Interrelation of ethylene glycol, urea and water transport in the red cell

The reflection coefficient, sigma j, which measures the coupling between the jth solute and water transport across a semipermeable membrane, varies between 0 and 1.0. Values of sigma j significantly less than 1.0 provide irreversible thermodynamic proof that there is coupling between the transport of solute and solvent and thus that they share a common pathway. We have developed an improved method for measuring sigma and have used it to determine that sigma ethylene glycol = 0.71 +/- 0.03 and sigma urea = 0.65 +/- 0.03, in agreement with many, but not all, previous determinations. Since both of these values are significantly lower than 1.0, they show that there is a common ethylene glycol/water pathway and a common urea/water pathway. Addition of first one and then two methyl groups to urea increases sigma to 0.89 +/- 0.04 for methylurea and 0.98 +/- 0.4 for 1,3-dimethylurea, consistent with passage through an aqueous pore with a sharp cutoff in the 6-7 A region.