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

Abstract

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 degrees C under in vivo-like conditions, secretion rates averaged 0.45 micromol x cm(-2) x h(-1), of which 0.25 micromol x cm(-2) x 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.

Mechanisms of transepithelial ammonia excretion and luminal alkalinization in the gut of an intestinal air-breathing fish, Misgurnus anguilliacaudatus

“…Electrophysiological measurements were performed under symmetric conditions (see Grosell and Genz, 2006) using voltage clamp (0mV) with mucosal reference while current was recorded. A 5mV potential was induced for 3s, at 60s intervals, through the epithelium from the mucosal to the serosal side.…”

“…The pH electrode and microburette of the pH-stat system were inserted into the mucosal chamber to measure the net base flux using a pH-stat technique (Grosell and Genz, 2006). In brief, the mucosal salines were maintained at 7.800±0.003pH units by the addition of 0.005moll…”

“…Total HCO 3 -and CO 3 2-were then measured as titratable alkalinity by double end-point titration as described previously (Grosell and Genz, 2006).…”

Measuring intestinal fluid transport in vitro: Gravimetric method versus non-absorbable marker

“…High molecular weight PEGs (3350-4000) are considered to be poorly absorbed by the intestine (Schedl, 1966;Krag et al 1975; Winne and Gorig, 1982;Furuichi et al 1984;Schiller et al 1997; Grosell and Genz, 2006), and easily determined in biological fluids by direct assay (Malawer and Powell, 1967) or labelling the PEG molecule with a radioisotope such as 3 H or 14 C (Wingate et al 1972;Krag et al 1975). The latter offers the greatest sensitivity and practical advantage, hence radiolabelled PEG has become a common choice in studies of gastrointestinal physiology, such as gut transit and digestibility in ruminants (Till and Downes, 1965;Pickard and Stevens, 1972), drinking rates in fish (Shehadeh and Gordon, 1969;Scott et al 2006;Genz et al 2008), and is widely regarded as a suitable volume marker for studies of intestinal absorption in vivo (Jacobson et al 1963;Schedl, 1966;Maddrey et al 1967;Schiller et al 1997).…”

“…However, examples of volume markers being successfully used in gut sac preparations are scarce and appear limited to 14 C-inulin (Aull, 1966) and more recently 14 C-PEG 4000 Grosell and Genz, 2006;Grosell and Taylor, 2007;. Although considered reliable in vivo, to the best of our knowledge there has not been any prior, direct validations of PEG as a volume marker for the gut sac technique.…”