Effects of vitamin A and ethanol on liver plasma membrane fluidity

To study possible mechanisms whereby vitamin A and ethanol may affect liver plasma membranes, rats were fed liquid diets containing either 6 international units of vitamin A per kcal or 5 times more, with or without ethanol (36% of total energy as isocaloric substitution for carbohydrate). Vitamin A supplementation resulted in 2- to 3-fold increases of liver plasma membrane free retinol (p less than 0.005) and retinyl esters (p less than 0.001), particularly esters of palmitate and oleate, whereas cholesterol esters did not change. The fluorescent probe 1,6-diphenyl-1,3,5-hexatriene revealed decreased fluidity as measured by an increase in fluorescence polarization which correlated significantly with retinyl palmitate plus oleate content in membranes. In rats fed ethanol chronically, we first verified our previous observation of a decrease in liver plasma membrane fluorescence polarization. We now find this effect to be associated with (and possibly due to) an increase of cholesterol ester content. In linear regression analysis, the change in fluorescence polarization correlated positively with vitamin A (p less than 0.02) and negatively with cholesterol ester contents (p less than 0.001). Ethanol feeding partially offset the effect of vitamin A supplementation on fluorescence polarization. We conclude from these observations that liver plasma membranes contain a significant amount of vitamin A, that vitamin A supplementation increases membrane fluorescence polarization and that chronic ethanol administration can interfere with this effect.