Renal excretion of uric acid in the rat: a micropuncture and microperfusion study.

Free-flow micropuncture experiments were done in rats of three strains infused with small amounts of urate [plasma urate (P urate) = 95 +/- 8 muM]. Urate concentrations in tubular fluid were measured by an accurate chemical fluorometric ultramicromethod. In fluid from surface glomeruli, the glomerular fluid-to-plasma urate ratio [GF/P) urate] was 0.99 +/- 0.03 (n=11), i.e., lower than expected for total ultrafiltrability of plasma urate. Along proximal convolutions, net reabsorption of 55% of filtered urate was demonstrated. Small amounts of urate may have been reabsorbed between late proximal and early distal sites. Net transepithelial movements of urate did not occur in distal tubules or collecting ducts. In microperfusion experiments on proximal tubules, both a reabsorptive flow of urate (loss of perfused [2-14C]urate) and a secretory flow (entrance of cold urate into perfusate) of the same order of magnitude were demonstrated. Neither flow was influenced by simultaneous water movements. Microperfusion of Henle's loops indicated a significant but very small net reabsorption.