H+ secretion in distal tubules: the effect of carbonic anhydrase in luminal perfusates

1. The ability of distal tubules in control rats to acidify luminal fluids is questioned in view of the absence of bicarbonate reabsorption and of a disequilibrium pH as shown by the equilibrium electrode method. However, the ability to establish a transepithelial pH gradient has been shown previously. These discrepancies are investigated in the present work. 2. Luminal pH was measured in cortical distal segments of rat nephron during stationary perfusion with carbonic anhydrase (1 mg/ml)-containing solutions. During perfusion with bicarbonate Ringer, the stationary pH was significantly higher in the presence of the enzyme, suggesting the existence of an acid disequilibrium pH in these segments. 3. Acidification half-times were also longer in the presence of the enzyme, yielding bicarbonate reabsorption rates of 0.24 +/- 0.07 (N = 29)nmol cm-2s-1 in early and 0.32 +/- 0.09 (N = 28) in late distal tubules. 4. During luminal perfusion with phosphate Ringer, significant transepithelial pH gradients were also observed but were unaffected by the enzyme. Thus, a disequilibrium pH was detected in distal tubules when perfused with bicarbonate-containing solutions. This acid disequilibrium pH can inhibit distal bicarbonate reabsorption by creating an adverse pH gradient. 5. In conclusion, the ability of distal tubules to establish a significant pH gradient will contribute to the titration of non bicarbonate buffers, i.e., to titratable acid formation.