Factors influencing the uptake of cholesterol by isolated brush border membranes from rabbit small intestine.

The uptake of [3H] cholesterol by isolated brush border membranes from rabbit small intestine was studied under different conditions. In initial experiments the uptake from taurocholate-micellized suspensions was found to be greatly enhanced by Ca2+. Maximum stimulation was obtained with 10 mM Ca2+ and this condition was adopted for some of the subsequent experiments. In the presence of this cation, the uptake was too rapid to allow kinetic studies and only equilibrium values could be measured. Addition of lecithins to the micelles had an inhibitory effect on the equilibrium uptake in the presence of Ca2+ and on the rate of uptake in the absence of this cation which increased with acyl chain length. Admixture of the sterol with dipalmitoylphosphatidylethanolamine stimulated the uptake whereas admixture with dipalmitoylphosphatidylglycerol as well as with 1-monoolein alone or together with oleic acid had little or no effect on uptake in the presence of Ca2+. Again with this cation present, preincubation of membranes with the dipalmitoyl analogue of lecithin increased the equilibrium uptake of the sterol and abolished the inhibitory effect of the choline lipid. This inhibitory effect could be seen only with untreated membranes when lecithin and cholesterol were micellized together. In the absence of Ca2+, preincubation with dipalmitoylphosphatidylcholine had no marked effect on the rate of cholesterol uptake and the suppressive effect of lecithin on this rate could be seen even with lipid-treated membranes provided the lecithin and sterol were micellized together.(ABSTRACT TRUNCATED AT 250 WORDS)