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Renal Transport of Urate in Humans

Françoise Roch-Ramel and Barbara Guisan

F. Roch-Ramel is Professor and B. Guisan is a specialized technician from the Institut de Pharmacologie et Toxicologie, Bugnon 27, CH1005 Lausanne, Switzerland.

	Abstract

 
The theory of the "four-component model" of urate excretion in humans is reevaluated, considering that a decrease in urate excretion induced by drugs like pyrazinamide or by endogenous compounds like lactate and ketone bodies might be a result of stimulation of urate reabsorption and not, as previously considered, of inhibition of urate secretion.

	Introduction

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

 
The renal urate handling in humans has been explained for many years by a "four-component model." The purpose of this review is to reevaluate the validity of the theory behind this hypothetical model in the light of the transport mechanisms that were characterized recently in the apical membrane (brush-border membrane; BBM) of human proximal tubule.

In humans, the uricase gene is not expressed, and uric acid is the end product of purine metabolism. It is a trioxypurine of which only the hydroxyl group in position 8 (pK_a 5.75) is dissociable at physiological pH. The kidney plays a predominant role in its elimination, two-thirds of the daily production of uric acid being excreted by the renal route and the other one-third being eliminated by the gastrointestinal tract. In the plasma, 98% of the uric acid is dissociated and present as sodium urate, which is freely filtered, being bound by <5% to plasma proteins. Thus uric acid enters the proximal tubule in its anionic form, its concentration in the ultrafiltate being close to that of plasma. Both uric acid and urate, being hydrophilic, hardly permeate the tubular cells in the absence of facilitated mechanisms.

The transport mechanisms for urate, as for organic anions in general, are localized in the proximal tubule (Fig. 1). In humans, urate is extensively reabsorbed, which results in excretion of ~10% of the filtered load of urate (FE_urate = renal clearance of urate/glomerular filtration rate = 10%) (1). In the mammalian proximal tubule, micropuncture and microperfusion studies, as well as earlier stop-flow techniques, demonstrated that urate is bidirectionally transported with a predominance of reabsorption, in species such as rats, dogs, and Cebus monkeys, or secretion, as in pigs and rabbits (13). The distal parts of the nephron and the collecting ducts are hardly permeable to urate. A simplified scheme of urate handling by the nephron of reabsorbing mammals is provided in Fig. 1. In humans, experiments on renal handling of urate hitherto have been limited to the study of the effects of urate loading and of the different drugs interfering with urate excretion in vivo. Such experiments demonstrated a secretory transport for urate in a group of male gout patients with low filtration rates infused with uric acid and a uricosuric drug, when the FE_urate rose from values below the filtration load (reabsorption) to values slightly exceeding the filtration load (secretion) (6). Urate secretion is also observed in hereditary hypouricemia, in which an increase in renal urate clearance is caused by a specific inborn error of urate transport (14). These observations suggest that urate is not only reabsorbed but also secreted in humans as in other mammals, reabsorption being predominant. Many attempts have been made to estimate the respective importance of the reabsorptive and secretory fluxes in pharmacological experiments, by measuring the effects of uricosuric and antiuricosuric drugs, in an endeavor to selectively inhibit the reabsorption or the secretion of urate.

View larger version (67K): [in this window] [in a new window]   FIGURE 1. Schematic representation of urate handling by the human nephron. Urate is secreted and reabsorbed along the proximal tubule (shown in gray). Reabsorption is predominant, and fractional excretion in the final urine approximates 10%. In some mammalian species secretion predominates (see text).

	Drug effect on urate excretion

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

	"The four components of urate excretion"

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

	Transport mechanisms in membranes of the human proximal tubule

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

 
In urate-reabsorbing nonprimate mammals, e.g., rats and dogs, a urate/anion exchanger has been identified in the BBM of the proximal tubule, which accepts multiple monovalent organic anions, aliphatic or aromatic, including p-aminohippurate, as well as chloride and hydroxyl ions (5). The physiological role of the anion exchanger was demonstrated in situ in tubular and peritubular capillary perfusion experiments. Such a urate/anion exchanger is absent in rabbits and pigs, which predominantly secrete urate. These species differences strongly suggest that the urate/anion exchanger is an essential mechanism for urate reabsorption. The presence of a similar urate/anion exchanger was demonstrated in human BBM vesicles prepared from normal sections of human kidneys excised because of carcinoma (Fig. 2, left, mechanism 1) (10). The substrate affinities for the human anion exchanger are largely identical to those of dogs, with the exception that p-aminohippurate and hydroxyl ions are not substrates. The aromatic substrates pyrazinoate, nicotinate, and orotate have about the same affinity as urate for the anion exchanger. The affinity of the aliphatic compounds lactate, ß-hydroxybutyrate, and acetoacetate is about one order of magnitude lower. These substrates, when loaded in the BBM vesicles, stimulate urate uptake (trans stimulation), but when added to the uptake medium, they inhibit urate uptake (cis inhibition) (10). Substrates such as lactate, nicotinate, and pyrazinoate, beside being substrates for the exchanger, are also transported by sodium cotransport and can stimulate urate uptake by BBM vesicles, when added to the uptake medium, in the presence of an inward directed sodium gradient (cis stimulation) (10, 12). The uricosuric drugs probenecid, benzbromarone, sulfinpyrazone, tienilic acid, salicylate, and losartan cis inhibit urate uptake when added to the uptake medium (11).

View larger version (83K): [in this window] [in a new window]   FIGURE 2. Scheme of membrane mechanisms involved in urate reabsorption. Top: urate transport mechanisms of human proximal tubules are shown on left. Mechanism 1, urate/anion exchanger; mechanism 2, apical voltage-sensitive pathway; mechanism 3, basolateral voltage-sensitive pathway. Physiological substrates for the exchanger (X-) are lactate, ß-hydroxybutyrate, acetoacetate, etc. Many of these substrates are compounds reabsorbed from the proximal tubule by sodium cotransport. On right, cooperation of a sodium-anion cotransport (4) with the urate/anion exchanger (1) is shown. From the lumen, urate enters the cells in exchange for X-, which enters the lumen. Urate leaves the cell through the basolateral voltage-sensitive pathway (3), and X- is recycled to the cell by sodium cotransport; from the cell it is again available for urate exchange. Bottom: administration of nicotinate and pyrazinoate (PZA-) provides additional substrates for the urate/anion exchanger. Both anions are taken into the cell by sodium cotransport, from where they stimulate urate uptake. From the lumen they are recycled again to the cell by sodium cotransport (4). In contrast, orotate, which is not transported with sodium, competes with urate for transport through the urate/anion exchanger (1) and thus decreases the reabsorption of urate. Uricosuric drugs such as benzbromarone and probenecid act in a similar way at the luminal surface of the tubular cells, reducing access of the urate to the urate/anion exchanger. Modified from Ref. 10 with permission.

	Physiological role of the apical urate/anion exchanger

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

 
We postulate that from the lumen, urate enters the proximal tubular cell in exchange for intracellular anions (X^-; Fig. 2, right, mechanism 1). This is the first step of urate reabsorption. From the tubular lumen anions such as lactate, ß-hydroxybutyrate, and acetoacetate are recycled by sodium cotransport (mechanism 4), thereby providing a constant gradient of intracellular anions, which drives urate into the cells. However, these anions are also produced by intracellular metabolism. When the amount of intracellular anions available for exchange with urate is increased, reabsorption might be stimulated, which results in a decrease in urate excretion and in hyperuricemia. This is what happens in humans during fasting or starvation because of the accumulation of ketone bodies or also during experimental infusion of ß-hydroxybutyrate or acetoacetate to normal subjects (3). We suggest that the hyperuricemia induced by lactate and ketone bodies results from a stimulation of urate reabsorption and not, as postulated earlier, by an inhibition of urate secretion.

	Mechanism of action of uricosuric and antiuricosuric agents

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

 
The urate/anion exchanger might be the site of action of uricosuric as well as antiuricosuric agents, depending on their presence either in the lumen or in the proximal tubular cells. This is illustrated by the comparison of the in vivo effect of orotate and nicotinate, two substrates that have a similar affinity for the urate/anion exchanger (12). In humans orotate is uricosuric, whereas nicotinate is antiuricosuric (3). The main difference between the two substrates is that nicotinate is reabsorbed from the proximal tubule by sodium cotransport, whereas orotate is not. Both nicotinate and orotate can stimulate urate transport through the urate/anion exchanger (Fig. 2, bottom), but only nicotinate is recycled into the cell by sodium cotransport. In contrast to nicotinate, orotate remains in the lumen, where it competes with urate cellular uptake, which explains its uricosuric effect. It has long been recognized that uricosuric drugs act from the tubular lumen. All uricosuric compounds investigated in the human BBM vesicle model inhibited urate uptake through the urate/anion exchanger. In vivo, they compete with urate in the lumen for the exchanger, either for binding or for transport, the net result being uricosuria.

Pyrazinoate, the active metabolite of pyrazinamide, is a pyrazine analog of nicotinate. Like nicotinate it is reabsorbed by a sodium cotransport mechanism, and most probably it stimulates urate uptake by the proximal cells in a way similar to nicotinate. When pyrazinamide is administered and metabolized to pyrazinoate, the cooperation of the urate/pyrazinoate exchange and of the sodium-pyrazinoate cotransport (Fig. 2, bottom) provides a constant increase in the anion gradient that stimulates urate reabsorption.

	Why is the uricosuric effect of drugs blunted by pyrazinamide?

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

 
Two points need to be considered. First, the reabsorptive capacity of the human kidney is large. When the filtered load of urate of normal subjects is doubled, following several days' ingestion of yeast RNA and pyrazinamide administration, the reabsorption of urate is also doubled, the reabsorptive transport still not being saturated (9). Second, when therapeutic doses of uricosuric drugs are administered to normal subjects, the reabsorption of urate is only partially inhibited (Table 2); an amount at least equal to 40–50% of the filtrated load is still reabsorbed. We postulate that the reabsorptive capacity of the human proximal tubule is such that even if part of the reabsorptive mechanism is inhibited by uricosuric drugs, the cellular recycling of pyrazinoate is efficient enough to stimulate the reabsorption of urate through the urate/anion exchanger.

	Urate secretion

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

 
Urate secretion was suggested to be important because of the strong decrease of urate excretion by pyrazinamide. It is now evident from the human BBM vesicle studies that pyrazinamide has a major effect on reabsorption and that urate secretion in humans has been largely overestimated. At present the role of secretion is undefined; we suggest that it contributes only in a minor way to urate excretion in normal individuals.

	Conclusions

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

 
The urate handling by the human kidney does not seem to proceed according to the four-component model proposed earlier. Secretion appears to be minor, and reabsorption and secretion might coexist along the whole proximal tubule. In humans urate reabsorption in the kidney is mediated mainly by the urate/anion exchanger of the apical membrane of the proximal tubule. Intracellular substrates for the exchanger stimulate the reabsorption of urate, whereas uricosuric agents inhibit urate reabsorption from the lumen. Urate leaves the cell through the basolateral voltage-sensitive pathway, along its electrochemical gradient.

	Acknowledgments

 
The authors thank Dr. H. A. Simmonds and Prof. J. Diezi for their critical reading of the manuscript.

The authors' work was supported by grants from the Swiss National Fund for Scientific Research.

	References

Top Introduction Drug effect on urate... "The four components of... Transport mechanisms in... Physiological role of the... Mechanism of action of... Why is the uricosuric... Urate secretion Conclusions References

 

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