Suppression of red cell diffusional water permeability by lipophilic solutes

The inhibitory effect of a series of neutral lipophilic solutes (methanol, ethanol, n-propanol, iso-propanol, n-butanol, iso-amylalcohol, n-hexanol, d iethylether, nitrobenzene, and pyridine) on the diffusional water permeabil ity (P-d. tot) of bovine erythrocyte membrane at 25 degreesC was studied in comparison to that of p-chloromercuri benzoate (pCMB). Permeability data w ere obtained by measuring the transmembrane diffusional water exchange time tau (exch) using an H-1-T-2 NMR technique. Maximal inhibition by similar t o 50% of P-d.tot was produced by 2 mM pCMB which completely blocked the mem brane water channels in 20 min, hence suggesting the channel-to-lipid diffu sional water permeability ratio of about 1:1. Furthermore, the maximal inhi bitory effect of pCMB in combination with the lipophilic solutes was lower than that of pCMB alone. As pCMB does not interfere with the lipid bilayer, and provided that it blocks the water channels in solute presence as well, this confirms that the solutes induce an increase in the lipid-mediated ba ckground water permeability contribution (P-d. lipid) by the formation of a queous leaks in the membrane hydrophobic barrier. However, faster but less efficient in permeability inhibition than pCMB (either alone or combined wi th solutes) were the lipophilic solutes alone. Taken together, the results indicate that the lipophilic solutes suppress the membrane total permeabili ty P-d. tot by two opposing effects: a reduction of its channel-mediated pa rt (P-,P-d (channel)) to the extent exceeding that of a simultaneous P-d. l ipid increase. The inhibitory potency of the solutes tested appears to be c orrelated with their solubility in the membrane medium. (C) 2000 Elsevier S cience S.A. All rights reserved.