Urea reflection coefficient for the human red cell membrane.

The reflection coefficient, sigma, is an irreversible thermodynamic parameter which measures the interaction between solute and solvent in passage across a membrane. The initial estimate of Goldstein and Solomon ((1960) J. Gen. Physiol. 44, 1-17) by the zero-time method gave sigma urea = 0.6 for the human red cell membrane and a more recent measurement by Levitt and Mlekoday ((1983) J. Gen. Physiol. 81, 239-253) using a different method gave sigma urea = 0.95. We have now developed a variant of the zero-time method which gives sigma urea = 0.70 +/- 0.02, which is significantly different from 1.0. There has been controversy as to whether urea permeates the human red cell by the same channel used by water or by a different route. The finding that sigma urea is significantly less than 1.0 (actually less than 0.95) makes it possible to discriminate between these two possibilities since completely independent transfer of urea and water mandates a value of sigma urea = 0.95. Values significantly lower than 0.95 can only be achieved if the transport of the solute, urea, is coupled to that of the solvent, water.