Bicarbonate secretion plays a role in chloride and water absorption of the European flounder intestine.

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