CFTR inhibition augments NHE3 activity during luminal high CO2 exposure in rat duodenal mucosa

We hypothesized that the function of duodenocyte apical membrane acid-base transporters are essential for H+ absorption from the lumen. We thus examined the effect of inhibition of Na+/H+ exchanger-3 (NHE3), cystic fibrosis transmembrane regulator (CFTR), or apical anion exchangers on transmucosal CO2 diffusion and HCO3- secretion in rat duodenum. Duodena were perfused with a pH 6.4 high CO2 solution or pH 2.2 low CO2 solution with the NHE3 inhibitor, S3226, the anion transport inhibitor, DIDS, or pretreatment with the potent CFTR inhibitor, CFTRinh-172, with simultaneous measurements of luminal and portal venous (PV) pH and carbon dioxide concentration ([CO2]). Luminal high CO2 solution increased CO2 absorption and HCO3- secretion, accompanied by PV acidification and PV PCO2 increase. During CO2 challenge, CFTRinh-172 induced HCO3- absorption, while inhibiting PV acidification. S3226 reversed CFTRinh-associated HCO3- absorption. Luminal pH 2.2 challenge increased H+ and CO2 absorption and acidified the PV, inhibited by CFTRinh-172 and DIDS, but not by S3226. CFTR inhibition and DIDS reversed HCO3- secretion to absorption and inhibited PV acidification during CO2 challenge, suggesting that HCO3- secretion helps facilitate CO2/H+ absorption. Furthermore, CFTR inhibition prevented CO2-induced cellular acidification reversed by S3226. Reversal of increased HCO3- loss by NHE3 inhibition and reduced intracellular acidification during CFTR inhibition is consistent with activation or unmasking of NHE3 activity by CFTR inhibition, increasing cell surface H+ available to neutralize luminal HCO3- with consequent CO2 absorption. NHE3, by secreting H+ into the luminal microclimate, facilitates net transmucosal HCO3- absorption with a mechanism similar to proximal tubular HCO3- absorption.