be the cornerstone of treatment.

Uric acid nephrolithiasis: current concepts and controversies

PURPOSE

Uric acid calculi with or without a calcium component comprise a significant proportion of urinary stones. Knowledge of the pathophysiology of stone formation is important to direct medical treatment. The aim of this review is to provide an update on the epidemiology, pathophysiology and management of uric acid renal stones.

MATERIALS AND METHODS

A MEDLINE search was performed on the topic of uric acid stones. Current literature was reviewed with regard to the epidemiology, pathophysiology, associated medical conditions and management of uric acid stones.

RESULTS

The incidence of uric acid stones varies between countries and accounts for 5% to 40% of all urinary calculi. Hyperuricuria, low urinary output and acidic urine are well known contributing factors. However, the most important factor for uric acid stone formation is persistently acidic urine. Gout and myeloproliferative disorders are associated with uric acid stones. Why most patients with gout present with acidic urine yet only 20% have uric acid stone formation remains unclear. The pathophysiological basis for persistent urine acidity also remains unclear although various mechanisms have been proposed. Urinary alkalization with potassium citrate or sodium bicarbonate is a highly effective treatment, resulting in dissolution of existing stones and prevention of recurrence.

CONCLUSIONS

Acidic urine is a prerequisite for uric acid stone formation and growth. Medical management with urinary alkalization for stone dissolution and prevention of recurrence is effective and should be the cornerstone of treatment.

Pathophysiologic basis for normouricosuric uric acid nephrolithiasis

BACKGROUND

Low urinary pH is the commonest and by far the most important factor in uric acid nephrolithiasis but the reason(s) for this defect is (are) unknown. Patients with uric acid nephrolithaisis have normal acid-base parameters according conventional clinical tests.

METHODS

We studied steady-state plasma and urinary parameters of acid-base balance in subjects with normouricosuric pure uric acid stones. We also tested the ability of these subjects to excrete ammonium in response to an acute acid load. We compared these parameters in patients with pure uric acid stones to patients with mixed uric acid/calcium oxalate stones, pure calcium stones, and normal volunteers.

RESULTS

Pure uric acid stone formers have a much higher incidence of either diabetes or glucose intolerance. After equilibration to a control diet, patients with uric acid stones have lower urinary pH and they excrete less of their acid as ammonium. This is compensated by higher titratable acidity and hypocitraturia. Despite their low baseline urinary pH, uric acid stone formers further acidify their urine after an acid load because of a severely impaired ammonia excretory response. Their characteristics are significantly different from normal volunteers and pure calcium stone formers. Patients with mixed uric acid/calcium stones exhibit intermediate characteristics.

CONCLUSION

We propose that certain patients with normouricosuric uric acid nephrolithiasis have a renal acidification disease. The primary defect lies in renal ammonium excretion, which may be linked to the insulin-resistant state. Although net acid excretion is maintained at the expense of increased titratable acidity and to some degree hypocitraturia, the compromise is acid urine pH and may result in uric acid nephrolithiasis.

The association between gout and nephrolithiasis: the National Health and Nutrition Examination Survey III, 1988-1994

BACKGROUND

Gout, an inflammatory arthritis, reportedly afflicts more than 2 million men and women in the United States. Previous reports have suggested an association between gout and kidney stone disease; however, these studies did not adjust for such important potential confounders as obesity and the presence of hypertension. To our knowledge, no published study has examined the independent association between gout and kidney stone disease.

METHODS

We used a national probability sample of the US population to determine the independent association between reported gout and history of kidney stone disease.

RESULTS

Among men and women 20 years and older, 5.6% (10 million) reported the previous passage of a kidney stone and 2.7% (5.1 million) reported a diagnosis of gout by a physician. Moreover, 8.6% of individuals who reported the passage of a kidney stone on two or more occasions had a history of gout. Conversely, the prevalence of previous kidney stones in subjects with reported gout was 13.9%. In the age-adjusted model, gout was associated with an increased odds ratio (OR) for previous kidney stones (OR, 1.97; 95% confidence interval [CI], 1.37 to 2.83). After further adjustment for sex, race, body mass index, and presence of hypertension, the OR for previous kidney stones in individuals with gout decreased to 1.49 (95% CI, 1.04 to 2.14).

CONCLUSION

Showing an independent association between kidney stone disease and gout strongly suggests that they share common underlying pathophysiological mechanisms. Identification of these mechanisms may lead to improved preventive strategies for both conditions.

Uric acid-related nephrolithiasis

Abnormalities in uric acid metabolism are associated with uric acid and calcium oxalate urolithiasis. Clinical stone formation depends on multiple identifiable risk factors that affect uric acid and calcium oxalate solubility. The understanding of urinary pH is critical to direct appropriate treatment of uric acid-related nephrolithiasis. Understanding uric acid metabolism and the pathophysiology of uric acid and calcium oxalate stone formation leads to a rational treatment approach to uric acid and hyperuricosuric calcium oxalate stone disease.

A study of the hypouricaemic effect of tiaprofenic acid

Tiaprofenic acid is a non-steroidal anti-inflammatory drug which also has hypouriceamic effect. Studies involving 10 healthy volunteers were designed to investigate the mode of this effect. We postulate that the site of action of tiaprofenic acid is at the cell membrane, the mechanism being an interference with the transport of uric acid from intra- to extra-cellular fluid thus limiting its passage into the plasma. The same mechanism acting on renal tubular cells impedes reabsorption thereby increasing uric acid clearance.

Distal renal tubular dysfunction: a common feature in calcium stone formers

Distal renal tubular acidosis has been reported as an uncommon cause of urinary calcium stone disease. However, this defect appears to be more frequent when appropriate tests are performed systematically. Twenty-nine patients with recurrent calcium stones have been separated into three groups: normocalciuric (group A), renal hypercalciuric (group B) and absorptive hypercalciuric (group C). Distal tubular functions were investigated by the (urine-blood) pCO2 gradient and by an ammonium chloride test. (Urine-blood) pCO2 gradient was (mean +/- SEM), 3.33 +/- 0.59 in group A, 2.95 +/- 0.34 in group B and 3.31 +/- 0.58 kPa in group C. All these values differ significantly from those observed in controls (4.11 +/- 0.28 kPa; P less than 0.05). After 3 days of ammonium chloride loading, ammonium excretion averaged 54.7 +/- 4.2 in group A, 54.4 +/- 4.3 in group B and 64.3 +/- 5.5 mumol min-1 in group C. Values obtained in the first two groups were significantly lower than that achieved by control subjects (76.4 +/- 14.9 mumol min-1). It is concluded that tubular dysfunctions defined as impairments in hydrogen ion secretion and ammonium excretion after an acid challenge are a common feature of the urinary calcium stone disease and play a contributory role in its pathogenesis.

Renal impairment and gout

A study of renal function of 51 patients with gout and an equal number of normouricaemic controls revealed significant differences. A relative impairment of the glomerular filtration rate and urine concentrating ability in the gouty subjects could not be wholly explained on the basis of aging or hypertension. Renal dysfunction was generally mild and was not associated with specific clinical characteristics higher levels of uric acid excretion, or hypertriglyceridaemia. Gout patients excreted urine with a significantly lower pH. This was associated with a relatively high excretion of titratable acid and a deficit of ammonium excretion, which was accentuated by ingestion of an acid load. Urate clearance was significantly reduced in gout, even when expressed as a fraction of the glomerular filtration rate.

The natural history of hyperuricemia among asymptomatic relatives of patients with gout

1. Hyperuricemia is common among the gouty relatives as reported by others (8-11). It is of interest to note that serum urate fluctuates periodically. Hyperuricemia is not necessarily maintained in a steady state throughout the years. Thus a single determination of serum uric acid can be misleading. 2. Development of gout from asymptomatic hyperuricemia is often correlated with the degree of hyperuricemia as observed from population or family studies (12-14). The data presented indicate that unequivocal hyperuricemia is more often accompanied by excessive excretion of uric acid, diminished excretion of ammonia and abnormally high plasma glutamic acid. All are undoubtedly important risk factors for gout. 3. The elevated glutamate could be due to a deficiency of glutamic dehydrogenase, as postulated by Pagliara and Goodman (15). In presence of intracellular accumulation of glutamate in glutamic dehydrogenase deficiency, renal production of ammonium may be reduced due to its inhibitory action on glutaminase 1. As a result of a renal block of ammonia formation, the glutamine in surplus may be diverted for uric acid synthesis. 4. Long-term studies indicate serum urate in most hyperuricemia relatives of gout can be modified by environmental factors, such as diet, weight and changes of life style. When hyperuricemia is under better control, the potential hazard of developing symptomatic gout may be circumvented.

The kinetics of intramolecular distribution of 15N in uric acid after administration of (15N) glycine. A reappraisal of the significance of preferential labeling of N-(3+9) of uric acid in primary gout

THE CONCEPT OF AN ABNORMALITY OF GLUTAMINE METABOLISM IN PRIMARY GOUT WAS FIRST PROPOSED ON THE BASIS OF ISOTOPE DATA: when [(15)N]glycine was administered to gouty subjects, there was disproportionately great enrichment of N-(3 + 9) of uric acid, which derive from the amide-N of glutamine. An unduly high concentration of (15)N in glutamine was postulated, and attributed to a hypothetical defect in catabolism of glutamine. Excess glutamine was proposed as the driving force of uric acid overproduction. WE HAVE REEXAMINED THIS PROPOSITION IN FOUR GOUTY SUBJECTS: one mild overproducer of uric acid with "idiopathic gout," one marked overproducer with high-grade but "partial" hypoxanthine-guanine phosphoribosyl-transferase deficiency, and two extraordinary overproducers with superactive phosphoribosylpyrophosphate synthetases. In the last three, the driving force of excessive purine biosynthesis is a known surplus of alpha-5-phosphoribosyl-1-pyrophosphate. Disproportionately high labeling of N-(3 + 9) was present in all four gouty subjects, most marked in the most flamboyant overproducers. The precursor glucine pool was sampled by periodic administration of benzoic acid and isolation of urinary hippuric acid. Similarly, the precursor glutamine pool was sampled by periodic administration of phenylacetic acid and isolation of the amide-N of urinary phenylacetylglutamine. The time course of (15)N enrichment of hippurate differed from that of the amide-N of glutamine. Whereas initial enrichment values of hippurate were very high, those of glutamine-amide-N were low, increasing to a maximum at about 3 h, and then declining less rapidly than those of hippurate. However, enrichment values of hippurate and of phenacetyl glutamine were normal in all of the gouty subjects studied. Thus, preferential enrichment of N-(3 + 9) in gouty overproducers given [(15)N]glycine does not necessarily reflect a specific abnormality of glutamine metabolism, but rather appears to be a kinetic phenomenon associated with accelerated purine biosynthesis per se.In addition, greater enrichment of N-9 than of N-3 on days 1 and 2 provided suggestive evidence for a second pathway for synthesis of the initial precursor of purine biosynthesis, phosphoribosylamine, perhaps utilizing ammonia rather than the amide-N of glutamine as nitrogen donor. In this limited study, the activity of this potential second pathway did not appear to be selectively increased in gout.

Plasma and urinary amino acids in primary gout, with special reference to glutamine

Measurement of the plasma free amino acids by column chromatography (AutoAnalyzer) in 32 patients with primary gout showed statistically significant increases or decreases in several components when compared with the spectrum in 18 control subjects, but the absolute amounts involved were small and the mean total plasma amino acid concentrations in both groups were the same. In the urine all major amino acid components, notably glutamine, serine, threonine, and leucine, were lower in our gouty than in our nongouty subjects, as were also the corresponding renal clearance ratios. These deficits could be reproduced by restricting dietary protein, so appear to be due largely to the some-what reduced mean dietary protein intake of our gouty subjects. However, the low renal clearance of glutamine, the most striking and consistent of the deficits in urinary amino acids noted, could not be accounted for by dietary or other relevant factors, and is interpreted as indicating increased tubular reabsorption of glutamine in primary gout. This interpretation was supported by the results of glutamine loading. The possible compensatory relationship of the abnormality in renal handling of glutamine to the deficiency in renal production of ammonia previously reported is discussed.