[Mechanism of renal uric acid excretion in the rat]

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.

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.

Renal urate transport during variations in urate synthesis in the rat

In order to determine the effect of intrarenal synthesis of urate upon the urinary urate excretion in the rat, we effected large changes in urate synthesis by increasing it with allopurinol. Hypoxanthine infusion increased plasma urate rapidly and also increased the urinary urate excretion and its renal clearance. However, when the plasma urate was maintained constant, hypoxanthine had no effect upon renal urate transport. Conversely, allpurinol infusion rapidly diminished the plasma urate, urinary urate excretion and its renal clearance. Again, the maintenance of a constant plasma urate concentration prevented any change in urate transport during allopurinol. The urinary degradative purine metabolic pattern was altered predictably by hypoxanthine and allopurinol. Assuming than any putative intrarenal component of urate synthesis would be affected predictably and consistently by hypoxanthine and allopurinol, these results suggest that changes in intrarenal urate synthesis are not an important determinant of urate excretion in the rat.

Handling of allantoin by the rat kidney. Clearance and micropuncture data

Renal excretion of allantoin was measured by tracer techniques. After injection of 2-C14 urate and H3 inulin, clearances of allantoin and inulin were measured and both proximal and distal tubules were micropunctured. In confirmation of earlier results 2-C14 urate injected into an intact animal is very rapidly converted to C14 allantoin: after 15 min more than 90% of urinary tracer is present as allantoin. It was further observed that 1) allantoin clearance is essentially identical with inulin clearance over a wide range of urine flows; 2) no net transport of allantoin occurs in either proximal or distal tubules. Clearly allantoin is handled by the rat kidney like inulin. The total excretion of filtered allantoin unlike that of filtered urate provides an easy and effective mechanisms for animals possessing the enzyme uricase to dispose of their purine loads.

The influence of the extracellular fluid volume on the tubular reabsorption of uric acid

Changes is tubular reabsorption of uric acid in response to alterations in the extracellular fluid volume (ECFV) were examined in rats by clearance studies and by direct intratubular microinjections. Contraction of the ECFV led to a rise in the serum uric acid concentration and a 47% decrease in the clearance of uric acid. The ratio of uric acid to inulin clearance also fell, indicating an increase in the net tubular reabsorption of urate. Volume expansion resulted in an increase in the urate clearance and a 37% decrease in the net tubular reabsorption of uric acid. To localize the site in the nephron where these changes occur, microinjections of [2-14C]urate were performed. The lack of conversion of radioactive urate to allantoin after microinjections was demonstrated by thin-layer chromatography. After contraction of the ECFV, urinary recoveries of uric acid were significantly decreased after microinjections into proximal tubular sites. In contrast, recoveries were increased from these proximal sites after volume expansion. No evidence for distral reabsorption was obtained in any group of animals. These studies demonstrate that net urate reabsorption is influenced by the state of hydration of the ECFV and that these alterations are mediated by changes in the rates of reabsorption in the proximal tubule.