DIFFERENT MECHANISMS OF UPTAKE OF STEARIC-ACID AND CHOLESTEROL INTO RABBIT JEJUNAL BRUSH-BORDER MEMBRANE-VESICLES

The rate of uptake of stearic acid and cholesterol solubilized in taurocholic acid (TC) was examined in rabbit jejunal brush border membrane vesicles (BBMV). For stearic acid (18:0) or cholesterol there was an initial rapid rate of uptake, which reached a plateau within approximately 1 min and remained stable thereafter. At low concentrations of 18:0 and 20 mM, but not 2 mM, TC, there was a curvilinear relationship between the concentration of 18:0 and uptake, whereas the relationship between cholesterol uptake and concentration was linear over a wide range of values. When the concentration of TC was held constant at increasing concentrations of 18:0 or cholesterol, there was a linear increase in the rate of uptake. When the concentration of 18:0 or cholesterol was held constant and the concentration of TC was increased from 2 to 20 mM, the uptake of 18:0 declined, but the rate of uptake of cholesterol increased. When the concentrations of 18:0 plus TC, or cholesterol plus TC, were both increased in unison and their ratio was held constant, their rate of uptake increased. Thus, (i) BBMV may be used to assess the rate of uptake of lipids; (ii) the partitioning of cholesterol from bile acid micelles into the BBMV appears to be by way of ''collision'' of the cholesterol with the membrane. In contrast, the uptake of 18:0 from the micelle into the membrane vesicles may be by both the collision and the aqueous/dissociation models; and (iii) 18:0 uptake may be mediated by both a concentration-dependent and a concentration-independent component. Thus, stearic acid and cholesterol seem to be taken up from bile acid micelles into rabbit BBMV by different mechanisms.