Potassium secretion by the decending limb or pars recta of the juxtamedullary nephron in vivo.

Potassium reabsorption by the juxtamedullary nephron up to the hairpin turn was studied by the micropuncture technique in the exposed renal papilla of rats. In 18 nondiuretic rats, the fraction of filtered potassium remaining at the end of the desdending limb averaged 113 +/- 9%, indicating either that potassium is not reabsorbed by the juxtamedullary proximal tubule and descending limb or that potassium is reabsorbed and secreted in those segments. Furosemide, a drug which inhibits NaCl reabsorption in the ascending limb downstream from the descending limb, significantly decreased the potassium remaining at the end of the descending limb from 106 +/- 12 to 72 +/-11% in seven rats. Benzolamide, a drug which inhibits reabsorption of NaHCO3 and water in the proximal tubule upstream from the descending limb significantly increased the potassium remaining from 103 +/- 13 to 177 +/- 32% in eight rats. These findings support the hypothesis that in the rat, potassium is normally reabsorbed by the proximal convoluted tubule and secreted in the pars recta or descending limb of the juxtamedullary nephron.