Free-flow micropuncture study of renal urate transport in the Munich-Wistar rat

In order to determine the nephron sites of urate reabsorption, the fractional rates of delivery of urate to specific segments of the superficial and deep nephrons were determined by the free-flow micropuncture technique in the Munich-Wistar rat. In nondiuretic control animals, the fractional delivery of urate to the late proximal tubule (34.1 +/- 3.6%), distal tubule (39.0 +/- 6.2%), base (32.6 +/- 3.2%), and tip (32.3 +/- 5.6%) of the papillary collecting duct were not different from one another. In animals in which the extracellular fluid volume was expanded with isotonic saline and in animals infused with the uricosuric diuretic MK-196, the fractional rates of urate delivery to any given nephron segment were significantly higher than in controls. There were no significant differences in either group, however, in the rates of delivery of urate to the specific nephron sites sampled. These studies indicate that urate absorption occurs primarily in the proximal convoluted tubule, but provide no evidence for nephron heterogeneity for urate transport. Expansion of the extracellular fluid volume and the infusion of MK-196 resulted in a higher fractional excretion of urate.