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Micropuncture of the proximal convoluted tubule during mannitol diuresis in rats in poor physiologic condition because of abdominal exposure of the kidney disclosed that steep potassium concentration gradients between tubular fluid and plasma could be generated, with concentration ratios as low as 0.5. This constitutes definitive evidence that proximal tubular potassium reabsorption must be active, since a passive process mediated by solvent drag could not depress the luminal concentration of potassium. Similar studies during mannitol diuresis in rats in good physiologic condition (flank exposure of the kidney) revealed that the proximal tubule, under these conditions, did not generate steep concentration gradients; the concentration ratios were the same as in nondiuretic rats (0.9); moreover, urinary potassium excretion rose linearly as urine mannitol diuresis mounted. These findings establish that in this group potassium reabsorption was sharply depressed by mannitol diuresis and suggest that the increased urinary potassium excreted by these rats may have been derived from the filtered potassium which escaped proximal reabsorption.
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Accepted: September 25, 1963
Received: July 18, 1963With the technical assistance of MARTHA HUDDLESTON
☆Supported in part by grant HTS-5469 from the National Institutes of Health and in part by a grant from the Dallas Heart Association.
© 1964 Published by Elsevier Inc.