Renal excretion of ascorbic acid in the rat: a micropuncture study.

Ascorbate concentration was measured by high-performance liquid chromatography and amperometry in plasma, tubular fluid, and urine from rats infused with ascorbic acid to steady-state levels. At a low concentration of ascorbate in plasma (Pasc = 0.2 mM) reabsorption occurred along the proximal convoluted tubule (PCT). The fractional delivery (FD) of ascorbate [(TF/P)asc/(TF/P)polyfructosan] to the late proximal convoluted tubule was 0.64 +/- 0.04, and the fractional excretion of ascorbate (FEasc) was 0.56 +/- 0.01. At higher Pasc (0.9 mM) net secretion occurred in the PCT, while the FDasc was 1.5 +/- 0.2 to the early and 1.8 +/- 0.2 to the late PCT. At still higher Pasc the secretory and the reabsorptive transports were saturated and the FDasc and FEasc approached unity, indicating that reabsorptive as well as secretory transport occurs in the proximal tubule. In clearance experiments the reabsorptive transport and secretory transport were inhibited by 2-nitroprobenecid. The drug induced a fall of FEasc when infused at a low rate (0.9 mumol X kg body wt-1 . min-1), which was followed by an increase in FEasc when the rate of infusion of 2-nitroprobenecid was increased to 3 mumol . kg body wt-1 X min-1.