Study of the paradoxical effects of salicylate in low, intermediate and high dosage on the renal mechanisms for excretion of urate in man

Gout: can management be improved?

Ongoing reviews of Cochrane collaboration show that there is still very little reliable information based on randomized controlled trials on which to base treatment decisions in acute and chronic gout. Recent studies have stressed that avoidance of factors contributing to development of gouty attacks such as diuretic therapy, weight gain, and alcohol consumption may lead to a decrease in gouty arthritis. Attention to minidose aspirin and its effect on serum uric acid levels was addressed. A low carbohydrate, high protein and unsaturated fat diet was recommended for gouty patients since they all enhance insulin sensitivity and therefore may promote a reduction in serum uric acid levels. Treatment of gout in transplant recipients brings into focus some of the issues regarding management of gout, because gout is a common problem among transplant patients.

The effect of mini-dose aspirin on renal function and uric acid handling in elderly patients

OBJECTIVE

Aspirin is known to have a bimodal effect on the renal handling of uric acid (UA). High dosages (>3 gm/day) are uricosuric, while low dosages (1-2 gm/day) cause UA retention. Although very-low-dose (mini-dose) aspirin is used increasingly as a platelet aggregation inhibitor, no studies have been published on whether aspirin's renal effects occur at dosages of <0.5 gm/day. The aim of the present study was to evaluate the effects of commonly used mini-dosages of aspirin on renal function and UA handling in elderly patients.

METHODS

The study included 49 elderly inpatients (age 61-94). Patients were excluded if they had renal failure, hyperuricemia, gout, or a history of bleeding, or if they were receiving anticoagulants, aspirin, or nonsteroidal antiinflammatory drugs. Previous medications and diet were kept unchanged. Aspirin was administered as follows: 75 mg/day (week 1), 150 mg/day (week 2), 325 mg/day (week 3), and 0 mg/day (week 4). Baseline and weekly samples of blood and urine were evaluated for UA, creatinine, blood urea nitrogen, creatinine clearance, UA excretion, UA clearance, and plasma levels of aspirin.

RESULTS

At the lowest dosage, aspirin caused a 15% decrease in the rate of UA excretion (P = 0.045 by t-test), which was associated with a slight but significant increase in serum levels of UA (P = 0.009). These effects on UA levels were gradually reduced with increasing dosages of aspirin (multivariate analysis of variance with repeated measures showed no statistically significant difference in the rate of UA excretion between weeks 1-3 and week 0 [baseline], but the difference in serum UA levels for the same comparison was statistically significant [P = 0.038]). Generally, creatinine and UA clearance rates paralleled each other during aspirin treatment. However, 1 week after aspirin was discontinued, creatinine clearance remained decreased while UA clearance returned to baseline. Plasma aspirin concentrations were low and variable. However, patients with above-median aspirin levels had significantly greater changes in serum creatinine levels, urinary UA excretion rates, and UA clearance rates following the first week of aspirin treatment. Hypoalbuminemia and concomitant treatment with diuretics enhanced the effects of aspirin on renal function and UA retention.

CONCLUSION

Mini-dose aspirin, even at a dosage of 75 mg/day, caused significant changes in renal function and UA handling within 1 week in a group of elderly inpatients, mainly in those with preexisting hypoalbuminemia. Given the widespread (and often unmonitored) use of mini-dose aspirin, especially among the elderly, these findings call for clinician alertness as well as for further studies to clarify the mechanisms underlying these phenomena.

Mechanism of the uricosuric action of E3040, a drug used to treat inflammatory bowel disease II: study using DBA/2N mice

In the initial phase of clinical studies, it was shown that E3040, a new type of anti-inflammatory drug, reduced plasma uric acid levels. The present study describes a comparison of the excretion of uric acid in the proximal tubules of the kidney after administration of E3040 and its conjugates, sulphate and glucuronide, with that of other general uricosuric agents in DBA/2N mice. The aim of this investigation was to elucidate the mechanism for the uricosuric action of E3040. It was found that E3040 increased the excretion rate of uric acid in a dose-dependent manner, and the excretion rates following 10 and 50 mg/kg doses were significantly greater than that of the control group. The paradoxical effect observed with probenecid was not seen in the E3040 dose-response curve for the uric acid excretion rate. Neither E3040-sulphate nor E3040-glucuronide increased the excretion rate of uric acid significantly, even at a high dose, such as 200 mg/kg. In the pyrazinoic acid suppression test, the uric acid excretion rate after concomitant administration of E3040 and pyrazinoic acid was significantly higher than that after administration of pyrazinoic acid alone, and the rate after concomitant administration was 30% of the level after administration of E3040 alone. The change in the excretion rate of uric acid after concomitant administration of E3040 and pyrazinoic acid was similar to that of AA193, a selective inhibitor of the presecretory reabsorption of uric acid. From these results, it appears that E3040 may exert its uricosuric action by reducing the presecretory reabsorption of uric acid rather than increasing its secretion.

Determination of folate transport pathways in cultured rat proximal tubule cells

Deficiency of the vitamin folic acid has recently been linked with increased incidence of neural tube defects and of cardiovascular disease, through elevated plasma homocysteine levels. The kidney has an important role in conserving folate to counteract development of deficiency. Urinary folate excretion is regulated by the degree of reabsorption of folate by the proximal tubule cell. To evaluate an in vitro model for studies of the regulation of urinary folate excretion, the present studies examined the transport of 5-methyltetrahydrofolate (5-CH3-H4PteGlu), the primary form of folate in the glomerular filtrate, by normal rat proximal tubule (RPT) cells in confluent monolayer cultures. Specific binding of 5-CH3-H4PteGlu to the apical membrane was saturable (K(D) = 27 nM), but intracellular transport was not saturated up to 100 nM concentrations. 5-CH3-H4PteGlu transport was decreased 50% by concentrations of folic acid that completely blocked 5-CH3-H4PteGlu binding by the apical folate receptor. Probenecid (10 mM), an anion exchange (reduced folate carrier) inhibitor, reduced 5CH3-H4PteGlu transport by 50% without significantly affecting binding. Aspirin (3 mM) did not alter 5-CH3-H4PteGlu transport, but significantly enhanced the inhibition due to probenecid. Similarly, indomethacin (5 microM) potentiated the inhibition of 5-CH3-H4PteGlu transport by probenecid. These data suggest that RPT cells take up 5-CH3-H4PteGlu by both the folate receptor and the reduced folate carrier, implying a role for both pathways in regulating urinary folate excretion.

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 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.

Saturable pharmacokinetics in the renal excretion of drugs

The renal excretion of drugs is the result of different mechanisms: glomerular filtration, passive back diffusion, tubular secretion and tubular reabsorption. Of these mechanisms the last 2 are saturable, as they involve carrier transport. This also implies that both tubular secretion and tubular reabsorption are susceptible to competition between similar substrates for a common carrier site. Furthermore, transport via these mechanisms is energy-dependent, so-called active transport, able to concentrate a drug. Tubular secretion takes place in the proximal tubule of the nephron. Many organic compounds are actively secreted, but there are separate carrier systems for anions and cations. Anions appear to be transported actively over the basolateral membrane and by a less efficient non-active carrier-mediated process (facilitated diffusion) over the brush border membrane. As a result of these mechanisms, anions tend to accumulate in proximal tubular cells. For cations, however, the active transport step operates over the brush border membrane, whereas the uptake of the cation in the cell occurs via facilitated diffusion over the basolateral membrane. Active reabsorption is most prominent for many nutrients and endogenous substrates (amino acids, glucose, vitamins), but various exogenous compounds also have a certain affinity for the reabsorptive carrier systems. Uricosuric drugs, for instance, interfere with carrier-mediated reabsorption of urate. The occurrence of saturable excretion routes causes dose-dependent, non-linear pharmacokinetics. In clinical pharmacokinetics, tubular secretion can adequately be described with the use of a Michaelis-Menten equation. This implies that a compound undergoing tubular secretion exhibits a concentration-dependent renal clearance. At low plasma concentrations the clearance will be maximal, and for several drugs may be as high as the effective renal plasma flow. Increasing concentrations cause decreasing renal clearance, until eventually the secretion mechanism becomes fully saturated. Then the excretion of the drug in urine will depend primarily on its net rate of filtration. It is important to realise that the non-linear kinetics will be evident from the plasma kinetics only when the saturable pathway contributes to at least some 20% of the total body clearance. Interactions with other substrates, however, are likely to occur even when only a very small amount of drug is transported by the carrier system. Non-linear kinetics inevitably lead to disproportionate accumulation.(ABSTRACT TRUNCATED AT 400 WORDS)

The activity of AA-193, a new uricosuric agent, in animals

A new uricosuric agent, 5-chloro-7,8-dihydro-3-phenylfuro[2,3-g]-1,2-benzisoxazole-7-carbo xylic acid (AA-193), was compared with other uricosurics in the rat, mouse and cebus monkey. In rats, probenecid and tienilic acid increased the urate excretion, but benzbromarone did not have the uricosuric activity. Thus, the presecretory reabsorption of urate is probably dominant in rats. We found that in rats AA-193 was the most potent uricosuric tested. In mice, probenecid not only had so-called paradoxical actions but stimulated urinary urate wasting after administration of pyrazinamide. These data suggest that the renal transport system of urate in the mouse is similar to that in man. AA-193 as well as benzbromarone enhanced the urate excretion dose-dependently, but the effects were different in pyrazinamide suppression tests in mice. In cebus monkeys, the uricosuric and hypouricemic effects of AA-193 were more potent than those of probenecid and similar to those of tienilic acid, but less than those of benzbromarone. Benzbromarone had a considerable role in postsecretory reabsorption in the monkey. These results suggest that AA-193 is a new class of uricosuric agent that controls the renal reabsorption of filtered urate particularly.

The association of nephrolithiasis and autosomal dominant polycystic kidney disease

Despite the frequency and morbidity of nephrolithiasis in autosomal dominant polycystic kidney disease (ADPKD), this association has not been subject to a detailed study. One hundred fifty-one of 751 ADPKD patients seen at the Mayo Clinic between 1976 and 1986 had nephrolithiasis. Seventy-four had passed calculi or had stones surgically removed. Stone analysis was available in 30 patients: uric acid, calcium oxalate, calcium phosphate, and struvite were present in 56.6%, 46.6%, 20%, and 10%, respectively. Calculi were observed in 71 of 79 patients with excretory urograms available for review. Faintly opaque and bull's eye stones, probably containing uric acid, were present in 12.7% and 14.1% of these patients, respectively. Precaliceal tubular ectasia was observed in 15.5%. Ninety-seven patients had preserved renal function (serum creatinine less than 1.5 mg/dL) at the initial evaluation. Six were excluded because they had other known causes of stone disease. The most common metabolic abnormality in the remaining 91 patients was hypocitric aciduria (ten of 15 patients with measurements). The urine pH in the first voided morning specimens (5.66 +/- 0.05) was significantly lower than that of an unselected control population (5.92 +/- 0.03, P less than 0.001). Hyperuricosuria, hyperoxaluria, and hypercalciuria were observed in six of 32 (18.8%), six of 31 (19.4%), and three of 39 (9.7%) patients with preserved renal function. The composition of the stones, the frequency of hypocitric aciduria, and the low urine pH (possibly related to the defect in excretion of ammonia described in ADPKD), suggest that metabolic, along with mechanical, factors are responsible for the frequent occurrence of nephrolithiasis in this disease.

The effect of tiaprofenic acid on uric acid excretion in man

The uricosuric effect of tiaprofenic acid was evaluated in a group of normouricaemic inpatients with various rheumatic disorders. Six patients aged 26 to 60 years were maintained on a standardised low-purine diet and, after a washout period of 72 hours, tiaprofenic acid was administered in 3 oral doses of 300 mg 12-hourly. A normal renal function, as assessed by serum creatinine and creatinine clearance determinations, was considered mandatory for entry into the study. The following parameters were evaluated before and after treatment: haematological values, blood urea nitrogen (BUN), serum and urinary creatine concentrations, serum uric acid concentration and the fractional excretion rate of uric acid. Our preliminary results showed a substantial increase of urinary uric acid excretion in the samples collected after treatment, especially in the first 4 hours.

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.

Uric acid levels in plasma and urine in rats chronically exposed to inorganic As (III) and As(V)

The effect of inorganic arsenic (III) and arsenic (V) on renal excretion and plasma levels of uric acid was examined in rats. Oral administration of 1200 micrograms As/kg/day for 6 weeks diminished uric acid levels in plasma by 67.1% and 26.5% of control after the administration of As(III) and As(V), respectively. Renal excretion of uric acid was significantly reduced during the first 3 weeks following As (III) administration, with a subsequent increase to approach control values at the end of the treatment. When As(V) was administered, the diminution in renal excretion was significant at 6 weeks.

The uricosuric effect of oxaprozin in humans

The effects of oxaprozin, a new investigational propionic acid analogue, and indomethacin on uric acid metabolism were compared in 12 healthy volunteers receiving either agent, first as a single dose and then daily for seven days. While indomethacin did not alter either serum or urinary uric acid values, oxaprozin caused a fall in serum uric acid levels from 5.8 +/- 0.2 mg/dL to 4.8 +/- 0.4 mg/dL (P less than 0.01). Urinary uric acid excretion rose from a baseline of 673 +/- 47 mg to 825 +/- 66 mg/24 h by day 7 of treatment (P less than 0.01). Since oxaprozin was associated with no change in glomerular filtration rate in these studies, the hypouricemic effect of oxaprozin is most likely a result of its direct uricosuric action. Because indomethacin and oxaprozin both are inhibitors of urinary prostaglandin E2 excretion, the data suggest that prostaglandin inhibition per se is not associated with changes in uric acid excretion.

The uricosuric action of azapropazone: dose-response and comparison with probenecid

Azapropazone is an anti-inflammatory agent with reported uricosuric properties. The aim of the present study was to extend these observations, by examining the dose-response and to compare the uricosuric effect of azapropazone with that of probenecid. Patients were given varying doses of azapropazone from 900-2400 mg daily for 4-day periods at separated intervals. Plasma uric acid levels were measured before and at the end of each treatment period. Three other patients maintained on low purine diets were given a 4-day course of 1200 mg azapropazone daily followed at an interval by a 4-day period of probenecid 1 g daily. Plasma uric acid levels and 24 h urinary uric acid excretion were compared. The mean fall in plasma uric acid level after four days of 900 mg azapropazone daily was 31.4% (n = 9) compared with 33.9% (n = 12) on 1200 mg daily; 42.3% (n = 10) on 1800 mg daily; and 46% (n = 6) on 2400 mg daily, indicating a graded dosage response. In the three patients on low purine diets the falls in plasma uric acid levels on probenecid 1 g daily were 50.5%, 46% and 29% compared with 33.5%, 32% and 20% respectively on azapropazone 1200 mg daily. Similarly the total amount of uric acid excreted in the urine by each patient during the 4-day period on probenecid 1 g daily was 14.01; 13.03 and 8.97 mmol compared with 23.53, 10.9 and 7.69 mmol on azapropazone 1200 mg daily.(ABSTRACT TRUNCATED AT 250 WORDS)

Coexistent rheumatoid arthritis and tophaceous gout: a case report

Rheumatoid arthritis and gout are both common rheumatic diseases, but their coincidence is rare. We report the case of a 67-year-old Caucasian woman with rheumatoid arthritis who later developed tophaceous gout. The tophi disappeared with remarkable rapidity on treatment with allopurinol.

Blunting of furosemide diuresis by aspirin in man

Experiments were performed on humans to study the blunting on the diuretic action of furosemide by prostaglandin synthetase inhibitors. Maximal water diuresis was instituted. At the peak of urine flow, clearance periods were performed during baseline conditions and repeated after the injection of aspirin and, subsequently, of furosemide. Control subjects did not receive aspirin. Urine flow rate (V), Cosm, and Na excretion (UNa) . V were significantly lower when the administration of the diuretic had been preceded by that of aspirin. In the absence of furosemide, however, aspirin did not influence renal hemodynamics nor Na and water reabsorption. Therefore, the same experimental protocol was repeated in paired experiments where each normal subject served as his own control, being studied twice, in the presence and absence of aspirin, respectively. The average changes in water and Na excretion induced by furosemide were not different when the patients were pretreated with aspirin as compared with those measured in the absence of prostaglandin inhibition. Changes occurring in individual experiments were significantly correlated (r = 0.95, P less than 0.01) with those in calculated furosemide clearance. Since aspirin, indomethacin, and meclophenamate are secreted by the organic acid transport system of the proximal tubule, competition for a common secretory mechanism, rather than prostaglandin inhibition, could mediate the blunting of furosemide diuresis.

Effect of combined administration of furosemide and aspirin on urinary urate excretion in man

Furosemide (20 mg) was administered intravenously to 7 healthy volunteers, before and after 4 days of oral administration of aspirin in uricosuric dosage (1 g daily). Aspirin prevented the decrease in urinary urate excretion induced by furosemide, without interfering with its natriuretic action. This finding is suggested to be the result of the opposite and independent effects of the two drugs on tubular reabsorption of urate.

Diflunisal: a review of pharmacokinetic and pharmacodynamic properties, drug interactions, and special tolerability studies in humans

1 In the fasting state, peak plasma levels of diflunisal were achieved within 2 hours. The drug was not metabolized and almost totally excreted in the urine as unchanged or conjugated drug. The terminal plasma half-life was approximately 8 hours. These results support a twice daily dose regimen.

2 During multiple dose administration the time required to achieve steady-state plasma levels varied with the dose. A dose regimen of 125 mg twice daily required 2-3 d, whereas a regimen of 500 mg twice daily required 7-9 d to reach a steady-state plasma level.

3 Clinically effective doses of diflunisal decreased the urinary excretion of the major prostaglandin E metabolite, 7α-hydroxy-5,11-diketotetranorprostane-1,16-dioic acid, and exhibited significant uricosuric activity. These same doses did not seem to cause tinnitus, nor did they significantly alter gastrointestinal blood loss, affect blood glucose, bleeding time, or platelet function.

4 Clinically significant drug interactions may be anticipated during concomitant administration with at least one oral anticoagulant (acenocoumarol), but probably not anticipated during the coadministration of oral antidiabetic agents, thiazide diuretics, and non-steroidal anti-inflammatory/analgesic agents.

5 Clinical and laboratory data accumulated during these studies indicated that diflunisal was well tolerated.

Renal transport of urate during diuretic-induced hypouricemia

The effect of two weeks administration of a uricosuric diuretic (SKF-62698) on renal urate handling has been examined in 11 normal men. Plasma urate concentrations had declined by more than 60 per cent after two weeks. Urate excretion per unit of glomerular filtration rate and urate clearance (Curate) per unit of glomerular filtration rate were increased after the administration of SKF-62698. The importance of intact tubular secretion of urate in producing these changes was assessed by administering pyrazinamide, an agent that curtails urate secretion, to each participant. The decrements in urate excretion and clearance produced by pyrazinamide both increased significantly, whereas the residual urate excretion rates and clearances not suppressible by pyrazinamide were only minimally altered by SKF-62698 treatment. These results suggest that the excretion of secreted urate was enhanced by prolonged administration of SKF-62698, probably secondary to the inhibition of postsecretory urate reabsorption. In addition, because the nonsuppressible urate excretion did not decline despite a 63 per cent reduction in the plasma urate, it is likely that the reabsorption of filtered urate also was impaired by SKF-62698.

Renal excretion of uric acid during prolonged fasting

Serum and urine uric acid were evaluated during prolonged therapeutic fasting in 15 obese patients. With increasing ketonemia the serum uric acid rose from a control value of 5.9 +/- 0.4 to 12.5 +/- 1.0 mg/100 ml at 7 days and then decreased progressively to 7.7 +/- 1.3 mg/100 ml by 28 days despite sustained ketonemia. The uric acid clearance were 5.5 +/- 0.9, 1.8 +/- 0.2, and 4.4 +/- 1.5 ml/min at days 0, 7, and 25 of fasting. At the same times the creatinine clearances were 114 +/- 11, 80 +/- 6, and 64 +/- 6.3 ml/min. There was no evidence of a renal tubular abnormality as assessed by glycosuria, bicarbonaturia, or increased phosphaturia. Urate binding to plasma proteins remained unchanged. Acute studies of the renal handling of uric acid revealed a uricosuric response to the administration of sodium lactate or sodium bicarbonate by intravenous infusion and low-dose acetylsalicylic acid orally. This renal tubular response departs significantly from that observed during the overnight fasted state and could not be accounted for by extracellular fluid volume expansion or the induced acid-base changes.

Simple analgesics

A number of drugs are available that act fairly specifically as "mild" analgesics, although this description by no means implies that their clinical effectiveness is limited to the relief of slight pain and trivial disability. They are effective by mouth and their action is mediated peripherally. Among the possible mechanisms of action, the inhibition of prostaglandin synthesis is currently regarded as most likely to be relevant. Some centrally acting drugs of the narcotic analgesic type, such as codeine and dextropropoxyphene are effective orally; they are usable in the same way as other mild analgesics and may be preferable for some types of pain. Many problems arise in the assessment and comparison of mild analgesics, both experimentally and clinically. Subjective assessments may be made on a pain scale by the patient himself, or by a trained observer. Individual variations are all-important, and the limitations of controlled trials need to be remembered. Alternative drugs and mixtures have little advantage over aspirin, but specific drug tolerance, in the long term, varies from patient to patient. Gastric irritation is most likely to occur with aspirin in the presence of chronic dyspepsia or acute precipitating causes such as alchoholic gastritis. Allergy also occurs in some susceptible individuals. The risk of renal damage with phenacetin is increasingly appreciated, and the possibility of hepatic damage from paracetamol is now recognised. Other side-effects and interactions are summarized in the review, and some notes are given on therapeutic and non-therapeutic use.

Pyrazinoate excretion in the chimpanzee. Relation to urate disposition and the actions of uricosuric drugs

These experiments were designed to define the renal disposition of pyrazinoic acid in a nonhuman primate that is phylogenetically close to man and to relate this to the effects of pyrazinoate on urate excretion. The renal clearance of pyrazinoate was almost always greater than the simultaneous glomerular filtration rate at plasma concentrations ranging from 1.9 to 960 mug/ml. Some inhibitors of tubular secretion, probenecid, MK-282 (an experimental, potent uricosuric drug), p-aminohippurate, iodopyracet, sulfinpyrazone, and mersalyl, reduced clearances of pyrazinoate to values far below filtration rate. Chlorothiazide, allopurinol, and salicylate did not. The clearance of pyrazinoate was not influenced by changes in urine flow. It is concluded that pyrazinoate is actively secreted and actively reabsorbed. Pyrazinoate had a dual effect on urate excretion. At concentrations in plasma less than 10 mug/ml there was a concentration related fall in urate/inulin clearance ratio, reaching values of 10-20% of control. Over the range of 10-100 mug/ml in plasma, the clearance of urate remained maximally depressed. At higher concentrations of pyrazinoate there was a concentration related increase in urate/inulin clearance ratio such that at pyrazinoate levels above 600 mug/ml a definite uricosuric response was obtained. Prior administration of pyrazinoate to give plasma levels of 20-140 mug/ml completely or almost completely prevented uricosuric responses to probenecid, PAH, chlorothiazide, and sulfinpyrazone. Iodopyracet, mersalyl, salicylate and N-acetyl-4-dibutylsulfamoyl-3-trifluoromethylbenzenesulfonamide (MK-282) retained significant uricosuric action, but the activities were probably less than normal. The results are consistent with a model of urate transport involving high rates of bidirectional transtubular flux.

Evidence for a postsecretory reabsorptive site for uric acid in man

The effects of administration of drug combinations on uric acid excretion were studied in order to test the hypothesis that a portion of renal tubular reabsorption of uric acid occurs distal to the uric acid secretory site. Oral administration of pyrazinamide (3 g) during probenecid uricosuria (probenecid 500 mg every 6 h) decreased urate excretion from 463 mug/min following probenecid medication alone to 135 mug/min following probenecid plus pyrazinamide (P < 0.01). When a greater uricosuric effect was induced with a 2 g oral dose of probenecid, the decrement in urate excretion which followed pyrazinamide administration (3 g) was more pronounced (2,528 mug/min following probenecid alone, 574 mug/min following probenecid plus pyrazinamide). Results were similar when an 800 mg oral dose of sulfinpyrazone was given in place of probenecid (1,885 mug/min following sulfinpyrazone alone, 475 mug/min following sulfinpyrazone plus pyrazinamide). Thus, apparent urate secretion (measured as the decrease in excretion of urinary uric acid resulting from pyrazinamide administration) appeared to vary, depending upon the degree of inhibition of reabsorption produced by probenecid or sulfinpyrazone. When small doses of aspirin were administered in place of pyrazinamide to produce secretory inhibition, the results were similar. Neither probenecid nor pyrazinamide significantly altered urate excretion when administered to patients with serum salicylate levels above 14 mg/100 ml. These results are interpreted as suggesting that renal tubular reabsorption of uric acid occurs at least in part at a postsecretory site and that a portion of secreted urate is reabsorbed. During maximum probenecid- or sulfinpyrazone-induced uricosuria, inhibition of urate secretion with either pyrazinamide or low doses of aspirin resulted in a decrease in uric acid excretion which exceeded total urinary uric acid during control periods by two- to fourfold. This suggests that renal tubular secretion of urate may greatly exceed uric acid excretion and that a large fraction of secreted urate is reabsorbed. The pyrazinamide suppression test underestimates urate secretion. Uricosuria induced by some drugs, including probenecid, sulfinpyrazone, and iodinated radioopaque dyes, appears to represent, at least in part, inhibition of postsecretory urate reabsorption.