Carbonic anhydrases and mucosal vanilloid receptors help mediate the hyperemic response to luminal CO2 in rat duodenum

Background & Aims: The duodenal mucosa is exposed to PCO2 > 200 mm Hg due to the luminal mixture of gastric acid with secreted bicarbonate, which augments mucosal protective mechanisms. We examined the hyperemic response to elevated luminal PCO2 in the duodenum of anesthetized rats luminally exposed to high CO2 saline to help elucidate luminal acid-sensing mechanisms. Methods: Blood flow was measured by laser Doppler, and intracellular pH of epithelial cells by measured by ratio microimaging. The permeant carbonic anhydrase (CA) inhibitor methazolamide, relatively impermeant CA inhibitor benzolamide, vanilloid receptor antagonist capsazepine, or sodium-hydrogen exchanger 1 (NHE-1) inhibitor dimethyl amiloride were perfused with or without the high CO2 solution. Results: The high CO2 solution increased duodenal blood flow, which was abolished by pretreatment with methazolamide or capsazepine or by dimethyl amiloride coperfusion. Sensory denervation with capsaicin also abolished the CO2 effects. Benzolamide dose-dependently inhibited CO2-induced hyperemia and at 100 nmol/L inhibited CO2-induced intracellular acidification. The membrane-bound CA isoforms IV, IX, XII, and XIV and cytosolic CA II and the vanilloid receptor 1 (TRPV1) were expressed in duodenum and stomach. Dorsal root ganglion and no-dose ganglion expressed all isoforms except for CA IX. Conclusions: The duodenal hyperemic response to luminal CO2 is dependent on cytosolic and membrane-bound CA isoforms, NHE-1, and TRPV1. CO2-induced intracellular acidification was inhibited by selective extracellular CA inhibition, suggesting that CO2 diffusion across the epithelial apical membrane is mediated by extracellular CA. NHE-1 activation preceding TRPV1 stimulation suggests that luminal CO2 is sensed as H+ in the subepithelium.