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First published online May 15, 2009
Journal of Experimental Biology 212, 1684-1696 (2009)
Published by The Company of Biologists 2009
doi: 10.1242/jeb.027730

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Review Article

High rates of HCO₃^– secretion and Cl^– absorption against adverse gradients in the marine teleost intestine: the involvement of an electrogenic anion exchanger and H⁺-pump metabolon?

M. Grosell^*, E. M. Mager, C. Williams and J. R. Taylor

RSMAS, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149, USA

^* Author for correspondence (e-mail: mgrosell{at}rsmas.miami.edu)

Accepted 19 March 2009

Anion exchange contributes significantly to intestinal Cl^– absorption in marine teleost fish and is thus vital for successful osmoregulation. This anion exchange process leads to high luminal HCO₃^– concentrations (up to 100 mmol l^–1) and high pH and results in the formation of CaCO₃ precipitates in the intestinal lumen. Recent advances in our understanding of the transport processes involved in intestinal anion exchange in marine teleost fish include the demonstration of a role for the H⁺-pump (V-ATPase) in apical H⁺ extrusion and the presence of an electrogenic (nHCO₃^–/Cl^–) exchange protein (SLC26a6). The H⁺-V-ATPase defends against cellular acidification, which might otherwise occur as a consequence of the high rates of base secretion. In addition, apical H⁺ extrusion probably maintains lower HCO₃^– concentrations in the unstirred layer at the apical surface than in the bulk luminal fluids and thus facilitates continued anion exchange. Furthermore, H⁺-V-ATPase activity hyperpolarizes the apical membrane potential that provides the driving force for apical electrogenic nHCO₃^–/Cl^– exchange, which appears to occur against both Cl^– and HCO₃^– electrochemical gradients. We propose that a similar coupling between apical H⁺ extrusion and nHCO₃^–/Cl^– exchange accounts for Cl^– uptake in freshwater fish and amphibians against very steep Cl^– gradients.

Key words: osmoregulation, water absorption, seawater, pH-stat titration, fish

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