Renal excretion of uric acid in the rat: a micropuncture and microperfusion study

Lack of xanthine dehydrogenase leads to a remarkable renal decline in a novel hypouricemic rat model

Uric acid (UA) is the final metabolite in purine catabolism in humans. Previous studies have shown that the dysregulation of UA homeostasis is detrimental to cardiovascular and kidney health. The Xdh gene encodes for the Xanthine Oxidoreductase enzyme group, responsible for producing UA. To explore how hypouricemia can lead to kidney damage, we created a rat model with the genetic ablation of the Xdh gene on the Dahl salt-sensitive rat background (SS^Xdh−/−). SS^Xdh−/− rats lacked UA and exhibited impairment in growth and survival. This model showed severe kidney injury with increased interstitial fibrosis, glomerular damage, crystal formation, and an inability to control electrolyte balance. Using a multi-omics approach, we highlighted that lack of Xdh leads to increased oxidative stress, renal cell proliferation, and inflammation. Our data reveal that the absence of Xdh leads to kidney damage and functional decline by the accumulation of purine metabolites in the kidney and increased oxidative stress.

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A novel rat model of hypouricemia was created by the gene ablation of the Xdh gene

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The SS^Xdh-/- rat showed a failure to thrive, kidney injury, and functional decline

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Multi-omics revealed increased inflammation and oxidative stress in SS^Xdh-/- rats

Pathophysiology of Gout

Multiple interacting checkpoints are involved in the pathophysiology of gout. Hyperuricemia is the key risk factor for gout and is considered a prerequisite for monosodium urate (MSU) crystal formation. Urate underexcretion through renal and gut mechanisms is the major mechanism for hyperuricemia in most people. Multiple genetic, environmental, and metabolic factors are associated with serum urate and alter urate transport or synthesis. Urate supersaturation is the most important factor for MSU crystal formation, and other factors such as temperature, pH, and connective tissue components also play a role. The nucleotide-binding oligomerization domain leucine-rich repeats and pyrin domain-containing protein 3 inflammasome plays a pivotal role in the inflammatory response to MSU crystals, and interleukin 1β is the key cytokine mediating the inflammatory cascade. Variations in the regulatory mechanisms of this inflammatory response may affect an individual's susceptibility to developing gout. Tophus formation is the cardinal feature of advanced gout, and both MSU crystals and the inflammatory tissue component of the tophus contribute to the development of structural joint damage owing to gout. In this article, we review the pathophysiologic mechanisms of hyperuricemia, MSU crystal formation and the associated inflammatory response, tophus formation, and structural joint damage in gout.

A model of uric acid transport in the rat proximal tubule

The objective of the present study was to theoretically investigate the mechanisms underlying uric acid transport in the proximal tubule (PT) of rat kidneys, and their modulation by factors, including Na+, parathyroid hormone, ANG II, and Na+-glucose cotransporter-2 inhibitors. To that end, we incorporated the transport of uric acid and its conjugate anion urate in our mathematical model of water and solute transport in the rat PT. The model accounts for parallel urate reabsorption and secretion pathways on apical and basolateral membranes and their coupling to lactate and α-ketoglutarate transport. Model results agree with experimental findings at the segment level. Net reabsorption of urate by the rat PT is predicted to be ~70% of the filtered load, with a rate of urate removal from the lumen that is 50% higher than the rate of urate secretion. The model suggests that apical URAT1 deletion significantly reduces net urate reabsorption across the PT, whereas ATP-binding cassette subfamily G member 2 dysfunction affects it only slightly. Inactivation of basolateral glucose transporter-9 raises fractional urate excretion above 100%, as observed in patients with renal familial hypouricemia. Furthermore, our results suggest that reducing Na+ reabsorption across Na+/H+ exchangers or Na+-glucose cotransporters augments net urate reabsorption. The model predicts that parathyroid hormone reduces urate excretion, whereas ANG II increases it. In conclusion, we have developed the first model of uric acid transport in the rat PT; this model provides a framework to gain greater insight into the numerous solutes and coupling mechanisms that affect the renal handing of uric acid.

Effects of the Dietary Approaches to Stop Hypertension (DASH) Diet and Sodium Intake on Serum Uric Acid

OBJECTIVE

Randomized trial data guiding dietary recommendations to lower serum uric acid (UA), the etiologic precursor of gout, are scarce. We undertook this study to examine the effects of the Dietary Approaches to Stop Hypertension (DASH) diet (a well-established diet that lowers blood pressure) and levels of sodium intake on serum UA.

METHODS

We conducted an ancillary study of a randomized, crossover feeding trial in 103 adults with prehypertension or stage I hypertension. Participants were randomly assigned to receive either the DASH diet or a control diet (typical of the average American diet) and were further fed low, medium, and high levels of sodium for 30 days, each in random order. Body weight was kept constant. Serum UA levels were measured at baseline and following each feeding period.

RESULTS

Trial participants were 55% women and 75% black with a mean ± SD age of 51.5 ± 9.7 years and a mean ± SD serum UA level of 5.0 ± 1.3 mg/dl. The DASH diet reduced serum UA (-0.35 mg/dl [95% confidence interval (95% CI) -0.65, -0.05], P = 0.02), with a higher effect (-1.29 mg/dl [95% CI -2.50, -0.08]) among participants (n = 8) with a baseline serum UA level of ≥7 mg/dl. Increasing sodium intake from the low level decreased serum UA during the medium sodium intake period (-0.3 mg/dl [95% CI -0.5, -0.2], P < 0.001) and during the high sodium intake period (-0.4 mg/dl [95% CI -0.6, -0.3], P < 0.001).

CONCLUSION

The DASH diet lowered serum UA, and this effect was greater among participants with hyperuricemia. Moreover, we found that higher sodium intake decreased serum UA, which enhances our knowledge of urate pathophysiology and risk factors for hyperuricemia.

Opposing effects of sodium intake on uric acid and blood pressure and their causal implication

Reducing uric acid is hypothesized to lower blood pressure, although evidence is inconsistent. In this ancillary of the DASH-Sodium trial, we examined whether sodium-induced changes in serum uric acid (SUA) were associated with changes in blood pressure. One hundred and three adults with prestage or stage 1 hypertension were randomly assigned to receive either the DASH diet or a control diet (typical of the average American diet) and were fed each of the three sodium levels (low, medium, and high) for 30 days in random order. Body weight was kept constant. SUA was measured at baseline and following each feeding period. Participants were 55% women and 75% black. Mean age was 52 (SD, 10) years, and mean SUA at baseline was 5.0 (SD, 1.3) mg/dL. Increasing sodium intake from low to high reduced SUA (-0.4 mg/dL; P < .001) but increased systolic (4.3 mm Hg; P < .001) and diastolic blood pressure (2.3 mm Hg; P < .001). Furthermore, changes in SUA were independent of changes in systolic (P = .15) and diastolic (P = .63) blood pressure, regardless of baseline blood pressure, baseline SUA, and randomized diet, as well as sodium sensitivity. Although both SUA and blood pressure were influenced by sodium, a common environmental factor, their effects were in opposite directions and were unrelated to each other. These findings do not support a consistent causal relationship between SUA and BP.

Hyperuricemia, the kidneys, and the spectrum of associated diseases: a narrative review

Hyperuricemia (elevated serum uric acid) is prevalent, and an important mediator of gout, an increasingly common condition. In addition, hyperuricemia is associated with metabolic syndrome, diabetes, hypertension, and kidney and cardiovascular diseases. Although it remains controversial whether hyperuricemia is a causal factor for kidney disease, the kidneys play a major role in the regulation of serum uric acid levels. Approximately two-thirds of the uric acid produced in humans is excreted by the kidneys. The handling of urate in the renal proximal tubule is extensive, as uric acid undergoes filtration, reabsorption, and secretion. Variations in renal urate handling have been shown to influence the risk of gout. In observational studies, hyperuricemia has been shown to predict kidney disease onset and progression, with a variety of mechanisms implicated. Because of this close association between hyperuricemia and kidney disease, and due to limited studies on the topic, it is important to conduct future studies on the treatment of hyperuricemia to slow kidney disease progression and improve cardiovascular survival in patients with chronic kidney disease. Furthermore, it is important to monitor for gout in patients with kidney disease and to follow the guidelines for treatment of hyperuricemia in this group of patients. This narrative review provides an in-depth discussion of the link between serum uric acid levels, renal handling of uric acid, and diseases associated with dysfunction in uric acid homeostasis.

Quantitative estimation of urate transport in nephrons in relation to urinary excretion employing benzbromarone-loading urate clearance tests in cases of hyperuricemia

Background

A four-component system for urate transport in nephrons has been proposed and widely investigated by various investigators studying the mechanisms underlying urinary urate excretion. However, quantitative determinations of urate transport have not been clearly elucidated yet.

Methods

The equation C_ua = {C_cr(1 − R₁) + TSR}(1 − R₂) was designed to approximate mathematically urate transport in nephrons, where R₁ = urate reabsorption ratio; R₂ = urate postsecretory reabsorption ratio; TSR = tubular secretion rate; C_ua = urate clearance, and C_cr = creatinine clearance. To investigate relationships between the three unknown variables (R₁, R₂, and TSR), this equation was expressed as contour lines of one unknown on a graph of the other two unknowns. Points at regular intervals on each contour line for the equation were projected onto a coordinate axis and the high-density regions corresponding to high-density intervals of a coordinate were investigated for three graph types. For benzbromarone (BBR)-loading C_ua tests, C_ua was determined before and after oral administration of 100 mg of BBR and C_uaBBR(∞) was calculated from the ratio of C_uaBBR(100)/C_ua.

Results

Before BBR administration, points satisfying the equation on the contour line for R₁ = 0.99 were highly dense in the region R₂ = 0.87–0.92 on all three graphs, corresponding to a TSR of 40–60 ml/min in hyperuricemia cases (HU). After BBR administration, the dense region was shifted in the direction of reductions in both R₁ and R₂, but TSR was unchanged. Under the condition that R₁ = 1 and R₂ = 0, urate tubular secretion (UTS) was considered equivalent to calculated urinary urate excretion (U_ex) in a model of intratubular urate flow with excess BBR; C_uaBBR(∞) = TSR was deduced from the equation at R₁ = 1 and R₂ = 0. In addition, TSR of the point under the condition that R₁ = 1 and R₂ = 0 on the graph agreed with TSR for the dense region at excess BBR. TSR was thus considered approximately equivalent to C_uaBBR(∞), which could be determined from a BBR-loading C_ua test. Approximate values for urate glomerular filtration, urate reabsorption, UTS, urate postsecretory reabsorption (UR₂), and U_ex were calculated as 9,610; 9,510; 4,490; 4,150, and 440 μg/min for HU and 6,890; 6,820; 4,060; 3,610, and 520 μg/min for normal controls (NC), respectively. The most marked change in HU was the decrease in TSR (32.0%) compared to that in NC, but UTS did not decrease. Calculated intratubular urate contents were reduced more by higher UR₂ in HU than in NC. This enhanced difference resulted in a 15.4% decrease in U_ex for HU.

Conclusion

Increased UR₂ may represent the main cause of urate underexcretion in HU.

Renal Urate Transport

Serum uric acid is determined by a balance between production and renal excretion. Luminal reabsorption of urate by the proximal tubule from the glomerular ultrafiltrate involves coupling between sodium-anion cotransport and urate-anion exchange. Apical sodium-coupled cotransport of lactate, ketoacids, nicotinate, and pyrazinoate increases intracellular levels of these anions in proximal tubular cells, stimulating the apical absorption of luminal urate via anion exchange. Hyperuricemia occurs when plasma levels of these anions increase; for example, hyperuricemia is a well-recognized concomitant of lactic acidosis and ketoacidosis. Relevant developments in the molecular and renal physiology of urate homeostasis are reviewed.

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.

Part 1. Uric acid and losartan

PURPOSE OF REVIEW

To characterize the mechanism and clinical impact of the angiotensin-receptor blocker losartan on both renal uric acid handling and thereby serum uric acid.

RECENT FINDINGS

Losartan effect on serum uric acid has been demonstrated at various stages of renal failure including most recently observations obtained in end-stage renal disease patients. Other angiotensin-receptor blockers do not alter renal handling of uric acid. The uricosuria, which accompanies losartan administration, has not been associated with adverse renal consequences, in part, because of the increase in urinary pH that follows its administration.

SUMMARY

Hyperuricemia is closely linked to both hypertension and cardiovascular disease. The development of hyperuricemia and its persistence are clearly renal processes. Likewise, the correction of hyperuricemia is often accomplished by increasing its renal excretion. A number of medications, by way of varying mechanisms, can alter renal urate handling and thereby influence serum uric acid values. Most recently, the angiotensin-receptor blocker losartan has been shown to reduce serum uric acid. The mechanism of this process relates to losartan alone and does not involve the E-3174 metabolite of this compound. This probenecid-like effect of losartan occurs shortly after drug administration, and is both transient and dose-dependent. This property of losartan, touted by some as a meaningful pharmacological distinction among the angiotensin-receptor blockers, remains to be proved, since, to date, the hypothesis that a reduction in serum uric acid alters the natural history of cardiovascular disease has not been formally tested.

Uric acid permeability coefficient in the rat papillary collecting duct

1. While it is believed that the mammalian distal nephron is not involved in uric acid transport, this has not been directly evaluated. Nevertheless, some studies are consistent with significant distal nephron transport. 2. As uric acid transport in man may be similar to the rat, undirectional uric acid permeability was evaluated by perfusion of the isolated rat papillary collecting duct. 3. Uric acid permeability was 0.61 +/- 0.04 micron/s, which was similar to sodium permeability (0.66 +/- 0.05 micron/s) but was less than chloride permeability (0.93 +/- 0.07 micron/s) and markedly less than water permeability (4.81 +/- 0.21 micron/s). Uric acid permeability was not changed following the addition of a maximal antidiuretic concentration of arginine vasopressin (200 microU/mL), nor was it changed by altering the uric acid concentration in the perfusate and bath. 4. These results demonstrate that the papillary collecting duct is permeable to uric acid. The coefficient of transport is sufficiently low and insensitive to arginine vasopressin and uric acid concentrations to suggest that any transport that occurs is probably passive and only of minor physiological significance.

Urate excretion by the cat kidney

1. The renal handling of urate by the cat kidney was investigated during continuous infusion of urate. 2. Fractional urate excretion (FE(UA)) in cats was 0.57 +/- 0.04, indicating net reabsorption of urate. In contrast, FE(UA) in rabbits was 1.76 +/- 0.08, reflecting net secretion of urate. 3. Fractional PAH excretion (FE(PAH)) was 3.94 +/- 0.26 in cats and 4.12 +/- 0.76 in rabbits, showing net secretion in both species. 4. FE(UA) in cats was dependent on urine flow, but was independent of plasma urate concentration. 5. The urate excretion in cats was enhanced by probenecid, but was insensitive to PAH and PZA. 6. The PAH excretion in cats was reduced by probenecid, but was unaltered by urate and PZA. 7. These results indicate that urate is handled in the cat kidney by a unique transport system which is distinct from that for organic anions.

Uptake of uric acid and p-aminohippurate (PAH) by renal cortical slices of various mammals

1. Accumulation of uric acid and PAH was measured in renal cortical slices of various mammalian species. 2. The slice to medium ratio of uric acid was above unity in the rabbit, guinea pig, pig and cow, suggesting an active accumulation of uric acid, while it was near or below unity in the rat and mongrel dog. 3. Uric acid uptake in the rabbit, guinea pig and cow was significantly inhibited by PAH. 4. Uric acid was a potent inhibitor of PAH uptake in the rabbit, guinea pig, dog and pig, but much less potent in the rat and cow. 5. Kinetic analysis showed that uric acid inhibited PAH uptake in a competitive manner in all species studied except for the cow showing a noncompetitive type. 6. These results indicate that uric acid and PAH share a common transport mechanism at the basolateral membrane of the rabbit, guinea pig and pig.

Uricosuric and diuretic activities of DR-3438 in dogs and rabbits

The purpose of this study was to evaluate the uricosuric and diuretic properties of the new diuretic agent DR-3438. In the conventional clearance studies in urate-loaded dogs, intravenous injection of DR-3438 (3-30 mg/kg) resulted in dose-related increases in fractional excretion of urate (FEua), urine flow and sodium excretion. At doses causing similar natriuresis, tienilic acid (50 mg/kg, i.v.) markedly increased the FEua value, whereas indacrinone (1 mg/kg, i.v.) had no significant effect on it. Trichloromethiazide (1 mg/kg, i.v.) and furosemide (0.3 mg/kg, i.v.) tended to decrease the FEua. Thus, the uricosuric activity of DR-3438 (30 mg/kg) was 0.6-fold that of tienilic acid and 3.4-fold that of indacrinone. In contrast, in urate-loaded rabbits that exhibit net tubular secretion of urate, intravenous DR-3438 (30 mg/kg) produced a significant decrease in FEua. Stop-flow studies in dogs revealed that DR-3438 (30 mg/kg) blocks both urate reabsorption and p-aminohippurate secretion in the proximal segment of the nephron and strongly inhibits reabsorption of water, sodium and potassium in the distal segments. These results suggest that DR-3438 exerts uricosuric activity through blocking urate transport in the proximal tubules and diuretic and saluretic activities by inhibiting water and sodium reabsorption in the distal segment of the nephron.

Effect of anion exchange inhibitors and para-aminohippurate on the transport of urate in the rat proximal tubule

The present studies were designed to examine the effect of some anion exchange inhibitors and para-aminohippurate (PAH) on urate transport in the proximal tubule of the rat utilizing microperfusion techniques. The addition of SITS, DIDS, or furosemide to the luminal perfusion solution resulted in a decreased rate of absorption of water and 2-14C-urate. In addition, the presence of PAH in the luminal microperfusion solution resulted in a lower rate of urate absorption. The absorptive flux of urate was significantly higher, however, when PAH was added to the solution microperfusing the capillaries. The capillary to lumen secretory flux of urate was significantly higher when PAH or unlabeled urate was added to the luminal perfusion solution and significantly lower when PAH was added to the capillary perfusion solution. The addition of SITS to either the capillary or luminal microperfusion solution resulted in lower secretory and absorptive fluxes of urate. These studies suggest that both the secretion and reabsorption of urate in the proximal convoluted tubule of the rat is influenced by some anion exchange inhibitors and PAH. The results are considered in conjunction with recent in vitro data suggesting that urate transport is mediated by a process of anion exchange.

Effect of flow rate and the extracellular fluid volume on proximal urate and water absorption

The in vivo microperfusion technique was used to examine the effect of variations in tubular flow rate and the extracellular fluid volume onf [2-14C]-urate and water absorption in the proximal tubule of the rat. In nondiuretic animals, fractional urate absorption was highest at the lowest perfusion rate examined and decreased as the rate of perfusion was increased. Increasing the initial concentration of urate in the perfusion solution had no effect on the fractional absorption of urate. Fractional water absorption was also inversely related to the rate of perfusion. Expansion of the extracellular fluid volume with isotonic saline resulted in rates of urate absorption similar to control values at any given microperfusion rate. Fractional water absorption showed the same flow rate dependency pattern observed in control animals, but at a significantly lower rate of absorption. These studies indicate that fractional urate absorption is dependent upon some parameter of tubular flow rate and that the relationship between urate absorption and perfusion rate is not related to the delivered load of urate per se and is not affected by the state of hydration of the extracellular fluid.

A micropuncture study of urate excretion by Cebus monkeys employing high performance liquid chromatography with amperometric detection of urate

Net renal reabsorption of endogenous urate was studied by the micropuncture technique in Cebus monkeys in the absence of osmotic diuresis. Most of filtered urate (more than 70%) was reabsorbed in the proximal convoluted tubules. Samples from early distal tubules contained 9% of filtered urate; approximately 18% being reabsorbed between the late proximal and early distal segments. There was no detectable reabsorption along the distal tubule. Fractional delivery of urate to late distal tubules was greater than fractional excretion, implying reabsorption of some 4% of filtered urate in the collecting system. However, we cannot exclude nephron heterogeneity as the cause of the difference. The foregoing results were obtained using the method of Pachla and Kissinger for the determination of urate. Urate is separated by high performance liquid chromatography and detected by an amperometric technique. We found the method to be sufficiently sensitive, precise and specific for renal micropuncture samples.

Renal transport of oxalate: effects of diuretics, uric acid, and calcium

Clearance experiments were performed in the rat to examine the effects of diuretics on the renal transport of oxalate. In addition, the effect of infusing either uric acid or calcium on the renal transport of oxalate was examined. During control periods, the fractional excretion of oxalate (FEOx) averaged 118.0 +/- 2.1%. Acute administration of either chlorothiazide, furosemide, or indanyl-oxyacetic acid (MK-196), a new uricosuric diuretic, resulted in a significant decrease in the FEOx in all groups to 104.8 +/- 2.4%, 111.3 +/- 2.1%, and 108.6 +/- 2.7%, respectively. Infusion of uric acid increased urinary uric acid excretion from 2.41 +/- 0.28 to 4.26 +/- 0.03 micrograms/min/g kidney wt (P less than 0.001) and decreased FEOx to 104.0 +/- 2.4% (P less than 0.001 compared to control). Infusion of calcium chloride increased urinary calcium excretion from 0.10 +/- 0.04 to 0.44 +/- 0.06 micrograms/min/g kidney wt (P less than 0.001) but had no effect on the FEOx which averaged 118.3 +/- 8.3% (P = NS compared to control). These studies show that the acute administration of chlorothiazide, furosemide, or MK-196, as well as increasing urinary uric acid excretion by uric acid infusion, are all associated with a decrease in the FEOx. Acutely increasing urinary calcium excretion, however, had no effect on oxalate transport.

Effect of splanchnicotomy on the renal excretion of uric acid in anaesthetized dogs

The renal excretion of uric acid was examined during acute i.v. urate (Ua) loading on unilaterally splanchnicotomized ("renal denervation") anaesthetized mongrel dogs. Glomerular filtration rate (GFR) in general was not different in innervated and denervated kidneys, whereas urine flow (V) and urinary excretion of sodium (UNaV) on the splanchnicotomized side were significantly increased at any plasma concentration of Ua. The excretion (UUaV) and tubular transport (TUa) of urate calculated for unit GFR were considerably increased and depressed, respectively, at normal plasma Ua level and during minor urate loading (plasma concentration up to 4.7 mg%). Above this plasma level, i.e. up to 24.6 mg%, no difference in net urate reabsorption between intact and sympathectomized organs was found. It is suggested that both reabsorption and secretion of Ua in denervated kidneys are diminished.

Factors affecting urate reabsorption in the rat kidney

1. Urate transport in the rat appears to be saturable. However, affinity of the transport system for urate is very low and transport far from saturated at physiological plasma concentrations. 2. Since increase of the nonionized fraction of uric acid by a factor of five failed to increase urate reabsorption, transport cannot be due to nonionic diffusion but rather involves ionized urate. 3. Increases in luminal flow rate markedly depress urate reabsorption in the loop of Henle, which results in wash out of medullary urate.

Dissociation of urate from sodium transport in the rat proximal tubule.=

The tubular transport of urate and sodium was examined by clearance, free-flow micropuncture, intratubular microinjection and precession techniques in control rats and in rats receiving a new uricosuric diuretic, indanyloxyacetic acid (MK-196). The i.v. infusion of MK-196 (50 mg/kg of body wt/hr) resulted in significant increases in the fractional excretion of sodium (FENa) from 0.98 +/- 0.01 to 11.86 +/- 2.88% (P less than 0.001) and in FEurate from 14.1 +/- 1.03 to 56.0 +/- 2.86% (P less than 0.001). End-proximal tubular fluid to plasma inulin (TF/Pinulin) ratios were 2.43 +/- 0.15 and 2.51 +/- 0.10 in control and drug-treated animals, respectively (P = NS). Total urinary urate recovery after MK-196 administration was higher following microinjections of [2-14C] urate into early proximal tubule sites: 70.5 +/- 2.7% in controls vs. 84.9 +/- 0.9 (P less than 0.001), and after microinjections into late proximal tubule sites: 82.8 +/- 2.9% vs. 91.3 +/- 1.9 (P less than 0.05). Urinary precession of urate from inulin was demonstrable following placement of isotopes of these compounds on the surface of the kidney in controls, but was abolished by MK-196. This agent, therefore, inhibits the reabsorption and secretion of urate in the proximal convoluted tubule, the net effect being a marked increase in urinary urate excretion. By contrast, its inhibitory effect on sodium reabsorption is exerted at a site or sites distal to the accessible portion of the proximal tubule. The demonstration of reduced urate reabsorption and normal sodium reabsorption in the proximal tubule suggests that the reabsorption of these constituents of the glomerular filtrate is not intimately linked at this nephron site.