Renal ammonium excretion after an acute acid load: blunted response in uric acid stone formers but not in patients with type 2 diabetes

Type 4 renal tubular acidosis and uric acid nephrolithiasis: two faces of the same coin?

PURPOSE OF REVIEW

The present review summarizes findings of recent studies examining the epidemiology, pathophysiology, and treatment of type 4 renal tubular acidosis (RTA) and uric acid nephrolithiasis, two conditions characterized by an abnormally acidic urine.

RECENT FINDINGS

Both type 4 RTA and uric acid nephrolithiasis disproportionately occur in patients with type 2 diabetes and/or chronic kidney disease. Biochemically, both conditions are associated with reduced renal ammonium excretion resulting in impaired urinary buffering and low urine pH. Reduced ammoniagenesis is postulated to result from hyperkalemia in type 4 RTA and from insulin resistance and fat accumulation in the renal proximal tubule in uric acid nephrolithiasis. The typical biochemical findings of hyperkalemia and systemic acidosis of type 4 RTA are rarely reported in uric acid stone formers. Additional clinical differences between the two conditions include findings of higher urinary uric acid excretion and consequent urinary uric acid supersaturation in uric acid stone formers but not in type 4 RTA.

SUMMARY

Type 4 RTA and uric acid nephrolithiasis share several epidemiological, clinical, and biochemical features. Although both conditions may be manifestations of diabetes mellitus and thus have a large at-risk population, the means to the shared biochemical finding of overly acidic urine are different. This difference in pathophysiology may explain the dissimilarity in the prevalence of kidney stone formation.

Urinary Ammonium in Clinical Medicine: Direct Measurement and the Urine Anion Gap as a Surrogate Marker During Metabolic Acidosis

Ammonium is the most important component of urinary acid excretion, normally accounting for about two-third of net acid excretion. In this article, we discuss urine ammonium not only in the evaluation of metabolic acidosis but also in other clinical conditions such as chronic kidney disease. Different methods to measure urine NH₄⁺ that have been employed over the years are discussed. The enzymatic method used by clinical laboratories in the United States to measure plasma ammonia via the glutamate dehydrogenase can be used for urine ammonium. The urine anion gap calculation can be used as a rough marker of urine ammonium in the initial bedside evaluation of metabolic acidosis such as in distal renal tubular acidosis. Urine ammonium measurements, however, should be made more available in clinical medicine for a precise evaluation of this important component of urinary acid excretion.

Urinary Risk Profile, Impact of Diet, and Risk of Calcium Oxalate Urolithiasis in Idiopathic Uric Acid Stone Disease

The role of diet in the pathogenesis of uric acid (UA) nephrolithiasis is incompletely understood. This study investigated the effect of dietary intervention on the risk of UA stone formation under standardized conditions. Twenty patients with idiopathic UA stone disease were included in the study. Dietary intake and 24 h urinary parameters were collected on the usual diet of the patients and a standardized balanced mixed diet. Although urinary UA excretion did not change, the relative supersaturation of UA decreased significantly by 47% under the balanced diet primarily due to the significant increase in urine volume and pH. Urinary pH was below 5.8 in 85% of patients under the usual diet, and in 60% of patients under the balanced diet. The supersaturation of calcium oxalate declined significantly under the balanced diet due to the significant decrease in urinary calcium and oxalate excretion and the increase in urine volume. Dietary intervention is a key component in the management of UA nephrolithiasis. Urinary calcium and oxalate excretion should also be monitored in patients with pure UA calculi to reduce the risk of mixed stone formation with calcium oxalate. Lower urinary pH in UA stone patients can only be partially explained by diet.

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.

Results of a Gene Panel Approach in a Cohort of Patients with Incomplete Distal Renal Tubular Acidosis and Nephrolithiasis

BACKGROUND

Distal renal tubular acidosis (dRTA) is characterized by an impairment of urinary acidification resulting in metabolic acidosis, hypokalemia, and inappropriately elevated urine pH. If not treated, this chronic condition eventually leads to nephrocalcinosis, nephrolithiasis, impaired renal function, and bone demineralization. dRTA is a well-defined entity that can be diagnosed by genetic testing of 5 genes known to be disease-causative. Incomplete dRTA (idRTA) is defined as impaired urinary acidification that does not lead to overt metabolic acidosis and therefore can be diagnosed if patients fail to adequately acidify urine after an ammonium chloride (NH4Cl) challenge or furosemide and fludrocortisone test. It is still uncertain whether idRTA represents a distinct entity or is part of the dRTA spectrum and whether it is caused by mutations in the same genes of overt dRTA.

METHODS

In this cross-sectional study, we investigated a group of 22 stone formers whose clinical features were suspicious of idRTA. They underwent an NH4Cl challenge and were found to have impaired urinary acidification ability. These patients were then analyzed by genetic testing with sequencing of 5 genes: SLC4A1, ATP6V1B1, ATP6V0A4, FOXI1, and WDR72.

RESULTS

Two unrelated individuals were found to have two different variants in SLC4A1 that had never been described before.

CONCLUSIONS

Our results suggest the involvement of other genes or nongenetic tubular dysfunction in the pathogenesis of idRTA in stone formers. However, genetic testing may represent a cost-effective tool to recognize, treat, and prevent complications in these patients.

Uric Acid and Urate in Urolithiasis: The Innocent Bystander, Instigator, and Perpetrator

Uric acid is an end product of purine metabolism in human beings. An unusual and still unexplained phenomenon is that higher primates have relatively high uric acid levels in body fluids owing to a combination of absence of degradation and renal retention. The physiologic purpose of high uric acid levels still is enigmatic, but the pathobiologic burden is a variety of crystallopathies owing to the low aqueous solubility of uric acid such as gouty arthritis and acute uric acid nephropathy. In the urinary space, three distinct conditions result from chronic uric acid and/or urate precipitation. The first and most common variety is uric acid urolithiasis. In this condition, urate is a victim of a systemic metabolic disease in which increased acid load to the kidney is coupled with diminished urinary buffer capacity owing to defective ammonium excretion, resulting in titration of urate to its sparingly soluble protonated counterpart, uric acid, and the formation of stones. Uric acid is the innocent bystander of the crime. The second variety is hyperuricosuric calcium urolithiasis, in which uric acid confers lithogenicity via promotion of calcium oxalate precipitation by multiple mechanisms involving soluble, colloidal, and crystalline urate salts. Uric acid is the instigator of the crime. The third and least common condition involves urate as an integral part of the urolith as an ammonium salt driven by high ammonium and high urate concentrations in urine. Here, uric acid is one of the perpetrators of the crime. Both known and postulated pathogenesis of these three types of urolithiasis are reviewed and summarized.

Diabetes Mellitus Might Not Be Associated with Any Stone Component in a Local District

BACKGROUND

Type II diabetes mellitus (DM) is a risk factor for urinary stones, but the pathogenesis remains unclear. The aim of our study was to present the distribution of stone components between DM and no DM group from a local stone center in China and to help the prevention department in decision-making.

METHODS

We reviewed the records of patients with upper urinary stones attending our hospital from January 2015 to September 2018. The patients with complete information were divided into 2 groups: type II DM group (DM group) and without DM group (no DM group). The distribution of stone components was analyzed.

RESULTS

Two hundred twenty-two patients were complicated with DM, whereas 1,894 (89%) were not. Significant difference was found in the distribution of hypertension and BMI (p = 0, p = 0, respectively). Distribution of sex, age, and stone components did not differ between the 2 groups. By the binary logistic analysis, increasing age and sex seemed to be the main risk factors influencing the stone components. Only the calcium stone seemed to be free of the -impact from age and sex. Occurrence of hypertension is a single risk factor for calcium stone from our analysis. Presence of diabetes and increasing BMI was not found to be significantly associated with the risk for any stone component.

CONCLUSIONS

In a local district, DM might not be the main factor associated with an increased risk for uric acid stone formation or any stone component. We should also consider the local characteristics of the stone distribution.

Uric acid stone disease: lessons from recent human physiologic studies

PURPOSE OF REVIEW

An overly acidic urine resulting in supersaturation of urine with respect to uric acid is the major mechanism responsible for uric acid nephrolithiasis. The present review summarizes findings from recent human physiologic studies examining the pathophysiology and reversibility of low urine pH in uric acid stone formers.

RECENT FINDINGS

Epidemiologic and metabolic studies have confirmed an increase in the prevalence of uric acid nephrolithiasis and reported its association with several features of the metabolic syndrome including dyslipidemia, hyperglycemia, hepatic steatosis, and greater visceral adiposity. Physiologic studies in uric acid stone formers have identified diet-independent excessive net acid excretion and concomitant reduction in urinary buffering from impaired renal ammoniagenesis as the two causes underlying the greater aciduria. Administration of the insulin sensitizer pioglitazone to uric acid stone formers reduced the acid load presented to the kidney and enhanced ammoniagenesis and ammonium excretion, resulting in significantly higher urine pH.

SUMMARY

Recent human physiologic studies have identified greater acid excretion and reduced urinary buffering by ammonia as two culprits of aciduria in uric acid nephrolithiasis that can be reversed by pioglitazone, raising new questions regarding the origin of the aciduria and opening the door to pathophysiology-based treatment of uric acid stones.

Discrepancy Between Stone and Tissue Mineral Type in Patients with Idiopathic Uric Acid Stones

Objectives: To describe the papillary pathology found in uric acid (UA) stone formers, and to investigate the mineral form of tissue deposits. Materials and Methods: We studied eight UA stone formers treated with percutaneous nephrolithotomy. Papillae were imaged intraoperatively using digital endoscopy, and cortical and papillary biopsies were taken. Biopsies were analyzed by light microscopy, micro-CT, and microinfrared spectroscopy. Results: As expected, urine pH was generally low. UA supersaturation exceeded one in all but one case, compatible with the stone material. By intraoperative imaging, the renal papillae displayed a heterogeneous mixture of plaque and plugging, ranging from normal to severe. All patients had mineral in ducts of Bellini and inner medullary collecting ducts, mainly apatite with lesser amounts of urate and/or calcium oxalate in some specimens. Papillary and cortical interstitial tissue injury was modest despite the tubule plugging. No instance was found of a stone growing attached to either plaque or plugs. Conclusions: UA stone formers resemble those with ileostomy in having rather low urine pH while forming tubule plugs that contain crystals that can only form at pH values above those of their bulk urine. This discrepancy between tissue mineral deposits and stone type suggests that local tubular pH exceeds that of the bulk urine, perhaps because of localized tubule injury. The manner in which UA stones form and the discordance between tubule crystals and stone type remain open research questions.

Net Acid Excretion and Urinary Organic Anions in Idiopathic Uric Acid Nephrolithiasis

BACKGROUND AND OBJECTIVES

Idiopathic uric acid nephrolithiasis, which is closely associated with obesity and the metabolic syndrome, is increasing in prevalence. Unduly acidic urine pH, the quintessential pathophysiologic feature of this disease, is in part explained by inadequate excretion of the principal urinary buffer ammonium. The role of net acid excretion in the pathogenesis of uric acid nephrolithiasis is incompletely understood.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

We compared acid-base parameters of patients with idiopathic uric acid nephrolithiasis with matched control subjects under controlled diets in an inpatient metabolic unit. Measurements included fasting blood and 24-hour urine chemistries and 24-hour urine metabolomic analysis. Comparisons between groups included analysis of covariance models controlling for urine pH or body mass index.

RESULTS

Subjects with idiopathic uric acid nephrolithiasis had lower urine pH (5.5 versus 5.9; P<0.001) and higher net acid excretion (60 versus 43 mEq/24 h; P<0.001), with the excess H+ carried by nonammonium buffers. In all subjects, there was a positive relationship of net acid excretion with higher body mass index in spite of strictly controlled equivalent dietary acid intake. This relationship was most evident among control subjects (r=0.36; P=0.03). It was attenuated in patients with idiopathic uric acid nephrolithiasis whose net acid excretion remained fixedly high and ammonium excretion remained low relative to net acid excretion, resulting in low urine pH over a wide body mass index range. Urinary metabolomics was performed to attempt to identify excess organic acids presented to the kidney in idiopathic uric acid nephrolithiasis. Among the tricarboxylic acid cycle intermediates and amino acid and lipid metabolites analyzed, 26 organic anions with acid dissociation constants values in the range of urine pH showed greater protonation. However, protons carried by the identified organic acids did not entirely account for the higher titratable acidity seen in idiopathic uric acid nephrolithiasis.

CONCLUSIONS

Higher acid load to the kidney, resulting in higher urinary net acid excretion, is an important factor in the pathogenesis of idiopathic uric acid nephrolithiasis.

Increased production and reduced urinary buffering of acid in uric acid stone formers is ameliorated by pioglitazone

Idiopathic uric acid nephrolithiasis is characterized by an overly acidic urine pH caused by the combination of increased acid production and inadequate buffering of urinary protons by ammonia. A large proportion of uric acid stone formers exhibit features of the metabolic syndrome. We previously demonstrated that thiazolidinediones improved the urinary biochemical profile in an animal model of the metabolic syndrome. In this proof-of-concept study, we examined whether the thiazolidinedione pioglitazone can also ameliorate the overly acidic urine in uric acid stone formers. Thirty-six adults with idiopathic uric acid nephrolithiasis were randomized to pioglitazone 30 mg/day or matching placebo for 24 weeks. At baseline and study end, participants underwent collection of blood and 24-hour urine in an inpatient research unit while consuming a fixed metabolic diet, followed by assessment of the ammoniagenic response to an acute oral acid load. Twenty-eight participants completed the study. Pioglitazone treatment improved features of the metabolic syndrome. Pioglitazone also reduced net acid excretion and increased urine pH (5.37 to 5.59), the proportion of net acid excreted as ammonium, and ammonium excretion in response to an acute acid load, whereas these parameters were unchanged with placebo. Treatment of patients with idiopathic uric acid nephrolithiasis with pioglitazone for 24 weeks led to a reduction in the acid load presented to the kidney and a more robust ammoniagenesis and ammonium excretion, resulting in significantly higher urine pH. Future studies should consider the impact of this targeted therapy on uric acid stone formation.

Nephrolithiasis against type 2 diabetes mellitus: on the effect of hypoglycemic therapy on lithogenesis

Aim. To study the effects of oral hypoglycemic agents that can affect the probability of recurrence of nephrolithiasis. Materials and methods. The article is based on the results of examination and treatment of 315 patients suffering from recurrent nephrolithiasis and medically compensated type 2 diabetes mellitus treated at the N.A. Lopatkin Institute of Urology and Interventional Radiology - the branch of the SMRC of Radiology, Ministry of Health of Russia and D.D. Pletnev City Hospital Moscow Healthcare Department in 2012-2017. The patients were divided into three groups according to the applied tool antidiabetic: metformin, glibenclamide, canagliflozin. The control group consisted of patients receiving insulin therapy. Results and discussion. The propensity of Metformin to reduce the pH of urine, which has a negative impact in the conditions of urate nephrolithiasis, which is most common in the population of patients with type 2 diabetes mellitus. Glibenclamide, on the contrary, somewhat latches urine. But changes in the reaction of urine under the influence of the drug do not go beyond normal values and are not clinically significant. Canagliflozin increases diuresis due to medication induced glycosuria and stimulates renal excretion of uric acid and its salts. However canagliflozin does not cause significant shifts in the pH of urine that may somewhat negates the increased risk of recurrence of urate stone formation in the background of the uricosuric effect of the drug. Conclusion. Drug therapy of type 2 diabetes mellitus significantly affects the properties of urine from patients with nephrolithiasis.

Metabolic syndrome and uric acid nephrolithiasis: insulin resistance in focus

Uric acid nephrolithiasis (UAN) is an increasingly common disease in ethnically diverse populations and constitutes about 10% of all kidney stones. Metabolic syndrome and diabetes mellitus are accounted among the major risk factors for UAN, together with environmental exposure, individual lifestyle habits and genetic predisposition. The development and overt manifestation of UAN appears to stem on the background of insulin resistance, which acts at the kidney level by reducing urinary pH, thus hampering the ability of the kidney to generate renal ammonium in response to an acid load. Unduly acidic urinary pH and overt UAN are both considered renal manifestations of insulin resistance. The mechanisms underlying increased endogenous acid production and/or defective ammonium excretion are yet to be completely understood. Although the development of UAN and, more in general, of kidney stones largely recognizes modifiable individual determining factors, the rising prevalence of diabetes, obesity and accompanying metabolic disorders calls for the identification of novel therapeutic approaches and intervention targets. This review aims at providing an updated picture of existing evidence on the relationship between insulin resistance and UAN in the context of metabolic syndrome and in light of the most recent advancements in our understanding of its genetic signature.

Mechanisms of acid-base regulation in peritoneal dialysis

Background

Peritoneal dialysis (PD) contributes to restore acid-base homeostasis in patients with end-stage renal disease. The transport pathways for buffers and carbon dioxide (CO2) across the peritoneal membrane remain poorly understood.

Methods

Combining well-established PD protocols, whole-body plethysmography and renal function studies in mice, we investigated molecular mechanisms of acid-base regulation in PD, including the potential role of the water channel aquaporin-1 (AQP1).

Results

After instillation in peritoneal cavity, the pH of acidic dialysis solutions increased within minutes to rapidly equilibrate with blood pH, whereas the neutral pH of biocompatible solutions remained constant. Predictions from the three-pore model of peritoneal transport suggested that local production of HCO3- accounts at least in part for the changes in intraperitoneal pH observed with acidic solutions. Carbonic anhydrase (CA) isoforms were evidenced in the peritoneal membrane and their inhibition with acetazolamide significantly decreased local production of HCO3- and delayed changes in intraperitoneal pH. On the contrary, genetic deletion of AQP1 had no effect on peritoneal transport of buffers and diffusion of CO2. Besides intraperitoneal modifications, the use of acidic dialysis solutions enhanced acid excretion both at pulmonary and renal levels.

Conclusions

These findings suggest that changes in intraperitoneal pH during PD are mediated by bidirectional buffer transport and by CA-mediated production of HCO3- in the membrane. The use of acidic solutions enhances acid excretion through respiratory and renal responses, which should be considered in patients with renal failure.

Impact of food quantity and quality on the biochemical risk of renal stone formation

OBJECTIVE

This study evaluated the role of body mass index (BMI) and dietary potential renal acid load (PRAL) with urinary saturation for calcium oxalate (US-CaOx), calcium phosphate (US-CaP) and uric acid (US-UA) in renal stone formers.

MATERIALS AND METHODS

A retrospective analysis was conducted of laboratory data collected on 442 renal stone-forming patients. Demographic information, BMI and 24 h urinary samples were collected from patients on their regular diets. PRAL was calculated as the Load of Acid to Kidney Evaluation (LAKE) score through a short questionnaire.

RESULTS

Urinary risk factors, but also inhibitors of calcium stone formation such as magnesium, tended to increase in relation to BMI (p = .000). Urinary pH (p = .002) and ammonium/sulfate ratio (p = .000) were negatively related to BMI. This resulted in a positive correlation between BMI and US-UA (p = .000), whereas US-CaOx and US-CaP were not influenced by BMI. LAKE score was positively correlated with US-CaOx (p = .022) and US-CaP (p = .000) as a consequence of the inverse relationship between LAKE score and citrate (p = .000). Multiple linear regression analysis identified BMI (p = .009) and male gender (p = .002) as independent predictors of US-UA, and LAKE score (p = .004) and age (p = .001) as independent predictors of US-CaP.

CONCLUSIONS

BMI, which depends on excessive intake of energy from food, is not related to an increased biochemical risk of calcium stone formation, which is more dependent on the renal acid load of the diet. In contrast, obesity is associated with an increased risk of uric acid stone formation due to insulin resistance, impaired ammoniagenesis and low urinary pH.

Role of Acid-Base Homeostasis in Diabetic Kidney Disease

PURPOSE OF REVIEW

Acid-base homeostasis is impaired in chronic kidney disease (CKD) and may contribute to disease progression. Diabetes, a major cause of CKD worldwide, may exacerbate acidosis further due to differences in acid production and excretion. Here, we review the role of abnormal acid-base homeostasis in the pathogenesis and progression of diabetes and diabetic kidney disease.

RECENT FINDINGS

Acidosis and dietary acid loading may contribute to the development and worsening of insulin resistance and hypertension, thereby promoting diabetes and diabetic CKD. However, although metabolic acidosis associates with progression of CKD generally, the results in diabetic CKD are mixed. Data suggests that metabolic acid production in diabetes may be higher than would be predicted based on dietary intake alone, and new observational data suggests that this higher diet-independent acid production could potentially be protective. The role of acid-base homeostasis in diabetic CKD progression is complex and must consider differences in endogenous acid production and excretion in diabetes. Ongoing observational and interventional studies in this field should consider the unique physiology of diabetes.

Medical management of urinary calculi: up to date 2016

Nephrolithiasis (NL) is one of the most prevalent nontransmissible diseases in western countries. It is being associated with other frequent diseases, including osteoporosis, cardiovascular disease, hypertension, diabetes mellitus, through a putative common link with metabolic syndrome and insulin resistance or altered mineral metabolism. This review will focus on classification, physicochemical basis, risk factors, laboratory and imaging investigations, medical management.Classification as to stone composition includes calcium, uric acid (UA), cystine (Cys), infected, 2-8 dihydroxyadenine and rare NL. According to pathophysiology, NL is classified as primary, secondary to systemic diseases or drugs, caused by renal or metabolic hereditary disorders.A stone can only form in supersaturated environment, and this is sufficient in UA, Cys and infected NL, but not in Ca-NL, which results from the imbalance between supersaturation and inhibition. All types are characterized by derangements of peculiar risk factors. Laboratory investigations aim at identifying type of NL, underlying risk factors and state of saturation, and pathophysiology. This justifies a rationale therapy able to dissolve some types of stones and/or produce reduction in recurrence rate in others.Medical management includes alkali and allopurinol for UA nephrolithiasis (UA-NL), thiols and alkali in Cys-NL, dietary and pharmacological intervention for Ca-NL. Thiazides and alkaline citrate salts are the most widely used drugs in Ca-NL, where they proved efficient to prevent new stones. Other drugs have only been used in particular subsets.Proper medical management and modern urological approaches have already notably improved clinical outcomes. Future studies will further clarify mechanisms of NL with expected new and targeted therapeutic options.

Assessment of Urinary Inhibitor or Promoter Activity in Uric Acid Nephrolithiasis

PURPOSE

We assessed decreased inhibitor activity or increased promoter activity in the urine of idiopathic uric acid stone formers compared to nonstone formers independent of urinary pH.

MATERIALS AND METHODS

A total of 30 idiopathic uric acid stone formers, and 9 obese and 12 lean nonstone formers collected 24-hour urine while on a metabolic diet. Three urine aliquots per subject were used to assess spontaneous nucleation (de novo crystal formation), crystal growth using a 0.1 mg/ml anhydrous uric acid seed and steady-state uric acid solubility (the maximum amount of uric acid dissolvable in urine) using a 5 mg/ml uric acid seed. All experiments were performed for 6 hours at a constant pH of 5.0. Uric acid concentration was measured in filtered aliquots at 0, 3 and 6 hours.

RESULTS

At baseline 24-hour urinary pH was significantly lower and uric acid saturation was significantly higher in idiopathic uric acid stone formers. No significant spontaneous nucleation developed and similar uric acid steady-state solubility was reached in the 3 groups. Idiopathic uric acid stone formers and lean nonstone formers showed a similar decrease in uric acid concentration during crystal growth. Obese nonstone formers started with a higher uric acid concentration and consequently demonstrated a greater decrease in the uric acid concentration for crystal growth.

CONCLUSIONS

This study suggests that there is no significant difference between idiopathic uric acid stone formers and nonstone formers in promoter or inhibitor activity in whole urine against uric acid stone formation when urine pH is maintained constant. The findings suggest that uric acid stone formation is dictated by high urinary saturation with respect to uric acid, which is driven primarily by low urine pH.

Role of insulin resistance in uric acid nephrolithiasis

Metabolic syndrome has been implicated in the pathogenesis of uric acid stones. Although not completely understood, its role is supported by many studies demonstrating increased prevalence of uric acid stones in patients with metabolic syndrome and in particular insulin resistance, a major component of metabolic syndrome. This review presents epidemiologic studies demonstrating the association between metabolic syndrome and nephrolithiasis in general as well as the relationship between insulin resistance and uric acid stone formation, in particular. We also review studies that explore the pathophysiologic relationship between insulin resistance and uric acid nephrolithiasis.