EFFECTS OF CHOLESTEROL ON TRANSMEMBRANE WATER DIFFUSION IN HUMAN ERYTHROCYTES MEASURED USING PULSED-FIELD GRADIENT NMR

The effect of cholesterol on the diffusional permeability of water in suspensions of human erythrocytes was studied by means of pulsed field gradient NMR, which unlike the relaxation NMR method avoids the use o f Mn2+ ions. The analysis allows the internal and external diffusion c oefficients, as well as the lifetime characterizing the rate of exchan ge between the two regions, to be extracted from the data. The cholest erol content of the erythrocyte membranes was altered by incubating th e cells with sonicated dispersions of cholesterol/dipalmitoyl phosphat idylcholine at 310 K. It was shown that decreasing the molar ratio of cholesterol to phospholipid (C/P ratio) of the membrane, from a mean v alue of 0.92 for normal cells (controls) to a value of 0.46, had littl e effect on the intracellular mean residence lifetime and the diffusio nal permeability. Enriching the cholesterol content of the membrane, h owever, had a marked effect on the exchange lifetime and the diffusion al permeability. At a C/P ratio of approximate to 1.5 the rate of tran sport was reduced approximate to 3.5-fold. A further increase of the c holesterol content, to a C/P ratio of approximate to 1.9, resulted in an enhancement of the rate of transport back to a normal (control) val ue, which was characterized by a lifetime of 8-9 ms. The combined inhi bition of the water permeability by cholesterol and pCMBS for cells wi th C/P ratios of 1.44 and 1.54, and by pCMBS alone for cells with a co ntrol C/P ratio resulted in the same value for P-d within experimental error.