Evidence for voltage regulation of carbonic anhydrase-independent acidification in turtle bladder

Despite 100% reabsorption of renal carbonic anhydrase activity, 70% of filtered bicarbonate is still reabsorbed. Recent data suggest that one of the mechanisms mediating carbonic anhydrase-independent bicarbonate reabsorption is the generation of favorable concentration gradients across the distal nephron following inhibition of carbonic anhydrase activity. Additional work suggests that the generation of favorable voltage gradients across the cortical collecting tubule may also mediate carbonic anhydrase-independent bicarbonate reabsorption. These studies were performed in turtle bladder to examine this issue. Acidification in this membrane was measured over a range of mucosa-negative potential difference of from 0 to 100 mV in the presence and absence of carbonic anhydrase activity. Under both conditions there was a linear increase in acidification as the potential difference increased. The rate of increase, however, was slightly less than 50% in the bladders subjected to carbonic anhydrase inhibition as compared to those with intact carbonic anhydrase activity. These data demonstrate that favorable electrical gradients enhance carbonic anhydrase-independent acidification as do favorable concentration gradients. We suggest that carbonic anhydrase-independent bicarbonate reabsorption in the kidney is mediated by a combination of favorable concentration and electrical gradients.