THE MECHANISM OF POTASSIUM REABSORPTION IN THE PROXIMAL TUBULE OF THE RAT*

Abstract

According to present concepts, renal excretion of potassium is mediated by a three-component system: complete filtration of plasma potassium at the glomerulus, virtually complete reabsorption in the proximal tubule, and active secretion in the distal portions of the nephron.Of these mechanisms, proximal reabsorption has been the most difficult to examine experimentally.Conventional clearance techniques have provided only indirect evidence for the existence and magnitude of this process.Berliner (1) cites the following experiments as support for essentially complete reab- sorption of filtered potassium in the proximal tu- bule: 1) in the dog infused with saline and given an organic mercurial diuretic, potassium excretion remains constant and independent of large varia- tions in filtered potassium (2), and 2) in the dog with a split bladder subjected to unilateral renal artery constriction, if sodium excretion is main- tained by infusions of sodium salts or by diuretics, potassium excretion on the experimental side does not change despite a 30 to 35 per cent reduction in filtered potassium (3).Since these observations suggest that potassium excretion is independent of filtered load, the conclusion is drawn that either potassium reabsorption is adjusted exactly to fil- tered load, or, more likely, that filtered potassium is completely reabsorbed proximal to the secretory site and does not contribute to excreted potassium.The stop-flow technique has been of definite value in localizing and characterizing distal potas- sium secretion.Since, however, proximal samples must pass through the secretory site before col- lection, potassium concentration patterns cannot accurately reflect proximal tubular events (4, 5).