Diffusional water permeability of mammalian red blood cells

An extensive programme of comparative nuclear magnetic resonance measurements of the membrane diffusional permeability for water (Pd) and of the activation energy (Ea,d) of this process in red blood cells (RBCs) from 21 mammalian species was carried out. On the basis of Pd, these species could be divided into three groups. First, the RBC's from humans, cow, sheep and "large" kangaroos (Macropus giganteus and Macropus rufus) had Pd values approximately 5 x 10(-3) cm/s at 25 degrees and 7 x 10(-3) cm/s at 37 degrees C. The RBCs from other marsupial species, mouse, rat, guinea pig and rabbit, had Pd values roughly twice higher, whereas echidna RBCs were twice lower than human RBCs. The value of Ea,d was in most cases correlated with the values of Pd. A value of Ea,d approximately 26 kJ/mol was found for the RBCs from humans and the species having similar Pd values. Low values of Ea,d (ranging from 15 to 22 kJ/mol) appeared to be associated with relatively high values of Pd. The highest values of Ea,d (33 kJ/mol) was found in echidna RBCs. This points to specialized channels for water diffusion incorporated in membrane proteins; a relatively high water permeability of the RBC membrane could be due to a greater number of channel proteins. There are, however, situations where a very high water permeability of RBCs is associated with a high value of Ea,d (above 25 kJ/mol) as in the case of RBCs from mouse, rat and tree kangaroo. Moreover, it was found that Pd in different species was positively correlated to the RBC membrane phosphatidylcholine and negatively correlated to the sphingomyelin content. This suggests that in addition to the number of channel proteins, other factors are involved in the water permeability of the RBC membrane.