title: Active urea transport and an unusual basolateral membrane composition in the gills of a marine elasmobranch.
abstract: In elasmobranch fishes, urea occurs at high concentrations (350-600 mM) in the body fluids and tissues, where it plays an important role in osmoregulation. Retention of urea by the gill against this huge blood-to-water diffusion gradient requires specialized adaptations to the epithelial cell membranes. Experiments were performed to determine the mechanisms and structural features that facilitate urea retention by the gill of the spiny dogfish Squalus acanthias. Analysis of urea uptake by gill basolateral membrane vesicles revealed the presence of a phloretin-sensitive (half inhibition 0.09 mM), sodium-coupled, secondary active urea transporter (Michaelis constant = 10.1 mM, maximal velocity = 0.34 micromol. h(-1). mg protein(-1)). We propose that this system actively transports urea out of the gill epithelial cells back into the blood against the urea concentration gradient. Lipid analyses of the basolateral membrane revealed high levels of cholesterol contributing to the highest reported cholesterol-to-phospholipid molar ratio (3.68). This unique combination of active urea transport and modification of the phospholipid bilayer membrane is responsible for decreasing the gill permeability to urea and facilitating urea retention by the gill of Squalus acanthias.
authors: Department of Zoology, University of Guelph, Guelph, Ontario N1G 2W1, Canada.
authors: G A
authors: GA
authors: Fines
authors: J S
authors: JS
authors: Ballantyne
authors: P A
authors: PA
authors: Wright
mesh_terms: D000081:Acetamides / Q000494:pharmacology; D000222:Adaptation, Physiological / Q000502:physiology; D000255:Adenosine Triphosphate / Q000378:metabolism / Q000494:pharmacology; D000818:Animals; D001693:Biological Transport, Active / Q000187:drug effects / Q000502:physiology; D015415:Biomarkers; D002250:Carbon Radioisotopes; D002412:Cations / Q000378:metabolism; D002462:Cell Membrane / Q000737:chemistry / Q000201:enzymology*; D002784:Cholesterol / Q000032:analysis / Q000378:metabolism; D002845:Chromatography; D004284:Dogfish / Q000378:metabolism*; D003576:Electron Transport Complex IV / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005880:Gills / Q000378:metabolism*; D005952:Glucose-6-Phosphatase / Q000378:metabolism; D009612:Nicotinamide-Nucleotide Adenylyltransferase / Q000378:metabolism; D010042:Ouabain / Q000494:pharmacology; D010693:Phloretin / Q000494:pharmacology; D010743:Phospholipids / Q000032:analysis / Q000378:metabolism; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism; D013890:Thiourea / Q000494:pharmacology; D014508:Urea / Q000493:pharmacokinetics*
chemical_list: D000081:Acetamides; D015415:Biomarkers; D002250:Carbon Radioisotopes; D002412:Cations; D004791:Enzyme Inhibitors; D010743:Phospholipids; D010042:Ouabain; D000255:Adenosine Triphosphate; D014508:Urea; D002784:Cholesterol; D003576:Electron Transport Complex IV; D009612:Nicotinamide-Nucleotide Adenylyltransferase; D005952:Glucose-6-Phosphatase; D000254:Sodium-Potassium-Exchanging ATPase; D013890:Thiourea; D010693:Phloretin