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Am J Physiol Regul Integr Comp Physiol. 2005 Apr;288(4):R936-46. Epub 2004 Dec 2.

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

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RSMAS, Division of Marine Biology and Fisheries, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149-1098, USA.


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 micromol x cm(-2) x h(-1) against 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 CO(2) 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.

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