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

The gulf toadfish (Opsanus beta) intestine secretes base mainly in the form of HC[O.sup.-.sub.3] via apical anion exchange to serve [Cl.sup.-] and water absorption for osmoregulatory purposes. Luminal HC[O.sup.-.sub.3] secretion rates measured by pH-stat techniques in Ussing chambers rely on oxidative energy metabolism and are highly temperature sensitive. At 25[degrees]C under in vivo-like conditions, secretion rates averaged 0.45 [micro]mol x [cm.sup.-2] x [h.sup.-1], of which 0.25 [micro]mol x [cm.sup.-2] x [h.sup.-1] can be accounted for by hydration of endogenous C[O.sub.2] partly catalyzed by carbonic anhydrase. Complete polarity of secretion of HC[O.sup.-.sub.3] and [H.sup.+] arising from the C[O.sub.2] hydration reaction is evident from equal rates of luminal HC[O.sup.-.sub.3] secretion via anion exchange and basolateral [H.sup.+] extrusion. When basolateral [H.sup.+] extrusion is partly inhibited by reduction of serosal pH, luminal HC[O.sub.3] secretion is reduced. Basolateral [H.sup.+] secretion occurs in exchange for [Na.sup.+] via an ethylisopropylamilorideinsensitive mechanism and is ultimately fueled by the activity of the basolateral [Na.sup.+]-[K.sup.+]-ATPase. Fluid absorption by the toadfish intestine to oppose diffusive water loss to the concentrated marine environment is accompanied by a substantial basolateral [H.sup.+] extrusion, intimately linking osmoregulation and acid-base balance. bicarbonate transport; chloride absorption; epithelial water transport; seawater ingestion; pH-stat titrations