The involvement of H+-ATPase and carbonic anhydrase in intestinal HCO3– secretion in seawater-acclimated rainbow trout

Abstract

SUMMARYPyloric caeca and anterior intestine epithelia from seawater-acclimated rainbow trout exhibit different electrophysiological parameters with lower transepithelial potential and higher epithelial conductance in the pyloric caeca than the anterior intestine. Both pyloric caeca and the anterior intestine secrete HCO 3 -at high rates in the absence of serosal HCO 3 -/CO 2 , demonstrating that endogenous CO 2 is the principal source of HCO 3 -under resting control conditions. Apical, bafilomycin-sensitive, H + extrusion occurs in the anterior intestine and probably acts to control luminal osmotic pressure while enhancing apical anion exchange; both processes with implications for water absorption. Cytosolic carbonic anhydrase (CAc) activity facilitates CO 2 hydration to fuel apical anion exchange while membrane-associated, luminal CA activity probably facilitates the conversion of HCO 3 -to CO 2 . The significance of membrane-bound, luminal CA may be in part to reduce HCO 3 -gradients across the apical membrane to further enhance anion exchange and thus Cl -absorption and to facilitate the substantial CaCO 3 precipitation occurring in the lumen of marine teleosts. In this way, membrane-bound, luminal CA thus promotes the absorption of osmolytes and reduction on luminal osmotic pressure, both of which will serve to enhance osmotic gradients to promote intestinal water absorption.

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

“…It has been noted that the preparation of DIDS is key to its efficacy (Grosell et al, 2009b), and such precautions were taken in the present study. We thus conclude that the HCO 3 -excretion mechanism is relatively DIDS insensitive in loach and similar to that seen in trout and flounder (Grosell et al, 1999;Grosell et al, 2009a).…”

“…All the inhibitor stocks used in this study were also recently used in similar in vitro studies on toadfish and trout, which confirmed their efficacy (Grosell and Genz, 2006;Grosell et al, 2009a;Genz et al, 2011).…”

“…as has been demonstrated in Platichthys flesus (Grosell and Jensen, 1999) and O. mykiss (Grosell et al, 2009a), although in Anguilla japonica (Ando and Subramanyam, 1990) and Citharichthys sordidus (Grosell et al, 2001) the lower dose of 10…”

“…However, the lack of effect of ETZ, a permeant inhibitor that would inhibit both intracellular and extracellular luminal CA, argues against a role for these proteins in net base and ammonia flux in the weatherloach, and that uncatalyzed rates of reaction are sufficient under high ammonia conditions. In contrast, in the marine toadfish (Grosell and Genz, 2006) and seawater-acclimated rainbow trout (Grosell et al, 2009a), using ETZ Grosell and colleagues demonstrated that cytosolic CA and, thus, catalyzed endogenous HCO 3 -production are important to HCO 3 -secretion. Ando and Subramanyam reported similar findings in A. japonica using acetazolamide, another CA inhibitor (Ando and Subramanyam, 1990).…”

“…However, in marine teleosts HCO 3 -secretion is a significant driver of Cl -uptake and thus osmoregulation, which is not the case for teleosts living in fresh water (Grosell, 2010). As a corollary, in a study on the euryhaline rainbow trout, salinity acclimation resulted in an increase in cytosolic and membrane-bound CA isoforms , which are involved in HCO 3 -secretion (Grosell et al, 2009a). Nevertheless, CA has been localized by enzyme histochemistry in the weatherloach digestive tract to both the 'stomach' (anterior intestine) and posterior intestine (Chang et al, 2006).…”