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.