title: Regulation of apical H⁺-ATPase activity and intestinal HCO₃⁻ secretion in marine fish osmoregulation.
abstract: The absorption of Cl(-) and water from ingested seawater in the marine fish intestine is accomplished partly through Cl(-)/HCO(3)(-) exchange. Recently, a H(+) pump (vacuolar-type H(+)-ATPase) was found to secrete acid into the intestinal lumen, and it may serve to titrate luminal HCO(3)(-) and facilitate further Cl(-)/HCO(3)(-) exchange, especially in the posterior intestine, where adverse concentration gradients could limit Cl(-)/HCO(3)(-) exchange. The H(+) pump is expressed in all intestinal segments and in gill tissue of gulf toadfish (Opsanus beta) maintained in natural seawater. After acute transfer of toadfish to 60 ppt salinity, H(+) pump expression increased 20-fold in the posterior intestine. In agreement with these observations was a fourfold-increased H(+)-ATPase activity in the posterior intestine of animals acclimated to 60 ppt salinity. Interestingly, Na(+)-K(+)-ATPase activity was elevated in the anterior intestine and gill, but not in the posterior intestine. Apical acid secretion by isolated intestinal tissue mounted in Ussing chambers fitted with pH-stat titration systems increased after acclimation to hypersalinity in the anterior and posterior intestine, titrating >20% of secreted bicarbonate. In addition, net base secretion increased in hypersalinity-acclimated fish and was ∼70% dependent on serosal HCO(3)(-). Protein localization by immunohistochemistry confirmed the presence of the vacuolar-type H(+)-ATPase in the apical region of intestinal enterocytes. These results show that the H(+) pump, especially in the posterior intestine, plays an important role in hypersaline osmoregulation and that it likely has significant effects on HCO(3)(-) accumulation in the intestinal lumen and, therefore, the continued absorption of Cl(-) and water.
authors.affiliation: Rosenstiel School of Marine and Atmospheric Sciences, University of Miami, Miami, Florida 33149, USA. s.guffey@umiami.edu
authors.forename: S
authors.initials: S
authors.lastname: Guffey
authors.forename: A
authors.initials: A
authors.lastname: Esbaugh
authors.forename: M
authors.initials: M
authors.lastname: Grosell
mesh_terms: D000818:Animals; D024541:Batrachoidiformes / Q000502:physiology*; D001639:Bicarbonates / Q000378:metabolism*; D015971:Gene Expression Regulation, Enzymologic / Q000187:drug effects; D005880:Gills / Q000201:enzymology / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D007413:Intestinal Mucosa / Q000378:metabolism; D007422:Intestines / Q000201:enzymology; D006180:Proton-Translocating ATPases / Q000235:genetics / Q000378:metabolism*; D054712:Salinity; D012965:Sodium Chloride / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000235:genetics / Q000378:metabolism; D014867:Water / Q000737:chemistry; D014882:Water-Electrolyte Balance / Q000502:physiology*
chemical_list: D001639:Bicarbonates; D014867:Water; D012965:Sodium Chloride; D006180:Proton-Translocating ATPases; D000254:Sodium-Potassium-Exchanging ATPase