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Biophys J, November 2002, p. 2856-2863, Vol. 83, No. 5

Importance of Intracellular Water Apparent Diffusion to the Measurement of Membrane Permeability

Jonathan V. Sehy,* Alison A. Banks,Dagger Joseph J. H. Ackerman,dagger § and Jeffrey J. Neil§||

 *Program in Molecular Cell Biology and  dagger Department of Chemistry, Washington University, St. Louis, Missouri 63130,  Dagger Department of Biology, Bowdoin College, Brunswick, Maine 04011,  §Departments of Radiology and  Internal Medicine, Washington University School of Medicine, St. Louis, Missouri 63110, and  ||Departments of Neurology and Pediatrics, St. Louis Children's Hospital, St. Louis, Missouri 63110 USA

The exchange of water across biological membranes is of fundamental significance to both animal and plant physiology. Diffusional membrane permeability (Pd) for the Xenopus oocyte, an important model system for water channel investigation, is typically calculated from intracellular water pre-exchange lifetime, cell volume, and cell surface area. There is debate, however, whether intracellular water motion affects water lifetime, and thereby Pd. Mathematical modeling of water transport is problematic because the intracellular water diffusion rate constant (D) for cells is usually unknown. The measured permeability may be referred to as the apparent diffusional permeability, P<UP><SUB><IT>d</IT></SUB><SUP><IT>′</IT></SUP></UP>, to acknowledge this potential error. Herein, we show that magnetic resonance (MR) spectroscopy can be used to measure oocyte water exchange with greater temporal resolution and higher signal-to-noise ratio than other methods. MR imaging can be used to assess both oocyte geometry and intracellular water diffusion for the same single cells. MR imaging is used to confirm the dependence of intracellular water lifetime on intracellular diffusion. A model is presented to relate intracellular lifetime to true membrane diffusional permeability. True water diffusional permeability (2.7 ± 0.4 µm/s) is shown to be 39 ± 6% greater than apparent diffusional permeability for 8 oocytes. This discrepancy increases with cell size and permeability (such as after water channel expression) and decreases with increasing intracellular water D.

Biophys J, November 2002, p. 2856-2863, Vol. 83, No. 5
© 2002 by the Biophysical Society   0006-3495/02/11/2856/08  $2.00


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