Mechanisms of transepithelial ammonia excretion and luminal alkalinization in the gut of an intestinal air-breathing fish, Misgurnus anguilliacaudatus.
The weatherloach, Misgurnus angulliacaudatus, is an intestinal air-breathing, freshwater fish that has the unique ability to excrete ammonia through gut volatilization when branchial and cutaneous routes are compromised during high environmental ammonia or air exposure. We hypothesized that transepithelial gut NH(4)(+) transport is facilitated by an apical Na(+)/H(+) (NH(4)(+)) exchanger (NHE) and a basolateral Na(+)/K(+)(NH(4)(+))-ATPase, and that gut boundary layer alkalinization (NH(4)(+) → NH(3) + H(+)) is facilitated by apical HCO(3)(-) secretion through a Cl(-)/HCO(3)(-) anion exchanger. This was tested using a pharmacological approach with anterior (digestive) and posterior (respiratory) intestine preparations mounted in pH-stat-equipped Ussing chambers. The anterior intestine had a markedly higher conductance, increased short-circuit current, and greater net base (J(base)) and ammonia excretion rates (J(amm)) than the posterior intestine. In the anterior intestine, HCO(3)(-) accounted for 70% of J(base). In the presence of an imposed serosal-mucosal ammonia gradient, inhibitors of both NHE (EIPA, 0.1 mmol l(-1)) and Na(+)/K(+)-ATPase (ouabain, 0.1 mmol l(-1)) significantly inhibited J(amm) in the anterior intestine, although only EIPA had an effect in the posterior intestine. In addition, the anion exchange inhibitor DIDS significantly reduced J(base) in the anterior intestine although only at a high dose (1 mmol l(-1)). Carbonic anhydrase does not appear to be associated with gut alkalinization under these conditions as ethoxzolamide was without effect on J(base). Membrane fluidity of the posterior intestine was low, suggesting low permeability, which was also reflected in a lower mucosal-serosal J(amm) in the presence of an imposed gradient, in contrast to that in the anterior intestine. To conclude, although the posterior intestine is highly modified for gas exchange, it is the anterior intestine that is the likely site of ammonia excretion and alkalinization leading to ammonia volatilization in the gut.
Laboratório de Ecofisiologia, CIMAR/CIIMAR - Centro Interdisciplinar de Investigação Marinha e Ambiental - Universidade do Porto, Portugal. wilson.jm.cimar@gmail.com
Jonathan M
JM
Wilson
Joana
J
Moreira-Silva
Inês L S
IL
Delgado
Sue C
SC
Ebanks
Mathilakath M
MM
Vijayan
João
J
Coimbra
Martin
M
Grosell
D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid / Q000494:pharmacology; D000388:Air; D000468:Alkalies / Q000378:metabolism*; D000584:Amiloride / Q000031:analogs & derivatives / Q000494:pharmacology; D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D002257:Carbonic Anhydrase Inhibitors / Q000494:pharmacology; D003531:Cypriniformes / Q000378:metabolism*; D055724:Electrophysiological Phenomena / Q000187:drug effects; D004848:Epithelium / Q000187:drug effects / Q000378:metabolism*; D005016:Ethoxzolamide / Q000494:pharmacology; D007413:Intestinal Mucosa / Q000378:metabolism*; D007422:Intestines / Q000166:cytology / Q000187:drug effects; D008560:Membrane Fluidity / Q000187:drug effects; D008954:Models, Biological; D012119:Respiration / Q000187:drug effects; D012704:Serous Membrane / Q000187:drug effects / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers / Q000378:metabolism
D000468:Alkalies; D002257:Carbonic Anhydrase Inhibitors; D017923:Sodium-Hydrogen Exchangers; D000641:Ammonia; D000584:Amiloride; D017878:4,4'-Diisothiocyanostilbene-2,2'-Disulfonic Acid; C039614:ethylisopropylamiloride; D005016:Ethoxzolamide