Micropuncture study of distal tubular potassium and sodium transport in rat nephron

Abstract

Samples of proximal and distal tubular fluid were collected from rats on a control or on a low-Na diet. All animals received inulin-C14. Plasma (P) and tubular fluid (TF) were analyzed for sodium and potassium by micro-flame photometry. Transtubular electrical potential differences were measured with glass microelectrodes. Potassium ions were always extensively reabsorbed along the proximal tubule and along Henle''s loop. The importance of net secretion of potassium was demonstrated during hypertonic mannitol, KC1, NaC1, and Na2SO4 loading. Distal tubular entry of potassium always occurs down an electrochemical potential gradient. Normally, and in the state of dietary Na depletion, net sodium reabsorption along the distal tubule exceeds simultaneous potassium secretion by an order of magnitude or more. Neither the amount of sodium entering the distal tubule nor the intratubular sodium concentration were found rate limiting if a one-to-one exchange of cellular potassium for intratubular sodium were the sole mode of potassium secretion across the distal tubular epithelium.