Introduction The esophageal submucosal glands (SMG) in humans and several other mammalian species secrete bicarbonate and mucins into the esophageal lumen. These secretions contribute to esophageal defense against refluxed gastric acid. We recently demonstrated HCO3- secretion in the SMG-bearing pig esophagus and showed that it could be stimulated approximately two-folds by carbachol. Bicarbonate secretion was blocked by removal of HCO3- from the serosal bath and by treatment with DIDS, a nonselective anion transport inhibitor. We have also identified by immunohistochemistry the presence of carbonic anhydrase (CA) II, and of Na+-(HCO3-)n and Cl-/HCO3- transporters in the glandular cells. To investigate further the cellular mechanisms of HCO3- secretion in SMG, we studied its dependence on Cl- and the role of CA.
Methods Pig esophagi were stripped of their muscularis propria, keeping the mucosa and submucosal glands intact. The esophagi were cannulated, mounted, and bathed in a chamber, which allowed independent perfusion of the esophageal lumen. Bicarbonate secretion was measured by using a recirculated unbuffered saline solution into the lumen and pH stat technique.
Results Mean basal bicarbonate secretion was 0.11 ± 0.01 μEq/cm2/hour (n = 5). Removal of serosal Cl- abolished HCO3- secretion within ˜ 60 minutes (0.018 ± 0.01 μEq/cm2/hour). Subsequent addition of carbachol in the absence of Cl- did not stimulate HCO3- secretion. The membrane permeable CA inhibitor methazolamide (0.2 mM) added serosally completely inhibited basal and carbachol stimulated HCO3- secretion. The impermeant CA inhibitor benzolamide (0.01 mM) partially inhibited HCO3- secretion from 0.07 ± 0.005 to 0.05 ± 0.01 μEq/cm2/hour, which was not further stimulated by carbachol.
Conclusions HCO3- secretion is mediated by carbonic anhydrase dependent mechanism(s). Intracellular and extracellular CA are involved in this process. Transport mechanisms responsible for HCO3- secretion are dependent on the presence of Cl-.
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