The stop flow technique was used to investigate the permeability characteristics of the dog nephron to various C14-labeled non-electrolytes. 12 minutes after clamping the ureter, creatinine, PAH, and C14 compound were injected intravenously. 2 minutes later, urine samples were collected. Urea and glycerol were able to enter the tubular urine along the entire nephron at rates which were commensurate with their molecular weights. No significant movement of larger molecules (D-arabinose, D-glucose, and mannitol) could be detected. However, after administration of twenty units of pitressin, D-arabinose was able to diffuse across the distal and proximal tubular epithelium.

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