Studies were undertaken to define the mechanism whereby bile acid facilitates fatty acid and cholesterol uptake into the intestinal mucosal cell. Initial studies showed that the rate of uptake (Jd) of several fatty acids and cholesterol was a linear function of the concentration of these molecules in the bulk phase if the concentration of bile acid was kept constant. In contrast, Jd decreased markedly when the concentration of bile acid was increased relative to that of the probe molecule but remained essentially constant when the concentration of both the bile acid and probe molecule was increased in parallel. In other studies Jd for lauric acid measured from solutions containing either 0 or 20 mM taurodeoxycholate and saturated with the fatty acid equaled 79.8±5.2 and 120.8±9.4 nmol.min-1.100 mg-1, respectively: after correction for unstirred layer resistance, however, the former value equaled 113.5±7.1 nmol.min-1.100 mg-1. Maximum values of Jd for the saturated fatty acids with 12, 16, and 18 carbons equaled 120.8±9.4, 24.1±3.2, and 13.6±1.1 nmol.min-1.100 mg-1, respectively. These values essentially equaled those derived by multiplying the maximum solubility times the passive permeability coefficients appropriate for each of these compounds. The theoretical equations were then derived that define the expected behavior of Jd for the various lipids under these different experimental circumstances where the mechanism of absorption was assumed to occur either by uptake of the whole micelle, during interaction of the micelle with an infinite number of sites on the microvillus membrane or through a monomer phase of lipid molecules in equilibrium with the micelle. The experimental results were consistent both qualitatively and quantitatively with the third model indicating that the principle role of the micelle in facilitating lipid absorption is to overcome unstirred layer resistance while the actual process of fatty acid and cholesterol absorption occurs through a monomer phase in equilibrium with the micelle.
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