To determine the relative importance of plasma and
luminal pH changes as factors regulating
potassium secretion by rat distal tubule, superficial tubules were continuously microperfused in vivo. The effects of changes in plasma pH were examined by producing acute systemic
metabolic acidosis or
alkalosis and holding
luminal flow rate, solute composition, and pH constant by microperfusion. Alternatively, the effect of
luminal solution pH was evaluated by microperfusing tubules with solutions buffered to either pH 6.5 or 8.0 at constant systemic acid-base balance. Net transport of Na and K and the pH of the
luminal fluid were measured. Results showed that
metabolic acidosis inhibited and metabolic
alkalosis stimulated
potassium secretion. Increased
luminal fluid pH, in contrast, did not stimulate
potassium transport. In experiments in which
metabolic acidosis produced a diuresis, urinary
potassium excretion was enhanced compared with hydropenic controls. Free-flow
micropuncture studies revealed that the rate of fluid delivery to the distal tubule was 45% greater during
acidosis compared with control and that
potassium secretion increased in both the distal and collecting tubule. Since the rate of fluid delivery is a potent stimulus of
potassium secretion in the distal tubule, it is concluded that the stimulus of increased delivery of fluid, observed in free-flow conditions, masked the inhibitory effect of
acidosis on
potassium transport.
Potassium transport by the distal tubule, during
acid-base disorders, is regulated by plasma pH and the rate of delivery of fluid but is not stimulated by alkalinization of the
luminal fluid.