Water permeability ofNecturus gallbladder epithelial cell membranes measured by nuclear magnetic resonance

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Abstract
In order to assess the contribution of transcellular water flow to isosmotic fluid transport acrossNecturus gallbladder epithelium, we have measured the water permeability of the epithelial cell membranes using a nuclear magnetic resonance method. Spin-lattice (T 1) relaxation of water protons in samples of gallbladder tissue where the extracellular fluid contained 10 to 20mm Mn2+ showed two exponential components. The fraction of the total water population responsible for the slower of the two was 24±2%. Both the size of the slow component, and the fact that it disappeared when the epithelial layer was removed from the tissue, suggest that it was due to water efflux from the epithelial cells. The rate constant of efflux was estimated to be 15.6±1.0 sec1 which would be consistent with a diffusive membrane water permeabilityP d of 1.6×103 cm sec1 and an osmotic permeabilityP os of between 0.3×104 and 1.4×104 cm sec1 osmolar1. Using these data and a modified version of the standing-gradient model, we have reassessed the adequacy of a fluid transport theory based purely on transcellular osmotic water flow. We find that the model accounts satisfactorily for near-isosmotic fluid transport by the unilateral gallbladder preparation, but a substantial serosal diffusion barrier has to be included in order to account for the transport of fluid against opposing osmotic gradients.

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