Tracer microinjection studies of renal tubular permeability

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Abstract
Renal tubular permeability was studied in rats with a tracer microinjection technique in which radioactive inulin and another isotope were simultaneously microinjected into proximal or distal convoluted tubules during osmotic diuresis and their excretion by that kidney measured. Noninulin radioactivity excreted with a time course similar to that of inulin is termed direct recovery and that excreted more slowly, delayed recovery. The absence of inulin excretion by the contralateral kidney demonstrated that there was no transtubular efflux of this substance under these conditions. Inulin transit time averaged 0.84 min and 0.33 min following proximal and distal microinjection, respectively. Excreted sodium 22 molecules apparently followed closely the path of inulin molecules, since they appeared in the urine simultaneously. There was no delayed recovery of sodium 22. There was considerable direct and delayed recovery of urea-C14, indicating its diffusion into the tubular epithelial cells and subsequent return to the lumen. There was very little delayed and almost no direct recovery of tritiated water under these conditions in which physiologically maximally effective amounts of ADH were probably present. The injected quantity of isotope minus its direct recovery is believed to approximate its total tubular efflux.