Changes in the viscosity of the plasma membrane of flounder (Platichthys flesus L.) erythrocyte induced by varying the content of membrane cholesterol or by benzyl alcohol. Correlation of the activity of the intrinsic Mg2+-ATPase and the viscosity.

The order parameter S and the correlation time tau from the ESR-spectra of 5-DS and 5-DD, respectively, are used as measures of the viscosity of the plasma membrane. It is suggested that the membrane undergoes a thermotropic phase transition which correlates with a change of the activation energy of the (Na+ + K+)-ATPase but not of the Mg2+-ATPase. The viscosity varies almost proportionally with the amount of cholesterol in the membrane when 0 less than mg C/mg PL less than 0.5. It is suggested that cholesterol is uniformly distributed in the membrane when mg C/mg PL less than 0.5 and that cholesterol-rich domains appear when mg C/mg PL greater than 0.5. Benzyl alcohol decreases the viscosity of the membrane. The effect depends on the presence of cholesterol in the membrane. The onset of the effect of benzyl alcohol on the native membrane is delayed--in terms of alcohol concentration--relative to the cholesterol-depleted membrane. Benzyl alcohol affects only the outer part of the membrane bilayer. Changes of the viscosity and of the activity of the Mg2+-ATPase are proportional. The effect of cholesterol-depletion on the activity of the Mg2+-ATPase cannot be ascribed only to the simultaneous lowering of the viscosity.