FILLED PORE APPROXIMATION - A THEORETICAL FRAMEWORK FOR SOLUTE-SOLVENT COUPLING IN NARROW WATER CHANNELS

A phenomenological model is presented of water and solute transport th at is applicable to water pores with radii less than similar to 2 Angs trom. This includes such examples as gramicidin A, the proximal tubule basolateral. membrane, and the aquaporin 1 (CHIP28) water channel. Th e model differs from the conventional single-file model by allowing fo r a variation of unoccupied volume within the pores. It is shown that the accessible or free portion of the unoccupied volume can be related to the mechanical frictional coefficients and thereby to the filtrati on and diffusive permeabilities by the filled pore approximation. In g eneral, the smallness of the unoccupied volume represents the compactn ess of the molecules within the pore and is indicative of the single-f ile character of the motion of water and solute moving together. When that volume is equal to a single water volume, the results are identic al to the conventional single-file model. An important result is that, despite very low diffusive permeabilities, the reflection coefficient of a solute can remain at similar to 0.5 if its frictional interactio n with the channel walls is comparable with its frictional interaction with neighboring water molecules. This is consistent with values prev iously reported for NaCl in cell membranes of proximal tubule. The mod el predicts a minimum effective pore radius for a water channel of 1.7 8 Angstrom and corresponds to a maximum filtration-to-diffusion permea bility ratio that is proportional to the length of the effective pore or channel. This limiting condition corresponds to a water channel com pletely filled by water and may be applicable to the aquaporin 1 water channel.