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.