HCO3−secretion in the esophageal submucosal glands

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Abstract
The mammalian esophagus has the capacity to secrete a HCO3−and mucin-rich fluid in the esophageal lumen. These secretions originate from the submucosal glands (SMG) and can contribute to esophageal protection against refluxed gastric acid. The cellular mechanisms by which glandular cells achieve these secretions are largely unknown. To study this phenomenon, we used the pH-stat technique to measure luminal alkali secretion in an isolated, perfused pig esophagus preparation. Immunohistochemistry was used to localize receptors and transporters involved in HCO3−transport. The SMG-bearing esophagus was found to have significant basal alkali secretion, predominantly HCO3−, which averaged 0.21 ± 0.04 μeq·h−1·cm−2. This basal secretion was doubled when stimulated by carbachol but abolished by HCO3−or Cl−removal. Basal- and carbachol-stimulated secretions were also blocked by serosal application of atropine, pirenzipine, DIDS, methazolamide, and ethoxzolamide. The membrane-impermeable carbonic anhydrase inhibitor benzolamide, applied to the serosal bath, partially inhibited basal HCO3−secretion and blocked the stimulation by carbachol. Immunohistochemistry using antibodies to M1cholinergic receptor or carbonic anhydrase-II enzyme showed intense labeling of duct cells and serous demilunes but no labeling of mucous cells. Labeling with an antibody to Na+-(HCO3−)n(rat kidney NBC) was positive in ducts and serous cells, whereas labeling for Cl−/HCO3−exchanger (AE2) was positive in duct cells but less pronounced in serous cells. These data indicate that duct cells and serous demilunes of SMG play a role in HCO3−secretion, a process that involves M1cholinergic receptor stimulation. HCO3−transport in these cells is dependent on cytosolic and serosal membrane-bound carbonic anhydrase. HCO3−secretion is also dependent on serosal Cl−and is mediated by DIDS-sensitive transporters, possibly NBC and AE2.