The renal mechanism for urate homeostasis in normal man

Identification of three distinct cell populations for urate excretion in human kidneys

In humans, uric acid is an end-product of purine metabolism. Urate excretion from the human kidney is tightly regulated by reabsorption and secretion. At least eleven genes have been identified as human renal urate transporters. However, it remains unclear whether all renal tubular cells express the same set of urate transporters. Here, we show renal tubular cells are divided into three distinct cell populations for urate handling. Analysis of healthy human kidneys at single-cell resolution revealed that not all tubular cells expressed the same set of urate transporters. Only 32% of tubular cells were related to both reabsorption and secretion, while the remaining tubular cells were related to either reabsorption or secretion at 5% and 63%, respectively. These results provide physiological insight into the molecular function of the transporters and renal urate handling on single-cell units. Our findings suggest that three different cell populations cooperate to regulate urate excretion from the human kidney, and our proposed framework is a step forward in broadening the view from the molecular to the cellular level of transport capacity.

Urate Handling in the Human Body

Elevated serum urate concentration is the primary cause of gout. Understanding the processes that affect serum urate concentration is important for understanding the etiology of gout and thereby understanding treatment. Urate handing in the human body is a complex system including three major processes: production, renal elimination, and intestinal elimination. A change in any one of these can affect both the steady-state serum urate concentration as well as other urate processes. The remarkable complexity underlying urate regulation and its maintenance at high levels in humans suggests that this molecule could potentially play an interesting role other than as a mere waste product to be eliminated as rapidly as possible.

Uricosuric action of losartan via the inhibition of urate transporter 1 (URAT 1) in hypertensive patients

BACKGROUND

The angiotensin receptor blocker losartan inhibited urate transporter 1 (URAT1) according to in vitro experiments. However, it is still unknown whether the inhibitory effect of losartan on URAT1 contributes to its uricosuric action in humans.

METHODS

Thirty-two patients with hypertension and nine patients with idiopathic renal hypouricemia (five with and four without hypertension) were enrolled for this study. Hypertensive patients were prescribed oral losartan (50 mg/day, n = 16) or candesartan (8 mg/day, n = 16). Before and after 1-month treatment, the serum concentration of urate (Sur) and creatinine (Scr), and the clearance value of urate (Cur) and creatinine (Ccr) were determined. Clearance studies using the URAT1 inhibitor benzbromarone (100 mg/day) or losartan (50 mg/day) loading test were also performed in these patients.

RESULTS

Blood pressure (BP) significantly decreased in the patients treated with either losartan or candesartan. Losartan significantly reduced Sur, which was associated with a concomitant increase in the Cur/Ccr ratio, whereas candesartan did not alter these parameters. In hypertensive patients with loss-of-function mutation of URAT1, losartan did not alter either Sur or Cur/Ccr, nor did benzbromarone. The lack of effect of URAT1 inhibitors on renal excretion of urate was independent of the renal function of hypouricemic patients. On the other hand, both losartan and benzbromarone increased Cur/Ccr ratio in hypertensive patients harboring the wild URAT1 gene, regardless of the presence of hypouricemia.

CONCLUSIONS

These findings suggested that losartan inhibited URAT1 and thereby it lowered Sur levels in hypertensive patients.

Endogenous Urate Production Augments Plasma Antioxidant Capacity in Healthy Lowland Subjects Exposed to High Altitude

BACKGROUND

Both tissue hypoxia in vitro, and whole-body hypoxia in vivo, have been found to promote the release of reactive oxygen species (ROS) that are potentially damaging to the cardiovascular system. Antioxidant systems protect against oxidative damage by ROS and may exhibit some degree of responsiveness to oxidative stimuli. Production of urate, a potent soluble antioxidant, is increased in hypoxic conditions. We aimed to determine whether urate is an important antioxidant defense in healthy subjects exposed to hypoxia.

METHODS

We conducted a cohort study of 25 healthy lowland volunteers during acute exposure to high altitude (4 days at 3,600 m, followed by 10 days at 5,200 m) on the Apex high-altitude research expedition to Bolivia. We measured markers of oxidative stress (8-isoprostane F2), serum urate concentration, and total plasma antioxidant activity by two techniques: 2,2'-amino-di-[3-ethylbenzthiazole sulfonate] spectrophotometry (total antioxidant status [TAS]) and enhanced chemiluminescence (ECL).

RESULTS

On ascent, F2-isoprostane levels were significantly elevated compared with those at sea level (p < 0.01). After 1 week at high altitude, plasma antioxidant capacity (AOC) by both TAS and ECL, and serum urate concentration were significantly elevated (each p < 0.01 vs sea level), and F2-isoprostane levels were reduced to values at sea level. There was a highly significant correlation between plasma urate and AOC at this stage (ECL, r(2) = 0.59, p = 0.0001; TAS, r(2) = 0.30, p = 0.0062).

CONCLUSIONS

Our results support the hypothesis that urate may act as a responsive endogenous antioxidant in high-altitude hypoxia.

Inverse association between coffee drinking and serum uric acid concentrations in middle-aged Japanese males

Consumption of caffeine-rich beverages, which have diuretic properties, may decrease serum uric acid concentrations. We examined cross-sectionally the relationship of coffee and green tea consumption to serum uric acid concentrations in 2240 male self-defence officials who received a pre-retirement health examination at four hospitals of the Self-Defence Forces between 1993 and 1994. The mean levels of coffee and green tea consumption were 2·3 and 3·1 cups/d respectively. There was a clear inverse relationship between coffee consumption and serum uric acid concentration. When adjusted for hospital only, those consuming less than one cup of coffee daily had a mean serum uric acid concentration of 60 mg/l, while that of those drinking five or more cups of coffee daily was 56 mg/l (P < 0·0001). No such relationship was observed for green tea, another major dietary source of caffeine in Japan. The relationship between coffee consumption and serum uric acid concentration was independent of age, rank in the Self-Defence Forces, BMI, systolic blood pressure, serum creatinine, serum total cholesterol and serum HDL-cholesterol concentrations, smoking status, alcohol use, beer consumption and intake of dairy products. These findings suggest that coffee drinking may be associated with lower concentrations of serum uric acid, and further studies are needed to confirm the association.

Effect of ethanol on metabolism of purine bases (hypoxanthine, xanthine, and uric acid)

There are many factors that contribute to hyperuricemia, including obesity, insulin resistance, alcohol consumption, diuretic use, hypertension, renal insufficiency, genetic makeup, etc. Of these, alcohol (ethanol) is the most important. Ethanol enhances adenine nucleotide degradation and increases lactic acid level in blood, leading to hyperuricemia. In beer, purines also contribute to an increase in plasma uric acid. Although rare, dehydration and ketoacidosis (due to ethanol ingestion) are associated with the ethanol-induced increase in serum uric acid levels. Ethanol also increases the plasma concentrations and urinary excretion of hypoxanthine and xanthine via the acceleration of adenine nucleotide degradation and a possible weak inhibition of xanthine dehydrogenase activity. Since many factors such as the ALDH2*1 gene and ADH2*2 gene, daily drinking habits, exercise, and dehydration enhance the increase in plasma concentration of uric acid induced by ethanol, it is important to pay attention to these factors, as well as ingested ethanol volume, type of alcoholic beverage, and the administration of anti-hyperuricemic agents, to prevent and treat ethanol-induced hyperuricemia.

Two male siblings with hereditary renal hypouricemia and exercise-induced ARF

Familial renal hypouricemia with exercise-induced acute renal failure (ARF) is rare. A 45-year-old man presented with abdominal pain, vomiting, and oliguria after severe exercise. The diagnosis was ARF based on high serum creatinine (SCr) level (5.1 mg/dL [451 micromol/L]). Renal function recovered completely within 2 weeks of conservative treatment (creatinine clearance [Ccr], 100.4 mL/min [1.67 mL/s]). After remission, laboratory results showed serum urate (SUA) of 0.8 mg/dL (48 micromol/L), and fractional excretion of uric acid (FE(UA)) of 46%. The final diagnosis was ARF associated with idiopathic renal hypouricemia. Other diseases that could increase the excretion of urate were excluded. Because only mild responses were observed both in pyradinamide and benzbromarone loading tests, he was considered to be a presecretory reabsorption disorder type. The younger brother (42 years old) also had episodes of low and middle back pain after severe exercise and experienced similar attacks at least 5 times since the age of 29. SCr level was elevated in every attack. Hypouricemia (SUA, 1.0 mg/dL [59 micromol/L]) and high urinary urate excretion (FE(UA), 65.7%) also were detected. Renal function recovered almost completely without any specific treatment. Radiologic examination of the 2 cases showed bilateral urolithiasis probably caused by the high urinary urate excretion. Sequence analysis of a urate anion exchanger known to regulate blood urate level (URAT1 gene) in both brothers showed homozygous mutation in exon 4 (W258Stop), resulting in a premature truncated URAT1 protein. Both their parents and their children showed heterozygous mutation of the URAT1 gene. This is the first report of the 2 male siblings of familial renal hypouricemia complicated with exercise-induced ARF, with definite demonstration of genetic abnormality in the responsible gene (URAT1).

Uric acid transport

PURPOSE OF REVIEW

The goal of this article is to review the physiology and describe newly defined molecular mechanisms that are responsible for renal urate transport.

RECENT FINDINGS

Four complementary DNAs have recently been cloned whose expressed proteins transport urate. Two of these proteins have been localized to the apical membrane of proximal tubular cells: one, a urate transporter/channel, a galectin, is an electrogenic transporter (an ion channel); the second is a urate-anion electroneutral exchanger, a member of the organic anion transporter family. The other urate transport proteins, organic anion transporters 1 and 3, are also members of the organic anion transporter family. These proteins have been localized to the basolateral membrane of proximal tubular cells: organic anion transporter 1 is an electroneutral organic anion exchanger; the mechanism of urate transport on organic anion transporter 3 remains to be determined.

SUMMARY

The molecular definition and localization of four urate transport proteins provides a basis for developing a molecular model of the bi-directional transport of urate in renal proximal tubules. It seems likely that the urate-anion exchanger is responsible for luminal reabsorption while the urate transporter/channel permits secretion of urate from the cell into the lumen. Since organic anion transporters 1 and 3 reside in the basolateral membrane, one or both may be relevant in the reabsorptive flux of urate into the peritubular capillary as well as in the cellular uptake of urate from the peritubular space, the first step in the process of urate secretion. Knowledge of the molecular basis of urate transport should provide greater insights into states of altered transport as well as assist in development of drugs to modify urate flux.

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.

Urate homeostasis in polycystic kidney disease: comparison with chronic glomerulonephritic kidney

Autosomal dominant polycystic kidney disease (ADPKD) might affect urate homeostasis and clearance. Renal tubular urate transport was studied by means of probenecid (PB) and pyrazinamide (PZA) tests in individuals with ADPKD and normal renal function as well as various degrees of renal failure (49 patients). Comparisons were made between polycystic and chronic glomerulonephritic kidney (CGNK), as well as with controls (men with normal renal function). Patients with ADPKD and normal renal function showed plasma urate levels within normal range and normal renal urate handling. In contrast higher plasma urate levels comparing to controls were found in patients with CGNK and normal renal function. During the evolution of renal failure ADPKD patients showed lower urate plasma levels and higher renal clearance as well as, fractional urate excretion, comparing to CGNK patients with the same degree of renal failure. In conclusion patients with ADPKD and normal renal function have normal urate handling and plasma urate levels within normal range. With increasing severity of disease and during evolution of renal failure CGNK patients showed higher urate plasma levels and lower clearances comparing to ADPKD patients. When renal disease becomes more advanced there was no difference in renal urate handling between ADPKD and CGNK patients.

Abnormal serum uric acid levels in children

Measurement of the serum uric acid level, most commonly considered in adult patients, is frequently obtained inadvertently for pediatric patients because it is a standard component of many multichannel chemistry profiles offered by clinical laboratories. Most standard references for normal uric acid values do not take into account the impact of the metabolic changes in children at different ages on the uric acid level. A substantial number of childhood conditions may produce perturbations in the serum uric acid level. Knowledge of normal serum uric acid levels and of the conditions affecting those levels in children enables a more focused pursuit of underlying abnormalities.

Renal hypouricemia: prevention of exercise-induced acute renal failure and a review of the literature

Isolated renal hypouricemia from defective uric acid reabsorption and/or secretion is a well-described entity, with a prevalence of 0.12% to 0.20% in Japan. It is rarely associated with exercise-induced acute renal failure (ARF). The etiology of ARF is debated. Prevention of ARF in renal hypouricemia has not been previously addressed. A 29-year-old Pakistani man had recurrent exercise-induced ARF. He was found to have isolated renal hypouricemia; serum uric acid 0.5 mg/dL, 24-hour urine uric acid 472 +/- 25 mg (+/- SD), and fractional excretion of uric acid 55.2% to 69.4%. Both pyrazinamide and probenecid decreased fractional excretion of uric acid and uric acid excretion rate (UV(Urate)) in our patient, suggesting either a partial presecretory and postsecretory reabsorption defect or increased secretion. We investigated renal uric acid excretion during exercise in our patient and four control subjects. All five subjects underwent a physical fitness test (PFT). Our patient developed ARF. Uric acid excretion rate increased in our patient, from 0.48 mg/min at baseline to 1.49 mg/min 4 hours after the PFT, as did the urine uric acid to urine creatinine ratio (UUa)/UCr) (0.29 to 1.49). In the controls, UV(Urate) and UUA/UCr were unchanged after the PFT: UV(Urate) was 0.46 +/- 0.10 mg/min at baseline and 0.59 +/- 0.04 mg/min 4 hours after the PFT, while UUA/UCr was 0.30 +/- 0.04 at baseline and 0.36 +/- 0.04 at 4 hours. All five subjects took allopurinol 300 mg daily for 5 days and repeated the PFT. In our patient, allopurinol prevented the ARF as well as the exercise-induced increases in UV(Urate) (0.28 mg/min to 0.22 mg/min) and UUA/UCr (0.25 to 0.17). In the controls, the UV(Urate) and UUA/UCr responses to exercise were not altered. We conclude that increased renal excretion of uric acid during exercise was responsible for the ARF in our patient with renal hypouricemia and that successful prophylaxis with allopurinol is possible.

Hyperuricemia in patients with hyperthyroidism due to Graves' disease

The effects of hyperthyroidism on uric acid metabolism were investigated. First, the serum uric acid level was measured in 92 patients with hyperthyroidism due to Graves' disease, eight patients with subacute thyroiditis, six patients with hypothyroidism, and 70 sex- and age-matched controls. Second, the correlation between serum thyroxine (T4) and serum uric acid was obtained in hyperthyroid Graves' disease patients before and during antithyroid drug therapy. Finally, uric acid clearance (CUA) was determined in untreated patients with hyperthyroidism due to Graves' disease. Serum uric acid was significantly elevated in patients with hyperthyroidism, and the elevation correlated well with serum T4 before treatment as a group and during treatment in each patient. A significant elevation of serum uric acid was not present in patients with a transient mild thyrotoxicosis due to subacute thyroiditis. Serum uric acid was significantly decreased in patients with hypothyroidism. Renal excretion of uric acid clearly increased in hyperthyroid patients, and CUA also increased. The increase in CUA corresponded to the increase in renal plasma flow (RPF), which was measured by p-aminohippuric acid clearance. The fractional excretion of uric acid as determined by CUA/glomerular filtration rate (GFR) was similar and within the normal range in hyperthyroid patients and normal controls. A significant inverse correlation between CUA and serum uric acid concentration was present in hyperthyroid patients as in normal controls, indicating that the renal handling of uric acid in the tubule affected uric acid excretion.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of BOF-4272 on the oxidation of allopurinol and pyrazinamide in vivo. Is xanthine dehydrogenase or aldehyde oxidase more important in oxidizing both allopurinol and pyrazinamide?

Allopurinol or pyrazinamide was administered to rats treated with BOF-4272 (a potent xanthine oxidase inhibitor) to investigate to what degree xanthine dehydrogenase participates in the oxidation of these agents. BOF-4272 markedly decreased the plasma concentration and the urinary excretion of both oxypurinol and 5-hydroxypyrazinamide. It also decreased the sum of the urinary excretion of allopurinol and oxypurinol and that of pyrazinamide and its metabolites, although it did not affect the sum of the plasma concentrations of allopurinol and oxypurinol at 105 min after administration of allopurinol or the plasma concentration of pyrazinamide during the period after the administration of pyrazinamide. These results suggested that BOF-4272 almost completely inhibited the oxidation of allopurinol and pyrazinamide and had some effect on the excretion and/or the tissue incorporation of these two compounds. Since the in vitro study demonstrated that BOF-4272 did not inhibit the activity of aldehyde oxidase, which oxidized both allopurinol to oxypurinol and pyrazinamide to 5-hydroxypyrazinamide, the results suggested that xanthine dehydrogenase was the more important enzyme in converting allopurinol to oxypurinol and pyrazinamide to 5-hydroxypyrazinamide.

Quantitative analysis of drug handling by the kidney using a physiological model of renal drug clearance

Published data on the renal clearance of creatinine, p-aminohippuric acid (PAH) and kanamycin in relation to glomerular filtration rate (GFR) in patients with various renal diseases were analysed by a physiological model of renal clearance. Fitting of the data by the general linear equation representing the model proposed by Levy [10] resulted in insignificant intercepts with the ordinate, indicating the unsuitability of the model for the detection of tubular secretory activity. Use of this model also did not lead to significant improvement in goodness of fit compared to simple proportionality of renal clearance and GFR. On the other hand, parameter estimates of the physiological model obtained from the data by nonlinear regression analysis revealed statistically significant tubular secretion both of PAH and creatinine. The much lower tubular secretory activity estimated from the kanamycin data did not reach statistical significance. For compounds exhibiting statistically significant tubular secretion, use of the physiologically based relationship between renal clearance and GFR significantly improved the goodness of fit to the data as compared to simple proportionality of both variables. It is concluded that analysis of the relationship between renal clearance of drugs and GFR using the physiological model of renal clearance can contribute to our knowledge of drug handling by the kidney, and may facilitate drug classification according to total extraction by this organ.

Familial hypouricaemia due to an isolated tubular defect of urate reabsorption

Hypouricaemia due to an isolated renal tubular defect is a rare condition. Several members from an affected family are described. The propositus is a 12-year-old girl with hypouricaemia (0.7-1.1 mg/dl) and increased fractional excretion of uric acid (50%). Pyrazinamide and sulphinpyrazone tests revealed an attenuated response in this subject to both drugs. The mother and one of the propositus' two brothers have the same defect. The other brother has uric acid levels at the lower limit of normal and increased fractional excretion of uric acid. These results suggest familial hypouricaemia due to a pre-secretory reabsorptive tubular defect, transmitted by autosomal dominant inheritance.

Single kidney function: early and late changes in urate transport after nephrectomy

Renal urate transport was studied by means of pyrazinamide (PZA) and probenecid (PB): (a) before and at 2, 6, 24 weeks (24 patients), (b) 1 to 30 years after uninephrectomy in 27 and 12 patients with Ccr greater than 80 and 30 to 70 ml/min, respectively. Uninephrectomy was followed by important tubular urate transport modifications during at least two weeks, which lead to a marked uricosuria as indicated by significant increase in FEur (mean value +/- SD, 0.228 +/- 0.059 vs. 0.097 +/- 0.014 and 0.099 +/- 0.019 in normals and chronically diseased solitary kidneys). Reduced response to PZA and PB suggests a diminished reabsorptive capacity for urate mainly at the presecretory site which persisted after FENa normalization. Tubular compensations were presumably complete at six weeks, since pattern of urate transport returned to normal with an almost complete reabsorption of filtered urate load (99%) and a percentage of postsecretory reabsorption (80%) very close to those seen in normal subjects with a pair of kidneys. The adjustment in urate excretion in solitary kidneys was achieved by a significant increase of secreted urate as compared with 50% of pre-uninephrectomy values. Thus, increased urate secretion by the remaining intact organ is sufficient to maintain urate balance with a normal serum level.

Drug-induced gout

A number of pharmacological agents can induce hyperuricaemia, and sometimes gout, usually by interfering with the renal tubular excretion of urate but also in some instances by increasing the formation of uric acid. Alcohol is well known to have this property and in recent years diuretic-induced hyperuricaemia has become a global phenomenon. Other drugs which can cause hyperuricaemia are salicylates, pyrazinamide, ethambutol, nicotinic acid, cyclosporin, 2-ethylamino-1,3,4-thiadiazole, fructose and cytotoxic agents. A special type of 'drug-induced gout' can follow the rapid lowering of serum uric acid by allopurinol or uricosuric drugs.

Uric acid handling in autosomal dominant polycystic kidney disease with normal filtration rates

PURPOSE

Patients with autosomal dominant polycystic kidney disease (ADPKD) are alleged to have more frequent or more pronounced alterations of uric acid homeostasis than are seen in most other types of chronic renal diseases. We performed this study to examine the hypothesis that individuals with ADPKD have abnormal uric acid homeostasis that is manifest before the development of renal insufficiency.

PATIENTS AND METHODS

We studied 301 subjects, 163 with ADPKD and 138 relatives without ADPKD (NADPKD), by ultrasonography. The subjects were interviewed and examined. Venous blood and two 24-hour urine collections were obtained for uric acid and creatinine determinations.

RESULTS

Presence of hyperuricemia, serum uric acid levels, uric acid clearance, and fractional excretion of uric acid did not differ between ADPKD and NADPKD subjects with normal renal function (creatinine clearance greater than 80 mL/minute/1.73 m2). Clearance of uric acid decreased and fractional excretion increased in subjects with decreased renal function in both groups. Female gender enhanced renal excretion of uric acid in both groups and hypertension depressed it except in men with ADPKD, who had higher fractional excretions of uric acid than did hypertensive NADPKD men.

CONCLUSIONS

Uric acid homeostasis is preserved in individuals with ADPKD with normal renal function when compared to unaffected family members. Hyperuricemia and decreased renal excretion of uric acid develop as renal function worsens in ADPKD, similar to that in control subjects. The expected depressing effect of hypertension on renal handling of uric acid was not seen in men with ADPKD, speculatively due to an effect of atrial natriuretic factor.

Uric acid metabolism in children

The renal excretion of uric acid in children differs quantitatively, and perhaps qualitatively, from that in adult humans. The younger the child, the greater the renal clearance of uric acid and the greater the excretion of uric acid expressed as mg per kg body weight. During infancy, the reduced ability to maximally concentrate the urine may protect against precipitation of uric acid crystals within the kidney. Conversely, the extremely high urinary uric concentrations places the very small infant at jeopardy during sudden increases in the filtered load of uric acid. Understanding the pharmacologic and physiologic modulators of renal uric acid clearance will allow the pediatrician to minimize the risk of uric acid nephropathy, and to understand the implications of uric acid in the serum or urine in children with fluid and electrolyte disorders. Certainly evaluation of serum and urinary uric acid concentrations is essential in any child with acute renal failure.

Hypouricemia in acquired immunodeficiency syndrome

Clinical evaluations of hypouricemia in patients with the acquired immunodeficiency syndrome (AIDS) have shown that it is a common disorder resulting from defective renal handling of uric acid. We prospectively studied renal urate handling in 23 patients and reviewed the records of 73 consecutive patients with AIDS or AIDS-related complex (ARC), who were seen in our AIDS clinic between March 1985 and April 1988, to determine the incidence, significance, and, when possible, the cause of hypouricemia. Hypouricemia was defined as serum urate less than or equal to 0.18 mmol/L (3 mg/dL). Renal clearance studies were performed in 23 patients, 10 hypouricemic and 13 nonhypouricemic. Eight patients (six with hypouricemia) underwent central venous pressure (CVP) monitoring, which was performed for clinical signs and symptoms of extracellular volume depletion. Fourteen (eight with hypouricemia) had daily urine urate measured. Hypouricemia was found in 21 (21.9%) of 96 patients. It was more common in females and intravenous (IV) drug abusers, and was associated with more opportunistic illnesses, particularly mycobacterium avium intracellulare (MAI) and cytomegalovirus (CMV) infections. Hypouricemia occurred in three patients with ARC and 18 patients with AIDS and was associated with cerebral atrophy in all 12 hypouricemic and 14 of 28 nonhypouricemic patients who had cranial computed tomography (CT) scans. During a comparable follow-up period, 71.4% of the hypouricemic as compared with 38.7% of nonhypouricemic patients died. Eleven developed hypouricemia as outpatients. Fractional excretion of uric acid (FEua) was elevated in the eight patients with CVP less than 1 cm of water, and in 10 of 10 with and nine of 13 without hypouricemia, despite CVP less than 1 cm water in eight.(ABSTRACT TRUNCATED AT 250 WORDS)

Co-occurrence of hypercalciuria and hypouricaemia in type 2 diabetic patients

The relationship between the urinary excretion of calcium (Ca2+) and uric acid was investigated in 151 Type 2 diabetic patients and 48 normal subjects. In the diabetic patients, uric acid clearance/creatinine clearance (Clurate/Clcr) was higher and the serum level of uric acid was lower than in the normal subjects (Clurate/Clcr: 10.9 +/- 5.8 vs 8.1 +/- 2.6%, p less than 0.001; serum uric acid: 3.4 +/- 86 vs 357 +/- 89 mumol l-1, p less than 0.001). Calcium clearance/Clcr (Clca/Clcr) also increased in the diabetic patients, as did urinary excretion rate, but the serum Ca2+ level was not different to normal control subjects (Clca/Clcr: 2.29 +/- 1.59 vs 1.56 +/- 0.98%, p less than 0.001; Ca2+ excretion rate: 2.24 +/- 1.67 vs 1.63 +/- 1.11 mmol day-1, p less than 0.01; serum Ca2+ level: 2.34 +/- 0.11 vs 2.33 +/- 0.08 mmol l-1). In the diabetic patients, Clcr positively and the serum uric acid negatively correlated with the urinary excretion of Ca2+ (p less than 0.001 for both correlations in the multivariate regression analysis). These data suggest that the diabetic patients have increased fractional excretion of both Ca2+ and uric acid.

Acute renal failure due to uric acid nephropathy in a patient with renal hypouricemia

This report is about a 23-year-old man who required hemodialysis in connection with an acute renal failure resulting from uric acid nephropathy without hyperuricemia. After recovering renal function he showed extreme hypouricemia (0.1-0.3 mg/dl) and elevated uric acid clearance (100-300 ml/min). The fractional excretion of uric acid (Cua/Ccr) could be suppressed by oral pyrazinamide and enhanced by probenecid. As no other renal tubular or metabolic abnormalities were detected, it is suggested that a markedly increased renal tubular urate secretion was responsible for the hypouricemia and also for the rare side-effect of an uric acid nephropathy in this patient.

Renal handling of uric acid in gout: impaired tubular transport of urate not dependent on serum urate levels

Metabolic studies were conducted in 56 patients with primary gout and in ten normal subjects to assess differences in the tubular transport mechanisms of urate. Renal handling of uric acid was examined by means of pyrazinamide and probenecid tests at increased and pharmacologically reduced serum urate concentrations in both groups. Patients with gout showed similar serum urate levels and glomerular filtration rates than controls at both serum urate levels. Pyrazinamide decreased urinary uric acid excretion to less than 1.0% of the urate filtered load in both groups at increased and diminished serum urate concentrations. The maximum uricosuric response promoted by probenecid at high serum urate levels was (mean +/- SD) 3,707 +/- 443 micrograms/min/1.73 m2 in controls and 2,215 +/- 738 micrograms/min/1.73 m2 in patients with gout (P less than 0.01). Forty-four patients had a daily uric acid excretion rate below 700 mg/1.73 m2, and all of them showed a diminished uricosuric response to probenecid. When serum urate was reduced in normal subjects and 30 patients to a mean of 2.1 and 2.3 mg/dL, respectively, probenecid elicited a significantly lower urate excretion rate in gout (532 +/- 202 micrograms/min/1.73 m2) than in controls (922 +/- 136 micrograms/min/1.73 m2; P less than 0.01). Among these 30 patients examined in their basal state and at decreased serum urate levels, uric acid excretion following probenecid was normal in six and diminished in 24 in both situations. The difference between maximum uricosuria and basal urate excretion was not increased in gouty patients.(ABSTRACT TRUNCATED AT 250 WORDS)

Sex differences in uric acid metabolism in adults: evidence for a lack of influence of estradiol-17 beta (E2) on the renal handling of urate

The serum urate concentration of adult women, which is lower than in men of a similar age, is thought to be related to a higher renal clearance of urate in women, possibly due to their higher plasma estrogen levels. Intersexual differences in the renal handling of uric acid was assessed in 9 normal adult women and 9 normal age-matched men. Women showed a significantly lower serum urate concentration as compared to men (3.5 +/- 0.3 v 4.9 +/- 0.7 mg/dL, P less than 0.001), higher fractional excretion of urate (9.8 +/- 1.0 v 7.3 +/- 0.8%, P less than 0.001), and significantly lower tubular urate postsecretory reabsorption (67.2 +/- 1.6 v 76.6 +/- 1.4% of secreted urate, P less than 0.01). To test whether plasma E2 has a uricosuric effect we administered estradiol valerate and estradiol benzoate to either oophorectomized or adult women. Plasma E2 levels and urinary total estrogen excretion increased significantly in both groups but the treatment failed to significantly modify serum urate or the fractional excretion of uric acid. Furthermore, in 4 normal adult women, the tubular phases that modulate the renal excretion of urate were not significantly influenced by increased plasma E2 levels. We conclude that in comparison to men of a similar age, the lower tubular urate postsecretory reabsorption of adult women is in accordance with the intersexual differences in uric acid metabolism. Plasma E2 does not influence renal handling of uric acid or serum urate levels.

Renal hypouricaemia in a patient with 48,XXYY syndrome

Studies on hypouricaemia observed in a patient with 48,XXYY syndrome revealed an abnormality in renal urate handling. His renal urate clearance was abnormally increased. Inosine administration and provocative tests using probenecid and pyrazinamide identified an isolated renal tubular abnormality with increased urate secretion. Since the serum urate in his brother with a normal sex chromosome constitution was also low, the association of renal hypouricaemia and 48,XXYY syndrome in this patient is probably coincidental. Although the brother was not investigated, these siblings may be a previously unreported case of familial hypouricaemia due to isolated renal hypersecretion.

Hypouricemia and renal tubular acidosis in primary biliary cirrhosis

A 51-year-old woman with primary biliary cirrhosis developed distal renal tubular acidosis and hypouricemia (1.4 mg per 100 ml) in the course of hepatic injury. Her renal clearance of uric acid reached 33.7 ml/min (mean +/- SD in five normal age-matched women: 9.2 +/- 3.1 ml/min). Pyrazinamide, an inhibitor of uric acid secretion, considerably reduced the uric acid clearance, while it was not enhanced by probenecid, a blocker of uric acid reabsorption. Thus, the hypouricemia may have been due to a defect of postsecretory reabsorption of uric acid in the renal tubules. The present case emphasizes the significance of hypouricemia and hyperuricosuria as indicators of renal tubular injury in primary biliary cirrhosis.

Paradoxical effects of pyrazinoate and nicotinate on urate transport in dog renal microvillus membranes

The effects of pyrazinoate and nicotinate on urate transport in microvillus membrane vesicles isolated from canine renal cortex were evaluated. An outwardly directed gradient of pyrazinoate stimulated uphill urate accumulation, suggesting urate-pyrazinoate exchange. An inside-alkaline pH gradient stimulated uphill pyrazinoate accumulation, which suggested pyrazinoate-OH- exchange. Pyrazinoate-OH- exchange and urate-OH- exchange were similarly sensitive to inhibitors, implying that both processes occur via the same transport system. In addition, an inward Na+ gradient stimulated uphill pyrazinoate accumulation, suggesting Na+-pyrazinoate cotransport. Inhibitor studies demonstrated that Na+-pyrazinoate cotransport takes place via the same pathway that mediates Na+-lactate cotransport in these membrane vesicles. Previously we found that urate does not share this Na+-dependent cotransport pathway. Nicotinate inhibited transport of pyrazinoate by the anion exchange pathway and the Na+ cotransport pathway, suggesting that it is a substrate for both transport systems. Finally, in the presence of an inward Na+ gradient, low doses of pyrazinoate or nicotinate stimulated urate uptake, and higher doses of pyrazinoate or nicotinate inhibited urate accumulation, thereby mimicking in vitro the paradoxical effects of drugs on renal urate excretion that have been observed in vivo. These findings indicate that the paradoxical effect of uricosuric drugs at low doses to cause urate retention may result at least in part from stimulation of urate reabsorption across the luminal membrane of the proximal tubular cell.

Hereditary renal hypouricemia in children

The renal handling of urate was investigated in four children with hereditary renal hypouricemia and in their parents. The urate/creatinine clearance ratios in the four patients were 1.02 +/- 0.28, 0.93 +/- 0.11, 1.03 +/- 0.24, and 1.46 +/- 0.26, markedly higher than those in control subjects. Except for a partial response to pyrazinamide (change in clearance ratio from 1.46 to 1.07) in one patient, pyrazinamide and benzbromarone did not affect the clearance ratios in our patients. In the parents, the urate/creatinine clearance ratios were intermediate between those of the patients and control subjects, but responses to pyrazinamide and benzbromarone were normal. These data indicate that our patients have a combined defect in renal urate reabsorption, and that one of them might be subclassified as having the hypersecretion of defect. Results also show that heterozygotes can be identified by testing their urate/creatinine clearance ratio.

Glomerular function and hyperuricaemia in sickle cell disease

Renal insufficiency is common in adults with homozygous sickle cell disease, and the contribution of glomerular failure to the hyperuricaemia which is often a feature of the disease has therefore been investigated. In a study of 64 patients between the ages of 15 and 66, serum urate concentration was dependent on renal urate clearance and also on creatinine clearance. The relation between serum urate and creatinine clearance was abnormal in patients with sickle cell disease and it is suggested that this might be caused by high single nephron glomerular filtration rates. Both the amount of urate excreted per millilitre of glomerular filtrate and the fractional excretion of urate increased with falling creatinine clearance, suggesting that the ability to increase tubular urate secretion was preserved. Patients with extensive tubular disease as shown by tubular proteinuria had serum urate concentrations which were not significantly different from those of age and sex matched non-proteinuric patients. Evidence that renal tubular disease interferes with urate secretion and causes hyperuricaemia in patients with sickle cell disease needs to be reinterpreted in the light of these findings.

Hypouricemia in cirrhosis reflects hemodynamic alterations

In a population of 27 consecutive patients with liver cirrhosis, systemic hemodynamics were investigated and correlated to uric acid concentrations, fractional uric acid excretion, and creatinine clearances. Mean serum uric acid concentration was lower than in normal controls, and this was related to abnormally high uric acid clearances. Uric acid concentrations correlated positively to total peripheral resistances and negatively to cardiac output. Fractional uric acid excretions correlated negatively to total peripheral resistances and positively to cardiac output. There was no correlation between creatinine clearances and any variable of systemic hemodynamics. Serum uric acid concentration and fractional uric acid excretion are dependent of the hemodynamic state in cirrhosis.

Therapeutics of hyperuricaemia and gout

Environmental factors, including diet, fluid intake and alcohol consumption, often contribute significantly to the development of hyperuricaemia. The contribution of these factors, together with body size and the renal excretory capacity for urate, can be assessed on the basis of the clinical history and examination, together with simple investigations. These include the measurement of both serum levels of urate and the urinary excretion of urate, and the effect of purine restriction on these measurements. Recognition of causative factors provides the potential for their correction. Should gout develop, serum urate levels must be reduced to normal. This usually, but not invariably, necessitates prolonged drug treatment. The major problem in maintaining serum urate levels within the normal range is that this depends upon the cooperation of the patient. Acute gout needs to be managed on its own merits, irrespective of the use of drugs which alter the serum urate concentration. Prophylactic therapy with colchicine reduces the frequency of acute attacks of gout, whether the serum urate level is normal or elevated.

Hypouricemia and hyperuricosuria as expressions of renal tubular damage in primary biliary cirrhosis

Renal tubular damage, in particular, renal tubular acidosis is associated with primary biliary cirrhosis (PBC), but hypouricemia has not been described. We studied four patients with PBC whose serum uric acid levels were 1.4 to 1.8 mg per dl, and compared their renal and liver functions with those of 11 patients with PBC whose serum uric acid levels were normal. In the patients with PBC and hypouricemia, uric acid clearance (Cua) and the ratio of Cua and creatinine clearance (Cua/Ccr) were high enough to cause hypouricemia. Elevated Cua/Ccr was suppressed by administration of pyrazinamide, a blocker of tubular secretion of uric acid, but was not affected by probenecid; the effects of drugs on Cua/Ccr were similar to those reported in Wilson's disease. Elevation in Cua/Ccr was associated with increased serum bilirubin and urinary copper excretion. These observations indicate that hypouricemia and hyperuricosuria, which may be caused by defective postsecretory reabsorption of uric acid, are additional indicators of renal tubular damage in PBC.

Renal uric acid clearance in human neonates

Renal clearance of uric acid was examined in 40 premature and term infants during the first 24 hours of life. Creatinine clearance and uric acid clearance increased with increasing gestational age. Serum uric acid concentration (r = -0.31, P less than 0.05) and fractional excretion of uric acid (r = -0.50, P less than 0.01) were inversely related to gestational age; FEUA was nearly 70% at 29 to 31 weeks gestational age, and decreased to a mean value of 39 +/- 14% (+/- SD) at 38 to 40 weeks gestational age. The decline in FEUA with advancing gestational age appears to represent alterations in net renal tubular transport of uric acid, because the filtered load of uric acid increased with gestational age. Postnatal clearance studies were performed in 18 premature infants of 29 to 35 weeks gestational age; FEUA also declined during early postnatal development (r = -0.44, P less than 0.01). In all studies, a relationship between FEUA and fractional excretion of sodium was observed. The high basal excretion of uric acid in infants may increase the risk of acute uric acid nephropathy when excretion demands are increased.

Gout and hyperuricemia associated with sickle-cell anemia

Since the initial description, in 1958, of gouty arthritis occurring in association with SCA, more than 12 cases have been reported. The high proportion of women and the relatively young ages are noteworthy. Since 1968, studies of patients with SCA have shown a high prevalence of hyperuricemia, beginning during childhood. The initial event in the development of hyperuricemia presumably is increased synthesis of nucleic acids occurring as part of the erythropoietic response to hemolysis. Catabolism of the nucleic acids generates urate. Increased production of UA normally is compensated for by increased urinary excretion of UA. This response occurs in patients with SCA, but during the third decade of life hyperuricosuria can be reduced, probably by damage to the renal tubules caused by infarction and hypoxia resulting from sickling. Impairment of the compensatory renal response leads to more severe and sustained hyperuricemia, and gouty arthritis may then develop. A number of questions about hyperuricemia and gout in SCA remain unanswered. The prevalence of gout among patients with SCA, both in general and in relation to age and sex, has not been determined. The relationships between specific aspects of SCA and of hyperuricemia and gout need to be determined. These include any effect of sickle cell crises on SUA and attacks of gout, and correlation of abnormalities in renal handling of urate with other indices of tubular function and with the pathologic anatomy of the kidney. Finally, it is important to learn whether hyperuricemia and hyperuricosuria contribute to the renal manifestations of SCA; if so, allopurinol might be useful in the prevention and treatment of the renal disease.

Lack of effect of salt intake on urinary uric acid excretion

Although it has been established that acute expansion of the extracellular fluid volume results in enhanced uric acid clearance, the effect of chronic volume expansion by a high salt diet on urinary uric acid excretion has not been examined. Eleven normal subjects were placed on a constant diet containing 10 mEq. sodium per day for 10 days, followed by 240 mEq. sodium daily for another 10 days. Measurements were performed on the final 3 days of each phase. Urinary sodium increased from 9 plus or minus 3 standard error to 221 plus or minus 9 mEq. per day (p less than 0.001), and uric acid clearance increased from 5.9 plus or minus 0.4 to 7.1 plus or minus 0.6 ml. per minute (p less than 0.01). However, serum uric acid decreased from 6.4 plus or minus 0.4 to 5.5 plus or minus 0.3 mg./dl. (p less than 0.001). Total urinary excretion of uric acid did not change (533 plus or minus 24 to 535 plus or minus 26 mg. per day). A high salt diet does not result in sustained hyperuricosuria, although it may predispose to kidney stone formation in other ways.

Allopurinol treatment and its effect on renal function in gout: a controlled study

Fifty-nine patients with primary gout were treated with either a combination of colchicine and allopurinol or colchicine alone. Assessments of renal function over 2 years revealed a statistically significant fall of glomerular filtration rate an urine concentrating ability in those receiving only colchicine. The renal function of patients given allopurinol did not change. Treatment with allopurinol resulted ina significant reduction of ammonium excretion, a phenomenon which could not be readily explained. Urate clearance also declined during allopurinol treatment, and the impaired urate clearance associated with gout became more evident. The most important observation was that allopurinol retarded an apparent decline of renal function. Presumably this was achieved through its hypouricaemic effect and implies that the hyperuricaemia of gouty patients is deleterious to the kidneys.