CFTR inhibition augments NHE3 activity during luminal high CO2exposure in rat duodenal mucosa

Abstract

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 CO2diffusion and HCO3−secretion in rat duodenum. Duodena were perfused with a pH 6.4 high CO2solution or pH 2.2 low CO2solution 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 CO2solution increased CO2absorption and HCO3−secretion, accompanied by PV acidification and PV Pco2increase. During CO2challenge, CFTRinh-172 induced HCO3−absorption, while inhibiting PV acidification. S3226 reversed CFTRinh-associated HCO3−absorption. Luminal pH 2.2 challenge increased H+and CO2absorption 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 CO2challenge, 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 CO2absorption. NHE3, by secreting H+into the luminal microclimate, facilitates net transmucosal HCO3−absorption with a mechanism similar to proximal tubular HCO3−absorption.