Demonstration of active potassium secretion in the late distal tubule.

Studies were performed in the rat, under steady-state conditions, to determine the mode of potassium transport in the late distal tubule. In these experiments the potential difference and potassium activity were measured simultaneously in steady-state fluid droplets in control rats, during acute infusion of KCl, and in potassium-deficient rats. The results demonstrate active potassium secretion in the late distal tubule of the control and KCl-infused animals, since the observed potassium activity of 11.14 +/- 1.39 in control animals and 25.26 +/- 2.96 meq/liter in potassium-loaded animals was significantly greater than the predicted values of 7.75 +/- 0.59 and 14.01 +/- 0.94 meq/liter in the same groups, respectively. In potassium-deficient animals, however, the observed potassium activity in luminal fluid of 4.03 +/- 0.55 meq/liter was not different from the predicted value of 4.26 +/- 0.32. In summary, this study demonstrates that potassium secretion in the late distal tubule of the rat occurs by an active process and fails to demonstrate active absorption in the same tubular segment.