EFFECT OF BARIUM ON POTASSIUM DIFFUSION ACROSS THE PROXIMAL CONVOLUTED TUBULE OF THE ANESTHETIZED RAT

The role of diffusion in transepithelial potassium flux and the import ance of potassium channels in the luminal cell membrane to this proces s were examined by applying a luminal microperfusion technique to surf ace tubules in kidneys of anesthetized rats. Potassium concentration g radients were applied by altering the concentration of KCI in perfusat es. To some perfusates, 2 mmol/l BaCl2 was added to block potassium ch annels in the luminal cell membrane. The mean applied potassium concen tration gradient was highly predictive of net potassium transport in t he absence of any change in fluid reabsorption, with an apparent potas sium permeability of 22 x 10(-5) cm/s. Thus potassium transport in the proximal tubule may have an important diffusive component. Luminal ba rium significantly reduced the concentration of potassium in collected fluid under conditions of net potassium secretion, although a substan tial barium-insensitive potassium permeability was also observed. Howe ver, the site of action of luminally applied barium is uncertain in pr oximal tubule, since barium was reabsorbed by the tubule at a rate of 13.6 pmol . mm(-1). min(-1). We conclude that diffusion is a significa nt driving force for potassium reabsorption in proximal tubule and tha t most diffusive potassium transport occurs via a barium-insensitive r oute, possibly the paracellular pathway.