Submitted on December 16, 2004
Accepted on January 22, 2005
Selective Permeability Barrier to Urea in Shark Rectal Gland
Joshua D. Zeidel1, John C. Mathai1, John D. Campbell2, Wily G. Ruiz3, Gerard L. Apodaca3, John Riordan4, and Mark L. Zeidel1*
1 Laboratory of Epithelial Cell Biology, Renal-Electrolyte Division and Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA; Mount Desert Island Biological Laboratories, Bar Harbor, ME, USA
2 Mount Desert Island Biological Laboratories, Bar Harbor, ME, USA
3 Laboratory of Epithelial Cell Biology, Renal-Electrolyte Division and Department of Medicine, University of Pittsburgh School of Medicine, Pittsburgh, PA, USA
4 Mount Desert Island Biological Laboratories, Bar Harbor, ME, USA; Mayo Clinic Foundation, Scottsdale, AZ, USA
* To whom correspondence should be addressed. E-mail: Zeidel{at}MSX.Dept-med.pitt.edu.
Elasmobranchs such as the dogfish shark Squalus Acanthius achieve osmotic homeostasis by
maintaining urea concentrations in the 300 - 400 mM range, thus offsetting to some degree ambient
marine osmolalities of 900 - 1000 mOsm/kg. 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
which 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 had shown that lipid bilayers
which 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, while basolateral membrane fractions were enriched for Na/K-ATPase.
Basolateral membrane osmotic water permeability (Pf) averaged 4.3 ± 1.3 X 10-3 cm/sec, while urea
permeability averaged 4.2 ± 0.8 X 10-7 cm/sec. The activation energy for water flow averaged 16.4
kcal/mole. Apical membrane Pf averaged 7.5 ± 1.6 X 10-4 cm/sec, and urea permeability averaged 2.2
± 0.4 X 10-7 cm/sec, with an average activation energy for water flow of 18.6 kcal/mole. 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 5
- fold 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.
