Summary
Urea added to the outside of toad skin bathed in Ringer's solution increases the permeability of the skin to urea and sucrose, and creates a net inward flow of sucrose with a flux ratio of 3 to 4. Two hypotheses aim at explaining the sucrose transport, the first as entrainment in an inwardly directed water flow (anomalous solvent drag) created by urea (driving species) diffusion in the lateral intercellular spaces, the second as a direct solute-solute interaction.
The anomalous solvent drag model was formulated mathematically and simulated on an analogue computer. This permitted quantitative examination of the model assumptions and predictions. The flux ratio for the driven species (sucrose) was a function of two dimensionless parameters including the geometrical restrictions on the intercellular spaces, concentration of driving species, and diffusion coefficient of driving and driven species. For a range of experimental values of the system parameters, the calculated flux ratios of driven species were in agreement with most experimental findings. The unilateral fluxes would have to be restricted to 10−3 to 10−4 of the simple skin area. The ratio between the sucrose and urea flux ratios predicted by the model was equal to the observed value.
The solute-solute interaction model requires a coupling coefficient of 10−3 to account for the observed flux ratio. It is concluded that the anomalous solvent drag model is compatible with the bulk of experimental evidence available, whereas solute-solute interaction as occurring in free diffusion is far from being quantitatively sufficient.
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Stender, S., Kristensen, K. & Skadhauge, E. Solvent drag by solute-linked water flow. J. Membrain Biol. 11, 377–398 (1973). https://doi.org/10.1007/BF01869831
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DOI: https://doi.org/10.1007/BF01869831