Effects of pH on potassium transport by renal distal tubule

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Abstract
To determine the relative importance of plasma and luminal pH changes as factors regulating K 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. Metabolic acidosis inhibited and metabolic alkalosis stimulated K secretion. Increased luminal fluid pH did not stimulate K transport. In experiments in which metabolic acidosis produced a diuresis, urinary K 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 K secretion increased in the distal and collecting tubules. Since the rate of fluid delivery is a potent stimulus of K secretion in the distal tubule, the stimulus of increased delivery of fluid, observed in free-flow conditions, apparently masked the inhibitory effect of acidosis on K transport. K 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.

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