Epithelial carbonic anhydrases facilitate P CO2 and pH regulation in rat duodenal mucosa

The duodenum is the site of mixing of massive amounts of gastric H + with secreted HCO 3 − , generating CO 2 and H 2 O accompanied by the neutralization of H + . We examined the role of membrane‐bound and soluble carbonic anhydrases (CA) by which H + is neutralized, CO 2 is absorbed, and HCO 3 − is secreted. Rat duodena were perfused with solutions of different pH and P   CO   2with or without a cell‐permeant CA inhibitor methazolamide (MTZ) or impermeant CA inhibitors. Flow‐through pH and P   CO   2electrodes simultaneously measured perfusate and effluent pH and P   CO   2. High CO 2 (34.7 kPa) perfusion increased net CO 2 loss from the perfusate compared with controls (pH 6.4 saline, P   CO   2≈ 0 ) accompanied by portal venous (PV) acidification and P   CO   2increase. Impermeant CA inhibitors abolished net perfusate CO 2 loss and increased net HCO 3 − gain, whereas all CA inhibitors inhibited PV acidification and P   CO   2increase. The changes in luminal and PV pH and [CO 2 ] were also inhibited by the Na + –H + exchanger‐1 (NHE1) inhibitor dimethylamiloride, but not by the NHE3 inhibitor S3226. Luminal acid decreased total CO 2 output and increased H + loss with PV acidification and P   CO   2increase, all inhibited by all CA inhibitors. During perfusion of a 30% CO 2 buffer, loss of CO 2 from the lumen was CA dependent as was transepithelial transport of perfused 13 CO 2 . H + and CO 2 loss from the perfusate were accompanied by increases of PV H + and tracer CO 2 , but unchanged PV total CO 2 , consistent with CA‐dependent transmucosal H + and CO 2 movement. Inhibition of membrane‐bound CAs augments the apparent rate of net basal HCO 3 − secretion. Luminal H + traverses the apical membrane as CO 2 , is converted back to cytosolic H + , which is extruded via NHE1. Membrane‐bound and cytosolic CAs cooperatively facilitate secretion of HCO 3 − into the lumen and CO 2 diffusion into duodenal mucosa, serving as important acid–base regulators.