Transport parameters in the human red cell membrane: solute-membrane interactions of hydrophilic alcohols and their effect on permeation

Abstract

A systematic study has been made of the three coefficients that describe the human red cell membrane transport of a series of short straight-chain hydrophilic alcohols: the permeability coefficient, ω i , the reflection coefficient, σ i , and the hydraulic conductivity, L p . Ethylene glycol transport is saturable with K m = 220 ± 50 mM ; there is a second, low-affinity, ethylene glycol site which inhibits water transport ( K l = 570 ± 140 mM , max. inhib. = 90 ± 10% ). σ eth gly = 0.71 ± 0.04 which is significantly less than 1 ( n = 6, P < 0.001), as are σ i for six other alcohols ( n = 23), thus providing strong thermodynamic evidence that water and these alcohols cross the red cell membrane, at least in part, in an aqueous channel. The solute/membrane frictional coefficient, f sm , for all seven alcohols has been determined and found to decrease monotonically as membrane permeability increases. The red cell membrane has been perturbed by treatments with phenylglyoxal and Bs 3 (bis(succinimidyl suberate)); these treatments are accompanied by correlated modulation of both ethylene glycol and urea permeability. In one set of experiments in control cells, urea permeability is correlated with water permeability; and, in another set, ethylene glycol permeability is correlated with water permeability. All of these observations support the proposiion that the urea class of solutes, the ethylene glycol class of solutes and water all cross the membrane through the same aqueous pore. A schematic model of the red cell pore, consistent with the experimental observations, is presented.