Characteristics of urate influx in the rat nephron

Permeability of the tubular epithelium to urate presented on the peritubular side was studied in anesthetized rats during mannitol diuresis by capillary microinjections of [14C]urate and [3H]inulin, [14C]PAH and [3H]inulin, or [14C]urate and [3H]PAH. Recovery of isotopes was determined in urine collected serially from the injected and contralateral kidney. Appearance and peak excretion of urate in the experimental but not in the contralateral kidney preceded inulin and coincided with PAH, indicating proximal permeability to urate. Recovery of urate was higher from the injected than from the contralateral kidney. Urate precession and recovery did not change after addition of PAH (1.5-6.4 mm) to [14C]urate-[3H]inulin solutions, whereas they decreased significantly in the experimental kidney after pyrazinoate (1.6-3.2 mM) addition. On the other hand, no effect of urate on [14C]PAH-[3H]inulin injections was detectable. These findings are suggestive of a carrier-mediated transtubular influx of urate in rat proximal tubule. Absence of competition with PAH may suggest differences in the secretory mechanisms for organic acids.