Water-solute coupling and ion selectivity in epithelia

Abstract

Water transport across epithelia results from local osmotic gradients set up within the epithelium by active solute transport. These gradients arise in long, narrow, dead-end channels, which function as standing-gradient flow systems. Mathematical analysis of a simple model of such a flow system suggests that the ultrastructural geometry of epithelia accounts for the principal features of water-to-solute coupling. ‘Backwards’ channels are expected to differ from ‘forwards’ channels in being hypotonic rather than hypertonic, in their capacity for osmotic filtration, and in being restricted to the active transport of major plasma constituents. Epithelial pumps and other biological systems distinguish remarkably between closely similar ions, such as Na + and K + , Cl - and I - , and Ca 2+ and Mg 2+ . Qualitative regularities in discrimination, the so-called selectivity sequences, and quantitative regularities, the so-called selectivity isotherms, make it possible to predict the potencies of untested ions in a given biological system, once the relative effects of two related ions are known for that system. The observed selectivity sequences for the principal inorganic ions of biological interest have been correctly reconstructed by comparing the ions’ hydration energies with their interaction energies with membrane sites of different strengths. Details of the selectivity isotherms may provide information about the molecular structure of the biological sites.