Flux Of Urate Research Articles

Evidence for active and passive urate transport in the rat proximal tubule

Studies utilizing the technique of simultaneous microperfusion of peritubular capillaries and tubular lumen of the proximal tubule of the rat were performed to determine if the absorption of urate was an active transport process and to determine the passive permeability coefficient for urate. When radioactive urate of equal specific activity and concentration was present in both perfusion solutions, the ratio of collected to initial concentrations of urate in the luminal perfusate (CO/CI) was 0.71 +/- 0.02. This gradient was higher than that predicted at equilibrium from the electrical potential difference determined in the in vitro perfused rabbit proximal tubule. The addition of para-chloromercuribenzoate (PCMB) to both solutions resulted in a significantly higher CO/CI of 0.90 +/- 0.02. This latter value is closer to the value predicted at electrochemical equilibrium. In separate studies, the unidirectional fluxes of urate were determined in the presence of PCMB. The calculated passive permeability coefficient averaged approximately 0.94 pmol . min-1 . mm-1 . mM-1 and was equal in both directions. These results indicate that in the rat proximal tubule urate absorption is an active transport process. In addition, there exists a passive permeation pathway for urate movement out of and into the proximal tubule.

Urate secretion in isolated rabbit renal tubules.

The flux of radioactive urate from bath to lumen was measured in isolated proximal convoluted (PCT), proximal straight (PST), and cortical collecting tubules (CCT). Urate was secreted in the PST as evinced by a) accumulation of urate in the lumen of perfused tubules to levels 2-4 times greater than in the bath, and b) inhibition of urate flux (bath to lumen) by cooling, by probenecid (2.5 X 10(-4) M), and by ouabain (10(-4) M). Urate secretion was strongly dependent on the rate of urinary flow, diminishing as flow rate was reduced below 2 nl/min, and reaching maximal rates when flow exceeded 5 nl/min. The isotopic urate (either [2-14C]urate or [6-14C]urate) was secreted without modification. Luminal urate was not impressively elevated above the concentration of the bath in PCT. Cortical collecting tubules were relatively impermeable to urate. It is concluded that urate secretion occurs predominantly in the PST of this species by a mechanism dependent on the uphill transport of the molecule from the bath into the urine.

Secretion of urate in the proximal convoluted tubule of the rat.

In order to examine the transepithelial secretory flux of urate in the rat proximal tubule, simultaneous perfusions of capillaries and lumens were performed. The capillary perfusate contained [2-14C]urate in concentrations of 0.305--2.94 mM. The secretory flux of urate increased as the concentration of urate in the capillary perfusion solution was increased from 0.305 to 1.235 mM but tended toward a plateau at higher concentrations. An apparent Km of 0.41 mM and Vmax of 4.7 pmol . min-1 . mm-1 were calculated from the observed net flux and an estimated passive permeability coefficient of 0.725 pmol . min-1 . mm-1. The addition of probenecid (10(-4) M) to the capillary perfusion solution inhibited urate secretion in a manner consistent with that of a competitive inhibitor. The addition of p-chloromercuribenzoate (10(-4) M) to the capillary perfusion solution inhibited the Vmax but not the Km, suggesting a non-competitive type of inhibition. These data provide the first estimates of the apparent transport constants for urate secretion in the rat proximal tubule determined in vivo. Urate secretion is mediated by a saturable carrier-mediated system. This carrier is not affected by the presence or absence of potassium in the perfusion solution but can be inhibited by the addition of either probenecid or p-shloromercuribenzoate.

Urate transport in the nephron

This paper summarizes the literature on the sties of urate transport, secretory and reabosrptive, in the nephron. In the animals studied thus far the bulk of urate transport occurs in the proximal tubules. Two patterns of transport have been uncovered. In one, urate is secreted and reabsorbed throughout the convoluted tubule (perhaps the pars recta as well). The direction of net transport depends on the kind of animal studied and in some instances on the experimental conditions. In the second pattern there is a strong secretory process in the pars recta, and net secretion of urate, when it occurs, is attributable to that segment. In some but not all animals it is clear that urate secretion in the proximal tubules occurs by a mechanism separate from that which secretes p-aminohippurate (PAH). One important unresolved question is: How general is the occurrence of separate secretory mechanisms for PAH and urate? Another unresolved question is: What are the magnitudes of the unidirectional fluxes of urate in the segments in which bidirectional transport occurs?

Peritubular transport of urate in rat kidneys.

Measurement in the rat of renal artery-to-vein transit times permitted evaluation of movement of 14C-labelled urate and of other solutes out of the peritubular inulin spaces in the renal cortex. No interaction between urate and PAH could be demonstrated at this level. Provided the peritubular disappearance may be equated to cellular uptake a mean unidirectional urate flux across peritubular membranes equal to about 1/3 of total renal load may be calculated. This value greatly exceeds the net secretory flux of urate. Such a fact can be explained by the likelihood of significant post-proximal reabsorption of secreted urate, and/or by the conclusion that peritubular urate permeability exceeds that of the luminal side. Assignment of the limiting step in urate secretion to the luminal cell membranes, in contrast to a determining role played by peritubular membranes in PAH transport, can account for the lack of interaction between PAH and urate at the peritubular membrane in rats.