Mechanism of augmented duodenal HCO3−secretion after elevation of luminal CO2

The proximal duodenum is exposed to extreme elevations of Pco2because of the continuous mixture of secreted HCO3−with gastric acid. These elevations (up to 80 kPa) are likely to place the mucosal cells under severe acid stress. Furthermore, we hypothesized that, unlike most other cells, the principal source of CO2for duodenal epithelial cells is from the lumen. We hence examined the effect of elevated luminal Pco2on duodenal HCO3−secretion (DBS) in the rat. DBS was measured by the pH-stat method. For CO2challenge, the duodenum was superfused with a high Pco2solution. Intracellular pH (pHi) of duodenal epithelial cells was measured by ratio microfluorometry. CO2challenge, but not isohydric solutions, strongly increased DBS to approximately two times basal for up to 1 h. Preperfusion of the membrane-permeant carbonic anhydrase inhibitor methazolamide, or continuous exposure with indomethacin, fully inhibited CO2-augmented DBS. Dimethyl amiloride (0.1 mM), an inhibitor of the basolateral sodium-hydrogen exchanger 1, also inhibited CO2-augumented DBS, although S-3226, a specific inhibitor of apical sodium-hydrogen exchanger 3, did not. DIDS, an inhibitor of basolateral sodium-HCO3−cotransporter, also inhibited CO2-augemented DBS, as did the anion channel inhibitor 5-nitro-2-(3-phenylpropylamino) benzoic acid. CO2decreased epithelial cell pHi, followed by an overshoot after removal of the CO2solution. We conclude that luminal CO2diffused in the duodenal epithelial cells and was converted to H+and HCO3−by carbonic anhydrase. H+initially exited the cell, followed by secretion of HCO3−. Secretion was dependent on a functioning basolateral sodium/proton exchanger, a functioning basolateral HCO3−uptake mechanism, and submucosal prostaglandin generation and facilitated hydration of CO2into HCO3−and H+.