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Am J Physiol Regul Integr Comp Physiol (May 18, 2006). doi:10.1152/ajpregu.00818.2005
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Submitted on November 22, 2005
Accepted on May 16, 2006

Ouabain sensitive bicarbonate secretion and acid absorption by the marine teleost fish intestine play a role in osmoregulation

Martin Grosell1* and Janet Genz1

1 Marine Biology and Fisheries, RSMAS, University of Miami, Miami, Florida, United States

* To whom correspondence should be addressed. E-mail: mgrosell{at}rsmas.miami.edu.

The gulf toadfish (Opsanus beta) intestine secretes base mainly in form of HCO3- via apical anion exchange to serve Cl- and water absorption for osmoregulatory purposes. Luminal HCO3- secretion rates measured by pH-stat techniques in Ussing chambers rely on oxidative energy metabolism and are highly temperature sensitive. At 25 °C under in vivo like conditions secretion rates average 0.45 µmol cm-2 h-1, of which 0.25 µmol cm-2 h-1 can be accounted for by hydration of endogenous CO2 partly catalyzed by carbonic anhydrase. Complete polarity of secretion of HCO3- and H+ arising from the CO2 hydration reaction is evident from equal rates of luminal HCO3- secretion via anion exchange and basolateral H+ extrusion. When basolateral H+ extrusion is partly inhibited by reducing serosal pH reduced luminal HCO3- secretion results. Basolateral H+ secretion occurs in exchange for Na+ via a 5-(N-ethyl-N-isopropyl) amiloride (EIPA) -insensitive mechanism and is ultimately fueled by the activity of the basolateral Na+:K+-ATPase. The fluid absorbed by the toadfish intestine to combat diffusive water loss to the concentrated marine environment is accompanied by a substantial basolateral H+ extrusion, intimately linking osmoregulation and acid-base balance.




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