Microinjection study of p-aminohippurate excretion by rat kidneys

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Abstract
Mixtures of [p-aminohippurate] (PAH) (14C or 3H) and inulin or mannitol (3H or 14C), and mixtures of radioactive mannitol and inulin were microinjected into superficial proximal and distal convolutions of diuretic rat kidneys and in control experiments were injected into polyethylene tubing through which saline was flowing. Percent recovery and mean transit time ([image]) were measured in serial samples of urine or effluent from the polyethylene tubing. Mean transit times for the 3 compounds were the same after injection into polyethylene tubing, but PAH and mannitol were less dispersed than inulin because of faster radial diffusion. Although there appeared to be no transtubular movement of inulin or mannitol, [image] for mannitol micro-injected into proximal tubules was longer than [image] inulin, suggesting a larger volume of distribution for mannitol. Radioactive PAH recoveries were complete in rats that had not received intravenous infusions of nonradioactive PAH; however, microinjected PAH appeared to be distributed in a larger volume than either mannitol or inulin. In rats preloaded with nonradioactive PAH 5% of the radioactive PAH microinjected into distal tubules was lost and the remainder distributed in a volume equal to that for inulin. Ten% of the PAH microinjected into proximal tubules was lost from PAH loaded rats, but the remainder was distributed in a larger volume than was inulin. It is concluded that there is transtubular efflux of PAH in the cortex and medulla. Raising plasma PAH concentration increases the loss of microinjected PAH presumably by competitively inhibiting its influx back into the tubular urine.