Transepithelial pH gradients in cortical distal tubules during metabolic alkalosis

1. The cortical distal tubule of the rat kidney participates in the regulation of acid-base balance, showing bicarbonate reabsorption, secretion or absence of transport under different experimental conditions. In the present study, we measured differences in transepithelial pH using double ion-exchange resin/reference microelectrodes in control and alkalotic (chronic plus acute) male Wistar rats and in alkalotic rats receiving a K+ supplement in diet and infusion. 2. pH was measured in the tubule lumen during stationary microperfusion with 25 mM bicarbonate Ringer solution, and in peritubular vessels next to the perfused tubules. 3. Differences in transepithelial pH were 0.70 +/- 0.12 (N = 16) pH units in early distal tubules (ED) and 1.03 +/- 0.050 (N = 15) in late distal tubules LD) of control rats, 0.22 +/- 0.056 (N = 17) in ED and 0.25 +/- 0.050 (N = 20) in LD of alkalotic rats, and -0.02 +/- 0.039 (N = 24) in ED and -0.02 +/- 0.040 (N = 24) in LD of K(+)-supplemented alkalotic rats. 4. In control rats, the transepithelial potential difference (PD) (-8.9 +/- 1.45 mV (N = 16) in ED and -32.7 +/- 2.99 mV (N = 15) in LD) was not large enough to explain transepithelial H+ and HCO3- gradients, suggesting the presence of an active transport mechanism responsible for their maintenance. 5. The present data show that the cortical distal tubule is able to establish transepithelial pH (HCO3-) differences, that these differences are reduced by alkalosis and abolished by alkalosis plus K+ supplementation, and that, although inversion of pH gradients (evidence for bicarbonate secretion) was observed in individual tubules, this inversion was not significant in the groups studied.