The Aqueous Pore in the Red Cell Membrane: Band 3 as a Channel for Anions, Cations, Nonelectrolytes, and Water*

Abstract

This article develops arguments for the existence of an aqueous pore in the red cell membrane as the principal route for passive flux of ions, water, and small nonelectrolytes and proposes a molecular model for the pore. In principle, such an aqueous pore would provide easy passage into and out of the cell for all solutes small enough to enter the channel. The red cell membrane, however, regulates the fluxes of cations and anions closely and discriminates carefully among other small solutes. These constraints have been incorporated into the model, which visualizes the channel and its associated regulatory system as governing passive transport of ions of either sign, as well as water and small nonelectrolytes into and out of the cell. The model, which was formulated to consolidate a number of observations already in the literature, has caused us to look for new interrelations between inhibitors specific to cation, anion, and nonelectrolyte transport. The results of these experiments, presented below, demonstrate that interrelations d o exist and provide evidence that supports the view that a common aqueous channel provides primary access to the red cell cytoplasm. REVIEW OF EVIDENCE FOR THE AQUEOUS POREIn 1957, Paganelli and Solomon' measured water diffusion across the human red cell membrane and proposed that the membrane was traversed by a n "equivalent pore" of 3.5 A radius. The pore dimensions were established from the ratio of the osmotic permeability coefficient, PI, measured by Side1 and Solomon,2 to the diffusional permeability coefficient, P d . Subsequently, Goldstein and Solomon' measured the reflection coefficient of a number of small hydrophilic solutes and found that these conformed t o expectations for an aqueous pore whose dimensions could be computed from the equations governing steric hindrance put forward by Pappenheimer et a/.' and R e n k h s Drawing primarily on these results, as well as a subsequent determination of P d by Barton and Brown,' Solomon' calculated that Pf/Pd = 3.4 and concluded that the red cell equivalent pore had a radius of 4.2-4.6 A.Further support for the concept of an aqueous pore was provided by Vieira et aLB who measured the apparent activation energy for water diffusion into the human red cell and found it to be 6 kcal/mol, slightly greater than the 4.5 kcal/mol characteristic of water diffusion in bulk s o l~t i o n .~ Vieira et a/. found the activation energy* Supported in part by National Institutes of Health grants 5R01 GM15692 and 2R01Annals New York Academy of Sciences for water diffusion into the dog red cell, whose equivalent pore radius was 6 A, to be 4.5 kcal/mol, the same as in bulk solution, and ascribed the slightly larger value in human red cells to interactions between the water molecule and the hydrophilic walls of the smaller pore. Wang9 measured water diffusion in bulk solution and computed the product D w~w / T (in which DW is the diffusion coefficient and q~, the viscosity of bulk water), which contains all the temperature-dependent terms in the Sto...

The permeability of red blood cells to chloride, urea, and water

“…Whether water and urea share AQP1, as suggested by some (Solomon, 1968;Solomon et al, 1983) but not by others (Macey, 1984;Galey and Brahm, 1985;Brahm and Galey, 1987;Finkelstein, 1987), or UT-B (Yang and Verkman, 1998;Yang and Verkman, 2002;Levin et al, 2007) makes no principal difference in the testing strategy of the hypothesis. Firstly, is P solute above that of lipid bilayers that have no transporters inserted and, if so, does the transport saturate?…”

“…A P f greater than P d indicates that the membrane contains pores, and the ratio P f :P d was interpreted as a measure of the width of the pores in the socalled 'equivalent pore theory' (Solomon, 1968). The pores were assumed to accommodate transport of water, small nonelectrolytes such as urea, and even anions (Brown et al, 1975;Poznansky et al, 1976;Solomon et al, 1983). We questioned the concept in a study of chick RBC and a preliminary qualitative study of RBC from different species Wieth and Brahm, 1977).…”

Effects of low ionic strength media on passive human red cell monovalent cation transport.

“…However, it is worthwhile to note that DIDS inhibits the low ionic strength effect by about 70 % (Jones & Knauf, 1985) which suggests a role for the anion transport protein Band 3. Also Solomon, Chasan, Dix, Lukacovic, Toon & Verkman (1983) have already speculated that the anion transport protein can be involved in cation transport. It is possible of course that the DIDS effect could be due to a more non-specific DIDS-membrane protein interaction.…”

Evidence from oocyte expression that the erythrocyte water channel is distinct from band 3 and the glucose transporter.

“…The plasma membrane of mammalian erythrocytes contains a specialized channel or pore for rapid transport of water (1,2).…”