Transport of water and urea in red blood cells

Abstract

Evidence for water channels in red blood cells is reviewed. In an entropically driven reaction, organic mercurials decrease water permeability, elevate the activation energy, and reduce the ratio of osmotic to diffusional water permeabilities to unity so that water transport properties of red blood cells are hardly distinguishable from lipid bilayers. It is concluded that mercurials close the water channels. A variety of kinetic, pharmacological, and comparative evidence converges on the conclusion that urea and other solutes are excluded from water channels. Urea apparently permeates the red cell membrane via a facilitated diffusion system, which plays an important role when red blood cells traverse the renal medulla; rapid urea transport helps preserve the osmotic stability and deformability of the cell, and it helps prevent dissipation of extracellular osmotic gradients. Water apparently traverses the channel via a single-file mechanism; the very low channel permeability of H+ is explained if the channel contains fixed charge, or alternatively, if the mobile water molecules within the channel do not form a continuum. An alternative unitary pore hypothesis for simultaneous transport of water, ions, and small solutes is also discussed.

Mercuric Chloride Effects on Root Water Transport in Aspen Seedlings

“…A low E a (Ͻ6 kcal mol Ϫ1 ) for water transport is among the typical features of membranes with water-transporting pores (Chrispeels and Agre, 1994;Verkman et al, 1996;Maurel et al, 1997;Niemietz and Tyerman, 1997), while transport through the membrane lipid bilayer is associated with high Arrhenius E a values. Mercurials can increase the E a of water permeation facilitated by water channels (Macey, 1984;Meyer and Verkman, 1987;van Hoek et al, 1990). However, in our study, HgCl 2 significantly reduced E a values for Q v in roots and more studies will be required for a proper explanation of these results.…”

“…Water transport via aquaporins is characteristically inhibited by mercurial reagents, which react with sulfhydryl groups in the channel proteins and result in closure of the channels. This closure inhibits water transport and increases E a to the level of that for transport through the lipid pathway (Macey, 1984). An inhibition of water transport by mercury was reported in cell membranes isolated from higher plants Niemietz and Tyerman, 1997) and in whole root systems (Maggio and Joly, 1995;Carvajal et al, 1996).…”

“…These include a high ratio of osmotic to diffusional water permeability (P f /P d Ͼ1), low Arrhenius activation energy (E a Ͻ 6 kcal mol Ϫ1 ) for water transport, and its reversible inhibition by mercury sulfhydryl reagents (for reviews, see Chrispeels and Agre, 1994;Verkman et al, 1996;Maurel, 1997). The transport of water through the lipid bilayer has a high E a , usually above 10 kcal mol Ϫ1 (Macey, 1984). Water transport can also be via water channel proteins (aquaporins), which have been found in the tonoplasts (Maurel et al, 1993) and plasma membranes (Kammerloher et al, 1994) of plants.…”

Lack of Aquaporin 3 in bovine erythrocyte membranes correlates with low glycerol permeation

“…The presence of an aquaglyceroporin in these red cells imply that this channel may additionally be involved in other mechanisms, possibly contributing to volume regulation after exposure to local osmotic stresses. After the initial osmotic shrinkage due to glycerol gradients stimulated by fasting, AQP3 expressing erythrocytes would be enabled to rapidly recover their volume, a similar phenomenon hypothesised for urea when red blood cells cross the kidney medulla [33]. AQP3 can also moderately permeate urea although to a lesser extent [1] and this function might also play a role in volume regulation in hRBC.…”

Two TIP‐like genes encoding aquaporins are expressed in sunflower guard cells

“…The swelling rate increments corresponded to a 6.4, 6.6 or 17.5 fold enhancement in osmotic water permeability ofthe oocyte membrane for SunTIP7 (6.21 10 -3 cm -1 sec), SunTIP20 (6.44 10 -3 cm -1 sec) and yTIP (17.05 10 -3 cm -1 sec, Figure 2), respectively. These permeabilities were lowered by 90% in the presence of 1 mM HgCI2 which is a characteristic inhibitor of water channel proteins (Macey, 1984).…”