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COMPARATIVE AND EVOLUTIONARY PHYSIOLOGY

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

Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida

Submitted 22 November 2005 ; accepted in final form 16 May 2006

The gulf toadfish (Opsanus beta) intestine secretes base mainly in the 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 averaged 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 reduction of serosal pH, luminal HCO₃^– secretion is reduced. Basolateral H⁺ secretion occurs in exchange for Na⁺ via an ethylisopropylamiloride-insensitive mechanism and is ultimately fueled by the activity of the basolateral Na⁺-K⁺-ATPase. Fluid absorption by the toadfish intestine to oppose diffusive water loss to the concentrated marine environment is accompanied by a substantial basolateral H⁺ extrusion, intimately linking osmoregulation and acid-base balance.

bicarbonate transport; chloride absorption; epithelial water transport; seawater ingestion; pH-stat titrations

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