HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text Full Text (PDF) All Versions of this Article: 289/1/F83    most recent 00456.2004v1 Alert me when this article is cited Alert me if a correction is posted Citation Map Services Email this article to a friend Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Zeidel, J. D. Articles by Zeidel, M. L. Search for Related Content PubMed PubMed Citation Articles by Zeidel, J. D. Articles by Zeidel, M. L.

Selective permeability barrier to urea in shark rectal gland

¹Laboratory of Epithelial Cell Biology, Renal-Electrolyte Division and Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, Pennsylvania; ²Mount Desert Island Biological Laboratories, Bar Harbor, Maine; and ³Mayo Clinic Foundation, Scottsdale, Arizona

Submitted 16 December 2004 ; accepted in final form 22 January 2005

Elasmobranchs such as the dogfish shark Squalus acanthius achieve osmotic homeostasis by maintaining urea concentrations in the 300- to 400-mM range, thus offsetting to some degree ambient marine osmolalities of 900–1,000 mosmol/kgH₂O. These creatures also maintain salt balance without losing urea by secreting a NaCl-rich (500 mM) and urea-poor (18 mM) fluid from the rectal gland that is isotonic with the plasma. The composition of the rectal gland fluid suggests that its epithelial cells are permeable to water and not to urea. Because previous work showed that lipid bilayers that permit water flux do not block flux of urea, we reasoned that the plasma membranes of rectal gland epithelial cells must either have aquaporin water channels or must have some selective barrier to urea flux. We therefore isolated apical and basolateral membranes from shark rectal glands and determined their permeabilities to water and urea. Apical membrane fractions were markedly enriched for Na-K-2Cl cotransporter, whereas basolateral membrane fractions were enriched for Na-K-ATPase. Basolateral membrane osmotic water permeability (P_f) averaged 4.3 ± 1.3 x 10^–3 cm/s, whereas urea permeability averaged 4.2 ± 0.8 x 10^–7 cm/s. The activation energy for water flow averaged 16.4 kcal/mol. Apical membrane P_f averaged 7.5 ± 1.6 x 10^–4 cm/s, and urea permeability averaged 2.2 ± 0.4 x 10^–7 cm/s, with an average activation energy for water flow of 18.6 kcal/mol. The relatively low water permeabilities and high activation energies argue strongly against water flux via aquaporins. Comparison of membrane water and urea permeabilities with those of artificial liposomes and other isolated biological membranes indicates that the basolateral membrane urea permeability is fivefold lower than would be anticipated for its water permeability. These results indicate that the rectal gland maintains a selective barrier to urea in its basolateral membranes.

elasmobranchs; apical and basal membrane permeability; membrane vesicles