Uptake of cholesterol into rabbit jejunum using three in vitro techniques: importance of bile acid micelles and unstirred layer resistance

The rate of uptake (Jd) of cholesterol into the intestine is influenced by the effective resistance of the unstirred water layer, the concentration of the probe molecule at the aqueous-membrane interface, and the passive permeability characteristics of the membrane. This study was undertaken to determine the influence of the bile acid micelle and the unstirred water layer on the Jd of cholesterol into rabbit jejunum, using everted sacs, full-thickness biopsies, and disks. When the bulk phase was stirred and the resistance of the unstirred layer was low, there was a linear relation between cholesterol concentration in the bulk phase and Jd when the concentration of taurodeoxycholic acid (TDC) was constant, but increasing TDC in the presence of a constant concentration of cholesterol was associated with a decline in Jd. When the concentration of both TDC and cholesterol was varied but the ratio of TDC to cholesterol was maintained constant, Jd increased slightly. The Jd of cholesterol was higher into sacs than into biopsies, which in turn was greater than into disks; Jd into disks was much lower when the resistance of the unstirred layer was high. These results suggest that 1) the bile acid micelle serves to provide a reservoir for partitioning of the cholesterol from the micelle into the aqueous phase from which the cholesterol is absorbed; 2) the Jd of cholesterol into disks of jejunum is influenced by the effective resistance of the unstirred water layer; and 3) although the quantity of cholesterol Jd varies markedly between sacs, biopsies, and disks, the qualitative aspects of the role of the bile acid micelle in cholesterol absorption were similar using the different in vitro techniques.