Journal of Lipid Research, Vol 21, 1097-1107, Copyright © 1980 by Lipid Research, Inc.
Influence of site and unstirred layers on the rate of uptake of cholesterol and fatty acids into rabbit intestine
AB Thomson
Passive permeation into the intestine is influenced by resistance of the
unstirred water layer (UWL) and the microvillus membrane. Failure to
account for UWL leads to gross underestimation of apparent passive
permeability coefficients, and to qualitative errors in the interpretation
of the nature of absorptive processes. A previously validated in vitro
technique was used 1) to determine the permeability characteristics of the
rabbit jejunum (J) and ileum (I) towards a homologous series of saturated
fatty acids; 2) to measure UWL in jejunum and ileum under conditions of
variable effective resistance of the UWL; and 3) to estimate the
preferential site of absorption of cholesterol in the intestine. The rate
of uptake, Jd, of fatty acids was similar in J and I when UWL was high;
although Jd was higher in J than I when UWL was low, the UWL was smaller in
I than J at each rate of stirring of the bulk phase, and the incremental
free energy change for the uptake of medium chain-length fatty acids was
similar in J and I, indicating similar passive permeability characteristics
along the length of the intestine. However, the intercept of the
relationship between chain length and uptake was significantly different
for the two sites, suggesting a greater surface area, Sm, of the membrane
in J than I. These findings persisted over a wide range of different
periods of incubation and of concentrations of cholesterol and bile acids.
The results suggest that i) cholesterol uptake in the J and I occurs from
an aqueous monomolecular phase into the microvillus membrane; and ii)
reported variations in the Jd of cholesterol along the intestine are likely
due to differences in UWL, Sm, or availability of substrate and not to
differences in the permeability properties of the intestine.
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