Experiments performed on isolated intestinal segments from the marine teleost fish, the European flounder (Platichthys flesus), revealed that the intestinal epithelium is capable of secondary active HCO3− secretion in the order of 0.2–0.3 μmol·cm−2·h−1 against an apparent electrochemical gradient. The HCO3− secretion occurs via anion exchange, is dependent on mucosal Cl−, results in very high mucosal HCO3− concentrations, and contributes significantly to Cl− and fluid absorption. This present study was conducted under in vivo-like conditions, with mucosal saline resembling intestinal fluids in vivo. These conditions result in a transepithelial potential of −16.2 mV (serosal side negative), which is very different from the −2.2 mV observed under symmetrical conditions. Under these conditions, we found a significant part of the HCO3− secretion is fueled by endogenous epithelial CO2 hydration mediated by carbonic anhydrase because acetazolamide (10−4 M) was found to inhibit HCO3− secretion and removal of serosal CO2 was found not to influence HCO3− secretion. Reversal of the epithelial electrochemical gradient for Cl− (removal of serosal Cl−) and elevation of serosal HCO3− resulted in enhanced HCO3− secretion and enhanced Cl− and fluid absorption. Cl− absorption via an anion exchange system appears to partly drive fluid absorption across the intestine in the absence of net Na+ absorption.
- HCO3− secretion
- chloride absorption
- carbonic anhydrase
- marine teleost
- Copyright © 2005 the American Physiological Society